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

OCTOBER 2008

1. Northern Hemisphere

The 500-hPa height pattern during October 2008 featured positive anomalies in the middle latitudes and negative anomalies at high latitudes (Fig. E9). The main areas of positive anomalies included the central North Pacific, much of North America , the eastern North Atlantic , and northern Asia . The main areas of negative anomalies included Alaska , the high latitudes of the North Atlantic , and the polar region. Surface temperatures during October were above average across northern Canada , much of Russia , and Siberia , and below-average in Alaska (Fig. E1). Rainfall was above average in the central U.S. and portions of northern Europe , and below average along the U.S. Gulf Coast, the Great Lakes Region, and portions of southern Europe (Fig. E3)

a. North Pacific/ North America

In the middle latitudes, the mean 500-hPa circulation during October featured above-average heights from the central North Pacific to the western United States (Fig. E9). At high latitudes the circulation featured a deep trough over Alaska , along with large positive height anomalies over central Canada in association with a disappearance of the mean Hudson Bay trough. This pattern was associated with well below-average temperatures over Alaska , and above-average temperatures in central and eastern Canada (Fig. E3).

b. Eurasia

The 500-hPa circulation featured above-average heights over the eastern North Atlantic , western Russia , and much of Siberia , and below-average heights across the high latitudes of the North Atlantic and Scandinavia (Fig. E9). This overall pattern was associated with a significant poleward transport of heat into northern Europe , and a pronounced northward shift of the mean jet stream across Scandinavia and Russia . This pattern contributed to well above-average temperatures from eastern Europe to eastern Siberia (Fig. E3). It also contributed to a slower-than-normal development of the Siberian snow pack, which likely further contributed to the anomalous warmth across northern Asia .

  2. Southern Hemisphere

The 500-hPa height field during October featured positive anomalies across the central South Pacific and southern Australia , and negative anomalies across the high latitudes of the North Pacific and much of Antarctica (Fig. E15). This pattern reflected an equatorward expansion of the circumpolar vortex. In the stratosphere, this expanded vortex (Fig. S8 middle) allowed for an extensive polar stratospheric cloud (PSC) cover to be maintained (Fig. S8 bottom). The PSC’s developed during the past few months in association with record low temperatures at both 2-hPa and 10 hPa (Fig. S4). These conditions led to an above average size of the Antarctic ozone hole (Fig. S8 top, Fig. S6 bottom), which reached a peak extent of 25 million square kilometers in mid-September.

Extratropical land surface temperatures were generally above average in all three continents during October (Fig. E1). The most significant anomalies were recorded in southeastern Australia in association with an anomalous upper-level ridge. That region also experienced well below-average precipitation during the month, with many areas recording totals in the lowest 10^th percentile of occurrences (Fig. E3).

In southern Africa , the rainy season normally extends from October through April. The 2008-09 rainy season started off slowly, with the lowest area-averaged totals for October recorded since 1979 (Figs. E4, E3). Rainfall in much of the monsoon region was in the lowest 30^th percentile of occurrences, with isolated areas recording totals in the lowest 10^th percentile of occurrences.