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Deviation of the temperature from deep undisturbed water during daylight warming.
Notice logarithmic scale.
Sea surface temperature (SST) is the water temperature close to the surface.

In practical terms, the exact meaning of surface varies according to the measurement method used. A satellite infrared radiometer indirectly measures the temperature of a very thin layer of about 10 micrometres thick (referred to as the skin) of the ocean which leads to the phrase skin temperature (because infrared radiation is emitted from this layer). A microwave instrument measures subskin temperature at about 1 mm. A thermometer attached to a moored or drifting buoy in the ocean would measure the temperature at a specific depth, (e.g. at 1 meter below the sea surface) — this temperature during the day is called temperature of the warm layer. The measurements routinely made from ships are often from the engine water intakes and may be at various depths in the upper 20 m of the ocean. In fact, this temperature is often called sea surface temperature, or foundation temperature. Note that the depth of measurement in this case will vary with the cargo aboard the vessel.

Deviation of the temperature from deep undisturbed water during night.
Notice logarithmic scale.

Measuring SST

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 that was 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 approximately 1 meter. Many different drifting buoys exist around the world that vary in design and the location of reliable temperature sensors varies. Furthermore, once deployed, it is very difficult to obtain information that reliably monitors the temperature sensor calibration. These measurements are sometimes 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 about 1990, there has also been an extensive array of moored buoys maintained across the equatorial Pacific Ocean designed to help monitor and predict the El Niño phenomenon. However, much more data is required for SST studies than El Niño studies and only a fraction of the data set required by numerical weather prediction and ocean forecasting models for SST is available from buoys. Only satellite SST data sets can provide this information.

Since the 1980s 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. Satellite measurements of SST are far more consistent and, in some cases, accurate than the in situ temperature measurements described above. The satellite measurement is made by sensing the ocean radiation in two or more wavelengths in the infrared part of the electromagnetic spectrum or other parts of the spectrum which can 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. able to 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 knot (20 km/h) would require 10 years to cover the same area a satellite covers in two minutes. The Group for High resolution SST Ghrsst-pp see [81702] provides operational access to nearly all satellite SST data sets in a common format and within 6 hours of acquisition by the satellite instrument. The National Oceanic and Atmospheric Administration (NOAA) has been providing research quality SST data continuously since 1981. As of 2009, version 5 of the Pathfinder project contains re-processed global SST data from 1981 through April 2009 at 4km resolution. This is the longest continuous data set of its kind. NASA's (National Aeronautic and Space Administration) Moderate Resolution Imaging Spectroradiometer (MODIS) SST satellites have been providing global SST data since 2000. These gridded products are available with a one-day lag. NOAA's GOES (Geostationary Orbiting Earth Satellites) satellites are geo-stationary and always fly above the Western Hemisphere. This enables to them to deliver SST data on an hourly basis with only a few hours of lag time. GHRSST, Pathfinder, MODIS and GOES data can all be obtained from NASA's Physical Oceanography Distributed Active Archive Center (PO.DAAC).

However, there are several difficulties with satellite based absolute SST measurements. First, in infrared remote sensing methodology the radiation emanates from the top "skin" of the ocean, approximately the top 0.01 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, reflected radiation, as well as sensible heat loss and surface evaporation. All these factors make it somewhat 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. However, some microwave techniques can measure SST and "see" through clouds.

As an aside, away from the immediate sea surface, general temperature measurements are accompanied by a reference to the specific depth of measurement (e.g. SST1m refers to an SST measurement made at a depth of 1 m). This is because of significant differences encountered between measurements made at different depths, especially during the daytime when low wind speed and high sunshine conditions may lead to the formation of a warm layer at the ocean's surface and strong vertical temperature gradients (a diurnal themocline).There are 3 layers to the ocean one is the surface layer the next on is the thermocline,and the last is the deep ocean. 75 percent of the ocean is made up of the deep ocean area/layer.

[[File:WOA05 sea-surf TMP AYool.png|thumb|right|Annual mean sea surface temperature for the World Ocean. Data from the World Ocean Atlas 2005.]]

SST and tropical cyclones

SSTs above are generally favorable for the formation and sustaining of tropical cyclones. Generally the higher the SST, the stronger the storm. However, there are many factors affecting the strength of such storms.

Remotely sensed SST can be used to detect the surface temperature signature due to hurricanes. In general, an 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. SST changes are short-lived and their ramifications are still not well understood.

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