NOAA Coral Reef Watch Daily Global 5km Satellite Marine Heatwave Watch Product (Version 1.0.1)

Daily Global 5km Satellite Monitoring
Marine Heatwave Watch
(Version 1.0.1, released September 21, 2021)

Regions:	Tropics • Global • East • West • Pacific • Indian • S. Atlantic • Coral Triangle • Caribbean • Florida • Hawaii • GBR
Jurisdictions:	South Florida • Gulf of Mexico • Puerto Rico & USVI • Guam & CNMI • Samoas • Hawaii • Northeast Pacific

Archived Maps (1985-present)

Data Access:

Marine Heatwave Category:		NetCDF files: HTTPS FTP \| Browse Images: HTTPS FTP
Climatology:		NetCDF files: HTTPS FTP

Product Description

Marine Heatwaves are prolonged periods of anomalously high sea surface temperature (SST). In effect, they are categorized anomalies of SST compared with a long-term, high, daily SST threshold. The daily global 5km-resolution Marine Heatwave Watch (MHW) product presented here is a generalized version of the NOAA Coral Reef Watch (CRW) daily global 5km satellite Coral Bleaching HotSpot product. It isn't aimed specifically at corals and the warm season for each coral reef worldwide, but instead provides a general description of oceanic heat stress that can occur at any time of year, at any given location, and is likely applicable to and impacting a broader range of marine life. The MHW product is derived by applying the Marine Heatwave algorithm of Hobday et al. (2018)¹ to the CRW daily global 5km CoralTemp satellite SST data product. It has near real-time and historic components and spans 1985 to the present.

Version History:

v1.0.1 (Released on September 22, 2021) Fixed minor compatibility issue loading data into GIS Applications (eg ArcGIS) by altering dimension names and coordinate variable names.

v1.0 (Released on November 19, 2019) Initial version.

Methodology

Climatology

CoralTemp SST values from January 1, 1985 to December 31, 2012 were used to derive the climatology. For each day of the year, both an average and a 90th percentile SST value were created. These two values were calculated from the SST in an 11-day window centered on the day of the year, extended over the 28 years of the climatology. This means that each day's average and 90th percentile values were calculated from 308 values (i.e., 11 x 28).

Note that the SST value for February 29 in each leap year was left out of the overall climatology calculation to simplify the daily climatology calculation. When needed, the climatology for February 29 is derived by averaging the climatology values for February 28 and March 1.

Since January 1, 1985 is the first day of the dataset, special consideration was given to calculating the climatology average and 90th percentile values for January 1, 2, 3, 4, and 5 of that year. For example, the average and 90th percentile values for January 1, 1985 should have included the SST values from December 27 to January 6 (the 11-day window around January 1) for each year of the climatology. However, since there are no SST values available for December 27-31, 1984, the average and 90th percentile values for January 1, 1985 had to be calculated from the 303 values that are available (i.e., 6 values in 1985 and 11 values from each of the other 27 years of the climatology, giving n=303). It follows then that n=304 for January 2, 1985; n=305 for January 3, 1985; and so on, until n=308 for January 6, 1985 and for every other day of the climatological year.

Calculation of Heatwave Category

For each day from January 1, 1985 to the present, we calculated the Marine Heatwave category for each 5km satellite pixel grid in the v3.1 daily global CoralTemp SST dataset.

We identified a Marine Heatwave if the SST for a particular day was greater than the 90th percentile value for that location. Once a Marine Heatwave was identified, it was categorized based on its intensity, after Hobday et al. (2018)¹. Intensity categories were defined based on the difference between the average and 90th percentile values for each 5km pixel (diff). If the SST for a particular day was ≥ (average + diff) and < (average + 2diff), it was categorized as being Marine Heatwave Category 1. If the SST for a particular day was ≥ (average + 2diff) and < (average + 3diff), it was categorized as being Marine Heatwave Category 2; and so on (Figure 1).

Figure 1: Pictorial description of Marine Heatwave categories (from Hobday et al., 2018¹).

Since the climatology does not have a value for February 29 in each leap year, when this value was needed, both the average and 90th percentile SST values were calculated using an average of the SST values from February 28 and March 1.

Dealing with Sea Ice

Due to the methodology used to derive the MHW product, its interpretation becomes problematic if all pixels are not approximately normally distributed. At the very least, if the methodology is applied to a pixel with non-normally distributed temperatures (e.g., a pixel with any sea ice) and then applied to a pixel with approximately normally distributed temperatures (e.g., a pixel with only water temperatures), then the interpretation of each of these pixels will be different, creating internal inconsistencies. For this reason, if sea ice was present in a 5km satellite grid location within the 11-day window surrounding any date in the 28-year climatology period, then the pixel was classified as ice and given a flag value of 1 in the mask array of the climatology. The sea ice mask in the MHW product is static and is a conservative estimate of the ice-free satellite pixels within the climatology. It is not an indication of sea ice for that pixel for a given day.

NetCDF files

Each day of the global 5km MHW product is output as a separate NetCDF file. Each daily file contains the Marine Heatwave Category values for all 5km satellite pixels for that day (Table 1).

There are three arrays of data in each file: latitudes, longitudes and Marine Heatwave Category.

Value		Meaning
-127		Land
-1		Climatology Sea Ice
0		No Marine Heatwave (SST below 90th percentile)
1		Category 1 Marine Heatwave (Moderate)
2		Category 2 Marine Heatwave (Strong)
3		Category 3 Marine Heatwave (Severe)
4		Category 4 Marine Heatwave (Extreme)
5		Category 5 Marine Heatwave (Beyond Extreme)

Table 1: Marine Heatwave Categories included within the NetCDF files.

Relevant references:

1. Hobday AJ, Oliver ECJ, Sen Gupta A, Benthuysen JA, Burrows MT, Donat MG, Holbrook NJ, Moore PJ, Thomsen MS, Wernberg T, Smale DA. 2018. Categorizing and naming marine heatwaves. Oceanography 31, 2. doi: 10.5670/oceanog.2018.205.

2. Hobday AJ, Alexander LV, Perkins SE, Smale DA, Staub SC, Oliver ECJ, Benthuysen JA, Burrows MT, Donat MG, Feng M, Holbrook NJ, Moore PJ, Scannell HA, Sen Gupta A, Wernberg T. 2016. A hierarchical approach to defining marine heatwaves. Progress in Oceanography 141, 227-238.

3. Oliver ECJ, Donat MG, Burrows MT, Moore PJ, Smale DA, Alexander LV, Benthuysen JA, Feng M, Gupta AS, Hobday AJ, Holbrook NJ, Perkins-Kirkpatrick E, Scannell A, Straub SC, Wernberg T. 2018. Nature Communications 9, 1324. doi: 10.1038/s41467-018-03732-9.