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

JULY 2008

1. Northern Hemisphere

The 500-hPa height pattern during July 2008 featured positive anomalies over the high latitudes of the central North Pacific, eastern Canada , and Greenland , and negative anomalies over Alaska , south-central Canada , and the eastern North Atlantic (Fig. E9). Over the central Pacific Ocean , the subtropical circulation at 200-hPa reflected a lingering La Niña signal, with cyclonic anomalies in both hemispheres flanking the suppressed convection over the central equatorial Pacific (Fig. T22).

The main surface temperature departures during July reflected warmer than average conditions in the southwestern U.S. , eastern Canada , and portions of Europe , and below-average temperatures in Alaska and extreme western Canada (Fig. E1). The main precipitation anomalies included above average totals in the Midwest and Gulf Coast regions of the U.S. (Fig. E5), portions of northern and eastern Europe , and eastern China , and below-average totals in the Pacific Northwest U.S. (Fig. E3).

a. North Pacific/ North America

Even though La Niña has dissipated, as measured by the Niño 3.4 SST index, there remained a complete disappearance of tropical convection from the central equatorial Pacific (Fig. T25). Impacts from this anomalous convection continued to be reflected in cyclonic streamfunction anomalies over the central subtropical Pacific of both hemispheres (Fig. T22).

The 500-hPa height pattern during July featured amplified troughs over Alaska and eastern North America , and a persistent ridge over the southwestern US.  This overall pattern was associated with a continuation of below-average precipitation in northern California and the Pacific Northwest (Fig. E5). In northern California many locations have 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. 

In contrast the Midwest and Great Lakes regions of the U.S. recorded above-average precipitation during July.  Both regions have seen well above-average precipitation since February (Fig. E5).

b. Southeastern China

In China , the eastern flank of the normal Asian monsoon ridge at 200-hPa was weaker than average for the second straight month (Fig. T22). This pattern has favored well above-average precipitation, with totals during both June and July in the upper 90^th percentile of occurrences.

  2. Southern Hemisphere

The 500-hPa height field during July featured large areas of above-average heights centered south of both New Zealand and South Africa , and below-average heights located over the eastern South Pacific and over the central South Atlantic and Indian Oceans (Fig. E15). In the subtropics, ongoing positive 200-hPa streamfunction anomalies over the central Pacific Ocean indicated an anomalous cyclonic circulation consistent with a lingering La Niña signal (Fig. T22).

The main surface temperature anomalies during July reflected exceptionally warm conditions over central South America , where departures exceeded the 90^th percentile of occurrences (Fig. E1). This region was situated downstream of the mean upper-level trough, and also experienced anomalous poleward flow at 850-hPa (Fig. T20). These conditions were associated with a poleward shift of the mean cold frontal boundary, and with an anomalous southward incursion of mild air into central South America .