Prolonged La Nina events and the associated heat distribution in the Tropical Indian Ocean

In the present study, heat redistribution in the Tropical Indian Ocean (TIO) associated with the prolonged La Nina events during 1958-2017 is examined using reanalysis/observations. It is found that the prolonged La Nina forcing strengthened the east-west thermocline gradient in the equatorial Indian Ocean and propelled the eastward extension of thermocline ridge of the Indian Ocean (TRIO) from its climatological location of southwestern TIO. The cyclonic winds over the southeastern TIO and the associated upwelling Rossby waves are primarily driving the TRIO intensification and its eastward extension. Anomalous subsurface warming, thermocline deepening, and the associated increase in the upper ocean heat content and sea-level in the eastern equatorial Indian Ocean, southeastern TIO and Bay of Bengal (BoB) are found to be the characteristic features of the prolonged La Nina events. Cross equatorial Sverdrup transport near the eastern boundary during the prolonged La Nina events has increased the heat content of BoB and is found to be a pathway of Pacific water entering the north Indian Ocean.

Automated summary

The study found that prolonged La Nina events strengthened the east-west thermocline gradient in the equatorial Indian Ocean, causing the eastward extension of the Indian Ocean thermocline ridge. The cyclonic winds in the southeastern Tropical Indian Ocean played a key role in intensifying and extending the thermocline ridge. The characteristic features of the prolonged La Nina events included anomalous subsurface warming, thermocline deepening, and increased upper ocean heat content and sea-level rise in specific regions of the Indian Ocean.