Extratropical Highlights –November 2019

 

1. Northern Hemisphere

The 500-hPa circulation during November featured above-average heights over western North America, Greenland, and western Russia, and below-average heights over eastern North America and Europe (Fig. E9). The main land-surface temperature signals included above-average temperatures in Alaska and eastern Europe, and below-average temperatures in the eastern U.S. and central Russia (Fig. E1). The main precipitation signals included above-average totals in southwestern U.S. and central Europe, and below-average totals in the northwestern and Gulf Coast regions of the U.S. (Fig. E3).

 

a. North America

The 500-hPa circulation during November featured an amplified ridge-trough pattern across North America, with a strong ridge in the west and an amplified Hudson Bay trough in the east (Fig. E9). This pattern contributed to above-average surface temperatures in Alaska, and to below-average temperatures in the eastern U.S. and southeastern Canada (Fig. E1). It also contributed to below-average precipitation within and downstream of the mean ridge axis (Fig. E3), with the northwestern U.S. recording totals in the lowest 5^th percentile of occurrences (Fig. E5).

 

b. Europe

The 500-hPa circulation during November featured an amplified wave pattern with below-average heights over Europe and above-average heights over western Russia. This pattern was associated with exceptionally warm surface temperature in eastern Europe (Fig. E1), and with above-average precipitation across central and southeastern Europe (Fig. E3). For the Southern Europe region as a whole, the area-averaged precipitation total was in the upper 95^th percentile of occurrences (Fig. E4).

 

c. Atlantic hurricane season

The 2019 Atlantic hurricane season produced 18 named storms, with six becoming hurricanes and three of those becoming major hurricanes. The 2019 Accumulated Cyclone Energy (ACE) value was about 135% of the 1981-2010 median. Based on this activity, NOAA classifies the season as being above normal. This marks the fourth consecutive above-normal season. The only other period in the record (1950-present) that produced four consecutive above-normal seasons was 1998-2001. An average season has 12 named storms, six hurricanes, and three major hurricanes.

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 weak vertical wind shear across the western Atlantic Ocean and Gulf of Mexico.

 

 

2. Southern Hemisphere

The 500-hPa height field during November featured above-average heights over the polar region and in the area west of southern Australia, and below-average heights to the south of eastern Australia, and over the three central ocean basins (Fig. E15). In Australia, the anomalous ridge-trough pattern contributed to drier than average conditions (Fig. E1) across much of the continent, with many areas recording totals in the lowest 10^th percentile of occurrences (Fig. E3).

The South African monsoon season runs from October to April. During November 2019, much of this area recorded below-average precipitation (Fig. E3), and area-averaged totals were in the lowest 30^th percentile of occurrences (Fig. E4). The region recorded even stronger deficits in October.

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 2019, the ozone hole disappeared in early November, which is far sooner than average. Overall, the spatial extent and duration of the 2019 ozone hole were significantly below average. This evolution reflected highly anomalous stratospheric conditions that began in early September in association with a stratospheric warming event, which led to a sharp reduction in the size of the polar vortex (Fig. S8 middle). These conditions resulted in a complete disappearance of PSC’s (Fig. S8 bottom) and a dramatic lessening of the ozone hole.