Geoscience Reference
In-Depth Information
In addition to the wind-driven surface currents, the
thermohaline circulation
is
caused by changes in density associated with the temperature and salinity of the
sea water. This originates at the poles as vertical flow that sinks to the middle
ocean or lower where it becomes horizontal. It originates as a result of density
increases likely due in part to direct cooling, but also as a result of increased salin-
ity occurring when sea water freezes and ejects additional salt into nearby saline
water. Although the downward branches of the thermohaline circulation are
largely restricted to high latitudes, the compensating upward branches occur more
widely across the globe. The thermohaline cycle is very slow, typically 1600 years,
but there is evidence that more rapid changes occur at time periods of decades
with an associated influence on climate at this time scale.
Oceanic influences on continental hydroclimate
Monsoon flow
The seasonal change in near-surface air temperature is markedly different between
oceanic and continental surfaces. Oceans have a large heat capacity and efficient cou-
pling with the atmosphere and this has the effect of moderating the seasonal cycle in
the temperature of the overlying air. The heat capacity of land surfaces is less so the
seasonal cycle can be much larger. Figure 9.9 shows isotherms of the difference
between the near-surface air temperature in January relative to that in July. Although
there are reductions of around 30°C in air temperature over the Arctic Ocean, else-
where changes in near-surface air temperature over oceans are on the order of 5-10°C.
However, the mid-continental reduction in near-surface air temperature from July to
January is more typically on the order of 40-50°C. Notice that the penetration of
maritime air into the European continent is greater than it is into the North American
continent presumably because there is less topographic obstruction to onshore flow
and the seasonal difference in near-surface air temperature is less as a result.
One important consequence of the substantial difference in temperature
between summer and winter over land surfaces relative to that over adjacent ocean
is the occurrence of monsoon air flow in the tropics. The largest monsoon system
is the Asian-Australian monsoon system (Fig. 9.10) which, recognizing the popu-
lation distribution of the world, is arguably therefore the hydroclimate phenome-
non that has most impact on humankind. In the northern hemisphere winter
season, there is a low level flow of dry, cool air from cold continent to the warmer
ocean and precipitation over the land is low. However, during the northern hemi-
sphere summer season the direction of flow is reversed, and warm, moist air flows
from the ocean over the warm land where the resulting upward motion of the
heated air produces heavy precipitation during the monsoon season. Monsoonal
flows of a broadly similar nature occur elsewhere in West Africa and in the North
American Monsoon System (NAMS), the latter being responsible for much of the
summer rain falling in northern Mexico and the southwestern USA.
