Extratropical Highlights –February 2017
1. Northern Hemisphere
The 500-hPa circulation during February
featured a highly amplified anomalous wave pattern (Fig. E9), and continued to be strongly
influenced by La Niña and the Madden Julian Oscillation (MJO). Above-average
heights were present over the high latitudes of the North Pacific, the eastern
U.S., and large portions of the polar region, while below-average heights were
present over most of Canada, the western U.S., and southern Europe.
This anomaly pattern projected onto
several teleconnection patterns (Fig. E7, Table E1). It projected onto the positive phases of the
North Atlantic Oscillation (NAO, +1.3) and the Tropical/ Northern Hemisphere
pattern (TNH, +2.2). It also projected onto the negative phases of the Polar/
Eurasian pattern (-2.2), the east-Atlantic pattern (-1.4), and the east
Atlantic/ Western Russian pattern (-1.4).
At 200-hPa, the circulation across
the subtropical Pacific Ocean in both hemispheres reflected La Niña. The La
Niña signal included highly amplified troughs east of the date line in both
hemispheres, in association with the disappearance of deep tropical convection
from the central equatorial Pacific (Fig.
T25). The La Niña signal also
included a focusing of the subtropical ridges over Australasia (Fig. T22),
in association with enhanced convection over the western tropical Pacific,
Indonesia, and the eastern Indian Ocean.
The main land-surface temperature signals
during February included above-average temperatures across the southern and
eastern U.S. and in the polar region, and below-average temperatures in the western
Canada and western Europe (Fig. E1). The main precipitation signals
included above-average totals in the central U.S. and southern Europe (Fig. E3).
a. North Pacific and North America
The 500-hPa circulation during February
featured above-average heights over the high latitudes of the North Pacific,
the Gulf of Alaska and the southeastern U.S., and below-average heights over
Canada and the western U.S. (Fig. E9). This type of highly amplified wave pattern is typical
of La Niña.
La Niña produces an enhanced
subtropical ridge over southeastern Asia along with an amplified mid-Pacific
trough (Fig. T22). These conditions act to retract
westward the East Asian jet steam. This anomalous jet
structure, in turn, acts to retract westward the mean downstream ridge and
trough positions, resulting in the essence of the anomalous 500-hPa circulation
pattern seen during February. This anomalous wave pattern projected strongly
onto two teleconnection patterns which tend to be favored during La Niña winter;
the positive phase of the TNH pattern and the negative phase of the PNA pattern
(Fig. E7, Table E1).
This overall pattern contributed to
anomalously warm surface temperatures in the eastern U.S. and to anomalously
cool temperatures in western Canada (Fig.
E1). It also contributed to above-average
precipitation in the central U.S. (Fig. E3). Area-averages totals in the Great Plains, Great
Lakes, Midwest, and Ohio Valley regions in the upper 90th percentile
of occurrences (Fig. E5).
b. Polar region
The mean February 500-hPa
circulation during February featured an anomalous zonal wave-1 pattern, with above-average
heights north of Eurasia and below-average heights extending from central
Canada to Greenland (Fig. E9). This pattern appears to have reflected a
downward extension of a stratospheric anomaly pattern that was prominent up to
at least the 30-hPa level (Fig. S2).
At 500-hPa, this pattern projected
strongly onto the negative phase of the Polar/ Eurasian pattern (-2.2). The anomalous
southerly flow between the anomalous trough and the downstream anomalous ridge
resulted in a massive influx of relatively mild air (and significantly warmer
temperatures) into the polar region at levels extending from the surface (Fig. E8) to 500-hPa.
circulation across the central North Atlantic and Europe reflected an anomalous
zonal wave-4 pattern, with above-average heights over the west-central North
Atlantic and Scandinavia, and below-average heights over Greenland and southern
Europe (Fig E9).
This pattern reflected an amplified jet stream over the western Atlantic (Fig. E10), along with a pronounced jet
exit region and split-flow pattern across the eastern Atlantic and Europe.
The northern branch of this split flow brought a large influx of milder air
into the polar region. The southern branch contributed to increased storminess
and above-average precipitation across southern Europe (Fig. E3). This marked the first time in
more than a year that area-averaged precipitation was above average across
southern Europe (Fig. E4).
2. Southern Hemisphere
The mean 500-hPa circulation during
February featured above-average heights over the Indian Ocean and below-average
heights over the high latitudes of the South Pacific Ocean (Fig. E15). At
200-hPa, the subtropical circulation featured an amplified trough over the central
and eastern South Pacific Ocean, and ridge over western
Australia (Fig. T22). This anomalous subtropical
circulation is typical of La Niña.
The South African monsoon season
runs from October to April. This area recorded above-average precipitation
during February, with area-averaged totals in the upper 80th
percentile of occurrences (Fig. E4). The most significant surpluses were observed in
the northern and central portions of the monsoon region (Fig. E3), where they helped to alleviate significant rainfall deficits
which had accumulated over the last two months.