Satellites & Bleaching


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In the Coral Bleaching section of this tutorial, you learned that coral bleaching is caused by unusually warm sea surface temperatures (SSTs). To determine what areas of the world are at risk for coral bleaching, we first define the "usual" temperatures in the world's oceans as a baseline (or reference). This is accomplished by calculating a set of 12 long-term monthly mean SSTs, or climatologies.

A climatology, by definition, is the long-term condition for a specified time period. NOAA CRW developed its climatology for the daily global 5km satellite coral bleaching heat stress monitoring products using 28 years of satellite data (1985-2012) from the daily global 5km CoralTemp satellite SST product. Then, following CRW's currently-establised methodology (Heron et al., 2015), we adjusted the climatology to use a baseline (or reference) time period of 1985-1990 plus 1993 only. (In other words, although we used data from 1985-2012 to derive the initial climatology, we adjusted the resulting, final climatology to represent the baseline (reference) time period of 1985-1990 plus 1993 only.) We then defined the Maximum Monthly Mean (MMM) SST climatology. This is the warmest of the 12 monthly mean SST climatology values for each satellite pixel around the world, indicating the upper limit of "usual" temperature.

So how did we create the 5km climatology? By following these steps:

    • We calculated the mean (average) SST for each individual calendar month between 1985 and 2012 for each individual satellite pixel in
      the world’s oceans.
    • Next, for each calendar month, we calculated both (a) an initial
      28-year monthly mean climatology, and (b) the linear regression of
      the monthly average temperatures over the 28-year period
      (1985-2012) of the CoralTemp SST dataset.
    • Finally, we re-centered the initial 28-year monthly mean climatology
      for a calendar month, from the time center (temporal mid-point) of 1985-2012 (which is 1998.5), to the time-center of 1985-1990 plus 1993 (which is 1988.286). The resulting, final climatology has a baseline (reference) time period of 1985-1990 plus 1993 only.
    • The 12 monthly mean SST climatologies described in step 3 above
      are used to derive CRW's daily global 5km SST Anomaly product.
      The maximum satellite pixel-based values from among the 12
      monthly mean SST climatologies form the MMM SST climatology.
      We use the MMM SST climatology to derive CRW's daily global 5km Coral Bleaching HotSpot and Degree Heating Week (DHW) products.
      The MMM SST climatology is static in time but varies in space
      (Strong et al., 1997).

By creating the 5km climatology this way, we were able to use a sufficiently long time series (1985-2012) of daily satellite SST values, to adequately capture inter-annual SST variability in the initial climatology. We also were able to maintain, in the final 5km climatology produced, the baseline (reference) time period of 1985-1990 plus 1993. This baseline time period is used in both our heritage 50km-resolution climatology and next-generation 5km-resolution climatologies (both current and historical).

The 5km climatologies are available as images and NetCDF4 data files on the CRW website.

To learn more about NOAA CRW's 5km climatologies, visit the Climatology description page or 5km methodology page.

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