1. Northern Hemisphere
circulation during December featured above-average heights in the polar region
and below average heights in the middle latitudes (Fig.
E9). Regional aspects of this pattern included
strong ridges in western North America and Greenland, and amplified troughs in
central North America, across the central North Atlantic and central Europe, and
central Siberia. This overall pattern reflected the strongest negative phase
(-2.4) of the Arctic Oscillation (AO) for any December since 1950 (Fig.
A2.1). Over the Atlantic sector this pattern
produced the strongest negative phase (tied with 1963) of the North Atlantic
Oscillation (-1.9) since 1950.
Over the central
and subtropical Pacific Ocean, the circulation also reflected a strong El Niņo
influence (Fig. T18).
Typical El Niņo-related features during December included anticyclonic
circulation anomalies over the central and eastern subtropical Pacific, and
cyclonic circulation anomalies (Fig. T22)
and below average heights (Fig. E9)
across the central extratropical North Pacific. These
conditions were associated with an eastward extension of the East Asian jet
stream, an eastward shift of the jet exit region to well east of the date line,
and a southward shift of the jet axis toward the southwestern U.S. (Fig.
temperature signals during December included above average temperatures in
Alaska, and well below average temperatures in western North America and central
Siberia (Fig. E1). The main precipitation signals
included above-average totals across the southern tier of the United States and
southern Europe, and below-average totals in much of Canada (Fig. E3).
a. North Pacific/
A strong El Niņo
and a record negative phase of the AO both influenced conditions across the
North Pacific Ocean and North America during December.
El Niņo is well known to influence the structure and location of the East Asian
jet stream, which in turn affects the circulation, temperature, and
precipitation patterns across the extratropical North Pacific and North America.
Normally, the core of the East Asian jet stream is located well west of the date
line, and the jet exit region is centered near the date line. During El Niņo,
enhanced convection over the central equatorial Pacific (Fig.
T25) acts to strengthen and extend
eastward the subtropical ridge (Fig. T22),
resulting in 1) an eastward extension of the East Asian jet core, 2) a shift of
the jet exit region toward the eastern Pacific, and 3) a southward shift of the
jet axis toward the southwestern U.S. (Fig.
T21). Also, an anomalous jet entrance
region is produced in the vicinity of the date line, and the resulting
ageostrophic circulation leads to below average heights and anomalous cyclonic
vorticity north of the jet core (Figs. E9, T22).
typically produce a more zonal flow across North America and a weaker Hudson Bay
trough, both of which lead to above average temperatures in Canada and the
northern United States. However, this signal was absent during December, largely
due to the record negative phase of the AO.
This AO was
associated with an amplified ridge in western North America, and with a westward
shift of the mean Hudson Bay trough axis to the central United States. The
enhanced meridional flow led to well below average temperatures across the
western half of North America (Fig. E1).
It also contributed to a continuation of above average precipitation across the
eastern United States (Fig. E5),
with some areas recording totals in the upper 90th percentile of
occurrences (Fig. E2).
circulation during December reflected a strong negative phase of the NAO, which
occurred in association with the hemispheric-scale AO. This pattern featured
positive height anomalies across Greenland and the polar region, and negative
height anomalies across the central North Atlantic and central Europe (Fig.
E9). These conditions reflected a strong ridge at
high latitudes, and a pronounced southward shift of the mean jet axis to
southern Europe (Fig. T21).
A main impact from these conditions was strong northerly flow and well below
average temperatures (exceeding -5°C) in central Siberia, with departures
throughout the region in the lowest 10th percentile of occurrences (Fig. E1). Another impact was increased
storminess and above average precipitation across southern Europe and
southwestern Russia (Fig. E3).
2. Southern Hemisphere
The 500-hPa circulation during December
featured generally below average heights at high latitudes and an anomalous
zonal wave-3 pattern in the middle latitudes (Fig.
E15). Regional aspects of this pattern include
above average heights over the central South Pacific and South Atlantic Oceans
and in the region south of Australia. The main temperature anomalies during the
month included warmer than average conditions in southern Africa, southeastern
Brazil, and large portions of coastal Australia (Fig.
E1). The main precipitation signals reflected
above average totals in central and southeastern South America and below average
totals in southeastern Africa (Fig. E3).
The South African rainy season lasts from
October to April. For the 2009-10 season, precipitation was near-average during
October, above average during November, and below average during December (Fig.
E4). Rainfall is typically below average in this
region during El Niņo.