Extratropical Highlights –November 2021

 

1. Northern Hemisphere

The 500-hPa circulation during November featured above-average heights over the North Pole, Sea of Okhotsk, Black Sea, North Atlantic Ocean, and western U.S., and below-average heights over the Gulf of Alaska, Arctic Ocean, Mediterranean Sea, and eastern U.S. (Fig. E9). The main land-surface temperature signals included above-average temperatures throughout much of central North America and Eurasia, and below-average temperatures in Alaska and the eastern U.S. (Fig. E1). The main precipitation signals included above-average totals in parts of Canada, Europe, and Russia, and below-average totals in Alaska and the eastern U.S. (Fig. E3).

 

a. North America

The 500-hPa circulation during November featured an amplified ridge-trough pattern across the U.S., with a strong ridge in the west and an amplified trough in the east (Fig. E9). This pattern contributed to well above-average surface temperatures in the west and Canada, and to slightly below-average temperatures in the eastern U.S. (Fig. E1). The trough over the Gulf of Alaska ushered in below-average temperatures and below-average precipitation for Alaska.  Despite the troughing over the eastern U.S., much of CONUS recorded below-average precipitation with totals in the lowest 30th percentile for the western U.S. and totals in the lowest 10th percentile for the eastern U.S. (Fig. E3).

 

b. Europe and Asia

The 500-hPa circulation during November featured an amplified wave pattern with above-average heights over the North Atlantic Ocean, below-average heights over the Mediterranean Sea, and above-average heights over the Black Sea.  This pattern was associated with warm surface temperatures across Eurasia and exceptionally warm temperatures in Russia (Fig. E1).  Much of the region recorded temperatures in the 70th percentile and the 90th percentile was reached in Russia (Fig. E1).  Several regions in Russia, namely western and eastern Russia, recorded above-average precipitation with some areas recording the highest 90th percentile of precipitation (Fig. E3). Near-normal temperatures were observed for Europe (Fig. E1) with below-average precipitation to the north and above-average precipitation in the southern parts of the region (Fig. E3). 

 

c. Atlantic hurricane season

The 2021 Atlantic hurricane season produced 21 named storms, with seven becoming hurricanes and four of those becoming major hurricanes. The 2021 Accumulated Cyclone Energy (ACE) value was about 147% of the 1951-2020 median. Based on this activity, NOAA classifies the season as being above normal. This marks the sixth consecutive above-normal season, extending the record stretch by another season. An average season has 14 named storms, 7 hurricanes, and three major hurricanes.  The fifth storm this year, Hurricane Elsa, was the earliest fifth named storm on record. The season then became quiet with no hurricanes present after Oct 5th.

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).  Conditions that favored more, stronger, and longer-lasting storms this year included a stronger west African monsoon system (Fig. E4), warmer Atlantic waters, and favorable trade winds.  Early in the season, weak vertical wind shear favored development, but then rapidly increased across the Gulf of Mexico during October, likely contributing to the quiet end of the season.

 

2. Southern Hemisphere

The 500-hPa height field during November featured above-average heights over the three central ocean basins, and below-average heights over the Southern Ocean (Fig. E15). In southern Australia, heights were slightly below-average while temperatures were also slightly below-average and precipitation was broadly above-average with percentiles reaching the 90th percent (Figs. E1, E3).  In interior Antarctica, heights were slightly above-average (Fig. E15).

The South African monsoon season runs from October to April. During November 2021, much of this area recorded below-average precipitation (Fig. E3), and area-averaged totals were near the lowest 20^th percentile of occurrences (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 2021, the ozone hole was predominantly above the 2010-2019 mean and was double the average size by the end of November.  The polar vortex was also above-average for November (Fig. S8 middle) while polar stratospheric clouds were near normal conditions for November and largely absent (Fig. S8 bottom).