1. Northern Hemisphere
The 500-hPa height pattern during
January 2009 featured positive anomalies across the high latitudes of the
North Pacific Ocean
and the southern
North Atlantic Ocean
, and negative anomalies from the eastern
This overall pattern reflected La Niņa, a strong positive phase (+1.9) of the
Tropical / Northern Hemisphere (TNH) teleconnection pattern partly in response
to La Niņa, and a strong positive phase (+1.6) of the East Atlantic (EA)
teleconnection pattern (Table E1,
The subtropical circulation at
200-hPa remained consistent with La Niņa, with enhanced mid-Pacific troughs in
both hemispheres flanking the suppressed convection over the central equatorial
Pacific, and a westward retraction of the subtropical ridges to Australasia (Fig.
T22). Associated with this pattern,
the East Asian jet stream remained weaker than average east of the date line, as
the jet core was retracted westward toward the western Pacific (Fig.
The main temperature signals
during January included above-average temperatures in the western
, and below-average temperatures in portions of eastern
(Fig. E1). The main precipitation
signals included above-average totals in portions of western and southern
, and below-average totals along the North American west coast and the U.S. Gulf
Coast (Fig. E3).
a. North Pacific/
The 500-hPa circulation during
January partly reflected La Niņa, with westward shifts in several main
circulation features, including the exit region of the East Asian jet stream and
the mean ridge normally over western
. The positive phase of the TNH teleconnection pattern was also present, as
manifested by the exceptionally large anomalies over both the Gulf of Alaska and
This pattern was associated with anomalous northwesterly flow into central
These conditions brought below
average temperatures to the northeastern
, and above average temperatures to the western
Below average precipitation was observed in western North America in the
vicinity of the mean ridge axis, and across the
upstream of and within the mean trough axis (Fig. E3). The
region typically receives below average wintertime precipitation during La Niņa,
as seen during the last three months (Fig.
E5). Some of the most significant precipitation
deficits have occurred in
, where exceptional drought has developed.
The 500-hPa circulation featured
below average heights from the eastern
(Fig. E9). This overall pattern reflected an
anomalously zonal flow across the
, and a southward shift of the
jet stream and storm track. As a result, both western and southern
recorded above average precipitation and increased winter storm activity during
2. Southern Hemisphere
The 500-hPa height field during January featured an
anomalous zonal wave-1pattern, with positive anomalies in the middle latitudes
and negative anomalies at high latitudes (Fig. E15).
, and anomalous anticyclonic circulation contributed to below average
precipitation during the month, with some areas recording totals in the lowest
10th percentile of occurrences.
, the rainy season extends from October through April. During January rainfall
was above average for the region as a whole, with area average totals reaching
the 80th percentile of occurrences (Fig.
E4). Above average totals during November
2008-January 2009 are consistent with La Niņa.