[A-OS12] Physical, biogeochemical, and ecological processes and variability in the Indian Ocean

[AOS12-05] Interpreting negative IOD events based on the transfer routes of wave energy in the upper ocean

*Zimeng Li¹, Hidenori AIKI¹ (1.Institute for Space-Earth Environmental Research, Nagoya university)

The present study adopts an energy-based approach to interpret the negative phase of Indian Ocean Dipole (IOD) events. This is accomplished by diagnosing the output of hindcast experiments from 1958 to 2018 based on a linear ocean model. The authors have performed a composite analysis for a set of negative IOD (nIOD) events, distinguishing between independent nIOD events and concurrent nIOD events with El Niño-Southern Oscillation (ENSO). The focus is on investigating the mechanism of nIOD events in terms of wave energy transfer, employing a linear wave theory that considers the group velocity. The proposed diagnostic scheme offers a unified framework for studying the interaction between equatorial and off-equatorial waves. Both the first and third baroclinic modes exhibit interannual variations characterized by a distinct packet of eastward energy flux associated with equatorial Kelvin waves. During October-December, westerly wind anomalies induce the propagation of eastward-moving equatorial waves, leading to thermocline deepening in the centraleastern equatorial Indian Ocean, a feature absent during neutral IOD years. The development of wave energy demonstrates different patterns during nIOD events of various types. In concurrent nIOD-ENSO years, characterized by strong westerly winds, the intense eastward transfer of wave energy becomes prominent as early as October. This differs significantly from the situation manifested in independent nIOD years. The intensity of the energy-flux streamfunction/potential reaches its peak around November and then rapidly diminishes in December during both types of nIOD years.