wind bursts driving warm water eastward along
the equator, which then influence the wind system.
Quantitative descriptions of the wind stress
fields have been obtained from ship observations
(Hellermann and Rosenstein, 1983; Josey et al.,
2000), from the outputs of operational atmospheric
analyses (e.g. Trenberth et al., 1990) and most
recently from satellite remote sensing (Liu and
Katsaros, Chapter 3.4). A comprehensive review of
the various products is given by WGASF (2000).
Winds directly drive the near-surface ocean
layer. The rotation of the earth causes a depthintegrated Ekman transport to the right (left) of
the wind in the northern (southern) hemisphere
(Ekman, 1905). A large-scale confirmation of this
Ekman transport, a fundamental tenet of oceanography, has come from surface drifter observations
(Niiler, Chapter 4.1, Figs 4.1.6–4.1.8) obtained
during WOCE and the Tropical Ocean-Global
Atmosphere (TOGA) programme. The dominant
wind stress pattern produces a convergence of the
Ekman transport in mid-latitudes and a divergence
poleward of the peak in the westerly winds. This
generates the major ocean circulation gyres (Section
1.2.4.1). The continuing inability to adequately
describe the wind field over the oceans significantly limits our ability to simulate and model
ocean circulation.
1.2.2.2 Heat transfer
The net flux of heat at the ocean surface results
from the balance between incoming short- and
long-wave radiation, the loss of heat by outgoing
long-wave radiation and through evaporation, and
to some extent by the surface turbulent fluxes of
sensible heat. There is a heat flux associated with
precipitation but on the large-scale this is generally
small (Curry and Webster, 1999). Each component of the surface heat budget depends on a
variety of oceanic and atmospheric parameters (Liu
and Katsaros, Chapter 3.4; Curry and Webster,
1999). The heat budgets of ocean and atmosphere
are coupled through these air–sea fluxes.
Radiation
The incoming (short-wave) solar radiation flux at
the top of the atmosphere is a function of latitude,
season and time of day. Averaged globally it has a
value of approximately 342 W m
92 (Fig. 1.2.1).
There have been only small variations in the solar
flux over the last millennia. Over the last two
million years, the radiation changes due to the
cycles of precession, obliquity and eccentricity in
the earth’s orbit around the sun (the Milankovich
cycles, with periods of tens to hundreds of thousands of years) have triggered long-term climate
variations.
Solar radiation passes through a clear atmosphere with little absorption. However atmospheric
water vapour, clouds and aerosols reflect, scatter
and absorb solar radiation, significantly reducing
the amount of heat reaching the surface (Fig. 1.2.1;
Curry and Webster, 1999). At the sea surface, a
fraction (the surface albedo – typically 5–8% when
the sun is nearly overhead) of the incoming solar
radiation is reflected or backscattered from the
ocean (Curry and Webster, 1999). Conditions
within the ocean determine where the remainder of
the radiation is absorbed. Waters with heavy sediment loads or experiencing strong phytoplankton
blooms absorb the solar radiation over a few
metres whereas in clear water the short-wave radiation can penetrate tens of metres below the surface
mixed layer. This relatively deep penetration is
important in understanding the heat balance of the
western equatorial Pacific Ocean and thus in simulating interannual climate variability associated
with El Niño-Southern Oscillation events (see e.g.
Section 4.2 of Godfrey et al., 1998).
The ocean also radiates energy upwards to the
atmosphere at long wavelengths. The flux is a
function of the surface temperature (Curry and
Webster, 1999). Long-wave radiation is strongly
absorbed in water and consequently the outgoing flux originates from the topmost millimetre
of the ocean. The temperature of this surface skin
is slightly different (typically cooler by about
0.2–0.3°C) from that of the mixed layer immediately below. Long-wave radiation is absorbed by
the atmosphere and by clouds and these in turn
radiate long-wave energy out to space and back to
the ocean where it is absorbed (again in the top
millimetre).
Clouds, in their many forms, are the most
important factor regulating both the incoming
solar and the long-wave radiation. For more than
a century ships’ officers have documented marine
clouds and over recent decades clouds have also
been observed using satellites. Unfortunately,
neither set of observations provides sufficient
information to assess the clouds’ optical thickness
to either the short- or long-wave radiation.
SECTION 1 THE OCEAN AND CLIMATE
14
the equator, which then influence the wind system.
Quantitative descriptions of the wind stress
fields have been obtained from ship observations
(Hellermann and Rosenstein, 1983; Josey et al.,
2000), from the outputs of operational atmospheric
analyses (e.g. Trenberth et al., 1990) and most
recently from satellite remote sensing (Liu and
Katsaros, Chapter 3.4). A comprehensive review of
the various products is given by WGASF (2000).
Winds directly drive the near-surface ocean
layer. The rotation of the earth causes a depthintegrated Ekman transport to the right (left) of
the wind in the northern (southern) hemisphere
(Ekman, 1905). A large-scale confirmation of this
Ekman transport, a fundamental tenet of oceanography, has come from surface drifter observations
(Niiler, Chapter 4.1, Figs 4.1.6–4.1.8) obtained
during WOCE and the Tropical Ocean-Global
Atmosphere (TOGA) programme. The dominant
wind stress pattern produces a convergence of the
Ekman transport in mid-latitudes and a divergence
poleward of the peak in the westerly winds. This
generates the major ocean circulation gyres (Section
1.2.4.1). The continuing inability to adequately
describe the wind field over the oceans significantly limits our ability to simulate and model
ocean circulation.
1.2.2.2 Heat transfer
The net flux of heat at the ocean surface results
from the balance between incoming short- and
long-wave radiation, the loss of heat by outgoing
long-wave radiation and through evaporation, and
to some extent by the surface turbulent fluxes of
sensible heat. There is a heat flux associated with
precipitation but on the large-scale this is generally
small (Curry and Webster, 1999). Each component of the surface heat budget depends on a
variety of oceanic and atmospheric parameters (Liu
and Katsaros, Chapter 3.4; Curry and Webster,
1999). The heat budgets of ocean and atmosphere
are coupled through these air–sea fluxes.
Radiation
The incoming (short-wave) solar radiation flux at
the top of the atmosphere is a function of latitude,
season and time of day. Averaged globally it has a
value of approximately 342 W m
92 (Fig. 1.2.1).
There have been only small variations in the solar
flux over the last millennia. Over the last two
million years, the radiation changes due to the
cycles of precession, obliquity and eccentricity in
the earth’s orbit around the sun (the Milankovich
cycles, with periods of tens to hundreds of thousands of years) have triggered long-term climate
variations.
Solar radiation passes through a clear atmosphere with little absorption. However atmospheric
water vapour, clouds and aerosols reflect, scatter
and absorb solar radiation, significantly reducing
the amount of heat reaching the surface (Fig. 1.2.1;
Curry and Webster, 1999). At the sea surface, a
fraction (the surface albedo – typically 5–8% when
the sun is nearly overhead) of the incoming solar
radiation is reflected or backscattered from the
ocean (Curry and Webster, 1999). Conditions
within the ocean determine where the remainder of
the radiation is absorbed. Waters with heavy sediment loads or experiencing strong phytoplankton
blooms absorb the solar radiation over a few
metres whereas in clear water the short-wave radiation can penetrate tens of metres below the surface
mixed layer. This relatively deep penetration is
important in understanding the heat balance of the
western equatorial Pacific Ocean and thus in simulating interannual climate variability associated
with El Niño-Southern Oscillation events (see e.g.
Section 4.2 of Godfrey et al., 1998).
The ocean also radiates energy upwards to the
atmosphere at long wavelengths. The flux is a
function of the surface temperature (Curry and
Webster, 1999). Long-wave radiation is strongly
absorbed in water and consequently the outgoing flux originates from the topmost millimetre
of the ocean. The temperature of this surface skin
is slightly different (typically cooler by about
0.2–0.3°C) from that of the mixed layer immediately below. Long-wave radiation is absorbed by
the atmosphere and by clouds and these in turn
radiate long-wave energy out to space and back to
the ocean where it is absorbed (again in the top
millimetre).
Clouds, in their many forms, are the most
important factor regulating both the incoming
solar and the long-wave radiation. For more than
a century ships’ officers have documented marine
clouds and over recent decades clouds have also
been observed using satellites. Unfortunately,
neither set of observations provides sufficient
information to assess the clouds’ optical thickness
to either the short- or long-wave radiation.
SECTION 1 THE OCEAN AND CLIMATE
14