Extratropical Highlights –December 2019

 

1. Northern Hemisphere

The 500-hPa circulation during December featured above-average heights across the central North Pacific Ocean, the northeastern U.S. and eastern Canada, and much of Europe, and below-average heights over the far eastern North Pacific and the high latitudes of the North Atlantic (Fig. E9). The main land-surface temperature signals included above-average temperatures across much of North America, Europe, and western Russia (Fig. E1). The main precipitation signals included above-average totals in southwestern and eastern U.S. and in central Europe, and below-average totals in the south-central U.S. (Fig. E3).

 

a. North America

The 500-hPa circulation during December featured above-average heights across the northeastern U.S. and eastern Canada, and below-average heights over the far eastern North Pacific (Fig. E9). This pattern reflected a flattening of the normal ridge-trough pattern across North America, along with an anomalously zonal flow across the continent (Fig. T21). These conditions contributed to above-average surface temperatures across most of the U.S. and Canada, with many areas recording departures in the upper 70^th percentile of occurrences (Fig. E1).

 

b. North Atlantic and Europe

The 500-hPa circulation during December featured above-average heights over Europe and western Russia, and below-average heights over the high latitudes of the North Atlantic (Fig. E9). This pattern was associated with an anomalously zonal flow of milder, marine air into the continent (Fig. E10), and with exceptionally warm surface temperatures throughout Europe and western Russia (Fig. E1). The largest departures (exceeding 4°C) were observed in eastern Europe and western Russia, and exceeded the 90^th percentile of occurrences.

 

2. Southern Hemisphere

The 500-hPa height field during December featured above-average heights over the polar region, across the southern half of Australia, and over the central South Pacific, and below-average heights over the high latitudes of the eastern South Pacific, the central South Atlantic, and the central Indian Ocean (Fig. E15).

In Australia, the upper-level (200-hPa) circulation featured a north-south dipole pattern of streamfunction anomalies, with anti-cyclonic anomalies across the south and cyclonic anomalies across the north (Fig. T22). This pattern was associated with a poleward shift to the south of the continent of the mean jet stream (Fig. T21), and with anomalous upper-level convergence and sinking motion across the eastern half of Australia (Fig. T23). Accompanying these conditions the South Pacific Convergence Zone, which is normally located near northeastern Australia and southern New Guinea, was shifted well northeast of normal (Fig. T25). The result was significant rainfall deficits across the eastern half of Australia, with most areas recording totals in the lowest 10^th percentile of occurrences (Fig. E3). Combined with exceptionally warm surface temperatures (Fig. E1), these conditions helped set the stage for widespread, uncontrolled wildfires in eastern Australia.

The South African monsoon season runs from October to April. During December 2019, area-averaged rainfall totals were in the lowest 30^th percentile of occurrences (Fig. E4). Area-averaged totals have been below average during the first three months of the rainy season.