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 2008

1. Northern Hemisphere

The 500-hPa height pattern during December 2008 featured positive anomalies across the high latitudes of the North Pacific Ocean , across the North Atlantic Ocean , and Scandinavia / western Russia , and negative anomalies over Canada , southern Europe , and eastern Siberia (Fig. E9). In the Pacific/ North America region, these anomalies partly reflect La Niña. The subtropical circulation at 200-hPa was also consistent with La Niña, with enhanced mid-Pacific troughs in both hemispheres flanking the suppressed convection over the central equatorial Pacific, and enhanced ridges over the western Pacific flanking the region of enhanced equatorial convection (Fig. T22). Associated with this pattern, the East Asian jet stream was notably weaker than average east of the date line, and the jet core was retracted westward toward the western Pacific (Fig. T21).

The pattern of land surface temperature anomalies during December closely resembled the mid-level circulation, with well above-average temperatures in western Russia and below-average temperatures across Canada (Fig. E1). The main precipitation signals during December included above average totals in the central and eastern U.S. and southern Europe , and below-average totals along the Canadian west coast and the U.S.Gulf Coast (Fig. E3).

a. North Pacific/ North America

The 500-hPa circulation during December reflected La Niña, with below average heights over the central subtropical Pacific Ocean and western North America , and above average heights over the high latitudes of the central North Pacific (Fig. E9). These anomalies reflected westward shifts in several main circulation features, including the exit region of the East Asian jet stream, the mean ridge normally over western North America , and the mean trough axis normally over the eastern United States . These conditions were consistent with the La Niña-related westward retraction of deep tropical convection toward Indonesia .

In North America , strong northerly flow resulted in below average temperatures across Canada and the northern tier of the western United States , with departures in many areas in the lowest 30^th percentile of occurrences (Fig. E1). The precipitation pattern (Fig. E3) also reflected the upper-air circulation, with above average totals across the central and eastern U.S. coinciding with the area downstream of the mean upper-level trough and also with the right entrance region of the anomalous North Atlantic jet stream (Fig. T21). In contrast, the Gulf Coast region of the U.S. recorded below average totals during December. The largest departures were recorded in Texas where totals were in the lowest 10^th percentile of occurrences. The Gulf Coast region typically receives below average wintertime precipitation during La Niña, as was also observed in 2008 (Fig. E5).

b. Eurasia

The 500-hPa circulation featured positive height anomalies across the central North Atlantic and western Russia , and negative height anomalies over southern Europe (Fig. E9). This overall pattern was associated with a significant poleward transport of heat into northern Europe , and with a pronounced northward shift of the mean jet stream across Scandinavia and Russia . These conditions contributed to a continuation (3 months) of well above-average temperatures across western Russia , and to a slower-than-normal development of the regional snow pack. This reduced snow cover likely contributed further to the anomalous warmth during the last three months.

Farther south, southern Europe recorded above average precipitation during December in association with a large-scale trough across the region. The largest surpluses were observed in the southern Lee of the Alps , where several major winter storms developed downstream of the mean trough axis.

  2. Southern Hemisphere

The 500-hPa height field during December featured an anomalous zonal wave-3 pattern, with positive anomalies across the central ocean basins and negative anomalies across the high latitudes of the North Pacific and southeastern Australia (Fig. E15). This anomaly pattern was also evident in November, and is generally consistent with La Niña.

In southern Africa , the rainy season extends from October through April. During December, rainfall was above average for the region as a whole, with area average totals reaching the 90^th percentile of occurrences (Fig. E4). The most significant surpluses were recorded in the East, which helped to reduce early-season deficits in central Mozambique . For the entire monsoon region, above average totals during both November and December were consistent with La Niña.