Extratropical Highlights –October 2020

 

1. Northern Hemisphere

The 500-hPa circulation during October featured above-average heights across the high latitudes of the North Pacific, North Atlantic, North Pole region, and western Russia, and below-average heights over central North America and Western Europe (Fig. E9). At 200-hPa, the subtropical circulation reflected La Niña. This signal included cyclonic streamfunction anomalies across most of the Pacific Ocean in both hemispheres, and anticyclonic streamfunction anomalies in both hemispheres across the Indian Ocean and Australasia. This anomaly pattern reflected: 1) Amplified mid-Pacific troughs in both hemispheres (Fig. T22) flanking the area of suppressed convection over the central equatorial Pacific (Fig. T25); and 2) Enhanced subtropical ridges over Australasia in association with enhanced convection across Indonesia and southeast Asia.

The main land-surface temperature signals included above-average temperatures in Alaska, Europe, and eastern Siberia, and below-average temperatures in central North America and portions of eastern China (Fig. E1). The main precipitation signals included above-average totals in portions of the central U.S., the Marine Continent and Western Europe, and below-average totals in the southwestern U.S. and Canada (Figs. E3, E6).

 

a. North America

The 500-hPa circulation during October featured an anomalous wave pattern extending from the eastern North Pacific to the North Atlantic. (Fig. E9). This pattern reflected amplified ridges over the eastern North Pacific and eastern North America, and an amplified trough in central North America.  These conditions contributed to above-average surface temperatures in Alaska and the eastern and western U.S., and to below-average temperatures in much of the central U.S. (Fig. E1). The amplified ridges and trough also delineated areas of below-average precipitation in the southwestern U.S. and Canada from areas of above-average precipitation in the Ohio Valley and eastern U.S. (Fig. E3).

 

b. Europe, Siberia, and West African monsoon

The 500-hPa circulation during October featured above-average heights over western Russia, and below-average heights over Western Europe (Fig. E9). This pattern was associated with above-average surface temperatures and precipitation in much of the European Continent, (Figs. E1, E4).

The west African monsoon season extends from June through October, with a peak during July-September. In this year, the west African monsoon system was enhanced from July-October with area-average rainfall totals above the 95^th percentile of occurrences in July- October (see Sahel region, Fig. E4).

 

c. Above-normal Atlantic hurricane activity

The 2020 Atlantic hurricane season has been above normal and extremely active, with 27 named storms, 11 hurricanes, and 4 major hurricanes recorded by the end of October. Also, a record of 11 named storms had struck the continental U.S. by the end of October, with two landfalling hurricanes striking Louisiana during the month. The 2020 season sets a record of five consecutive above-normal Atlantic hurricane seasons, and marks an unprecedented 18^th above-normal season since the high-activity era for Atlantic hurricanes began in 1995.

The enhanced 2020 activity reflects a combination of the ongoing warm phase of the Atlantic Multi-Decadal Oscillation (AMO) and La Niña. Historically, this combination of conditions sets the stage for an extremely active season, as seen again this year.

Warm AMO conditions are a main climate pattern behind the ongoing Atlantic high-activity era. Key features of this pattern were again evident during October, including above-average SSTs across much of the tropical Atlantic Ocean and Caribbean Sea (Fig. T18) and an enhanced West African monsoon (see Sahel region, Fig. E4). These features produce an inter-related set of atmospheric anomalies across the tropical Atlantic, which favors increased Atlantic hurricane activity. These anomalies include an enhanced ridge at 200-hPa (Fig. T22) and weaker low-level tropical easterly trade winds. This combination acts to decrease the vertical wind shear and also to significantly increase the cyclonic shear along the equatorward flank of the 700-hPa African Easterly Jet. The result in an increased number of storms that develop from African easterly waves, which subsequently strengthen as they move westward over progressively warmer waters and decreased vertical wind shear. La Niña acts to amplify the upper-level ridge (Fig. T22) and further decrease the vertical wind shear over the western tropical Atlantic and Caribbean Sea, which further contributes to increased Atlantic hurricane activity.

 

2. Southern Hemisphere

The 500-hPa height field during October featured above-average heights over New Zealand, the middle latitudes of the central South Pacific, southern South American, and south of Africa, and below-average heights in the high latitudes of the South Pacific and South Atlantic (Fig. E15). In the subtropics, the 200-hPa streamfunction pattern reflected La Niña, with anticyclonic anomalies across the Indian Ocean and Indonesia and cyclonic anomalies across the Pacific Ocean.

Overall warmer and drier than average conditions were observed in southern South America and south of Africa, with some areas recording temperature departures in the upper 90^th percentile of occurrences (Fig. E1) and precipitation totals in the lowest 10^th percentile of occurrences (Fig. E3).

The South African monsoon season runs from October to April. During October, this area recorded below-average precipitation, with some locations recording totals in the lowest 10^th percentile of occurrences (Fig. E3).

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). By the end of this October, the size of the ozone hole was about 20 million square kilometers, larger than the 2008-2017 average size of 12.5 million square kilometers.

Overall, the spatial extent and duration of the ozone hole were larger than average. During October, these conditions were associated with an enlarged size of the polar vortex (30 million square kilometers compared to the average of 26 million) (Figs. S8 middle), along with a well above average amount of polar stratospheric clouds (PSCs) (Figs. S8 bottom, E15).