The Observed Climatology - Climate Dynamics - page 33

Atlantic deep water formation , and its presence is indicated by the high-
latitude vertical isotherms and isohalines in Figures 2.23b and c. The North
Atlantic deep water flows south to the Antarctic circumpolar region, where it
joins the westerly flow of the Antarctic circumpolar circulation. Water sinks
to even greater depths (~4-5 km) in this region, forming the Antarctic bottom
water . The deep and bottom waters of the Antarctic circumpolar westerly flow
upwell to the surface in either the Indian Ocean or the subtropical and tropical
Pacific. The upper-ocean return flow that balances the sinking in the North At-
lantic consists of the low-latitude easterly low from the Pacific, past Indonesia
(known as the Indonesian Throughflow) ) into the Indian Ocean, around the
southern tip of Africa into the South Atlantic Ocean. Approximately 20% of
the mass flux of the Gulf Stream is thought to be associated with the thermo-
haline circulation, with the remainder wind driven.
Another facet of the ocean circulation that is important for climate is the
upwelling of cold water and downwelling of warm water. Upwelling can be
wind driven or mixing driven, the latter in association with the thermohaline
circulation. Wind-driven upwelling occurs when the wind blows warm surface
water away from a coast and cooler waters rise as a result. Downwelling oc-
curs when the wind blows toward a coast. Upwelling also takes place when the
wind blows along a coast and, driven by Coriolis accelerations (discussed in
chapter 6 ), surface waters flow away from the coast (see chapter 8) . One ex-
ample of this type of wind-driven upwelling occurs off the east coast of Africa
near Somalia, where the low-level southerly wind ( Fig. 2.13b) forces the ocean
surface waters to flow eastward because of Coriolis acceleration. Upwelling of
colder water results, which accounts for the cooler sea surface temperatures in
the western Indian Ocean during Northern Hemisphere summer (Fig. 2.16) .
Similar upwelling occurs off the Peru and California coasts, and in the Gulf of
Guinea (south of Africa's westward bulge). The nutrients brought closer to the
surface by coastal upwelling attract marine life and support the fishing indus-
tries of many countries.
2.3 THE HYDROLOGIC CYCLE
Water cycles among the interdependent components of the climate system and
is an important agent in producing interactions among them ( Fig. 1.1) .
The volume of water in the hydrosphere , defined as all the water in all
phases (solid, liquid, and vapor) in the earth system down 2 km into the crust,
is estimated to be 1.4 # 10 10 km 3 . Approximately 97.5% of this water is saline,
so only 2.5% is fresh water. Table 2.1 gives the estimated distribution of water
in various reservoirs.
Estimated annual mean global fluxes and reservoirs of water in the climate
system are shown in Figure 2.24. Over land, precipitation is greater than evap-
oration or, more properly, evapotranspiration , by about 36 # 10 15 kg/year. The
opposite is true over the oceans, where evaporation rates exceed precipitation
rates. Note that although the oceans cover approximately 70% of the earth's
surface, 86% of the total evaporation is from the ocean. The atmosphere trans-
ports the excess water evaporated from the oceans to the land, indicating the