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

JUNE 2008

1. Northern Hemisphere

The 500-hPa height pattern during June 2008 featured positive anomalies across eastern Siberia and the high latitudes of the North Pacific, Greenland , and southern Europe , and negative anomalies over North America , northern Europe , and Scandinavia (Fig. E9). Over the Pacific Ocean , the subtropical circulation at 200-hPa continued to reflect La Niña, with enhanced mid-Pacific troughs and cyclonic circulation anomalies in both hemispheres flanking the suppressed convection over the central equatorial Pacific (Fig. T22).

The main surface temperature departures during June reflected warmer than average conditions across the southern and eastern US, eastern Europe and northeastern Russia , and below-average temperatures over the north-central US, western Canada , and northern India (Fig. E1). The main precipitation anomalies included above average totals in the midwestern US , eastern Canada , and southeastern China , and below-average totals in northern California , the Gulf Coast states, the southeastern US, and northeastern Europe (Fig. E3).

a. North Pacific/ North America

The La Niña signal was again prominent across the subtropical Pacific Ocean during June. La Niña is associated with a westward retraction of deep tropical convection toward Indonesia , and with 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. 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. During La Niña, the core of the East Asian jet stream is often retracted westward toward Asia , (Fig. T21). This relationship is normally most evident in winter and spring, but was also pronounced in June in response to the ongoing La Nina-related pattern of tropical convection.

Associated with these conditions, the 500-hPa heights were above average over the high latitudes of the eastern North Pacific, and below-average over the north-central US.  This overall pattern was associated with a continuation of below-average precipitation in northern California and the Gulf coast region of the US . Both of these areas have recorded significant precipitation deficits during the past several months. In northern California many locations had recorded almost no rainfall since late March, which set the stage for numerous (1781) uncontrolled wildfires that burned over 702,000 acres during June and early July. 

The 500-hPa circulation over the United States also exhibited considerable variability during June. In early June, the circulation featured a trough in the west and a ridge in the east. This pattern was associated with a persistent frontal boundary and significant low-level moisture transport across the Midwest . These conditions contributed to a series of major rainfall events and severe flooding in Iowa . Overall, the Midwest has recorded well above-average precipitation since February (Fig. E5).

b. North Atlantic and Europe

The 500-hPa circulation during June featured a north-south dipole pattern of height anomalies over Europe , with below-average heights to the north and above-average heights to the south (Fig. E9). This pattern mainly contributed to well above-average temperatures in eastern Europe, with many areas recording departures in the upper 90^th percentile of occurrences (Fig. E1).

b. Southeastern China

During June, the eastern flank of the normal Asian monsoon ridge at 200-hPa was weaker than average in association with a broad trough over eastern China (Fig. T22). This pattern favored an anomalous southward penetration of frontal boundaries and mid-latitude storms which, in turn, favored enhanced convection and above-average rainfall over southeastern and eastern China .

  2. Southern Hemisphere

      The 500-hPa height field during June exhibited a strong zonal symmetry, with above average heights in the middle latitudes and below average heights over Antarctica (Fig. E15). In the subtropics, ongoing positive 200-hPa streamfunction anomalies over the central Pacific Ocean indicated an anomalous cyclonic circulation and enhanced mid-Pacific trough consistent with La Niña (Fig. T22). In southern Australia an anomalous 500-hPa trough contributed to anomalously warm and dry conditions during June (Figs. E1, E3).