TC genesis SST threshold and SST trends

Because daily SST values are not available from the ERSSTv5 dataset, spatial coverage of the 26.5 °C threshold value is calculated from daily mean ERA5²¹ SST fields, which have a spatial resolution of 0.25°. ERA5 SSTs are drawn from several different analyses over the period 1950-2020, including HadISST2.1.0.0 prior to 1961, HadISST2.1.1.0 between 1961 and mid-2007⁴⁷, and OSTIA since mid-2007⁴⁸. The native temporal resolutions of these analyses are one month, five days, and one day, respectively. However, because SSTs change slowly with time, using these datasets downscaled to a uniform daily resolution is unlikely to introduce significant observational bias into the SST threshold analysis. The proportion of the 10–36°N, 100–70°W box for which this criterion is satisfied is calculated by applying a land mask and excluding the Pacific Ocean and dividing the number of daily gridpoints exceeding 26.5 °C by the total number of non-land, non-Pacific gridpoints. Trends of ERSSTv5 monthly mean SSTs are calculated from ordinary least squares regressions against year. Box or global average SST trends are calculated after excluding land gridpoints. Supplementary Fig. ⁷ was the only instance in which ERA5 SST values rather than ERSSTv5 monthly means were used as a source of SST values.