Extratropical Highlights – November 2011

 

1. Northern Hemisphere

The 500-hPa circulation during November featured a persistent zonal wave-4 anomaly pattern (Fig. E11), with above average heights over the central North Pacific, eastern Canada, Europe, and eastern Asia (Fig. E9). Below average heights were observed over northwestern North America, the central North Atlantic, central Russia, and eastern Siberia. This overall pattern projected onto three prominent teleconnections, including the positive phases of the North Atlantic Oscillation (NAO, +1.3) and East Atlantic/ Western Russia pattern (+2.1), and the negative phase (-1.3) of the East Pacific-North Pacific (EP) pattern (Table E1, Fig. E7).

The circulation over the Pacific/ North American sector showed links to La Niña. This link is most evident at 200-hPa, where a 3-celled pattern of extratropical height anomalies spanning the central Pacific to eastern north America is seen to emanate from a large area of cyclonic streamfunction anomalies over the central subtropical Pacific (Fig. T22). These cyclonic anomalies reflect an enhanced mid-Pacific trough and a marked westward retraction of the East Asian jet core (Fig. T21), both of which are a well-known response to the La Niña- related suppression of tropical convection across the central equatorial Pacific (Fig. T25).

The main surface temperature signals during November included warmer-than-average conditions across eastern Canada, central Europe, Scandinavia, and China, and below average temperatures in Alaska and the Middle East (Fig. E1). The main precipitation signals included above-average totals in the mid-western U.S., southern Europe, and eastern China, and below-average totals along the U.S. Gulf Coast and much of Europe (Fig. E3).

 

a. North Pacific and North America

The mean 500-hPa circulation during November featured above average heights over the central North Pacific and eastern North America, and below average heights over Alaska and western Canada (Fig. E9). This pattern projected onto the negative phase (-1.3) of the East Pacific-North Pacific (EP) teleconnection pattern (Table E1, Fig. E7). When viewed in combination with the cyclonic streamfunction anomalies over the central subtropical North Pacific, this overall pattern can be interpreted as an anomalous wave train emanating from the tropical Pacific in association with La Niña (Fig. T22).

La Niña is associated with deep tropical convection focused over Indonesia and the eastern Indian Ocean, along with a disappearance of tropical convection from the central equatorial Pacific (Fig. T25). This westward retraction in the area of deep convection acts to amplify the mean mid-Pacific troughs at 200-hPa in both hemispheres (Fig. T22), which in the NH results in a westward retraction the east Asian jet stream and its associated jet exit region (Fig. T21). This jet structure favors corresponding westward shifts in the downstream ridge and trough axes normally located over western and eastern North America, respectively. During November, for example, these features were located over the central North Pacific and western North America, respectively.

These overall conditions were associated with above average surface temperatures across eastern Canada and below average temperatures in Alaska (Fig. E1). Large portions of eastern Canada have recorded positive temperature departures above the 90^th percentile of occurrences for the last two months. Also during November, above-average precipitation was recorded in the Tennessee and Ohio Valleys, along with below-average precipitation along the U.S. Gulf Coast (Fig. E3). These precipitation signals are typical of the wintertime response to La Niña.

b. North Atlantic and Europe

The 500-hPa circulation during November featured a large-amplitude wave pattern extending from eastern North America to Mongolia (Fig. E9). Prominent features of this pattern included a strong ridge over Europe and deep troughs over both the central North Atlantic and western Russia. These anomalies  projected onto the positive phases of the North Atlantic Oscillation (NAO, +1.3) and the East Atlantic/ Western Russia pattern (+2.1) (Table E1, Fig. E7).

This overall circulation was associated with an enhanced northward transport of mild air into Scandinavia, resulting in surface temperature departures above the 90^th percentile of occurrences (Fig. E1). It was also associated with an enhanced southward transport of colder air and below average temperatures across southwestern Russia and the Middle East. The mean ridge and trough positions also strongly controlled the precipitation patterns, with above-average totals observed over southern Europe and south-central Russia in the areas downstream of the mean trough axes, and well below-average totals across the remainder of Europe and western Russia in the vicinity of the amplified ridge axis (Fig. E3).

 

2. Southern Hemisphere

 

The 500-hPa circulation during November featured above average heights over Antarctica, eastern Australia, the central South Pacific Ocean, and the central South Atlantic Ocean (Fig. E15). It also featured below average heights extending from southern Africa to South America. At 200-hPa the subtropical circulation featured an extensive area of cyclonic streamfunction anomalies across the central South Pacific Ocean in association with La Niña (Fig. T22).

In Australia, an east-west dipole pattern of surface temperature anomalies was present during November, with above average temperatures in the east and below average temperatures in the west (Fig. E1). These conditions were associated with a broad upper-level trough-ridge couplet, which spanned the continent from west to east. Most of central and eastern Australia also recorded above-average precipitation, with many areas recording totals in the upper 90^th percentile of occurrences (Fig. E3).

  The SH ozone hole was at record high size during the first half of the month (Fig. S8, top), covering approximately 15 million square kilometers in mid-November. This size is approximately double the average for the time of the year, and occurred in association with a record large SH polar vortex (Fig. S8, middle). The ozone hole then rapidly weakened during the 3^rd week of November is response to a sharp reduction in size of the polar vortex. By the end of November, the ozone hole was near the average size of 5 million square kilometers.

Overall, the 2011 ozone hole developed rapidly in mid-August, which is slightly later than its normal onset in early August. It then reached peak extent from mid-September to early October, spanning approximately 24 million square kilometers. The ozone hole remained persistent and large during mid-October through mid-November, with a record areal extent of 20 million square kilometers throughout the period. The rapid decay of the ozone hole in late November reflected the normal late-spring weakening of the polar vortex.