Extratropical Highlights –November 2015

 

1. Northern Hemisphere

The mean 500-hPa circulation during November featured above-average heights over the central North Pacific, eastern North America, and southern Europe, and below-average heights over the western U.S., the polar region and central Russia (Fig. E9). Over the North Atlantic sector, the anomaly pattern projected onto the strong positive phase of the North Atlantic Oscillation (NAO, +1.7) and the East Atlantic (+1.5) teleconnection pattern (Table E1, Fig. E7).

At 200-hPa, a significant El Niño response continued in the streamfunction (Fig. T22) and wind (Figs. T21) fields. This response featured anticyclonic anomalies over the eastern half of the subtropical North and South Pacific, straddling the region of enhanced convection (Fig. T25).

The main land-surface temperature signals during November included well above-average temperatures across Canada, the eastern half of the U.S., Europe, and Scandinavia, along with below-average temperatures in the western U.S. (Fig. E1). The main precipitation signals included above-average totals in the central and southeastern U.S., northeastern Europe, western Russia, China, and Japan, and below-average totals across southern Europe (Fig. E3).

 

a. North Pacific/ North America

At 500-hPa, the circulation during November featured above-average heights across the central North Pacific Ocean and eastern North America, and below-average heights over the western U.S. (Fig. E9). At 200-hPa, the circulation featured anticyclonic streamfunction anomalies across the subtropical eastern North Pacific in association with El Niño (Fig. T22).

Over North America, the 500-hPa height pattern reflected an amplified trough in the West and a disappearance of the mean trough normally situated over the eastern U.S. This overall circulation pattern contributed to anomalously warm conditions across most of Canada, with most to the eastern half of the country recording departures in the upper 90^th percentile of occurrences (Fig. E1). Anomalously warm and wet conditions were observed across the eastern half of the U.S. (Fig. E3), with portions of the Southern Plains recording precipitation totals in the upper 90^th percentile of occurrences.

Overall, area-averaged precipitation totals exceeded the 90^th percentile of occurrences in the Great Plains, Midwest, Southeast, and Gulf Coast regions of the U. S. (Fig. E5). According to the U.S. Drought Monitor, exceptional drought continued across central California and western Nevada. Severe or extreme drought persisted in Washington, Oregon, Idaho and western Montana.

 

b. North Atlantic

The 500-hPa circulation during November featured above-average heights across southern Europe and below-average heights over the high latitudes of the North Atlantic. This pattern projected onto the strong positive phase of the North Atlantic Oscillation (NAO, +1.7) and the East Atlantic (+1.5) teleconnection pattern (Table E1, Fig. E7). These conditions resulted in well above-average surface temperatures across Europe and Scandinavia (Fig. E1), with many locations recording departures in the upper 90^th percentile of occurrences.  They also contributed to well below-average precipitation across southern Europe, where area-averaged totals were the lowest in the 1971-present record (Fig. E4).

 

c. China/ Japan

Well above-average precipitation was recorded in China and Japan during November, with many locations recording totals in the upper 90^th percentile of occurrences (Fig. E3). Area-average precipitation totals in China were the largest in the 1971-present record (Fig. E4), with the most significant departures recorded in the east. These conditions resulted from an amplified trough-ridge pattern that extended across eastern Asia and the western North Pacific (Figs. E9, T21), which produced increased storminess and an enhanced low-level inflow of moisture across eastern China and Japan (Fig. T20).

 

2. Southern Hemisphere

The mean 500-hPa circulation during November featured above-average heights across southern Australia and the eastern South Pacific, and below-average heights over the high latitudes of South Pacific (Fig. E15). At 200-hPa, a significant El Niño response was evident in the streamfunction field across the eastern half of the Pacific Ocean (Fig. T22). This response featured anticyclonic anomalies over the subtropical North and South Pacific straddling the region of enhanced convection (Fig. T25).

In the SH, this pattern was associated with 1) a strengthening and eastward extension of the South Pacific jet stream to well east of the date line, and 2) an eastward shift of that jet’s exit region to the eastern South Pacific (Fig. T21). This jet stream pattern represents major dynamical and kinematic changes in the mid- and upper-level circulation during El Niño, and it also represents a fundamental manner in which El Niño’s circulation impacts are communicated downstream.

The main surface temperature signals during November included well above-average temperatures across Australia, with departures in eastern Australia exceeding the upper 90^th percentile of occurrences (Fig. E1).

The South African rainy season lasts from October to April. During November 2015, exceptionally dry conditions were recorded in southern Africa with many locations recording totals in the lowest 10^th percentile of occurrences (Fig. E3). These deficits follow record low totals in October (Fig. E4).

The Antarctic ozone hole typically develops during August and reaches its peak aerial extent in September and early October (Fig. S8). The ozone hole then typically weakens rapidly after mid-October. In contrast to this normal evolution, the ozone hole decreased only slightly during October and November 2015 (Fig. S6), and still spanned approximately 10 million square kilometers at the end of November. This size is largest in the 2005-2014 record (Fig. S8, top), and it is nearly three times that of the 2005-2014 average. This record ozone hole was associated with the largest SH polar vortex observed in the 2005-2014 record (Fig. S8, middle), and also with a record coverage of polar stratospheric cloud (Fig. S8, bottom). This record-size ozone hole follows a near-average size in 2014, and below-average sizes during both 2012 and 2013.