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

APRIL 2009

1. Northern Hemisphere

The 500-hPa height field during April 2009 featured positive anomalies over the Gulf of Alaska , the central North Atlantic , Europe , and eastern Asia , and negative anomalies over the high latitudes of the North Atlantic and central Russia (Fig. E9). Over the Pacific Ocean and North America , the circulation continued to reflect a westward retraction of the East Asian jet stream (Fig. T21) and a westward shift of the downstream ridges and troughs. These conditions partly reflect a lingering La Niña signal.

The main temperature signals during April included above average temperatures in Europe and northeastern Asia , and below average temperatures in northwestern North America and in the vicinity of the Caspian Sea (Fig. E1). The main precipitation signals included above-average totals in the central U.S. and east of the Caspian Sea , and below-average totals in western Canada / southeastern Alaska and Europe / western Russia (Fig. E3).

a. North Pacific/ North America

At 200-hPa, La Niña again contributed to enhanced mid-Pacific troughs in both hemispheres flanking the suppressed equatorial convection, and a westward retraction of the subtropical ridges toward 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 Associated with this pattern, the East Asian jet stream was retracted westward toward the western Pacific (Fig.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 Gulf of Alaska of the mean ridge normally located over western North America , and a westward shift of the mean Hudson Bay trough toward western North America (Fig. E9).

These conditions were associated with cooler (Fig. E1) and drier (Fig. E3) than average conditions from southern Alaska to California . They were also associated with well above average precipitation in large portions of the central and northern Plains regions of the United States (Figs. E3, E6).

b. North Atlantic and Eurasia

Over the North Atlantic and Europe , the circulation during April featured a 4-celled pattern of 500-hPa height anomalies (Fig. E9). This pattern reflected an anomalously zonal jet stream configuration across the North Atlantic , and a pronounced split-flow pattern over Europe . The anomalous ridge over Europe contributed to well above average temperatures in eastern Europe (Fig. E1), and to an extensive area of significantly below average precipitation across Europe and western Russia (Fig. E3). Also during April an anomalous wave pattern contributed to cooler and wetter than average conditions in the vicinity of the Caspian Sea , and to well above average temperatures in northeastern Asia .

  2. Southern Hemisphere

The circulation during April continued to reflect some aspects of La Niña, with an enhanced ridge over the central South Pacific and an anomalous trough over the high latitudes of the eastern South Pacific (Figs. E15, T22). Also evident during the month was a zonal wave-4 pattern, with above average heights over southern South America , the eastern South Atlantic , and the eastern Indian Ocean , and below average heights over the western South Atlantic and western Indian Ocean .

A main La Niña signal was again evident in the pattern of below average precipitation in southeastern South America (Fig. E3). This region has received below average rainfall in every month since November 2008, and during April 2009 the totals were in the lowest 10^th percentile of occurrences (Fig. E4).

In southern Africa , the rainy season extends from October through April. Rainfall during April was slightly below-average for the region as a whole, although portions of Zimbabwe and Mozambique recorded well below average totals (Figs. E3, E4). Overall, the 2008-09 rainy season has been above average, which is consistent with La Niña.