热带西太平洋和东印度洋对ENSO发展的影响

分析了从1955～1998年44 a热带西太平洋次表层海温距平资料,指出在这一期间共发生了11次El Nino事件,这些事件的初始海温正距平无一例外地都在“暖池”次表层160 m附近生成,发展到一定强度,到西太平洋出现异常的持续西风距平后,暖的海温距平沿着气候的温跃层向东向海表传播,经过1a左右的时间,传播到赤道东太平洋海表的El Nino 3或1、2海域形成通常认为的El Nino事件。进一步分析1960～1999年的850 hPa风场指出,在热带西太平洋西风距平爆发前,赤道东印度洋已经有距平西风持续维持了2a左右(西南季风),然后距平西风快速地向东发展产生热带西太平洋的距平西风,或真正的强西风,致使西太平洋出现El Nino爆发不可缺少的条件之一,在这40 a中,这一过程对所有的El Nino事件无一例外,同样的分析方法也用到这44 a中的La Nina事件,冷的海温距平也都首先在“暖池”的次表层形成,然后沿着温跃层向东向表层传播,在热带西太平洋出现东风前,印度洋也早已是持续的东风。

Contrasting the Indian and western North Pacific summer monsoons in terms of their intensity of interannual variability and biennial relationship with ENSO

The intensity of interannual variability(IIV)of the monsoon and monsoon–ENSO biennial relationship(MEBR)were examined and compared for both the Indian summer monsoon(ISM)and western North Pacific summer monsoon(WNPSM)during 1958–2018.Covariability of the IIV and MEBR were identified for the two monsoons.When the MEBR was strong(weak),the IIV of the monsoon was observed to be large(small).This rule applied to both the ISM and WNPSM.Out-ofphase relationships were found between the ISM and the WNPSM.When the IIV and MEBR of the ISM were strong(weak),those of the WNPSM tended to be weak(strong).During the period with a stronger(weaker)ENSO–Atlantic coupling after(before)the mid-1980 s,the IIV and MEBR of the WNPSM(ISM)were observed to be stronger.The increasing influences from the tropical Atlantic sea surface temperature(SST)may trigger the observed seesaw pattern of the ISM and WNPSM in terms of the IIV and MEBR multidecadal variability.The results imply that tropical Atlantic SST may need to be given more attention and consideration when predicting future monsoon variability of the ISM and WNPSM.

The Interannual Variations of Summer Precipitation in the Northern Indian Ocean Associated with ENSO

Using rainfall data from the Global Precipita- tion Climatology Project （GPCP）, NOAA extended reconstruction sea surface temperature （ERSST）, and NCEP/NCAR reanalysis, this study investigates the interannual variation of summer rainfall southwest of the Indian Peninsula and the northeastern Bay of Bengal associated with ENSO. The composite study indicates a decreased summer rainfall southwest of the Indian Penin- sula and an increase in the northeastern Bay of Bengal during the developing phase, but vice versa during the decay phase of E1 Nifio. Further regression analysis dem- onstrates that abnormal rainfall in the above two regions is controlled by different mechanisms. Southwest of the Indian Peninsula, the precipitation anomaly is related to local convection and water vapor flux in the decay phase of E1 Nifio. The anomalous cyclone circulation at the lower troposphere helps strengthen rainfall. In the northeastern Bay of Bengal, the anomalous rainfall depends on the strength of the Indian southwest summer monsoon （ISSM）. A strong/weak ISSM in the developing/decay phase of E1 Nifio can bring more/less water vapor to strengthen/weaken the local summer precipitation.

The critical role of Indian summer monsoon on the remote forcing between Indian and Northwest Pacific during El Nio decaying year

Recent studies have found a connection between Indian Ocean Basin Warming and the anomalous Northwest Pacific Anticy- clone （ANPWA） during El Nifio decaying year. This study focuses on the necessary condition for this connection by using ob- servation and numerical simulation. The seasonal transition of the Indian Ocean sea surface wind is critical to the climatic ef- fect of Indian Ocean Basin Warming. When the South Asian Summer Monsoon reaches its peak, the background wind be- comes desirable for basin warming, which then affects the climate in the Northwest Pacific. Via the Kelvin waves and Ekman divergence, the wind anomalies exist in the lower atmosphere east of the Indian Ocean warm Sea Surface Temperature （SST） anomalies, and intensify and sustain the ANWPA throughout the E1 Nifio decaying summer. This impact plays an important role in the inter-annual variability of the East Asian Summer Monsoon.