Extratropical Highlights

  Table of Indices  (Table 3)

  Global Surface Temperature  E1

  Temperature Anomalies (Land Only)  E2

  Global Precipitation  E3

  Regional Precip Estimates (a)  E4

  Regional Precip Estimates (b)  E5

  U.S. Precipitation  E6

  Northern Hemisphere

  Southern Hemisphere

  Stratosphere

  Appendix 2: Additional Figures

Extratropical Highlights

JANUARY 2010

1. Northern Hemisphere

The 500-hPa 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. T21).

The main 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. E3).

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).

This anomaly 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 States.

b. Europe

The 500-hPa 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 and precipitation.

  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.