PROGRESS IN THE STUDY ON THE FORMATION OF THE SUMMERTIME SUBTROPICAL ANTICYCLONE
LIU Yimin and WU Guoxiong
State Key Lab of Atmospheric Sciences and Geophysical Fluid Dynamics (LASG)
Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
The studies on the subtropical anticyclone are reviewed. New insights in recent studies are introduced. It is stressed that either in the free atmosphere or in the planetary boundary, descent cannot be considered as a mechanism for the formation of the subtropical anticyclone. Then the theories of thermal adaptation of the atmosphere to external thermal forcing and the potential vorticity forcing are developed to understand the formation of the subtropical anticyclone in the three-dimensional domain. Numerical experiments are designed to verify these theories. Results show that in the boreal summer, the formation of the strong South Asian High (SAH) in the upper troposphere and the subtropical anticyclone over the western Pacific (SAWP) in the middle and lower troposphere is due, to a great extent, to the convective latent heating associated with the Asian monsoon, but affected by orography and the surface sensible heating over continents. On the other hand, the formation of the subtropical anticyclone at the surface over the northern Pacific and in the upper troposphere over North America is mainly due to the strong surface sensible heating over North America, but affected by radiation cooling over the eastern North Pacific. Moreover, by considering the different diabatic heating in synthesis, a LOSECOD quadruple heating pattern is found over each subtropical continent and its adjacent oceans in summer. A distinct circulation pattern accompanies this heating pattern. The global summer subtropical heating and circulation may be viewed as “mosaics” of such quadruplet heating and circulation patterns respectively. At last some important issues for further researches in understanding and predicting the variations of the subtropical anticyclone are raised.
Key words: subtropical anticyclone, quadruplet heating, mosaics circulation
Along the subtropics of the Northern and Southern Hemispheres there exist belts of subtropical anticyclone. The existence of mountains, air-sea interaction, land surface processes, land-sea contrast, and sea-ice and snow cover etc. changes the energy budget of the atmosphere, and breaks the belts into enclosed subtropical anticyclones. In the boreal summer near the surface, there are two pronounced anticyclones, one is the sea surface subtropical anticyclone over the western North Pacific (SAWP), and the other is the subtropical anticyclone over North Atlantic (SANA). The SAWP alone covers about twenty to twenty five per cent of the northern globe. In the free atmosphere in the boreal summer, the circulation over East Asia is characterized by the existence of two persistent subtropical anticyclone systems. One is the remarkable South Asian anticyclone (SAA) in the upper troposphere just over the region to the north of the Bay of Bengal; and the other is the SAWP in the middle and lower troposphere. The seasonal variations of these two systems are closely linked to the onset and withdrawal of the Asian summer monsoon. Their spatial and temporal variations are associated not only with the disastrous weathers in the area, such as typhoon and torrential rain, but also with severe climate anomalies, such as drought and flooding over vast areas. Therefore, subtropical anticyclone and its dynamics have long been the subjects of meteorological studies. Ye et al. (1958a, 1958b) found that, the abrupt northward movement from winter to summer of the subtropical anticyclone in the Asian monsoon area is accompanied with the abrupt changes in circulation patterns. Tao et al. (1962a, 1962b, 1963) and Huang et al. (1962, 1963) studied the SAWP and revealed its seasonal variation in intensity, structure, location and its structure in association with the distribution of summer rainfall in China. As a matter of fact, the activities of the SAWP are also closely linked with the weather and climate anomalies in Korea and Japan (e.g., Kurihara and Tsuyuki, 1987; Kurihara, 1989; Nikaidou, 1988). Many factors have also been proposed to explain the variation and formation of subtropical anticyclone as reviewed by Liu and Wu (2000). These include the circulation interactions (Tao and Zhu, 1964), the impact of the Tibetan Plateau (Krishnamurti, 1973; Ye and Gao, 1979; Wu and Zhang, 1998; Ye and Wu, 1998), etc. In the recent years, the influence of the East Asian monsoon (Li and Luo, 1988; Yu and Wang, 1989; Nikaidou, 1989; Qian and Yu, 1991; Hoskins, 1996; Wu et al., 1999, Liu et al., 2001; 2002; Rodwell and Hoskins, 2001) has also been emphasized.
However, due to the limitations in available data and development of our sciences, our knowledge on subtropical anticyclone is still poor. Its formation mechanism is unclear, and its forecast is still unsatisfied. By the middle 1990s of last century, the NCEP/NCAR reanalysis data set (Kalnay et al., 1996) became available, and a climate system model of Global-Ocean- Atmosphere-Land-System (GOALS model) that had coupled the three climate sub-systems together was completed (Wu and Zhang et al., 1997; Zhang et al., 2000). Conditions for pursuing climate study had been improved tremendously in China. To advance our understanding on subtropical anticyclone, in 1995 the National Natural Science Foundation of China (NSFC) decided to set up a Key Project entitled “Formation and variation of subtropical anticyclone”. Many new research results had been obtained by the end of 1999 when the project was completed. The material presented here provides a summery on the research results concerning the formation of the summertime subtropical anticyclones. In Section II, after introducing the general concepts and dynamics of the zonal mean subtropical anticyclone, the contrasts in observation as well as dynamics between the local subtropical anticyclones and meridional circulations over the western and eastern Pacific are made. In Section III some relevant dynamics are presented for understanding the three-dimensional features of the subtropical anticyclone. The effects of the surface sensible heating, deep convective condensation heating and radiation cooling are discussed in Sections IV to VI, respectively. Since these results indicate that different diabatic heatings play different roles in the formation of subtropical anticyclones and should be considered in synthesis, Section VII employs the reanalysis data of NCEP/NCAR to demonstrate the distributions of individual as well as total diabatic heating against circulations in the summer subtropics. Discussions and conclusions are presented in Section VIII.
II. DISTRIBUTIONS OF THE SUBTROPICAL HIGH AND THE HADLEY CIRCULATION
Wu et al. (2003) recently provided some criteria for the study of the subtropical anticyclone, and used them to discuss the relation between the zonal mean subtropical anticyclones and the Hadley cells. These are reviewed in this section, and extended to compare the distributions of the local subtropical anticyclones against the meridional circulations over the western and eastern Pacific at the longitudes 135oE and 125oW, respectively.
1. General Concepts of Subtropical Anticyclone
In the free atmosphere the zonal mean flow can be described by using the geostrophic relation, and the equation of the divergence of meridional mass flux ( ) in a steady state can be obtained as:
the symbols used here are conventional in meteorology. Equation (1) implies that under the constrain of geostrophic relation, the convergence of mass flux due to the exertion of the Coriolis force upon the zonal flow ( ) is balanced by the divergence of mass flux produced by the pressure gradient force ( ), as depicted schematically in Fig. 1a.
Within the planetary boundary layer, friction impacts need to be considered and Eq.(1) is modified as
where k is a friction coefficient. Thus the surface subtropical anticyclone is accompanied with strong horizontal divergence of mass flux ( ) within the boundary layer. Since the mass transport into the boundary layer at the top of the Ekman layer is equal to the divergence of the cross-isobaric mass transport in the layer, i.e.,
the location of the subtropical anticyclone in the planetary boundary layer is then characterized by strong descent at the top of the Ekman layer. It is worthwhile to point out that, although at a steady state the two terms in Eq.(3) balance each other, the descent at the top of the planetary boundary layer cannot be used as a mechanism to explain the formation of subtropical anticyclone. This is because both the descent and the cross-isobaric flow in association with the anticyclone are secondary non-divergence circulations, and do not contribute to the mass built-up in the layer, as depicted schematically in Fig.1b.
Fig.1. Schematic diagram showing the dynamic mechanism for the maintenance of the zonal mean subtropical anticyclone and the meridional Hadley Cell.
(a) In the north- south direction at the ridgeline of subtropical anticyclone, the convergence of meridional mass flux owing to the inertial effects of the Earth's rotation (-f u, dotted white arrow) is balanced by its divergence due to the pressure gradient force ( , solid white arrow). (b) In the planetary boundary layer, the cross- isobaric flow that diverges from the subtropical anticyclone outwards is balanced by the descent at the top of the planetary boundary layer into the layer. (c) According to the thermal wind relation, the ridge of subtropical anticyclone tilts with increasing height towards beneath warmer region, forming westerly shear in winter and easterly shear in summer when crossing the ridgeline upwards in the Asian monsoon area. (d) The inertial torques associated with the horizontal branches of the Hadley Cell (blank white arrow) are balanced by the generation of angular momentum due to friction at the surface (dotted arrow), and by the divergence of angular momentum from tropics to mid- latitudes in the upper troposphere (solid arrow).
2. Location and Intensity of Subtropical Anticyclone
The meridional wind component vanishes at the latitude where the zonal mean center of the subtropical anticyclone is located. Thus in both the free atmosphere and the planetary boundary layer, the location of the center of subtropical anticyclone in the sense of zonal mean can be defined from the zonal wind distribution by using the following criteria:
The vertical variation in the location of subtropical anticyclone can be understood by using the thermal wind relation:
For simplicity, p-coordinate is adopted in Eq.(5), in which is potential temperature and , the specific volume of the air. Equation (5) means the ridgeline of the subtropical anticyclone tends to tilt towards warmer latitude with increasing height (Fig. 1c). Thus in the Asian monsoon region in the boreal summer, there should be easterly shear across the ridgeline upwards when the land surface of the southern part of Asia gets warmer than the sea surface of North Indian Ocean (Mao et al., 2002).
The intensity of the subtropical anticyclone can be measured by the convergence or accumulation of the meridional mass flux
Here density is assumed to be independent of y in the vicinity of the ridgeline of subtropical anticyclone. Formula (6) together with (1) implies that the intensity of subtropical anticyclone can be measured by . Furthermore, in p-coordinate this is equivalent to . For simplicity or can also be used as an intensity index.
3. Distribution of the Subtropical Anticyclone and Meridional Circulation
Wu et al. (2002, 2003) have shown that in the zonal and annual mean case, the descending arm of the Hadley circulation and the subtropical anticyclone deviate each other in the free atmosphere, and coincide only in the planetary boundary layer. In their study, the deviation of the geopotential height ( ) at latitude y and at pressure level p from its value at the equator (y=0) and at the same level ( ), i.e. , was used to present the distributions of the subtropical anticyclone. The advantage of using such a deviation of geopotential height is that such a field can demonstrate the three-dimensional structure of the subtropical anticyclone much clearer (Liu P., 1999). In this section in Fig. 2, the annual mean climate distributions of along 135oE and 125oW are plotted versus the in situ meridional circulations. It is prominent that the isopleth u=0 coincides almost everywhere with the maximum at the same level. It becomes evident from Fig. 2 that the criteria (4) is adequate in defining the location of the subtropical anticyclone even in local sense. As in the zonal mean case, the two ridgelines in the two hemispheres are approximately symmetric to the equator. In Fig. 2, significant difference between the two hemispheres in the domain of positive geopotential height deviation can be observed below 500 hPa. Particularly at 1000 hPa along 125oW, although its pole-ward rim in the Southern Hemisphere is bounded by 44oS, in the Northern Hemisphere it extends northward to approach 60oN. This can be attributed to the existence of the strong surface subtropical anticyclone over Northeast Pacific in the boreal summer.
The deviation fields along the ridgelines in the two hemispheres are approximately symmetric to the equator as well, and decrease with increasing height below 700 hPa. They are over 40 gpm at the surface with centers located at 30o, but about 10 gpm at 700 hPa near 20 o. In this layer, the ridgelines in the two hemispheres both tilt equatorward with increasing height. The weakening in intensity of the subtropical anticyclone with increasing height is in accordance with the equator- ward tilting of its ridgeline. This is because as the ridgeline is approaching the equator with increasing height, the Coriolis parameter and air density both become smaller. According to (6), for the same zonal wind shear, there is less convergence of mass flux along the ridgeline, leading to the weakening in intensity. Above 300 hPa along 135oE, the deviation increases with height in the two hemispheres, in accordance with their poleward tilting.
Fig.2. Annual mean distributions averaged from 1980 to 1997 of the meridional deviation of geopotential height from its equatorial value at the same level (shading, unit: 10 gpm), the meridional circulations (light streamline with vectors), and the ridgeline of subtropical anticyclone identified by the curve u=0 (heavy dashed curve) along (a) 135oE and (b) 125oW.