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Section 10.3 Ocean temperatures

Sea Surface Temperature (SST)

The temperature of the ocean is reported as Sea Surface Temperature (SST), typically measured a 1-3 meters below the surface (but as deep as 20 meters). Three distinct layers can be recognized in the thermal profile of the oceans:

  • the surface zone or mixed layer (top 100 meters) that has the highest temperatures, and the largest variations in temperature as a function of altitude;

  • the thermocline, a zone of rapidly decreasing temperatures with depth;

  • the deep zone (below 1000 meters depth) where the temperature is consistently 1-3 C regardless of latitude

Sea Surface Temperature Anomaly (SSTA)

Another measure of ocean temperature is the Sea Surface Temperature Anomaly (SSTA): which the difference between the nightime SST and the monthly mean nighttime SST based on climate observations over several years. In other words, the SSTA compares the current temperature to "normal" or expected temperatures for a given time of year. Unusually high SSTA values can lead to thermal stress of coral reefs and coral bleaching events.

SST patterns

A number of persistent SST patterns emerge from ocean observations:

  • middle latitudes (and subtropics):

    • western coasts are bordered by cool water

    • eastern coasts are bordered by warm water

  • tropical latitudes: western coasts bordered by warm water

  • polar latitudes: eastern coasts bordered by cold water

These temperature patterns can be explained by ocean circulation more generally:

  • ocean current patterns generally resemble wind patterns

  • global-scale wind patterns blow over large regions and push the ocean in the same direction as the wind

  • a gyre is an ocean circulation that forms a closed loop across an ocean basin; these spin in the same direction as anticyclones, sending warm water poleward and cool water equatorward

  • winds of subtropical highs tend to cause cold water to flow toward the equator along western coasts

Ocean currents and the Ekman spiral

  • friction between the air and the sea surfaces causes the air to move

  • the Coriolis effect creates a tendency for the water to veer to the right (in the northern hemisphere) or the left (in the southern hemisphere)

  • the movement of water influences the next layer of water underneath; this underlying layer will also experience a Coriolis tendency to turn

  • over the column of water, a pattern emerges in the shape of a spiral

  • the net result: water tends to move to the right (NH) or left (SH) in Ekman transport

Circulation and upwelling

Ocean currents can act to push surface water toward or away from the coast, which will tend to lead to downwelling or upwelling, respectively. This upwelling plays an important role in primary (biological) production, by bringing cold, nutrient-rich water to surface waters (where photosynthesis can play a larger role).

Good examples of circulation-induced upwelling is along the coast of California, where cold water moving down the coast has a tendency to veer right (west) and promote upwelling, or the Humboldt current along the west coast of South America, where the cold water moving along the coast has a tendency to veer left (west), promoting upwelling and (under normal conditions) maintaining a westward flow of cold water along the equatorial zone.