1. Northern Hemisphere
The 500-hPa
circulation pattern during March featured positive height anomalies across
the central North Pacific and the polar region, and negative anomalies over
the western
U.S.
, across the central
North Atlantic
, and across northern
Eurasia
(Figs. E9, E11).
In the subtropics, cyclonic 200-hPa streamfunction anomalies across the
eastern half of the North Pacific and anticyclonic anomalies across the
western North Pacific and southeastern
Asia
reflected the ongoing Pacific cold episode (La Niņa) (Figs. T22,
T25). These conditions were associated with a
pronounced westward retraction of the East Asian jet core, and a westward
shift of the associated jet exit region to well west of
Hawaii
. Farther downstream a pronounced westward shift of the mean upper-level
ridge and trough axes was also evident.
The main surface temperature
departures during March reflected warmer than normal conditions across the
eastern half of
Canada
and central
Asia
, and cooler than normal conditions across western
North America
, northern
Europe
, and western
Russia
(Fig. E1). The main precipitation departures
included above-average totals in
Hawaii
, central
California
, and western Europe, and below-average totals across the southeastern and
eastern
U.S.
(Figs. E3, E5, E6).
a. Pacific/
North America
The circulation across the
North Pacific strongly reflected the ongoing Pacific cold episode. In
particular, the westward retraction and confinement of deep tropical
convection to the western Pacific resulted in the combination of an
amplified subtropical ridge over the western Pacific and an amplified trough
across the central and eastern subtropical Pacific (Fig. T22).
Accompanying this pattern, the East Asian jet stream was retracted to well
west of the date line, and the associated jet exit region was shifted
westward to well west of
Hawaii
(Fig. T21). Also in the extratropics a
corresponding westward retraction of the mean downstream ridge and trough
positions at 500-hPa was also evident, with the mean ridge shifted from
western North America to the high latitudes central and eastern North
Pacific, and the mean trough shifted from Hudson Bay to western North
America (Fig. E9). This pattern reflected a
complete disappearance of the mean
Hudson Bay
trough.
The main surface temperature
departures reflected this anomalous circulation, with below-average
temperatures observed across western
North America
and above-average temperatures observed across the eastern half of
Canada
(Fig. E1). Precipitation was well above
average in the vicinity of
Hawaii
(Fig. E3), which was situated just
downstream of the mean upper-level subtropical trough axis and the area of
largest cyclonic streamfunction anomalies. Monthly precipitation surpluses
far exceeded 100 mm across the
Hawaiian Islands
, with the entire region recording totals above the 90th
percentile of occurrences. In contrast, the southeastern and eastern
U.S.
recorded below-average precipitation in association with descending motion
downstream of the mean ridge axis.
b.
North Atlantic
/
Eurasia
The 500-hPa circulation during March featured a persistent pattern of
above-average heights over
Greenland
, and generally below-average heights across the central
North Atlantic
(Fig. E9). This pattern reflected a negative
phase (-1.3) of the North Atlantic Oscillation (NAO) (Table E1,
Fig. E7). The area of negative height anomalies
also extended eastward across northern
Eurasia
. This pattern was associated with a strong low-latitude westerly flow into
central
Europe
, resulting in above-average precipitation in that region. It was also
associated with anomalously northerly flow of arctic air into
Scandinavia
and western
Russia
, where temperatures in many regions were below the 10th
percentile of occurrences. Over central Asia, a persistent upper-level ridge
situated south of the area of negative height anomalies contributed to
significantly above-average temperatures during the month, with many regions
recording temperatures that were more than 3°-5°C above average, and above
the 90th percentile of occurrences.
2. Southern Hemisphere
In the subtropics, the mean 200-hPa circulation during
March was consistent with ongoing La Niņa conditions, as indicated by an
enhanced ridge over
Australia
and a very strong trough across the central and eastern South Pacific (Fig.
T22). This pattern was associated with a large
area of positive extratropical height anomalies at 500-hpa across the
central South Pacific (Fig. E15) similar to
that observed in the Northern Hemisphere. The 500-hPa circulation pattern
also featured positive height anomalies over the central
South Atlantic
and negative anomalies south of both
Africa
and
South America
.
In southern
Africa
the rainy season normally lasts from October to April. Area-averaged
rainfall was above normal during March, marking five consecutive months of
above-average rains (Fig. E4). An enhanced
South African rainy season is consistent with ongoing La Niņa conditions.
In
Australia
, rainfall was above average in the north in association with La Niņa, and
below average in the south. Surface temperatures were generally above
average across eastern
Australia
, with values in many areas exceeding the 70th percentile of
occurrences.
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