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 2008

1. Northern Hemisphere

The 500-hPa height pattern during April 2008 featured positive anomalies over the high latitudes of the North Pacific, eastern Canada , Greenland , and northeastern Europe , and negative anomalies over western Canada , the eastern North Atlantic , and central Siberia (Fig. E9). The circulation over the North Pacific and North America was again consistent with marked a weakening and westward retraction of the East Asian jet stream typical of La Nina (Fig. T21). 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 (Fig. T22).

The main surface temperature departures during April reflected significantly warmer than average conditions over eastern Canada and eastern Europe, and below-average temperatures over western Canada and Siberia (Fig. E1). The main precipitation anomalies included above average totals in the midwestern US and over southern Europe , and below-average totals in the northwestern and Gulf Coast areas of the US (Fig. E3).

a. North Pacific/ North America

The La Niña signal was again prominent across the Pacific Ocean during April. La Niña is associated with a westward retraction of deep tropical convection toward Indonesia , and a complete disappearance of tropical convection from the central equatorial Pacific (Fig. T25). These conditions result in a westward retraction of deep tropospheric heating, and hence a westward retraction of the 200-hPa subtropical ridge toward Indonesia (Fig. T22). Over the central equatorial Pacific, the reduction in convective heating contributes to an increased strength of the mid-Pacific troughs in both hemispheres.

The strength, structure, and position of the East Asian jet stream are strongly linked to conditions in the tropics and subtropics. For example, the jet core coincides with the strongest north-south height gradient at 200-hPa, which is heavily influenced by the height anomalies in the subtropics. The jet exit region coincides with the area of strong diffluence between the subtropical ridge and trough axes. During La Niña, the core of the East Asian jet stream is often retracted westward toward Asia, and the heart of the jet exit region is shifted westward to west of the date line (Fig. T21).

In North America , the circulation during April featured a westward shift of the mean 500-hPa ridge and trough axes, such that the mean Hudson Bay trough was located over the western US (Fig. E9). This shift, which is consistent with the anomalous East Asian jet stream, contributed to below-average temperatures over western Canada and above-average temperatures in eastern Canada (Fig. E1). It also contributed to above-average precipitation across the midwestern and Great Lakes regions of the US (Figs. E3, E5). In contrast, below-average precipitation was observed in the Pacific Northwest and Gulf Coast regions of the US . This marks the sixth consecutive month with below-average precipitation in the Gulf Coast region, which is consistent with La Nina (Fig. E5).

b. North Atlantic and Europe

The 500-hPa circulation during April featured an anomalous trough over the eastern North Atlantic and an anomalous ridge over Greenland . This pattern reflected a strong negative phase (-1.07) of the North Atlantic Oscillation (Table E1). These conditions were associated with increased storminess and above-average precipitation over portions of southern and central Europe .

  2. Southern Hemisphere

    The 500-hPa circulation pattern during April generally featured an anomalous zonal wave-3 pattern in the middle latitudes, with above-average heights over the high latitudes of the eastern South Pacific, the eastern South Atlantic , and the polar region, and below-average heights east of the three continents (Fig. E15). In the subtropics, the ongoing pattern of positive streamfunction anomalies over the central Pacific reflected an enhanced mid-Pacific trough consistent with La Niña (Fig. T22). In eastern Australia, a persistent upper-level trough led to a continuation of significantly cooler and drier than average conditions (Figs. E1, E3).

The South African rainy season extends from October to April, and is often stronger than average during La Niña. The region received near-normal precipitation during March and April, which followed below-average totals in February (Fig. E4). Overall the 2007-08 rainy season has been above average, with significant surpluses recorded from October through January.