Extratropical Highlights –March 2017

 

1. Northern Hemisphere

The 500-hPa height pattern during March featured large anomalies over much of the NH extratropics (Fig. E9). Above-average heights were present over the southwestern U.S., eastern Canada and Greenland, from the Middle East to Japan, and over the middle and high latitudes of the North Pacific.  Below-average heights were present over both the western and eastern U.S., Europe, and northern Asia.

This anomaly pattern projected onto a record positive phase of the East Atlantic/ West Russia pattern (+4.0 std. dev) (Table E1, Fig. E7). It also projected onto the negative phases of both the North Atlantic Oscillation (NAO, -1.4) and the Pacific/ North American pattern (PNA, -1.2).  A negative phase of the PNA teleconnection pattern is typical of La Niña.

At 200-hPa, the circulation across the subtropical Pacific Ocean in both hemispheres continued to reflect La Niña. The La Niña signal included amplified troughs east of the date line in the subtropics of both hemispheres (Fig. T22), in association with the disappearance of deep tropical convection from the central and eastern equatorial Pacific (Fig. T25). The La Niña signal also included a focusing of the subtropical ridges over Australasia (Fig. T22), in association with enhanced convection over the western tropical Pacific and Indonesia.

The main land-surface temperature signals during March included above-average temperatures in Alaska, Mexico, the south-central U.S., and across southern Asia, and below-average temperatures in western Canada, the eastern U.S., and much of northern Europe and northwestern Russia (Fig. E1). The main precipitation signals included above-average totals in the northwestern U.S., western Canada, and throughout central and southern Europe (Fig. E3).

 

a. North Pacific and North America

The 500-hPa circulation during March featured above-average heights over the middle and high latitudes of the North Pacific and Mexico, and below-average heights over both the western and eastern U.S. (Fig. E9). This overall pattern projected onto the negative phase (-1.2) of the PNA teleconnection pattern, which is typical of La Niña.

La Niña produces an amplified mid-Pacific trough (Fig. T22), which act to retract westward the East Asian jet steam (Fig. T21). This anomalous jet structure, in turn, acts to retract westward the mean downstream ridge and trough positions, resulting in the essence of the anomalous 500-hPa circulation pattern during March (Fig. E7, Table E1).

During March, the overall circulation pattern contributed to above-average surface temperatures in Mexico and the south-central U.S., and to below average surface temperatures in the eastern U.S. and western Canada (Fig. E1). It also contributed to above-average precipitation in the Pacific Northwest U.S. and western Canada (Fig. E3).

During the last few months, severe or extreme drought has spread across the southwestern U.S. from Arizona to northern Texas and south-central Kanas. This overall area recorded 50%-75% of normal rainfall during March (Fig. E6).Within this region, exceptional drought has developed in western Oklahoma, where less than 25% of normal rainfall was recorded during March (Fig. E6). Severe drought was also present in southeastern Georgia and southeastern South Carolina in response well below-average precipitation during March.

 

b. Eurasia

            The 500-hPa height anomaly pattern featured a pronounced north-south dipole pattern characterized by above-average heights extending from the Middle East to Japan and by below-average heights extending from central Europe to eastern Siberia. This pattern projected onto a record positive phase of the East Atlantic/ West Russia pattern (+4.0 std. dev) (Table E1, Fig. E7).

In Europe, the amplified trough contributed to well above-average precipitation, with area-averaged totals across southern Europe in the upper 90^th percentile of occurrences (Fig. E4). This region also recorded well above-average totals in February.

 

2. Southern Hemisphere

The mean 500-hPa circulation during March featured above-average heights over the Indian Ocean and the central South Pacific, and below-average heights over the high latitudes of the eastern South Pacific (Fig. E15). At 200-hPa, the subtropical circulation featured an amplified trough over the central and eastern South Pacific Ocean, and ridge over western Australia (Fig. T22). This anomalous subtropical circulation is typical of La Niña.

The South African monsoon season runs from October to April. This area recorded above-average precipitation during March (Fig. E3), with area-averaged totals near the 70^th percentile of occurrences (Fig. E4). To date, the South African rainy season was above average during February and March, and below average during November and January.