Extratropical Highlights
– February 2015
1. Northern Hemisphere
The mean 500-hPa circulation during
February featured an extensive anomalous wave pattern extending from the
western North Pacific to western Asia (Fig.
E9). Regional aspects of this pattern included
above-average heights across the high latitudes of the North Pacific, western
North America, the central North Atlantic, and western Russia. Below-average
heights were observed over the central North Pacific, eastern North America,
Greenland, and southern Europe.
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 appear to be linked to enhanced convection over the central
equatorial Pacific (Fig. T25).
The main land-surface temperature signals
during February included above-average temperatures across Alaska, western
North America, Scandinavia, and most of Asia, and below-average temperatures
across eastern North America (Fig. E1). The main precipitation signals included above-average
totals in the eastern U.S. and southern Europe, and below-average totals in Southern
California, the Gulf Coast and Midwestern regions of the U.S., and northeastern
Europe (Fig. E3).
a. North Pacific/ North America
The mean 500-hPa circulation during
February featured a north-south dipole pattern of height anomalies over the
North Pacific, and an anomalous ridge-trough pattern across North America (Fig. E9). Over
the North Pacific, these conditions were associated with an eastward extension
of the east Asian Jet (Fig. T21), which was linked to an eastward extension of the mean
subtropical ridge to well east of the date line (Fig. T22). These conditions appear to be
coupled to enhanced convection over the central equatorial Pacific (Fig. T25).
Over North America, the 500-hPa
circulation featured an amplified ridge in the west and an amplified trough in
the east. This pattern was associated with above average surface temperatures
across western North America and Alaska, with the largest departures (exceeding
+4oC) observed in western Alaska and the west-central U.S. (Fig. E1). The
pattern was also associated with exceptionally cold surface temperatures across
eastern North America, with much of the eastern half of the U.S. recording
departures in the lowest 10th percentile of occurrences.
Monthly precipitation totals in
North America (Fig. E3)
were above-average in the mid-Atlantic and northeastern regions of the U.S., locations
situated downstream of the mean upper-level trough axis. In contrast, monthly
totals were below average in southern California and portion of the U.S. Gulf
Coast.
Consistent with these conditions, the
U.S. Drought Monitor indicated a continuation of exceptional drought in central/
southern California and western Nevada, with extreme drought extending
northward into Oregon and southwestern Idaho. Exceptional or extreme drought
also continued in north-central Texas and large portions of western Oklahoma.
b. Eurasia
The 500-hPa circulation featured an
extensive north-south dipole pattern of height anomalies across the North
Atlantic and western Russia, with above-average heights in the middle latitudes
and below-average heights extending from eastern Canada to central Siberia (Fig. E9). This
pattern was associated with an extensive southwesterly flow of milder air into northern
Europe for a second straight month (Fig.
E10). This warmth contributed to well
above-average surface temperatures across all of central Asia, with much of the
region recording monthly departures exceeding 5oC (Fig. E1).
2. Southern Hemisphere
The mean 500-hPa circulation during
February featured an anomalous zonal wave-3 pattern, with above-average heights
south of Africa and eastern Australia and also over the high latitudes of the
eastern South Pacific (Fig. E15). Below-average heights were observed over much
of Antarctica. At 200-hPa, cyclonic streamfunction anomalies
were observed east of Australia and across southern Africa (Fig. T22). Over Australia, the
resulting amplified wave pattern contributed to an extensive area of descending
motion and suppressed precipitation across the eastern half of Australia (Fig. E3).
The South African rainy season
lasts from October to April. Rainfall during February was below average for the
region as a whole (Fig. E4), with
the main deficits recorded across southern Africa (Fig. E3). The suppressed rainfall appears to be linked to
an anomalous upper-level trough centered over and west of southern Africa (Fig. T22).
To date, the South African rainy season was below average during October- November
and January- February. Rainfall was
above average in December (Fig. E4).