1. Northern Hemisphere
circulation during January featured above-average heights across northern Canada
and the high latitudes of the North Atlantic, and below average heights from the
central North Pacific Ocean to southeastern Europe (Fig.
E9). This overall pattern reflected the
combination of El Niño and a strong negative phase (-1.1) of the North Atlantic
Oscillation (NAO) (Table E1, Fig. E7).
Regional aspects of this pattern include a deep trough over the Gulf of Alaska,
a significantly weaker Hudson Bay trough, high-latitude blocking over the
Atlantic sector, and strong wintertime jet streams shifted well south of normal
over both the eastern Atlantic and eastern Pacific regions (Fig.
temperature signals during January included above average temperatures across
Canada, northern Africa and southern Asia, and below average temperatures in
southeastern U.S. northern Europe, and central Russia (Fig.
E1). The main precipitation signals included
above-average totals in the southwestern U.S. and southern Europe, and well
below average totals in southern Alaska, Scandinavia, and northwestern Russia (Fig.
a. North Pacific/ North America
A strong El Niño
influenced the circulation during January across the North Pacific Ocean and
North America. Typical El Niño
impacts included a 4-celled pattern of 500-hPa height (Fig. E9) and 200-hPa streamfunction (Fig. T22) anomalies, with
above average heights over the subtropical North Pacific and eastern Canada, and
below average heights over the eastern North Pacific and the southeastern U.S. (Fig. E9).
pattern is consistent with El Niño’s impacts on the structure and location of
the East Asian jet stream. 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 Niño, convection is enhanced over the central equatorial
Pacific (Fig. T25),
which acts to strengthen and extend eastward the subtropical ridge across the
Pacific basin (Fig. T22).
As seen in January, these conditions lead to 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). As a result, the
Pacific storm track was shifted well south and east of normal, which contributed
to above average precipitation and increased storminess in the southwestern U.S.
and California (Fig. E3).
Downstream of major
jet streams, one normally sees a split-flow pattern with a broad ridge to the
north and trough to the south. During January, the pattern of positive height
anomalies over eastern Canada and negative height anomalies over the
southeastern U.S. is consistent with the observed dynamical changes in the East
Asian jet stream. These anomalies reflected a much weaker than normal Hudson Bay
trough over eastern Canada, and an amplified trough over the southeastern United
circulation during January also reflected a strong negative phase of the NAO (Table E1, Fig. E7),
as indicated by the combination positive height anomalies extending from eastern
Canada to northwestern Russia, and negative height anomalies extending from the
southeastern U.S. to southeastern Europe (Fig.
E9). These anomalies reflected a strong ridge from
Greenland to Scandinavia, and a deep trough across the central North Atlantic.
They were associated with a pronounced southward shift and zonal elongation of
the North Atlantic jet stream, with the main jet axis extending from the U.S.
Gulf Coast to northern Africa (Fig. T21). January marks the
second consecutive month with an NAO index below -1.0, and the third month in
the last four with negative index values exceeding that threshold.
During January, a
main impact from these conditions was pronounced north-south dipole patterns of
temperature (Fig. E1)
and precipitation (Fig. E3)
anomalies across the North Atlantic, Europe, northern Africa, and Russia.
Overall, the higher latitudes received below average temperatures and
precipitation, while the middle latitudes recorded above average temperatures
2. Southern Hemisphere
The 500-hPa circulation during January
featured an anomalous zonal wave-3 pattern, with above average heights located
generally over the central ocean basins and below average heights located
poleward of the three continents (Fig. E15).
The main temperature departures during the month included a continuation of
warmer than average conditions in southeastern Brazil, southeastern Africa, and
southern Australia (Fig. E1).
The main precipitation signals included above average totals in southeastern
South America and southern Africa (Fig. E3).
The South African rainy season lasts from
October to April. For the 2009-10 rainy season, precipitation was near-average
during October, above average during November and January, and below average
during December (Fig. E4).
Rainfall tends to below average in this region during El Niño.