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

JANUARY 2008

1. Northern Hemisphere

The 500-hPa heights during January 2008 featured an anomalous wave-3 pattern in the middle and high latitudes, with positive anomalies over the central North Pacific, the western North Atlantic , and northwestern Russia , and negative anomalies over the western US, the northeastern North Atlantic , and eastern Siberia (Fig. E9). The anomaly pattern across the North Pacific Ocean and North America is consistent with La Ni�a. The subtropical circulation at 200-hPa was also consistent with La Ni�a, with enhanced mid-Pacific troughs in both hemispheres flanking the suppressed convection over the central equatorial Pacific, and enhanced ridges over the western Pacific flanking the region of enhanced equatorial convection (Fig. T22).

The main surface temperature departures during January reflected warmer than average conditions in eastern Canada , Europe , Scandinavia , and northwestern Russia , and below-average temperatures across south-central Asia (Fig. E1). The main precipitation anomalies included above average totals over much of the western US and southeastern China , and below-average totals in the Plains states and the southeastern US (Fig. E3).

a. North Pacific/ North America

The La Ni�a signal was again prominent across the Pacific and North America during January. La Ni�a 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 Ni�a, the core of the East Asian jet stream is often retracted westward toward Asia , as was again seen during January (Fig. T21) and the heart of the jet exit region is shifted westward to west of the date line.

The downstream circulation features are retracted westward as well. This includes a shift of the mean ridge from western North America to the central/eastern North Pacific, and a shift of the mean Hudson Bay trough to central or even western North America (Fig. E9). During January, these conditions were associated with a complete disappearance of the mean ridge from western North America , and a broad trough across the US . This pattern produced

above-average precipitation over much of the western US (Figs. E1, E5). It was also associated with below-average precipitation across the Plains states, with much of the region recording significant precipitation deficits in two of the last three months (Fig. E5). In parts of the southeastern US, ongoing precipitation deficits helped to sustain drought conditions.

  b. North Atlantic and Europe

The circulation during January featured a north-south dipole pattern of 500-hPa height anomalies over the North Atlantic Ocean , with below-average heights centered east of Greenland and above-average heights extending across the middle latitudes (Fig. E9). A strong blocking ridge was also evident over northwestern Russia . This pattern is similar to that observed in December 2007.

During January, these conditions were associated with a split-flow pattern over the eastern North Atlantic , with the northern branch of the jet stream entering the continent over northern Europe and southern branch entering over central and southern Europe .  

The enhanced poleward heat transport associated with the northern branch of the jet stream led to above-average temperatures and above-average precipitation over Scandinavia and western Russia (Fig. E1). Within the strong southern branch of the jet stream, a deep trough was centered over south-central Asia , with anomalous northerly flow evident upstream of the trough axis. This pattern contributed to well below-average temperatures across the region, with departures in many areas in the lowest 10^th percentile of occurrences.

2. Southern Hemisphere

      The 500-hPa height field during January featured negative anomalies centered over Antarctica and positive anomalies spanning the middle latitudes (Fig. E15). In the subtropics, the pattern of negative (anticyclonic) 200-hPa streamfunction anomalies over the central Indian Ocean and positive (cyclonic) streamfunction anomalies over the central Pacific was consistent with La Ni�a. Similar conditions were also observed in December.

In Australia , an anomalous ridge-trough pattern led to warmer and drier than average conditions in the west, and cooler and wetter than average conditions in the east.

The South African rainy season extends from October to April, and is often stronger than average during La Ni�a. During January, well above-average totals again covered much of the monsoon region (Fig. E3), and departures in many areas exceeded the 70^th percentile of occurrences. Area-averaged totals for the entire monsoon region were the largest since 1979 for the second straight month, and were above average for the fourth straight month (Fig. E4).