Working Group I: The Scientific Basis

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7.3 Oceanic Processes and Feedbacks

The ocean influences climate and climate change in various ways. Ocean currents transport a significant amount of heat, usually directed poleward and thus contributing to a reduction of the pole-to-equator temperature gradient; a remarkable exception exists however in the South Atlantic where heat is transported equatorward, i.e. up-gradient, into the North Atlantic. Because of its large heat capacity, ocean heat storage largely controls the time-scales of variability to changes in the ocean-atmosphere system, including the time-scales of adjustment to anthropogenic radiative forcing. The ocean is coupled to the atmosphere primarily through the fluxes of heat and fresh water which are strongly tied to the sea surface temperature (see Section 7.6.1), and also through the fluxes of radiatively active trace gases such as CO2 (see Chapter 3) which can directly affect the atmospheric radiation balance. All ocean processes which ultimately can influence these fluxes are relevant for climate change. Processes in the ocean surface layer which are associated with seasonal time-scales hence are of obvious relevance. As the budgets in the surface layer depend on the exchange with deeper layers in the ocean, it is also necessary to consider the processes which affect the circulation and water mass distribution in the deep ocean, in particular when the response of the climate system at decadal and longer time-scales is considered. Moreover, processes governing vertical mixing are important in determining the time-scales on which changes of, for example, deep ocean temperature and sea level evolve.

Since the SAR, the assessment of the status of ocean processes in climate models has changed in two ways. On the one hand, advances in model resolution and in the representation of sub-grid scale processes have led to a somewhat improved realism in many model simulations. On the other hand, however, growing evidence for a very high sensitivity of model results to the representation of certain small-scale processes, in particular those associated with the THC, has been found. As a consequence, considerable uncertainties still exist concerning the extent to which present climate models correctly describe the oceanic response to changes in the forcing.

7.3.1 Surface Mixed Layer

The surface mixed layer is directly influenced by the atmospheric fluxes which are connected to the ocean interior by vigorous three-dimensional turbulence. That turbulence is driven primarily by the surface wind stress and convective buoyancy flux, and includes the wave driven Langmuir circulation (e.g., Weller and Price, 1988; McWilliams et al., 1997). As a result, the upper ocean often becomes well mixed.

The heat budget of the mixed layer is determined by horizontal advection, surface heating, entrainment and the vertical heat flux at the mixed-layer base. Entrainment occurs when there is sufficient turbulent energy to deepen the mixed layer and can result in rapid cooling when accompanied by upwelling from Ekman pumping. This pumping velocity at the mixed-layer base results from divergent mixed-layer flow driven by the wind stress. The shallowing of the mixed layer leads to a transfer of water from the mixed layer to the interior of the ocean. The water that passes the deepest mixed-layer depth will not be re-entrained within a seasonal cycle and is subducted. Large subduction rates are found where horizontal gradients in mixed layer depth are large. Thus variations in mixed-layer depth are of primary importance in setting the structure of the interior of the ocean. This process can temporarily shield heat anomalies generated in the mixed layer from the atmosphere. The subduction process itself is relatively well understood (Spall et al., 2000), although the role of sub-grid processes in modifying the subduction process needs further clarification (Hazeleger and Drijfhout, 2000).

The surface buoyancy flux (combined net heat and fresh water flux) effectively drives a cross-isopycnal mass flux by converting mixed-layer water from one density class to another. Waters of intermediate density are transformed into both lighter waters and heavier waters. In general, but especially in the Indian and Pacific Oceans, the thermal and haline contributions are additive in forming light tropical waters, but opposed in forming heavy polar waters. Despite this cancellation about 15 Sv (1 Sverdrup = 1 Sv = 106m3s-1) of North Atlantic Deep Water (NADW) is formed thermally. With more cancellation, less than a few Sverdrups of Antarctic Bottom Water (AABW) is formed in the Southern Ocean. However, nearly 30 Sv of Antarctic Intermediate Water is formed mostly by the haline effect.

In summary, proper parametrization of turbulence in the surface mixed layer is crucial to correctly simulate air-sea exchange, SST and sea ice (e.g., Large et al., 1997; Goosse et al., 1999), and thereby reduce the need for flux adjustments in coupled models. While several schemes are in use (Large and Gent, 1999), a systematic intercomparison of the properties, behaviour and accuracy of these parametrizations is still lacking.

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