Sea Surface Temperature

What is Sea Surface Temperature ?

The general definition of sea surface temperature (SST) is that it is the water temperature at 1 meter below the sea surface. However, there are a variety of techniques for measuring this parameter that can potentially yield different results because different things are actually being measured.

The earliest technique for measuring SST was dipping a thermometer into a bucket of water manually drawn from the sea surface. The first automated technique for determining SST was accomplished by measuring the temperature of water in the intake port of large ships. This measurement is not always consistent, however, as the depth of the water intake as well as exactly where the temperature is taken can vary from vessel to vessel. Probably the most exact and repeatable measurements come from fixed buoys where the depth of water temperature measurement is always 1 meter and very robust electrical temperature probes are used. These measurements are usually beamed to satellites for automated and immediate data distribution. A large network of coastal buoys in U.S. waters is maintained by the National Data Buoy Center (NDBC).

Since the 1980's satellites have been increasingly utilized to measure SST and have provided an enormous leap in our ability to view the spatial and temporal variation in SST. The satellite measurement is made by sensing the ocean radiation in two or more wavelengths in the infrared part of the electromagnetic spectrum which can be then be empirically related to SST. These wavelengths are chosen because they are 1) within the peak of the blackbody radiation expected from the earth and 2) transmit well through the atmosphere. The satellite measured SST provides both a synoptic view of the ocean and a high frequency of repeat views, allowing the examination of basin-wide upper ocean dynamics not possible with ships or buoys. For example, a ship traveling at 10 knots would require 10 years to cover the same area a satellite covers in two minutes.

However, there are several difficulties with satellite based absolute SST measurements. First, because all the radiation emanates from the top "skin" of the ocean, approximately the top 0.1 mm or less, it may not represent the bulk temperature of the upper meter of ocean due primarily to effects of solar surface heating in the daytime, and back radiation and sensible heat loss at night as well as from the effects of surface evaporation. This makes it difficult to compare to measurements from buoys or shipboard methods, complicating ground truth efforts. Secondly, the satellite cannot look through clouds, creating a "fair weather bias" in the long term trends of SST. Nonetheless, these difficulties are small compared to the benefits in understanding gained from satellite SST estimates.

SST and Hurricanes

Remotely sensed SST can be used to detect the surface temperature signature due to hurricanes. In general, a SST cooling is observed after the passing of a hurricane primarily as the result of mixed layer deepening and surface heat losses. In some cases upwelling caused by a surface wind field divergence perhaps in conjunction with bathymetric effects can also be a source of cooling.

The SST changes primarily have important biological implications for hospitable/inhospitable conditions for many organisms including species of plankton, seagrasses, shellfish, fish and mammals. Although the SST changes are short-lived their ramifications are still not well understood.

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