Effects of climate modes on interannual variability of upwelling in the tropical Indian Ocean.

This paper investigates interannual variability of the tropical Indian Ocean (IO) upwelling through analyzing observational datasets from 1993-2016 using the conventional Static Linear regression Model (SLM) and Bayesian Dynamical Linear Model (DLM), and performing model experiments using a linear ocean model. Strong interannual variability is observed over the mean upwelling zone of the Seychelles-Chagos thermocline ridge (SCTR) and in the seasonal upwelling area of the Eastern tropical IO (EIO), with enhanced EIO upwelling accompanying weakened SCTR upwelling. Surface winds associated with the El Niño and Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are the major drivers of upwelling variability. ENSO is more important than the IOD over the SCTR region, but they play comparable roles in the EIO. Completely new results are that Eastern Pacific (EP) El Niños overall have much stronger impacts than Central Pacific (CP) El Niños, since the former (latter) has subsidence over the Indo-Pacific warm pool (western Pacific and Indian subcontinent) and thus drive strong (weak) easterlies in the equatorial IO. The 1997 EP El Niño however has a much larger impact on IO upwelling than the 2015 EP-CP “hybrid” El Niño, due to their different effects on the IO convection and winds. Rossby wave propagation has strong impact on upwelling in the western basin, which causes errors in the SLM and DLM because neither can properly capture wave propagation. While the first two baroclinic modes capture over 80-90% of the upwelling variability, intermediate modes (3-8) are needed to fully represent upwelling over the IO.