Tropical Indian Ocean Interannual Variability Derived from Long-term Ocean and Atmosphere Analyses

Details Tropical Indian Ocean Interannual Variability Derived from Long-term Ocean and Atmosphere Analyses Tropical Indian Ocean Interannual Variability Derived from Long-term Ocean and Atmosphere Analyses

Dec 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufmos22f..05s&link_type=abstract

American Geophysical Union, Fall Meeting 2002, abstract #OS22F-05

Physics

3337 Numerical Modeling And Data Assimilation, 3339 Ocean/Atmosphere Interactions (0312, 4504), 4215 Climate And Interannual Variability (3309), 4231 Equatorial Oceanography, 4522 El Niño

Scientific paper

We have used upper ocean heat content (HC) and SST fields from a 41-year (1958-1998) global ocean data assimilation (ODA) analysis to examine the tropical Indian Ocean (IO) interannual variability. The ODA analysis was produced at COLA with continuous assimilation of historical ocean temperature measurements into an ocean general circulation model. The in situ temperature profiles are derived from NODC's World Ocean Database and real-time data, the Global Temperature-Salinity Profile Program, and the TOGA TAO array daily measurements. SSTs from COADS in situ measurements and CPC's gridded weekly fields have also been used. A validation using the TOPEX/POSEIDON altimetry has shown that the ODA HC fields capture major features of the tropical IO fluctuations in the 1990's, especially the anomalous events in 1994 and 1997-98. Combining these oceanic data with the NCAR-NCEP reanalyzed surface fluxes as well as the CPC precipitation for 1979-1998, we have expanded the analysis of the IO variability into the earlier period (since 1958), when continuous measurements of oceanic fields were not available. Our analysis demonstrates that a coupled ocean-atmosphere interannual oscillation is the major variability in the tropical Indian Ocean in the past 40 years. At the peak phase, anomalous equatorial zonal winds over the central and the eastern ocean and anomalous trade winds to the south induce zonal gradients of the SST and HC anomalies near the equator and an east-west shift of the convection. This interannual oscillation is the dominant signal from the boreal autumn to the next spring. The westward propagating HC anomalies causes a phase delay between the peaks of the surface cooling near the eastern coast and the warming near the western coast near the equator. During its propagation, the southern branch of the HC anomalies is strengthened by the anomalous wind curl of the equatorial and southeast trade wind anomalies over the southern ocean. As a result, the southern HC anomalies are maintained near the western coast for a much longer period. This IO oscillation is significantly correlated with the El Niño/Southern Oscillation (ENSO) variability in the Pacific Ocean. However, both the IO period and its ENSO connection exhibit a long-term modulation. Since late the 1970s, the IO oscillation had a nearly 4-year period and it was closely connected with ENSO. In the 1960's, however, the IO oscillation was more biennial and only weakly linked to ENSO. This long-term modulation coincided with the global climate shift during the 1970s.

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