1. Northern Hemisphere   

The 500-hPa height field during July featured an anomalous wave-like pattern across most of the middle and high latitudes, with positive anomalies over the high latitudes of the central North Pacific, western North America, Greenland, the polar region, and central Russia, and negative anomalies in the Gulf of Alaska, the eastern U.S., and northern Europe (Figs. E9, E11).

The main surface temperature departures during July reflected warmer than average conditions in Alaska, western North America, eastern Europe, and western Russia, and cooler than average conditions in the eastern United States (Fig. E1). The main precipitation anomalies included above average totals in the south-central U.S., northern Europe, and southern Scandinavia, and below average totals in the north-central and eastern U.S., southern Europe, and Mongolia (Figs. E3, E6).

a. North America

The 500-hPa circulation pattern during July featured an anomalous wave pattern with deep troughs in the Gulf of Alaska and the eastern U.S., and a very strong ridge over western North America (Fig. E9). The surface temperature anomalies were very consistent with this pattern, with above-average temperatures in the western U.S., Alaska, and western Canada, and below-average temperatures in the eastern United States. Temperatures across much of the western U.S. and western Canada were generally 2°-4°C above average, with many regions recording values in the upper 90th percentile of occurrences. Similar anomalous warmth was recorded in western Alaska. In contrast, anomalous northwesterly flow and a deeper-than-average upper-level trough brought below-average temperatures to the eastern United States.

This anomalous wave pattern also strongly controlled the precipitation patterns across North America (Figs. E3, E5). Above-average precipitation was recorded in western Canada in the area immediately downstream of the mean trough axis, and below-average precipitation was recorded over large portions of the north-central and eastern U.S. in the region between the mean upper-level ridge and trough axes. Long-term precipitation deficits have led to moderate drought conditions across much of the eastern U.S. east of the Mississippi River, and to severe drought in portions of the upper Midwest and Inter-Mountain regions of the western United States.

b. Eurasia

The 500-hPa circulation pattern during July featured a strong trough/ jet stream configuration over the eastern North Atlantic and western Europe. This pattern led to well above-average precipitation in northern Europe and southern Scandinavia, with severe flooding recorded in areas of England and northern Europe during the month. In contrast, a persistent upper-level ridge over the eastern Mediterranean Sea contributed to below-average precipitation and above-average temperatures in that region.

2. Southern Hemisphere

The 500-hPa height anomaly pattern during July featured a strong zonal symmetry, with above-average heights over Antarctica, and below-average heights in the middle latitudes (Fig. E15). This pattern projected strongly onto the negative phase of the Antarctic Oscillation (AAO). The circulation across Australia featured an anomalous trough-ridge-trough pattern, with the ridge located over the middle of the continent and the troughs centered just off the west and east coasts. This pattern led to a continuation of well below-average precipitation in eastern Australia, with large portions of the region recording totals in the lowest 10th percentile of occurrences (Fig. E3).

A strong ridge-trough pattern flanking southern South America was also evident. This circulation contributed to below-average temperatures across the southern half of the continent, with departures in many areas in the lowest 30th percentile of occurrences. A very unusual cold air outbreak brought snow to Buenos Aires, Argentina for the first time in 89 years.