During La Niña episodes there is an absence of convective activity over the eastern
half of the equatorial Pacific, and an overall westward retraction of deep tropical
convection and deep tropospheric heating toward the western Pacific. This pattern of
anomalous heating acts to retract the subtropical ridges in both hemispheres to west of
the date line, and to reduce the north-to-south temperature difference east of the date
line in the subtropics of both hemispheres. These conditions are generally most prominent
during the respective hemisphere's winter season, when they contribute to a westward
retraction of the midlatitude jet stream toward the western Pacific. Overall, these
conditions reflect an increased east-west contrast in both temperature and winds across
Pacific basin, and are an important factor affecting the winter weather patterns and storm
tracks in the middle latitudes over both North America and South America.
During La Niña episodes a large-scale upper-level cyclonic circulation (C's) anomaly
is evident in both hemispheres over the subtropical latitudes of the central Pacific.
Features of this cyclonic anomaly dipole include easterly wind anomalies along its
poleward flanks in the middle latitudes of both hemispheres and westerly wind anomalies
across the equatorial eastern Pacific. In the subtropics and middle latitudes, these
anomalies reflect the westward retraction of both the subtropical ridge and wintertime jet
stream to west of the date line. In the Tropics they are consistent with an increased
strength of the equatorial Walker Circulation typical of La Niña episodes.