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

JANUARY 2009

1. Northern Hemisphere

The 500-hPa height pattern during January 2009 featured positive anomalies across the high latitudes of the North Pacific Ocean and the southern North Atlantic Ocean , and negative anomalies from the eastern U.S. to southern Europe (Fig. E9). This overall pattern reflected La Niña, a strong positive phase (+1.9) of the Tropical / Northern Hemisphere (TNH) teleconnection pattern partly in response to La Niña, and a strong positive phase (+1.6) of the East Atlantic (EA) teleconnection pattern (Table E1, Fig. E7).

The subtropical circulation at 200-hPa remained consistent with La Niña, with enhanced mid-Pacific troughs in both hemispheres flanking the suppressed convection over the central equatorial Pacific, and a westward retraction of the subtropical ridges to Australasia (Fig. T22). Associated with this pattern, the East Asian jet stream remained weaker than average east of the date line, as the jet core was retracted westward toward the western Pacific (Fig. T21).

The main temperature signals during January included above-average temperatures in the western U.S. and central Siberia , and below-average temperatures in portions of eastern North America and Europe (Fig. E1). The main precipitation signals included above-average totals in portions of western and southern Europe , and below-average totals along the North American west coast and the U.S. Gulf Coast (Fig. E3).

a. North Pacific/ North America

The 500-hPa circulation during January partly reflected La Niña, with westward shifts in several main circulation features, including the exit region of the East Asian jet stream and the mean ridge normally over western North America . The positive phase of the TNH teleconnection pattern was also present, as manifested by the exceptionally large anomalies over both the Gulf of Alaska and eastern North America (Fig. E9). This pattern was associated with anomalous northwesterly flow into central North America (Fig. T21)

These conditions brought below average temperatures to the northeastern U.S. and southeastern Canada , and above average temperatures to the western U.S. (Fig. E1). Below average precipitation was observed in western North America in the vicinity of the mean ridge axis, and across the Gulf Coast upstream of and within the mean trough axis (Fig. E3). The Gulf Coast region typically receives below average wintertime precipitation during La Niña, as seen during the last three months (Fig. E5). Some of the most significant precipitation deficits have occurred in Texas , where exceptional drought has developed.

b. Eurasia

The 500-hPa circulation featured below average heights from the eastern U.S. to southern Europe (Fig. E9). This overall pattern reflected an anomalously zonal flow across the North Atlantic , and a southward shift of the North Atlantic jet stream and storm track. As a result, both western and southern Europe recorded above average precipitation and increased winter storm activity during the month.

  2. Southern Hemisphere

The 500-hPa height field during January featured an anomalous zonal wave-1pattern, with positive anomalies in the middle latitudes and negative anomalies at high latitudes (Fig. E15). In southeastern Australia , and anomalous anticyclonic circulation contributed to below average precipitation during the month, with some areas recording totals in the lowest 10^th percentile of occurrences.

In southern Africa , the rainy season extends from October through April. During January rainfall was above average for the region as a whole, with area average totals reaching the 80^th percentile of occurrences (Fig. E4). Above average totals during November 2008-January 2009 are consistent with La Niña.