Extratropical Highlights – July 2014

 

1. Northern Hemisphere

The 500-hPa circulation during July featured above-average heights over western North America, eastern Canada, and Scandinavia, and below-average heights over central North America and central Russia (Fig. E9). The main land-surface temperature signals included well above-average temperatures in the western U.S., Scandinavia, and eastern Europe, and below-average temperatures in the central U.S. and central Russia (Fig. E1). The main precipitation signals included above-average totals in the south-central and northeastern U.S., and in central Europe, and below-average totals in the north-central U.S. and northeastern Europe (Fig. E3).

 

a. North Pacific/ North America

The mean 500-hPa circulation during July featured a ridge in western North America and a trough in the east (Fig. E9). This anomalous wave pattern differs markedly from climatology, in which the mean summertime ridge is situated over central North America east of the Rocky Mountains. This anomaly pattern was associated well above-average surface temperatures in western North America, with many areas recording departures in the upper 90^th percentile of occurrences (Fig. E1). It was also associated with well below average surface temperatures in the central U.S., with many areas recording departures in the lowest 10^th percentile of occurrences. Much of the region between the mean ridge and trough axes recorded below-average precipitation during the month, with the exception of the south-central U.S. which recorded 175% of normal rainfall (Fig. E6).

According to the U.S. Drought Monitor (http://droughtmonitor.unl.edu), this above-average precipitation helped to lessen drought conditions in southern Plains states. The most significant improvements occurred in Texas and Oklahoma, where only small regions of extreme or exceptional drought persisted at the end of the month. However, a massive area of exceptional drought continued in central/ southern California and northwestern Nevada, and severe or extreme drought extended across the U.S. Southwest, California, southern Oregon, central Washington, and southern Idaho.

 

b. Europe

            A blocking pattern extended from the eastern North Atlantic to central Russia, with the mean ridge centered over Europe and troughs located over the eastern North Atlantic and central Russia (Fig. E9). This anomaly pattern was associated with well above-average surface temperatures over Scandinavia and eastern Europe, and with well below-average temperatures over central Russia (Fig. E1). It was also associated with a north-south dipole pattern of precipitation anomalies, with well below-average totals over northern Europe and western Russia, and well above-average totals over central Europe (Fig. E3).

 

c. Northern Africa

Rainfall in the African Sahel has been well below average during this year’s rainy season, which peaks during July-September. During July, area-averaged totals were record low dating back to 1979 (Figs. E3, E4). The suppressed west African monsoon circulation was clearly evident in the pattern of 200-hPa velocity potential anomalies (Fig. T24), and appears to have had significant impacts on the large-scale circulation in both hemispheres. One classic signature of a suppressed monsoon circulation is seen in the pattern of 200-hPa streamfunction anomalies, which featured cyclonic anomalies in the subtropics of both hemispheres from the western Atlantic Ocean to Africa (Fig. T22).

 

2. Southern Hemisphere

The mean 500-hPa circulation during July featured above average heights over the central South Atlantic, and below-average heights over the central South Pacific (Fig. E15). In southeastern Australia, exceptionally warm and dry conditions continued during the month. Temperature departures were generally within the upper 70^th percentile of occurrences, and precipitation totals were in the lowest 10^th percentile of occurrences (Figs. E3, E4).