Dick Reynolds

Daily SST Analyses: The Good, the Bad & the Ugly

Two new high resolution sea surface temperature (SST) analysis products have been developed using optimum interpolation (OI). The analyses have a spatial grid resolution of 0.25 degree and temporal resolution of 1 day. One product uses Advanced Very High Resolution Radiometer (AVHRR) infrared satellite SST data. The other uses AVHRR and Advanced Microwave Scanning Radiometer (AMSR) on the NASA Earth Observing System satellite SST data. Both products also use in situ data from ships and buoys and include a large-scale adjustment of satellite biases with respect to the in situ data. Because of AMSR's near all-weather coverage, there is an increase in OI signal variance when AMSR is added to AVHRR. Thus, two products are needed to avoid an analysis variance jump when AMSR became available in June 2002. Daily error fields are also produced which include bias, sampling and random errors.

The AVHRR-only product uses Pathfinder AVHRR data (currently available from January 1985 through December 2006) and operational AVHRR data for 2007 onwards. Pathfinder AVHRR was chosen over operational AVHRR, when available, because Pathfinder agrees better with the in situ data. The AMSR&AVHRR product begins with the start of AMSR data in June 2002. In this product, the primary AVHRR contribution is in regions near land where AMSR is not available. However, in cloud-free regions, use of both infrared and microwave instruments can reduce systematic biases because their error characteristics are independent.

The latest version (version 2) has several improvements. These improvements include the use of 3 days of satellite and in situ data instead of 1 to reduce the day-to-day noise. This version also smoothes the 7-day satellite bias correction to reduce a strong 7-day period in the original bias correction. In addition, version 2 includes a step to reduce ship SST biases with respect to buoy SSTs. Justification for the version 2 changes are discussed. The daily OI analyses are further evaluated by intercomparisons. Results show that recent Pathfinder AVHRR gradients are lower than operational Navy AVHRR gradients. In addition, comparisons with analyses from other centers show that increasing the spatial grid resolution does not always increase the analysis resolution. Many users may not understand the difference between grid resolution and analysis resolution.