1. Northern Hemisphere
The mean 500-hPa circulation during
April featured above-average heights across the central North Pacific and
Europe, and below-average heights over the high latitudes of the North Pacific,
eastern Canada, the middle and high latitudes of the North Atlantic, and
western Russia (Fig. E9). At 200-hPa, anti-cyclonic streamfunction
anomalies in both hemispheres over the central subtropical Pacific reflected eastward
extensions of the mean subtropical ridges (Fig.
T22). These conditions have persisted
for two months and have links to El Niño (Fig.
The main land-surface temperature signals
during April included above-average temperatures across the eastern half of the
U.S., Alaska, Scandinavia, western Europe, and central
Siberia (Fig. E1).
The main precipitation signals included above-average totals in the
southeastern and eastern U.S., and below-average totals in the northwestern U.S.,
and southern Europe (Fig. E3).
a. North Pacific/ North America
Over the North Pacific, the mean
500-hPa circulation during April featured above-average heights in the middle
latitudes and below-average heights at high latitudes (Fig. E9). Over the central subtropical
Pacific, anti-cyclonic streamfunction anomalies at 200-hPa reflected El Niño (Fig. T22).
Over the U.S., a broad trough was
present across the west, with much of the southeastern U.S. situated in an area
of anomalous southwesterly flow downstream of the trough axis (Fig. E14). This
circulation contributed to above-average surface temperatures across the
eastern half of the U.S., with monthly departures generally above the 70th
percentile of occurrences (Fig. E1). Similar levels of anomalous warmth were
observed across most of Alaska.
The southeastern quadrant of the
U.S., in addition to above-average surface temperatures, recorded above-average
precipitation during April. Most areas recorded departures in the upper 70th
percentile of occurrences, while the western Gulf Coast region experienced
totals in the upper 90th percentile of occurrences (Fig. E3). Area-averaged
monthly rainfall totals in the Gulf Coast region were the largest in the
1979-2015 record, while those in the Southeast and Ohio Valley regions were in
the upper 90th percentile of occurrences (Fig. E5). In contrast, the Pacific Northwest
region of the U.S. recorded below-average precipitation for the second month,
and southern California recorded a continuation of below-average precipitation
for the 4th straight month.
The U.S. Drought Monitor indicated a
continuation of exceptional drought in central/ southern California and western
Nevada, with extreme drought extending northward into central Oregon and severe
drought extending northward into central Washington State. Also, areas of exceptional
or extreme drought persisted in north-central Texas and large portions of western
Oklahoma, while severe drought covered portions of southern Kansas.
The 500-hPa circulation featured above-average
heights across Europe, and below-average heights across the North Atlantic,
Greenland, and western Russia (Fig. E9). This pattern was associated with well
below-average precipitation in southern Europe (Fig. E3), where area-averaged totals were the lowest in the 1979-2015
record (Fig. E4). It was
also associated with above-average surface temperatures in central Siberia in
the region downstream of the mean trough axis. This area has recorded warmer
than average conditions for the past three months.
2. Southern Hemisphere
The mean 500-hPa circulation during
April featured an anomalous zonal wave-3 pattern, with above-average heights located
generally south of the three continents, and
below-average heights over southeastern Australia and Antarctica (Fig. E15).
At 200-hPa, anti-cyclonic
streamfunction anomalies over the central subtropical Pacific reflected El Niño
Anti-cyclonic streamfunction anomalies were also observed over western Australia and in the area west of South Africa (Fig. T22).
In Australia, the amplified ridge
was associated with an anomalous flow of air from the Great Australian Bight
into the central part of the continent (Fig.
T20). This flow resulted in below
average surface temperatures across the region (Fig. E1).
The South African rainy season
lasts from October to April. Rainfall during April was above average for the
region as a whole (Fig. E4), with
the main surpluses recorded in the northeast (Fig. E3). Overall, the 2015 South African rainy season featured
above average rainfall in December and April, and below average totals during
October- November and January- March (Fig.