Extratropical Highlights –February 2020

 

1. Northern Hemisphere

The 500-hPa circulation during February featured above-average heights in the middle latitudes and below-average heights over the polar region (Fig. E9). Core regions of above-average heights included the central and eastern North Pacific, southern Europe, and northeastern Asia. Core regions of below-average heights included most of Canada, the high latitudes of the North Atlantic, Greenland, and Scandinavia. This overall pattern projected onto several teleconnection patterns, including the positive phases of the North Atlantic Oscillation (NAO), the East Atlantic (EA) pattern, and the Tropical/ Northern Hemisphere (TNH) pattern, as well as the negative phases of the East Pacific (EP) and Scandinavia teleconnection patterns (Fig. E7, Table E1).

The main land-surface temperature signals during February included above-average temperatures in the eastern U.S. and throughout Eurasia (Fig. E1). The main precipitation signals included above-average totals in the southeastern quadrant of the U.S., northern Europe and western Russia, and below-average totals in the far western U.S. and across southern Europe (Fig. E3).

 

a. North America

The 500-hPa circulation during February featured above-average heights over the eastern North Pacific, the western U.S., and the southeastern U.S., and below-average heights over most of Canada (Fig. E9). This pattern projected onto the positive phase of the Tropical/ Northern Hemisphere (TNH) pattern and the negative phase of the East Pacific (EP) teleconnection pattern (Fig. E7, Table E1). Along with this overall pattern was the presence of a broad trough across the continental U.S.

These conditions were associated with an enhanced low-level flow of milder, marine air into western Canada and the northwestern U.S. (Fig. T20), along with anomalous southwesterly winds throughout the troposphere across the eastern half of the U.S. in the region downstream of the mean trough axis (Figs. T20, T21, E10). These conditions contributed to above-average surface temperatures in the eastern U.S. (Fig. E1), and to increased storminess and above-average precipitation over the southeastern quadrant of the U.S. (Fig. E3). In the Midwest, Ohio Valley, and Southeast regions, area-averaged precipitation totals were well above the 90^th percentile of occurrences (Fig. E5). The Midwest region has recorded totals above the 90^th percentile of occurrences for the last two months.

 

b. North Atlantic/ Europe/ northern Asia

The 500-hPa circulation during February featured above-average heights across the middle latitudes of the North Atlantic and southern Europe, and below-average heights over the high latitudes of the North Atlantic, Greenland, and Scandinavia (Fig. E9). This overall pattern projected onto the positive phases of the North Atlantic Oscillation (NAO) and the East Atlantic (EA) pattern, and onto the negative phase of the Scandinavia teleconnection pattern (Fig. E7, Table E1). In addition, above-average heights were observed across eastern Asia

These conditions were associated with a significantly enhanced onshore flow of milder marine air into Europe (Figs. T20, T21, E10), and with a nearly complete disappearance of the flow of arctic air into western Russia and central Siberia. The result was exceptionally warm surface temperatures across northern Europe, Russia and most of northern Asia (Fig. E1), with most locations recording departures in the upper 90^th percentile of occurrences (Fig. E1). For northern Europe and much of Eurasia, this represents a continuation of exceptional warmth that has persisted for several months.

In northern Europe, the significantly enhanced jet stream winds (Fig. E10) were also associated with increased storminess and well above-average precipitation (Fig. E3), with area-averaged totals above the 90^th percentile of occurrences (Fig. E4). In contrast, below-average precipitation totals across southern Europe coincided with the anomalous 500-hPa ridge axis located south of the enhanced jet stream (Fig. E9).

 

2. Southern Hemisphere

The 500-hPa height field during February featured above-average heights in the area south of Australia and across the central South Pacific (Fig. E15). In southeastern Australia, well above-average precipitation was recorded during February (Fig. E3). This increased precipitation, along with near-average totals in January, helped to moderate a devastating fire season.

The South African monsoon season runs from October to April. During February 2020, area-averaged rainfall totals were near average (Fig. E4). Previously, area-averaged totals had been below average during the first four months of the 2019-2020 rainy season.