28
DYNAMICAL OCEANOGRAPHY
The main characteristics of the surface forcing of the ocean circulation,
the annual mean patterns of wind stress, heat flux and freshwater flux are
presented section 2.1. In section 2.2, the patterns of the ocean currents in
the different basins are described with a focus on major surface currents,
the thermohaline circulation and the associated heat- and freshwater transport. Section 2.3 provides an overview of the role of the ocean circulation
in climate and naturally leads to the formulation of a few central problems in dynamical oceanography (section 2.4) which help to organize the
structure of this book.
2.1. Forcing fields
The large-scale ocean circulation is driven by a surface wind stress and affected
by surface fluxes of heat and fresh water.
2.1.1. Wind stress
The magnitude and direction of the shear stress which is exerted by the atmospheric winds on the ocean surface depends on the wind velocities. If U =(U, V )
indicates the wind velocity at 10 m height, then a semi-empirical bulk relation
provides the wind stress τ (in Pa = Nm −2 )as
Ex. 2.1
τ = C D ρ a |U|U,
(2.1)
where ρ a =1.2 kgm 3 is the density of air and C D ≈ 10 −3 is a friction coefficient
that depends on |U|.
The annual average zonal wind stress on the ocean-atmosphere surface from
observations (Trenberth et al., 1989) is plotted in Fig. 2.1a and the meridional
wind stress in Fig. 2.1b. Maximum values of the zonal wind stress are about 0.3
Pa and occur at the Southern Hemisphere. Typical values are about 0.1 Pa and the
general pattern is the same during the whole year: strong easterly winds near the
equator, strong westerly winds at midlatitudes and weaker easterly winds at polar
latitudes. The meridional wind stress is somewhat weaker than the zonal wind
stress and largest values occur over the Southern Ocean.
2.1.2. Heat flux
The total net heat flux Q oa in Wm −2 which is positive when heat is transported
from the atmosphere into the ocean) is the sum of several components, i.e.,
Q oa = Q H + Q E + Q LW + Q SW .
(2.2)
In this expression, Q H is the sensible heat flux, Q E is the latent heat flux, Q LW
is the longwave heat flux and Q SW is the shortwave heat flux. The magnitudes
DYNAMICAL OCEANOGRAPHY
The main characteristics of the surface forcing of the ocean circulation,
the annual mean patterns of wind stress, heat flux and freshwater flux are
presented section 2.1. In section 2.2, the patterns of the ocean currents in
the different basins are described with a focus on major surface currents,
the thermohaline circulation and the associated heat- and freshwater transport. Section 2.3 provides an overview of the role of the ocean circulation
in climate and naturally leads to the formulation of a few central problems in dynamical oceanography (section 2.4) which help to organize the
structure of this book.
2.1. Forcing fields
The large-scale ocean circulation is driven by a surface wind stress and affected
by surface fluxes of heat and fresh water.
2.1.1. Wind stress
The magnitude and direction of the shear stress which is exerted by the atmospheric winds on the ocean surface depends on the wind velocities. If U =(U, V )
indicates the wind velocity at 10 m height, then a semi-empirical bulk relation
provides the wind stress τ (in Pa = Nm −2 )as
Ex. 2.1
τ = C D ρ a |U|U,
(2.1)
where ρ a =1.2 kgm 3 is the density of air and C D ≈ 10 −3 is a friction coefficient
that depends on |U|.
The annual average zonal wind stress on the ocean-atmosphere surface from
observations (Trenberth et al., 1989) is plotted in Fig. 2.1a and the meridional
wind stress in Fig. 2.1b. Maximum values of the zonal wind stress are about 0.3
Pa and occur at the Southern Hemisphere. Typical values are about 0.1 Pa and the
general pattern is the same during the whole year: strong easterly winds near the
equator, strong westerly winds at midlatitudes and weaker easterly winds at polar
latitudes. The meridional wind stress is somewhat weaker than the zonal wind
stress and largest values occur over the Southern Ocean.
2.1.2. Heat flux
The total net heat flux Q oa in Wm −2 which is positive when heat is transported
from the atmosphere into the ocean) is the sum of several components, i.e.,
Q oa = Q H + Q E + Q LW + Q SW .
(2.2)
In this expression, Q H is the sensible heat flux, Q E is the latent heat flux, Q LW
is the longwave heat flux and Q SW is the shortwave heat flux. The magnitudes