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Tropical Atlantic (TASI) SST index

The TASI SST anomaly index is an indicator of the meridional surface temperature gradient in the tropical Atlantic Ocean. It is calculated as the difference of the NAT and SAT indices.

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.

The TASI index was defined in a paper by Chang, Ji, and Li (Nature, 1997), where it were associated with a potential decadal 'dipole' mode of coupled variability in the tropical Atlantic. This study can be found on the Journal Nature's Web site.

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 0.1304 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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