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 2007

       1. Northern Hemisphere  

The 500-hPa heights during December 2007 featured positive anomalies over the central and eastern North Pacific, the eastern US, and Scandinavia , and negative anomalies over western Canada and the high latitudes of the North Atlantic Ocean (Fig. E9). The anomaly pattern from the central extratropical Pacific Ocean to eastern North America is consistent with La Niña. At 200-hPa the subtropical circulation across Australasia and the Pacific Ocean was also consistent with La Niña, with anticyclonic streamfunction anomalies flanking the region of enhanced convection over Indonesia and cyclonic streamfunction anomalies flanking the region of suppressed convection over the central equatorial Pacific (Fig. T22).

The main surface temperature departures during December reflected warmer than average conditions in Alaska , the eastern US, Scandinavia , and much of northern Asia , and isolated areas of below-average temperatures over the western US and central Russia (Fig. E1). The main precipitation anomalies included above average totals across the northwestern and northeastern US, and below-average totals over large portions of Europe and western Russia (Fig. E3).

a. North Pacific/ North America

The La Niña signal was prominent across the Pacific and North America during December. La Niña is associated with a westward retraction of deep tropical convection toward Indonesia and the eastern Indian Ocean , and a complete disappearance of tropical convection from the central equatorial Pacific (Fig. T25). These conditions result in a westward retraction of deep tropospheric heating, and hence a westward retraction of the subtropical ridge toward Indonesia (Fig. T22). Over the central equatorial Pacific, the reduction in convective heating contributes to an increased strength of the mid-Pacific trough.

The strength, structure, and position of the East Asian jet stream are strongly linked to conditions in the tropics and subtropics. For example, the jet core coincides with the strongest north-south height gradient at 200-hPa, which is heavily influenced by the height anomalies in the subtropics. The jet exit region coincides with the area of strong diffluence between the subtropical ridge and trough axes. Therefore, during La Nina the core of the East Asian jet stream is often retracted well westward toward Asia , as was seen during December (Fig. T21) and the heart of the jet exit region is shifted westward to well west of the date line.

The downstream circulation features are retracted westward as well. This includes a shift of the mean ridge from western North America to the central/eastern North Pacific, and a shift of the mean Hudson Bay trough to central or even western North America (Fig. E9). During December, these conditions were associated with above-average precipitation over much of the northern tier of the US (Fig. E6), and above-average temperatures in the southeast (Fig. E1).

b. North Atlantic and Europe

The circulation during December featured a north-south dipole pattern of 500-hPa height anomalies over the North Atlantic Ocean , with below-average heights centered over Greenland and above-average heights extending across the middle latitudes (Fig. E9). A strong blocking ridge was also evident over Scandinavia . This pattern was associated with a northward shift of the East Atlantic jet entrance region over eastern North America , which contributed to the above average temperatures in that region. It was also associated with a split-flow pattern over the eastern North Atlantic , with the northern branch of the jet stream affecting Scandinavia , and the southern branch affecting central and southern Europe .  

     2. Southern Hemisphere     

    The 500-hPa height field during December featured negative anomalies centered over Antarctica and positive anomalies spanning the middle latitudes (Fig. E15). In the subtropics, the pattern of negative (anticyclonic) 200-hPa streamfunction anomalies over the central Indian Ocean and positive (cyclonic) streamfunction anomalies over the central Pacific was consistent with La Niña.

    The South African rainy season extends from October to April, and is often stronger than average during La Niña. During December, well above-average totals covered much of the monsoon region (Fig. E3), and departures in many areas exceeded the 90^th percentile of occurrences. Area-averaged totals for the entire monsoon region were the largest since 1979, and were above average for the third straight month (Fig. E4).