Extratropical Highlights – December 2013
1. Northern Hemisphere
The 500-hPa circulation during December
exhibited a 4-celled anomaly pattern extending from the western North Pacific
to Europe. This pattern featured above-average heights across the high
latitudes of the North Pacific, the Gulf of Alaska, the central North Atlantic,
and Europe (Figs. E9,
E11). It also featured below-average heights
across the western North Pacific, eastern Canada, and the high latitudes of the
North Atlantic. These anomalies projected strongly onto several main
teleconnection patterns. Over the Pacific and North America sectors, the
dominant teleconnections included the positive phase of the Tropical/ Northern
Hemisphere pattern (+1.8), and the negative phases of both the West Pacific (-2.1)
and Pacific/ North American pattern (PNA, -1.2) (Table E1, Fig. E7).
Over the North Atlantic, the dominant teleconnections included the positive
phases of the North Atlantic Oscillation (NAO, +0.8) and East Atlantic pattern
The main land-surface temperature signals
during December included well above-average temperatures across Scandinavia and
central Asia, and below-average temperatures in much of Canada (Fig. E1). The
main precipitation signals included above-average totals in the eastern U.S.,
and below-average totals in the northwestern U.S., Europe, and western Russia (Fig. E3).
a. North Pacific/ North America
The mean 500-hPa circulation during
December featured a strong height anomaly pattern with above-average heights
across the high latitudes of the North Pacific and Gulf of Alaska, and
below-average heights over the western North Pacific and eastern Canada (Fig. E9). This
pattern projected onto the positive phase of the Tropical/ Northern Hemisphere
pattern (+1.8), and the negative phases of both the West Pacific (-2.1) and the
Pacific/ North American pattern (PNA, -1.2) (Table E1, Fig. E7).
These patterns were associated with
an amplified exit region of the East Asian jet stream (Fig. T21), a marked weakening
of the Aleutian Low, a strong and persistent ridge extending northward from the
Gulf of Alaska to the polar region, and a strengthening of the Hudson Bay
trough. This marks the second month in which the Aleutian low was weak or
absent, and the Hudson Bay trough was enhanced.
The resulting amplified
ridge-trough pattern across North America brought strong northwesterly flow and
below-average temperatures to much of Canada and portions of the western and
central U.S. (Fig. E1).
In the western U.S., strong sinking
motion and below-average precipitation were observed downstream of the mean
ridge axis. The Pacific Northwest recorded totals in the lowest 10th
percentile of occurrences (Figs. E3, E5), marking the third
straight month of below-average precipitation in that region. Below-average
totals in California and Nevada contributed to a worsening of drought
conditions, with large areas in both states experiencing extreme or severe
drought during the month.
In contrast, enhanced precipitation
was recorded in the eastern U.S. in the area downstream of the mean trough
axis. Much of the eastern seaboard recorded totals in the upper 70th
percentile of occurrences.
The U.S. Midwest and Plains states
have recorded near- or above-average precipitation during the past four months
This has helped to considerably lessen the severity of ongoing drought
conditions. However, the U.S. Drought Monitor indicates that moderate or severe
drought still persisted in portions of Iowa, Minnesota, Illinois, and northern
Missouri. Moderate or severe drought
persisted across much of the southwestern quadrant of the U.S., including the
Plains states from Texas to northern Nebraska.
b. North Atlantic/ Eurasia
The mean 500-hPa circulation
featured above-average heights across the central North Atlantic and Europe,
and below-average heights across the high latitudes of the North Atlantic (Fig. E9). This
pattern projected onto the positive phase of the North Atlantic Oscillation
(NAO, +0.8) and the East Atlantic pattern (+1.2) (Table E1, Fig. E7).
This circulation was associated
with an enhanced North Atlantic jet stream extending from the eastern U.S. to
Great Britain and then northward into Scandinavia (Fig. E10). This southwesterly flow
brought milder Atlantic air and well above-average surface temperatures across Scandinavia
and northern Russia (Fig. E1). Much of central and eastern Russia and Siberia
recorded departures exceeding +5oC for the second straight month.
This exceptional warmth has led to a slower-than average build-up of the winter
snowpack, especially along its southern boundary.
In Europe, an amplified mid-level
ridge and its associated strong sinking motion again brought well below-average
precipitation to the region (Figs. E1). Area-averaged totals in northern Europe were in
the lowest 20th percentile of occurrences during December, while
totals in southern Europe were in the lowest 5th percentile of
occurrences (Fig. E4).
December marks the sixth consecutive month of below-average precipitation for
both northern and southern Europe (Fig. E4).
2. Southern Hemisphere
The mean 500-hPa circulation during
December featured above-average heights over southern South America and
below-average heights over the high latitudes of the South Atlantic (Fig. E15). At
200-hPa, the circulation featured an enhanced tough to the immediate east of
Australia (Fig. T22).
Anomalous sinking motion upstream
of the amplified trough contributed to below-average precipitation across eastern
Australia, with many areas recording totals in the lowest 10th
percentile of occurrences (Fig. E3). Northeastern Australia has recorded
below-average precipitation in seven of the last eight months (Fig. E4).
In central Argentina, exceptionally
hot and dry conditions were observed during December, with temperature
departures exceeding the 90th percentile of occurrences (Fig. E1) and
precipitation totals below the 10th percentile of occurrences (Fig. E3).
These dry conditions coincided with anomalous sinking motion upstream of an
amplified trough axis (Fig. E9).
The South African rainy season
lasts from October to April. During December, much of the region recorded
totals in the upper 70th percentile of occurrences, with large
interior regions recording totals in the upper 90th percentile of
occurrences (Fig. E3).
For the entire South African monsoon region, area-averaged totals were in the
upper 80th percentile of occurrences (Fig. E4).
To date the 2013-14 rainy season has produced above-average totals
during October and December, and below-average totals during November.