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

FEBRUARY 2010

1. Northern Hemisphere

The 500-hPa circulation during February featured an anomalous zonal wave-1 pattern with above-average heights at high latitudes and below average heights in the middle latitudes (Fig. E9). This overall pattern reflected the combination of El Niño and a record negative phase of the Arctic Oscillation (-4.4) (Fig. A2.1). Consistent with this AO signal, the second strongest (after 1978) negative phase of the North Atlantic Oscillation (NAO) for February was also recorded (-2.0) (Table E1, Fig. E7).

A similar combination of climate patterns has been in place since December 2009. During December-February (DJF) 2009-10, the AO index averaged -3.4 and the NAO index averaged -1.7, both of which are record values dating back to 1950.

During February 2010, regional manifestations of the combined El Niño/AO/NAO signals included a deep trough over the Gulf of Alaska, a significantly weaker than average Hudson Bay trough, high-latitude blocking over the Atlantic sector, and wintertime jet streams shifted well south of normal over both the Pacific and Atlantic Oceans (Fig. T21). Consistent with this anomalous circulation, the main temperature signals during February included above average temperatures across Canada and Alaska, and below average temperatures in the central and eastern U.S., Scandinavia, and much of northern Russia (Fig. E1). The main precipitation signals included above-average totals along the U.S. Gulf Coast region and southern Europe, and below average totals in southwestern Alaska and western Canada (Fig. E3).

a. North Pacific/ North America

Typical El Niño impacts during February included a 4-celled pattern of 500-hPa height anomalies, with above average heights over the subtropical North Pacific Ocean and eastern Canada, and below average heights over the eastern North Pacific and 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 February, 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 southern U.S. (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 February, the combination of positive height anomalies over eastern Canada and negative height anomalies over the southeastern U.S. is consistent with the El Niño-related changes in the East Asian jet stream. These anomalies reflect a weaker than normal Hudson Bay trough, and an amplified trough over the southeastern United States.

Across eastern North America the circulation anomalies during February were also associated with a record negative phase of the Arctic Oscillation (-4.4) (Fig. A2.1), and with a strong negative phase of the North Atlantic Oscillation (-2.0) (Table E1, Fig. E7). In combination with El Niño, these conditions contributed to above average temperatures across Canada and to below average temperatures across the eastern half of the U.S. They also contributed to three major snow storms along the eastern seaboard of the U.S., and to above average precipitation and increased precipitation across the southern part of the country.

b. Europe

During February, the AO/ NAO signal dominated the circulation across the North Atlantic Ocean and Europe (Table E1, Fig. E7). Specific aspects of this signal included an blocking ridge at high latitudes and a deep trough extending from the southeastern U.S. to southeastern Europe (Fig. E9). These conditions 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 southern Europe (Fig. T21). February marks the third consecutive month with an NAO index below -1.0, and DJF 2009-10 marks the largest negative NAO (and AO) index dating back to 1950.

During February 2010, these conditions were associated with 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 lower latitudes recorded above average temperatures and precipitation.

  2. Southern Hemisphere

The 500-hPa circulation during February featured above average heights over Antarctica, New Zealand, and the eastern South Pacific, and below average heights from South America to the eastern South Atlantic (Fig. E15). Regionally, an amplified trough over the eastern South Pacific contributed to cooler (Fig. E1) and wetter (Fig. E3) than average conditions across extratropical South America, with portions of southern Argentina recording temperature departures in the lowest 10^th percentile of occurrences and precipitation totals in the upper 90^th percentile of occurrences. In Australia, precipitation was generally well below average in the west and well above average in the southeast. This pattern is associated with an amplified trough centered in the western part of the continent.

In southern Africa, the rainy season lasts from October to April. During February 2010, totals were near average with slightly below average precipitation recorded in portions of Mozambique and above average precipitation recorded in extreme South Africa. The South African rainy season tends to below average during El Niño.