Joint Seminar: Effect of climate modes on interannual variability of upwelling in the tropical Indian Ocean

This study investigates interannual variability of the tropical Indian Ocean (IO) upwelling
through analyzing satellite and in situ observations from 1993 to 2016 using the conventional
Static Linear Regression Model (SLM) and Bayesian Dynamical Linear Model (DLM), and
performing experiments using a linear ocean model. The analysis also extends back to 1979,
using ocean–atmosphere reanalysis datasets. 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 El Niño–Southern Oscillation
(ENSO) and the IO 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.
Upwelling anomalies generally intensify when positive IODs co-occur with El Niño events. For
the 1979–2016 period, eastern Pacific (EP) El Niños overall have stronger impacts than central
Pacific (CP) and the 2015/16 hybrid El Niño events, because EP El Niños are associated with
stronger convection and surface wind anomalies over the IO; however, this relationship might
change for a different interdecadal period. Rossby wave propagation has a strong impact on
upwelling in the western basin, which causes errors in the SLM and DLM because neither can
properly capture wave propagation. Remote forcing by equatorial winds is crucial for the EIO
upwelling. While the first two baroclinic modes capture over 80%–90% of the upwelling
variability, intermediate modes (3–8) are needed to fully represent IO upwelling.

Date

01.12.2020

Time

15:15–16:15 h

Place

Virtual Seminar
Hamburg

Speaker

Xiaolin Zhang

Organizers

Armin Köhl

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