This monthly SST anomaly map is typically updated on the 16-18th day of each month.
The data source for the SST map is NOAA. The map is derived from daily sea surface temperature at 0.25 degree grid. (OI SST V2.1). The anomalies are based on 1971-2000 climatology.
References:
NOAA OI SST V2 High Resolution Dataset data provided by the NOAA PSL, Boulder, Colorado, USA, from https://www.ncei.noaa.gov/products/optimum-interpolation-sst
Huang, B., Liu, C., Banzon, V., Freeman, E., Graham, G., Hankins, B., Smith, T., and Zhang, H. (2021). Improvements of the Daily Optimum Interpolation Sea Surface Temperature (DOISST) Version 2.1. Journal of Climate 34, 8, 2923-2939, available from https://doi.org/10.1175/JCLI-D-20-0166.1