ABSTRACT
Introduction Weather maps brought to us daily via television and newspapers have made us all aware of the changing state of the atmosphere (Figure 4.1). Cyclones and anticyclones (low and high pressure systems) evolve and interact, produce severe winds, fronts with associated rain, and vary the weather we experience on time-scales from a few hours to several days. Less well-known is that the ocean is populated by very similar systems. The oceanic equivalent of atmospheric highs and lows, the ocean weather is again an ever-changing pattern. As in the atmosphere, intense storms can develop in the ocean to produce strong currents. Fronts separating warm and cold water masses are a common occurrence. The oceanic systems, however, have a much smaller horizontal scale than do their atmos-
Figure 4.1 A familiar sight for those in the UK, a lowpressure system and associated rain-bearing fronts sweeping across the country. This illustrates the surface pressure field for a day in January. The low-pressure system, or cyclone, in the centre of the picture is approximately 2000 km across. The wind circles the low pressure in an anticlockwise sense. Interaction between high and low pressure systems dictates the weather we experience on a daily basis. The insert, expanded by a factor of 10, shows a detail from an infra-red image of the sea surface taken from a satellite. Light areas correspond to cool water and dark areas to warm water. Using the sea surface temperature as a tracer, we can clearly see the imprint of an ocean eddy, 100 km across, where the ocean currents associated with the eddy have caused the warm waters to the south and the cool waters to the north to spiral around each other; an oceanic cyclone (in the insert note also the sharp transition from warm to cool water, an ocean front, and the smaller scale structures, 10 km, along the front). Oceanographers call the cyclones and anticyclones of the ocean meso-scale eddies. These ocean eddies are dynamically equivalent to the weather systems in the atmosphere. However, ocean eddies have very different space-and time-scales; typically, horizontal scales of 10-200 km, current speeds of a few tens of centimetres per second, and circulation times of tens of days. In the vertical, some are restricted to the upper levels of the ocean, while others extend to the bottom.
