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 70th
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 90th 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 80th 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.