Geoscience Reference
In-Depth Information
Northern hemisphere
Southern hemisphere
N
Low
x
High
x
x
High
N
Low
x
Path of high or low pressure
Center of high or low pressure
x
Warm front
Low pressure corridor
Surface wind flow
Cold front
Schematic view of the dynamic structure of a mobile polar high and its associated mid-latitude low (based on Leroux, 1993).
Fig. 2.6
polar highs produce stronger monsoons, although in a
more restricted tropical belt.
with air then moving in the upper troposphere, to the
east Pacific. Convection is so intense that updrafts
penetrate the tropopause into the
stratosphere
, mainly
through
supercell thunderstorms
. These convective
cells are labeled
stratospheric fountains
.
Throughout the year, mobile polar highs tend to
stack up over the equatorial ocean west of South
America. The center of average high pressure here
shifts seasonally less than 5° in latitude. The highs are
locked into position by positive
feedback
. Cold water in
the eastern Pacific Ocean creates high pressure that
induces easterly airflow; this process causes upwelling
of cold water along the coast, which cools the air. On
the western Pacific side, easterlies pile up warm water,
thus enhancing
convective instability
, causing air to
rise, and perpetuating low pressure. Air is thus contin-
ually flowing across the Pacific, as an easterly trade
wind from high pressure to low pressure. This circula-
tion is zonal in contrast to the
meridional airflow
inherent within the Palmén-Newton general circula-
tion model. The persistent easterlies blow warm
surface water across the Pacific Ocean, piling it up in
The
Southern Oscillation
Introduction
The Earth's general atmospheric circulation in the
tropics and subtropics, as shown above, can be simply
described. However, the actual scene is slightly more
complex. The intensity of mobile polar highs within
a hemisphere varies annually with the apparent
movement of the sun north and south of the equator.
There is also a subtle, but important, shift in the inten-
sity of heating near the equator. In the northern
hemisphere summer, heating shifts from equatorial
regions to the Indian mainland with the onset of the
Indian monsoon. Air is sucked into the Indian subcon-
tinent from adjacent oceans and landmasses, to return
via upper air movement, to either southern Africa or
the central Pacific. In the northern hemisphere winter,
this intense heating area shifts to the Indonesian-
northern Australian 'maritime' continent (Figure 2.7A),
