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South Western Indian Ocean (SWIO) SST index

The SWIO SST anomaly index is an indicator of the surface temperatures in a region east of South Africa and south of Madagascar. It is calculated with SSTs in the box 31°E - 45°E, 32°S - 25°S.

In the full series case, monthly averages are shown by the plus signs, three month running mean filtered index values are shown by the shaded regions. In the "last two years" case, weekly averages are shown by the plus signs and the monthly average values are shown by the shaded regions.The weekly standard deviation of the index over the period 1991-2020 is indicated on the plot.

Above average austral summer rainfall over parts of subtropical southern Africa has been associated with periods of high SWIO.

This region has been identified as important in research by local scientists and others starting in the late 1980s. These include studies by Walker (1989), Walker (J. Geophys. Res., 1990), Jury et al. (J. Clim. Appl. Met., 1993), Mason and Tyson (J. Geophys. Res., 1992), Mason 1995 (Int. J. Clim., 1995), Rocha and Simmonds (Int. J. Clim., 1997), Reason and Mulenga (Int. J. Clim., 1999), Reason (J. Clim., 2001), Behera and Yamagata (Geophys. Res. Lett., 2001), Reason (Geophys. Res. Lett., 2001), Fauchereau et al. (Int. J. Clim., 2003), Hermes and Reason (J. Clim., 2005), and Washington and Preston (J. Geophys. Res., 2006).

Other sources of information (indices and forecasts) relevant to this region can be found at the: South African Weather Service Web site and the NOAA African Weather and Climate Web page.

The CLIVAR Variability in the African System (VACS) panel coordinates research on the variability and predictability of African climate, and its relationship to the global climate system. The CLIVAR VACS Web site can be accessed by clicking this link

Data source

The index is calculated using the Reynolds OIv2 SST analysis, made available by NOAA/ESRL, and is updated weekly (last update 12-MAY-2024).   get series (netcdf 24kB) »  get error (netcdf 24kB) »


The anomaly is calculated relative to a monthly climatological seasonal cycle based on the years 1991-2020. The monthly climatology is linearly interpolated to determine weekly anomalies.Spatial averaging of the gridded analysis was weighted by surface area.


The uncertainty of the index is estimated to be 0.1358 degrees C over the period 1982-present, and is based on the uncertainty estimate obtained from the objective interpolation analysis.    more details »

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