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Climate Diagnostics Bulletin
Climate Diagnostics Bulletin - Home Climate Diagnostics Bulletin - Tropics Climate Diagnostics Bulletin - Forecast


  Extratropical Highlights

  Table of Indices  (Table 3)

  Global Surface Temperature  E1

  Temperature Anomalies (Land Only)  E2

  Global Precipitation  E3

  Regional Precip Estimates (a)  E4

  Regional Precip Estimates (b)  E5

  U.S. Precipitation  E6

  Northern Hemisphere

  Southern Hemisphere


  Appendix 2: Additional Figures

Extratropical Highlights



Extratropical Highlights –February 2017


1. Northern Hemisphere

The 500-hPa circulation during February featured a highly amplified anomalous wave pattern (Fig. E9), and continued to be strongly influenced by La Niña and the Madden Julian Oscillation (MJO). Above-average heights were present over the high latitudes of the North Pacific, the eastern U.S., and large portions of the polar region, while below-average heights were present over most of Canada, the western U.S., and southern Europe.

This anomaly pattern projected onto several teleconnection patterns (Fig. E7, Table E1). It projected onto the positive phases of the North Atlantic Oscillation (NAO, +1.3) and the Tropical/ Northern Hemisphere pattern (TNH, +2.2). It also projected onto the negative phases of the Polar/ Eurasian pattern (-2.2), the east-Atlantic pattern (-1.4), and the east Atlantic/ Western Russian pattern (-1.4).

At 200-hPa, the circulation across the subtropical Pacific Ocean in both hemispheres reflected La Niña. The La Niña signal included highly amplified troughs east of the date line in both hemispheres, in association with the disappearance of deep tropical convection from the central 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, Indonesia, and the eastern Indian Ocean.

The main land-surface temperature signals during February included above-average temperatures across the southern and eastern U.S. and in the polar region, and below-average temperatures in the western Canada and western Europe (Fig. E1). The main precipitation signals included above-average totals in the central U.S. and southern Europe (Fig. E3).


a. North Pacific and North America

The 500-hPa circulation during February featured above-average heights over the high latitudes of the North Pacific, the Gulf of Alaska and the southeastern U.S., and below-average heights over Canada and the western U.S. (Fig. E9). This type of highly amplified wave pattern is typical of La Niña.

La Niña produces an enhanced subtropical ridge over southeastern Asia along with an amplified mid-Pacific trough (Fig. T22). These conditions act to retract westward the East Asian jet steam. 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 seen during February. This anomalous wave pattern projected strongly onto two teleconnection patterns which tend to be favored during La Niña winter; the positive phase of the TNH pattern and the negative phase of the PNA pattern (Fig. E7, Table E1).

This overall pattern contributed to anomalously warm surface temperatures in the eastern U.S. and to anomalously cool temperatures in western Canada (Fig. E1). It also contributed to above-average precipitation in the central U.S. (Fig. E3). Area-averages totals in the Great Plains, Great Lakes, Midwest, and Ohio Valley regions in the upper 90th percentile of occurrences (Fig. E5).



b. Polar region

The mean February 500-hPa circulation during February featured an anomalous zonal wave-1 pattern, with above-average heights north of Eurasia and below-average heights extending from central Canada to Greenland (Fig. E9). This pattern appears to have reflected a downward extension of a stratospheric anomaly pattern that was prominent up to at least the 30-hPa level (Fig. S2).

At 500-hPa, this pattern projected strongly onto the negative phase of the Polar/ Eurasian pattern (-2.2). The anomalous southerly flow between the anomalous trough and the downstream anomalous ridge resulted in a massive influx of relatively mild air (and significantly warmer temperatures) into the polar region at levels extending from the surface (Fig. E8) to 500-hPa.


b. Europe

            The circulation across the central North Atlantic and Europe reflected an anomalous zonal wave-4 pattern, with above-average heights over the west-central North Atlantic and Scandinavia, and below-average heights over Greenland and southern Europe (Fig E9). This pattern reflected an amplified jet stream over the western Atlantic (Fig. E10), along with a pronounced jet exit region and split-flow pattern across the eastern Atlantic and Europe. The northern branch of this split flow brought a large influx of milder air into the polar region. The southern branch contributed to increased storminess and above-average precipitation across southern Europe (Fig. E3). This marked the first time in more than a year that area-averaged precipitation was above average across southern Europe (Fig. E4).


2. Southern Hemisphere

The mean 500-hPa circulation during February featured above-average heights over the Indian Ocean and below-average heights over the high latitudes of the South Pacific Ocean (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 February, with area-averaged totals in the upper 80th percentile of occurrences (Fig. E4). The most significant surpluses were observed in the northern and central portions of the monsoon region (Fig. E3), where they helped to alleviate significant rainfall deficits which had accumulated over the last two months.


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