Extratropical Highlights –February 2019

 

1. Northern Hemisphere

The 500-hPa circulation during February featured an extensive height anomaly pattern that extended from the western North Pacific to Europe. Above-average heights were present across the central North Pacific, the Gulf of Alaska, the eastern U.S., and Europe, and below-average heights were present over the central subtropical North Pacific, western North America, the high latitudes of the western North Atlantic, and northeastern Asia. (Fig. E9).

Over the North Pacific/ North America region, the anomaly pattern projected onto the strong positive phases of the West Pacific (+2.1) and Tropical/ Northern Hemisphere (TNH, +1.6) teleconnection patterns, and also onto the negative phase of the Pacific/ North American pattern (PNA, -1.7) (Table E1, Fig. E7). These TNH and PNA phases are opposite to those typically seen during El Niño, and essentially masked the El Niño signal east of the date line.  Nonetheless, the El Niño signal was present over the western Pacific Ocean, as indicated by anticyclonic streamfunction anomalies in the subtropics of both hemispheres (Fig. T22) flanking the region of suppressed equatorial convection (Fig. T25).

The main land-surface temperature signals during February included above-average temperatures across most of Alaska, the southeastern U.S., Europe, and portions of northeastern Asia, and below-average temperatures across the western half of Canada and the northwestern U.S.  (Fig. E1). The main precipitation signals included above-average totals in the western and north-central and eastern U.S., and below-average totals in western Canada (Fig. E3).

 

a. North Pacific and America

The 500-hPa circulation during February featured above-average heights over the Gulf of Alaska and the eastern U.S., and below-average heights over the central subtropical North Pacific and western North America. (Fig. E9). These anomalies are opposite to those typically seen during El Niño. During February, this pattern was associated with a westward retraction of the Pacific jet stream (Fig. T21), and reflected an amplification of the downstream circulation anomalies typically associated with a westward-retracted jet.

This anomalous circulation contributed to above-average surface temperatures in Alaska and the southeastern U.S., with departures in the upper 90^th percentile of occurrences in the Southeast (Fig. E1). In contrast, below-average temperatures were present across western Canada and the northwestern U.S., with most areas recording temperatures in the lowest 10^th percentile of occurrences.

Also, precipitation was well above average in the western U.S. (Fig. E3), with area-averaged totals in the upper 90^th percentile of occurrences in the Pacific Northwest, Inter-Mountain, and Southern California regions (Fig. E5). The north-central U.S. also recorded well above-average precipitation during February (Fig. E3), with area-averaged totals above the 90^th percentile of occurrences in the Midwest, Oho Valley, and Great Lakes regions (Fig. E5).

According to the U.S. Drought Monitor, the area of severe-to-exceptional drought diminished considerably in the four-corner region of Utah, Colorado, Arizona, and New Mexico, and also in Oregon.

 

b. North Atlantic and Eurasia

The 500-hPa height pattern featured above-average heights over Europe and below-average heights over the high latitudes of the western North Atlantic (Fig. E9). This pattern resulted in above-average surface temperatures throughout Europe, with most areas recording departures in the upper 70^th percentile of occurrences (Fig. E1).

 

2. Southern Hemisphere

The mean 500-hPa circulation during February generally featured above-average heights over Antarctica and below-average heights in the middle latitudes (Fig. E15). At 200-hPa, cyclonic streamfunction anomalies over the western subtropical South Pacific were associated with El Niño (Fig. T22).

Much of northern Australia recorded below-average precipitation during February, (Fig. E3), in association with a widespread pattern of suppressed convection over the western tropical Pacific and Indonesia (Fig. T25).

The South African monsoon season runs from October to April. During February, rainfall was below average for the region as a whole (Fig. E4). The most significant departures were observed in the northern portion of the monsoon region, with totals in the lowest 10^th percentile of occurrences in northern Mozambique and central Madagascar (Fig. E3). To date, the monsoon region has recorded below-average precipitation during October-December and February, and only slightly above-average totals in January.