Extratropical Highlights –March 2016

 

1. Northern Hemisphere

The 500-hPa circulation during March featured above-average heights across the subtropical North Pacific Ocean, the eastern U.S., the high latitudes of the North Atlantic, and central/ eastern Asia. The circulation also featured below-average heights across the eastern North Pacific and southern Europe (Fig. E9).

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 central subtropical North and South Pacific straddling the region of enhanced convection (Fig. T25), along with cyclonic anomalies at higher latitudes of the North Pacific. This pattern was associated with an eastward extension of the East Asian jet steam to well east of the date line. 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 land-surface temperature signals during March included a continuation of above-average temperatures across much of North America and central Asia (Fig. E1). The main precipitation signals included above-average totals in the northwestern, Midwest and Gulf Coast portion of the U.S., and southern Europe (Fig. E3).

 

a. North Pacific/ North America

At 500-hPa, the circulation during March featured above-average heights across the subtropical North Pacific Ocean and eastern North America, and below-average heights over the eastern North Pacific (Fig. E9). At 200-hPa, the circulation featured anticyclonic streamfunction anomalies across the subtropical central North Pacific in association with El Niño, along with cyclonic anomalies over the high latitudes of the North Pacific (Fig. T22). This pattern was associated with an eastward extension of the East Asian jet steam to well east of the date line, along with an amplified jet exit region situated immediately upstream of the western U.S. (Fig. T21). This anomalous jet exit region is indicated by the combination of southwesterly wind anomalies upstream of California north of the mean jet axis and northwesterly/ northerly wind anomalies upstream of Mexico south of the mean jet axis.

This overall circulation was associated with an extensive onshore flow of marine air into North America, which contributed to increased storminess and above-average precipitation (in the upper 90^th percentile of occurrences) in the U.S. Pacific Northwest (Figs. E3, E5). It also contributed to a continuation of well above-average surface temperatures across the continent. The most significant temperature departures (in the upper 90^th percentile of occurrences) were observed in Alaska, western and central Canada, and the eastern half of the U.S. (Fig. E1).

The circulation during March also featured a weaker Hudson Bay trough that was shifted west of normal to the central U.S. This trough was associated with above-average precipitation in the Midwest and Gulf Coast regions of the U.S. (Fig. E3) with area-averaged totals exceeding the 90^th percentile of occurrences in both regions (Fig. E5).

According to the U.S. Drought Monitor, exceptional drought continued across central California, while severe drought persisted in western Nevada. Moderate drought persisted in eastern Oregon, and expanded across New Mexico and Arizona.

 

 

b. Eurasia

The 500-hPa circulation during March featured above-average heights over Scandinavia, and below-average heights over southern Europe (Fig. E9). This pattern was associated with a pronounced split flow upstream of Europe. This pattern produced increased storminess and above-average precipitation across southern Europe (Fig. E3). It also produced an extensive southwesterly flow of milder, marine air into northern Europe, which then spread eastward into central Asia (Fig. E10). These conditions, combined with an early snow-melt along the southern portion of the Asian snowpack, were associated with a continuation of well above-average surface temperatures across central and eastern Asia, with many locations recording departures above +4^oC and also above the 90^th percentile of occurrences (Fig. E1).

 

2. Southern Hemisphere

The mean 500-hPa circulation during March featured above-average heights in the middle latitudes and below-average heights over Antarctica. At 200-hPa, the El Niño response was evident in the ongoing pattern of anticyclonic streamfunction anomalies across the central subtropical South Pacific Ocean (Fig. T22).

The main surface temperature signals during March included significantly above-average temperatures in eastern Africa, Madagascar, and southeastern Australia (Fig. E1).The main precipitation signals during March included above-average totals in extreme southern South Africa and in southwestern Australia (Fig. E3).

The South African rainy season extends from October to April. During March, the area as a whole recorded above-average precipitation (Fig. E4), with surpluses observed mainly in extreme southern Africa. To date, the South African rainy season has featured below-average precipitation from October-February, and above-average precipitation in March. (Fig. E4). A below-average rainy season is typical of El Niño.