Extratropical Highlights –December 2020

 

1. Northern Hemisphere

The 500-hPa circulation during December featured above-average heights over the northwestern U.S., the northwestern North Atlantic and western Europe, and below-average heights across eastern Siberia, the middle and high latitudes of North Pacific and the northeastern North Atlantic (Fig. E9). The main land-surface temperature signals included above-average temperatures across much of North America, western Europe and eastern Siberia, and below-average temperature across the eastern Europe to northern China (Fig. E1). The main precipitation signals included near-average totals in much of North America and Asia, and below-average totals in western Europe (Fig. E3).

 

a. North America

The 500-hPa circulation during December featured above-average heights in the western U.S. and eastern Canada, and near-average heights over the eastern North America (Fig. E9). This pattern reflected an amplification of the normal ridge-trough-ridge pattern across North America. These conditions contributed to above-average surface temperatures across most of the U.S. and Canada, 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 December featured above-average heights over western Europe and western Russia, and below-average heights over the Scandinavia (Fig. E9). This pattern was associated with an amplification of the ridge over western Europe and milder, marine air into the higher latitudes (Fig. E10), and with warm surface temperatures throughout western Europe (Fig. E1). The largest departures (exceeding 4°C) were observed in western Europe, and exceeded the 90^th percentile of occurrences (Fig. E1).

 

2. Southern Hemisphere

The 500-hPa height field during December featured strengthened meridional gradients with below-average heights over the polar region, and above-average heights over the high latitudes with maximum centers in the South Indian Ocean and the central South Atlantic Ocean (Fig. E15).

The South African monsoon season runs from October to April. During December 2020, area-averaged rainfall totals were in the 80^th percentile of occurrences (Fig. E4). Area-averaged totals have been below-average in October and above-average in November and December.

The Antarctic ozone hole typically develops during August and reaches peak size in September. The ozone hole then gradually decreases during October and November, and dissipates on average in early December (Fig. S8 top). During December 2020, the ozone hole size was much larger than average. Overall, the spatial extent and duration of the 2020 ozone hole were above average since September 2020. This is associated with a strengthened polar vortex (Fig. S8 middle) and an expanded area of polar stratospheric clouds PSC) (Fig. S8 bottom) that began in August.