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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

  Stratosphere

  Appendix 2: Additional Figures

Extratropical Highlights

APRIL 2017

1

Extratropical Highlights ľApril 2017

 

1. Northern Hemisphere

The 500-hPa circulation during April featured above-average heights over the eastern U.S., western Europe, and portions of eastern Asia, and below-average heights across the central North Pacific, Greenland, and Scandinavia/ northwestern Russia (Fig. E9). Over the North Pacific, this anomaly pattern projected onto the positive phase of the East Pacific-North Pacific (+1.0) teleconnection pattern (Table E1, Fig. E7). Over the North Atlantic, the anomaly pattern projected onto the strong positive phase of the North Atlantic Oscillation (NAO, +1.7) and also onto the negative phase of the Scandinavia teleconnection pattern (-1.4).

The main land-surface temperature signals during April included above-average temperatures in western Alaska, the eastern half of the U.S., western Europe, and the eastern half of Asia (Fig. E1). The main precipitation signals included above-average totals in much of the U.S., southern Canada, and northwestern Russia (Fig. E3).

 

a. North Pacific/ North America

The 500-hPa circulation during April featured an extensive and amplified trough across the central North Pacific Ocean, a trough over the central U.S., and an amplified ridge over eastern North America (Fig. E9). This pattern reflected an amplification of the Aleutian Low and a marked weakening of the mean Hudson Bay trough. This marks the third straight month in which the Hudson Bay trough has been exceptionally weak or non-existent. Overall, these conditions were associated with an anomalous low-level flow of milder marine air into the northwestern U.S., along with weaker jet stream winds across the eastern U.S. (Fig. T20).

These conditions contributed to above-average surface temperatures over much of the central and eastern U.S., with many areas recording departures in the upper 90th percentile of occurrences (Fig. E1). They also contributed to above-average precipitation across much of the continental U.S. (Fig. E3).

Above-average precipitation was again recorded in the Pacific Northwest U.S. (Fig. E3), with totals in the upper 90th percentile of occurrences for the third straight month (Fig. E5). This region has recorded above-average totals in every month since October 2016 (7-months). Elsewhere, area-averaged totals exceeded the 90th percentile of occurrences in the Inter-Mountain region, the Great Plains, the Midwest and the Great Lakes region (Fig. E5). Totals exceeded the 80th percentile of occurrences in the Ohio Valley region.

According to the U.S. Drought Monitor, drought ended in the central and southern Plains states, and weakened or ended in large portions of the southeast and northeast. In the southeastern U.S., severe drought was recorded in southern Georgia, while the area of severe or extreme drought expanded in central Florida.

 

b. North Atlantic/ Eurasia

The 500-hPa circulation during April featured above-average heights across the central North Atlantic, western Europe, and below-average heights over Greenland, Scandinavia, and northwestern Russia (Fig. E9). This pattern projected onto the strong positive phase of the North Atlantic Oscillation (NAO, +1.7) and also onto the negative phase of the Scandinavia teleconnection pattern (-1.4).

These conditions were associated with well above-average surface temperatures in western Europe and much of eastern Asia, with large areas recording departures above the 90th percentile of occurrences (Fig. E1). They were also associated with well above-average precipitation in northwestern Russia and well below-average precipitation in western Europe (Fig. E3).

 

2. Southern Hemisphere

The mean 500-hPa circulation during April featured above-average heights east of New Zealand, and in the areas south of both South America and Africa. It featured below-average heights over the eat-central South Pacific and the eastern half of Antarctica (Fig. E15).

In north-eastern Australia, anomalously dry conditions were present during April, with precipitation totals in many regions in the lowest 10th percentile of occurrences (Fig. E3).

The South African monsoon season runs from October to April. This area recorded above-average precipitation during April, with the largest surpluses occurring in the western and central portions of the monsoon region (Fig. E3). Overall for the 2016-17 monsoon season, rainfall was below-average in October and March, near-average in November-December, and above-average in January, February, and April.

 

 

 


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