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 90th 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 10th
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 10th percentile of occurrences for
the second straight month (Fig. E4). Also, above-average temperatures covered most of
Eurasia, with departures exceeding the 90th 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).