AIR/SEA INTERACTION El Nino

OBJECTIVES To Understand what El Nino is and how it develops To understand why we should study El Nino or ENSO To understand how ENSO events are defined To understand the global effects of ENSO To understand NAO

OUTLINE What is El Nino? SOI vs. JMA/Nino 3.4 Importance of ENSO Events Effects of ENSO Events NAO

What is El Nino ? A disruption of the ocean-atmosphere system in the tropical Pacific Normally, northeasterly trade winds combined with upwelling off of the coast of Peru keep waters cool in the central and eastern Pacific During El Nino, trade winds relax in the western Pacific leading to warmer waters in the central and eastern Pacific

Normal Conditions

El Nino Conditions

La Nina Conditions

DEFINING ENSO Events SOI JMA/ Nino 3.4 (ONI)

SST vs. SOI Index

An Example of the Evolution of Equatorial Pacific SST Departures (oC) October 2008- February 2009, negative sea surface temperature (SST) anomalies dominated across the central and east-central equatorial Pacific Ocean February- April 2009, negative SST anomalies weakened across the equatorial Pacific Ocean Since the beginning of June 2009, positive SST anomalies have been at least +0.5°C across the equatorial Pacific. Time Longitude

Rossby Waves Also known as planetary waves, owe their origin to the shape and rotation of the Earth Travel from East to West Slow moving - the speed varies with latitude and increases equatorward, but is of the order of just a few cm/s (or a few km/day)

Influence of Rossby Waves

Equatorial Kelvin Waves Special gravity waves excited by an abrupt change in the overlying wind field Travel west to east Fast moving - speeds of approximately 2-3 m/s (250 km/day) at the surface Maximum amplitude at the Equator

Equatorial Kelvin Wave

Influence of Kelvin Waves Warmest water is usually 100-150 m below the surface

Rossby and Kelvin Waves

Ending Warm ENSO Events Rossby waves generated by the relaxation of the trade winds in the central Pacific (red oval) travel west to the Indonesian archipelago and are reflected back as Kelvin waves. Now, the Kelvin waves bring cold water back towards the surface (in the blue region), initiating the cooling that shuts down the warm ENSO event and re-strengthens the trade winds.

Why study El Nino? Weather impacts human lives California flooding Storm damage Loss of life Major impacts on the economy Agriculture Construction Energy distribution Outdoor recreation Retail U.S. economic impacts of the 97/98 El Nino - $25 Billion !! Both gains and losses Department store sales up 15% during mild Midwest winter Snow equipment sales down 35% during mild Midwest winter Skiing was up in the west…but down in the Midwest

Effects of El Nino

North Atlantic Oscillation

The North Atlantic Oscillation The NAO is the dominant mode of winter climate variability in the North Atlantic region ranging from central North America to Europe and much into Northern Asia. The NAO is a large scale see-saw in atmospheric mass between the subtropical high and the polar low. The corresponding index varies from year to year, but also exhibits a tendency to remain in one phase for intervals lasting several years.

The Positive NAO Index Stronger than usual subtropical high pressure center and a deeper than normal Icelandic low The increased pressure difference results in more and stronger winter storms crossing the Atlantic Ocean on a more northerly track (enhanced westerlies). This results in warm and wet winters in Europe and colder and drier winters in northern Canada and Greenland The eastern US experiences mild and wet winter conditions

The Negative NAO Index A weak subtropical high and a weak Icelandic low. The reduced pressure gradient results in fewer and weaker winter storms crossing on a more west-east pathway. Brings moist air into the Mediterranean and cold air to northern Europe The US east coast experiences more cold air outbreaks and hence snowy weather conditions. Greenland, however, will have milder winter temperatures

The NAO Index Look at the graph above, do you see any recognizable trends?