Extratropical Highlights –October 2016

 

1. Northern Hemisphere

The 500-hPa circulation during October featured above-average heights over much of the high latitudes and across eastern North America, and below-average heights over central and eastern Asia, and over the Gulf of Alaska (Fig. E9). In the subtropics, the 200-hPa circulation featured a pronounced westward retraction of the mean subtropical ridge over Australasia, along with amplified mid-Pacific troughs in both hemispheres (Fig. T22). This pattern is consistent with the La Niña-related pattern of enhanced convection over Indonesia and suppressed convection across the central equatorial Pacific (Fig. T25).

The main land-surface temperature signals during October included above-average temperatures in eastern North America, the southern half of the United States, and China, and below-average temperatures in western Canada and large portions of south-central and southeastern Russia (Fig. E1). The main precipitation signals included above-average totals in the northwestern U.S., western Canada, eastern Europe, and eastern China, and below-average totals in the central and southeastern U.S., Scandinavia, and northern Russia (Fig. E3).

 

a. North America

The 500-hPa circulation during October featured above-average heights across the eastern half of North America and a strong trough over the eastern Gulf of Alaska (Fig. E9). This pattern was associated with well above-average surface temperature across much of the U.S. and the eastern Canada, with much of the central and southern U.S. recording departures exceeding the 90^th percentile of occurrences (Fig. E1). It was also associated with above-average precipitation in the northwestern U.S. and western Canada (downstream of the trough axis), and with below-average precipitation across the central and southeastern U.S. (Fig. E3). Precipitation totals in the U.S. Gulf Coast states were generally in the lowest 10^th percentile of occurrences, which acted to intensify and expand drought conditions from central Mississippi to western South Carolina.

According to the U.S. Drought Monitor, exceptional or extreme drought continued during October across central and southern California, and expanded to cover the area from central Mississippi to southeastern Tennessee and western South Carolina. Severe or extreme drought persisted in New York, northern New Jersey and much of New England. Conversely, long-term drought conditions lessened considerably in the northwestern U.S.

 

b. China

The 500-hPa circulation during October featured above-average heights over northeastern China and a broad, amplified trough over central Asia (Fig. E9). This overall pattern was associated with an anomalous low-level southerly flow of moist subtropical air into central and eastern China (Fig. T20). These conditions were associated with well above-average surface temperatures across much of China, with many locations recording departures in the upper 90^th percentile of occurrences (Fig. E3). They were also associated with above-average precipitation in eastern China, with portions of the northeast recording totals in the upper 90^th percentile of occurrences (Fig. E3).

 

 

 

2. Southern Hemisphere

The mean 500-hPa circulation during October featured an anomalous zonal wave-3 pattern in the middle latitudes, which included an amplified trough south of Australia (Fig. E15).  This trough contributed to a continuation of below-average surface temperatures in portions of the southwest (Fig. E1).

The Antarctic ozone hole typically develops during August and reaches peak size in late September (Fig. S8). It then typically decreases in size during October and November, and dissipates in early December. During October 2016, the size of the ozone hole was below the average of the 2006-2015 period, with a rapid drop in size seen in mid-October.