Extratropical Highlights –December 2016
1. Northern Hemisphere
The 500-hPa circulation during December
featured above-average heights across the central North Pacific, the southern
U.S., Europe, China, and below-average heights over Canada, the high latitudes
of the North Atlantic, and central Asia (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 December included above-average temperatures across southeastern Asia,
and below-average temperatures in the northwestern U.S. and in central Russia (Fig. E1). The
main precipitation signals included above-average totals in both the
northwestern and eastern U.S., and below-average totals in Europe (Fig. E3).
a. North America
The 500-hPa circulation during December
featured above-average heights over the central North Pacific and the southern
U.S., and below-average heights across Canada. (Fig. E9). These conditions contributed to below-average
surface temperature in the northwestern U.S. (Fig. E1), along with above-average precipitation in both the northwestern
and eastern U.S. (Fig. E3).
According to the U.S. Drought
Monitor, severe or extreme drought extended from central Louisiana to eastern
Kentucky and western North Carolina. Long-tern exceptional drought persisted in
central/ southern California. 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 December
featured above-average heights over Europe and China and an amplified trough
over central Russia (Fig. E9). This overall pattern was associated with
above-average surface temperatures in Scandinavia and much of southeastern
Asia, and below average surface temperatures in central Russia (Fig. T20).
It was also associated with well below-average
precipitation across Europe, with many locations recording totals in the lowest
10th percentile of occurrences (Fig.
E3).
2. Southern Hemisphere
The mean 500-hPa circulation during
December featured above-average heights over Antarctica, the high latitudes of
the eastern South Pacific, and eastern Australia, and below-average heights over
the South Atlantic and 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).