Impact of the Atlantic and West Pacific Warming on the Interdecadal Increase of Spring Aerosol Loadings in the North Indian Ocean since early 21st Century

An increasing trend of aerosol loadings associated with the growth of industrial emissions due to the fast development of the economy in India, Southeast Asia, and China during the recent decades has been observed with growing scientific attention. Studies of the associated tropospheric aerosol variations and long-term trends help us ascertain the impact of modernization and industrialization on the regional and global climate. In this study, a 38-year aerosol optical thickness (AOT) climate data record (CDR) observed by the Advanced Very High-Resolution Radiometer (AVHRR) over the global ocean is used to explore the inter-annual and inter-decadal aerosol variabilities in the surrounding ocean regions. At first, an empirical orthogonal function (EOF) analysis is performed over the North Indian Ocean (NIO), South China Sea (SCS), and the neighboring marginal seas of the west Pacific Ocean during March-April-May (MAM). The results show an AOT uniform pattern in the first mode and a north-south seesaw pattern between the North Indian Ocean and equatorial tropical regions in the second mode. They explain 54.65% and 11.62% of the total variance, respectively. However, both of these two leading modes present a close relationship to the El Niño–Southern Oscillation (ENSO) events. The former is primarily related to the sea surface temperature anomaly (SSTA) in the equatorial-eastern Pacific Ocean and tropical South Atlantic Ocean (SAO), and the latter to the SSTA in the southeastern Pacific Ocean (SEPO) and the North Atlantic Ocean (NAO). This NAO SSTA shows a typical horseshoe-like pattern of Atlantic multidecadal oscillation (AMO) characterized by negative SSTA in southeastern Newfoundland and positive SSTA in the subpolar, subtropical, and eastern NAO. The third mode shows an east-west contrast of aerosol variations between the tropical Indian Ocean and the SCS-Philippine Sea. It is primarily associated with the SSTA in the tropical NAO and the northern Indian Ocean (NIO) and explains 5.51% of the total variance.

The time series of the second EOF mode and the results from the analysis on the aerosol trend and regional mean suggest a distinct interdecadal increase of aerosol loading (IIAL) over the NIO in MAM since the early 2000s. To further investigate the relationship between the NIO IIAL and the AMO, we compared the mean SST and circulation between 2002-2020 (P2) and 1983-2001 (P1). Compared to those in P1 which is primarily associated with a relatively cold AMO phase, a zonal-oriented teleconnection pattern is revealed with two anomalous cyclones located in the northern middle Atlantic Ocean and western Siberia in P2 in 850-hPa wind field, and is accompanied by a notable increase of geopotential height and air temperature along the subtropical region with a maximum center located in the northwestern Tibetan Plateau (TP) at 500 hPa. The latter is also linkable to a northward extension of the North Africa subtropical high. As a result, a firm surface air warming belt is observed spanning the eastern African coast, southern Central Asia, and northeast India, with a noticeable dry trend in the Arabian Peninsula (AP) and South Asia. Therefore, more aerosol emissions are expected to be induced and transported to the neighboring regions of the NIO.

We also perform correlation analysis between the second mode time series and various fields in each time period. It is shown that dry and warming conditions in AP-northern South Asia, which naturally benefit an increase in the aerosol in the NIO in P1 (1983-2001), are primarily modulated by an ENSO event with a remarkable north-south contrast of strength of convective activities between the equatorial tropical Indian Ocean and subtropical AP-northern South Asia. But in P2 (2002-2020), a zonal contrast of anomalous convective activity between the ECA-South Asia and the Bay of Bengal-SCS is dominant. Meanwhile, associated with a warming AMO in P2, the correlation maps display a zonal teleconnection pattern in which a conspicuous cold air temperature-low geopotential height center is visible primarily in Central Asia both at 500 hPa and 200 hPa. To its south, a highly positive correlation is found in the subtropics extending from the Red Sea to the eastern TP while respective significant 500-hPa air temperature and 200-hPa height increases are visible in the eastern TP. Our study suggested that, while coincident with a climate regime shift, the warming AMO might play an important role in causing the interdecadal spring aerosol increase in the NIO since the early 21st century.