Chapter 6 - Temperature, Salinity, and Density

6.4 The Oceanic Mixed Layer and Thermocline

Wind blowing on the ocean stirs the upper layers leading to a thin mixed layer at the sea surface having constant temperature and salinity from the surface down to a depth where the values differ from those at the surface. The magnitude of the difference is arbitrary, but typically the temperature at the bottom of the layer must be no more than 0.02-0.1°C colder than at the surface. Note that the both temperature and salinity must be constant in the mixed layer. We will see later that mean velocity is not constant. The mixed layer is roughly 10-200 m thick over most of the tropical and mid-latitude belts.

The depth and temperature of the mixed layer varies from day to day and from season to season in response to two processes:

1. Heat fluxes through the surface heat and cool the surface waters. Changes in temperature change the density contrast between the mixed layer and deeper waters. The greater the contrast, the more work is needed to mix the layer downward and visa versa.
2. Turbulence in the mixed layer mixes heat downward. The turbulence depends on the wind speed and on the intensity of breaking waves. Turbulence mixes water in the layer, and it mixes the water in the layer with water in the thermocline.

The mid-latitude mixed layer is thinnest in late summer when winds are weak, and sunlight warms the surface layer (Figure 6.7). At times, the heating is so strong, and the winds so weak, that the layer is only a few meters thick. In fall, the first storms of the season mix the heat down into the ocean thickening the mixed layer, but little heat is lost. In winter, heat is lost, and the mixed layer continues to thicken, becoming thickest in late winter. In spring, winds weaken, sunlight increases, and a new mixed layer forms.

Figure 6.7 Growth and decay of the mixed layer and seasonal thermocline from November 1989 to September 1990 at the Bermuda Atlantic Time-series Station (BATS) at 31.8°N 64.1°W. Data were collected by the Bermuda Biological Station for Research, Inc. Note that pressure in decibars is nearly the same as depth in meters (see §6.8 for a definition of decibars).

Below the mixed layer, water temperature decreases rapidly with depth except at high latitudes. The range of depths where the rate of change, the gradient of temperature, is large is called the thermocline. Because density is closely related to temperature, the thermocline also tends to be the layer where density gradient is greatest, the pycnocline.

The top of the thermocline varies slightly with the seasons as seen in the region between 20 and 200 decibars in Figure 6.7. This is the seasonal thermocline. The permanent thermocline extends from below the seasonal thermocline to depths of 1500-2000m (Figure 6.8). At high latitudes, such as at the AAC station in the figure, there may be a cooler, fresher layer above the permanent thermocline.

Figure 6.8 Typical temperature and salinity profiles in the open ocean. AAC: At 62.0°S, 170.0°E in the Antarctic Circumpolar Current on 16 January 1969 as measured by the R/V Hakuho Maru. Warm Pool: At 9.5°N, 176.3°E in the tropical west Pacific warm pool on 12 March 1989 as measured by Bryden and Hall on the R/V Moana Wave. BATS: At 31.8°N, 64.1°W near Bermuda on 17 April and 10 September 1990 as measured by the Bermuda Biological Station for Research, Inc. Data are included with Java Ocean Atlas.

The mixed layer tends to be saltier than the thermocline between 10° and 40° latitude, where evaporation exceeds precipitation. At high latitudes the mixed layer is fresher because rain and melting ice reduce salinity. In some tropical regions, such as the warm pool in the western tropical Pacific, rain also produces a thin fresher mixed layer.

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