Extratropical Highlights –October 2017

 

1. Northern Hemisphere

The 500-hPa circulation during October featured above-average heights over the Gulf of Alaska, eastern Canada, and western Europe, and below-average heights over northeastern Europe and western Russia (Fig. E9). At 200-hPa, the circulation featured an amplified ridge extending from the Middle East to northeastern China (Fig. T22). The 200-hPa circulation also featured amplified mid-Pacific troughs in the subtropics of both hemispheres, a characteristic feature of La Niña.

The main land-surface temperature signals during October included above-average temperatures in western Alaska, eastern Canada, and Europe (Fig. E1). The main precipitation signals included above-average totals in the northeastern and mid-western U.S., northeastern Europe, and large portions of China, and below-average totals in central and southern Europe (Fig. E3).

The Atlantic hurricane season remained active during the first half of October, with Major Hurricane Ophelia making landfall as an extratropical storm in Ireland with maximum sustained winds above 100 mph. Ophelia is the most eastward Atlantic major hurricane on record. By the end of October, the 2017 Atlantic hurricane season had produced 16 named storms, of which 10 were hurricanes and six were major hurricanes. The season produced the most hurricanes since 2012 and the most major hurricanes since 2005. The seasonal ACE was 245% of the 1981-2010 median, tying several other seasons as the second largest ACE in the 1950-2017 record.

 

a. North Pacific and North America

The 500-hPa circulation during October featured above-average heights over the Gulf of Alaska and eastern Canada. This anomaly pattern contributed to a well-defined trough over central North America. (Fig. E9). This overall pattern contributed to anomalously warm conditions in Alaska, the northeastern U.S., and eastern Canada (Fig. E1). It contributed to above-average precipitation in the mid-western and eastern U.S. (Fig. E3), with the Great Lakes region recording totals in the upper 90^th percentile of occurrences (Fig. E5).

 

b. Europe

The 500-hPa circulation featured above-average heights over western Europe and below-average heights over northeastern Europe and western Russia (Fig. E9). This pattern was associated with exceptionally warm (Fig. E1) and dry conditions (Fig. E3) in southern and western Europe, with southern Europe recording rainfall totals in the lowest 10^th percentile of occurrences (Fig. E4). The pattern was also associated with above-average precipitation in northeastern Europe and western Russia.

 

c. Southeastern Asia/ China

The 200-hPa circulation featured an amplified ridge extending from the Middle East to northeastern China (Fig. T22). Over China, this pattern was associated with a northward shift of the East Asian jet stream, and with anomalous upper-level divergence (Fig. T21). At 850-hPa, the amplified ridge was associated with an extensive onshore flow of marine air into China (Fig. T20). These conditions contributed to well above-average precipitation in large portions of the country, with country-wide totals above the 90^th percentile of occurrences (Fig. E4).

 

 

2. Southern Hemisphere

The mean 500-hPa circulation during October featured above-average heights in the area southeast of Australia, and below-average heights over southern South America (Fig. E15). At 200-hPa the circulation featured an amplified trough in the subtropics that extended across the central and eastern South Pacific Ocean. This pattern is typical of La Niña, and was associated with a reduced strength of the mid-latitude jet stream across most of the Pacific basin.

The main surface temperature signal during October was well above-average temperatures in eastern Australia (Fig. E1). Rainfall in northeastern Australia was well above average, with area-averaged totals above the 90^th percentile of occurrences (Fig. E4). This marks the first month for above-average precipitation in this region since March 2017.

The Antarctic ozone hole typically develops rapidly during August and reaches peak size in September. The ozone hole then gradually decreases during October and November, and dissipates in early December (Fig. S8). During October 2017, the ozone hole size was smaller than the 2007-2016 mean, but still averaged about 12 million square kilometers. This below-average size was associated with a reduced size of the SH polar vortex and with a sharply reduced amount of polar stratospheric cloud (Fig. S4).