Highlights –October 2020
1. Northern Hemisphere
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
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
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 95th
percentile of occurrences in July- October (see Sahel region, Fig. E4).
c. Above-normal Atlantic hurricane activity
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 18th 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
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
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
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.
and drier than average conditions were observed in southern South America and
south of Africa, with some areas recording temperature departures in the upper
90th percentile of occurrences (Fig.
E1) and precipitation totals in the lowest 10th
percentile of occurrences (Fig. E3).
African monsoon season runs from October to April. During October, this area
recorded below-average precipitation, with some locations recording totals in
the lowest 10th percentile of occurrences (Fig. E3).
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.
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).