365 Days of Climate Awareness 46 - The Structure of the Ocean

The ocean is, with a few exceptions, stratified by density, the least dense on the surface, the densest on the bottom. Generally the uppermost water is warmest, and the deepest is the coldest, though salinity is a factor as well. Stratification tends to be most intense in the tropics, where the sun's warming effect is greatest, and the least near the poles. Mean salinity of the ocean is roughly 35 parts per thousand (ppt), giving mean ocean water a density of 1035 grams per liter (g/l), or 1035 kg/m². Ocean surface temperatures range from 4°Celsius (39 deg Fahrenheit) to 35°C (95°F). Deep ocean water averages about 4°C.

Typical vertical structure of the ocean.

Water absorbs red light, including infrared radiation, and more readily transmits blue light. This is the reason for the blue tint of deeper water, and also why the ocean surface warms so readily by sunlight: it absorbs the infrared component. The wind mixes the upper water, via waves and currents, deepening this warm surface mixed layer. The depth and specific properties of this mixed layer vary greatly with time and by location. In tropical zones, where wind is generally weak and the sun is strongest, the mixed layer might be 50 m deep. In mid- to upper latitudes the mixed layer is typically 100-200 m deep, but can in the coldest regions span the ocean's full depth. In the summer, with lighter wind, the mixed layer shallows; in winter, when the winds are stronger, the mixed layer is typically deeper. A hurricane will deepen the surface mixed layer to 200 m or more.

South-to-north profile of the vertical structure of the Atlantic Ocean.

Beneath the mixed layer is the thermocline (Greek: thermos, heat; kline, couch), sometimes known as the pycnocline (Greek pyknos: dense), a layer of rapidly decreasing temperature, and increasing density. Like the surface layer, the depth and thickness of the thermocline varies with time and location, but typically extends from about 200 m to 1000 m depth. Beneath the thermocline is the deep layer, where ocean water is typically about 4°C.

When deep convective events happen, in the northeast Atlantic, and in the Weddell Sea of Antarctica, and a few other locations worldwide, the thermocline disappears as surface water migrates directly to the sea floor. In the Arctic in wintertime, the thermocline, weak in summer, disappears completely and ocean water is homogeneous from top to bottom. This is very different from the highly stratified nature of tropical ocean water.

Tomorrow's post: properties of ocean water.

Be well!