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

MARCH 2008

1. Northern Hemisphere

The 500-hPa height pattern during March 2008 featured positive anomalies over the northwestern and eastern North Pacific, and over much of Asia , and negative anomalies over Canada and Europe (Fig. E9). The circulation over the western North Pacific was associated with a weakening and westward retraction of the east Asian jet stream (Fig. T21). The circulation across the eastern North Pacific remained consistent with La Niña. 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, and enhanced ridges over the western Pacific and Australasia flanking the region of enhanced equatorial convection (Fig. T22).

The main surface temperature departures during March reflected significantly warmer than average conditions across Asia , and below-average temperatures over Canada (Fig. E1). The main precipitation anomalies included above average totals from the upper Midwest to New England, and across much of Europe, and below-average totals in the southwestern and southeastern US and in the Middle East (Fig. E3).  

            a. North Pacific/ North America

The La Niña signal was again prominent across the Pacific Ocean during March. 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-hPasubtropical ridge toward Indonesia (Fig. T22). Over the central equatorial Pacific, the reduction in convective heating contributes to an increased strength of the mid-Pacific trough.

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). During March, an extensive area of above-average heights extending across eastern Asia and the northwestern North Pacific also contributed to this jet structure (Fig. E9).

In North America , the circulation during March featured above-average heights extending into the southwestern US, and below-average heights (and temperatures) across Canada . This pattern contributed to above-average precipitation from the Great Lakes to New England , and to below-average precipitation in both the southwestern and southeastern US (Fig. E3). For the Gulf Coast region, this marks the fifth consecutive month with below-average precipitation (Fig. E5).

b. Eurasia

The 500-hPa circulation during March featured a large-scale pattern of negative height anomalies across Europe and positive height anomalies from the Caspian Sea to eastern Siberia (Fig. E9). This pattern was associated with significantly above-average temperatures across most of Asia , with departures in many areas (> 5°C) exceeding the 90^th percentile of occurrences (Fig. E1). It was also associated with above-average precipitation across northern Europe and northwestern Russia , and below-average precipitation in the Middle East .

  2. Southern Hemisphere

        The 500-hPa circulation pattern during March generally featured above-average heights over the central ocean basins and below-average heights in the higher extratropics (Fig. E15). In the subtropics, the ongoing pattern of negative 200-hPa streamfunction anomalies over the central Indian Ocean and positive streamfunction anomalies over the central Pacific was consistent with La Niña (Fig. T22). In eastern Australia , a persistent upper-level trough led to a continuation of cooler than average temperatures and below-average precipitation (Fig. 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, 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.