What is Ocean Temperature?

Ocean Temperature is a measure of the energy due to the motion of molecules in the ocean. Satellites enable measurement of sea surface temperature (SST) from approximately 10 µm below the surface (infrared bands) to 1mm (microwave bands) depths using radiometers. The spatial patterns of SST reveal the structure of the underlying ocean dynamics, such as, ocean fronts, eddies, coastal upwelling and exchanges between the coastal shelf and open ocean. SST is a vital component of the climate system as it exerts a major influence on the exchanges of energy, momentum and gases between the ocean and atmosphere. The heat and moisture exchanges are a main driver of global weather systems and climate patterns.

How is Ocean Temperature Measured?

Prior to the 1980s, measurements of sea surface temperature (SST) were derived from instruments on shorelines, ships and buoys. The first automated method of gathering SST was by measuring water flowing through the input ports of ocean faring ships.  While this method obtained a significant quantity of useful SST data there were some shortcomings. The depth of the input ports of different ships can vary greatly from ship to ship.  In a stratified ocean these different depths can have different temperatures. This method also resulted in rigorous sampling along major shipping routes but a dearth of information about the vast majority of the world's oceans.

Since the 1980's most of the information about global SST has come from satellite observations. Instruments like the Moderate Resolution Imaging Spectroradiometer on board (MODIS) onboard NASA’s Terra and Aqua satellites orbit the Earth approximately 14 times per day, enabling it to gathering more SST data in 3 months than all other combined SST measurements taken before the advent of satellites.

The motion of electrically charged particles produces electromagnetic radiation of various wavelengths. The electromagnetic spectrum comprises the range of these wavelengths. From longest to shortest the general wavelength categories are radio, microwave, infrared, visible, ultraviolet, x-ray, and gamma ray.

The ocean and most other objects emit radiation in the infrared and the microwave wavelengths. The amplitude of these wavelengths vary with the temperature of the ocean and therefore can be used to measure it.  Satellite sensors can measure these bands from space.  Infrared radiation of the ocean comes from the top 10 microns of the surface.  Microwave radiation results from the topmost 1-millimeter layer. Infrared satellite sensors have better spatial resolution but are more susceptible to cloud contamination than microwave. This is due to the absorption of the ocean emitted infrared energy by clouds.

Today in addition to satellite and shipboard measurements there are thousands of floats in the oceans measuring temperature and salinity. These are used to validate satellite instruments in addition to sampling throughout the water column. The surface drifters from the Global Drifter Program (GDP) provide regularly about 60,000 night time SST measurements per month at a shallower depth of 0.2 m. becoming the biggest contribution to the in-situ global SST, Ocean Currents, and Salinity measurements.

A major accomplishment in the distribution of satellite derived SSTs occurred with the Group for High Resolution Sea Surface Temperature (GHRSST) project. The project provides all SST data sets in a common format that allows for easy accessibility across different computer platforms and operating systems. To provide data sets that are suitable for climate modeling a necessary requirement is that climate data records come with a description of the errors associated with each SST value. The GHRSST project, to accommodate using these data sets in climate and ocean modeling, provides a full characterization of the errors associated with each pixel.  It is important to remember that satellites can only measure temperature at or close to the surface. Other instruments, or models, must be used to determine temperature at depth.

How is Ocean Temperature measured below 5 meters?

Global ocean temperature data below the surface are primarily measured using moorings and drifters.  Moorings are good for measuring time series through the depths of the water column and one particular longitudinal/latitudinal location.  Most deeper ocean temperature data are measured from drifters.  There are over 3000 drifters in the ocean today.  Ocean drifters are usually placed at a particular place in the ocean and then descend to a predefined depth where they record a time series of water temperature while moving with the currents at that depth.  One disadvantage of drifters is that most shut off their sensors between 5m deep and the surface to avoid fouling.

If a particular area or line of interest is to be measured underwater, autonomous gliders and/or forward propelled vehicles may be used.  These carry temperature-recording devices along with depth and salinity sensors, clocks and GPS.  These vehicles enable scientists to plan specific routes in which to make measurements.