South Atlantic Ocean Dipole Index (SAODI)

The South Atlantic Ocean Dipole (SAOD) is the mechanism of warming of the surface waters off the coasts of West/Central Equatorial Africa associated with concurrent cooling of similar magnitude off the Argentina-Uruguay-Brazil coasts. These sea surface temperature (SST) patterns are coupled to the atmospheric circulation field and regional climates.

Sea surface temperature (RED, warming; BLUE, cooling), mean sea level pressure and surface winds anomalies associated with the SAOD.

A simple measure of the dipole, the SAOD Index (SAODI) is defined by differencing the domain-averaged normalized SST anomaly (SSTA) of the two centres of intense warming and cooling associated with the SAOD, viz:

SAODI=[SSTA]_NEP-[SSTA]_SWP

where the square brackets indicate domain averages, the subscripts show the two regions over which the SSTA averages are computed. These domains are described by their locations in the South Atlantic Ocean as the northeast pole (NEP:  10ºE–20ºW, 0º–­­­­­­­­15ºS) and the southwest pole (SWP: 10º–40ºW, 25ºS –­ ­­­­­­­40ºS). As shown below, this index is closely reproduced by the time series of the SAOD-mode determined by the singular value decomposition of the South Atlantic Ocean SST and mean sea level pressure.

Evolution of 7-month Gaussian pass-filtered monthly time series of the SAODI (red) and the singular value decomposition expansion coefficient of the SAOD mode (inversed for graphical presentation).

Related References:

• Nnamchi, H.C., J. P. Li. and R. N. C. Anyadike, 2011: Does a dipole mode really exist in the South Atlantic Ocean? J. Geophys. Res., 116, doi: 10.1029/2010JD015579. PDF
• Nnamchi, H.C. and J. P. Li, 2011: Influence of the South Atlantic Ocean Dipole on West African summer precipitation. J. Climate, 24, 1184-1197, doi:10.1029/2010JD015579. PDF 

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