La Niña impacts on Pacific Northwest climate in spring:

2011 and the historical record

Jim Johnstone

JISAO-UW

jajstone@uw.edu

Niño 3.4 index

Niño 3.4 index

SST anomalies

January 3, 2011

Sea-level pressure (SLP) anomalies

March 2011

SOI

Niño 3.4 index

SOI

Feb-Apr SOI

Strongest spring La Niña on record (to 1866)

Lowest Apr-Jun Tmax in WA (to 1900)

Pac NW Precip

Pac NW Tmax

Airport cloud ceilings(hours per day)

SeaTac

Airport cloud ceilings(hours per day)

SeaTac

Portland

US Tmax anomalies

Apr-Jun 2011

Record values

(back to 1951)

Texas drought

April tornadoes

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

La Nina winter composite

2011 Apr-Jun

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)

Top 5 Tmax