What do the currents of the ocean really look like?


Well let's start by saying that they are not the nice steady currents of the books and school atlas. It would be better to start by thinking of a weather map with high and low pressure regions acting like eddies and pushing the clouds around in a fairly random manner. This is especially true of the deep ocean (say below 1500m) where the mixing due to the eddy field is much stronger than any mean current.

In fact the eddies are very similar to the high and low pressure systems of the atmosphere, only in the ocean they are usually only 100 km across instead of the 1000 km of the atmosphere. Why? - well maybe I will get to that below.

So think first of a lot of turbulence. The video loops linked to this page give some idea of how there work. On top of this turbulence we get some mean currents driven mainly by the wind. These are the currents of the text books and atlases. In practice they really only show up where they are really strong - for exampke in the top 1000m next to western boundaries or along the equator. Good examples are the Gulf Stream of the US east coast, the Kurishio off Japan and the Agulhas off south-east Africa.

The Antarctic Circumpolar Current is also a good strong current (at eight times times the strength of the Gulf Stream) and extending four times the depth, but even that shows a strong eddy field.

So on a small scale, over a period of a month or so, individual parcels of water move in a fairly random manner. However over a longer time scale, say a year or more, they slowly move around the great ocean basins following the large scale gyres of the text books and atlases.

Very long timescales

On even longer timescales, say tens to hundreds of years, we also see parcels of water changing their depth within the ocean. At high latitudes in the Atlantic, i.e. north of Iceland and off Labrador, the saline Atlantic surface waters are cooled to near freezing and become so dense that they sink down to depths of 2000 to 3000 meters forming what is called North Atlantic Deep Water. The North Pacific is relatively fresh so this sinking does not occur. However around Antarctica even denser water is formed on the continental shelves of the Weddell and Ross Seas. This sinks to near 5000 meters where it is called Antarctic Bottom water. It eventually spreads out to form most of the bottom water of the ocean.

Surace water can move down to these depths in the matter of a few days. In contrast the return leg can take hundreds of years. Two mechanisms are involved which are roughly of equal importance. The first is the slow downward movement of heat in the ocean due to breaking internal waves. This slowly heats up the deep water and as its density is reduced the water slowly moves back to the surface.

The second mechanism is a suction effect due to the wind. In the tropics North Atlantic Deep Water is found at depths near 2000 meters but as one goes south, where surface temperatures are cooler, the water is found at shallower depths. Eventually in the Southern Ocean, say near 60 degrees South, the water lies so close to the surface that it can be driven north by the strong westerly winds.

When this happens, the water that is driven north has to be replaced, and this has the effect of upwelling water from deeper within the ocean. Figure 2 (when I draw it) is a sketch of how this sinking and upwelling might look.

The overall picture

So what happens when all these flows are added together. At short timescales we have the eddies pushing the water around in a random manner. At longer timescales the water moves, like a jostling crowd, around the large gyres of the ocean. And at even longer timescales some of the water overflows from one basin to another, occasionally sinking, at other times warmed by the turbulence or brought up to the surface by the wind, until eventually it may complete a circuit of the whole ocean.

To be concluded on another wet summer afternoon.

This page lifted from:  http://www.noc.soton.ac.uk/