Sub-monthly Variability in the South American Monsoon System

論文發表人: 馬席研(加州大學洛杉磯分校大氣及海洋科學系博士班)

 

http://www.bom.gov.au/events/9icshmo/

 

本研究利用觀測資料與全球大氣與地表耦合模式的模擬結果，在月內 (sub-monthly) 的時間尺度下，探討亞馬遜雨林中心低層持續的西風與東風期間，南美季風系統的變異度。

觀測與模擬的降雨異常合成圖在巴西西北部和中部與東南部都呈現一個雙極類型。在盛行西風（東風）期間，巴西中部與東南部顯示降雨的正異常(負異常)，這同時也顯示南大西洋輻合區降雨的增強。此外，在盛行西風期間，季風區高層高壓也同時增強，並且位於南美上空的亞熱帶噴射氣流也同樣增強並且更接近赤道地區。

在亞馬遜地區東、西風盛行期間之前，南太平洋850毫巴重力位高度場的時間序列合成圖與該地區主要的低頻變異度(又稱為太平洋南美類型Pacific-South American patterns)非常類似。因此我們認為這個發現顯示了南太平洋溫帶大氣環流的低頻變異度與南美亞馬遜季風風場與降雨變異度的關連性。

 

We examine the sub-monthly variability of the South American Monsoon System (SAMS) with emphasis on three-day periods or longer during which the anomalous wind at low-levels in central Amazonia is consistently from the west or the east (westerly and easterly wind regimes, WWRs and EWRs, respectively). Our research methodology is based on observational, Reanalysis, and model data.  The latter is provided by simulations of an atmospheric general circulation model coupled to an advanced land surface model (AGCM/LSM).

For both wind regimes, the composites of observed and simulated anomalous precipitation show a dipole pattern with poles in northwestern and central-southeastern Brazil. During WWRs (EWRs), precipitation anomalies at the central-southeastern pole are positive (negative), indicating a stronger (weaker) South Atlantic Convergence Zone (SACZ). Furthermore, during WWRs the upper-level monsoon high is stronger and the subtropical jet in the South American sector is stronger and closer to the equator.

Lag composites of geopotential height anomalies at 850mb prior to the establishment of the wind regimes in Amazonia show the development of structures in the South Pacific that resemble the principal modes of variability in that region, which are usually referred to as the Pacific South American (PSA) patterns. This finding suggests that links exist between the wind regimes and the modes of intraseasonal variability over the South Pacific and over South America. Thus, it is proposed that the wind regimes over Amazonia are associated with low-frequency variability in the extratropical circulation.