Geoscience Reference
In-Depth Information
wave number of two and to move toward the equator as they flow over continents,
and away from the equator as they flow over oceans. This tendency is more
pronounced in the northern hemisphere winter. Within this average flow pattern,
the jet stream is continually developing, meandering, and decaying giving relatively
less consistent lower-level winds. The jet streams have a fundamental influence on
hydrometeorological variability.
Ocean circulation
As mentioned in Chapter 1, the oceans are the principal source of 'memory' in the
hydroclimatic system because of their capability to store and subsequently release
large amounts of heat energy. The thermal structure of the oceans is fundamentally
different to that of the atmosphere. Energy from the Sun is captured near the ocean
surface and some warms the atmosphere from below. As a result, air nearer the
ocean surface tends to be warmer and lighter in daytime conditions, and the
atmosphere is unstable and well-mixed throughout the boundary layer as a result.
On the other hand, solar energy warms the oceans from above, so the temperature
of the water is higher nearer the surface and the surface layer of the ocean is stable.
Some mixing of energy downward does occur as a result of the turbulence caused
by surface winds but the depth to which this occurs is limited. Consequently, the
oceans are divided into a
mixed layer
(typically between 100 m and 1000 m deep)
which is separated from the deep ocean below by the
thermocline
, i.e., the steep
negative temperature gradient that gives a stable interface between these two layers
and suppresses mixing, see Fig. 9.7.
At low latitudes, solar heating is strong and fairly constant through the year, and
the stability of the thermocline is able to keep the mixed layer fairly shallow with
limited seasonality in its depth. In middle latitudes the strength of the solar heating
changes seasonally. In the hemispherical summer, the mixed layer is again fairly
shallow as at low latitudes. However, in the hemispherical winter solar heating is
less, so the distinction between the mixed layer and deep ocean is reduced; because
the temperature gradient and stability of the thermocline is therefore less, surface
winds can mix warmer surface water to greater depth.
Sea-surface temperature (SST) provides an important lower boundary
condition on the atmosphere over the ocean surfaces that cover 70% of the globe.
As discussed in Chapter 1, there is a strong coupling between oceans and
atmosphere because of the effective exchange of energy fluxes and momentum.
Since air near open water is close to saturation, the surface temperature of the
ocean defines not only the lower boundary condition for sensible heat transfer
but also for latent heat transfer. The isotherms of annual average SST run roughly
east-west across the large oceans, from greater than 29°C at the equator to almost
-2°C near the poles where the presence of salt in the ocean depresses the freezing
point, see Fig. 9.8. However, isotherms are modified near continents in response
to wind-driven currents in the upper few hundred meters of the ocean. In the
most southerly latitudes of the southern hemisphere (not shown in Fig. 9.8), the
