Extratropical Highlights –April 2016
1. Northern Hemisphere
The 500-hPa circulation during April
featured above-average heights across the subtropical North Pacific Ocean, the western
U.S., the high latitudes of the North Atlantic, and the Mediterranean Sea (Fig. E9). The
circulation also featured below-average heights across the high latitudes of
the North Pacific, eastern Canada, and the eastern North Atlantic.
At 200-hPa, a significant El Niño
response continued in the streamfunction (Fig.
T22) and wind (Figs. T21) fields. This response featured anticyclonic anomalies over
the central subtropical North and South Pacific straddling the region of
enhanced convection (Fig. T25), along with cyclonic anomalies at higher latitudes of the North and
South Pacific. This pattern was associated with an eastward extension of
the East Asian jet steam to well east of the date line. This jet stream pattern
represents major dynamical and kinematic changes in the mid- and upper-level
circulation during El Niño, and it also represents a fundamental manner in
which El Niño’s circulation impacts are communicated downstream.
The main land-surface temperature signals
during April included a continuation of above-average temperatures across much
of North America and central Asia (Fig. E1). The main precipitation signals included above-average
totals in the U.S. Plains and Gulf Coast states, southwestern Europe, and China
a. North Pacific/ North America
At 500-hPa, the circulation during April
featured above-average heights across the subtropical North Pacific Ocean and western
North America, and below-average heights over the high latitudes of the North
Pacific (Fig. E9).
At 200-hPa, the circulation featured anticyclonic streamfunction anomalies
across the subtropical central North Pacific in association with El Niño, along
with cyclonic anomalies over the high latitudes of the North Pacific (Fig. T22).
This pattern was associated with an eastward extension of the East Asian jet
steam to well east of the date line, along with an amplified jet exit region
upstream of the western U.S. (Fig. T21). This anomalous jet exit region is
indicated by the combination of southwesterly wind anomalies upstream of
California north of the mean jet axis and northwesterly/ northerly wind
anomalies upstream of Mexico south of the mean jet axis.
This overall circulation was
associated with an extensive onshore flow of marine air into Canada and Alaska,
and with increased storminess across the central and southern U.S. These
conditions were associated with well above-average surface temperatures in
Alaska and western Canada (Fig. E1), and
with well above-average precipitation in the U.S. Great Plains and Gulf
Coast states (Fig. E3).
Area-averaged precipitation totals in the Great Plains were the largest for
April in the 1979-present record. Area-averaged totals in the Gulf Coast region
were above the 80th percentile of occurrences for the second
straight month (Fig. E5).
According to the U.S. Drought
Monitor, exceptional or extreme drought continued across central California
during April, while extreme drought persisted in western Nevada. However, the
spatial coverage of these drought areas continued to decrease during April. Also,
moderate drought persisted in southeastern Oregon, and in the southern half of
both New Mexico and Arizona.
The 500-hPa circulation during April
featured above-average heights over central Siberia and a broad trough over
China (Fig. E9).
These conditions were associated with a continuation of well above-average
surface temperatures in central Asia (Fig.
E1). They were also associated with well
above-average precipitation in China, with much of central and southern China
recording totals above the 70th and 90th percentile of
occurrences (Fig. E3).
Above-average totals in China were the largest in the 1979-present record for
April (Fig. E4).
2. Southern Hemisphere
The mean 500-hPa circulation during
April featured above-average heights over the high latitudes of the eastern
South Pacific, eastern Australia and New Zealand, across the South Atlantic,
and over the eastern Indian Ocean (Fig. E15). At 200-hPa, the El Niño response was evident
in the ongoing pattern of anticyclonic streamfunction anomalies across the central
subtropical South Pacific Ocean, and with cyclonic anomalies over the central
South Pacific (Fig. T22).
The main surface temperature
signals during April included significantly above-average temperatures in southern
Africa, Madagascar, and eastern Australia (Fig.
E1). The main precipitation signals during April included
well above-average totals in southern South America, and below-average totals
in eastern and northern Australia (Fig. E3).
The South African rainy season
extends from October to April. During April, the area as a whole recorded near-normal
precipitation (Fig. E4).
Overall, the 2015-16 South African rainy season was below normal, which is
typical of El Niño. Below-average precipitation was recorded from October-February,
above-average precipitation was recorded in March, and near-normal
precipitation was recorded in April.