Intraseasonal Oscillation of Tropospheric Ozone over the Indian Summer Monsoon Region

Abstract

Boreal summer intraseasonal oscillation (BSISO) of lower tropospheric ozone is observed in the Indian summer monsoon (ISM) region on the basis of ERA-Interim reanalysis data and ozonesonde data from the World Ozone and Ultraviolet Radiation Data Centre. The 30–60-day intraseasonal variation of lower-tropospheric ozone shows a northwest–southeast pattern with northeastward propagation in the ISM region. The most significant ozone variations are observed in the Maritime Continent and western North Pacific. In the tropics, ozone anomalies extend from the surface to 300 hPa; however, in extratropical areas, it is mainly observed under 500 hPa. Precipitation caused by BSISO plays a dominant role in modulating the BSISO of lower-tropospheric ozone in the tropics, causing negative/positive ozone anomalies in phases 1–3/5–6. As the BSISO propagates northeastward to the western North Pacific, horizontal transport becomes relatively more important, increasing/reducing tropospheric ozone via anticyclonic/cyclonic anomalies over the western North Pacific in phases 3–4/7–8.

As two extreme conditions of the ISM, most of its active/break events occur in BSISO phases 4–7/1–8 when suppressed/enhanced convection appears over the equatorial eastern Indian Ocean and enhanced/suppressed convection appears over India, the Bay of Bengal, and the South China Sea. As a result, the BSISO of tropospheric ozone shows significant positive/negative anomalies over the Maritime Continent, as well as negative/positive anomalies over India, the Bay of Bengal, and the South China Sea in active/break spells of the ISM. This BSISO of tropospheric ozone is more remarkable in break spells than in active spells of the ISM, due to the stronger amplitude of BSISO in the former.

摘 要

本文利用ERA再分析资料和WOUDC臭氧探空资料, 分析了印度季风区夏季对流层臭氧的季节内振荡(BSISO)特征. 结果表明对流层低层臭氧存在着30-60天的季节内振荡, 臭氧异常呈西北-东南向分布, 在季风区向东北方向传播. 最显著的臭氧异常在海洋大陆和西北太平洋区域. 臭氧异常在热带从地面延伸到300hPa, 而在热带外地区臭氧异常主要在500hPa以下. 在热带, BSISO引起的降水是导致臭氧异常的主要原因, 使得在1-3/5-6位相出现了臭氧负/正异常. 然而当BSISO传播到西北太平洋区域时, BSISO引起的大气环流异常对臭氧异常的形成更为重要, 通过反气旋性/气旋性环流异常导致了臭氧在3-4/7-8出现正/负异常. 大多数印度季风的活跃/中断发生在BSISO的4-7/1-8位相, 此时减弱/加强的对流出现在赤道东印度洋, 加强/减弱的对流出现在印度, 孟加拉湾和中国南海. 海洋大陆对流层臭氧出现显著的负/正异常, 印度, 孟加拉湾和中国南海对流层臭氧出现显著的正/负异常. 由于季风中断期BSISO的振幅比季风活跃期更大, 对流层臭氧的季节内振荡也更加显著.