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Tropical Northern Atlantic Index (TNA)

The TNA SST anomaly index is an indicator of the surface temperatures in the eastern tropical North Atlantic Ocean. It is calculated with SSTs in the box 55°W - 15°W, 5°N - 25°N.

The TNA and TSA indices were defined in a paper by Enfield et al. (JGR, 1998), where dipolar patterns across the tropical Atlantic were found with a periodicity of 8-12 years for the boreal winter-spring, and 2.3 years for the boreal summer-fall.

Data source

The index is calculated using the Reynolds OIv2 SST analysis, made available through the IRI Data Library, and is updated weekly (last update 14-SEP-2016).   get series (netcdf 24kB) »  get error (netcdf 24kB) »


The anomaly is calculated relative to a climatological seasonal cycle based on the years 1982-2005. The weekly series was linearly interpolated to a daily series to calculate the climatological seasonal cycle with daily resolution. The leap day (29 February) was treated as a special case and linearly interpolated between the climatology of 28 February and 1 March. Spatial averaging of the gridded analysis was weighted by surface area. The standard deviation of the index over the period 1982-2005 is indicated on the plot.


The uncertainty in the estimate of the index is shown in gray, surrounding the zero x-axis, and is based on the uncertainty estimate of the analysis. For the surface indices, it is generally small compared to the value of the index.    more details »

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