The 500-hPa circulation during March 2004 featured above-average heights
in the lower and middle latitudes (Figs. E10,
E12), and below-average heights over
northern Canada and central Siberia. Over the eastern North Pacific
anomalous cyclonic streamfunction anomalies were observed in the subtropics
of both hemispheres (Fig. T22). This
inter-hemispheric symmetry is consistent with the enhanced convection over
the western Pacific and lack of convection over the eastern Pacific. In
contrast during February anticyclonic circulation anomalies were observed
across nearly the entire subtropics in both hemispheres. These
month-to-month fluctuations in the subtropical circulation are consistent
with the ongoing MJO activity.
Prominent temperature departures during March included warmer than
average conditions across Alaska and Canada, nearly all of Europe, Asia,
and China, and a continuation of warmer than average SSTs over large
portions of the North Atlantic (Fig. E1).
a. Pacific/North America
The upper-level circulation during March featured a persistent
north-south dipole of streamfunction anomalies over the eastern North
Pacific and western North America. This pattern reflected anticyclonic
streamfunction anomalies in the middle latitudes and cyclonic
streamfunction anomalies in the subtropics. These anomalies projected onto
the strong positive phase (+2.2) of the East Pacific teleconnection pattern
(Table E1, Figs. E7, E8).
This anomalous circulation was associated with a complete disappearance of
the East Asian jet core east of the date line, and with a northward shift
of the jet axis into southwestern Canada (Fig. T21).
The resulting enhanced westerlies across western and central Canada (Fig.
E11) contributed to above-average surface temperatures across the
region, and also brought enhanced precipitation to western Canada and the
Inter-Mountain region of the United States (Fig. E3).
The Inter-Mountain region has experienced above-average precipitation in
four of the last five months (Fig. E5) and
also recorded above-average totals during the 2002-03 winter. This region
had previously experienced large precipitation deficits during the
prolonged 1998-2001 Pacific cold episode. Southern California also
experienced above-average precipitation during March, and has received
rainfall surpluses in three of the last four months. Farther east
below-average precipitation was observed during March from the mid-western
U.S. to New England. Monthly totals were generally below the 30th
percentile of occurrences in these areas, with isolated regions reporting
totals below the 10th percentile.
The circulation across Asia featured a zonally-elongated pattern of
500-hPa height anomalies, with above-average heights in the subtropics and
middle latitudes, and below-average heights at high latitudes. This
anomalous circulation was associated with enhanced upper-level westerlies
north of the Tibetan Plateau, and reduced westerlies south of the Plateau (Fig.
T21).This circulation contributed to significantly above-average
surface temperatures (+3 to +5 C) from the Caspian Sea to eastern China and
Mongolia, with temperatures in most places exceeding the 90th
percentile of occurrences. Much of central Russia and China also
experienced well above-average surface temperatures during February.
2. Southern Hemisphere
In the Southern Hemisphere the 500-hPa circulation during Maech featured
an anomalous zonal wave-3 pattern, with below-average heights south of the
three continents and above-average heights over the central ocean basins (Fig.
E16). This circulation was associated with enhanced rainfall across
southern Australia and southern Africa (Fig. E3).
In southern Africa the rainy season normally lasts from October to
March. Area-averaged rainfall during March was well above-average, with
totals generally above the 90th percentile of occurrences (Fig.
E3). The enhanced rainfall and associated axis of upper-level
divergence (Fig. T23) was situated in the
area of ascending motion between the anomalous upper-level trough and ridge
axes (Figs. T21, T22).
Overall, the 2003-2004 rainy season featured below-average rains during
November and December, and above-average rains during January-March.