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 2008

1. Northern Hemisphere

The 500-hPa height pattern during February 2008 featured positive anomalies across the eastern North Pacific, the central North Atlantic , and Europe , and negative anomalies over the high latitudes of the North Pacific and Alaska , central Canada , and Greenland (Fig. E9). The circulation across the North Pacific Ocean is 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 February reflected warmer than average conditions in Europe and northern Russia , and below-average temperatures over central Canada and south-central Asia (Fig. E1). The main precipitation anomalies included above average totals over portions of the northwestern US, and from the upper Midwest to New England , and below-average totals across southern Europe (Fig. E3).

            a. North Pacific/ North America

The La Niña signal was again prominent across the Pacific Ocean during February. 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 wintertime 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). The downstream circulation features such as the mean ridge over western North America and the Hudson Bay trough are retracted westward as well (Fig. E9).

During February, these conditions were associated with a continuation of above-average precipitation over the Inter-mountain region of the western US, and with well above-average precipitation from the Great Lakes to New England (Figs. E3, E5). They were also associated with below-average temperatures over central Canada (Fig. E1).

b. North Atlantic and Europe

The 500-hPa circulation during February featured a north-south dipole pattern height anomalies over the North Atlantic Ocean , with below-average heights centered over Greenland and above-average heights extending across the middle latitudes (Fig. E9). This overall pattern reflects the ongoing positive phase of the North Atlantic Oscillation (NAO), which has been prevailed for the past six months (Fig. E7, Table E1). Also during February large positive height anomalies were also evident across Europe .

These conditions were associated with an anomalously strong northeasterly transport of mild marine air into northern Europe and Scandinavia which, when combined with the strong ridge over Europe , resulted in a continuation of well above-average temperatures across Europe and northern Russia  (Fig. E1). Also, anomalous sinking motion downstream of the European ridge resulted in significant precipitation deficits (10^th percentile of occurrences) across southeastern Europe .

  2. Southern Hemisphere

      The 500-hPa height field during February featured a continuation of negative anomalies over Antarctica , and an anomalous zonal wave-4 pattern in the middle latitudes (Fig. E15). In the subtropics, 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 Australia , a persistent upper-level trough and anomalous low-level southerly flow from the Great Australian Bight led to a continuation of cooler than average conditions in the east. The mean trough axis also demarked the areas of above- 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 well above average rains during October 2007-January 2008, but recorded a deficit during February with totals in the lowest 20^th percentile of occurrences (Fig. E4).