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Monsoon currents

Image of the month - April 2007

Monsoon is one of the major climatic features on Earth, vital for people living around the Indian Ocean. It involves both ocean and atmophere, with a seasonal shift : the Summer monsoon (May-September) is wet whereas the Winter monsoon (November- February) is dry. In winter, ocean is hotter than land, and winds blow from the NorthEast, from land to sea. In summer the reverse occurs, blowing wet air over land from the SouthWest with much more force, and carrying welcome, if heavy, rains. Variations in this cycle - too much or too little rain -, can have a catastrophic impact on the countries neighbouring the Indian Ocean. Studying this cycle and predicting its shifts is thus of foremost importance for the South Asian people.

Monthly mean of Sea Level Anomalies and geostrophic currents for January (winter monsoon) and July (summer monsoon) between 1993-2016. Credits Marine Copernicus Service/CNES/CLS, 2017)

Like the winds and rain, ocean currents also change with season, flowing eastward in summer from the western Arabian Sea to the Bay of Bengal, and westward in winter. Altimeters able us to study these currents and their variations; used in coupled ocean-atmosphere forecasting models, their data could help estimate the next monsoon.

Asian Summer monsoon knows interannual variations for a variety of reasons (Eurasian snow cover, winds circulation, ENSO, IOD, ...). It's widely known that one of the strongest influences on the Asian Monsoon is the ENSO oscillation. During La Niña years, the western Pacific Ocean is warmer (see all maps here). The Indian Ocean Dipole (IOD) also plays an important role as a modulator of the Summer Monsoon rainfall : rainfall remains above normal during a positive IOD. A three-way link between ENSO, the Indian Summer Monsoon Rainfall and the Indian Ocean Dipole can be established but it is complex.


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