Extratropical Highlights –March 2016
1. Northern Hemisphere
The 500-hPa circulation during March
featured above-average heights across the subtropical North Pacific Ocean, the
eastern U.S., the high latitudes of the North Atlantic, and central/ eastern
Asia. The circulation also featured below-average heights across the eastern North
Pacific and southern Europe (Fig. E9).
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 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 March 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 northwestern, Midwest and Gulf Coast portion of the U.S., and southern
Europe (Fig. E3).
a. North Pacific/ North
At 500-hPa, the circulation during March
featured above-average heights across the subtropical North Pacific Ocean and eastern
North America, and below-average heights over the eastern 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 situated immediately
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 North America,
which contributed to increased storminess and above-average precipitation (in
the upper 90th percentile of occurrences) in the U.S. Pacific
Northwest (Figs. E3,
E5). It also contributed to a continuation of well
above-average surface temperatures across the continent. The most significant temperature
departures (in the upper 90th percentile of occurrences) were
observed in Alaska, western and central Canada, and the eastern half of the
U.S. (Fig. E1).
The circulation during March also
featured a weaker Hudson Bay trough that was shifted west of normal to the
central U.S. This trough was associated with above-average precipitation in the
Midwest and Gulf Coast regions of the U.S. (Fig. E3) with area-averaged totals exceeding the 90th
percentile of occurrences in both regions (Fig. E5).
According to the U.S. Drought
Monitor, exceptional drought continued across central California, while severe
drought persisted in western Nevada. Moderate drought persisted in eastern
Oregon, and expanded across New Mexico and Arizona.
The 500-hPa circulation during March
featured above-average heights over Scandinavia, and below-average heights over
southern Europe (Fig. E9). This pattern was associated with a pronounced split
flow upstream of Europe. This pattern produced increased storminess and
above-average precipitation across southern Europe (Fig. E3). It also produced an extensive
southwesterly flow of milder, marine air into northern Europe, which then
spread eastward into central Asia (Fig. E10). These conditions, combined with an early
snow-melt along the southern portion of the Asian snowpack, were associated
with a continuation of well above-average surface temperatures across central
and eastern Asia, with many locations recording departures above +4oC
and also above the 90th percentile of occurrences (Fig. E1).
2. Southern Hemisphere
The mean 500-hPa circulation during
March featured above-average heights in the middle latitudes and below-average
heights over Antarctica. At 200-hPa, the El Niño response was evident in the ongoing
pattern of anticyclonic streamfunction anomalies across the central subtropical
South Pacific Ocean (Fig. T22).
The main surface temperature
signals during March included significantly above-average temperatures in
eastern Africa, Madagascar, and southeastern Australia (Fig. E1).The main precipitation signals during
March included above-average totals in extreme southern South Africa and in
southwestern Australia (Fig. E3).
The South African rainy season
extends from October to April. During March, the area as a whole recorded
above-average precipitation (Fig. E4), with
surpluses observed mainly in extreme southern Africa. To date, the South
African rainy season has featured below-average precipitation from October-February,
and above-average precipitation in March. (Fig.
E4). A below-average rainy season is typical of