Amita Prabhu

Recent trends and tele-connections among South and East Asian summer monsoons in a warming environment

Recent trends, variations and tele-connections between the two large regional sub-systems over the Asian domain, the South Asian and the East Asian monsoons are explored using data for the 1901–2014 period. Based on trend analysis a dipole-type configuration with north-drought and south-flood over South as well as East Asia is observed. Two regions over South Asia, one exhibiting a significant decreasing trend in summer monsoon rainfall over northeast India and the other significant increasing trend over the northern parts of the west coast of India are identified. Similarly two regions over East Asia, one over South Korea-southern parts of Japan and the other over South China are also identified both indicating a significant increasing trend in the summer monsoon rainfall. These trends are examined post 1970s. Possible factors associated with the recent trends are explored. Analysis of sea surface temperature (SST), mean sea level pressure and winds at lower troposphere indicates that the entire monsoon flow system appears to have shifted westwards, with the monsoon trough over South Asia indicating a westward shift by about 2–3° longitudes and the North Pacific Subtropical High over East Asia seems to have shifted by about 5–7° longitudes. These shifts are consistent with the recent rainfall trends. Furthermore, while the West Indian Ocean SSTs appear to be related with the summer monsoon rainfall over northern parts of India and over North China, the West Pacific SSTs appear to be related with the rainfall over southern parts of India and over South Korea- southern Japan sector.

Potential role of the February–March Southern Annular Mode on the Indian summer monsoon rainfall: a new perspective

Relationship between the Southern Annular Mode (SAM) and the India summer monsoon rainfall (ISMR) has been examined based on the data period 1949–2013. While the entire data period indicates a significant increasing trend in SAM, recent decades 1983–2013 indicate no trend. The relationship between the two strengthened considerably since 1983. Results reveal that the February–March SAM is significantly related with the subsequent ISMR. A positive (negative) SAM during February–March is favorable (unfavorable) for the ensuing summer monsoon rainfall over the Indian sub-continent. The delayed response is relayed through the central Pacific Ocean. We propose a hypothesis that states: when a negative (positive) phase of February–March SAM occurs, it gives rise to an anomalous meridional circulation in a longitudinally locked air–sea coupled system over the central Pacific that persists up to the subsequent boreal summer and propagates from the sub-polar latitudes to the equatorial latitudes inducing a warming (cooling) effect over the central equatorial Pacific region. In turn, this effect concomitantly weakens (strengthens) the monsoon rainfall over the Indian sub-continent. Thus, the February–March SAM could possibly serve as a new precursor to foreshadow the subsequent behavior of the Indian summer monsoon.

Variability and teleconnections of South and East Asian summer monsoons in present and future projections of CMIP5 climate models

Coupled Model Inter-comparison Project Phase 5 (CMIP5) model outputs of the South and East Asian summer monsoon variability and their tele-connections are investigated using historical simulations (1861-2005) and future projections under the RCP4.5 scenario (2006-2100). Detailed analyses are performed using nine models having better representation of the recent monsoon teleconnections for the interactive Asian monsoon sub-systems. However, these models underestimate rainfall mainly over South Asia and Korea-Japan sector, the regions of heavy rainfall, along with a bias in location of rainfall maxima. Indeed, the simulation biases, underestimations of monsoon variability and teleconnections suggest further improvements for better representation of Asian monsoon in the climate models. Interestingly, the performance of Australian Community Climate and Earth System Simulator version 1.0 (ACCESS1.0) in simulating the annual cycle, spatial pattern of rainfall and multi-decadal variations of summer monsoon rainfall over South and East Asia appears to more realistic. In spite of large spread among the CMIP5 models, historical simulations as well as future projections of summer monsoon rainfall indicate multi-decadal variability. These rainfall variations, displaying certain epochs of more rainfall over South Asia than over East Asia and vice versa, suggest an oscillatory behaviour. Teleconnections between South and East Asian monsoon rainfall also exhibit a multi-decadal variation with alternate epochs of strengthening and weakening relationship. Furthermore, large-scale circulation features such as South Asian monsoon trough and north Pacific subtropical high depict zonal oscillatory behaviour with east-west-east shifts. Periods with eastward or westward extension of the Mascarene High, intensification and expansion of the upper tropospheric South Asian High are also projected by the CMIP5 models.

Connection between Antarctic sea-ice extent and Indian summer monsoon rainfall

Antarctic sea‐ice extent (AnSIE) is an important parameter influencing global climate. The present study is carried out to find whether any connection between the AnSIE and the Indian summer monsoon rainfall (ISMR) exists. Temporal cross‐correlations between the anomalies of the ISMR and AnSIE as a whole and its different sectors are computed for the period 1988 to 2005. A coherent propagating pattern is clearly evident between the AnSIE and ISMR, as well as the rainfall over most of the homogeneous geographical regions of India. Furthermore, this study reveals that the sea‐ice extent (SIE) of the western Pacific Ocean sector in the month of March has a strong association with that of the ISMR in the same year. Year 2002 was a major drought year, which none of the regional or general circulation models could simulate. AnSIE variation has provided a strong signal to imply that 2002 would be a deficit monsoon year.

Can the Southern annular mode influence the Korean summer monsoon rainfall

We demonstrate that a large-scale longitudinally symmetric global phenomenon in the Southern Hemisphere sub-polar region can transmit its influence over a remote local region of the Northern Hemisphere traveling more than 100° of latitudes (from ~70°S to ~40°N). This is illustrated by examining the relationship between the Southern Annular Mode (SAM) and the Korean Monsoon Rainfall (KMR) based on the data period 1983-2013. Results reveal that the May-June SAM (MJSAM) has a significant in-phase relationship with the subsequent KMR. A positive MJSAM is favorable for the summer monsoon rainfall over the Korean peninsula. The impact is relayed through the central Pacific Ocean. When a negative phase of MJSAM occurs, it gives rise to an anomalous meridional circulation in a longitudinally locked air-sea coupled system over the central Pacific that propagates from sub-polar to equatorial latitudes and is associated with the central Pacific warming. The ascending motion over the central Pacific descends over the Korean peninsula during peak-boreal summer resulting in weakening of monsoon rainfall. The opposite features prevail during a positive phase of SAM. Thus, the extreme modes of MJSAM could possibly serve as a predictor for ensuing Korean summer monsoon rainfall.