Extratropical Highlights –December 2016

 

1. Northern Hemisphere

The 500-hPa circulation during December featured above-average heights across the central North Pacific, the southern U.S., Europe, China, and below-average heights over Canada, the high latitudes of the North Atlantic, and central 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 December included above-average temperatures across southeastern Asia, and below-average temperatures in the northwestern U.S. and in central Russia (Fig. E1). The main precipitation signals included above-average totals in both the northwestern and eastern U.S., and below-average totals in Europe (Fig. E3).

 

a. North America

The 500-hPa circulation during December featured above-average heights over the central North Pacific and the southern U.S., and below-average heights across Canada. (Fig. E9). These conditions contributed to below-average surface temperature in the northwestern U.S. (Fig. E1), along with above-average precipitation in both the northwestern and eastern U.S. (Fig. E3).

According to the U.S. Drought Monitor, severe or extreme drought extended from central Louisiana to eastern Kentucky and western North Carolina. Long-tern exceptional drought persisted in central/ southern California. In the northeastern U.S., severe drought persisted in southern New York, northern New Jersey and portions of New England, with extreme drought evident in Connecticut.

 

b. Eurasia

The 500-hPa circulation during December featured above-average heights over Europe and China and an amplified trough over central Russia (Fig. E9). This overall pattern was associated with above-average surface temperatures in Scandinavia and much of southeastern Asia, and below average surface temperatures in central Russia (Fig. T20). It was also associated with well below-average precipitation across Europe, with many locations recording totals in the lowest 10^th percentile of occurrences (Fig. E3).

 

 

2. Southern Hemisphere

The mean 500-hPa circulation during December featured above-average heights over Antarctica, the high latitudes of the eastern South Pacific, and eastern Australia, and below-average heights over the South Atlantic and Indian Ocean (Fig. E15).

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).