Oceanic forcing by airsea exchange of moisture and heat

2.4.1 Moisture exchange

We have repeatedly seen the climatic importance of latent heat, transferred to the atmosphere through evaporation. Evaporation and precipitation also help to drive the ocean circulation by creating horizontal density gradients. Evaporation, by removing water and concentrating the dissolved salts, increases salinity, and hence density. Precipitation, by adding water, reduces the salinity, and therefore density.

Density gradients produced in this way are less important to the regional ocean circulation than wind-induced flow, as will be seen in §2.11. Nevertheless, there are locations where these processes, evaporation especially, are the key to understanding the circulation. In §1.3.2 we saw how the Mediterranean Sea influences circulation in the North Atlantic ultimately because of its extreme evaporation. Similarly, large evaporation rates also occur in the Red Sea and the Persian Gulf.

The degree of excess of evaporation over precipitation is dependent on the atmospheric circulation. Its variation with latitude is shown in Fig. 2.9. Peaks in evaporation occur in the sub-tropical high pressure belts, while excess rainfall occurs in the mid-latitude westerly wind belts and the ITCZ (Inter-Tropical Convergence Zone). A longitudinal variation also exists. The Atlantic is about 5% saltier than the other oceans because of higher evaporation relative to precipitation, P. This difference drives the present global thermohaline circulation (Fig. 1.14). Change to the longitudinal distribution of E-P may be responsible for dramatic alterations to this circulation pattern over the last 20 000 years (Chapter 6).

2.4.2 Heat exchange

Section 2.3 may convey the impression that the ocean heats or cools the atmosphere but is not affected in return because of the high thermal inertia of the

Fig. 2.9. Zonally averaged precipitation (P, solid line) and evaporation (E, dotted line), and the net freshwater balance (P — E, dot-dash line), in cm/year. [Data from Baumgartner and Reichel, 1975.]

Fig. 2.9. Zonally averaged precipitation (P, solid line) and evaporation (E, dotted line), and the net freshwater balance (P — E, dot-dash line), in cm/year. [Data from Baumgartner and Reichel, 1975.]

Latitude

Fig. 2.10. Schematic diagram of the variation of mixed layer depth through the year. Note that each winter's maximum depth is not the same. In the example illustrated the second winter sees some water permanently lost from the influence of the atmosphere; if a future winter mixes back to greater depths the water then entrained into the mixed layer is likely to be of different origin.

Fig. 2.10. Schematic diagram of the variation of mixed layer depth through the year. Note that each winter's maximum depth is not the same. In the example illustrated the second winter sees some water permanently lost from the influence of the atmosphere; if a future winter mixes back to greater depths the water then entrained into the mixed layer is likely to be of different origin.

ocean. On the global scale this is not the case - the oceanic and atmospheric circulations are strongly coupled, as we will see in Chapter 5. However, to a large degree the oceanic coupling to the atmosphere for basin-scale processes is through the wind (§2.8) - momentum transfer - rather than heat and water transfer.

Local heating and cooling of the ocean by the atmosphere is, nonetheless, important for climate. The seasonal change to the supply of thermal energy to the upper ocean away from the equatorial regions determines the characteristics of its mixed layer. This is the zone of the ocean immediately below the air-sea interface, analogous to the atmospheric planetary boundary layer, where properties tend to be well mixed. In mid-summer the heating of the surface produces a thin layer of less dense water. This is well mixed by the wind but the mixing region is sealed by the narrow zone of rapid temperature (and therefore density) change at the base of the mixed layer. Cooling of this layer in the autumn reduces the density contrast at its base. This eventually leads to the mixed layer merging with the region below, or, in extreme circumstances, overturning to mix with a substantial extent of the water column. The warming of the surface in spring leads to the mixed layer becoming less dense. Rapid warming will lead to the formation of a new, shallow, mixed layer in the upper part of the winter layer. This formation of the summer mixed layer can occur over a few days given a warm, calm period of weather. The seasonal cycle in the mixed layer is illustrated schematically in Fig. 2.10. In Chapter 4 we will see that this cycle is of fundamental importance for biological processes in the upper ocean, controlling the variation of

Fig. 2.11. Vertical profile of temperature at 33°10.7' N, 43° 12.5' Win the North Atlantic on 7 August 1983. The mixed layer is very shallow - less than 20 m in depth. The seasonal thermocline extends from the base of the mixed layer to 100 m, while the permanent thermocline is found from 400 to 900 m depth.

Fig. 2.11. Vertical profile of temperature at 33°10.7' N, 43° 12.5' Win the North Atlantic on 7 August 1983. The mixed layer is very shallow - less than 20 m in depth. The seasonal thermocline extends from the base of the mixed layer to 100 m, while the permanent thermocline is found from 400 to 900 m depth.

primary productivity through the year, and the supply of nutrients to the euphotic zone.

Locations where the mixed layer mixes only with water immediately below it can appear to possess two regions of rapid temperature change, as shown in Fig. 2.11. The deeper one, the permanent thermocline, corresponds to the depth of maximum winter mixing. The shallower zone, the seasonal thermocline, only appears in the summer and corresponds to the minimum summer mixing. The water between the two thermoclines in the summer, in being advected to another region, or because of variability in the severity of successive winters, can become detached from the region of seasonal surface influence to become a new sub-surface water mass in the ocean.

Fig. 2.12 shows the typical depth of winter mixing over the North Atlantic. In northern latitudes this is several hundred to over a thousand metres. In §1.3.2 we saw how this mixing was an integral part of the global circulation through the formation of North Atlantic Deep Water in the Norwegian and Greenland

80°W 70°W 60°W 50°W 40°W 30°W 20°W 10°W 0° 10°E 20°E

Longitude

Fig. 2.12. Average mixed layer depth over the North Atlantic Ocean at the time of maximum mixing (March). The depth of the mixed layer is determined by the depth by which the temperature has dropped by 0.5°C from the surface temperature. Note the absence of data in the Labrador Sea and around the Greenland coast, due to sea-ice. Deep convection occurs on either side of southern Greenland. Using data from Levitus etal. (1994).

Seas (see Figs. 1.14 and 1.16). This type of extreme winter mixing, which also occurs in the Gulf of Lyon in the western Mediterranean, is due to pronounced cooling events in the atmosphere. Strong winds assist the overturning but the reduction of density contrasts by surface cooling is a necessary precursor. The combination of these two driving forces of the mixing means that the horizontal extent of overturned regions is likely to be restricted. Few observations of actual overturning events have been made but they suggest that regions less than a score of kilometres in diameter may be involved. In §2.5 we will explore in more detail how density is altered through surface temperature and salinity changes.

Was this article helpful?

0 0
Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook


Post a comment