Abnormal Weather

October 22, 2007

Teleconnections

• Teleconnections: relationship between weather or climate patterns at two widely separated locations– ENSO– PDO– AMO

Ocean Currents

ENSO events

• ENSO events are a disruption of the ocean-atmosphere system in the tropical Pacific

• El Niño = warm phase, La Niña = cool phase

• El Nino Southern Oscillation

– El Nino – ocean

– Southern Oscillation – atmosphere

ENSO• Changes in pressure patterns alter path of jet

stream

• Mainly concentrated in the Pacific Ocean

• Measured using Southern Oscillation Index– Differences in pressure observed in Tahiti and

Darwin, Australia– Also monitored using remote sensing data

ENSO

Occurs once every 3 to 5 years (but varies)

• Major ENSO events in last 25 years: 1982–1983, 1986–1987, 1991–1993, 1997–1998, 2002–2003

• 1997–1998 was most intense on record

• No two ENSO events are alike. They all differ in size, location, and duration

ENSO

ENSO

ENSO

Normal

ENSO

Normal Year

Normal Year

ENSO

ENSO Year

ENSO Year

ENSO Year

• Australia-Drought and bush fires • Indonesia, Philippines-Crops fail, starvation follows • India, Sri Lanka-Drought,fresh water shortages • Tahiti-6 tropical cyclones • South America-Fish industry devastated • Across the Pacific-Coral reefs die • Colorado River basin-Flooding, mud slides • Gulf states-Downpours cause death, property damage • Peru, Ecuador-Floods, landslides • Southern Africa-Drought, disease, malnutrition

Wildfires

• Precipitation increases over the American southwest during ENSO events

• Increase in fuels

• Succeeding La Nina or normal weather patterns dries fuels

• Preconditioning

• Fire suppression

ENSO and Crop Yields

• El Nino events are associated with LOW grain yields in Asia and Australia and HIGH grain yields in North America

• In the SE Coastal Plain, El Nino years tend to be cool (and wet) and La Nina years warm (and dry) between October and April

ENSO and Crops

• In the SE, crops yields are higher after an El Nino winter and lower after a La Nina year

• The neutral ENSO phase has above average vegetation response

La Nina

• La Niña brings extreme normal conditions

• Not all El Niño events are followed by La Niña events

• La Niña events increase wildfires in the SE and Atlantic hurricanes

La Nina

La Nina

Pacific Decadal Oscillation (PDO)

• 20 to 30-year cycles

• Discovered 1996

• Involves two regions of sea-surface temperatures and related air pressure:– 1) the northern and tropical western Pacific– 2) the eastern tropical Pacific, along the west

coast of North America

PDO• 1947 – 1977: Cool (negative) Phase -

region 1 had higher than normal temperatures and region 2 had lower than normal temperatures

• 1997 – 1999: Warm (positive) Phase – region 1 had lower than normal temperatures and region 2 had higher than normal temperatures

• Despite several years of cool phase values, we are still considered to be in warm phase

PDO and ENSO

• PDO impacts the strength of ENSO events

• When PDO is in the warm phase, El Nino’s are stronger– Southern US has wetter than normal winters;

Northern US is drier

• When PDO is in the cool phase, El Nino’s are weaker

Atlantic Multidecadal Oscillation (AMO)

• 20 – 40 year cycles

• Has warm (positive) and cool (negative) phases

• AMO impacts ENSO strength and increases hurricane activity

• During warm phases, the US experiences decreased rainfall

• Been reconstructed back to 1650 AD

• Measure of SST in North Atlantic between Equator and 70°N

AMO

• AMO warm phases: 1860-1880 and 1930-1960

• AMO cool phases: 1905-1925 and 1970-1990

• AMO involves strengthening (warm phase) and weakening (cool phase) of the North Atlantic Thermohaline Circulation

AMO

• During an AMO warm phase, El Nino events are muted, providing less winter precipitation

• During an AMO cool phase, El Nino events are not affected

• When AMO is positive and PDO is negative, the majority of the US experiences drought conditions– Example: 1930s Dust Bowl

AMO

• AMO entered warm phase in 1995

• Global temperatures expected to mimic those between 1930-1960 and increase

• This means a decrease in precipitation for the eastern US

• Important implications for water managers

• Florida exempt