1. Northern Hemisphere
500-hPa circulation during April featured above-average heights in the polar
region, across the central North Pacific, and over Europe and northern Africa,
and below average heights in the Gulf of Alaska, the western U.S., and northern
China/ Mongolia (Fig. E9). These anomalies projected
strongly onto several teleconnection patterns, including the negative phases of
the North Atlantic Oscillation (NAO) and the East Pacific/Central Pacific
(EP-NP) pattern, and the positive phases of the East Atlantic (EA), West Pacific
(WP), and Pacific/North American (PNA) patterns (Table
E1, Fig. E7). Each of
these signals has been exceptionally persistent during the past several months,
with the NAO, EA, and PNA patterns prevailing since July 2009. The negative
EP-NP pattern has been present since November 2009, and the positive WP pattern
has been present since January 2010.
main temperature signals during April were consistent with the upper-level
circulation, and included above average temperatures in Canada, the northeastern
U.S., and Europe, and below average temperatures in China and Mongolia (Fig. E1). The main precipitation signals
during the month included above-average totals in the western U.S., the Plains
states, and eastern China, and below average totals across the eastern half of
the U.S., most of Europe, and western Russia (Fig. E3).
a. North Pacific/
extratropical circulation during April featured a continuation of above average
heights at 500-hPa over the central North Pacific Ocean and Canada, and below
average heights over the high latitudes of the North Pacific, and over the
eastern North Pacific/ western U.S. (Fig.
anomaly pattern projected onto three leading modes of variability, which
included the positive phases of both the PNA and WP patterns, and the negative
phase of the EP-NP pattern. These patterns have been exceptionally persistent
for the past several months, with the positive PNA pattern prevailing since July
2009 in association with the ongoing El Niņo. Over
North America, aspects of the PNA pattern during April (combined with the
negative NAO) included a reversal in the normal ridge-trough positions,
including a disappearance of the mean Hudson Bay trough.
conditions again contributed to above average temperatures in Canada, with large
portions of the country recording departures in the upper 90th
percentile of occurrences (Fig. E1).
They also contributed to a continuation of below average temperatures along the
U.S. Gulf Coast and in the Gulf of Mexico, where SST departures remained in the
lowest 10th percentile of occurrences.
anomalous ridge-trough configuration also contributed to a pronounced east-west
dipole of precipitation anomalies in the U.S., with above average totals in the
west and below average totals in the east (Fig.
E3). Area-averaged totals in the Pacific
Northwest, Southern California, and Inter-Mountain regions were in the upper 90th
percentile of occurrences (Fig. E5), while area-averaged totals in
the mid-Atlantic and Northeast states were in the lowest 10th
percentile of occurrences.
April, conditions across the North Atlantic and Europe were associated with a
2-celled pattern of 500-hPa height anomalies (Fig.
E9), and with a 3-celled pattern of 200-hPa
streamfunction anomalies (Fig. T22). Over the North
Atlantic, these anomalies reflected enhanced ridges at high latitude and in the
subtropics, and a broad trough in the middle latitudes. This circulation
projected strongly onto the negative phase of the NAO and the positive phase of
the EA pattern (Table E1, Fig. E7).
Both of these patterns have persisted since July 2009.
April, regional aspects of these signals also included above average heights
over Europe, and an extensive southwesterly and southerly flow into Europe (Fig. E9). These conditions contributed to
above average temperatures across the continent, with portions of southern
Europe recording departures in the upper 90th percentile of
occurrences (Fig. E1). They also contributed to below
average precipitation across Europe and portions of western Russia, with
Southern (Northern) Europe recording area-averaged totals in the lowest 10th
(20th) percentile of occurrences (Fig.
related circulation feature that has been observed since February is a nearly
complete disappearance of the northeasterly trade winds that normally extend
southward from the eastern extratropical North Atlantic into the tropical
Atlantic. The result has been significantly reduced upwelling near the West
African coast, the near-absence of cold water advection into the tropical
Atlantic, and reduced oceanic heat flux into the atmosphere. These conditions
contributed to record warm SSTs in the tropical Atlantic during March and April,
with area-averaged departures during April reaching +1.3°C.
China and Mongolia
extratropical circulation during April featured a deep 500-hPa trough centered
over Mongolia and northern China (Fig. E9),
which brought below average temperatures to the region (Fig.
E1). Anomalous southerly flow and an enhanced jet
entrance region downstream of the trough axis (Fig.
T21) contributed to an axis of well
above average precipitation that extended across eastern China and southern
Japan (Fig. E3).
For China as a whole, area-averaged precipitation during April was in the upper
90th percentile of occurrences (Fig. E4).
2. Southern Hemisphere
The 500-hPa circulation during April featured above average heights in the area
south of Australia, and below average heights across the high latitudes of the
South Pacific and in the area southwest of South Africa (Fig.
E15). At 200-hPa, the circulation also featured
an enhanced ridge and anomalous anticyclonic circulation over tropical southern
Africa and Madagascar (Figs. T21,
In southern Africa,
the official rainy season lasts from October to April, and rainfall is often
below average during El Niņo. During April 2010, rainfall totals for the
monsoon region as a whole were in the upper 90th percentile of
occurrences (Fig. E4),
with the most significant surpluses found in Mozambique and interior
southeastern Africa (Fig. E3).
For the entire 2009-10 South African rainy season, precipitation was above
average in two months (November and April), generally near-average in four
months (October and January-March), and below average in one month (December).