Extratropical Highlights – July 2013

 

1. North Pacific/ Northern Hemisphere

The 500-hPa circulation during July featured above-average heights across the high latitudes of the North Pacific, eastern Canada, and western Europe, and below-average heights across central Russia (Figs. E9, E11). At 200-hPa, cyclonic streamfunction anomalies were present in both hemispheres over the western and central subtropical Pacific Ocean (Fig. T22). This pattern was consistent with the combination of enhanced convection over Indonesia and suppressed convection over the central equatorial Pacific (Fig. T25).

The main land-surface temperature signals during July included above-average temperatures in the western U.S., Alaska, Europe and central/ eastern China, and below-average temperatures in the southeastern U.S. (Fig. E1). The main precipitation signals included above-average totals across the southern U.S., and drier-than average conditions in the northwestern U.S., eastern Alaska, and eastern Europe (Fig. E3). In the U.S., long-term precipitation deficits led to a continuation of extreme and exceptional drought in large areas of the southwestern U.S. and the southern Great Plains, and to a continuation of severe drought in much of the remaining area of the West.

 

a. North America

The mean 500-hPa circulation during July featured a strong ridge centered over the Rocky Mountains and a deep trough over the midwestern U.S. (Fig. E9). This pattern reflected a westward shift of the mean summertime ridge axis and an amplification of the Hudson Bay trough. The circulation also featured above-average heights across the high latitudes of the North Pacific and extending into the northwestern U.S. This pattern was associated with a weakening of the mean trough over the Gulf of Alaska, and with an overall northward shift of the mean westerly winds and storm track.

Consistent with these conditions, the northwestern U.S. experienced exceptionally warm and dry conditions during July, with surface temperatures well above average the 90^th percentile of occurrences (Fig. E1) and precipitation totals in the lowest 10^th percentile of occurrences (Figs. E3, E5). In contrast, the southern tier of the U.S. recorded well above-average totals in association with the anomalous upper-level trough, with amounts exceeding the 90^th percentile of occurrences in portions of the southwest and southeast. Area-average totals exceeded the 90^th percentile of occurrences in the Ohio Valley, the Southeast, and the Gulf Coast region (Fig. E5).

Much of the western half of the U.S. continued to be impacted by severe- extreme- or exceptional drought. At the end of July, the “U.S. Drought Monitor” indicated exceptional drought from central New Mexico to northwestern Kansas, and extreme drought from northern Texas northward to western Nebraska and southern Wyoming. Severe drought was recorded across much of the remaining region between southern Oregon and Nebraska and extending southward to Mexico.

 

b. China and Japan

The Asian monsoon ridge remained stronger than average during July, as indicated by positive streamfunction anomalies at 200-hPa extending from the Middle East to northeastern China (Fig. T22). This pattern was associated with a northward shift of the mean belt of westerly winds across central Asia (Fig. T21). These conditions contributed to a continuation of above-average surface temperatures in central and eastern China, with some areas recording departures in the upper 90^th percentile of occurrences (Fig. E1).

 

2. Southern Hemisphere

The mean 500-hPa circulation during July featured above-average heights in the middle latitudes and below-average heights over the high latitudes of the South Pacific (Fig. E15). This pattern may have had links to the Tropics via strong cyclonic streamfunction anomalies across the subtropical South Pacific (Fig. T22), which occurred in association with enhanced convection over Indonesia and suppressed convection near the date line (Fig. T25).

The main surface temperature and precipitation anomalies reflected exceptionally warm and dry conditions in eastern Australia (Fig. E1). These conditions were associated with positive 500-hPa height anomalies and an enhanced low-level ridge (Fig. T21), which extended from eastern Australia to South America.