Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 257

sun

b 1

Sensible heating,
evaporation

β2

β3

Parallel and uniformly
sloping
isobars and isopycnals

Isobars increasingly
divergent from
surface topography,
slope becoming opposite
in sign from barotropic
slope conditions

B

A

indicates flow into
page in N. Hemisphere

Curved isopycnals as density of
water at any depth increases in x -direction

BAROCLINIC CONDITIONS BAROTROPIC CONDITIONS

indicates flow out of
page in N. Hemisphere

Pressure gradient
to right,
decreasing rightward

Where isobars horizontal,
no gradient
and therefore no flow

Pressure gradient to left,
decreasing leftward

Fig. 6.22 Baroclinic conditions: isobaric and isopycnal surfaces are NOT parallel in water bodies with laterally varying density. Illustrated here
by a salinity gradient caused by solar evaporation similar to the situation in the eastern Mediterranean.

barotropic cases is balanced by the local Coriolis force to
define a geostrophic flow of magnitude u

Contours of ocean surface
(geopotentials)

( g / f )tan

Coriolis

along a path parallel to the isobars.
Large-scale oceanic circulation in the subtropical gyres
is due to pressure-driven flow outward from the centers of
ocean surface topography caused by Ekman transport
(Section 6.2.2). Surface flow is then set up, initially as gra-
dient currents that run down the surface slope and then is
turned by the action of the Coriolis force to run parallel to
the dynamic sea surface topography and to the regional
ABL flow (Fig. 6.23). Like their atmospheric equivalents
(Section 6.1) such surface currents are also geostrophic.

Pressure

Geostrophic flowlines spacing
proportional to velocity

Westerlies

6.4.3 Western amplification of geostrophic currents

The broad pattern of global surface ocean topography and
currents is shown in Figs 6.24 and 6.25. The circum-
polar Antarctic Current is strong because of the extreme
degree of wind forcing in response to great lateral pressure
gradients at these high polar latitudes (Section 6.1). But
how can we explain the intensification of surface flow on
the western borders of the oceans, manifest in strong west-
ern boundary currents such as the North Atlantic Gulf
Stream and Pacific Kuroshio ? It is common to measure
speeds of over 1 m s 1 (3.6 km h 1 ) in these currents.
The Gulf Stream, for example, is usually a continuous,

Trades

Fig. 6.23 Balance between Coriolis force and pressure gradient caus-
ing geostrophic flow around an ocean “bump.”