Extratropical Highlights –January 2020

 

1. Northern Hemisphere

The 500-hPa circulation during January featured above-average heights across the North Pacific Ocean, eastern North America, and much of Europe, and below-average heights over the polar region, southern Alaska, and western Canada (Fig. E9). At 200-hPa, large troughs were present across the eastern half of the Pacific Ocean in the subtropics of both hemispheres (Fig. T22). These conditions were associated with a pronounced cross-equatorial flow east of the date line and also over northern South America (Fig. T21). Such pronounced cross-equatorial flow is relatively uncommon.

The main land-surface temperature signals during January included above-average temperatures across much of North America, northern Europe, and the northern half of Asia, and below-average temperatures across Alaska (Fig. E1). The main precipitation signals included above-average totals in the northwestern and central U.S., and below-average totals in southern Alaska, Florida, and southern Europe (Fig. E3).

 

a. North America

The 500-hPa circulation during January featured above-average heights across eastern North America and the eastern North Pacific, and below-average heights over southern Alaska and western Canada (Fig. E9). This pattern reflected a flattening of the normal ridge-trough pattern across North America, along with an anomalously zonal flow coming into the continent and a reduced flow of Canadian air into the U.S. (Fig. T21). These conditions contributed to above-average surface temperatures across most of North America, with many areas recording departures in the upper 70^th percentile of occurrences (Fig. E1).

 

b. North Atlantic and Europe

The 500-hPa circulation during January featured above-average heights over much of Europe, and below-average heights over Greenland (Fig. E9). This pattern was associated with an anomalous southwesterly flow of milder, marine air into Scandinavia (Fig. E10), which contributed to exceptionally warm surface temperatures (departures exceeding 5°C) across northern Europe and northern Asia (Fig. E1).

 

2. Southern Hemisphere

The 500-hPa height field during January featured above-average heights over southeastern Australia, the high latitudes of the eastern South Pacific, and below-average heights over the western South Atlantic (Fig. E15). In eastern Australia, well above-average temperatures persisted during January. However, rainfall returned to near-average after being significantly below-normal in Nov.-Dec. 2019 (Fig. E4). This increased precipitation has helped somewhat to moderate a devastating fire season.

The South African monsoon season runs from October to April. During January 2020, area-averaged rainfall totals were in the lowest 30^th percentile of occurrences (Fig. E4). Area-averaged totals have been below average during the first four months of the 2019-2020 rainy season.