Middle-latitude climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 123

west of New Zealand), shifts westward and decreases in amplitude. The area
covered by the circumpolar westerly belt expands, especially toward the equa-
tor. El Ni˜o periods tend to dominate, but by themselves to do not control the
situation. Tyson et al.( 1997 ) state that the Quasi-biennial Oscillation (QBO) has
a more important influence. The regional circulation over the South Island of
New Zealand is enhanced southwesterly air flows, creating increased cloud,
lower temperatures, and increased precipitation on the west coast and slopes of
the mountains, but drier conditions in the east and northeast (Jones et al. 1999 ).
In the upper troposphere and lower stratosphere, a single jet stream prevails.
Transfer of heat, momentum, and moisture toward the poles is reduced.
Over Southern Africa, negative pressure anomalies south of the continent
dominate, westerly winds are stronger, and periods of drought occur. The New
Zealand glaciers advance after drought occurs in Southern Africa, with a
response time of 4 to 5 years lag. Over Antarctica, the combination of negative
AAO, a single jet stream, a more compact CPT and stronger zonal flow all
combine to minimize the transport of moisture southward past the coast
(Fauchereau et al. 2003 ; Goodwin et al. 2004 ). Sodium ion deposition is
reduced, indicating lower precipitation amounts.
Virtually the opposite occurs in all three locations when wave number 3
activity in the CPV is enhanced. In the New Zealand region, enhanced meridio-
nal circulation is created as the wave ridge shifts eastward. Both the SPCZ
(Chapter 3 ) and the climatological subtropical highs in the Atlantic and Indian
Oceans shift south and east. The surface pressure anomalies over the SW Pacific
Ocean become positive, and the occurrence of blocking highs increases, both in
frequency and strength. NE wind anomalies are created, and the periods and
strength of SW airflow are reduced. This leads to increased precipitation to the N
and NE, but the west coast has sunnier skies, higher temperatures, and reduced
precipitation. Glaciers on the west coast of the South Island retreat. In the upper
troposphere, the jet stream is split, enhancing the transfer of heat, momentum,
and moisture toward the polar region.
Over Southern Africa, the shift of the wave number 3 ridge eastward
encourages more E and NE airflow, enhancing moisture influx into the region,
with resultant increased precipitation. In the Antarctic region, the CPT expands,
zonal westerlies are weaker, the number of cyclones occurring in the CPT is
reduced (see Chapter 5 ), and enhanced ridging occurs between the Tasman Sea
and East Antarctica. The AAO is positive. The stronger meridional flow
encourages moisture flux southward across the Antarctic coast, creating
increased Na þ and precipitation deposition.
4.6.7 Are sunspot variations a forcing mechanism?
The reasons for the variations in the CPV and the shifting of standing waves in
both hemispheres relates to the forcing mechanisms that drive the atmospheric