Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 251

Northern hemisphere anticyclonic wind Northern hemisphere cyclonic wind

wind

Ekman water
transport

wind

wind

Ekman
transport

Ekman transport inward

Absolute vorticity increases, moment of inertia
decreases and angular velocity increases

Ekman transport outward

Surface convergence Absolute vorticity decreases, moment of inertia increases,

and angular velocity decreases

Surface Surface divergence

Surface

Downwelling

Thermocline

Deep
mixing
possible

Upwelling

Thermocline

Fig. 6.16 Ekman transport and vorticity changes associated with northern hemisphere anticyclonic and cyclonic winds.

6.2.3 Atmospheric boundary layers and heat
exchange: General

undergo extreme development to tropical cyclonic storms,
variously called hurricanes or typhoons. For example,
between 5 and 10 hurricanes typically develop in the
southern North Atlantic each year. Hurricanes are revolv-
ing storms (i.e. vortices) of great ferocity (surface wind-
speeds 33-70 m s 1 ) sourced over the global tropical
oceans. Because of their danger to humans and despite
their infrequence, of all meteorological phenomena their
correct forecast is probably of the greatest importance.
Cyclones grow from spatially concentrated seed banks of
cumulonimbus clouds on the downflow margins of the
trade wind belts, where sufficient passage has occurred
over warm tropical ocean so that saturation vapor pres-
sures are high. The position of the late summer Bermuda
High plays an important role in guiding the storm tracks
from east to west and hence northwest landwards toward
the Caribbean Islands and southeast North America.
During strong El Nino Southern Oscillation (ENSO) (El
Niño, see Section 6.2.5) years the Bermuda High is forced
eastward and storm tracks rarely impact land; vice versa for
weak ENSO years (El Niña). Tropical cyclones cannot be
generated within about 5

The conventional view of the Oceanic Boundary Layer
(OBL) thermal reservoir is that it gives up its thermal
energy in a one-way heat transfer to the overlying ABL,
mostly in the form of latent heat of evaporation. The evap-
orated seawater in air thus carries most of the transferred
heat energy, linking the ocean thermal system directly with
the atmosphere. We may apply this concept of an
ocean-atmosphere heat engine between the limits of the
ocean surface and the tropopause. We have noted previously
(Section 6.1) that there exists strong evidence that warming
of the Indo-Pacific oceans might be responsible, through a
teleclimatic forcing connection, for recent decadal change
in the NAM and SAM . At the same time we must also stress
the role of atmospheric flow and forced convection on cool-
ing ocean water in polar latitudes when cold polar air jets
chilled by passage over snow or ice or cooled by descent
then pass over the ocean. The resulting loss of heat from the
surface ocean by conductive transfer and forced convection
is a major process in the production of unstable surface cold
water that sinks to form Arctic and Antarctic deepwater.

or so of the Equator because
here the Coriolis force is insignificantly small to zero and a
geostrophic balance cannot be reached. Their energy is
derived from latent heat transfer above the very warmest
tropical oceans in late summertime and thus any cyclic per-
turbations to seasonal Sea Surface Temperatures (SSTs),
like those associated with ENSO oscillations, have an
important influence on the frequency of hurricane genesis.
Cyclones grow upward above the very warm ocean water

6.2.4 The tropical ocean-atmosphere heat engine:
Tropical cyclones

Many thousands of tropical thunderstorms are generated
each year in the intertropical “heat engine” but only a few