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Keywords:

  • IOD;
  • El Niño;
  • Walker circulation;
  • Ekman pumping;
  • convection

Abstract

An Indian Ocean Dipole (IOD) event often evolves without any forcing from the Pacific. This study demonstrates that Arabian Sea spring warming and associated summer barrier layer (BL) play an important role in the evolution of such IODs. We propose an alternate mechanism for IOD initiation. Anomalous Walker circulation over the Indian Ocean during IOD years forced by Arabian Sea warming in spring and summer plays a crucial role in initiating the easterly wind anomalies in the eastern equatorial Indian Ocean. The western warming and the associated convection induce easterly wind anomalies much before the significant eastern cooling during IOD years, which strongly supports the role of western warming in the IOD evolution. IOD years are characterized by anomalous thickening of summer BL over the Arabian Sea. The anomalous BL formation during IOD years is attributed to the anomalous negative Ekman pumping velocity in the Arabian Sea. This negative Ekman pumping velocity forced downwelling deepens isothermal layer depth and thickens the BL. In addition to this, strong precipitation during IOD years strengthened the stratification and supported the BL formation. These favourable conditions for BL formation are absent during El Niño years. Anomalous BL is mainly responsible for the deepening of thermocline and subsurface warming in the Arabian Sea during IOD years.