Extratropical Highlights – January 2015

 

1. Northern Hemisphere

The mean 500-hPa circulation during January featured an extensive anomalous wave pattern extending from eastern Asia to Europe. Regional aspects of this pattern included above-average heights across the high latitudes of the North Pacific, Alaska, western North America, the central North Atlantic, and eastern Asia (Fig. E9). The pattern also featured below-average heights over the central subtropical north Pacific, and across the high latitudes of the North Atlantic from eastern Canada to Scandinavia. This pattern projected onto the positive phases of the North Atlantic Oscillation (NAO, (+1.57) and the East-Atlantic teleconnection pattern (+1.11) (Table E1, Fig. E7).

The main land-surface temperature signals during January included above-average temperatures across eastern Alaska, western Canada, the western U.S., western Russia and east-central Asia (Fig. E1). The main precipitation signals included above-average totals in eastern Europe and western Russia, and below-average totals in the western and southeastern portions of the U.S. (Fig. E3).

 

a. North Pacific/ North America

The mean 500-hPa circulation during January featured a north-south dipole pattern of height anomalies over the North Pacific, and an anomalous ridge-trough pattern across North America (Fig. E9). Over North America, this anomaly pattern reflected an amplified ridge in the west and an amplified trough in the east. This pattern was associated with above average surface temperatures across western North America, with the largest departures (exceeding +4^oC) observed in western Canada and eastern Alaska (Fig. E1). It was also associated with exceptionally warm and dry (Fig. E3) conditions in the western U.S., with many areas recording temperature departures in the upper 70^th percentile of occurrences and precipitation totals in the lowest 10^th percentile of occurrences.

Consistent with these conditions, the U.S. Drought Monitor indicated a continuation of exceptional drought in central/ southern California and western Nevada, with extreme drought extending northward into Oregon. Exceptional or extreme drought also continued in north-central Texas and western Oklahoma.

 

b. North Atlantic/ Europe

The 500-hPa circulation featured a north-south dipole pattern of height anomalies across the North Atlantic, with above-average heights in the middle latitudes and below-average heights extending from eastern Canada to Scandinavia. This pattern projected onto the positive phase of the NAO (+1.57) and the east Atlantic teleconnection pattern (+1.11) (Table E1, Fig. E7). It was associated with amplified Icelandic Low, an enhanced ridge over the east-central North Atlantic, and a split flow pattern centered over western Europe (Fig. E10). It was also associated with an enhanced transport of milder, marine air into Europe, which contributed to anomalously warm (Fig. E1) and wet (Fig. E3) conditions in eastern Europe and western Russia.

 

c. East-central Asia

            Exceptionally warm surface temperatures were observed across east-central Asia during January, with many locations recording departures in the upper 90^th percentile of occurrences (Fig. E1). This warmth was related to a weakening of the climatological mean trough over northeastern Asia, which typically brings cold air into northern China and Mongolia (Figs. E9, T23).

 

2. Southern Hemisphere

The mean 500-hPa circulation during January featured above-average heights over the high latitudes of the South Pacific, and below-average heights over the central South Atlantic Ocean (Fig. E15). The South African rainy season lasts from October to April. Rainfall during January for was below average the region as a whole (Fig. E4), with the main deficits recorded in the south and the main surpluses recorded in northern Mozambique and Madagascar (Fig. E3). To date, the South African rainy season was below average during October, November, and January, and above average in December (Fig. E4).