人為氣膠對印度夏季季風自肇始前至消退之影響

Abstract

本研究使用國家大氣研究中心(NCAR)社區地球系統模式來研究人為氣膠對印度夏季季風的影響。研究中設計兩組實驗皆以氣候年循環海溫為模式下邊界條件進行30年模擬，實驗組(AERO)以2000年氣候平均人為氣膠排放資料驅動印度地區人為氣膠排放，控制組(CTL)則以1850年氣候平均人為氣膠排放資料驅動，實驗中僅考慮氣膠直接效應。實驗結果顯示在AERO中的印度夏季季風集合平均肇始日與CTL相似，各年的肇始日機率分佈相比CTL分佈更廣。與CTL相比，AERO中印度夏季季風於初春肇始期間環流轉變較急遽，而於初秋減退期間轉變較平緩。印度夏季季風肇始前，氣膠驅使的環流變化特徵於東北印度半島上空顯示上升運動以及阿拉伯海上空反旋式環流加強。阿拉伯海上空增強之反旋式環流導致季風肇始前增加16%自然源沙塵自中亞地區傳送至北印度地區。季風肇始前境外沙塵沿反氣旋環流由境外移入印度半島加上人為排放氣膠在青藏高原南側累積的輻射加熱(elevated heat pump)效應與半島上的氣膠黯化(aerosol dimming)效應影響區域氣候：高原南側氣流受輻射加熱上升；印度半島上氣膠吸收輻射增溫大氣冷卻地面，使得大氣受到氣膠輻射加熱導致海陸溫差加大產生氣旋式上升運動。前述氣膠-輻射-環流的交互作用越強則印度夏季季風肇始日期越早發生。在夏季季風期間，氣膠因降雨洗除作用及季節環流改變(南風分量增加)而使濃度降低且向北延伸，導致高原輻射加熱效應局限於西北印度地區且半島上氣膠黯淡效應減弱季風氣旋環流與降水（氣膠增加低層大氣穩定度、中高層大氣氣膠輻射改變海陸溫差效應不明顯）。夏季季風消退期間類似的氣膠黯淡效應持續作用，使得印度半島東側跟孟加拉灣產生南(0°-10°N)-北(10°-25°N)氣旋-反氣旋距平，此情況與印度夏季季風於九月消退時之環流特徵相似有利於季風消退提前。

The NCAR Community Earth System Model is used to study the influences of anthropogenic aerosols on the Indian summer monsoon (ISM). We perform two sets of 30-year simulations subject to the prescribed perpetual SST annual cycle. One is triggered by the year 2000 climatology anthropogenic aerosol emissions data over the Indian Peninsula (referred to as AERO), and the other one is by the year 1850 (referred to as CTL). Only aerosol direct effects are included in the experiments. In our results, the transition of ISM in AERO relative to the CTL exhibits a similar ensemble-mean onset date with a larger spread, and more abrupt onset in late spring, and an earlier but more gradual withdrawal in early fall. The aerosols-induced circulation changes feature an upward motion over the northeastern Indian Peninsula and strengthened anticyclonic circulation over the Arabia Sea in the pre-monsoon season, and a northward shift of monsoon flow in the developed monsoon period along with strengthened local meridional circulation over northern India. The strengthened anticyclonic circulation over Arabia Sea caused a 16% increase in natural dust transport from the Middle East in the pre-monsoon season. In the pre-monsoon period, dust transported along anticyclonic circulation into India and the accumulation of anthropogenic aerosols influence regional climate by elevated heat pump over the Tibet and the aerosol dimming effect over India Peninsula. The elevated aerosol heating induces ascending motion over the southern Tibetan area, while aerosol dimming-induced surface cooling and atmospheric warming leads to an increased land-sea temperature difference and cyclonic ascent over India. The stronger aerosols-radiation-circulation interaction corresponds to earlier onset of the Indian Summer Monsoon. During the developed monsoon period, aerosol concentrations decrease due to rainfall washout effects and shift northward by southwesterly monsoon flow. As a result, the radiative heating effect on the plateau is limited to the northwestern region of India, and the dimming effect over the peninsula weakens cyclonic monsoon circulation and precipitation due to the increased atmospheric stability and insignificant land-sea temperature difference. During the monsoon withdrawal, similar aerosol dimming effect results in a dipole pattern of south (0°-10°N) to north (10°-25°N) cyclonic-anticyclonic anomalies in Eastern Indian Peninsula and the Bay of Bengal. The above aerosol induced circulation change resembles the tendency change of flow in September, favoring early monsoon withdrawal.