During La Nina conditions, the east to west flow present during neutral conditions
is intensified.
This anomalous wind regime is responsible for anomalies in sea surface temperature;
above average in the western Pacific and below average in the eastern Pacific. In
accordance with the shifts in sea surface temperature, rainfall will be relatively
higher in the western Pacific and relatively lower in the eastern Pacific.
During La Nina, the deviations of sea surface temperature and rainfall from normal
are generally less dramatic than the deviations that occur during El Nino.
During La Nina, the following conditions are likely:
Lower Atmospheric Circulation: Enhanced east to west flow- Surface air pressure is anomalously high over the central equatorial Pacific, for
example Tahiti, and anomalously low in the western equatorial Pacific, for example
Australia. This increased gradient in surface air pressure drives a stronger than
average flow of surface air moving from higher pressure to lower pressure, or east
to west across the equatorial Pacific Ocean.
Surface Water Circulation: Enhanced east to west flow- The lower atmospheric circulation has a direct effect on the surface water beneath
it. Along the equatorial Pacific Ocean, the east to west movement of air in the lower
atmosphere along the equatorial Pacific Ocean works on the water below it. During
La Niña conditions, this means an enhanced net surface water transport from east to
west resulting in cooler than average surface water temperatures in the central and
western Pacific.
Upper Atmospheric Circulation: Enhanced Eastward flow- Low level winds converge over the warm waters of the western Pacific and rise. The
forced ascent of the very moist air creates heavy rainfall in the region, wringing
the air of moisture. The now dry air diverges out of the top of the convective region,
moving out over the eastern Pacific to sink over the cooler than average waters.
Thermocline: Increase in slope, slanted down from the eastern Pacific to the western
Pacific- The thermocline marks the transition between the warm upper water and the cold deep
water in the Pacific Ocean. The increase in slope enhances upwelling of cooler, nutrient rich deep water towards the surface layer in the eastern equatorial
Pacific.
