Extratropical Highlights –November 2017

 

1. Northern Hemisphere

The 500-hPa circulation during November featured an anomalous zonal wave-4 pattern. Above-average heights were present over the high latitudes of the central North Pacific, the western U.S., the central North Atlantic, and the polar region. Below-average heights were present over the Gulf of Alaska, Hudson Bay, Scandinavia, and eastern Asia (Fig. E9). This overall anomaly pattern projected onto the strong negative phases of the Polar/ Eurasian Pattern (-2.2) and the Pacific/ North American pattern (PNA, -2.0) (Table E1, Fig. E7).

At 200-hPa, the circulation featured amplified mid-Pacific troughs in the subtropics of both hemispheres (Fig. T22), along with a westward retraction of the East Asian jet stream (Fig. T21).  These conditions reflect the disappearance of deep tropical convection from the central equatorial Pacific and are typical of La Niña. They are also consistent with the negative phase of the PNA teleconnection pattern.

The main land-surface temperature signals during November included above-average temperatures in western Alaska, the western continental U.S., and western Russia, and below-average temperatures in western Canada (Fig. E1). The main precipitation signals included above-average totals in the northwestern U.S., and below-average totals in central and eastern U.S. (Fig. E3).

The 2017 Atlantic hurricane season ended as the fifth strongest since 1853. The season produced 17 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 2017 seasonal ACE value was 245% of the 1981-2010 median, which is the largest since 2005.

 

a. North Pacific and North America

The 500-hPa circulation during November featured above-average heights over the high latitudes of the central North Pacific and over the southwestern U.S., and below-average heights over the Gulf of Alaska. This anomaly pattern projected onto a strong negative phase (-2.0) of the PNA teleconnection pattern (Fig. E9). This pattern typifies La Niña conditions, and is consistent with the pronounced westward retraction of the East Asian jet steam typically seen during La Niña (Fig. T21).

This overall pattern contributed to anomalously warm conditions in Alaska and the western U.S. (Fig. E1). It contributed to above-average precipitation in the northwestern U.S. and to well below-average precipitation across the southwestern, central and eastern U.S. (Fig. E3). Overall, the Southwest, Great Plains, Midwest, Gulf Coast, Southeast, and Ohio Valley regions each recorded area-averaged totals that were in the lowest 10^th percentile of occurrences (Fig. E5).

 

2. Southern Hemisphere

The mean 500-hPa circulation during November featured above-average heights in the area southeast of Australia, and below-average heights over the high latitudes of the central South Pacific and over the southwestern South Atlantic (Fig. E15). At 200-hPa the circulation featured an amplified trough over the central South Pacific Ocean (Fig. T22). This pattern is typical of La Niña.

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). The 2017 ozone hole completely dissipated by mid-November. Overall, the 2017 ozone hole was smaller than the 2007-2016 mean, but still averaged about 15-20 million square kilometers at its peak. 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 that had disappeared by mid-October (Fig. S4).