Extratropical Highlights – June 2013

 

1. North Pacific/ Northern Hemisphere

The 500-hPa circulation during June featured above-average heights across Alaska and northwestern Canada, the southwestern U.S., the central North Atlantic, and northwestern Russia, and below-average heights over the polar region and southern Greenland (Figs. E9, E11).

The main land-surface temperature signals during June included above-average temperatures across Alaska, northwestern Canada, the southwestern U.S., western Russia and central China (Fig. E1). The main precipitation signals included well above-average totals in the eastern U.S., and drier-than average conditions in the northwestern U.S., southeastern China, and Japan (Fig. E3). In the U.S., long-term precipitation deficits led to a continuation of extreme and exceptional drought in the central and southern Great Plains, New Mexico, and northern Nevada, 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 June featured above-average heights across Alaska, northwestern Canada, and the southwestern U.S. (Figs. E9, E11). This pattern was associated with an amplified ridge-trough configuration across the U.S., and strongly influenced the monthly temperature and precipitation patterns across North America.

Surface temperature departures in Alaska, northwestern Canada, and the southwestern U.S. all exceeded the 90^th percentile of occurrences during June (Fig. E1). Monthly rainfall totals in south-central Alaska and portions of the northwestern U.S. were well below-average, with some areas recording totals in the lowest 10^th percentile of occurrences (Fig. E3). For the Inter-Mountain region of the western U.S., area-averaged rainfall totals were comparable to the lowest in the historical record dating back to 1979 (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 June, the “U.S. Drought Monitor” indicated exceptional drought from northern Texas northward to western Nebraska, eastern Wyoming, southeastern Colorado, and New Mexico.  Extreme drought was recorded across much of the remaining region between southern Oregon and Nebraska and extending southward to Mexico.

In contrast, the eastern U.S. during June was situated downstream of the mean trough axis, and recorded well above-average precipitation. Precipitation totals were generally more than 175% above average from North Carolina to Maine (Fig. E6). Area-averaged totals approached the 100^th percentile of occurrences for the Ohio Valley, Mid-Atlantic, and Northeast regions (Fig. E5).

 

b. China and Japan

The Asian monsoon ridge was stronger than average during June, as indicated by positive streamfunction anomalies at 200-hPa extending from the Middle East to Japan (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 above-average surface temperatures in central China, with some areas recording departures in the upper 90^th percentile of occurrences (Fig. E1). They also contributed to below-average precipitation across southeastern China and Japan (Fig. E3).

 

2. Southern Hemisphere

The mean 500-hPa circulation during June featured above-average heights over the central and eastern South Pacific Ocean, the South Atlantic Ocean, and the area poleward of Australia, and below-average heights over the high latitudes of the eastern South Pacific and in the area south of South America (Fig. E15). The main surface temperature anomalies included above-average temperatures in the Amazon Basin, southern Argentina, and east-central Australia (Fig. E1). Monthly precipitation totals were above average in southern Brazil and southeastern Australia, and below average in southern South America and southwestern Australia (Fig. E3).