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Problem

In ERA-Interim the Sea Surface Temperature (SST) over land should be Null or ‘missing_value’ -32767.

However, this is only the case at time 00/12, step 0. At steps 3/6/9/15/18/21 the SST values over land are zero deg C.

Cause

The 00/12 values are analysis values which we get from different sources (NCEP, NOAA, and since 2009 from UK Met Office OSTIA) and re-grid. See http://onlinelibrary.wiley.com/doi/10.1002/qj.828/full, specifically sections 2.2 and 3.4.

The 03/06/09/15/18/21 values are forecast values calculated by our forecasting system IFS.

There is a glitch in the application of the land-sea mask in the forecast, with two effects:

  1. The SST is set to 0 degrees C over land
  2. Along the coastline and inland water bodies points are interpolated horizontally between over-land points (0 degrees) and over-water points (valid SST), the resulting SST is too low.

Workarounds

Option 1: If you only need the analysis, not  the forecast, use OSTIA itself, which has a much higher spatial resolution and is available for free from the Copernicus Marine Services, details here.

Option 2: Use the land sea mask to mask out land points. Beware however, the results of this can be difficult to interpret if the values have been interpolated from the native grid.