Optimum Interpolation Sea Surface Temperature (OISST)
The NOAA 1/4° daily Optimum Interpolation Sea Surface Temperature (or daily OISST) is an analysis constructed by combining observations from different platforms (satellites, ships, buoys) on a regular global grid. A spatially complete SST map is produced by interpolating to fill in gaps.
The methodology includes bias adjustment of satellite and ship observations (referenced to buoys) to compensate for platform differences and sensor biases. This proved critical during the Mt. Pinatubo eruption in 1991, when the widespread presence of volcanic aerosols resulted in infrared satellite temperatures that were much cooler than actual ocean temperatures (Reynolds 1993).
Three other maps at the same 1/4° spatial resolution complement the daily OISST:
- Anomalies (i.e., the daily OISST minus a 30-year climatological mean) represent departures from "normal" or average conditions. Computation of several climate indices, such as the El Niño index, utilize SST anomalies.
- The error field provides a measure of confidence or quality, allowing users to exclude (using a threshold) or to minimize (using weights) the impact of daily OISST values with greater interpolation errors.
- The seven-day median of daily sea ice concentrations serves as the basis for proxy SSTs in the marginal ice zone, where observations are lacking. The proxy SST allows interpolation of temperatures from the open ocean to the seasonal sea ice margin.
Two Kinds of Daily OISST
There are two kinds of daily OISST, named after the relevant satellite SST sensors. These are the Advanced Very High Resolution Radiometer (AVHRR) and Advanced Microwave Scanning Radiometer on the Earth Observing System (AMSR-E). AVHRR has the longest record (from late 1981 to the present) of SST measurements from a single sensor design. Infrared instruments, like AVHRR, can make observations at relatively high resolution but cannot see through clouds. Microwave instruments like AMSR-E can measure SSTs in most weather conditions (except heavy rain) but not adjacent to land.
- AVHRR-Only refers to the OISST that uses satellite SSTs from AVHRR.
- AVHRR+AMSR uses AVHRR and additional data from AMSR-E, available from 2002 to 2011. Thus, in AVHRR+AMSR, observations near land come from AVHRR, while AMSR-E has superior spatial coverage over the open ocean. The combined use of infrared and microwave in cloud-free regions reduces systematic biases due to the independent error characteristics of the two sensors. After AMSR-E lost its full functionality in Oct 2011, AVHRR+AMSR production ended. Plans exist for rerelease of this product under another name using follow-up microwave instruments.