Extratropical Highlights –November 2016

 

1. Northern Hemisphere

The 500-hPa circulation during November featured above-average heights across the central North Pacific, North America, the central North Atlantic, and northern Siberia, and below-average heights over the high latitudes of the North Pacific, the Gulf of Alaska, and central and eastern Asia (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 November included above-average temperatures across North America, and below-average temperatures across large portions of south-central and southeastern Russia (Fig. E1). The main precipitation signal was below-average totals over the eastern half of the U.S. (Fig. E3).

 

a. North America

The 500-hPa circulation during November featured a 4-celled anomaly pattern, with above-average heights over the central North Pacific and North America and below-average heights over the Gulf of Alaska and the western North Atlantic (Fig. E9). This pattern projected onto the negative phase (-1.4) of the Pacific/ North American (PNA) teleconnection pattern, and was associated with a disappearance of the mean Hudson Bay trough.

These conditions contributed to well above-average surface temperature across North America, with large portions of both Canada and the U.S. recording departures above the 90^th percentile of occurrences (Fig. E1). They also contributed to below-average precipitation over the eastern half of the U.S. in the area downstream of the mean ridge axis, with many locations recording totals in the lowest 10^th percentile of occurrences (Fig. E3). Overall, area-averaged totals were in the lowest 10^th percentile of occurrences in the Southeast, Ohio Valley and Great Lakes regions, and area-averaged totals were in the lowest 20^th percentile of occurrences in the mid-Atlantic region (Fig. E5).

According to the U.S. Drought Monitor, ongoing precipitation deficits in the Southeast acted to intensify and expand drought conditions during November, with much of the region recording extreme or exceptional drought. Extreme drought extended from central Louisiana to eastern Kentucky and western North Carolina, while exceptional drought extended across northern Alabama and the northern half of Georgia extending northward into the southern Appalachians. Long-tern exceptional drought conditions also persisted in central/ southern California. Elsewhere, severe or extreme drought persisted in southern New York, northern New Jersey and portions of New England.

 

b. Asia

The 500-hPa circulation during November featured above-average heights over northern Siberia and a broad, amplified trough over central/ eastern Asia (Fig. E9). This overall pattern was associated with anomalous northerly flow and well below average surface temperatures across central Asia (Fig. T20). It was also associated with a continuation of above-average precipitation in China, where area-averaged totals have been in the upper 90^th percentile of occurrences since September (Fig. E4).

 

 

2. Southern Hemisphere

The mean 500-hPa circulation during November featured above-average heights over Antarctica and the central Indian Ocean, and below-average heights from southern Africa to the high latitudes of the South Pacific (Fig. E15). This overall pattern was associated with an amplified trough over Australia, which contributed to below-average surface temperatures (Fig. E1) and below-average precipitation across southern Australia (Fig. E3).

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 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. The 2016 ozone hole dissipated in mid-November, in association with a markedly reduced size of the SH polar vortex (Fig. S8).