Extratropical Highlights –November 2020

 

1. Northern Hemisphere

The 500-hPa circulation during November featured above-average heights from the central U.S. to the northwestern Atlantic Ocean, from western Europe to eastern Siberia, and over the middle-latitudes of the North Pacific, and below-average heights over the North Pole and the high latitudes of the North Pacific (Fig. E9). The main land-surface temperature signals included above-average temperatures in the central and eastern U.S. and the high-latitudes of Eurasia, and below-average temperatures in central Asia (Fig. E1). The main precipitation signals included below-average from the southern U.S. to the northeastern U.S. via the Ohio Valley, and over the southern Europe, and above-average totals over the Caribbean Sea (Fig. E3).

 

a. North America

The 500-hPa circulation during November featured an enhanced meridional gradient over North America, with strong positive anomalies in the central and eastern U.S. and negative anomalies in the subpolar region (Fig. E9). This pattern contributed to above-average surface temperatures in the central and eastern U.S., and to below- or near-average temperatures in the northwestern U.S. (Fig. E1). It also contributed to below-average precipitation from the southern U.S. to the northeastern U.S. via the Ohio Valley (Fig. E3, E6), with the northeastern U.S. recording totals in the lowest 5^th percentile of occurrences (Fig. E5).

 

b. Eurasia

The 500-hPa circulation during November featured above-average heights across Europe and extending to central Siberia. This pattern was associated with exceptionally warm surface temperatures across Europe and the high latitudes of Asia, and below average surface temperatures in portions of central Asia (Fig. E1). For the southern Europe region as a whole, the area-averaged precipitation total was in the lowest 5^th percentile of occurrences (Fig. E4).

 

c. Atlantic hurricane season

The 2020 Atlantic hurricane season was extremely active with a record high of 30 named storms, with 13 becoming hurricanes and six of those becoming major hurricanes. An average season during 1981-2010 has 12 named storms, six hurricanes, and three major hurricanes. Out of the 30 named storms in 2020, 12 made landfall in the contiguous U.S., breaking the record of nine set in 1916. The Accumulated Cyclone Energy (ACE) value in 2020 was 190% of the 1981-2010 median. NOAA classifies the season as being above normal and extremely active. This marks a record fifth consecutive above-normal season from 2016 onward. It also marks the fifth consecutive season in which at least one Category 5 hurricane formed. During the season, 27 tropical storms established a new record for the earliest formation by storm number. This season also featured a record 10 tropical cyclones that underwent a rapid intensification, tying it with 1995.

The above-normal activity is consistent with the ongoing high-activity era for Atlantic hurricanes, which began in 1995 in association with a transition to the warm phase of the Atlantic Multi-Decadal Oscillation (AMO). This unprecedented active season was also fueled by a La Niña that developed in August.  The conditions that favored more, stronger, and longer-lasting storms this year also included a stronger west African monsoon system (Fig. E4), warmer Atlantic waters, and weak vertical wind shear across the tropical Atlantic Ocean, Caribbean Sea, and the Gulf of Mexico.

 

 

2. Southern Hemisphere

The 500-hPa height field during November featured a dipole pattern in the South Polar region, with above-average heights over the Indian Ocean’s side and below-average heights over the Pacific Ocean’s side.  Above-average heights were also observed from the high latitudes of the southwestern Pacific to Australia, and over the southwestern Atlantic Ocean, while below-average heights were present over the southeastern Indian Ocean (Fig. E15). In Australia, the anomalous circulation pattern contributed to drier and warmer than average conditions across the eastern half of the continent (Fig. E1, E4).

The South African monsoon season runs from October to April. During November 2020, much of this area recorded near- or above-average precipitation (Fig. E3). For the season to date, area-averaged rainfall was below average during October and near-average during November (Fig. E4).

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 November 2020, the ozone hole size was much larger than average, with a record size of 27 million square kilometers during the second half of the month. 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.