Why We Observe the Ocean
Sea Surface Temperature and Currents
• The ocean communicates with the atmosphere via its surface. In this regard, sea surface temperatures (SSTs) are of particular interest because (1) the ocean covers 71 percent of the Earth’s surface; therefore, SSTs constitute a large component of global average temperature, which is a fundamental measure of global climate change; (2) SSTs determine the fluxes of heat and water to/from the atmosphere, which, in turn, control sequestration of heat in the ocean, impact amounts and patterns of precipitation, and influence large-scale circulation patterns in the atmosphere; (3) SSTs are a key ocean property that controls air-sea exchange of carbon dioxide, which, in turn, influences sequestration of CO2 in the ocean; (4) SSTs are an indicator of patterns of climate variability with known impacts on humans and ecosystems, e.g., El Niño, Pacific Decadal Oscillation (PDO), North Atlantic Oscillation (NAO); and (5) SSTs are known to correlate with tropical cyclone activity.
• SST at any location reflects how much heat enters or escapes the surface of the ocean (for example, as a consequence of direct solar radiation or greenhouse forcing that causes heating from above); how much heat is transported to or from other locations as a consequence of winds and ocean currents; and how much heat escapes from the bottom of the ocean mixed layer into the deeper ocean as a consequence of downwelling or vertical diffusion of heat.
• From the standpoint of numerical weather prediction, SSTs are an essential boundary condition on weather models; indeed, SST is the key ocean property that must be specified in order to model the physical behavior of the atmosphere. As a consequence, and because the ocean mixes much more slowly than the atmosphere, seasonal climate predictions may be possible because the slowly changing ocean boundary conditions retain a measure of deterministic predictability. In addition, decadal climate variability reflects ocean (SST) anomalies, which, in turn, reflect changes in ocean circulation and subsurface heat transport that occur on longer time scales.
• Thus while observed SSTs alone provide the information necessary to characterize the ocean’s influence on weather, long-term observations of ocean properties and elucidation of ocean processes that underlie prediction of future SSTs are essential to making seasonal climate predictions and to characterizing decadal climate variability and change.
• Because surface currents transport large amounts of heat from the tropics to subpolar latitudes, understanding their trends and variability is critical to prediction of SST anomalies. Indeed, surface current anomalies have been observed to lead SST anomalies, and therefore serve as early indicators of phase shifts in El Niño, and perhaps other climate cycles.
• Observations of surface currents are essential for evaluation of parameterized processes, or phenomena such as wind-driven currents and spatial patterns of seasonal circulation, in coupled ocean-atmosphere climate models.
• Knowledge of surface currents is also essential to computation of dispersal of ocean pollutants, enhancement of fishery models, and improvement of air-sea rescue operations.