By Owen Embury
Over 70% of the Earth’s surface is ocean and knowing its temperature is important for understanding both weather and climate. Historically sea surface temperature (SST) has been measured in situ – from ships and more recently from automated buoys.
Ship-based measurements provide the longest available record with usable measurements dating back to the 1850s when the seafaring nations of the time agreed on various standard measurements techniques at the Brussels Maritime Conference (Rayner et al., 2006; Maury, 1858; Maury, 1859). However, researchers using ship-based records must still account for the changing coverage and instrumentation used on ships (Kennedy et al., 2011). Most ships taking SST observations are Voluntary Observing Ships (VOS) engaged on other business, and as such the observations are concentrated along the major shipping lanes which have changed over the years. Secondly, the measurement methods have changed – from hauling a bucket of water up to the deck, to automated measurement of the temperature of the water used to cool the ship’s engine.
The primary source of in situ SST data these days comes from drifting buoys. Development of modern drifting buoys started in the late 1970s. These were primarily intended for measuring ocean currents, often with a simple temperature sensor attached. A common design was reached in the early 1990s and the network maintained by the Global Drifter Program reached “full” coverage of 1250 buoys around 2005.
A modern alternative to in situ measurements is remote sensing of surface temperature from satellites. Space-based instrumentation for estimating SST works by measuring the thermally emitted radiation at the top of the atmosphere. This will include contributions from both the surface and the atmosphere or any clouds in the way. Typically the instruments work in the infrared with measurement bands centred near wavelengths of 3.7, 11, and 12 microns. These cannot see through clouds, but otherwise can observe the whole globe over a day or so.
The European Space Agency started the Climate Change Initiative project in 2010 to produce satellite-based climate data records (CDRs) for a range of Essential Climate Variables (ECVs) including SST. The SST-CCI project is lead by the University of Reading and includes partners at the Met Office, University of Leicester, Meteo France, DMI, Norwegian Meteorological Institute, Brockmann Consult, Space ConneXions, and the National Oceanography Centre (Figure 1).
Figure 1. Global SST anomaly timeseries from the NCEO – SST-CCI data set
The first phase of the SST-CCI project released a climate data record covering 1991 to 2010 (Merchant et al., 2014). These data are currently available from ESA. The second phase is currently ongoing and will release an extended dataset covering the 1980s to 2016 later this year.
Kennedy et al., 2011 – doi:10.1029/2010JD015220
Maury, M. F. , 1858. Explanations and Sailing Directions to Accompany the Wind and Current Charts, volume 1. Cornelius Wendell.
Maury, M. F. , 1859. Explanations and Sailing Directions to Accompany the Wind and Current Charts, volume 2. Cornelius Wendell.
Merchant et al. 2014 doi 10.1002/gdj3.20 http://centaur.reading.ac.uk/37690/
Rayner et al., 2006 doi 10.1175/JCLI3637.1