Extratropical Highlights –January 2017
1. Northern Hemisphere
The 500-hPa circulation during January
featured above-average heights over the high latitudes of the eastern North
Pacific, eastern Canada, the high latitudes of the North Atlantic, and China,
and below-average heights over the western U.S., the polar region, and the
Mediterranean Sea (Fig. E9). In the subtropics, the 200-hPa circulation
featured a pronounced westward retraction of the mean subtropical ridge over
Australasia, along with amplified mid-Pacific troughs in both hemispheres (Fig. T22).
This pattern is consistent with the La Niña-related pattern of enhanced
convection over Indonesia and suppressed convection across the central
equatorial Pacific (Fig. T25).
The main land-surface temperature signals
during January included above-average temperatures across northern Europe, southeastern
Asia, and China and below-average temperatures in the northwestern U.S. and
central Russia (Fig. E1). The main precipitation signals included above-average
totals in both the western and central U.S., and below-average totals in northern
and western Europe (Fig. E3).
a. North America
The 500-hPa circulation during January
featured above-average heights over the Gulf of Alaska and eastern Canada, and below-average
heights in the western U.S. (Fig. E9). This pattern reflected a disappearance of the
climatological mean ridge over the western U.S., and a
weaker trough over the eastern U.S. These conditions were accompanied by an
anomalously zonal flow across the country (Figs.
E10, T22).
These conditions contributed to
below average surface temperatures in the northwestern U.S. (Fig. E1), and to increased storminess and
above-average precipitation in both the western and central U.S. (Fig. E3). Area-averaged precipitation totals
were near record (1971-2016) levels in both the Southern California and
Inter-Mountain regions during January (Fig.
E5), following well above-average precipitation in
both regions during December. The Pacific Northwest and Great Plains regions
also recorded above-average precipitation during January, with above-average
totals in the upper 80th percentile of occurrences.
According to the U.S. Drought
Monitor, the areal extent of long-term exceptional drought in California has
ended. However, by the end of January severe drought was still present along the southern half of coastal California. In the southern
Plains States, moderate or severe drought covered large portions of Oklahoma
and portions of western Kansas. In the southeastern U.S., moderate or severe
drought extended from central Alabama to eastern Tennessee and western North
Carolina. In the northeastern U.S., severe drought persisted in southern New
York, northern New Jersey and portions of New England, with extreme drought
evident in Connecticut.
b. Eurasia
The 500-hPa circulation during January
featured above-average heights across the high latitudes of the North Atlantic
and below-average heights over the Mediterranean Sea (Fig. E9). This pattern was associated
with a pronounced split-flow pattern over Europe, with anomalous southwesterly
flow of milder marine air into northern Europe and an anomalous northwesterly
flow into central and southern Europe. These conditions contributed to
above-average surface temperatures in Scandinavia (Fig. E1), and to below-average precipitation over large portions of Europe
(Fig. E3).
Over eastern Asia, the upper-level
circulation featured a broad trough over India and an amplified ridge over
eastern China (Fig. T22). This pattern contributed to well
above-average precipitation across southeastern Asia in the area between the
mean ridge and trough axes (Fig. E3). It also contributed to well above-average
surface temperatures in China, with much of the country recording departures in
the upper 90th percentile of occurrences (Fig. E1).
2. Southern Hemisphere
The mean 500-hPa circulation during
January featured above-average heights over the high latitudes of the central South
Pacific, eastern Australia, and the high latitudes of the Indian Ocean, and
below-average heights south of Australia and over the eastern Indian Ocean (Fig. E15). In
the subtropics, the 200-hPa circulation featured a pronounced westward
retraction of the mean subtropical ridge over Australasia, along with amplified
mid-Pacific troughs in both hemispheres (Fig.
T22). This pattern is consistent with
the La Niña-related pattern of enhanced convection over Indonesia and
suppressed convection across the central equatorial Pacific (Fig. T25).
In Australia, the overall
large-scale circulation pattern contributed to above-average precipitation in
the west in the area between the mean trough and ridge axes (Fig. E3). It
also contributed to above-average surface temperatures in the east beneath the
mean upper-level ridge (Fig. E1).
The South African monsoon season runs
from October to April. This area recorded well above-average precipitation
during January, with above-average totals near the 90th percentile
of occurrences (Figs. E3, E4). To date, the
region received below-average precipitation in October and near-average
precipitation during November and December. This region typically records
above-average precipitation during La Niña.