1. Northern Hemisphere
The 500-hPa circulation
pattern during February featured persistent positive height anomalies across
the central North Pacific, over
, and in the area west of the Tibetan Plateau, and persistent negative
anomalies over eastern
, most of
, and eastern
(Figs. E9, E11).
In the subtropics, cyclonic streamfunction anomalies at 200-hPa extended
(Fig. T22). Over the North Pacific and
these circulation anomalies were consistent suppressed equatorial convection
near the date line, in association with a combination of the Pacific cold
episode and descending motion associated with the MJO.
The main surface temperature
departures during the month reflected warmer than normal conditions across
and the area around the Tibetan Plateau, and cooler than normal conditions
across large portions of the
, and western
(Fig. E1). The main precipitation departures
included above-average totals in eastern
, and below-average totals in the southwestern and central
(Figs. E3, E5, E6).
In the extratropics, the
500-hPa circulation pattern featured a pronounced north-south dipole of
height anomalies across the western and central North Pacific, which
reflected a strong positive phase (+2.5) of the West Pacific (WP)
teleconnection pattern (Table E1, Fig. E7).
In the lower latitudes, cyclonic streamfunction anomalies at 200-hPa
reflected a below-average strength of the mean subtropical ridge across the
western and central North Pacific, and an above average strength of the mean
trough over the eastern Pacific (Fig. T22).
These conditions were associated with an anomalously weak East Asian jet
stream, and with a westward retraction of that jet core to well west of the
date line (Fig. T21). Over the eastern
Pacific, the associated split-flow configuration resulted in an anomalous
flow of mild air into northern
, where temperatures were 2°-4°C above average during the month (Fig. E1).
, the dominant circulation feature was an amplified trough over the eastern
. The enhanced northwesterly flow upstream of this trough contributed to an
anomalous southward displacement of the mean surface cold frontal boundary,
which led to slightly below-average temperatures from the
to the eastern
. Also, anomalous descending motion upstream of the mean trough axis,
combined with reduced storminess, led to below-average precipitation across
the western and central states (Figs. E3).
The most significant
precipitation anomalies occurred in the drought-stricken southwestern
and southern Plains states, where February totals were generally less
than 25% of normal (Fig. E6). In the
Southwest, area-average totals were below the 10th percentile of
occurrences for the sixth consecutive month (Fig. E5).
For the entire Plains region, area-average totals have also been below
normal for the past six months, with the largest deficits occurring in the
southern Plains. In contrast, the below average totals in the
during February signaled a reprieve from exceptionally heavy totals that had
affected the region since October 2005.
The 500-hPa circulation pattern during February featured a persistent
pattern of above-average heights over
, and generally below-average heights across the central
(Fig. E9). A pronounced split flow was also
evident over the eastern
. The northern branch of the jet stream headed north of
and then strongly southward into western
(Fig. E10). The southern branch of the jet
stream was situated over northern
. This circulation represented a nearly complete disappearance of the normal
flow of marine air into
, and resulted in below-average temperatures throughout
2. Southern Hemisphere
The mean 500-hPa circulation pattern during February
featured above-average heights south of
, and an extensive area of below-average heights across the
(Fig. E15). Over
, the mean upper-level circulation featured a ridge in the west and a trough
just off the east coast (Fig. T22).
Anomalously cool (Fig. E1) and wet (Fig. E3)
conditions were observed in
in the area upstream of the ridge axis, and anomalously warm and dry
conditions were observed across the eastern half of
in the area between the ridge and downstream trough axis.
the rainy season normally lasts from October to April. Area-averaged
rainfall was above normal during February, marking five consecutive months
of above-average rains (Fig. E4). An
enhanced South African rainy season is consistent with ongoing La Niņa