La Niña impacts on Pacific Northwest climate in spring:
2011 and the historical record
Jim Johnstone
JISAO-UW
[email protected]
Sea-level pressure (SLP) anomalies
March 2011
SOI
Feb-Apr SOI
Strongest spring La Niña on record (to 1866)
Lowest Apr-Jun Tmax in WA (to 1900)
Airport cloud ceilings(hours per day)
SeaTac
Airport cloud ceilings(hours per day)
SeaTac
Portland
2011 monthly 500 mb anomalies (i.e. pressure)
Feb
Mar
Apr
May
Jun
Jul
April-Jun 500 mb anomalies
Strong La Niña that persisted into April
Persistent and strong anomalies over the Pacific NW (and beyond) through June
Cause and effect?
Do Feb-Apr La Niña conditions occur with Apr-Jun anomalies over the Pacific NW?
Composite La Niña (Top 10)
Dec-Feb 500 mb
Composite La Niña (Top 10)
Dec-Feb 500 mb
Winter La Niña composites (Dec-Feb)
Precip
Tmax
Composite La Niña (Top 10)
Feb-Apr 500 mb
Composite La Niña (Top 10)
(Feb-Apr) Feb-Apr
Mar-May
Apr-Jun
500 mb
Tmax
Apr-Jun 2011
Apr-Jun response to Feb-Apr La Niña
Composite La Niña (Top 10)
(Feb-Apr) Feb-Apr
Mar-May
Apr-Jun
Tmax
500 mb anomalies Apr-Jun 2011
Conclusion:
Apr-Jun climate patterns similar to a typical La Niña pattern(Trough PNW, Ridge SE)
But historical data shows limited effects of La Niña in spring
La Niña US tornadoes
Spring (MAM) Z500 composite anomalies (normalized)