Extratropical Highlights –February 2016
1. Northern Hemisphere
The 500-hPa circulation during February
featured above-average heights across the subtropical North Pacific Ocean,
western North America, the North Atlantic Ocean, and central Asia. The
circulation also featured below-average heights across the eastern North Pacific,
the eastern U.S., and northern Europe (Fig.
E9). This overall anomaly pattern projected strongly
onto the positive phase of the Pacific/ North American teleconnection pattern
(PNA, +1.9), which is a typical response to El Niño (Table E1, Fig. E7). The anomaly pattern also
projected onto the positive phases of the North Atlantic Oscillation (NAO,
+1.3) and the east Atlantic teleconnection pattern (EA, +1.6), and onto the negative
phase of the Polar/ Eurasia Pattern (-2.3).
At 200-hPa, a significant El Niño
response continued in the streamfunction (Fig. T22)
and wind (Fig. 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 February included above-average temperatures across much of North
America, eastern Europe, and central Asia (Fig. E1). The
main precipitation signals included above-average totals in the northeastern portion
of the U.S. and Europe (Fig. E3).
a. North Pacific/ North America
At 500-hPa, the circulation during February
featured above-average heights across the subtropical North Pacific Ocean and western
North America, and below-average heights over the eastern North Pacific and the
southeastern U.S. (Fig. E9). At 200-hPa, the circulation featured
anticyclonic streamfunction anomalies across the
subtropical central North Pacific in association with El Niño, along with negative
anomalies over the high latitudes of the North Pacific (Fig. T22). This pattern was
associated with an eastward extension of the wintertime East Asian jet steam to
well east of the date line (Fig. T21). This overall anomaly pattern
projected strongly onto the positive phase of the Pacific/ North American
teleconnection pattern (PNA, +1.9), which is a typical response to El Niño (Table E1, Fig. E7).
This overall circulation was
associated with an extensive onshore flow of marine air into North America,
resulting well above-average surface temperatures across the continent. The
most significant departures were observed in across Alaska and western Canada, where
departures were in excess of +5oC and in the upper 90th
percentile of occurrences (Fig. E1).
Farther east, negative height
anomalies at 500-hPa over eastern North America reflected an amplification of
the mean Hudson Bay trough. This feature was associated with above-average
precipitation in the northeastern portion of the U.S. (Fig. E3). Area-averaged totals exceeded the 80th
percentile of occurrences in the Great Lakes Region, the Ohio Valley Region, the
Mid-Atlantic Region and the Northeast Region (Fig. E5).
According to the U.S. Drought
Monitor, exceptional drought continued across central California and extreme
western Nevada, while severe drought persisted in southeastern Oregon, western
Montana and northwestern Utah.
b. North Atlantic
The 500-hPa circulation during February
featured above-average heights across the North Atlantic Ocean,
and below-average heights over northern Europe. These conditions were
associated with an exceptionally strong westerly flow of jet stream winds and
low-level winds into Europe for a second straight month. The result was
above-average precipitation across Europe (Fig.
E3),
along with well above-average surface temperatures across the western half of
Eurasia (Fig. E1). The
largest surface temperature anomalies were observed in central Asia, were
departures exceeded 5oC and were in the upper 90th
percentile of occurrences.
2. Southern Hemisphere
The mean 500-hPa circulation during
February featured above-average heights south of South Africa,
and below-average heights over the high latitudes of the South Pacific. At
200-hPa, a significant El Niño response was evident in the ongoing pattern of anticyclonic
streamfunction anomalies across the central subtropical
South Pacific Ocean (Fig. T22).
The main precipitation signals during
February included above-average totals across southern South America, and
below-average totals over much of eastern and western Australia and also over South Africa (Fig. E3).
Consistent with El Niño, below-average
precipitation was again observed during February in the South African monsoon
region (Fig. E4).
The most significant deficits were recorded in the East, which is where the
heaviest monsoon rainfall is normally recorded. To date, the South African
monsoon region has received well below-average precipitation in every month
from October-February (Fig. E4). The
South African rainy season extends from October to April.
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