Causes for the intense interannual upwelling events in the tropical Indian Ocean
Event: 2019 Ocean Surface Topography Science Team Meeting
Session: Science II: Large Scale Ocean Circulation Variability and Change
Presentation type: Type Oral
Satellite altimetry observations, in situ datasets and reanalysis products reveal intense interannual upwelling events in the tropical Indian Ocean during 1993-2016. These events primarily occur in the mean upwelling zone of the Seychelles-Chagos thermocline ridge (SCTR) and in the seasonal upwelling area of the Eastern tropical IO (EIO) coastal area, with enhanced EIO upwelling accompanying weakened SCTR upwelling. In the fall of 1997, sea level increased by ~0.2 - 0.3m over the SCTR island nations and dropped by a similar amount in the EIO coastal area, which not only caused inundation but also significantly reduced (enhanced) upwelling in the SCTR region (EIO coast). The variability and upwelling directly affect sea surface temperature, biological activities, and air-sea CO2 flux. 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. The co-occurrence of El Nino and positive IOD in 1997 is the major cause for the intense upwelling event. Of particular interest is 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.