Extratropical Highlights – December 2011

 

1. Northern Hemisphere

The 500-hPa circulation during December featured a persistent pattern of above-average heights in the middle latitudes and below-average heights at high latitudes (Figs. E9, E11). Regional circulation features included an amplified ridge-trough pattern over the eastern North Pacific and central North America in association with La Niña, and a strong north-south dipole of height anomalies over the North Atlantic in association with a strong positive phase (+2.3) of the North Atlantic Oscillation (NAO) (Table E1, Fig. E7).

In the subtropics, the upper-level circulation over the Pacific Basin featured cyclonic streamfunction anomalies in both hemispheres near the Date Line, and anticyclonic streamfunction anomalies in both hemispheres over Australasia (Fig. T22). This 4-celled anomaly pattern is linked to La Niña. It reflects a combination of 1) enhanced mid-Pacific troughs in both hemispheres flanking the suppressed convection over the central equatorial Pacific, and 2) enhanced subtropical ridges in both hemispheres flanking the enhanced convection over the western equatorial Pacific and Indonesia (Fig. T25).

The main land-surface temperature signals during December included well above-average temperatures across Canada, Europe, Scandinavia, and central Siberia, and below-average temperatures in the Middle East (Fig. E1). The main precipitation signals included above-average totals in the mid-western U.S. and northern Europe, and below-average totals in the western and southeastern U.S., southern Europe, the Middle East, and northwestern Russia (Fig. E3).

 

a. North Pacific and North America

The mean 500-hPa circulation during December featured a strong ridge-trough pattern with axes over the eastern North Pacific and the central/ southwestern U.S. (Fig. E9). This overall pattern is linked to the La Niña-related pattern of cyclonic streamfunction anomalies over the central subtropical North Pacific and anticyclonic streamfunction anomalies over eastern Asia (Fig. T22).

La Niña is associated with deep tropical convection focused over Indonesia and the eastern Indian Ocean, along with a disappearance of tropical convection from the central equatorial Pacific (Fig. T25). This westward retraction in the area of deep convection acts to amplify the mean mid-Pacific troughs at 200-hPa in both hemispheres (Fig. T22), which in the NH results in a westward retraction the east Asian jet stream and an amplified jet exit region (Fig. T21). This jet structure favors corresponding westward shifts in the downstream ridge and trough axes normally located over western and eastern North America, respectively. During December, for example, these features were located over the eastern North Pacific and the central/ southwestern U.S., respectively.

Accompanying these conditions, the mean jet stream and storm track entered North America over Alaska and western Canada, which is well north of their climatological position over the Pacific northwestern U.S. (Figs. E10, E13). The strong southwesterly flow of mild air associated with this pattern (Fig. E8) led to well above-average temperatures across Alaska and Canada (Fig. E1).

The anomalous upper-level circulation led to well below-average precipitation in the western U.S., with much of the coastal region recording totals in the lowest 10^th percentile of occurrences (Fig. E3). Conversely, the westward shift of the upper-level trough axis (to the southwestern U.S.) contributed to above-average precipitation across the central U.S., including the Tennessee and Ohio Valleys. These precipitation signals are typical of the wintertime response to La Niña. The southern Plains also recorded above-average precipitation during the month, although that region continues to experience severe-to-exceptional drought.

 

b. North Atlantic and Europe

The 500-hPa circulation during December featured a persistent north-south dipole pattern of height anomalies, with above-average heights extending from the eastern U.S. to southern Europe and below-average heights extending from eastern Canada to Scandinavia (Figs. E9, E11). This pattern reflects a strong positive phase (+2.3) of the North Atlantic Oscillation (NAO) (Table E1, Fig. E7). The NAO has now been positive for the last four months.

This overall circulation was associated with an enhanced northward transport of mild air into Europe and Scandinavia, resulting in well above-average surface temperatures across western and northern Eurasia (Fig. E1). It was also associated with a north-south dipole pattern of precipitation anomalies, with above-average totals recorded across northern Europe and Scandinavia and below-average totals recorded across southern Europe and the Middle East (Fig. E3). In northern Europe, area-averaged totals were in the upper 90^th percentile of occurrences, while in southern Europe they were in the lower 30^th percentile of occurrences (Fig. E4).

 

2. Southern Hemisphere

The 500-hPa circulation during December featured an anomalous zonal wave-3 pattern in the middle latitudes, with above-average heights generally located south-southwest of the continents, and below-average heights located throughout the polar region and over the high latitudes of the eastern South Pacific (Fig. E15).

At 200-hPa, the subtropical circulation  featured 1) an amplified ridge over the Indian Ocean in response to above-average convection over the western equatorial Pacific and Indonesia, and 2) an enhanced mid-Pacific trough in response to suppressed convection over the central equatorial Pacific (Figs. T22, T25).  Similar anomaly patterns were also evident in the Northern Hemisphere, and are consistent with La Niña.

In Australia, an east-west dipole pattern of surface temperature anomalies was present during December, with above-average temperatures in the far west and below-average temperatures in the east (Fig. E1). This pattern was associated with a broad upper-level ridge-trough couplet that spanned the continent from west to east.

In South America, much of the southern part of the continent recorded precipitation totals in the lowest 10^th percentile of occurrences (Fig. E3). These deficits were linked to a poleward shift of the mean storm track and jet stream (Fig. E16), which occurred in association with an amplified upper-level ridge centered over the continent.

The South African rainy season lasts from October to April. During December, rainfall for the region as a whole was near average (Fig. E4). To date, precipitation for the 2011-12 rainy season has been near-average during October, November, and December. Seasonal rainfall is typically above average in this region during La Niña.