Extratropical Highlights – December 2013

 

1. Northern Hemisphere

The 500-hPa circulation during December exhibited a 4-celled anomaly pattern extending from the western North Pacific to Europe. This pattern featured above-average heights across the high latitudes of the North Pacific, the Gulf of Alaska, the central North Atlantic, and Europe (Figs. E9, E11). It also featured below-average heights across the western North Pacific, eastern Canada, and the high latitudes of the North Atlantic. These anomalies projected strongly onto several main teleconnection patterns. Over the Pacific and North America sectors, the dominant teleconnections included the positive phase of the Tropical/ Northern Hemisphere pattern (+1.8), and the negative phases of both the West Pacific (-2.1) and Pacific/ North American pattern (PNA, -1.2) (Table E1, Fig. E7). Over the North Atlantic, the dominant teleconnections included the positive phases of the North Atlantic Oscillation (NAO, +0.8) and East Atlantic pattern (+1.2).

The main land-surface temperature signals during December included well above-average temperatures across Scandinavia and central Asia, and below-average temperatures in much of Canada (Fig. E1). The main precipitation signals included above-average totals in the eastern U.S., and below-average totals in the northwestern U.S., Europe, and western Russia (Fig. E3).

 

a. North Pacific/ North America

The mean 500-hPa circulation during December featured a strong height anomaly pattern with above-average heights across the high latitudes of the North Pacific and Gulf of Alaska, and below-average heights over the western North Pacific and eastern Canada (Fig. E9). This pattern projected onto the positive phase of the Tropical/ Northern Hemisphere pattern (+1.8), and the negative phases of both the West Pacific (-2.1) and the Pacific/ North American pattern (PNA, -1.2) (Table E1, Fig. E7).

These patterns were associated with an amplified exit region of the East Asian jet stream (Fig. T21), a marked weakening of the Aleutian Low, a strong and persistent ridge extending northward from the Gulf of Alaska to the polar region, and a strengthening of the Hudson Bay trough. This marks the second month in which the Aleutian low was weak or absent, and the Hudson Bay trough was enhanced.

The resulting amplified ridge-trough pattern across North America brought strong northwesterly flow and below-average temperatures to much of Canada and portions of the western and central U.S. (Fig. E1).

In the western U.S., strong sinking motion and below-average precipitation were observed downstream of the mean ridge axis. The Pacific Northwest recorded totals in the lowest 10^th percentile of occurrences (Figs. E3, E5), marking the third straight month of below-average precipitation in that region. Below-average totals in California and Nevada contributed to a worsening of drought conditions, with large areas in both states experiencing extreme or severe drought during the month.

In contrast, enhanced precipitation was recorded in the eastern U.S. in the area downstream of the mean trough axis. Much of the eastern seaboard recorded totals in the upper 70^th percentile of occurrences.

 

The U.S. Midwest and Plains states have recorded near- or above-average precipitation during the past four months (Fig. E5). This has helped to considerably lessen the severity of ongoing drought conditions. However, the U.S. Drought Monitor indicates that moderate or severe drought still persisted in portions of Iowa, Minnesota, Illinois, and northern Missouri.  Moderate or severe drought persisted across much of the southwestern quadrant of the U.S., including the Plains states from Texas to northern Nebraska.

 

b. North Atlantic/ Eurasia

The mean 500-hPa circulation featured above-average heights across the central North Atlantic and Europe, and below-average heights across the high latitudes of the North Atlantic (Fig. E9). This pattern projected onto the positive phase of the North Atlantic Oscillation (NAO, +0.8) and the East Atlantic pattern (+1.2) (Table E1, Fig. E7).

This circulation was associated with an enhanced North Atlantic jet stream extending from the eastern U.S. to Great Britain and then northward into Scandinavia (Fig. E10). This southwesterly flow brought milder Atlantic air and well above-average surface temperatures across Scandinavia and northern Russia (Fig. E1). Much of central and eastern Russia and Siberia recorded departures exceeding +5^oC for the second straight month. This exceptional warmth has led to a slower-than average build-up of the winter snowpack, especially along its southern boundary.

In Europe, an amplified mid-level ridge and its associated strong sinking motion again brought well below-average precipitation to the region (Figs. E1). Area-averaged totals in northern Europe were in the lowest 20^th percentile of occurrences during December, while totals in southern Europe were in the lowest 5^th percentile of occurrences (Fig. E4). December marks the sixth consecutive month of below-average precipitation for both northern and southern Europe (Fig. E4).

 

2. Southern Hemisphere

The mean 500-hPa circulation during December featured above-average heights over southern South America and below-average heights over the high latitudes of the South Atlantic (Fig. E15). At 200-hPa, the circulation featured an enhanced tough to the immediate east of Australia (Fig. T22).

Anomalous sinking motion upstream of the amplified trough contributed to below-average precipitation across eastern Australia, with many areas recording totals in the lowest 10^th percentile of occurrences (Fig. E3). Northeastern Australia has recorded below-average precipitation in seven of the last eight months (Fig. E4).

In central Argentina, exceptionally hot and dry conditions were observed during December, with temperature departures exceeding the 90^th percentile of occurrences (Fig. E1) and precipitation totals below the 10^th percentile of occurrences (Fig. E3). These dry conditions coincided with anomalous sinking motion upstream of an amplified trough axis (Fig. E9).

The South African rainy season lasts from October to April. During December, much of the region recorded totals in the upper 70^th percentile of occurrences, with large interior regions recording totals in the upper 90^th percentile of occurrences (Fig. E3). For the entire South African monsoon region, area-averaged totals were in the upper 80^th percentile of occurrences (Fig. E4).  To date the 2013-14 rainy season has produced above-average totals during October and December, and below-average totals during November.