WIND AND BUOYANCY-FORCED UPPER OCEAN 3217 h\1), only a small fraction of the sea surface is Further Reading covered by stage B whitecaps (0.04 or 4%), and an Andreas EL, Edson JB, Monahan EC, Rouault MP and even smaller fraction of that surface is covered by Smith SD (1995) The spray contribution to net evapor- stage A whitecaps (0.002 or 0.2%). Yet the total ation from the sea: review of recent progress. Bound- area of all the world’s oceans is very great ary-Layer Meteorology 72: 3}52. 14 2 (3.61;10 m ), and as a consequence the total area Blanchard DC (1963) The electriRcation of the atmo- of the global ocean covered by whitecaps at any sphere by particles from bubbles in the sea. Progress in instant is considerable. If a wind speed of 7 m s\1 is Oceanography 1: 73}202. taken as a representative value, then at any instant Bortkovskii RS (1987) Air}Sea Exchange of Heat and some 7.0;1010 m2, i.e. some 70 000 km2, of stage Moisture During Storms, revised English edition. A whitecap area is present on the surface of the Dordrecht: D. Reidel [Kluwer]. global ocean. Following from this, and including Liss PS and Duce RA (eds) (1997) The Sea Surface and such additional information as the terminal rise Global Change. Cambridge: Cambridge University Press. velocity of bubbles, it can be deduced that some Monahan EC and Lu M (1990) Acoustically relevant ; 11 2 2 7.2 10 m , i.e. some 720 000 km of individual bubble assemblages and their dependence on meteoro- bubble surface area are destroyed each second in logical parameters. IEEE Journal of Oceanic Engineer- all the stage A whitecaps present on the surface ing 15: 340}349. of all the oceans, and an equal area of bubble Monahan EC and MacNiocaill G (eds) (1986) Oceanic surface is being generated in the same interval. Whitecaps, and Their Role in Air}Sea Exchange Pro- The vast amount of bubble surface area destroyed cesses. Dordrecht: D. Reidel [Kluwer]. each second on the surface of all the world’s Monahan EC and O’Muircheartiaigh IG (1980) Optimal oceans, and the great volume of water (some power-law description of oceanic whitecap coverage 2.5;1011 m3) swept by all the bubbles that burst dependence on wind speed. Journal of Physical } on the sea surface each second, have profound Oceanography 10: 2094 2099. Monahan EC and O’Muircheartaigh IG (1986) White- implications for the global rate of air}sea exchange caps and the passive remote sensing of the ocean sur- of moisture, heat and gases. An additional prelimi- face. International Journal of Remote Sensing 7: nary calculation following along these lines, sug- 627}642. gests that all the bubbles breaking on the sea surface Monahan EC and Van Patten MA (eds) (1989) Climate each year collect some 2 Gt of carbon during their and Health Implications of Bubble-Mediated Sea}Air rise to the ocean surface. Exchange. Groton: Connecticut Sea Grant College Program. See also Thorpe SA (1982) On the clouds of bubbles formed by breaking wind waves in deep water, and their role in Heat and Momentum Fluxes at the Sea Surface. air}sea gas transfer. Philosophical Transactions of the Wave Generation by Wind. Royal Society [London] A304: 155}210. WIND AND BUOYANCY-FORCED UPPER OCEAN M. F. Cronin, NOAA Pacific Marine Environmental buoyancy forcing are ultimately felt throughout the Laboratory, WA, USA entire ocean, the most immediate impact is in the J. Sprintall, University of California San Diego, surface mixed layer, the site of the active air}sea La Jolla, CA, USA exchanges. The mixed layer is warmed by sunshine Copyright ^ 2001 Academic Press and cooled by radiation emitted from the surface and by latent heat loss due to evaporation (Figure doi:10.1006/rwos.2001.0157 1). The mixed layer also tends to be cooled by sensible heat loss since the surface air temperature is Introduction generally cooler than the ocean surface. Evaporation and precipitation change the mixed layer salinity. 0001 Forcing from winds, heating and cooling, and rain- These salinity and temperature changes deRne the fall and evaporation, have a profound inSuence ocean’s surface buoyancy. As the surface loses buoy- on the distribution of mass and momentum in the ancy, the surface can become denser than the sub- ocean. Although the effects from this wind and surface waters, causing convective overturning and 3218 WIND AND BUOYANCY-FORCED UPPER OCEAN mixing to occur. Wind forcing can also cause sur- where it is sometimes difRcult to distinguish forcing face overturning and mixing, as well as localized from response. Because water has a heat capacity overturning at the base of the mixed layer through and density nearly three orders of magnitude larger sheared-Sow instability. This wind- and buoyancy- than air, the ocean has thermal and mechanical generated turbulence causes the surface water to be inertia relative to the atmosphere. The ocean thus well mixed and vertically uniform in temperature, acts as a memory for the coupled ocean}atmosphere salinity, and density. Furthermore, the turbulence system. can entrain deeper water into the surface mixed We begin with a discussion of air}sea interaction 0003 layer, causing the surface temperature and salinity through surface heat Suxes, moisture Suxes, and to change and the layer of well-mixed, vertically wind forcing. The primary external force driving the uniform water to thicken. Wind forcing can also set ocean}atmosphere system is radiative warming from up oceanic currents and cause changes in the mixed the sun. Because of the fundamental importance of layer temperature and salinity through horizontal solar radiation, the surface wind and buoyancy forc- and vertical advection. ing is illustrated here with two examples of the 0002 Although the ocean is forced by the atmosphere, seasonal cycle. The Rrst case describes the seasonal the atmosphere can also respond to ocean surface cycle in the north PaciRc, and can be considered conditions, particularly sea surface temperature a classic example of a one-dimensional (involving (SST). Direct thermal circulation, in which moist air only vertical processes) ocean response to wind and rises over warm SSTs and descends over cool SSTs, buoyancy forcing. In the second example, the sea- is most prevalent in the tropics. The resulting atmo- sonal cycle of the eastern tropical PaciRc, the atmo- spheric circulation cells inSuence the patterns of sphere and ocean are coupled, so that wind and cloud, rain, and winds that combine to form the buoyancy forcing lead to a sequence of events that wind and buoyancy forcing for the ocean. Thus, the make cause and effect difRcult to determine. The oceans and atmosphere form a coupled system, impact of wind and buoyancy forcing on the surface a0157f0001 Figure 1 Wind and buoyancy forces acting on the upper ocean mixed layer. Solar radiation (Qsw), net longwave radiation (Qlw ), latent heat flux (Q lat), and sensible heat flux (Q sen) combine to form the net surface heat flux (Q0). Qpen is the solar radiation penetrating the base of the mixed layer. WIND AND BUOYANCY-FORCED UPPER OCEAN 3219 mixed layer and the deeper ocean is summarized in and atmosphere have different surface temperatures, the conclusion. sensible heat Sux will act to reduce the temperature gradient. Thus when the ocean is warmer than the ^ air (which is nearly always the case), sensible heat Air Sea Interaction Sux will tend to cool the ocean and warm the Surface Heat Flux atmosphere. Likewise, the vapor pressure at the air}sea interface is saturated with water while the S 0004 As shown in Figure 1, the net surface heat ux air just above the interface typically has relative entering the ocean (Q0) includes solar radiation humidity less than 100%. Thus, moisture tends to (Qsw), net long-wave radiation (Qlw ), latent heat evaporate from the ocean and in doing so, the ocean S ux due to evaporation (Qlat), and sensible heat loses heat at a rate of: Sux due to air and water having different surface temperatures (Qsen ): "! Qlat L( fwE) [2] " # # # Q0 Qsw Qlw Qlat Qsen [1] S where Qlat is the latent heat ux, L is the latent heat R of evaporation, fw is the freshwater density, and The Earth’s seasons are largely de ned by the \2 S E is the rate of evaporation. Qlat has units W m , annual cycle in the net surface heat ux associated \1 \2 and ( fw E) has units kg s m . This evaporated with the astronomical orientation of the Earth rela- moisture can then condense in the atmosphere to tive to the Sun. The Earth’s tilt causes solar radi- form clouds, releasing heat to the atmosphere and ation to strike the winter hemisphere more obliquely affecting the large-scale wind patterns. than the summer hemisphere. As the Earth Because sensible and latent heat loss are turbulent 0008 orbits the sun, winter shifts to summer and summer processes, they also depend on wind speed relative shifts to winter, with the sun directly overhead to the ocean surface, S. Using similarity arguments, at the equator twice per year, in March and again S the latent (Qlat) and sensible (Qsen) heat uxes can in September. Thus, one might expect the seasonal be expressed in terms of ‘bulk’ properties at and cycle in the tropics to be semiannual, rather than near the ocean surface: annual. However, as discussed later, in some parts of the equatorial oceans, the annual cycle " ! dominates due to coupled ocean}atmosphere}land Qlat aLC#S(qa qs) [3] interactions. " ! 0005 Solar radiation entering the Earth’s atmosphere is Qsen a cpaC& S(Ta Ts) [4] absorbed, scattered, and reSected by water in both R its liquid and vapor forms.