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

DECEMBER 2009

1. Northern Hemisphere

The 500-hPa 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 Niño influence (Fig. T18). Typical El Niño-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. T21).

The main 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/ North America

A strong El Niño and a record negative phase of the AO both influenced conditions across the North Pacific Ocean and North America during December. El Niño 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 Niño, 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).

These conditions 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 90^th percentile of occurrences (Fig. E2).

  b. Eurasia

The 500-hPa 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 10^th 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 Niño.