Extratropical Highlights –December 2017

 

1. Northern Hemisphere

The 500-hPa circulation during December featured an anomalous wave-4 pattern. Above-average heights were present over the western North Pacific, western North America, the eastern North Atlantic, and northwestern Russia. Below-average heights were present over the central North Pacific, Hudson Bay, Scandinavia, and eastern Asia (Fig. E9).

At 200-hPa, the circulation featured amplified troughs east of the date line in the subtropics of both hemispheres (Fig. T22).  This pattern is typical of La Niña, and reflects the disappearance of deep tropical convection from the central equatorial Pacific.

The main land-surface temperature signals during December included above-average temperatures in Alaska, northwestern Canada, and western Russia, and below-average temperatures in the U.S. Great Lakes region (Fig. E1). The main precipitation signals included above-average totals in western Russia, and below-average totals in western Canada and in the southwestern and central/ eastern portions of the U.S. (Fig. E3).

 

a. North Pacific and North America

The 500-hPa circulation during December featured above-average heights over western North America and below-average heights over eastern North America. Highly amplified wave patterns such as this often reflect the westward retraction of the East Asian jet steam typically seen during La Niña (Fig. T21).

This overall pattern contributed to anomalously warm conditions in Alaska, western Canada, and the southwestern U.S., and to anomalously cool conditions in the U.S. Great Lakes region (Fig. E1). It also contributed to below-average precipitation in western North America and across the central/ eastern portions of the U.S. (Fig. E3).

 

b. Eurasia

            An amplified wave pattern at 500-hPa extended from the eastern North Atlantic to eastern Asia (Fig. E9). Features of this pattern included an amplified ridge over the eastern North Atlantic and a strong tough extending southward from Scandinavia. This pattern was associated with well above-average surface temperatures (Fig. E1) and above-average precipitation across eastern Europe and western Russia (Fig. E3).

 

2. Southern Hemisphere

The mean 500-hPa circulation during December featured above-average heights in the vicinity of New Zealand and below-average heights over much of the high latitudes (Fig. E15). At 200-hPa the circulation featured an amplified trough over the east-central South Pacific Ocean (Fig. T22),  which is typical of La Niña.