Section 2.3 Interannual Variability 

2.3.1 El Nino Southern Oscillation (ENSO) 

(i) Observations(ii) Theory for ENSO(iii) Impacts

2.3.2 Interannual variability in Atlantic SSTs

2.3.3 The North Atlantic Oscillation (NAO)

 

2.3.2 Interannual variability in Atlantic SSTs

So far we have emphasized interannual variability in the Pacific – in association with ENSO

It should be noted that other ocean basins exhibit interannual variability (e.g. Atlantic, Indian Ocean etc).

Here we briefly highlight some key “modes” of variability in the Atlantic.

2.3.2 Interannual variability in Atlantic SSTs

Unlike the Pacific the Atlantic is not dominated by any single mode of climate variability such as ENSO.

This region is subject to multiple competing influences.

See: Sutton et al 2000: The Elements of Climate Variability in the Tropical Atlantic Region, Journal of Climate

2.3.2 Interannual variability in Atlantic SSTs

Three different forced SST patterns have been identified:

(a) Remote response to ENSO(b) An Atlantic Dipole SST pattern(c) A response to Equatorial Atlantic SST anomalies (La hermanita!)

There relative importance varies according to season.

The North Atlantic Oscillation

most slides courtesy Martin Visbeck

2.3.3 The North Atlantic Oscillation

Useful website for NAO provided by David Stephenson (University of Exeter):

http://www1.secam.ex.ac.uk/cat/NAO

And the NOAA website:

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.shtml

General CirculationThe surface pressure

• NorthernWinter (January)

• High over Land, Low over Ocean

What is the North Atlantic Oscillation ?

A sea saw of atmospheric mass which alternates between the polar and subtropical regions.Changes in the mass and pressure fields lead to variability in the strength and pathway of storm systems crossing the Atlantic from the US East coast to Europe.

The NAO is most noticeable during the winter season (November - April) with maximum amplitude and persistence in the Atlantic sector.

The North Atlantic Oscillation Index

An Index can be constructed that represents the phase of the NAO. Most commonly the NAO index is based on the surface pressure (SLP) difference between the Subtropical (Azores) high and the Subpolar (Iceland) low.

Very often the pressure readings from two stations one on Iceland and the other either the Azores, Lisbon or Gibraltar are used to construct the NAO index. The twice daily reading are averaged from November through March and the difference is then the winter NAO index.

The North Atlantic Oscillation Index

The NAO index shows large variations from year to year. This interannual signal was especially strong during the end of the 19th century.

Sometimes the NAO index stays in one phase phase for several years in a row. This decadal variability was quite strong at the beginning and end of the 20th century.

One might also interpret the recent 30 years as a trend in the NAO index possibly linked to "global warming".

The positive NAO index phaseThe positive NAO index phase shows a 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.This results in warm and wet winters in Europe and in cold and dry winters in northern Canada and Greenland.The eastern US experiences mild and wet winter conditions.

The negative NAO index phase

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

Impacts of the NAO

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Us East Coast Impacts of the NAO

The US East coast experiences milder winter conditions during a positive NAO index phase.The amount of snow cover is reduced.Colder than usual tropical ocean temperatures reduce the number of hurricanes in the following summer.Cold ocean temperatures in the spawning grounds over the Grand Banks cause less cod reproduction.

Impacts of the NAO in Europe

Northern Europe experiences mild and wet winter during the positive NAO index phase.This has dramatic consequences for hydro-electric power generation and heating oil consumption.South-Eastern Europe receives less rain and hence causes significant problems with drinking water supply and reduced stream flow volume in the Middle East.Harvest yield of grapes and olives have been shown to depend significantly on the NAO.

NAO and Energy in Norway

Norway experience cold winters during a negative NAO phase.

Heating Oil consumption in Norway varies by 30% in good (anti) correlation with the NAO.

Correlation with precipitation results in variability in hydropower generation.

NAO and Water Resources in Turkey and the Middle East

Precipitation in Turkey is well correlated with the NAO.

As a result spring stream flow in the Euphrates River varies by about 50% with the NAO.

An upward trend in the NAO will lead to drought conditions in the Middle East.

Atlantic Ocean SSTs and the NAO

Some scientists have suggested that the storage and propagation of temperature anomalies by the ocean gives an important feed back to the atmosphere and is responsible for the decadal signal.If correct one could make use of the "slow ocean dynamics" to predict aspects of the NAO.

NAO movieAnimation of sea level pressure and surface winds during an idealized NAO cycle of 12 year duration.The lower panel shows the land temperature response and the propagation of SST anomalies in the ocean.The ocean is simulated by the Lamont Ocean model (LOAM)All other data are regressions from the NCEP/NCAR reanalysis.

NAO and global warming

Some scientist argue that changes in the stratospheric circulation can influence the phase of the NAO.Ozone depletion and increase of CO2 both result in a strong polar night vortex which might cause the NAO to prefer a positive state.Will "global warming" cause a persistent positive NAO phaes?

Summary

The North Atlantic Oscillation is the largest mode of climate variability in the Atlantic Sector and possibly in the whole northern hemisphere.Its impacts reach from the upper atmosphere to the bottom of the ocean and reach from America over to Europe and far into Asia.The dynamics of the NAO are not fully understood and in partiuclar its sensitivity to ocean, land or changes in the sea-ice conditions need more study.Some scientists argue that the NAO is strongly coupled to the stratosphere and will be significantly influenced by "global warming".Other scientists see evidence for coupling with the North Atlantic Ocean.It has also been suggested that tropical ocean temperatures can influence the phase of the NAO.It is unlikely that we will ever be able to predict the NAO with the same accuracy as we do for ENSO today.