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

FEBRUARY 2009

1. Northern Hemisphere

The 500-hPa height field during February 2009 featured a zonal wave-1 pattern at high latitudes with positive anomalies covering eastern Canada and Greenland and negative anomalies covering eastern Siberia and the Bering Sea (Fig. E9). In the middle latitudes the circulation featured positive anomalies across the central North Pacific Ocean , and across southern Asia , and negative anomalies from California to southern Europe . This overall pattern was associated with La Niña, a strong positive phase (+2.2) of the West Pacific (WP) teleconnection pattern (Table E1, Fig. E7), and a split-flow configuration across the North Atlantic Ocean .

The main temperature signals during February included above-average temperatures in the south-central U.S. , eastern Canada , and across southern Eurasia , and below-average temperatures in western Canada and across Siberia (Fig. E1). The main precipitation signals included above-average totals in southern and eastern Europe, and below-average totals along the U.S. Gulf Coast and eastern seaboard, and across southern China (Fig. E3).

a. North Pacific/ North America

At 200-hPa, La Niña again contributed to a pronounced inter-hemispheric symmetry of the circulation anomalies across the Pacific Ocean . In the subtropics, aspects of this circulation included enhanced mid-Pacific troughs in both hemispheres flanking the suppressed equatorial convection, and a westward retraction of the subtropical ridges to Australasia (Fig. T22). Associated with this pattern, the East Asian jet stream was retracted westward toward the western Pacific (Fig. T21). These conditions contributed to a westward shift to the central North Pacific of the mean ridge normally located over western North America , and to a weaker-than-average strength of the Hudson Bay trough (Fig. E9).

These conditions were associated with above average temperatures in the south-central U.S. and eastern Canada , and below average temperatures in western Canada . They were also associated with ongoing precipitation deficits in the Gulf Coast and mid-Atlantic regions. The Gulf Coast region typically receives below average wintertime precipitation during La Niña, as seen during the last four months (Fig. E5). Some of the most significant deficits have occurred in Texas , where exceptional drought has developed.

c. Asia

The circulation during February also featured negative 500-hPa height anomalies across central and eastern Siberia and positive anomalies across central and eastern Asia . This pattern partly reflected a strong positive phase of the WP teleconnection pattern (Table E1, Fig. E7). It was associated with a north-south dipole of temperature anomalies, with Siberia and northern Asia recording well below average temperatures, while central and southern Asia recorded significantly above average temperatures with departures generally in the upper 90^th percentile of occurrences. Across southern and central China , the persistent positive height anomalies also led to a third consecutive month of below average precipitation (Figs. E3, E4).

  2. Southern Hemisphere

    The circulation during February again showed a strong connection to La Niña, with enhanced ridges over western Australia and the central South Pacific, and enhanced troughs over the subtropical South Pacific and over the high latitudes of the central and eastern South Pacific (Figs. T22, E15).

    In southern Africa , the rainy season extends from October through April. During February rainfall was near-average for the region as a whole, with above average totals in the west and below average totals in the east (Fig. E4). Overall, the 2008-09 rainy season has been above average, consistent with La Niña.