Extratropical Highlights –December 2015
1. Northern Hemisphere
The 500-hPa circulation during December
featured large-amplitude height anomalies throughout the hemisphere (Fig. E9). Above-average
heights were present over the central North Pacific, eastern North America, central/
southern Europe, and Mongolia. Below-average heights were present over the high
latitudes of the North Pacific, the western U.S., the polar region, the high
latitudes of the North Atlantic, and central Russia. Over the North Atlantic
sector, the anomaly pattern projected onto the strong positive phase of the North
Atlantic Oscillation (NAO, +2.0) and the East Atlantic (+3.1) teleconnection
pattern (Table E1,
Fig. E7).
This is the strongest positive value of the EA pattern recorded for any month since
the records began in 1950
At 200-hPa, a significant El Niño
response continued in the streamfunction (Fig.
T22) and wind (Figs. T21) fields. This response featured anticyclonic anomalies over
the eastern half of the subtropical North and South Pacific, straddling the
region of enhanced convection (Fig. T25), along with cyclonic anomalies at higher latitudes of the Pacific
in both hemispheres. In the NH, this pattern was associated with an
eastward extension of the wintertime East Asian jet steam to well east of the
date line. This jet stream pattern represents major dynamical and kinematic
changes in the mid- and upper-level circulation during El Niño, and it also
represents a fundamental manner in which El Niño’s circulation impacts are
communicated downstream.
The main land-surface temperature signals
during December included well above-average temperatures across the eastern
half of North America, Europe, and most of central Asia (Fig. E1). The main precipitation signals
included above-average totals in the northwestern U.S., the eastern half of the
U.S., Great Britain, and China, and below-average totals across central and southern
Europe (Fig. E3).
a. North Pacific/ North America
At 500-hPa, the circulation during December
featured above-average heights across the central North Pacific Ocean and eastern
North America, and below-average heights over the high latitudes of the North
Pacific and the western U.S. (Fig. E9). At 200-hPa, the circulation featured
anticyclonic streamfunction anomalies across the subtropical eastern North
Pacific in association with El Niño, along with negative anomalies over the
high latitudes of the North Pacific (Fig.
T22). This pattern was associated
with an eastward extension of the wintertime East Asian jet steam to well east
of the date line. It was also associated with enhanced westerly winds
throughout the troposphere in the western U.S. (Figs. T20, T21). These conditions contributed to
increased storminess in the western U.S., which is typical of El Niño.
Over North America, the 500-hPa height
pattern reflected an amplified trough in the West and a disappearance of the
mean Hudson Bay trough in the East. At 850-hPa, this overall circulation pattern
contributed to 1) an anomalous flow of warm, moist air Pacific air into the
continent, 2) an anomalous southerly flow of warmer air from the Gulf of Mexico
into the eastern U.S., and 3) a lack of cold air transport from Canada to the
U.S. (Fig. T20).
These conditions resulted in anomalously
warm surface temperature across most of Canada and the eastern half of North
America, with most areas recording departures in excess of +5oC and in
the upper 90th percentile of occurrences (Fig. E1). They also contributed to anomalously
stormy and wet conditions in the northwestern U.S. and across the eastern half
of the U.S., with many areas recording precipitation totals in the upper 90th
percentile of occurrences (Fig. E3).
Overall, area-averaged
precipitation totals exceeded the 90th percentile of occurrences in
the Pacific Northwest, Great Plains, Midwest, Great Lakes, Southeast, and Ohio
Valley regions of the U. S. (Fig. E5). Excess precipitation caused extensive flooding
of the Mississippi River, but also removed any lingering drought conditions
from the Plains states. Also according to the U.S. Drought Monitor, drought
conditions were lessened in Washington, Oregon, and Idaho, while exceptional
drought continued across central California and western Nevada. Severe or extreme
drought persisted in south-central and eastern Oregon and in western Montana.
b. North Atlantic
The 500-hPa circulation during December
featured above-average heights across southern Europe and below-average heights
over the high latitudes of the North Atlantic and Greenland. This pattern
projected onto the strong positive phase of the NAO (+2.0) and the East
Atlantic (+3.1) teleconnection pattern (Table
E1, Fig. E7). This is the strongest positive value of the EA
pattern recorded for any month since 1950.
These conditions were associated with an
exceptionally strong southwesterly flow of jet stream winds and low-level winds
into Great Britain, along with a focused and northward-shifted storm track across
northern Europe. These conditions resulted in a north-south dipole pattern of
precipitation, with above-average totals in Great Britain and a continuation of
below-average totals across central and southern Europe (Figs. E3, E4).
They also resulted in above-average temperatures across Europe, with warm
anomalies extending eastward across central Asia (Fig. E1).
c.
China
Well above-average precipitation
was recorded in eastern China during December (Fig. E3), with totals reaching near-record levels (1971-present record)
for the second straight month (Fig. E4). During
December, the surplus precipitation was linked to a combination of
above-average heights at 500-hPa across Mongolia (Fig. E9) and an amplified trough over western China at 200-hPa (Fig. T22). This pattern was
associated with a northward shift in the exit region of the East Asian jet (Fig. T21), and with a corresponding
northward shift in the area of heavy rainfall typically found along the equatorward
flank of that jet exit region.
2. Southern Hemisphere
The mean 500-hPa circulation during
December featured above-average heights across southeastern Australia and the high
latitudes of the South Pacific, and below-average heights over the central South
Pacific (Fig. E15).
At 200-hPa, a significant El Niño response was evident in the streamfunction
field across the eastern half of the Pacific Ocean (Fig. T22). This response
featured anticyclonic anomalies over the subtropical North and South Pacific
straddling the region of enhanced convection (Fig. T25), along with cyclonic anomalies at higher latitudes of the Pacific
in both hemispheres. In the SH, this pattern was associated with 1) a
strengthening and eastward extension of the South Pacific jet stream to well
east of the date line, and 2) an eastward shift of that jet’s exit region (Fig. T21).
The main surface temperature signals
during December included above-average temperatures in southeastern Australia
and South Africa (Fig. E1). The main precipitation signals included
above-average totals in southeastern South America for the 3rd
straight month, along with a continuation of below-average precipitation across
South Africa (Fig. E4).
Both of these signals are consistent with El Niño.
The South African rainy season
lasts from October to April. During December 2015, exceptionally dry conditions
were recorded in southern Africa with many locations recording totals in the
lowest 10th percentile of occurrences (Fig. E3). These deficits follow significantly
below-average totals in both October and November (Fig. E4).
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