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 basic daily OISST methodology is described in Reynolds et al. (2007), but minor modifications were introduced in the current version, version 2 (Reynolds 2009). 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).
The daily OISST belongs to a family of products that are sometimes referred to as "Reynolds SST" after Richard W. Reynolds, who developed the early OISST methodology with T. Smith at the National Centers for Environmental Prediction (NCEP). The earliest OISST analyses were at lower spatial and temporal resolution. Reynolds later moved to NCDC, where he helped develop the 1/4° daily OISST. Precursors to the daily OISST include the NOAA 1° weekly OISST (version 2) available at NCEP. NCEP also runs a copy of the daily OISST software for their internal use. This and the fact that both weekly and daily analyses are both at version 2 often leads to confusion. The weekly OISST is not a seven-day average of the daily OISST. The weekly OISST gets its name because it uses a week of satellite observations averaged onto a 1° grid prior to interpolation; the daily OISST uses a day of satellite observations averaged onto the 1/4° grid. There are other methodological differences between the two products, discussed in the references previously mentioned.
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.
Input Data Preparation
The three inputs to the daily OISST require different preparation, as described in Reynolds et al. (2007). The in situ SSTs need to be checked for outliers, and then averaged onto the 1/4° grid, separately for ships and buoys. The ship data are adjusted to the buoys by subtracting a constant 0.17°C. Similarly, satellite temperatures are averaged onto the analysis grid, after which they are bias-adjusted to the in situ data. For the marginal ice zone, where there is little in situ data, proxy SSTs are generated from sea ice concentrations. The interpolation step combines the prepared inputs.
Climatology and Anomalies
The climatology used to compute the daily OISST anomalies, available from NCEP, represents the 1971–2000 base period and is interpolated to 1/4° daily grid file from the 1° monthly file. The climatology is described in Xue et al. (2003).