The Bimodality of the 100 hPa South Asia High and its Relationship to the Climate Anomaly over East Asia in Summer.

Abstract

The NCEP/NCAR pentad mean reanalysis data from 1980 to 1994 are employed to examine the activities of the 100 hPa South Asia High (SAH) during the boreal summer. The results show that there exists bimodality in the longitude location of the SAH. According to the two preferred regions for the SAH, the SAH is classified into the Tibetan Mode (TM) and the Iranian Mode (IM), respectively. The studies on the maintenance mechanism, both from circulation structure and thermal structure, manifest that the SAH has the feature of warm preference. The diagnosis based on the thermodynamic equation further reveals that the TM is closely related to the diabatic heating of the Tibetan Plateau, whereas the IM is more associated with the adiabatic heating in the free atmosphere, as well as the diabatic heating near the surface.The statistical composites, and case studies, corresponding to the two modes show that the SAH bimodality is strongly related to the climate anomalies over East Asia. In the case of TM, the southerly airflow over the subtropics and the northerly airflow over the middle-high latitude at 850 hPa are enhanced, forming a convergence zone along 30 N and resulting in increased rainfall extending from the south Japan, Korea Peninsula, and Yangtze-Yellow river valley of China to the Tibetan Plateau. In the case of IM, at 850 hPa the mid-latitude East Asia is dominated by a huge anomalous anticyclone, thus results in the decreased rainfall over the area.

Dynamic variability of the Asian monsoon anticyclone observed in potential vorticity and correlations with tracer distributions

“…[34] The bimodality in the location of the anticyclone found in earlier studies [Zhang et al, 2002] in 100 hPa geopotential height is not reflected in the 360 K (i.e., about 150 hPa) low PV area investigated here (see Figure 6, top panel). At 380 K, which is closer to 100 hPa as used in Zhang et al [2002], the probability distribution of low PV air shows two maxima, one at around 50 ı E and one around 105 ı E (see Figure 15).…”

“…At 380 K, which is closer to 100 hPa as used in Zhang et al [2002], the probability distribution of low PV air shows two maxima, one at around 50 ı E and one around 105 ı E (see Figure 15). However, low PV area at 380 K as diagnostic for the anticyclone strength suffers from the following deficit: Over the summer season, mean PV in the monsoon region increases.…”

“…Oscillations with periods of 10-20 and 30-60 days were found by Annamalai and Slingo [2001], and they concluded that these are driven by variations in the monsoon convection. Zhang et al [2002] found that the location of the center of the anticyclone at 100 hPa is bimodal and classified a Tibetan Mode and Iranian Mode according to the location of the center. Associated with these modes, Yan et al [2011] found tracer anomalies that are shifted in longitude depending on the location of the anticyclone.…”

Connection between the Asian Summer Monsoon and Stratosphere-Troposphere Circulation over the Asian Region

“…The same bimodal high pressure anomalies can also be seen at 200 hPa in the upper troposphere (not shown). In contrast, Zhang et al (2002) showed the existence of bimodality in the longitude of the South Asia High in northern summer. Statistical inspection of the South Asian High by Zhang et al (2002) reveals the Tibetan mode (near 90 E) and Iranian mode (near 60 E).…”

“…In contrast, Zhang et al (2002) showed the existence of bimodality in the longitude of the South Asia High in northern summer. Statistical inspection of the South Asian High by Zhang et al (2002) reveals the Tibetan mode (near 90 E) and Iranian mode (near 60 E). The results by Zhang et al (2002) for the climatological field and our findings for anomaly fields (Fig.…”