Ocean Mixed Layer

The ocean mixed layer (OML), the ocean region adjacent to the air–sea interface, is typically tens of meters deep, and due to the fact that it is well mixed, the temperature and salinity (and therefore the density) are fairly uniform. The rapidly changing regions below these uniform regions of temperature, salinity, and density are called the thermocline, halocline, and pycnocline, respectively. The mixing is primarily shear-driven, since the wind stress at the surface is the primarymixing agent, although at night significant convective mixing driven by the heat loss to the atmosphere takes place. The OML is heated near the surface by both short-wave (SW) and long-wave (LW) radiative fluxes, and deeper in the water column from solar radiation in the visible part of the spectrum penetrating into theOML.This solar heating produces a diurnal cycle that varies in importance and magnitude at different latitudes. The cooling, however, is driven from heat and evaporative losses at the surface. Seasonal variation of the OML due to radiative heating is also important, although the importance depends on the latitude. The OML mediates the exchange of mass, momentum, energy, and heat between the atmosphere and the ocean and hence plays a central role in long-term climate andweather. Because of the high heat capacity of water (2.5m of the upper ocean has the same heat capacity as the entire troposphere), and because the oceans compose over two-thirds of the surface of the globe, most of the solar heating on Earth passes through the OML.Oceans are heat reservoirs, gaining heat during spring and summer and losing it slowly during fall andwinter, and therefore act like a flywheel in matters related to weather on time scales of weeks and longer. The OML also plays an important role in the oceanic food chain. Primary production by phytoplankton is the first link in this chain. The need for an energy source in producing biomass restricts primary production to the upper few tens of meters (the euphotic or photic zone), in which the solar insolation is strong enough to assist carbon fixation. The mixing at the base of the OML is also crucial to biological productivity. TheOML is normally nutrient-poor, and it is the injection of nutrients from the nutrient-rich waters below the seasonal thermocline that permits higher levels of primary productivity. In fact, it is the upwelling regions (which compose just a few percent of the world’s oceans), where nutrient-rich waters are forced into theOMLandbrought into the photic zone, that provide most of the fish catch around the world. Biological productivity is important from a climatic point of view over time scales of decades or more. Carbon fixing constitutes a biological pathway for removing some of the anthropogenic CO2 introduced into the atmosphere. There also exists an inorganic pathway, since there is a significant uptake of CO2 in the cold subpolar oceans, some of which are also regions of deep and intermediatewater formation. It is likely that the ocean acts as an important CO2 sink on the globe and accounts for a significant fraction of the ‘missing’ anthropogenic CO2 input to the atmosphere. However, quantification of the magnitude of this sink requires accurate OML models coupled to accurate ecosystem and air–sea transfer models. Finally, the OML constitutes the first link in the chain of oceanic pollution.Most of the pollution in the global oceans takes place in the coastal oceans through the OML, and therefore the fate of any pollutants accidentally or intentionally deposited in the OML depends on the mixing and dispersion in the OML.