Middle-latitude climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 115

they are in the tropics. The extent and variations in sea ice also have important
effects. Although this section focuses on the mid-latitudes, it is important to
remember that links and complex interactions with both the tropics and the polar
regions exist.
4.6.1 The role of the oceans
The review by Bigg et al.( 2003 ) provides an excellent background to the role of
oceans in SHmid-latitude circulation and climate. Aside from the interchange of
gases, particulates, heat, and water between the ocean surface and the atmo-
sphere, the ocean redistributes heat across the mid-latitude zone, and creates a
thermal lag on climate and climate change. In the SH, two great cold ocean
currents, originating from the circumpolar ocean circulation, dominate. These
are the Benguela Current, which moves north along the west coast of Southern
Africa, and the Humboldt Current, which strongly influences stability, tempera-
ture, and moisture along the west coast of South America. By contrast, the warm
currents on the east sides of continents tend to have more influence on regional
and local scales.
Bigg et al.( 2003 ) and Fauchereau et al.( 2003 ) define two major differences in
the role of the ocean in the mid-latitudes compared to the tropics. First, SST is
not as important because its influence tends to be overwhelmed by several other
influences on circulation, some of which are listed in the previous section .
Divergent circulations in the mid-latitude baroclinic atmosphere are consider-
ably less important than in the tropics, where barotropic circulations are highly
influenced by surface conditions. The mid-latitude atmosphere also has con-
siderably greater internal variability than the tropics, associated with shifts in the
troughs and ridges of Rossby wave number 3 for example.
Second, the response timing of oscillations in climate in the SH mid-latitudes
is controlled mainly by ocean influences. Changes occur over a longer time scale
than in the tropics, at the level of inter-decadal rather than inter-annual. For
example, the pattern and timing of Rossby wave propagations are strongly
influenced by the ocean.
The ocean influence varies spatially, with regions such as the SW Indian
Ocean and the central Atlantic Ocean showing stronger coupling and better
phasing of climate variability than other areas. In these regions, Fauchereau
et al.( 2003 ) show that latent heat influences on the atmosphere are most
important on the western edge of SST anomalies, with a reasonably close
correlation to boundary-layer wind anomalies. SST warming and/or reduced
westerly winds lead to reduced latent heat flux, and SST cooling and increased
westerly winds create the opposite. However, the significance of ocean influ-
ences on mid-latitude atmospheric forcing in this area is still fairly weak. ENSO
links explain approximately 20% of the total variance for the Atlantic, and 17%
for the SW Indian Ocean, with maximum mid-latitude correlations reaching