Impact Of ENSO Events On East Asian Winter Monsoon

The ENSO phenomenon, which is known to be the most prominent signal inair-sea interactions, plays an important role in the interannual variability of East Asianwinter monsoon. This paper focuses on the features of the East Asian winter monsoonanomalies during El Nifio/La Nina events. Main results as follows:(1) The winters in 1997/1998 and 1998/1999, corresponding to El Nifio and LaNina episodes, respectively, are the two typical rain-abundant and rain-scarce seasons,respectively, in the southern part of China. The analysis results show that rich rainfallin winter is correlated to warm ENSO event, weak and eastward shifted westerlytrough/ridge, weakened East Asia winter monsoon, intensified subtropical High. Inthe rain scarce winter, however, the circulation systems in both upper and lower layersare different. The study also reveals that the water vapor in south China in winter ismainly from the southwesterly flow in foreside of the south branch westerly troughs,and the tuming flow over South China Sea and Indo-China peninsula. The channels ofmoisture transportation vary significantly with height. The intensified south branchwesterly flow and the anti-cyclonic circulation anomaly in the Philippine Sea in thewinter of 1997/1998 are favorable for moisture transportation towards the continentalChina. The two moisture transportation branches are dramatically weakened in thewinter of 1998/1999 due to the weak westerly flow and a cold high system dominatedin the lower layer in the southeast coast of China. Such significant inter-annualvariation of moisture transport is a key factor in resulting in the obvious difference ofprecipitation between the two winters.(2) Taking the winter of 1997/1998 and 1998/1999 as typical cases, East Asianwinter circulations are simulated by using the regional climate model (RegCM_NCC)of the National Climate Center of China, which has 16 vertical levels and a horizontalresolution of 60km. The simulation results show that the model not only simulates thedifferences of East Asian winter monsoon in the two winters, especially in the featuresrelated to 500-hPa East Asian trough, subtropical high and surface cold high overMongolia, but also reveals the features of low level circulation that are believed to beclosely related to ENSO events. The simulation reproduces the variation of cold surgegenesis in the different background of SST and atmospheric circulation in the twoseasons, as well as the three areas in which there is high frequency of cold surgeoccurrence. The study also reveals that the simulation gives realistic spatialdistribution of air temperature, the position of the rain belts and the abundant (scarce)rainfall in 1997/1998 (1998/1999) winter, indicating that the regional climate modelenjoys some level of skill in simulating winter climate variability in China underENSO episodes.(3) The responses of East Asian winter monsoon to ENSO events are analyzedusing the composite method based on the Oceanic Nino Index (ONI) andNCEP/NCAR reanalysis data with classification of the winters from 1950-2005 into5 categories of strong E1 Nifio, strong La Nifia, weak El Nifio, weak La Nifia andnon-ENSO event. The results show that response of the East Asian circulation in lowlatitudes to ENSO events has quasi-linear characteristics, i.e. response of the tropical atmosphere to the warm events is reverse to that of cold events, and that to weakENSO events is similar to strong ENSO events except for weakened intensity.However, distribution of circulation anomalies in middle and high latitudes does notshow obvious phase reversal for warm and cold ENSO events, indicating thatresponse of the atmosphere to SST anomaly is complex. The signals of significantresponse of the extratropical atmosphere to strong El Nifio events are representedmainly by enhancement of the westerly flow in middle and high latitudes in East Asia,positive anomalous temperature in northern areas of Asia, eastward extension ofanomalous wind shear in the middle and lower reach of the Yangtze River. Theanomalous Philippine Sea anticyclone, working together with intensified trough insouth branch of westerlies, is favorable for the southerly flow in the South China Seaand along the southeast coast of China. But, during the strong La Nifia events, theridge in the middle and lower reach of the Yangtze River is not evident, and thesouthern part of China and the South China Sea are covered by northerly anomaly,with shrunk area that passes significance test. Under weak El Nino events, thesouthwesterly flow in the northwest of the anticyclonic anomaly in the Philippine Seais shifted eastward, and separated with southwesterly flow ahead of the trough insouth branch of westerlies, resulting in the southerly anomaly in South China and theSouth China Sea less notable than that under strong El Nino events. It is veryinteresting to see anomalous systems in weak La Nina cases to be similar to thosewith strong El Nino but with inverse phase.