(Version 1.0.1, released September 21, 2021)
Marine Heatwaves are modelled off their terrestrial namesakes and are prolonged periods of anomalously high sea
surface temperature (SST). In effect, they are categorized SST anomalies (c.f., the NOAA Coral Reef Watch (CRW)
daily global 5km SST Anomaly product).
They also are a more general version of the CRW daily global 5km Coral Bleaching HotSpot
product, but rather than being aimed specifically at corals, they provide a more generalized description of marine heat stress
that is likely applicable to a broader range of marine life. The next important step, therefore, is to develop a generalized accumulation of
heat stress, much like the
daily global 5km Degree Heating Week (DHW) product accumulates HotSpots.
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 calculation of the climatology so as to simplify the calculation of the daily climatology. 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. As one 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 could only 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, the Marine Heatwave category was calculated for each 5km satellite pixel grid in the v3.1 daily global 'CoralTemp' SST dataset.
A Marine Heatwave was identified 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).
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.