Extratropical Highlights – July 2012

 

1. Northern Hemisphere

The 500-hPa circulation during July featured above-average heights over central North America, Greenland, and from southeastern Europe to the high latitudes of the central North Pacific, and below-average heights extending from Great Britain eastward to Alaska (Figs. E9, E11). Over the North Atlantic, the circulation continued to reflect a negative phase (-1.3) of the North Atlantic Oscillation (NAO) (Fig. E7, Table E1).

The main land-surface temperature signals during July included well above-average temperatures across the northern half of the United States and most of Canada, southeastern Europe, and large portions of Asia (Fig. E1). The main precipitation signals (Fig. E3)  included above-average totals in the northwestern Europe and central China, and below-average totals in the central, southeastern, and northeastern regions of the U.S. (Fig. E6).

 

a. North America

The mean 500-hPa circulation during July featured an amplified summertime ridge over central North America (Figs. E9, E11). July weather across North America is strongly linked to the position of strength of the mean summertime ridge. An ridge, as seen in July, is associated with a northward shift of the mean jet stream into central/ northern Canada (Fig. E14). It is also associated with very hot and dry conditions within and downstream of the mean ridge axis.

During July, surface temperatures exceeded the 90^th percentile of occurrences across much of the northern U.S. and portions of central Canada, with the largest departures (more than 3˚C) observed in the north-central U.S. and south-central Canada (Fig. E1). Rainfall was well below average in the central U. S., as well across eastern Canada, with both regions recording totals in the lowest 10^th percentile of occurrences (Fig. E3).

In the U.S., area-averaged rainfall totals in the Midwest were below average for a fourth straight month, and totals in the Great Plains were well below average for a third straight month (Fig. E5). These exceptionally hot and dry conditions led to extreme or exceptional drought across large portions of the U.S. Midwest and Great Plains.

 

b. North Atlantic and Eurasia

The 500-hPa circulation during July featured a continued negative phase (-1.3) of the NAO (Fig. E7, Table E1), which mainly reflected persistent positive height anomalies over Greenland (Figs. E9, E11). The circulation also featured an extensive north-south dipole pattern of height anomalies, with below-average heights extending from Great Britain eastward to Alaska, and above-average heights extending from southeastern Europe to the high latitudes of the central North Pacific.

This overall height pattern was associated with above-average precipitation across northern Europe, and a with below-average precipitation in southern Europe. Area-average precipitation in southern Europe was in the lowest 10^th percentile of occurrences for the second straight month (Fig. E4). Also, above-average temperatures covered most of Eurasia, with departures exceeding the 90^th percentile of occurrences across the Mediterranean Sea area, and in portions of central China.

 

c. African Sahel

The west African monsoon system reaches peak strength during July-September. Overall, that monsoon has been enhanced since 1995 in association with the warm phase of the Atlantic Multi-Decadal Oscillation (AMO). The monsoon circulation was again enhanced during July 2012 (Fig. T24), with well above-average precipitation evident across the African Sahel and Sudan regions (Fig. E1). For the entire west African monsoon region, area-average totals during July were the largest dating back to 1979 (Fig. E4). This region has recorded above-average precipitation for three straight months.

 

2. Southern Hemisphere

The mean 500-hPa circulation during July featured a zonally-symmetric pattern of height anomalies, with above-average heights across the middle latitudes and below-average heights in the polar region (Fig. E15). In Australia, the main precipitation signals included below-average totals along the southwest coast (Fig. E3). Also, surface temperatures were well below-average in northwestern Australia (Fig. E1).