Extratropical Highlights –March 2017

 

1. Northern Hemisphere

The 500-hPa circulation during March featured above-average heights over the high latitudes of the central North Pacific, most of the United States, Europe, and much of central and northern Asia (Fig. E9). The circulation featured below-average heights over the North Pacific from Japan to just east of the date line, the Gulf of Alaska, the western North Atlantic and the polar region. Over the North Pacific the 4-celled anomaly pattern projected strongly onto the negative phases of the West Pacific (-2.1) and East Pacific-North Pacific (-1.0) teleconnection patterns (Table E1, Fig. E7).

In the subtropics, the 200-hPa circulation again feature amplified ridges over Australasia, along with amplified mid-Pacific troughs, in both hemispheres (Fig. T22). This pattern is consistent with the lingering La Niña-related pattern of enhanced convection over Indonesia and suppressed convection across the central equatorial Pacific (Fig. T25).

The main land-surface temperature signals during March included above-average temperatures across most of the continental U.S., Europe, and much of Russia, and below-average temperatures in southern Alaska, western Canada, and the northeastern U.S. (Fig. E1). The main precipitation signals included above-average totals in the northwestern U.S., and below-average totals centered over Florida (Fig. E3).

 

a. North Pacific/ North America

The 500-hPa circulation during March featured a 4-celled pattern of height anomalies across the North Pacific and North America. This pattern included above-average heights over the high latitudes of the North Pacific and across most of the U.S., and below-average heights across the mid-latitude western and central North Pacific and also over the Gulf of Alaska (Fig. E9). This pattern projected strongly onto the negative phases of the West Pacific (-2.1) and East Pacific-North Pacific (-1.0) teleconnection patterns (Table E1, Fig. E7).

This overall anomaly pattern was associated with an amplified East Asian jet stream west of the date line, along with a very strong exit region to that jet stream (Fig. T21). Over central and eastern North America, the height anomaly pattern reflected a disappearance of the mean Hudson Bay Trough for the second straight month. Overall, these conditions were associated with an anomalous low-level flow of milder marine air into the northwestern U.S., and with anomalous westerly flow across the northern U.S. (Fig. T20).

These conditions contributed to above-average surface temperatures over much of the central and western U.S., with many areas recording departures in the upper 90^th percentile of occurrences (Fig. E1). Conversely, below-average temperatures were observed across southern Alaska, western Canada, and portions of the northeastern U.S. 

The anomalous circulation also contributed to a continuation of above-average precipitation in the Pacific Northwest U.S. (Fig. E3), with totals in the upper 90^th percentile of occurrences for the second straight month (Fig. E5). This region has recorded above-average totals in every month since October 2016 (6-months). Conversely, below-average precipitation was again recorded in the southeastern U.S., with the most significant deficits (lowest 10^th percentile of occurrences) observed in Florida.

According to the U.S. Drought Monitor, record long-term drought in the western U.S. has ended. However, moderate drought persisted across large portions of Oklahoma and south- western Kansas, and developed in eastern Colorado. In the southeastern U.S., severe or extreme drought extended from eastern Alabama to the western Carolina’s, and severe drought developed in south-central Florida. In the northeastern U.S., moderate drought extended from western Virginia to New Hampshire, with severe drought persisting in western Connecticut.

 

b. Eurasia

The 500-hPa circulation during March featured above-average heights across much of Eurasia (Fig. E9). This pattern was associated with well above-average surface temperatures throughout the region, with large portions of Europe and much of Russia recording departures above the 90^th percentile of occurrences (Fig. E1).

 

2. Southern Hemisphere

The mean 500-hPa circulation during March featured above-average heights over the central Indian Ocean, the western South Pacific, and in the area south of South America, and below-average heights across the high latitudes of the South Pacific and South Atlantic Ocean (Fig. E15).

In eastern Australia, the overall large-scale circulation pattern contributed to anomalously warm and wet conditions, with precipitation totals in many regions in the upper 90th percentile of occurrences (Fig. E3). Area-averaged totals in the Northeast Australia region were above average for the first time since September 2016 (Fig. E4).

The South African monsoon season runs from October to April. This area recorded below-average precipitation during March, with the largest deficits occurring in southern Africa (Fig. E3). To date for the 2016-17 monsoon season, the region received below-average precipitation in October and March, near-average precipitation in November-December, and above-average totals in January-February.