30
DYNAMICAL OCEANOGRAPHY
where ρ a (kgm −3 )andC pa (Jkg −1 K −1 ) are the density and heat capacity of air,
|U| (ms −1 ) is the wind speed, γ a (Km −1 ) the (dry or moist) atmospheric ‘lapse
rate’ and C H is a dimensionless semi-empirical transfer coefficient.
In Fig. 2.2, the long term monthly average values of Q oa are plotted for January,
April, July and October (Oberhuber, 1988). In the annual average, there is a net
positive heat flux in the equatorial region and a net negative flux at high latitudes.
In the Southern (Northern) Hemisphere summer, there is a net input of heat in the
Southern (Northern) Hemisphere. Maximum values of Q oa are about 200 Wm −2
and these occur in January over areas near the Gulf Stream (Atlantic) (Fig. 2.2)
and Kuroshio (Pacific), providing a strong zonal asymmetry of the heat flux in
both basins. There are other sources of heat into the oceans, such as geothermal
activity, but these are of minor importance for the ocean circulation and we will
not consider them.
2.1.3. Freshwater flux
The evaporation E (ms −1 ) over the ocean can be estimated by a bulk formula
of the form
E = C E |U|(q s − q a ),
(2.4)
where C E is a semi-empirical exchange coefficient, q a the relative humidity in
the atmosphere and q s the saturation relative humidity at the ocean-atmosphere
boundary. The latent heat flux Q E in (2.2) is related to the evaporation E through
Ex. 2.3
Q L = ρ a L f E,
(2.5)
where L f =2 .5 × 10 6 Jkg −1 is the latent heat of evaporation. It is hard to
determine the precipitation, P , from direct observations and in most cases an
empirical relation between precipitation and temperature/pressure is used.
The long term monthly mean values of P − E in mm/month are plotted for
January, April, July and October (Oberhuber, 1988) in Fig. 2.3. There is quite
a zonally uniform pattern in the Atlantic as well as in the Pacific. The zones of
high precipitation near the equator are related to the Intertropical Convergence
Zone (ITCZ) and can be seen as maxima of the freshwater flux with amplitudes
of 200 mm/month (Fig. 2.3b). The central midlatitude regions are zones of net
evaporation and there is less seasonal variation than in the heat flux. At very high
latitudes precipitation again dominates over evaporation. There are other sources
of freshwater into the oceans, such as river outflow, but these are again of minor
importance for the large-scale ocean circulation and we will not consider them.
DYNAMICAL OCEANOGRAPHY
where ρ a (kgm −3 )andC pa (Jkg −1 K −1 ) are the density and heat capacity of air,
|U| (ms −1 ) is the wind speed, γ a (Km −1 ) the (dry or moist) atmospheric ‘lapse
rate’ and C H is a dimensionless semi-empirical transfer coefficient.
In Fig. 2.2, the long term monthly average values of Q oa are plotted for January,
April, July and October (Oberhuber, 1988). In the annual average, there is a net
positive heat flux in the equatorial region and a net negative flux at high latitudes.
In the Southern (Northern) Hemisphere summer, there is a net input of heat in the
Southern (Northern) Hemisphere. Maximum values of Q oa are about 200 Wm −2
and these occur in January over areas near the Gulf Stream (Atlantic) (Fig. 2.2)
and Kuroshio (Pacific), providing a strong zonal asymmetry of the heat flux in
both basins. There are other sources of heat into the oceans, such as geothermal
activity, but these are of minor importance for the ocean circulation and we will
not consider them.
2.1.3. Freshwater flux
The evaporation E (ms −1 ) over the ocean can be estimated by a bulk formula
of the form
E = C E |U|(q s − q a ),
(2.4)
where C E is a semi-empirical exchange coefficient, q a the relative humidity in
the atmosphere and q s the saturation relative humidity at the ocean-atmosphere
boundary. The latent heat flux Q E in (2.2) is related to the evaporation E through
Ex. 2.3
Q L = ρ a L f E,
(2.5)
where L f =2 .5 × 10 6 Jkg −1 is the latent heat of evaporation. It is hard to
determine the precipitation, P , from direct observations and in most cases an
empirical relation between precipitation and temperature/pressure is used.
The long term monthly mean values of P − E in mm/month are plotted for
January, April, July and October (Oberhuber, 1988) in Fig. 2.3. There is quite
a zonally uniform pattern in the Atlantic as well as in the Pacific. The zones of
high precipitation near the equator are related to the Intertropical Convergence
Zone (ITCZ) and can be seen as maxima of the freshwater flux with amplitudes
of 200 mm/month (Fig. 2.3b). The central midlatitude regions are zones of net
evaporation and there is less seasonal variation than in the heat flux. At very high
latitudes precipitation again dominates over evaporation. There are other sources
of freshwater into the oceans, such as river outflow, but these are again of minor
importance for the large-scale ocean circulation and we will not consider them.