Extratropical Highlights –January 2021

 

1. Northern Hemisphere

The 500-hPa circulation during January featured above-average heights across the mid-latitudes of the North Pacific Ocean, eastern North America- the polar region, and West Europe, and below-average heights over the high latitudes of the North Pacific, eastern Siberia, the mid-latitudes of the North Atlantic, and the Scandinavia Peninsular (Fig. E9). At 200-hPa, large anomalous troughs were present across the eastern half of the Pacific Ocean in the subtropics of both hemispheres (Fig. T22).

The main land-surface temperature signals during January included above-average temperatures across much of high-latitudes of North America, northern Europe, and the northern half of Asia, and below-average temperatures across Asia and the northwestern U. S.  (Fig. E1). The main precipitation signals included above-average totals in the Great Plains and Mid-west of the U.S., portions of western Europe, the Indian Peninsular, and the Maritime Continent, and below-average totals in the northeastern U. S., Middle East, and portions of southeast Asia (Fig. E3, E5, E6).

 

a. North America

The 500-hPa circulation during January featured above-average heights across eastern North America, and below-average heights over Alaska and mid-latitudes of the North Atlantic (Fig. E9). This pattern reflected a flattening of the trough across eastern North America and the North Atlantic (Fig. T22). These conditions contributed to above-average surface temperatures across the high-latitudes of North America, with many areas recording departures in the upper 70^th percentile of occurrences (Fig. E1).

 

b. Europe and Asia

The 500-hPa circulation during January featured above-average heights over much of West Europe, and below-average heights over the high latitudes of northeastern Asia and eastern Siberia (Fig. E9). This pattern was associated with an anomalous southwesterly flow of milder, marine air into the high-latitudes of the Eurasian Continent and an anomalous northerly flow of cold air into Asia (Fig. E10), which contributed to exceptionally warm surface temperatures (departures exceeding 9°C) across Russia, and anomalous cold surface temperature in Asia (Fig. E1).

 

2. Southern Hemisphere

The 500-hPa height field during January featured above-average heights over southeastern Australia, and the high latitudes of the Atlantic and central South Pacific Oceans, and below-average heights over the high latitudes and polar region (Fig. E15). Such anomalous pattern reflected enhancements of the polar low and the associated zonal flow. Overall, the temperatures over the lands were below- or near-average in January (Fig. E1). Rainfall was above -average over much of Africa (Fig. E3, E4).

The South African monsoon season runs from October to April. During December 2020-January 2021, area-averaged rainfall totals were in the highest 80^th percentile of occurrences (Fig. E4). The area-averaged totals have been above average during the last 3 months.