Synoptic Characteristics of Heat Waves over the Indochina

Abstract:

The characteristics and synoptic behavior of heat waves (HWs) before (PRE) and after (POST) the onset of rainy season over Indochina Peninsula are studied by the NCEP Climate Forecast System Reanalysis dataset. Rotated empirical orthogonal function is used to identify the ‘key’ regions with spatially coherent and high amplitude fluctuations in daily maximum surface air temperature. The analysis through the 1979-2010 period indicates a notable increase in the frequency, number of HW days, duration, and event-severity of HWs, and averaged daily maximum temperature in both PRE and POST subseasons.

Analysis results reveal that HWs in PRE and POST subseasons exhibit different characteristics and synoptic behaviors, respectively. In the PRE subseason, HWs are characterized by below-normal pressure and precipitation near the surface and anomalous lower-level divergence center. Anomalously southerly flow appears over the South China Sea, implying that the weakening winter monsoon flow may be responsible for PRE HWs. Whereas, POST HWs are suggested to be induced by the dry and sinking air caused by the weakened summer monsoon flow. The POST HWs regions are overlain by prominent below-normal precipitation, above-normal pressure near the surface, with anomalous easterly flow appearing over the Indian Ocean, Indochina and South China Sea. This weakened monsoon flow is associated with an anomalous lower-level anticyclone and a divergence center over the land.