The Earliest Onset Areas and Mechanism of the Tropical Asian Summer Monsoon

Transcription

1 NO.2 QIAN Yongfu, ZHANG Yan, JIANG Jing, YAO Yonghong and XU Zhongfeng 129 The Earliest Onset Areas and Mechanism of the Tropical Asian Summer Monsoon QIAN Yongfu( ), ZHANG Yan( ), JIANG Jing( ), YAO Yonghong( ), and XU Zhongfeng( ) Department of Atmospheric Sciences, Nanjing University, Nanjing (Received May 8, 2005) ABSTRACT The multi-yearly averaged pentad meteorological fields at 850 hpa of the NCEP/NCAR reanalysis dada and the TBB fields of the Japan Meteorological Agency during are analyzed. It is found that if the pentad is taken as the time unit of the monsoon onset, then the tropical Asian summer monsoon (TASM) onsets earliest, simultaneously and abruptly over the whole area in the Bay of Bengal (BOB), the Indo-China Peninsula (ICP), and the South China Sea (SCS), east of 90 E, in the 27th to 28th pentads of a year (Pentads 3 to 4 in May), while it onsets later in the India Peninsula (IP) and the Arabian Sea (AS), west of 90 E. The TASM bursts first at the south end of the IP in the 30th to 31st pentads near 10 N, and advances gradually northward to the whole area, by the end of June. Analysis of the possible mechanism depicts that the rapid changes of the surface sensible heat flux, air temperature, and pressure in spring and early summer in the middle to high latitudes of the East Asian continent between 100 E and 120 E are crucially responsible for the earliest onset of the TASM in the BOB to the SCS areas. It is their rapid changes that induce a continental depression to form and break through the high system of pressure originally located in the above continental areas. The low depression in turn introduces the southwesterly to come into the BOB to the SCS areas, east of 90 E, and thus makes the SCS summer monsoon (SCSSM) burst out earliest in Asia. In the IP to the AS areas, west of 90 E, the surface sensible heat flux almost does not experience obvious change during April and May, which makes the tropical Indian summer monsoon (TISM) onset later than the SCSSM by about a month. Therefore, it is concluded that the meridian of 90 E is the demarcation line between the South Asian summer monsoon (SASM, i.e., the TISM) and the East Asian summer monsoon (EASM, including the SCSSM). Besides, the temporal relations between the TASM onset and the seasonal variation of the South Asian high (SAH) are discussed, too, and it is found that there are good relations between the monsoon onset time and the SAH center positions. When the SAH center advances to north of 20 N, the SCSSM onsets, and to north of 25 N, the TISM onsets at its south end. Comparison between the onset time such determined and that with other methodologies shows fair consistency in the SCS area and some differences in the IP area. Key words: the tropical Asian summer monsoon, the SCS summer monsoon, the India summer monsoon, the South Asian high, mechanism analyses 1. Introduction The Asian summer monsoon (ASM) can be divided into the East Asian summer monsoon (EASM) and the South Asian monsoon (SASM). The EASM is consisting of the tropical SCS summer monsoon (SC- SSM) and the subtropical East Asian summer monsoon (STEASM). In addition, Lau and Yang (1996) defined the Southeast Asian summer monsoon in the tropical ASM (TASM), and Wang and Xu. (1997b) defined the Northwest Pacific summer monsoon in the STEASM. Nevertheless, they are both the components of the EASM. Many scholars have made systematical studies on the EASM, such as Chen et al. (1991) and Ding and Murakami (1994). Tao and Chen (1987), Ma and Ding (1997) summarized the achievements made by the Chinese scholars in early and nowadays. Most scholars have recognized that the ASM begins earliest in the SCS (SCSSM) and then expends northward to the east coastal areas of China (STEASM) and at the same time westward (see Tao and Chen, 1987; Wu and Liang, 1998; Wang and Ding, Sponsored by the NSFC Key Project under No and the National Key Developing Programme for Basic Science project under No. 2004CB

2 130 ACTA METEOROLOGICA SINICA VOL ; Liu et al., 1998; Zhu et al., 1997; Jin, 1999). However, there are still certain debates on the source areas where the ASM onsets earliest. Some of researchers regard the Bay of Bengal (BOB) as the monsoon source area (see Zhang and Wu, 1998, 1999) instead in the SCS, while some consider the Indo-China Peninsula (ICP) area (see He and Luo, 1996). Nevertheless, almost all researchers call the TASM as the SCSSM. According to our knowledge the debates come from the time unit of monsoon onset. Many scholars like to use day as the unit and therefore the onset dates they defined are different from each other and so are the source areas where the TASM bursts earliest. If we take pentad as the time unit of monsoon onset, would the onset time of the TASM be the same in the above areas and what would the properties of the onset process be? It needs more studies on the source areas and the onset time of summer monsoon by further analysis of developing process of the low level southwesterly and the burst of convection. It is also necessary to depict the mechanism why the ASM onsets earliest in certain areas. To most meteorologists, especially in China, the SCSSM can be regarded as the tropical East Asian summer monsoon (TEASM) and onsets earlier than the TISM by about one month, no matter it onsets first in the BOB, or in the ICP, or in the SCS. Qian et al. (2001) discussed the mechanism of why the SC- SSM onsets earlier than the tropical Indian summer monsoon (TISM) by about one month. They pointed out that the different features of time variations of the surface sensible heat fluxes in the meridional belts containing the SCS and the India Peninsula (IP) areas are responsible for the difference of the onset time. It is found that the time of sign transition of the north to south surface sensible heat flux gradient along 30 N is much earlier in the SCS meridional belt than that in the IP belt. It is the different feature of time variations in different areas of the surface sensible heat flux that determines the earlier onset of the SCSSM than the TISM. Therefore, the SCSSM and the TISM are both the tropical monsoons although the physical factors and processes in the middle latitude are of important influence on the onset time and the source area. In this paper attentions will be paid again to this fact as well as the accompanied time evolutions of the air temperature and the sea level pressure when the mechanism is concerned. Scholars have studied the onset time and the source areas of the SCSSM from various aspects. Some of them discussed the onset process from the northsouth temperature difference averaged in the middle and high troposphere between two latitudes that are selected differently by different scholos. For example, He et al. (1987) selected 25 N and 5 N, Li and Yanai (1996) 30 N and 5 N, while He and Luo (1996) 20 N and 5 N. Therefore, the conclusions are not unique due to different latitudes and data used. Moreover, the scholars took into account only the thermal contrast between the tropical and the subtropical regions. The influences from the middle latitude are improperly neglected. Most scholars discussed the onset features of the SCSSM by analysis of the low-level wind fields and the convective activity. However, when the onset time and the onset source area are determined, some of them used dynamic quantities; some used thermal ones, while some combined both (see Yao and Qian, 2001). Due to the different quantities and methodologies used by different scholars the onset time and the source area are naturally different from one another to some extent. The temporal variation of the upper level circulation, such as the seasonal evolution, is much simpler and more stable than that of the low-level circulation. The South Asian high (SAH) at 100 hpa is the main upper level circulation system. The temporal and spatial variations of the SAH can be described with a few characteristic parameters, such as the center position, the intensity, and the area. All the parameters of the SAH have obvious seasonal, interannual, and interdecadal variations that are closely linked to both the surface thermal heating fields and the thermal structures of the middle to high troposphere as found by Qian et al. (2002) and Zhu et al. (1980). Two abrupt changes in the seasonal evolution of the SAH are depicted, one during April to May and the other during May to June (Zhang et al., 2002). What connections are there between the two abrupt changes

3 NO.2 QIAN Yongfu, ZHANG Yan, JIANG Jing, YAO Yonghong and XU Zhongfeng 131 and the onsets of monsoon? Whether or not it is possible to use the characteristic parameters of the SAH in determining the onset time and the source region? Whether or not the onset time and the source region such determined are in consistence with that by other techniques? Above problems are necessary to further study in this paper. The data we used are the pentad mean meteorological fields reprocessed by use of the daily NCEP/NCAR reanalysis dataset (see Kalnay et al., 1996) in (wind components in ) and the pentad mean TBB data of the GMS from the of Japan Meteorological Agency of which are also reprocessed. The data domains are limited in 20 S-60 N and E for the former and in 20 S-60 N and E for the latter which are sufficiently large enough for the Asian monsoon study. Resolutions of both data are 2.5 (lat.) 2.5 (long.), and the qualities are carefully checked and guaranteed. 2. Onset time and source area determined with low level wind fields and convective activities As pointed out in the introduction that there are still some debates on the onset time and the source area of the TASM, in this section we intend to carefully reanalyze the onset process by use of the lowlevel wind field and the convective activity in order to objectively determine the onset time and area of the SCSSM. The pentad is taken as the unit of the onset time. The time variation of 850 hpa multi-yearly pentad mean wind vectors averaged in the latitudinal belt of N is shown in Fig.1. It is seen that in early April (Pentad 19) weak southwesterly exists already in the ICP. While east southeasterly appears in the SCS and easterly dominates in the Philippines (Phi). In the IP, the west part of ICP, and the BOB, the wind speed is very weak. In the IP and the Arabian Sea (AS) the main flow is northerly. Such a distribution of wind field keeps no change until Pentad 5 of April (Pentad 23). In Pentad 6 of April (Pentad 24) the southwesterly flow expands westward to 90 E. In Pentad 1 of May (Pentad 25) the southwesterly in the ICP, the west regions of the SCS and the BOB begin to intensify. In Pentad 4 of May (Pentad 28), it increases further and expands eastward to the middle SCS at 115 E. After then the southwesterly continuously expands eastward and intensifies. From Pentad 2 of June (Pentad 32) the inclined northerly over the AS and the IP is greatly reduced and the westerly component of flow increases at the same time. Very strong southwesterly appears in a wide region between E, especially in the SCS and the BOB, while the ICP is occupied mainly by the strong inclined westerly. At this time the TASM onsets thoroughly in the tropical areas east of 80 E. From Pentad 3 of June (Pentad 33) the inclined westerly over the AS and the IP intensifies continuously which implies that the TISM is in developing process. Therefore, seen from the time variation of the mean low level southwesterly in N it may be concluded that the SCSSM onsets first in the ICP, then in Pentad 24 westward expands to the BOB of 90 E and intensifies. In Pentad 4 of May (Pentad 28) weak southwesterly occupies the middle SCS at 115 E and stronger one appears only after Pentad 2 of June. Comparing the meridional and the zonal wind components it is depicted that the meridional component is much larger than the zonal one in the SCS area. In the IP obvious southwesterly never appears, hence the appearance of southwesterly cannot be taken as the index of onset of the TISM. If the weakening of inclined northerly and strengthening of the westerly are taken as the monsoon onset index, then the TISM bursts out in the first or the second pentad of June. Fig.1. The longitude-time variation of the multiyearly pentad mean wind vectors at 850 hpa averaged in the N 1atitude belt.

4 132 ACTA METEOROLOGICA SINICA VOL.19 It is well known that both IP and SCS have large south to north domain. The former starts from the equator and ends at 27.5 N, while the latter from 5 S to 22.5 N. The time variation given in Fig.1 represents only the middle to north tropical regions. What features do the time variations of the low level wind field in the whole tropical belt have? Whether or not the TASM onsets simultaneously from the south to the north in the belt? We are going to answer those questions next. In order to answer the above questions, the timelatitude variations of the 850 hpa multi-yearly pentad mean wind vector in 6 typical regions of the Phi (125 E), the SCS (115 E), the ICP (105 E), the BOB (90 E), the IP (80 E), and the AS (65 E) are further analyzed (figures omitted). It is found that the Phi is continuously dominated by the anticyclonic circulation of the subtropical West Pacific high (SWPH) before Pentad 32, and from Pentad 33 inclined southerly and southwesterly appear from the south to the north. The SCS is also continuously dominated by the anticyclonic circulation of the SWPH before Pentad 27, in Pentad 28 the anticyclonic circulation disappears suddenly; and the inclined tropical southerly enters the area. In the north the southwesterly is evident. From Pentad 32 the strong tropical westerly reaches at the south SCS while the inclined southerly dominates the north. In the ICP the southwesterly exists already in Pentad 19 between 15 N and 25 N, however, it is obviously the subtropical southwesterly which enters the area after rounding the Tibetan Plateau (TP). The tropical westerly enters the area only from Pentad 28 and becomes stronger inclined westerly. In the BOB the tropical westerly expands from the south to the north gradually. Before Pentad 28 the subtropical westerly is very strong, after then the two westerly currents combines together rapidly with the subtropical one being the dominant. With continuously expanding northward the subtropical westerly occupies the area south of 25 N totally in Pentad 36. In the IP, the case is similar to that in the BOB, but the combination of the two westerly currents happens in Pentad 31. In the AS, the area south of 15 N is dominated by the northeasterly before Pentad 28 that is a turning branch of subtropical westerly after rounding the TP. In Pentad 30 the tropical westerly begins to enter the area and in Pentad 32 it becomes west southwesterly. Therefore, if the entrance of the inclined westerly is taken as an index of onset of the TASM, then the monsoon onset time is the same in the BOB, the ICP and the SCS areas, east of 90 E, in Pentad 28. The TISM advances gradually from south to north in the IP and the AS areas with the earliest onset in the south end in Pentad 31 and thoroughly onsets in the whole region after one month. No abruptness is observed. The onset time of the SCSSM has large difference in individual years. However, it is still in the same pentad in the BOB to the SCS areas east of 90 E in the latitude belt of 5-20 N, and obviously lags in the IP and the AS areas. Figure 2 shows the developing process of the 850 hpa flow fields in the BOB, the ICP, the SCS, and the IP in 1994 in which the SCSSM onsets much earlier than normal. It is seen that the onset time in the BOB, the ICP, and the SCS areas is simultaneously in Pentad 25, while in the IP is in Pentad 31. The conclusion is the same as obtained from the time variations of the multi-yearly averaged low-level flow fields in the 6 representative areas. The black body temperature (TBB) can be used to judge the burst time of convection and intensity. The onset of summer monsoon is usually followed by strong convection. Therefore, TBB can be used in studies of summer monsoon properties. In this paper the seasonal variation of TBB is used. It is supposed that when the monthly departure of the TBB from the annual mean changes from positive to negative, the summer monsoon onsets. It is reasonable at least in tropics. Figure 3 gives time-latitude variations of the multi-yearly pentad mean monthly departure of TBB in the ICP (a), the BOB (b), the SCS (c), and the IP (d). It can be seen that in the north of N of the ICP the convection starts to intensify in Pentad 23, but it does not burst until Pentads in the south (Fig.3a). The convection in the BB bursts out at first in land north of 25 N and then over ocean in Pentads (Fig.3b). The case is similar in the SCS; the convection begins first in land north of 25 N

5 NO.2 QIAN Yongfu, ZHANG Yan, JIANG Jing, YAO Yonghong and XU Zhongfeng 133 and then over ocean in Pentads as well (Fig.3c). The burst of convection in the north of the ICP before Pentad 27 is caused by the cyclonic convergent flows there. While convection over the land in the north of the SCS and the BOB is induced by the convergent zone of inclined westerly along 30 N. Therefore, the tropical convection, that is, the tropical summer monsoon bursts abruptly almost at the same time of Pentads in the above three areas. The monsoon onset time so determined is basically the same as that from time variations of the inclined westerly or southerly. The developing process of convection in the Fig.2. The latitude-time variations of the l994 pentad mean wind vectors at the 850 hpa averaged in the representative regions: (a) the South China Sea, (b) the Indo-China Peninsula, (c) the Bay of Bengal, and (d) the India Peninsula. Fig.3. The latitude-time variations of the multi-yearly pentad mean departures (K)from the annual mean of the TBB in the Indo-China Peninsula (a), the Bay of Bengal (b), the South China Sea (c), and the India Peninsula (d).

6 134 ACTA METEOROLOGICA SINICA VOL.19 IP is entirely different. From Fig.3d it is seen that before Pentad 27 there is no convection north of 10 N indicating the later season transition. To south of 10 N weakconvection appears in Pentads 21-24, but is interrupted for 3 pentads. Till Pentad 27,convection near the equator intensifies and advances northward. Convection strengthens abruptly in Pentads and reaches 25 N indicating the earlier onset of monsoon in south. In Pentad 33 convection intensifies largely again and northward expands to the whole IP area. In Pentads convection center moves to the middlenorth part indicating the entire onset of monsoon in the area. Therefore, from the point of view of convection, the tropical India summer monsoon (TISM) onsets gradually from south to north in Pentad 31, and expands to the whole area in Pentads It is also in consistence with the conclusion obtained from analyses of the wind field. It is seen from the above discussion that taking pentad as unit the TASM onsets simultaneously in the BOB, the ICP, and the SCS regions, east of 90 E, in the third to the fourth pentads of May (i.e., Pentads of the year) with abruptness in the multi-yearly average; while in individual years the onset pentad of the TASM may be earlier or later, in 1994 it onsets in Pentad 25. On the other hand, in the IP and the AS regions, west of 90 E, the TASM onsets first in their south parts and advances gradually northward, the mean onset time is about in the third to the fourth pentad of June (i.e., Pentads of the year), but it onsets in Pentad 31 in Therefore, it may be concluded that the meridian of 90 E is the demarcation line between the EASM (the SCSSM) and the SASM (the TISM). 3. Possible mechanisms of the first onset of the TASM in the regions from the BOB to the SCS From the above analyses it is seen that the TASM onsets simultaneously earliest in the BOB to the SCS regions, east of 90 E, during the third to fourth pentads of May. What is the mechanism? Some scholars in the past emphasized the impact of the ocean when they discussed the formation and anomaly of monsoon. The statistical facts also proved that in the El Niño years the summer monsoon onsets later and weaker, while in the La Niña years, it onsets earlier and stronger. However, that is the general case only. It is well known that the basic cause resulting in monsoon is the thermal contrast between ocean and land. The former has much larger heat capacity than the latter and the latter has much larger thermal inhomogenesis than the former. Therefore, the ocean provides large amount of moisture and heat to the atmosphere, although its temperature does not change a lot and has no large seasonal variation compared to the land. Thus, a new concept should be emphasized that the obvious seasonal anomaly and the large thermal inhomogenesis of the land surface temperature would largely change the thermal contrast between ocean and land, and hence, the onset time and the intensity of monsoon. According to this new viewpoint the land in the middle to high latitudes may also influence the TASM and the thermal contrast between plain land and mountains is also an affecting factor of monsoon (see Qian et al., 2001). The surface air temperature is one of the indicators of the thermal features of land and ocean. Figure 4 is the composite time variations of the pentad mean surface air temperature before and after the SC- SSM onset averaged in It is found that the surface air temperature over the East Asian continent north to the SCS begins to rapidly increase in Pentad 8 before the SCSSM onset time. The earliest increase of the temperature happens in the east land of China in N and E with the 12 C isothermal inclined northward and reached to 30 N or further north at 115 E. At 120 E there is a large value zone of temperature gradient with quasi-south to north direction (Fig.4a). In Pentad 6 before the onset time the 12 C isothermal expands westward to near 110 E and between 105 E and 120 E the temperature continues to rise to form a closed isothermal of 16 C in N (Fig.4b). In Pentads 4 and 2 prior to the onset time the 16 C isothermal advances further north and expands further westward forming an evident

7 NO.2 QIAN Yongfu, ZHANG Yan, JIANG Jing, YAO Yonghong and XU Zhongfeng 135 Fig.4. The composite surface temperature ( C) distributions averaged in l (a)-(d) For 8, 6, 4, and 2 pentads before the monsoon onset, (e) for the onset pentad, and (f) for 2 pentads after the monsoon onset, respectively. high temperature area in the latitude belt be tween 25 N and 40 N, the west to east temperature gradient along 120 E increases further (Figs.4c, d). By the onset pentad of monsoon the 20 C isothermal advances northward once more in that area (Fig.4e). There has been a temperature trough along the coast of the East Asia and East China and gradually deepens forming an obvious thermal contrast zone. After the onset of monsoon the distributive pattern of temperature keeps no change (Fig.4f). Therefore, before the onset time of monsoon the surface air temperature in the east part of China is higher than that over both the West Pacific and the land west to it and forces the land surface high pressure to weaken and the low pressure to form in that area. The low pressure in turn attracts the southwesterly to come from the tropics and enter into the SCS. This is the main mechanism resulting in the earliest onset of the TASM in the BOB to the SCS regions. Therefore, the rapid heating of the land surface in the middle to high latitudes between 110 E and 120 E has key impact on the onset time and the source area of the TASM, although the heating is not in the tropical and subtropical regions. The seasonal variation of the mean sea level pressure (SLP) in from March to June is also analyzed (figure omitted). It is seen that a high-pressure system occupies both the continental area and the ocean in the middle and high latitudes, while a lowpressure belt controls the subtropical and tropical regions in March and April. In May the tropical lowpressure zone breaks through the high-pressure belt in the middle and high latitudes between 90 E and 140 E and a land low-pressure center forms at the same time in that area. In order to further depict the impact of heating over the land in the middle to high latitudes on the monsoon onset, the January to June variation of the zonal departures of the SLP along 50 N is analyzed, too (figure omitted). It is seen that

8 136 ACTA METEOROLOGICA SINICA VOL.19 the land high pressure is continuously located in E before March. In April, the low pressure breaks through the high at that latitude first between 115 E and 145 E, and by the end of May expands westward to 100 E. Therefore, the time variations of the SLP and the surface air temperature are well in correspondence, it clearly indicates that the land in the middle and high latitudes plays a key role in the large-scale land-ocean thermal contrast and therefore in determination of the earliest onset time and the source area of the TASM. Due to different topographic heights of the weather stations the surface air temperatures cannot really reflect the thermal contrasts. However, the integrated temperature (i.e., the temperature difference between a certain pentad and the first pentad of the year) may be taken as the speed of the temperature variation; and therefore can be taken as the index of thermal forcing. Figure 5 is the distribution of the integrated temperature in the mean onset pentad of monsoon. It is seen that the surface temperature changes a little south of 25 N, while it rises very rapidly north of 30 N, especially in the narrow and long zone in N, reaching 30 C east of 90 E. The evident integrated temperature gradient is located in the area south of 40 N and east of 90 E indicating the temperature increase in the north much faster than that in the south and the rapid transition of temperature gradient in south-north direction. Such kind of temporal variation of the surface air temperature Fig.5. The integrated temperature departures in the onset pentad of the SCSSM (the fourth pentad in May) from that in the first pentad of the year ( C). and spatial distribution of the integrated temperature makes the land low form and the middle to high latitude high break first in that area. Therefore, the TASM begins earliest in the regions east of 90 E. It is also seen from the figure that the integrated temperatures in both the south and the north of 30 N, west of 90 E, are not much different showing that the south-north integrated temperature gradient is small in the meridional belt containing the IP and the sign transition of the temperature gradient has not taken place yet by the onset time of the SCSSM. It is the above case that determines the simultaneous earliest onset of the TASM in the BOB to SCS regions east of 90 E and the simultaneous later onset of the TISM in the IP and the AS west of 90 E. In order to further prove the more important heating effect over the land than over the ocean on the onset of the TASM, here we take 1991 and 1994 as the representative years of the later onset and the earlier onset, respectively, and analyze the time variations of the integrated anomalies of the surface air temperature and the SLP in Pentads both over the land key area (25-40 N and E) and the ocean key area (10-25 N and E) as shown in Fig.6. It is evidently seen from Fig.6 that in the earlier onset year (1994), the integrated temperature anomaly (ITA) over the land key area increases very fast during Pentads (Fig.6a), and the integrated SLP anomaly at the same time decreases rapidly also (Fig.6b). Compared with the land key area, the magnitudes of the ITA and the integrated SLP anomaly over the ocean key area are much smaller, although with the same trends of time variation (Figs.6c, d). Therefore from Pentad 20 the difference of the ITA between land and ocean becomes positive and its value increases fast (Fig.6e), while the difference of the SLP anomalies becomes negative during Pentads and reaches minimum in Pentad 27 (Fig.6f). In the later onset year (1991), the ITA in the land key area decreases very fast and is totally negative during Pentads 22-26, while the integrated SLP anomaly at the same time increases also rapidly and is totally positive. The ITA over the ocean key area almost does not change,

9 NO.2 QIAN Yongfu, ZHANG Yan, JIANG Jing, YAO Yonghong and XU Zhongfeng 137 Fig.6. The time variations of the surface air temperature ( C; a, c), the sea level pressure (hpa; b, d), anomalies in the land key area (a, b), the ocean key area (c,d), and their differences of the integrated anomalies(e,f). while the integrated SLP anomaly has much lower amplitude than that over the land key area although with the same trend. Therefore the ITA difference between land and ocean decreases rapidly to negative values since Pentad 21, while the SLP anomaly difference increases very fast to positive. Therefore, the heating anomalies over the land play a decisive role in the thermal contrast abnormality between land and ocean. In the earlier onset year the land exhibits positive heating anomaly from April, while in the later onset year the negative. The same conclusion may be obtained from the analysis of the different sensible heating anomalies over land and ocean. It is well known that the surface heating is one of the main factors influencing the time variations of the surface air temperature(sat) and the SLP, the sensible heat (SH) flux can be used to heat the atmosphere above the surface directly, while the surface latent heat (LH) flux supplies energy to the atmosphere by transferring water vapor, as long as condensation takes place in the atmosphere, it obtains the energy. Next our focus is put on the analysis of time variations of the SH in various dekads in April and May (see Fig.7). It is seen that the SH distribution pattern

10 138 ACTA METEOROLOGICA SINICA VOL.19 does not change almost from the first dekad in March to the third dekad in May, west of 90 E, over the IP, the Tibetan Plateau (TP), the Iranian Plateau (IrP), and the Arabian Sea (AS). There is a maximum SH center over the IP, south of 30 N, and a minimum center is located over the IrP, north of 30 N. The north to south SH gradient has not been changing its sign. However, over the land areas, east of 90 E, two maximum value centers of the SH are located in the northwest ICP and mid-east China with the center located in 30 N and 110 E, respectively, in the first dekad of April. The value of the former exceeds 100 W m 2 and is stronger than the latter. After then, the former in the northwest ICP becomes weaker and weaker while the latter stronger and stronger (see Figs.7a, b, c and d). By the second dekad of May (Fig.10e) the former disappears and the latter becomes the only maximum center of the SH flux with a large value and domain. It is the continuously increasing and expanding SH center that makes the North China centered in 30 N and 110 E become the area with the fastest temperature increasing and SLP decreasing. Consequently, the gradients along north to south direction of both SAT and SH flux change their sign first in the East Asia. It is such an evident and total difference of the spatial and temporal variations of the SH flux on both sides of 90 E that results in the earliest onset of the TASM in the BOB to SCS regions and the later onset of the TISM by about one month, and therefore, the 90 E meridian is indeed the demarcation of the EASM and the SASM. The spatial distribution and the time evolution of the LH flux averaged in each ten days in April and May are analyzed too (figure omitted). It is seen that in the southeast ICP, South China, and Northeast China there are large value centers. The center value of the LH flux in the Southeast ICP and South China east of 90 E, becomes larger and larger with time and advances northward. It creates favorable condition for the first onset of the TASM in the areas east of 90 E. Fig.7. The multi-yearly averaged spatial and temporal variations of the surface sensible heat flux density (W m 2 ) with a, b, c and d, e, f denoting the first, second and third dekads of April and May, respectively.

11 NO.2 QIAN Yongfu, ZHANG Yan, JIANG Jing, YAO Yonghong and XU Zhongfeng Relationships between the onset of the TASM and the time variations of the SAH It is well known that at the high levels in summer the dominant circulation pattern is the SAH or anticyclone especially at 100 and 200 hpa. In mid summer the SAH centers are mainly concentrated over the TP and the IrP forming the so-called bimodality of the SAH (Zhang et al., 2002). Figure 8 shows the geographical distributions of the pentad mean SAH centers in all each year (a), including the secondary centers, and the multi-yearly pentad mean SAH centers (b) at 200 hpa determined by use of the stream functions from 1958 to It is seen from Fig.8 that the SAH centers in Pentads scatter to high extent and most of the centers are located in the south part of the ICP, south of 15 N. The centers begin to concentrate in Pentads over the ICP and the BOB with seldom centers over the western Pacific. In Pentads the centers are most located over two areas, that is, the TP and its vicinities, east of 80 E, and the IrP, west of 70 E, between 25 N and 30 N. In Pentads all the centers are mainly located in the north of 30 N. Therefore, it is proper to suppose that the onset time of both TASM and TISM may be determined by the latitudes of the SAH center (called the SAH scheme). That is, when the center passes across 20 N the TASM onsets and when it passes through 25 N the TISM onsets. By doing so, we obtain the onset pentads of the SCSSM (curve ) and the TISM (curve ) in as shown in Figs.9a and 9b, respectively. The onset pentads determined by He et al., by Yao et al. (by use of the moist potential vorticity (MPV), see Yao and Qian, 2001), and by use of the anomalous zonal component shear (the shear scheme, see Webster and Yang, 1992) between 850 and 200 hpa are also shown in the figure as comparison. It is seen from Fig.9 that in the SCS area the earliest onset time determined by the SAH scheme is Pentad 24; the latest is Pentad 31; the mean is Pentad 28. The years with earlier onset time of the SCSSM (one or more pentads earlier than normal) are 1961, 1965, 1970, 1973, 1974, 1975, 1985, 1990, and 1995, while the years with later onset time (one or more pentads later than normal) are 1968, 1976, 1982, 1983, 1987, 1991, 1992, 1993, 1996, and The most years with earlier onset of the SCSSM take place before the 1980 s, while the most years with later onset mainly after 1980 s indicating the evident interdecadal variation of the SCSSM onset and the rather important effects of the colder or warm modes of the sea surface temperature (SST) in the tropical eastern Pacific on monsoon. Compared to He s and Yao s onset time most onset pentads are basically in consistence, except in In the IP area the earliest onset time determined by the SAH center is Pentad 27; the latest is Pentad 35; the mean is in Pentad 31.1, they are also in good consistence with the onset time determined with the low level wind and the TBB fields. While with the Yao s MPV scheme the earliest, the latest and the mean onset time are Pentads 27, 34, and 30.73, respectively. With the shear scheme they are Pentads 25, 32, and 28.38, indicating that the onset pentads of the TISM determined with the SAH and MPV schemes are basically the same, while one or Fig.8. The geographic distributions of the anticyclone centers at 200 hpa. (a) For all pentads and (b) for the 40-year pentad averages from April to July in The symbols + represent pentads 19-23, 24-30, 31-36, and 37-42, respectively)

12 140 ACTA METEOROLOGICA SINICA VOL.19 Fig.9. The SCS (a, 20 N) and the Indian (b, 25 N) summer monsoon onset time determined with the positions of the 200 hpa anticyclone centers in For comparison, the onset time determined with Yao s MPV, He s, and Webster s methods is also shown in the figure by denoting Yao or MPV, He and shear, respectively. two pentads earlier in the shear scheme. The years with earlier onset time of the TISM (one or more pentads earlier than normal) by the SAH centers are 1963, 1964, 1965, 1974, and 1989, while the years with later onset time (one or more pentads later than normal) are 1966, 1972, 1980, 1981, 1983, 1985, 1987, 1990, 1991, 1992, 1993, 1994, 1995, and 1997 indicating evident interdecadal variation of the TISM onset as the SCSSM. Therefore a certain connection between ESl Niño and TASM onset time may exist. 5. Concluding remarks From analyses of the temporal and spatial properties of the low-level 850 hpa wind and the TBB, it is found that the TASM onsets earliest in Pentads over the BOB, the ICP, and the SCS regions, east of 90 E, with abruptness and simultaneousness in the whole area. While the TISM onsets at first in the south part of the IP and the AS areas in Pentads 30-31, then expands northward gradually and by the end of June onsets over the whole South Asia. Therefore, the mean onset time of monsoon in that area can be considered in Pentads When the SCSSM onsets, the low level southwesterly increases. When the TISM onsets, the inclined low-level westerly strengthens. Due to the large-scale feature of the TASM it is reasonable to take the pentad as the time unit of monsoon onset and take the whole area containing the BOB to SCS regions east of 90 E as the monsoon source area. On this base the TASM onsets first in the BOB to SCS regions east of 90 E in Pentads and is about one month earlier than the TISM which onsets in mid June in the IP and the AS areas west of 90 E. The meridian of 90 E, hence, is a demarcation line between EASM and SASM. Over the continental area of East Asia in the mid and high latitudes between 110 E and 120 E both SH flux and surface temperature increase rapidly, while the SLP decreases fast, in spring and in early summer resulting in the first burst through of the tropical low pressure belt into the high pressure one in that longitudes, forming a land low pressure system there, and consequently attracting the southwesterly into the area, east of 90 E, from the south. It is such seasonal variations of the SH, the SAT and the SLP that is regarded as the main mechanism inducing the earliest onset of TASM in the BOB to the SCS regions, east of 90 E. In the IP area, west of 90 E, the spatial distribution of the SH almost keeps no change with time during April and May. The time of sign change of the SH gradient along north and south is much later than that in the BOB to the SCS areas, east of 90 E, making the onset time of the TISM be much later than that of the SCSSM by about one month. Therefore, from the seasonal variations of the spatial distributions of the SH it is reasonable to consider the areas from the

13 NO.2 QIAN Yongfu, ZHANG Yan, JIANG Jing, YAO Yonghong and XU Zhongfeng 141 BOB to the SCS, east of 90 E, as the source region of the ASM, while the areas including the IP and the AS, west of 90 E, as the later onset region. Therefore, the meridian of 90 E is also a demarcation line between EASM and SASM from the seasonal variations of the spatial distribution of the SH. Meanwhile, it is proved from the analysis of the seasonal variation of the SAT that the continental heating anomaly plays a major role in the thermal contrast anomalies between land and ocean, in the earlier onset year the positive heating anomaly appears from April, while in the later onset year the negative anomaly appears. The SCSSM onset time can be approximately determined by the crossing of the SAH center through 20 N, while that of the TISM through 25 N. Compared with other methods except for the shear scheme the onset times of the SCSSM and the TISM determined with the SAH center position in pentads are basically the same in the sense of average. It is also found that the onset time of the TASM has something to do with the El Nino events. The earlier onset of the SCSSM appears in the colder mode of the SST in the tropical eastern Pacific, while the later onset in the warm mode. Acknowledgement. We would like to sincerely acknowledge the NCEP-NCAR for providing the reanalysis datasets, the Japan Meteorological Agency for providing the pentad mean TBB data of the GMS and the Data Department of the Nanjing Institute of Meteorology for transferring all the above data for this study. REFERENCES Chen L. X., Zhu Q. G., Luo H. B., et al., 1991: The East Asian Monsoon. Meteorological Press, Beijing, China. 362pp. (in Chinese) Ding Y. H., M. Murakami, 1994: The Asian Monsoon. China Meteorological Press, Beijing, 263pp. He H. Y., J. W. McGinnis, Z. Song, and M. Yanai, 1987: Onset of the Asian summer monsoon in 1979 and the effect of the Tibetan Plateau. Mon. Wea. Rev., 115(9), He Jinhai and Luo Jingjia, 1996: The SCS monsoon onset, the developing features of the Asian monsoon and mechanism discussions of their formations. New progresses in the Asian monsoon study. China Meteorological Press, Beijing, (in Chinese) Jin Zhuhui, 1999: Climatic features of the SCS summer monsoon depicted by the TBB data. Onset, evolution and interaction with oceans of the SCS summer monsoon. China Meteorological Press, Beijing, (in Chinese) Lau K-M, and S. Yang, 1996: The Asian monsoon and predictability of the tropical ocean-atmosphere system. Quart. J. Roy.Meteor. Soc., 122(532), Li C., and M. Yanai, 1996: The onset and interannual variability of the Asian summer in relation to landsea thermal contrast. J. Climate, 9(2), Liu Xia, Xie An, Ye Qian, and M. Murakami, 1998: The climatic characteristics of summer monsoon onset over South China Sea. J. Tropical Meteor., 14(1), (in Chinese) Ma H. N., and Ding Y. H., 1997: The present status and future of research of the East Asian monsoon. Adv. Atmos. Sci., 14(2), Kalney E, et al., 1996: The NCEP/NCAR 40-year reanalysis project. Bull. Amer. Meteor. Soc., 77(3), Qian Yongfu, Wang Shiyu, and Shao Hui, 2001: A possible mechanism affecting the earlier onset of South weasterly monsoon in the South China Sea compared to the Indian monsoon. Meteor. Atmos. Physics., 76 (3-4), Qian Yongfu, Zhang Q., Zhang X. H., 2002: Seasonal Variation and heat preference of the South Asia High. Adv. Atmos. Sci., 19(5), Tao Shiyan and Chen Longxun, 1987: A review of recent research of the East Asian summer monsoon in China. Monsoon Meteorology. Chang CP and Krishnamurlti TN, Eds. Oxford University Press, Wang B., and Wu R. G., 1997a: Peculiar temporal structure of the South China Sea summer monsoon. Adv. Atmos. Sci., 14, Wang B., and Xu X. H., 1997b: Northern Hemisphere summer monsoon singularities and climatological intraseasonal oscillation. J. Climate, 10(5), Wang Q.W. and Ding Y. H., 1996: Comparative analysis of summer monsoon evolutions in the South China Sea and the Bay of Bengal. New progresses in the Asian monsoon study. China Meteorological Press, Beijing, (in Chinese)

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