AIR-SEA INTERACTION STUDIES IN CHINA
PU Shuzhen1, ZHAO Jinping1, YU Weidong1, ZHAO Yongping2 and YANG Bo1
1. First Institute of Oceanography, State Oceanic Administration, Qingdao 266061, China
2. Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China
ABSTRACTIt is summarized in this paper that the progresses of large-scale air-sea interaction studies have been achieved in China in the four year period from July 1998 to July 2002, including seven aspects in the area of the air-sea interaction i. e. the field experiments and oceanic cruise surveys, air-sea interaction related to the tropical Pacific, monsoon-related air-sea interaction, air-sea interaction in the north Pacific Ocean, air-sea interaction in the Indian Ocean, and the air-sea interactions in global oceans. However more attention has been paid to the second and the third aspects because a large number of papers in the reference literature for preparing and organizing this paper are concentrated in the tropical Pacific Ocean, such as ENSO process with its climatic effects and dynamics, and the monsoon-related air-sea interaction. The literature also involves various phenomena with their different spatio-temporal scales from each other such as intraseasonal and annual, and interannual, interdecadal variabilities in the atmosphere/ ocean interaction system, reflecting the contemporary themes in the four years period at the beginning of an era from the post TOGA to CLIVAR. Apparently it is a difficult task to summarize a great progress of this area, as it is extracted from a large quantity of the literature into such a short article, although authors have tried hard. Anyway the significance of the progresses is much greater and better than that of this article.
Key words: review of air-sea interaction, various spatio-temporal scales, atmosphere/ocean variability, climatic abnormality
The air-sea interaction study is an important scientific field in China. The research activities of Chinese scientists are vigorous and productive in this field. A large number of the scientists have participated in the international programs related to the large scale air-sea interaction studies such as TOGO, COARE, WOCE, CLIVAR and the others for many years, and organized domestic projects funded by National Natural Science Foundation of China, Chinese Academy of Sciences, China Meteorological Administration, State Oceanic Administration of China, and other agencies. This report is intended to briefly introduce the progress in the large-scale air-sea interaction studies made in China according to the literature published by Chinese scientists in the 4-years period from July 1998 to July 2002(the contribution in this scientific field by scientists from Taiwan, Hongkong and Macao of China is not included for the references of this report).
I . THE FIELD EXPERIMENTS AND CRUISE SURREYS
The South China Sea Monsoon Experiment (SCSMEX) is an international cooperative project in which China plays a major role to study the air-sea interaction processes related to the South China Sea monsoon. Three research vessels, i.e., R/V <Xiangyanghong No. 14>, R/V <Science No. 1>, and R/V<Experiment No. 3> have been involved in the field experiment of the project standing to sea for 2 months in 1998. Measurements of physical oceanography, meteorology and marine chemistry were conducted, including CTD casts, ADCP profilings, sea surface meteorological observations, high altitude soundings and marine chemistry water samplings. The multi-ship and multi-discipline observations lay the foundation of the basic researches for understanding the processes of the monsoon transition and its northward progression in the South China Sea in late spring and early summer, and the oceanic response to the monsoon atmospheric circulation. The research vessels for the field experiment of SCSMEX are supported by Chinese Academy of Sciences and State Oceanic Administration of China.
In addition to the SCSMEX field observations, a program “Pacific-Indian Warm Pool. Its Current System and Air-sea Interaction” is funded by Chinese Ministry of Sci. & Tech. and National Natural Science Foundation. A pilot study of this program started in 2001-2002, 3 Argo-floats have been deployed in the warm pool region on the board of R/V <Snow Dragon> when the ship navigates toward the Antarctica and some XBT drops deployed in the tropics as well. The program will help further understand the ENSO dynamics and the variability in the tropical waters.
The Southern Ocean near Prydz Bay is another sea area where Chinese scientists frequently conduct air-sea field experiments on the board of Chinese R/V <Snow-Dragon>. A program “Physical Process Study of Air-sea-ice Interaction in the Antarctic Ocean (1996-2000) ” was supported by State Oceanic Administration of China, and 5 summertime cruises of the southern ocean expeditions have been completed in the 5 years, obtaining full depth CTD data and water samples for chemistry.
II . THE AIR-SEA INTERACTION STUDIES RELATED TO THE TROPICAL PACIFIC OCEAN
The tropical Pacific Ocean is the nearest tropical sea area to China, where the oceanic abnormality is closely associated with the climatic variability in East Asia including China. Chinese oceanographers have drawn much attention to the Ocean for a long time. The study activities can be divided into the following aspects.
1. Intraseasonal Variability in the Tropical Pacific Ocean
The TOPEX/POSEIDON altimetric data have been used to analyze the spatial distribution of the intraseasonal oscillation in the tropical Pacific Ocean (Liu and Wang 1999, Hu and Liu 2002). Liu and Wang (1999) also used the model simulation output from POM with eddy-resolution for this purpose. As their results show, the quasi-30 day oscillation can be found in the zonal belts about 5°N and 5°S to the east of 160°W by means of the data analysis. The quasi-90 day oscillation is evidenced in the zonal belts about 20°N and 20°S, and it is more obvious in the western Pacific Ocean of the 20°N belt. The signal for the quasi-60 day oscillation distributed in the 10°N zonal belt and the 20°S belt. It is weaker than the quasi-90 day signal although it is distinct in the data. The signal in the 10°N belt is more obvious than that in the 10°S belt. In addition, the quasi-60 day oscillation can be also found in the eastern equatorial Pacific Ocean (5°N-5°S, 170°W-120°W). Hu and Liu (2002) report that the 90 days oscillation with its annual variability is closely related to the ENSO occurrence. In addition to the altimetry data, the satellite OLR data (1979-1993) are also used to find out the source of the intraseasonal oscillation (ISO) in the eastern tropical Indian Ocean and the western tropical Pacific Ocean. The water area where the intraseasonal oscillation with a period of 6.5-12.5 pentads is the most vigorous in the eastern tropical Indian Ocean, ISO with their respective periods can also be obviously evidenced in the tropical ocean to the northwest of Australia, in the tropical Pacific Ocean to the northeast of Australia, in the south part and the north part of the South China Sea, in the Luzon strait, and the area to the south of Japan (Wang, Liu and Xu, 2000).
ISO and semiannual oscillation on the 200 hPa in the tropical atmosphere, and their dependence upon SSTA of the eastern equatorial Pacific Ocean are analyzed by means of the estimate for the oscillation kinetic energy and the calculation for the correlation between the energy and the SSTA. It is revealed that the positive SSTA corresponds to weaker atmospheric oscillations and the negative SSTA to stronger oscillations (Li C. and Li G., 1999).
2. Interannual Variabilities of Wind, Sea Temperature etc. and ENSO Process Study
Of wind, sea temperature etc., the ENSO interannual variability of the tropical Pacific Ocean continues to be one of the most interesting subjects in the air-sea studies. Long time series of SST, precipitation, wind, oceanic heat, geopotential h eight and other meteorological or oceanographic observational elements are widely used to determine their intrannural variabilities and the ENSO-evolution process. Yin and Ni (2001) used the NCEP/NCAR monthly mean SST and 1000 hPa wind data (1979-1998) to study the interannual variability in the tropical Pacific Ocean, Indian Ocean and Atlantic Ocean, to describe the characteristics related to the ENSO events, and to estimate the correlation among the three oceans. It is reported that the contemporary correlation between the regional SSTA representative of the Indian Ocean and the regional SSTA representative of the eastern equatorial Pacific Ocean is positive and weakly negative between the regional SSTA representative of the eastern equatorial Atlantic and the regional SSTA representative of the eastern equatorial Pacific. The positive correlation reaches the greatest when the regional SSTA of the equatorial Indian Ocean appears 3 months lags behind the regional SSTA for the eastern equatorial Pacific Ocean and the negative correlation reaches the maximum when the regional SSTA for the equatorial Atlantic appears 6 months leads the regional SSTA for the equatorial Pacific Ocean.
It is reported (Li et al., 1999) that the monthly mean SSTA (from COADS) and the 500 hPa height (H500) field (from National Meteorology Centre of China) are analyzed by means of the CEOF method to study the low-frequency variabilities of SSTA in the global tropical waters and the H500 field in the northern hemisphere (Li et al., 1999). The both quasi-biennial and 3-7 year oscillations with phase-lock characteristics can be found in the SSTA and H500 field. The H500 variability lags about 2 months behind the SSTA. The component for the 3-7 year oscillation of H500 has a stationary wave pattern like the PNA pattern, while the component for the quasi-biennial oscillation of H500 has a progressive wave feature.
Since the heat content in the tropical Pacific Ocean is an important causative element of heat exchange between the upper ocean and the atmosphere above, its variability receives great attention of Chinese oceanographers. Scientific evidence shows that the heat content in the upper layer above the thermocline of the equatorial Pacific Ocean is accumulated before an El Niño event appears, and its variability a few months later leads to the SSTA of the eastern equatorial Pacific Ocean (Pu et al., 1999). The location for the maximum lag correlation between the heat anomaly in a water column of the area off equator and that of the eastern equatorial area tends to move in an anti-clockwise way in the northern tropical Pacific Ocean, and in a clockwise way in the southern tropical Pacific Ocean, completing circling in about 4 years. It is also reported (in the submitted paper by Yu and Qiao) that a similar cycling related to ENSO can be found for the heat content in the upper layer above 400 m depth of the tropical Pacific Ocean.
Li, Yu and Zhang (2001) studied the interannual variability of MJO using the daily mean oceanic rainfall data (1979-1991) of Microwave Sounding Unite and the pentad SST data from Climate Prediction Center Merged Analysis of Global Precipitation (1982-1992). It is found that MJO is limited to the west of the dateline in a normal year while it can extend further eastward and reach the central-eastern tropical Pacific Ocean in an ENSO year. The most significant correlation between the interannual variability of MJO and SST in the central-eastern Pacific Ocean is located in the El Niño 3 region. A numerical simulation result confirms the interannual variability of MJO in the tropical Pacific rainfall by use of the CCM3 model forced by the observational SST. It is also reported (Wu et al., 2002) that a global AGCM (L9R15 version) forced by COADS SST has been integrated for the period from 1945 to 1993. It is found that there are the interannual and interdecadal variabilities of the simulated surface wind over the tropical Pacific Ocean. The model output has been compared with the observations, showing a good agreement between the two.
The ENSO or ENSO-related studies remain to be a focus of the air-sea interaction research in China, which result in increase of the knowledge about the ENSO processes and dynamics. Particularly the relevant papers are concentrated in the oceanic subsurface temperature variability and its close relation with the ENSO occurrence. Pu et al. (1998) discussed the evolution of the 1997/1998 ENSO events using the observational data. It is concluded in the case study that the variability of the sea temperature anomaly in the tropical oceanic thermocline and the Kelvin wave propagation play an important role in the ENSO evolution, and the subsurface warming, which follows the westerly burst, leads the surface warming in the tropical ocean. Li (2002), Li and Mu (1999) point out that subsurface oceanic temperature anomaly in the equatorial Pacific Ocean is closely related to the occurrence of ENSO. It is the positive (negative) subsurface anomaly that results in the El Niño (La Niña) occurrence. They attribute the interannual variability of the subsurface temperature in the tropical Pacific Ocean to the East Asia monsoon abnormality. Chao, Q. and Chao J. (2001) analyse the 1955-1998 SST in the western tropical Pacific Ocean, concluding that the positive subsurface temperature anomalies have appeared at about 120 m depth in the western tropical Pacific Ocean before SST becomes warmer in an ENSO event. Using the XBT data of the upper tropical Pacific Ocean, Chen et al. (2002) reveal that the sea temperature anomaly of the north equatorial current (10°N) is closely associated with the subsurface temperature anomaly of the western Pacific warm pool. In the early stages of an ENSO cycle, the warmer water located in the thermocline of the certral and eastern Pacific Ocean can be transported westward to the western Pacific warm pool region by the north equatorial current, accumulating in the region, and extending toward the equator. Therefore the warmer thermocline water transport by the north equatorial current is one of the mechanisms for the ENSO occurrence.
The sea surface wind anomalies, which play an important role in the ENSO occurrence and evolution, also provoke the attention of Chinese scientists. Zhang and Huang (1998) and Pu et al. (1999) report that the westerly wind anormalies in the western tropical Pacific Ocean initially appear in the western and central Pacific Ocean during an ENSO event, and shift eastward. The SST in the eastern equatorial Pacific Ocean increases after the wind anormaly got stronger and gradually propagated eastward. The SST reaches its highest when the wind anormaly was located in the central Pacific Ocean. Then SST decreases in the eastern and central Pacific Ocean, and the easterly anormaly appears in the western tropical Pacific Ocean. Finally the normal conditions are recovered. According to the model simulation the dynamical effects of the wind anormaly on an ENSO cycle have been described, and the Kelvin wave reflection and the Rossby wave generation, and their effects on the ENSO duration in the tropics discussed by Zhang and Huang (1998), Huang et al. (1998), Yan, Huang and Zhang (2001) and Huang, Zhang and Yan (2001). Using an intermediate coupled model Yan and Zhang (2002a) hindcast the 1997/1998 ENSO event and discuss the wind anormaly importance for the ENSO cycle, when the model was forced by the observational wind anormaly during the period (1971-2000). The model is able to simulate all the ENSO events with a correlation of about 0.63 between the observational El Niño 3 SSTA and the model output. Yan and Zhang (2002b) also analyse the seasonal variability of the westerly anormaly during a composite ENSO event in the western tropical Pacific Ocean and find out the similarity of the seasonal variability between the westerly anormaly and the monthly mean divergence. The characteristics of the seasonal variability can successfully be simulated by a simple equatorial ocean-atmosphere coupled model. Their model results show that the model heating terms being dependant upon the basic climatic state is necessary in simulating the seasonal variability of the westerly anormaly, playing an important role in forming the westerly anormaly in the western equatorial Pacific Ocean at the beginning of an ENSO event. Then in the developing or mature phase of an ENSO cycle, SSTA in the eastern equatorial Pacific Ocean is more important to forming the westerly anormaly. In addition to the zonal wind anormaly the meridional wind anormaly in the tropical ocean is also analyzed. By using the singular value decomposition method, Zhang, Chao and Tan (2001) analyze the meridional wind anormaly in the tropical Pacific Ocean and the El Niño 3 index and indicate that the variability of the meridional wind anormaly occurs about 6 months leads the SST variability. Zhang and Chao (2001) also utilize an analytical model forced by equatorial convergence of an fideal meridional wind field to do the diagnostic analysis on the effects of Rossby waves excited by the meridional wind anormaly on the SST variability. Zhang, Ding, Zhao and Huang (2002) analyze the SST and the meridional wind of the tropical Pacific Ocean to obtain their periodic variability by means of Singular Spectrum Analysis (SSA) method. The analysis shows that the variabilities with the quasi-4 year period, the quasi-2 year period and the interdecadal time scale are significant. The variability of the quasi-4 year period has the greatest variance contribution. They also compare the SSTA results with the results from the regional meridional winds respectively corresponding to the western tropical Pacific Ocean, the northeast tropical Pacific Ocean and the southeast tropical Pacific Ocean to find the spatial variability of SSTA and the regional wind characteristics of the SSTA distribution.
Using COADS data and SOI of National Climate Center of China, Zhao and Chen (1998) classified either the ENSO events or the La Niña events occurred since 1956 into two types, i. e. the eastern area type and the central area type. They find that La Niña with the unusually cold SSTA in the central and eastern tropical Pacific Ocean has occurred one year before the ENSO onset of the eastern area type, and ENSO with the unusually warm SSTA in the central and eastern tropical Pacific Ocean has occurred one year before the La Niña onset of the eastern area type. In contrast with the eastern area type, the central and western tropical Pacific SSTA becomes essential for an central area type event with the positive SSTA in the central and western Pacific before the central area type ENSO and with the negative SSTA there before the central area type La Niña.
On the basis of the data from National Meteorological Center Zhu et al. (1998) apply the objective analysis to describe the characteristics of the 1976/1977 ENSO event and the 1982/1982 ENSO event and the difference in the background of the interdecadal variability, in which the ENSO events occurred. The 1976/1977 ENSO event occurred in such a case that SST field was relatively cold, the westerly anormaly could be found on the 850 hPa level over the equatorial Pacific Ocean (0°-80°W), and the SSTA oscillation with a 4-year period was being propagated eastward. The 1982/1982 ENSO occurred in such a case that SST field was relatively warm, the easterly anormaly could be found on the 850 hPa level over the equatorial Pacific Ocean (0°-80°W), and the SSTA oscillation was being propagated westward.
In addition to these observational studies and the numerical model simulations for ENSO, Wang et al. (1991) presented an analytical model to explain the ENSO-like oscillation between the South China Sea and the western tropical Pacific Ocean. The analytical solution of the model coincides with the data analysis. Wang et al. (2000) discuss the parameters in the model and propose a method for determining the parameters suitable for the inteannual time scale oscillation.
3. Climatic Effects of both ENSO and Oceanic Variability in the Tropical Pacific Ocean
A large number of papers are concentrated in the climatic effects of ENSO and oceanic variability in the tropical Pacific Ocean. It is convenient for reviewing the papers to classify them into 3 categories: oceanic background of the 1998 heavy flood in the Changjiang River Basin, the ENSO effects on the Climate and the others.(1) Oceanic background of the 1998 heavy flood in the Changjiang River Basin
The 1998 flood is the heaviest flood of the river basin in the 20 century but the 1954 flood in China. It is found that when the flood occurred in 1998, the 1997/1998 ENSO event was coming to an end. The background of the air-sea interaction, in which the flood was caused, provokes the attention of Chinese scientists. Huang et al. (1998) analyse the observational data in order to find out the climatic and hydrological features and the cause of the catastrophic flood occurring in the 1998 summer. They find that the flood occurred during the transition of the 1997/1998 ENSO event from its mature phase to its decaying phase, when SST in the tropical Pacific Ocean, especially the subsurface temperature in the western tropical Pacific Ocean, became cooler, the convective activities around Philippines weakened, thus the western subtropical high was situated further southward. As a result from the abnormal climatic situation, abundant moisture which was carried by the Asia summer monsoon from the Bay of Bengal and the south China Sea converged with moisture from the western tropical Pacific Ocean, initially converging and flowing into the middle and lower reaches of the river and then into the upper and middle reaches, and caused the continuously severe rainfall in the river basin and catastrophic flood. using IAP/LASG GOALS model forced by global observational SST Guo, Zhao and Wang (2002) reproduce the heavy rainfall over the Yangtze River Valley in the 1998 summer and the subtropical high abnormality over the western Pacific Ocean. A series of numerical experiments are conducted on the basis of the observed SST in the selected oceanic regions and the climatic SST in the rest in order to test the regional effects on the rainfall. In addition to the regional effects, two more numerical experiments are also designed on the basis of the observed SST in the selected time periods and the climatic SST in the rest in order to test the temporal effects on the rainfall. The results show that SSTA in the Indian Ocean plays a more significant role than the other oceanic area for the rainfall during the flood, the SSTA in the Indian Ocean and the western Pacific Ocean is more closely related to the subtropical high abnormalities over the western Pacific Ocean than that in the other area. It is also found that SSTA in the 1998 summer plays a more important role for the rainfall to cause the flood rather than SSTA in the 1997 winter and the 1998 spring.
(2) ENSO effects on climate
Huang, Zhang R. and Zhang Q. (2000) analyzed the observed SSTA, the subsurface temperature data, the NCEP/NCAR reanalysis data and the data of the daily precipitation in China to describe their characteristics during the 1997/1998 ENSO cycle and to discuss the impact on the East Asia climate in the 1998 summer. Their analysis shows that the vapor transport by the Asia summer monsoon was very weak in both the North China and the southern Korea Peninsula in the 1997 summer when the strongest ENSO event of the 20th century rapidly developed, which caused the drought and the hot weather in the area, and a large amount of water vapor is continuously transported from the Bay of Bengal, the South China Sea, and the western tropical Pacific Ocean into the Yangtze River valley of China, South Korea and Japan in the 1997/1998 winter when the strongest ENSO event of the 20th century rapidly decayed. The situation was associated with the southward displacement of the western Pacific subtropical high, resulting in the severe floods of those regions. Zhang et al. (1999) study the seasonal precipitation occurring in China when an ENSO process reaches its mature phase in spring, summer, autumn or winter. They find that positive precipitation anomalies in the composite precipitation patterns are all located in the South part of China no matter an ENSO process reaches its mature phase in spring, autumn, or winter, and the anticyclonic anomaly of the lower troposphere is all situated to the north of the maritime continent. It is the anticyclonic anomaly that intensifies the western Pacific subtropical high, makes it further westward extend, and results in the positive precipitation anomaly in the South China. When an ENSO process reaches its mature phase in summer, the composite precipitation pattern is different from those in the other three seasons in China. The negative precipitation anomalies in the pattern are located respectively in the more southern part and the northern part of China, while the positive precipitation anomalies tend to be located in between i. e. in the lower reaches of the Yangtze River and the Huaihe River Valley. Although the western Pacific subtropical high is intensified, it covers the southeastern periphery of China where the precipitation decreases. Meanwhile the Indian summer monsoon can not reach the northern part of China providing less moisture inflows.
Chen, Le, Jia and Peng (2002) analyze the frequency of cyclone occurrence in the southern hemisphere (70°—170°E, 0°—80°S) and its correlations with the SST in the eastern equatorial Pacific Ocean and with the southern oscillation index, respectively. They find that the correlations depend on the latitudinal zones where the cyclones occur and the correlation coefficient between the SST and the cyclone frequency reaches its maximum when SST lags 24 months behind the frequency of the cyclone occurrence.
(3) Variabilities in the western Pacific warm pool and its climatic effects
Li, Mu and Zhou (1999) study the seasonal and interannual variabilities of oceanic temperature in the western Pacific warm pool and their impacts on climate including the influence on the East Asia monsoon, the teleconnection with the atmospheric circulation and the effect on ENSO occurrence. They find that the subsurface temperature in the western Pacific warm pool is closely related to the SST in the eastern equatorial Pacific Ocean rather than the local SST, the positive (negative) SSTA in the warm pool is related to the stronger (weaker) summer monsoon with the EPA wave-train pattern stretching further (less) northward to higher (lower) latitudes, and the occurrence of ENSO is closely related to the subsurface temperature anomaly in the warm pool and the quick developing of ENSO to the eastward propagation of the subsurface temperature anomaly.
Zhao, Chen and Bai (2000) study the long term variabilities in the western tropical Pacific warm pool, and find that its temperature decreases in the first decade of the 20th century and increases by about 0.5℃ in the 8th decade of the 20th century. When the warm pool temperature increases, the subtropical high extends further westward in the northwest Pacific Ocean and its ridge is further southward displaced, resulting in a change of the precipitation pattern in China, i. e. less rain in the north and more rain in the south. They also find that the El Niño events get more frequent and stronger in the warm period and the La Niña events get more frequent and stronger in the cold period.
Weng, Zhang and Yan (1998) study not only the variability of the subtropical high, but also the variability of the area of the western tropical Pacific warm pool. The spectral analysis of the time series of the both data shows the same spectral peak at a period of 46.7 months. The subtropical high becomes stronger (weaker) when the warm pool area is larger (less). Zhang et al. (1999) identify the 2 climatic precipitation regions of North China by the comparative analysis method, and find that one of the two regions is closely related to the heat content of the western tropical Pacific warm pool with the best negative correlation between the wet season precipitation in this region and the warm pool heat content in October of the previous year. The wet season precipitation of the another region is closely related to the heat transport of Kuroshio in winter of the same year.
Liu and Wang (2000) analyse the seasonal variation of SST around the Indonesia Archipelago. They find that the maximum longitudinal SST difference between the north and the south is located at 110°E in boreal winter while it is in 130°E—145°E and 110°E in boreal summer. The seasonal variation is advantageous to the formation and maintainace of the cross equator air-flow toward the north at 0°, 105°E.
(4) SST variability in the tropical oceans with the variability of the subtropical high over the northwest Pacific Ocean and their climatic effects
Ying and Sun (2000) analyze the observational data to study the response of the subtropical high over the northwest Pacific Ocean to the SST of the tropical oceans. They find the strength of the subtropical high is closely related to SSTA of the central and eastern equatorial Pacific Ocean, the Indian Ocean and the western Atlantic Ocean. The subtropical high becomes stronger for the SST pattern of El Niño type, and weaker for the SST pattern of La Niña type. Li Y. and Li C. (1999) analyze the air temperature variability of Sichuan Province, China and SST variability in the western tropical Pacific Ocean. They find that within the 50 years both of them have the same tendency toward their decreases from the warmest decade of the 1950s to the coldest decade of the 1980s, with the relatively abrupt change in the 1970s. When SST in the western tropical Pacific Ocean becomes increased (decreased), the subtropical high over the western Pacific Ocean is extended further westward (eastward) and the air temperature of Sichuan Province becomes warmer (cooler). By using the wavelet analysis and EOF methods, Zhao and Wu (2002) analyze the SSTA variability of the tropical Pacific Ocean. They also analyse the variabilities of the subtropical high index and precipitation of the Changjiang River Basin and the North China. They conclude that their variabilities with a periodicity of 60 years play an important role in the abundance of the rainfall in the river basin and droughts of the North China in the recent years.
III . MONSOON-RELATED AIR-SEA INTERACTION
Both the South China Sea monsoon and the East Asia monsoon are important climatic systems with annual cycle, which influence the precipitation and the agricultural economics in China. As the monsoon cycle is affected by the thermal condition of the upper oceans adjacent to China, the monsoon-related air-sea interaction is one of the most important subjects in oceanography and meteorology in China.
1. East Asia Monsoon
Peng, Sun and Ni (2000) propose a new definition for the East Asia monsoon index, which can better express the annual and interannual variations of the subtropical high over the northwest Pacific Ocean, and discuss the correlation among the SSTA in the eastern equatorial Pacific Ocean, the subtropical high in summer, and the East Asia summer monsoon. Their results show that the East Asia summer monsoon becomes weaker, and the subtropical high is stronger and extends further southward and westward in summer if SSTA increases in the eastern equatorial Pacific Ocean in spring the same year. Following the SSTA increase in spring, the rainfall becomes increased in the Changjiang River Basin in summer and decreased in the South China, the Hetao area and the area to the east of Hetao. The reverse is also true if the SSTA decreases in spring. A numerical diagnostic study on the effects of the duration of the positive SSTA in the eastern equatorial Pacific Ocean in winter and spring on the East Asia summer climate is conducted by Long and Li (1999). Their simulating results by the LAP 2 layer AGCM and the 9 layer spectrum AGCM show that the atmospheric response and climate abnormality in East Asia affected respectively by the positive SSTA durations (January only, January-February, January-April) are quite similar to each other. They find that the strength of the positive SSTA might play a more important role in the summer climate rather than that of the positive SSTA duration. Therefore the greater attention should be paid to the stronger positive SSTA even if its duration might be shorter. Based on the monthly precipitation data (1951-1998) at the 115 stations of the Eastern China and the global SST data of COADS, Yu et al. (2001) calculate lag correlation coefficient between the regional precipitation and the regional SST, and study the oceanic predominant region in forcing the eastern China summertime monsoon rainfall. They find that the seasonal variation of rainfall at any one of the 4 sub-regions are similar to each other in the eastern China, with a monsoon rainfall peak in June or July. They work out the respective lag correlation coefficients between the summertime monsoon rainfall in the Changjiang River sub-region and the mean SST in each of the 7 sub-regions in the Pacific Ocean and the Indian Ocean with SST leading the rainfall, and obtain the regression equation of the summertime monsoon rainfall for the sub-region, which depends upon mean SST at each oceanic sub-region with their respective leading times. The hindcast of the summertime monsoon rainfall by using the regression equation is consistent with the observed one. Li and He (2001) study the interdecadal features of the East Asia monsoon and its monsoon rainfall over the North China and their relations with the North Pacific SSTA on the basis of the NCEP/NCAR reanalysis data (2.5°´2.5°), and the SST data and rainfall data of the National Climate Center of China. They find that the East Asia summertime monsoon had been stronger before 1926 together with the rainfall abundance of North China. And it gets weaker after 1976 together with rainfall deficiency of the North China. They also find that the correlation between the rainfall of North China and the SSTA in the North Pacific Ocean had been much better before 1976.
2. Interaction between Asia Monsoon and ENSO
A large number of papers have concentrated on the interaction between the Asia monsoon and ENSO. Based on rainfall data (1951-1999) of China, Huang (2001) analyzed the association of drought or flood with various phases of a ENSO cycle. He finds that the summertime monsoon rainfall tends to be abundant in the Changjiang River and the Huaihe River Basins of China, Japan and Korea if it coincides with the developing phase of an ENSO cycle, and it tends to be deficient in the North China and the south China. A reverse summertime rainfall distribution can be found in the ENSO decaying phase. Therefore the heavy disaster of drought or flood often occurs in China in the ENSO developing phase or decaying phase (Ni and Sun, 2000). Jin and Tao (1999) also find that the rainfall in the East China is closely related to the ENSO cycle. Tao and Chang (1998) analyze the geopotential height fields and the atmospheric circulation patterns for both El Niño and La Niña, indicating that a certain circulation pattern corresponds to the winter monsoon weakening in East Asia in an ENSO year. To explain the abnormality of Asia monsoon circulation in an ENSO cycle, Ren and Huang (1999) attribute it to the abnormal convection pattern forced by the SSTA distribution related to ENSO in the tropical Pacific Ocean, i. e. the thermal convection strengthening in the middle and eastern equatorial Pacific Ocean and the weakening in the western equatorial Pacific Ocean resulting in a dipole of the convective activity or the thermal source over the tropical Pacific Ocean, which is favorable for strengthening anti-cyclonic circulation over the south China Sea and the southwest monsoon air-flow over the Changjiang River and the Huihe River Basins. Wang et al. (2001) further indicate the ENSO effect on the East Asia summertime monsoon circulation lags behind the ENSO evolution, i.e. the anti-cyclonic circulation becomes strengthened in the Northeast Asia, and the subtropical high becomes further westward extended in summer after an ENSO event has reached its mature phase. As the Asia summertime monsoon usually brings a lot of water vapor into India, the South Asia and the East Asia, influencing the vaper transport for the above area, the ENSO effects on the summertime monsoon circulation will be an important role in the regional vapor transport. Huang et al. (1998) find that the longitudinal vapor transport by the East Asia monsoon is much greater than the latitudinal one, and the water vapor convergence, which causes the monsoon rainfall and depends upon both the monsoon circulation pattern and the water vapor advection path.
In addition to studies of vapor transport and its sources related to the summer monsoon, the coincidence of the ENSO occurrence with strengthening of the winter monsoon in the East Asia is investigated by means of the observational data (1950-1989) analysis (Mu and Li, 1999). Mu and Li find that the winter monsoon has become stronger (weaker) in the East Asia before an El Niño(La Niña) event occurs. They indicate that the ENSO signals are better reflected and included in the interannual anomaly of the winter monsoon in the East Asia. Zou, Wu and Ni (2002) further confirm that the Asia winter monsoon coming down southward can excite the strong convective activities in the western tropical Pacific Ocean and influence the ENSO cycle. They point out that the influence is reflected in the quasi-biennial oscillation mode. Therefore the Asia winter monsoon plays an important role in the ENSO occurrence. On the other hand, ENSO will influence the East Asia monsoon. Using the AGCM heated by the typical SSTA of the eastern Pacific Ocean during a ENSO event, Long and Li (1999) study the effect of a typical ENSO event on the following monsoon activity in the East Asia. They find that in the summer after an ENSO event has occurred, the subtropical high becomes stronger. Its ridge tends to be shifted northward and its area extends further westward causing rainfall deficiency in the East China and the North China. In winter after La Niña has occurred, rainfall deficiency happens in the Changjiang River and Huaihe River Basins, causing a different rainfall distribution.
A review of the studies about the interaction between the ENSO cycling and the Asia monsoon was published (Huang and Chen, 2002) and more details about the recent progress in this aspect can be found there.
3. South China Sea Monsoon
Zhao et al. (2000a) analyzed the SSTA field in May in the South China Sea and the eastern tropical Pacific Ocean by means of the EOF method to study the South China Sea monsoon onset and its association with the SSTA field. They find that the positive maximum value of the first characteristic vector is centered at Sumatra of Indonesia showing its closest relation with the monsoon onset. The South China Sea monsoon starts earlier (later) when SST becomes warmer (cooler) in the sea area around Sumatra than usual. They also (2000b) analyses seasonal and interannual variations of the South China Sea warm pool with sea temperatare of greater than 28℃ and discuss their relation with the South China Sea monsoon onset. It is shown that the warm pool has an significant seasonal variation with its largest area from June to May and the smallest area from December to February. The rapid transitional months are March-April and October-November respectively, leading the monsoon seasonal reverse. The monsoon onset can be closely related to the warm pool interannual variability. The summer monsoon starts later (earlier) than usual if the warm pool becomes warmer (cooler) in the previous winter and spring. The subsurface variability of the tropical Pacific-Indian Ocean and its correlation with the strength of the South China Sea monsoon are analysed by Zhang, Li and Wang (2001, 2002). They find (2001) that the thermocline depths of the central equatorial Pacific Ocean and the Bay of Bangal are positively and closely correlated with the strength of the monsoon, and can be used as a predictor for monsoon prediction. They also (2002) find that the sea temperature at the 120 m water depth is best correlated with the monsoon strength. By means of the composite analysis method, they classify all the South China Sea monsoon cases (1995-1998) into 4 kinds, of which the scientific senses of the subsurface sea temperature field at the 120 m depth and the wind field on the 850 hPa level for the monsoon and ENSO prediction are discussed.
IV. DECADAL-INTERDECADAL VARIABILITY IN THE PACIFIC OCEAN
Based on the comprehensive data analysis of the surface air temperature, the subtropical high activities over the Northwestern Pacific and the SSTA in the equatorial Pacific a clear quasi-decadal oscillation signals is shown by Li C. (1998). He also find that there are some indicators of the climate change in the East Asia, such as the surface air temperature, the precipitation and the onset date of Meiyu in the Yangtze River Basin, which have the quasi-decadal oscillation similar to that mentioned above.
Some signals of the interdecadal variability in the Pacific Ocean are examined by the CEOF method and the composite analysis method for the XBT temperature data obtained from 1950 to 1993 (Wang and Liu, 2000). Such signals are propagated southwestward from the central north Pacific and subducted to the subtropical region. They find that the themal anormalies subducted from the central north Pacific to the east of dateline can only reach 18°N in the western tropical ocean. There has been no further southward propagation than 18°N due to a certain barrier. However the origin of the interdecadal oceanic signal in the western tropical Pacific can be traced to the southern tropical Pacific. These variabilities reflect the nature of the thermocline circalation. Yang et al. (2002) apply the data (1950-1998) from the Maryland Ocean Data Assimilation and the NCEP/NCAR atmospheric reanalysis data to study spatio-temporal structures of interannual and interdecadal variations in the global ocean-atmosphere system. They conclude that the interannual variations in the global upper oceans are characterized by the ENSO mode, which dominated in the tropical Pacific while the oceanic interdecadal variations are mainly confined to the mid-to-high latitudes, the off-equator Pacific and Atlantic. On the other hand, the atmospheric intenannual and interdecadal variations both appear in the mid-to-high latitudes especially over the polar regions, with less coherence between the interannual air temperature variation and the interannual sea level pressure variation, but with better coherence between the interdecadal two. The interannual variation of the mid-to-high latitude atmosphere seems to be an instinct variation, which can not be related to the interannal variation of the tropical upper ocean as the ENSO features can. They also find that an abrupt changes in the global air-sea system happened in the 1970s, such as the air temperature increase in a large area of the world especially in Siberia and the polar regions, the SST increase in the eastern tropical Pacific Ocean and the western coastal waters of North America, South America and Africa, the air-temperature decrease in the North Tibet and Greenland, and the SST decrease in mid latitude water of the north Pacific Ocean and high latitude water of the south hemisphere.
Based on the data (1951-1995) of the geopotential height of 500hPa and the monthly mean SST Zhu et al. (2002) apply the SVD method to study the coupled pattern of the mid and low latitude atmosphere/ocean interaction and its decadal variation. They find that the PNA teleconnection pattern is closely related to El Niño and the Pacific-Japan (PJ) teleconnection pattern to La Niña. They also find that the abrupt changes of the coupled patterns happened in the middle 1970s, with the significant SST increase in the central and eastern tropical Pacific Ocean and the decrease in the middle north Pacific Ocean. The PNA pattern in winter becomes strengthened after the middle 1970s and no PJ pattern appears before the middle 1970s.
More details of the interdecadal variability of the air-sea interaction system in the Pacific region can be found in the paper by Li H. et al. (2001).
V. AIR-SEA INTERACTION IN THE NORTH PACIFIC OCEAN
Zhang, Yu and Liu (1998) analyze the sea surface turbulent heat flux anomalies (including sensible and latent) and their effects on SSTA in the North Pacific Ocean in winter using the data extracted from a long-term integration of a coupled ocean-atmosphere general circulation model. The relative importance of the sea surface heat flux components for determining the SSTA variability has been learnt to be compared with the other oceanic dynamical processes. They find that the turbulent heat fluxes play a major role in the SST variability in the most of the extratropical North Pacific Ocean except in a central patch of the basin where the effect of the oceanic horizontal advection on SST can not be negligible. The latent and sensible heat flux anomalies of the model are closely related to the SST tendency rather than SSTA in the extratropical Pacific Ocean during wintertime. Their results coincide with the analysis results by Cayan from COADS and by Reynolds et al. from the NCEP data. It is supported by the model runs that the atmospheric role in the ocean is dominant in the air-sea interaction over the extratropical ocean in winter rather than the oceanic forcing on the above atmosphere, while ocean plays a dominant role in the tropical ocean. The first EOF mode of the SST tendency from the model output and its correlations with the sea level pressure is similar to the results by Wallace et al. from the observational data. Therefore it is the anomalous large-scale atmospheric circulation that affects SSTA in the most of the extra tropical Pacific Ocean especially in the western Pacific through the sea surface turbulence heat fluxes. Jiang (1998) studies the heat transfer between air and sea in the mid-latitude North Pacific Ocean. He finds that the heat transfer from sea to air has significant seasonal features, this is to say, it is the greatest in December and the least in June. The spatial distribution of the heat transfer is closely related to the distribution of oceanic currents. It gets greater in the warm current area and less in the cold current. The seasonal variations of the heat transfer between air and sea is more significant near the west boundary where the Kuroshio or the Oyashio flows than near the east boundary where the Alaska Current or the California Current flows. The meridional gradient of the heat transfer becomes the greatest in 37.5°—43.5°N west of 160° in March and almost vanishes in June. The sea surface heat budget also has seasonal variation with the net heat from sea to air in December in any area of the North Pacific Ocean and the net heat from air to sea in June in any area of the ocean. The heat budget ratio, which depends upon the latitude of the sea area, has a different tendency in December from that in June.
In addition to the extra-tropical water heat budget studies, Xie and Liu (2001) detect a wind wake of 3000km trailing westward behind the Hawaiian Islands this is longer by a lot of times than that observed anywhere else. The wind wake drives an eastward oceanic current and draws the warm water 8000 km away from the Asia coast, leaving marked changes in the surface and sub-surface temperature. Located in the steady trade wind zone, Hawaii triggers a kind of air-sea interaction, which provide the feedback to sustain the influence of these small islands over a long stretch of the North pacific.
VI. AIR-SEA INTERACTIONS IN THE INDIAN OCEAN
Zhou and Zhang (2002) study the air-sea heat flux exchange of the Indian Ocean on the basis of the UWM/COADS data. It is found that SSTA is closely related to the net sea surface heat flux in the tropical Indian Ocean, especially in the wintertime in the central-eastern Indian Ocean and in the summertime in the western and northern Indian Ocean. However in the other area of the ocean, the evolution of SSTA has less correlation with the net sea surface heat flux. It is suggested that the SSTA evolution be dominated by the oceanic dynamical processes in the other area. The normalized covariance analysis shows that the latent heat prevails over the sensible heat, short wave radiation or long wave radiation in forcing the atmosphere in the tropical Indian Ocean. Although the sensible heat flux is less in quantity than the latent in 20°N—20°S, its zonal mean is more significantly correlated to SST rather than the latent heat flux.
Li and Mu (2001a and 2001b) study the SST variation with its dipole oscillation feature in the equatorial Indian Ocean based on the observational data as long as about 100 years. The dipole oscillation feature , which shows the higher SST in the west of the ocean and the lower SST in the east during its positive phase and the higher SST in the east and the lower SST in the west during its negative phase, becomes more obvious in September-November and less obvious in January-April. Generally speaking the amplitude of the positive phase is greater than that of the negative one. The interannual variation of the dipole has a 4-5 year periodicity and the interdecadal a 25-30 year periodicity. A significant impact of the dipole on the Asian monsoon activity can be found because the wind field in the lower troposphere over the South Asia, the Tibetan high in the upper troposphere and the subtropical high over the northwest Pacific Ocean are all related to the dipole. There are also some scientific evidences for the dipole effects on the atmospheric circulation in the North America and the southern Indian region (including Australia and south Africa). Wu et al. (2000) apply the thermal adaption theory to study the impact of SSTA in the Indian Ocean on the weather of the South China and the subtropical high over the western Pacific Ocean. It is shown that in the first stage of the atmospheric thermal adaption to the SSTA in the ocean the in situ anomalous cyclonic circulation of the lower troposphere and the deep convective precipitation to its east due to the southerlies strengening will be generated and in the second stage of the adaption to the latent heating from the condensation of the precipitation anomalous anticyclonic circulation will be producecd at 500 hPa over the western Pacific Ocean and at 200 hPa over the South Asia. Therefore, it is concluded that the both stages of the atmosphieric thermal adaption to the SSTA in the Indian Ocean are the important mechanisms for the abnormality of the subtropical high with the corresponding climate variability over the East Asian monsoon region.
Xiao et al. (2000) apply the IAP-GCM9L model to simulate the atmospheric response to the SSTA pattern during the ENSO occurrence phase with the warm SST in the western Indian Ocean and the cool SST in the eastern and to find out the effects of the SSTA pattern on the Asia climate. It is reported from the published literature that the equatorial Pacific SSTA pattern during ENSO brings about the global climatic abnormality. If the SSTA pattern of the Indian Ocean is taken into account, the climate abnormality will enhance in the Bay of Bengal, the Indo-China Peninsula, Indonesia, India, and China. The main climate abnormality includes the serious drought in the Indio-China Peninsula, especially in the North China, and the rainfall increase in a zone along the Changjiang River from the Southwest to the Southeast China.
The interdecadal variability in the tropical Indian Ocean is analyzed by Wang, Wu and Xu (1999) on the basis of the long-term climatic observational data. It is shown by the study that the strong interannual signals formed at the sea surface can reach the seasonal thermocline depth, where the sea temperature anomalies will keep a longer time memory as the interdecadal feature. The longer time memory at the thermoclime can be imitated by a damping process with exponential decay in time from the peak phase of the selected strong interannual events. Therefore a possible dynamical explanation for the interdecadal variability in the tropical Indian Ocean is proposed that the irregular interannual signals can lead to a slowly evolving climatic background with the interdecal time scale feature through the damping processes in the seasonal thermocline.
VII. AIR-SEA INTERACTION IN GLOBAL OCEANS
The air-sea interaction in global oceans is mainly studied as the regional features and their comparison. The diagnostic studies on the indian-Pacific interaction system are usually conducted on the basis of the observational data and/or model output, and the other aspects.
Ma et al. (2001) analyze the monthly mean wind data of the 1000 hPa from NCEP/NCAR and the SST from COADS by the singular value decomposition method (SVD) and display the regional difference of the main parameters and the first singular vectors respectively for the western, central and eastern tropical Pacific Oceans, the Atlantic Ocean and the Indian Ocean. It is shown that the global tropical oceans can be classified into 3 kinds of interaction depending upon how the air-sea interaction works. ENSO is the main process for the air-sea interaction in the central and eastern Pacific Ocean, both the ENSO processes and the longer time scale process play an important role in the air-sea interaction in the Indian Ocean, and the more than 2 components work together for the air-sea interaction in the Atlantic Ocean, so that ENSO cycling becomes less obvious.
Zhang and Qian (2001) made a comprehensive analysis of the monthly mean data of surface wind, surface air temperature, SST, humidity, and sensible and latent heat flux by adopting the lag correlation method to determine the 7 key regions for the global air-sea interaction. It is reported that SST is closely related to the air temperature in any of above regions, especially in the central and eastern Pacific Ocean and the southern Indian Ocean, and it is better correlated to the surface wind in the central and western Pacific Ocean than that in the other regions in the sense of its dynamic forcing to ocean. And it is better correlated to the sensible and latent heat in the eastern Pacific Ocean, the western Pacific Ocean, the northwest Pacific Ocean and the southern Indian Ocean than that in the others regions as well. It is further learnt from the lag correlation analysis that some meteorological elements can be used as predictors for the SST forecasting. Wu and Wang (1998) analyzed the data (1949-1989) of the summertime 500 hPa geopotential height field over the mid-latitudes and SSTA in the northern Pacific Ocean and the northern Atlantic Ocean by using the rotated principle component method. The cross correlation technique is also employed for the data. As it is shown from this results, the major spatial patterns of the summertime 500 hPa geopotential height anornaly can be patterned as the subtropical (ST), the polar and American (PA), the 4 wave (FW), and the 3 wave (TW). While the major patterns of the summertime SST in the north Pacific Ocean are patterned as the eastern equatorial Pacific (EEP), the Bay of Alaska (BAL), the central tropical Pacific (CTP), and the northern North Pacific (NNP) , and those of SST in the North Atlantic Ocean are patterned as the equatorial Atlantic (EAL), the Caribbean Sea (CAR), the eastern North Atlantic (ENA), and the central North Atlantic (CNA) . The summertime correlation is not so close as the wintertime one between the geopotential height anormalies and the SST anormalies, and it is especially insignificant for the correlation between the equatorial SSTA and the 500 hPa geopotential height of the mid-latitudes. The best correlations between SST and the 500 hPa height is spatially localized in some degree in the North Pacific Ocean, while those seems to be well organized with wave chain patterns.
Yin et al. (2001) comprehensively analyze the monthly mean data (1979-1998) of SST, OLR and the 1000 hPa zonal wind from the NCEP/NCAR reanalysis data. The interdecadal difference of the Indian Ocean dipole can be found by comparison of the dipole features, which were less marked in the 1980s and more marked in the 1990s. It is suggested that there be interaction between the tropical Indian Ocean and the tropical Pacific Ocean, and the intensification of the Pacific ENSO in the 1990s can be attributed to the influence of the strengthening of the Indian Ocean dipole.
On the basis of the observational data Wu and Meng (2001a) studied the seasonal variation of the Indian Ocean SST and estimated the correlation between the equatorial Indian Ocean SSTA and the eastern equatorial Pacific Ocean SSTA, which shows a significant positive correlation and is associated with the strong coupling between the monsoon zonal circulation over the equatorial Indian Ocean and the Walker circulation over the Pacific Ocean. The two circulation cells work together much like a pair of gears operating over the tropical Indian and Pacific Oceans (denoted as GIP). As shown in their results, the ENSO events are closely linked with the GIP operation. A cold (warm) event corresponds to a “positive (negative)” rotation of GIP. The surface airflow of the two cells converge to form the up-branch of the cells, near the Indonesia Archipelogoes, where "the gearing point" is located. The analysis of the ENSO events since 1980 shows that "the gearing point" appearing first over the Indian side before an ENSO event will propagate gradually eastward during the event evolution phase and the associated anomalies of SST and U-component of the 850 hPa wind to the east of the gearing point will move eastward approaching the dateline. Finally, the ENSO event occurs in the eastern equatorial Pacific Ocean. They conclude that the anomaly in the monsoon zonal airflow over the Indian Ocean can affect the air sea interaction over the eastern Pacific Ocean in virtue of GIP and trigger off the warm event. Beside the observational data analysis, Meng and Wu (2000) also analyze the model output from the IAP/LASG GOALS model and find that GIP is well represented in the model. The results from the sensitivity experiments for the regional anomalous zonal winds reveal that through the GIP gearing, the atmospheric anomaly in either the equatorial Indian Ocean or the equatorial Pacific Ocean will cause the SST anomaly in the other. Therefore GIP is considered as the atmospheric bridge linking the ENSO events in the Pacific Ocean on one end and the zonal wind of Asia monsoon over the Indian Ocean on the other. Zhou et al. applied the IAP/LASG/GOALS model to study the relationship between the thermohaline circulation (THC) and climate variability. They find that the strength of the north Atlantic THC is negatively correlated to the North Atlantic Oscillation (NAC). Based on this negative correlation and the instrument-measured climatic data such as air pressure and SST, they estimated the THC variability of the 20th century with the strengthening periods in 1967-1903 and 1934-1972 and the weakening periods in 1904-1933 and 1973-1994.
In addition to the studies of air-sea interaction in the tropical and mid-latitude oceans, some studies are concentrated in the tele-connection between the polar region and the other area. For example, Chen and Qin (2000) analyzed the monthly SST data of the tropical Indo-Pacific Ocean from COADS to study the SST variability in the tropics and its relation with the Antarctic sea-ice extention, especially the Ross Sea ice. They find that the most significant correlation appears when the tropical SST 16 months lags behind the Antarctic sea ice. Wu et al. (1999) analyzed the data (1954-1989) of the sea ice in the Polar regions and the rainfall in North China and calculated the correlation between them. They find that the sea ice extent in the Kara Sea and the Barents Sea in winter is negatively correlated to the rainfall of the Haihe River and the Liaohe River valleys in the following August and the correlation of the sea ice extent in Baffin Bay and the Davis strait in winter to the rainfall of the mid-upstream of the Yellow River in the following July with the best corrlation of about -0.6585. They (2001) also applied the IAP two-layer GCM to study the influence of the variability of both the Arctic sea-ice thickness and extent on the atmospheric circulation over the East Asia. Their model results indicate that a more reasonable description of the sea ice thickness will improve the simulated patterns for the Siberia high and the Iceland low in winter, and intensify both the summer monsoon and the winter monsoon over the East Asia. Furthermore the spatial variability of the sea ice thickness can excite the teleconnection wave trains over the Euro-Asian continent with planetary wave propagation from the western Pacific to the eastern in the lower latitudes. The control run experiments also show that a larger (less) sea ice extent in the Barents Sea in winter will result in the positive (negative) SLP anomaly over the central North Pacific, the Aleutian low weakening (deepening), the light (heavy) sea ice condition in the Bering Sea in the following spring, the deepening (weakening) of the thermal depression over Asia, the further northward (southward) displacement and the strengthening (weakening) of the subtropical high over the western Pacific Ocean in the following summer.
Fu et al. (1986) studied the general features of occurrence frequency, spatial distribution of locations, life-cycle and cloud patterns of polar lows over the Japan Sea and its adjacent Northwest Pacific Ocean in the 1995/1996 winter on the basis of the data from the field observation and satellite sensing. It is shown that the polar lows most frequently develop in mid-winter over 35-45°N in Japan Sea and 30-50°N in the Pacific, and rarely form over the Eurasian continent. Their life-cycle is usually 2-3 days over the Pacific Ocean and 1-2 day over Japan Sea because of the narrow width of the Japan Sea and the influence of the Japan Islands on their decaying. Generally speaking, the polar lows over the Japan Sea have the tight, spiral cloud patterns in the satellite images with a clear "eye" at the mature stage. The greater air-sea temperature difference in the Japan Sea affected by the Tsushima Warm Current in winter provides the favorable heating conditions for their formations.
A large number of papers have been published and great efforts made in the latest 4 years by Chinese oceanographic and meteorological communities in the air-sea interaction studies. Although great progress has been made almost in all the aspects of this research field in China, there are still some important questions or deficiencies left for the coming study especially in the field observation and the forecasting practice by the air-sea interaction theory. On the other hand it is too difficult to summarize all the activities of this field in a short report like this although it could have been managed better in organizing the report as the authors hoped. The authors would like to take this chance to thank Prof. LIU Qinyu, Ms. LI Wei, Prof. ZHAO Yongping, and Prof. WANG Dongxiao for their kindly providing the relevant information in the paper collection for this report.
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