SEASONAL TRANSITION OF EAST ASIAN SUBTROPICAL MONSOON AND ITS POSSIBLE MECHANISM

As one of the Asian Monsoon members^[1], the East Asian Monsoon has been further divided into SCS tropical monsoon (SCSTM) and the East Asian subtropical monsoon (EASM) ^[2-3]. Different Eas Asian monsoon members interact with each other and mutually result in the flood and drought in eastern China^[4]. Through observational experiments and international cooperation studies, scientists have obtained basic knowledge about the characteristics of the Asian monsoon and its primary mechanisms^[5-9]. However, the exact time of seasonal transition of EASM and its possible mechanism have not been clarified yet.

In recent years, Wu et al.^[10], based on the 'thermal adaptation' theory, have proposed an origina view on the formation of subtropical monsoons. They pointed out that the long wave radiation cooling (LO in eastern oceans, the sensible heating (SE) in western continents, the condensation heating (CO) in eastern continents and the dual type of heating (LO+CO) in western oceans comprised the so-called'Four Leaves LOSECOD'heating in boreal summer, which is related to the zonal land-sea thermal contrast in the extratropical region and is the main cause of the coexistence of monsoon and desert. However, when and how does the 'LOSECOD' heating set up as wel as the heating pattern in Asian-Pacific region change from the 'west cold east warm' into the 'east cold west warm'? Subsequently, a great number of relative issues still remain to be further investigated.

In fact, on the onset time of the East Asian subtropical summer monsoon, scientists have been in profound dispute for a long time, and two major considerable controversial views are as follows: Tian et al.^[11] and Wan et al.^[12] pointed out, by analyzing the features of spring rainfall in the south of China that the spring rain does not belong to the East Asian summer monsoon rainfall. Wu et al.^[10] also classified the spring rain into a winter type of the atmospheric circulation based on the F-GOALS model sensitivity experiments; On the contrary, Ding et al.^[13] firstly called the persistent rainfall over the southern China during April to June an early summer rainy season Chen et al.^[6] suggested that the East Asian subtropica monsoon rainy season began in early April over the southern China first. Zhao et al.^[14] pointed out that the southwest summer monsoon set up in the subtropics first, along with the beginning of a subtropical rainy season. Wang et al.^[15] proposed that in late March and early October, along the east coast of Asia, not only the sensible heat flux but also the latent heat flux presents a distinct inverse of land-sea thermal contrast He et al.^[16] asserted that the seasonal transition of the zonal thermal contrast between the East Asian continent and the Western Pacific Ocean happened over the subtropical region first around the end of March and early April, with prevalence of northerly in winter turning into southerly in summer at lower troposphere, and the emergence of convective precipitation at the same time, which all marked the establishment of the East Asian subtropical summer monsoon. As mentioned above, one point of view held that the transition of East Asian atmospheric circulation from winter to summer occurred in May, and the EASM set up after the SCS summer monsoon; while the other point of view admitted that the EASM established at the end of March to the beginning of April. Therefore, when does the East Asian subtropical monsoon seasonal transition actually occur?And what is its possible mechanism?These are the urgent issues needed to be further researched and investigated.

In this paper, the key EASM region is defined according to the dominant seasonal transition feature of the meridional wind. Based on the 'thermal wind' rule in the atmospheric dynamics and 'thermal adaptation' theory, a new 'thermal-wind-precipitation' relationship has been deduced. At last, the seasonal transition features of EASM and its possible mechanism have been illustrated.

Daily reanalysis datasets from the NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research, USA), which include the variables of air temperature, geopotential height, and 10-meter wind (with a horizontal resolution 2.5º×2.5º), and the pentad data of the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) (with a horizontal resolution 2.5º×2.5º) from 1979 to 2009 are applied.

The pentad mean data were calculated from daily data.

As we all know, there are two basic features over the monsoon regions; the first one is significan seasonal transition of the lower-layer wind, especially the wind direction reverses from winter to summer while the second one is a dry period turning into a we period corresponding with the wind changing from winter to summer. As indicated in Figure 1, the easterly occurred mostly in the lower layers and the westerly dominated in the middle and higher layers As to the meridional wind, there is south wind in lower layers from March to September, and northerly in the rest of the months; while in the upper layers there is north wind from April to October, and south wind in the remaining months, and all the levels are characterized by significant alternation of the meridional wind. Based on its significant seasona reverse of meridional wind, the key EASM region is objectively defined by whether there is a robus change in the meridional wind at 925 h Pa and a meridional vertical wind shear between 925 h Pa and200 h Pa.

Figure 1.  Time-height cross-section of zonal (a) and meridional (b) wind over the EASM region (20–35°N, 110–120°N) y-coordinate:h Pa, x-coordinate:pentad; The shades in (a) and (b) indicate the westerly and southerly, respectively. Units:m/s.

The pentad evolution of the meridional wind i illustrated in Figure 2, which shows that no seasona transition exists between 100–110°E at the lowe layer, and all the latitudes are characterized by south wind throughout the year. We conclude that the region over 100–110°E is not the EASM region. In the region of 110–120°E, 20–35°N, there is significant change of the low-layer meridional wind and its vertical wind shear. To the north of 35°N, the change of the meridional wind is weak, which we identify as the edge of the EASM. Besides, the change of the meridional wind is also significant over the western Pacific Ocean (120–150°E). Due to the distribution of observations and for the sake of convenience, the region (110–120°E, 20–35°N) is consequently defined as the key region of EASM.

Figure 2.  Climatological evolution of the meridional wind (line with hollow circles, units:m/s) at 925 h Pa and its vertical shear between 925 h Pa and 200 h Pa (925 h Pa-200 h Pa, line with solid circles, units:m/s).

By simplifying the quasi-geostrophic vorticity equation, the relationship between the vertical motion and vertical shear of the meridional wind can be deduced, i.e.

where w is the vertical motion, v is the meridional wind.

According to the thermal wind principle,

where T is air temperature, f is geostrophic vorticity, and R is a constant parameter.

It is noticed that there is a strong transition of zonal land-sea thermal contrast from winter to summer, while there is no signal change of meridional land-sea thermal contrast. In addition, in the subtropical region, the observed wind can be approximated as the geostrophic wind. Therefore, Eq. (2) can be rewritten as:

By combining the Eq. (3) and Eq. (1), a new relationship Eq. (4) is inferred as:

Thus, in Eq. (4), a thermal-wind-precipitation relationship is established by combining the thermal and dynamical processes, which include zonal land-sea thermal contrast, vertical shear of meridional wind, and precipitation associated with the summer monsoon. To validate Eq. (4) and for the sake of convenience, we choose 500 h Pa area mean air temperature contrast between the (80–110°E, 20–35°N) and (120–150°E, 20–35°N) region to stand for the whole-column thermal contrast between land and sea. For the whole troposphere, we take the meridional wind vertical shear as the parameter of monsoon meridional circulation of EASM, and the300 h Pa vertical motion as the inner response. Hence, Eq. (4) is deduced as

where T_L is whole-layer mean air temperature over land, Ts is air temperature over the sea.

Based on Eq. (5), the thermal-wind-precipitation relationship is shown in Figure 3.

Figure 3.  Seasonal transition of the three factors in the 'thermal-wind-precipitation' relationship and pentad mean CMAP anomaly over the key EASM region (20–35°N, 110–120°E) (Cross:land-sea thermal difference (left vertical axis), units:deg K; solid circle:omega (left vertical axis), units:0.01 m/s; hollow circle:the high and low difference (right vertical axis), units:m/s; columnar:pentad precipitation anomaly (right vertical axis), units:mm/p; the transition time of the factors is indicated by two big circles.

As can be seen from Figure 3, the zonal sea-land thermal contrast, the vertical shear of meridional wind, and the 300 h Pa vertical movement along with the rainfall anomaly all presented a dominant seasonal reverse from winter (summer) to summer (winter). During late March and early April, the zonal land-sea thermal contrast and the vertical meridional wind shear both reversed from positive to negative, while the w and the anomaly rainfall changed from negative to positive.

It has been known that the most fundamental definition of monsoon is the reverse of wind direction (e.g., from winter to summer). We first analyzed the wind direction on each of the layers at different periods of time.

Figure 4 shows the averaged wind direction change with height in different pentads. The thermal wind principle shows that the wind direction rotates anticlockwise with height throughout the winter half-year, associated with cold advection. By contrast, in summer, the wind direction on the low level (e.g., 925 h Pa) is southeast or south, west wind dominates in the middle layer (e.g., 500 h Pa) of the troposphere, and there is either northerly or westerly in the upper layer (e.g., 200 h Pa). It is identified that the transition time of EASM happened right after the 12th pentad and before the 22nd pentad.

Figure 4.  Vertical profile of wind direction over the key region of EASM.

During seasonal transition, the reverse of wind direction and thermal advection on the lower layer of the troposphere is most obvious (Table 1). As shown in the 925 h Pa wind field (Figure 5), in early January (1st pentad), the entire East Asia was covered by cold advection; in early March (12th pentad), the EASM region, except for a small area at the southwest side, was generally controlled by cold advection. However, in early April (the 22nd pentad), the EASM region was fully covered with warm advection, the EASM was completely established, and warm advection was centered in the Hunan, Guangdong, and Guangxi region. In late June and early July (36th pentad, i.e.during the SCS summer monsoon), warm advection was further developed, the maximum value center was located along the south of China to the north of China to the Huaihe River basin; while in late August (48th pentad), over the EASM region, warm advection decreased significantly, but the whole eastern China was still covered with warm advection; in early October (54th pentad), warm advection in the EASM region retreated, which was replaced by uniform cold advection. At this time, the mainland of the eastern China entered a rapidly cooling period in which the East Asian subtropical winter monsoon began to establish.

Figure 5.  Horizontal wind (vector), temperature (contour; units:K) and thermal flux (shaded, units:10^-5 K/s) at 925 h Pa (1P, 12P, 22P, 36P, 48P, and 54P represents the 1st, 12th, 22nd, 36th, 48th, and 54th pentad, respectively).

The above analysis confirmed that both the wind angle and the corresponding thermal flux characterized the seasonal transition of EASM from winter (summer) to summer (winter). In order to further clarify the exact time of seasonal transition of EASM, we designed a diagram to characterize the temporal evolution of area-averaged wind direction over the EASM region (Figure 6).

Figure 6.  The pentad mean evolution of wind direction angle over the key region of EASM (hollow circle:10-m wind angle, solid circle:925 h Pa wind angle, cross:850 h Pa wind angle, fork:200 h Pa wind angle).

As shown in Figure 6, at 925 h Pa, the north wind suddenly changed into the south wind between the14th pentad and the 16th pentad over the EASM region before changing into a southwesterly wind during summer (6th-7th pentad). In the 48th pentad, an abrupt change of wind occurred from southeasterly to northeasterly. The ground 10-m wind field also shows an obvious change from winter to summer: before the 16th pentad, and the wind direction is northeasterly, which changed into southerly around the 18th pentad and northeasterly before the 54th pentad. Although not as obvious as the 10-m wind field and 925 h Pa wind field, a seasonal change at 850h Pa and 200 h Pa appears:the wind angle rotated almost 180 degrees just in three pentads (half a month). The summer and winter types of EASM are stabilized in late March to early April and late September to early October, respectively, representing the transition of EASM from winter (summer) to summer (winter).

As the evolution of wind direction at each of the layers is characterized as significant meridional change, the transformation of wind angle in high and low layers roughly reflected a transition in EASM meridional circulation. From the meridional circulation field in different periods (Figure 7) we can find that in early January (1st pentad), there are downward flows above the EASM region; the higher layers are with the southerly while the lower layers are with the northerly. In early March (12th pentad), a weak southerly appeared in the low layer, meanwhile week convection is generated at the middle-and lower-layer over the EASM region. In early April (22nd pentad), an obvious upward motion appeared over the EASM region. Southerly wind is prevalent at the low layer while there is weak northerly wind at the high layer. It is also noticed that at 200 h Pa, a clear closed circulation circle appeared around 20°N; to its south (north), there are downward (upward) airflows. This closed circulation is an excellent indicator for the setup of EASM. At the end of May and beginning of June (36th pentad), a time when SCS summer monsoon has already fully established, although there is consistent upward motion from the equator to 35°N, subtropical summer monsoon circulation and tropical summer monsoon circulation are still separated. By this time, the high (low) level has been occupied by the northerly (southerly) wind. At the end of August to early September (48th pentad), the south wind at the low layer are decelerated and the subtropical summer monsoon convection came into a decay stage. From the end of September to early October (54th pentad), it turned to the northerly wind at the low level when the subtropical winter monsoon was established.

Figure 7.  The meridional circulation and vertical motion in different periods (shaded areas stand for the rising motion, unit:10^-2m/s).

The seasonal transition of East Asian subtropical monsoon starts in the lower troposphere. In order to understand its building process and corresponding weather system, the evolution of the 10-m wind field is examined (Figure 8). As shown in Figure 8, in early January (1st pentad), the whole East Asia was controlled by a continental high. In early March (12th pentad), a mainland cold high moved eastward, when the northerly wind was replaced by northeasterly over eastern China; In early April (22nd pentad), the continental cold high moved towards the north of East China Sea, when eastern China was controlled by the southeasterly wind, which marked the establishment of the subtropical summer monsoon. At the end of June and early July (36th pentad), the cold high merged into the western Pacific subtropical high and the eastern China was entirely controlled by the southerly wind. In late September to early October (54th pentad), a cold high with a closed center was located over Shandong province, eastern China was controlled by northwesterly wind, and the subtropical monsoon circulation transformed from summer to winter. From the last panel of Figure 7 that shows the moving tracks of the cold high from the 12th pentad to 22nd pentad, it is known that it was around the 18th pentad that the continental cold high moved eastward to the sea.

Figure 8.  10-m wind field in different periods and moving track of a cold high (H) center.

Through the above research, we analyzed the seasonal transition features of EASM and clarified that the transition period of EASM (from the winter type to the summer type) is around 18th to 22nd pentad (from late march to early April). Besides, based on previous work^[11], we preliminarily present here a mechanism (Figure 9) for the establishment of the subtropical summer monsoon. As we can see from Figure 9, when the Sun passes from the Southern Hemisphere to the Northern Hemisphere, the surface thermal contrast (including the effects of the Tibetan Plateau) causes sensible heat to distribute asymmetrically in the zonal direction, which changes the non-uniform distribution of heat from a pattern of 'west cold-east warm' into one of 'west warm-east cold'. Due to the sensible heating forcing over the continent, a low-layer cold high moved eastward into the sea. As a result, the southerly wind at the low layer is stimulated over the subtropics. The southerly wind conveys heat from lower latitudes, inducing positive feedback in the 'west warm-east cold' pattern. Meanwhile, the southerly wind at the low layer changed the sign of vertical wind shear, inducing rising motion and rainfall which ultimately released latent heat and warmed the high-level atmosphere. Through this 'thermal adaptation' process, latent heating not only strengthened the southerly wind at the low layer, but also induced the northerly wind at the high layer.

Figure 9.  The mechanism of East Asian subtropical summer monsoon onset.

Based on the definition of the EASM region and the onset time of the subtropical summer monsoon, the formation and development mechanism of the subtropical summer monsoon is examined and discussed. Major conclusions are given as follows:

(1) According to the characteristics of changes in the meridional wind of EASM, the area (20–35°N, 110–150°E) is defined as the EASM area, and the35–40°N is a border area of EASM

(2) A 'thermal-wind-precipitation' relationship, i.e.the change phase of the land-sea temperature contrast, the high and low layer vertical wind shear, and the vertical movement associated with the monsoon rains, is identified for the EASM region. The wind direction experiences significant seasonal change:during the winter monsoon, wind direction rotates anticlockwise with height, representing cold advection; while in the summer monsoon, wind direction rotates clockwise with height, representing warm advection.

(3) In late march to early April, at the low level, the wind direction changes almost 180°. The seasonal transition of the EASM circulation occurs simultaneously.

(4) Due to the surface thermal contrast and the seasonal transition in solar radiation, the heating type over the EASM region changes from a pattern of being 'west cold-east warm' to one of being 'west warm-east cold'. The latter induces the southerly wind at the low layer. As a result, the wind direction rotates clockwise with height, representing warm advection that conveys heat from the south to the north. Meanwhile, the 'thermal adaptation' process produces the upward motion and rainfall, releasing latent heat and warming the upper layer. Both of the two processes have positive feedback effects on the thermal distribution pattern of being 'warm in the west and cold in the east'.

In this paper, we made a preliminary examination of the seasonal transition mechanism of EASM. However, the effects of various external forcing factors and internal positive feedback processes on the formation of EASM are still not quite clear. In our future work, we will utilize large-scale circulation models to gain better understanding of the formation mechanism of EASM.