Extratropical Highlights –September 2015

 

1. Northern Hemisphere

The mean 500-hPa circulation during September featured above-average heights over the high latitudes of the North Pacific, the northeastern quadrant of the U.S., eastern Canada, Scandinavia, and western Russia, and below-average heights over western Canada and central Europe (Fig. E9).

At 200-hPa, a significant El Niño response continued in the streamfunction field throughout the global tropics and subtropics. This response featured a zonal wave-1 pattern of streamfunction anomalies in both hemispheres (Fig. T22), with anticyclonic anomalies over the subtropical North and South Pacific straddling the region of enhanced convection (Fig. T25), and cyclonic anomalies extending from the America’s to Australasia. 

The main land-surface temperature signals during September included well above-average temperatures across much of the U.S., eastern Canada, eastern Europe, and western Russia (Fig. E1). The main precipitation signals included above-average totals in China, and below-average totals in the south-central U.S. (Fig. E3).

 

a. North Pacific/ North America

At 500-hPa, the circulation during September featured above-average heights over the high latitudes of the North Pacific, the northeastern quadrant of the U.S., and eastern Canada, and below-average heights over western Canada (Fig. E9). Over most of North America, this anomaly pattern reflected an amplified trough in the West, and a disappearance of the mean Hudson Bay Low in the east. Across the southern U.S., the mean circulation during September featured a ridge in the southwest and a trough across the Gulf Coast.

This overall circulation pattern contributed to exceptionally warm surface temperatures across the southwestern, central and northeastern U.S., and also across the eastern half of Canada (Fig. E1). Temperature anomalies across these areas generally exceeded +3C and were in the upper 90^th percentile of occurrences.

Precipitation was below average across the central U.S. and eastern Canada, with departures in many regions falling into the lowest 30^th percentile of occurrences (Fig. E3). According to the U.S. Drought Monitor, this below-average precipitation initiated moderate- or- extreme drought across portions of eastern Texas, much of Mississippi, central Louisiana, and southern Arkansas. Elsewhere, severe or extreme drought persisted in Washington, Oregon, Idaho and western Montana. Also, exceptional drought continued across much of central California and western Nevada.

 

b. North Atlantic

In association with El Niño, the 200-hPa circulation featured cyclonic streamfunction anomalies extending across the tropical and subtropical North Atlantic to southern Asia (Fig. T22). Across the Atlantic hurricane Main Development Region (MDR, which spans the Caribbean Sea and tropical Atlantic Ocean between 9°N-21.5°N), this pattern contributed to an amplification of the Tropical Upper-Tropospheric Trough (TUTT), which now extends well southward into the western MDR.

Over the western MDR, these conditions were associated with above-average westerly winds at 200-hPa (Fig. T21) and enhanced low-level easterly trade winds (Fig. T20). This wind pattern produced enhanced vertical wind shear, and was also associated with anomalous upper-level convergence (Fig. T23) and sinking motion, across large portions of the MDR. This combination of conditions is expected to continue, and to produce a below normal Atlantic hurricane season.

 

c. Eurasia

The 500-hPa circulation during September featured above-average heights over Scandinavia and western Russia, and below-average heights over central Europe (Fig. E9). This pattern contributed to exceptionally warm surface temperatures across eastern Europe and western Russia, with many locations recording departures in the upper 90^th percentile of occurrences (Fig. E1).  

 

2. Southern Hemisphere

The mean 500-hPa circulation during September featured a zonal wave-3 pattern of height anomalies at higher latitudes (Fig. E15). It also featured below-average heights across the central South Pacific and over Antarctica. At 200-hPa, a significant El Niño response was evident in the streamfunction field throughout the global tropics and subtropics. This response featured a zonal wave-1 pattern of streamfunction anomalies in both hemispheres (Fig. T22), with anticyclonic anomalies over the subtropical North and South Pacific straddling the region of enhanced convection (Fig. T25), and cyclonic anomalies extending from the America’s to Australasia.

The El Niño response also included an extended area of below-average heights at 500-hPa (and cyclonic streamfunction anomalies at 200-hPa) across the central South Pacific (Figs. E15, T22). This pattern, combined with above-average heights/ anticyclonic streamfunction anomalies in the Tropics/ subtropics, 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 wintertime 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 Antarctic ozone hole typically develops during August and reaches peak aerial extent in September and October. By the end of September 2015, the ozone hole (Fig. S6) spanned approximately 25 million square kilometers, which is comparable to the largest in the 2005-2014 record (Fig. S8, top). This large ozone hole was associated with the largest SH polar vortex observed in the 2005-2014 record (Fig. S8, middle) and with a well above-average coverage of polar stratospheric cloud (Fig. S8, bottom). These conditions were accompanied by well below-average stratospheric temperatures for the past several months (Fig. S4, right).