Environmental Engineering Reference
In-Depth Information
western North America in summertime when greenhouse gas concentrations
are enhanced.
The same argument for a poleward shift in the storm tracks in the
Northern Hemisphere in a warmer climate also applies in the Southern
Hemisphere. Because the jet and storm tracks are intimately related and
in the southern hemisphere they are circumpolar and vary weakly with
longitude, this pattern of climate change is called the Southern Annular
Mode (SAM; named after the leading pattern of circulation variability in
the Southern Hemisphere due to wave-mean flow interactions—largely in
the troposphere). There is ample evidence that the SAM-like changes in the
mid- and high-latitude circulation in the Southern Hemisphere that were
observed in the latter third of the 20th century (a poleward shift in the jet)
was in part due to the human-induced depletion of stratospheric ozone (see,
e.g., Miller et al., 2006 and references therein). Hence, a poleward shift
in the jet in the Southern Hemisphere that would otherwise be expected
because of increasing greenhouse gases will be somewhat tempered until
about mid-21st century—when the stratospheric ozone is expected to be
nearly recovered.
Many of the AR4 climate models forced by increasing greenhouse gases
show that the Meridional Overturning Circulation (MOC) in the Atlantic
Ocean will slow down over the 21st century in direct response to increased
buoyancy in the sinking regions of the North Atlantic (Meehl et al., 2007).
The increased buoyancy is due to both warming and freshening, with the
former being more important than the latter. The slowdown in the MOC
will have several impacts including a somewhat muted change in surface
temperature in the North Atlantic and in maritime western Europe compared
to other oceanic regions and modifications in the gradients in tropical sea
surface temperature, which will further influence hurricane activity in the
tropical Atlantic. The poleward movement of the jet in the Southern Hemi-
sphere will cause the Antarctic Circumpolar Current to narrow and shift
southward, impacting the sea-ice extent, the temperature of the water in
contact with the floating ice sheets, and the exchange of carbon between
the atmosphere and ocean.
There is much interest in knowing, as the planet warms: “How will
the El Niño/Southern Oscillation (ENSO) and its teleconnected climate
impacts change?” Unfortunately, this question cannot as yet be answered
with confidence because almost all of the present climate models have poor
representations of ENSO—as measured by the incongruities between the
observed and simulated spatial and temporal characteristics of the observed
ENSO and those simulated by the climate model. These nature-model incon-
