(a) Which are the main cool surface currents in lii the subtropical North and South Pacific, and (ii) the subtropical North and South Atlantic?

ibl What do these currents have in common?

As far as the transport of heat is concerned, the 'warm' and 'cool' surface currents shown in Figure 3.1 are only part of the story. In certain high-latitude regions, water that has been subjected to extreme cooling sinks and flows equatorwards in the thermohaline circulation (Chapter 1). In order to know the net poleward heat transport in the oceans at any location, we would need to know the direction and speed of flow of water, and its temperature, at all depths. In fact, the three-dimensional current structure of the oceans is complex and has only relatively recently begun to be investigated successfully.

If you compare Figures 2.3 and 3.1, you will see immediately that the surface wind field and the surface current system have a general similarity. The most obvious difference is that because the flow of ocean currents is constrained by coastal boundaries, the tendency for circular or gyral motion seen in the atmospheric circulation is even more noticeable in the oceans. However, the way in which ocean currents are driven by the atmospheric circulation is not as obvious as it might at first appear, and in this Chapter we consider some of the mechanisms involved.

Figure 3.2 The paths of drifting derelict sailing vessels (and a few drifting buoys) over the period 1883-1902. This chart was produced using data from the monthly Pilot Charts of the US Navy Hydrographie Office, which is why the paths are shown as straight lines between points. The paths are extremely convoluted and cross one another, but the general large-scale anticyclonic circulation of the North Atlantic may just be distinguished.

This is perhaps a suitable point to emphasize that maps like Figures 2.3(a) and (b) and 3.1 represent average conditions. If you were to observe the wind and current at, for instance, a locality in the region labelled 'Gulf Stream' on Figure 3.1, you might well find that the wind and/or current directions were quite different from those shown by the arrows in Figures 2.3 and 3.1, perhaps even in the opposite direction. Moreover, currents should not be regarded as river-like. Even powerful, relatively well-defined currents like the Gulf Stream continually shift and change their position to greater or lesser extents, forming meanders and eddies, or splitting into filaments. The actual spatial and temporal variations in velocity (speed and direction) are much more complex than could be shown by the most detailed series of current charts. The drift paths in Figure 3.2 convey something of the variability of surface currents in the North Atlantic, while the satellite image on the book cover shows the complexity of the Gulf Stream flow pattern at one moment in time.

Despite the complex and variable nature of the marine environment, certain key theoretical ideas have been developed about the causes and nature of current flow in the ocean, and have been shown to be reasonable approximations to reality, at least under certain circumstances. The most fundamental of these are introduced in Sections 3.1 to 3.4.

Azores direction of wave propagation

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