Physical Oceanography: An Overview
2.5 Zonal and Meridional Variations
layer, which is isothermal (or nearly so); the seasonal thermocline, where the temperature changes relatively rapidly below the base of the mixed lay- er; and the deep permanent thermocline, where the temperature changes are more gradual with depth and exceedingly slow with time. While there are only small variations between the summer and winter thermoclines in the tropics, there are very large alterations in the subpolar and even the mid- latitude profiles. It is in this upper region of the sea that the main portion of the incoming solar energy is stored and redistributed by currents and air/sea interchanges.
22 Principles of Ocean Physics
at the poles in the winter, with the the north-south asymmetry arising from the f 3.2% variation in total radiation due t o the ellipticity of the orbit. The outgoing radiation in Fig. 2.5, on the other hand, is much more uniform in latitude than is the incident radiation, which implies that the equator-to- pole temperature gradient has acted to smooth out the distribution of heat resulting from the radiation deposited in the atmosphere. The heat trans- port required to reduce the gradient is carried by both ocean and atmosphere, and it is this driving force of north-south temperature gradient that is the major source of the dynamical motions of both geophysical fluids. Figure 2.7 shows, for the Northern Hemisphere, the poleward transport of heat due to both fluids. As we shall see, the heat is carried mainly by meridional (north-south) circulation in the atmosphere and the oceanic equivalent, i.e., boundary currents in the sea, with both the average circulations and their fluctuating components playing roles in the transports. These motions con- stitute the great atmospheric and oceanic circulation systems, but the multi- ple processes that establish and maintain them are indirect, complicated, and
I 1 I I I I
//x/ /zooh\x\\
3u Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month
Fig. 2.6 Average daily solar irradiance in W m-2 for a unit horizontal surface at the top of the atmosphere, versus latitude and month. The dotted vertical lines show the equinoxes and solstices; the dark regions represent the polar zones during ab- sence of sunshine. The north-south asymmetry is largely due to the eccentricity of the earth’s orbit, with the perihelion occurring during the Southern Hemisphere sum- mer. [Adapted from Mllankovitch, M., in Hendbuch der Klirnatologie (1930).]
I I I I I I I I
Atmos,>heric, fluctuations
~ Oceanic, mean
North latitude, A (deg)
Fig. 2.7 Rate of poleward (meridional) transport of energy by the atmosphere and the ocean as a function of latitude. The outer curve is net transport deduced from radiation measurements from satellites; the middle curve derives from atmospheric mean circulation; lower curve is for atmospheric eddy transport The remainder is at- tributed to oceanic mean and eddy transports, but direct measurements to support this are not generally available. [Adapted from Vonder Haar, T. H., and A. H. Oort, J.
fhys Oceanogr. (1973).1
tightly coupled to one another. It is these processes that we shall discuss in the remainder of this section.
According t o the diagram of the atmospheric heat engine (Fig. 2.3), about 50% of the incoming solar radiation is absorbed by the earth’s surface, most of it through the large heat capacity of the ocean, where it is stored for redis- tribution to the atmosphere. The single most important mechanism in the redistribution is evaporation, which is active where there are high tempera- tures and copious water supplies. The major regions of the earth where this occurs are the tropical oceans, the Amazon and Congo River basins, and the so-called “Far Eastern Island Continent,’’ the myriad of islands and ar- chipelagos in the tropical Far East. Here the absorption of solar radiation by land and sea, and the attendant high daily and average temperatures, re- sult in large evaporation rates, rising air, strongly buoyant cumulonimbus cloud formation, and heavy rainfall. Because of this, the ocean has its war- mest surface waters in the tropical Eastern Indian Ocean and Western Pa- cific. This rising, unstable convective motion carries water vapor high into the atmosphere, with the maximum release occurring near mid-altitudes (ap- proximately 500 mbar pressure height), but with the tops of the convective
24 Principles of Ocean Physics
systems often reaching altitudes of 12 to 15 km (40,000 to 50,000 ft), even injecting some water vapor into the stratosphere. The condensation of the vapor into liquid water and the subsequent rainfall releases the latent heat of evaporation, L , , to the surrounding air and warms it even more, fre- quently leading to superbuoyant conditions and convective instabilities.
Air must flow into these buoyant regions at low levels to replace the up- wardly displaced moist air, which departs from the unstable convective “chim- neys”.at mid to high altitudes. A return flow at height then occurs, along with a degree of poleward migration of air and vapor caused by vortex-line stretching from Coriolis forces (which will be explained more fully in Chap- ter 6 ). The air finally sinks toward the surface near the cooler western coasts of continents to complete the circulation. The zonal (east-west) cellular pat- terns thereby established, called Walker cells, are shown schematically in Fig. 2.8.
Walker cells 12
Height z /km)
0 30 60 90 120 150 180 150 120 90 60 30 0 30 60
East West East
Longitude, $ (degl Africa Far East
archipelago
South America Africa Eauatorial zonal section
Fig. 2.8 Schematic diagram of Walker cell circulation. Heating and evaporation of water are greatest over the Amazon and Congo River basins and the Far Eastern Island Continent. Convection carries water vapor to high altitudes; surface flows oc- cur to replace rising air. The circulation is closed in the upper atmosphere.
Thus the process of convective motion, with its release of latent heat, is the fundamental means by which the ocean communicates a flux of mois- ture, momentum, and energy to the atmosphere. It is active in the forma- tion of tropical cyclones (called hurricanes in the Atlantic and Eastern Pacific, typhoons in the Western Pacific, and cyclones in the Indian Ocean), but is not confined to the tropics, although it is most pronounced there. As an ex- ample of temperate zone convection, Fig. 2.9 shows an image made from the geosynchronous satellite GOES at thermal infrared wavelengths, illus- trating cold polar air progressing out across a much warmer ocean and result- ing in very large evaporation and formation of linear cloud streets over the Gulf Stream and Gu!f of Mexico. Thus strong exchanges between sea and air are not confined to the tropics, but occur when there is a negative differ-
ence between the temperature of the air, T,, and of the sea, T,. The rate of transfer also depends on surface wind stress, as will be discussed further in Chapter 3.
Fig. 2.9 Image from geosynchronous satellite, GOES, at infrared wavelengths, showing large evaporative fluxes occurring over the warm ocean during a cold, polar air outbreak. [Figure courtesy of National Oceanic and Atmospheric Administration.]
Returning t o the atmospheric circulation: The idealized east-west Walker cell is modified by north-south and low-high circulations induced by con- tinuity of flow and by the Coriolis force. The rising air leaving the rotating surface near the equator must conserve its absolute angular momentum and, finding itself at a larger distance from the surface of the earth at altitude, moves poleward. At high altitudes, it radiatively cools by emitting long-wave infrared radiation and thereby becomes denser. By the time the poleward- flowing air has reached 25" to 30" latitude, it has acquired an appreciable easterly or zonal component, has dried considerably through rainfall, and
26 Principles of Ocean Physics
begins to sink. The zone of dry, sinking air is a desert region globally, the atmospheric moisture having been wrung out of the air along the way. This meridional cell is called a Hadfey cell, and is a region of net energy release into the atmosphere. In the simple Hadley cell picture, the return surface winds would have only an equatorward component, but in actuality are modi- fied by interacting with the Walker circulation t o take on strong zonal com- ponents as well. Over the oceans, these combined lower level Walker-Hadley wind components form the trade winds, and blow from approximately the east-northeast in the Northern Hemisphere and the east-southeast in the Southern Hemisphere, to converge just north of the equator in the Inter- tropical Convergence Zone (ITCZ). Because of the asymmetrical distribu- tion of land masses and attendant heat storage between the hemispheres, the ITCZ is actually located at an average of several degrees north of the equa- tor, and is effectively the thermal and meteorological equator. It is a region of persistent cloudiness and rainfall and is known t o mariners as the dof- drums. Figure 2.10 sketches the Hadley cells and the surface winds for the Northern Hemisphere; the upper level winds tend t o be oppositely directed.
Poleward of the zones of dry, sinking air, a second set of meridional cells develops called Ferrel cells, and in the Northern Hemisphere extend from approximately 25" or 30"N t o 60"N; these cells are much weaker than the directly driven Hadley cells. In the polar regions is a third set known ap- propriately as polar cells. In the Ferrel cell there is a rising of relatively cold air in high latitudes and a sinking of relatively warm air in lower middle lati-
Fig. 2.10 Diagram of Northern Hemisphere meridional cellular circulation, show- ing the tropical Hadley cells, the mid-latitude Ferrel cells, and the polar cells. Upper and lower level winds have directions suggested by the top and bottom arrows at- tached to the cells.
tudes, thereby requiring net work. It is the region of high meteorological variability, strong westerlies, the subpolar jet streams, and the production of mid-latitude cyclones, or low-pressure storm systems. In the Southern Hemisphere, the latitudinal center of the Ferrel cell falls over the infamous
“roaring forties” of maritime lore. The three-cell regime results from zonal radiation and temperature gradients combined with the Coriolis force, and is a consequence of the relatively high spin rate of the earth. If the earth were spinning significantly less rapidly, there would be only one cell, the Had- ley cell, rather than the three. Figure 2.11, a schematic of an average vertical section of the atmosphere from north t o south, illustrates how tropical con- vection elevates the tropopause, or the top of the lower atmosphere; the three- cell circulation is suggested as well.
Jet stream Tropopause
’ a
Ferrel cell
I
9075 60 45 30 15 0
N o r t h latitude, A (deg)
Polar easterlies Westerlies Trades I T C Z
Fig. 2.11 Vertical section of three-cell circulation from equator to pole, showing elevation of tropopause in the tropics relative to polar regions.
Figure 2.12 shows north-south cross sections of mean zonal and meridi- onal wind speeds, ( u ) and ( u ) , respectively, in meters per second, and the transport rate of atmospheric mass, in units of 10” kilograms per second.
The vertical coordinate is atmospheric pressure in decibars or 0.1 bar (1 bar
= lo5 N m-’ = l o 5 Pa). Figure 2.13 gives the mean global distribution of surface pressure in millibars ( = 10 - 3 bar), surface streamlines, and mean surface wind velocity, as indicated by arrows; a constant pressure of 1000 mbar must be added t o the numbers shown to obtain the actual pressure.
It can be seen that the ITCZ is a region of lower average pressure, while the “horse latitudes” near 30”N and 30”s (cf. Fig. 2.10) average near 1020 mbar. These surface pressure and wind distributions provide the basic me- chanical forcing functions for the upper ocean, and we will return to study their effects in more detail at a later time.
The sum total of the mean general circulation of the troposphere is a com- plicated, three-dimensional flow field; Fig. 2.14 (Plate 1) attempts to illus- trate this. While it is only the surface of the sea that is in intimate contact with the atmosphere and which exchanges fluxes of moisture, energy, and momentum with it, one also sees that the entire air mass up to the tropopause is strongly conditioned by the ocean on all time scales ranging through daily, seasonal, and climatological periods. Conversely, the ocean is in turn condi-
28 Principles of Ocean Physics
'080 70 60 50 40 30 20 10 0
South North
Latitude, A (deg)
10 20 30 40 50 60 70 80
Fig. 2.12 Cross sections of (a) zonal wind component, (u), and (b) meridional wind component, ( v ) , in m s - 1 , as averaged over the year and zonally around the globe.
(c) Mass transport in 1010 kg s - 1 . Jet stream axes are near 250 rnbar in both hemispheres. [From Piexoto, J. P., and A. H. Oort, Rev. Mod. Phys. (1984).]
tioned by atmospheric radiation, wind stress, temperature, pressure, and rain- fall, although its response times are much longer than those of the atmosphere. The ocean thus is the flywheel or the inertia on the global heat engine and serves to smooth out the high-frequency fluctuations of the at- mosphere. The two fluids are strongly coupled, having both positive and nega- tive feedback loops and greatly differing time constants.