The North Atlantic Oscillation Martin Visbeck Department of Earth and Environmental Sciences Lamont-Doherty Earth Observatory of Columbia University with contributions from: Gerd Krahmann, Heidi Cullen, Naomi Naik and Jessica Cherry many thanks to my colleagues: Yochanan Kushnir and Richard Seager

Outline of the presentation What is the North Atlantic Oscillation ? Review of Atmospheric Circulation. Impacts of the NAO: Temperature Precipitation Storminess Economics NAO and the North Atlantic Ocean. NAO, the stratosphere and global warming. Conclusions Martin Visbeck 25 April, 2017

What is drives our Climate? The planet receives short wave radiation from the Sun. The incoming Energy depends strongly on the latitude and season. Martin Visbeck 25 April, 2017

What balances the received Energy? Any surface emits long wave radiation which depends strongly on its temperature. A balanced climate is reached when incoming and outgoing radiation are equal. Martin Visbeck 25 April, 2017

Is that balance true for all place on Earth? The strong temperature gradient between the tropics and pole can induce atmospheric motion. Martin Visbeck 25 April, 2017

General Circulation The surface energy balance Atmosphere/Ocean Heat Transport I ~ sT4

The atmospheric wind system A three cell structure emerges. Hadley, Polar Mid-Latitude Martin Visbeck 25 April, 2017

General Circulation The surface pressure Northern Winter (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. Martin Visbeck 25 April, 2017

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 (Island) 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 in then the winter NAO index. Martin Visbeck 25 April, 2017

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". Martin Visbeck 25 April, 2017

The positive NAO index phase The positive NAO index phase shows a stronger than usual subtropical high pressure center and a deep 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. Martin Visbeck 25 April, 2017

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. Martin Visbeck 25 April, 2017

Impacts of the NAO . Martin Visbeck 25 April, 2017

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. Warmer than usual ocean temperatures cause more frequent occurrence of "red tides" in the summer. 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. Martin Visbeck 25 April, 2017

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. Martin Visbeck 25 April, 2017

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. Martin Visbeck 25 April, 2017

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. Martin Visbeck 25 April, 2017

Temperatures in the Atlantic Ocean Winter temperatures become colder along the pathway of the Gulf Stream / North Atlantic Current System because the ocean warms the overlying atmosphere. Martin Visbeck 25 April, 2017

NAO impact on Atlantic Ocean SSTs Ocean surface temperatures (SST) changes with the phase of the NAO. During a positive year the ocean warms just east of the US east coast and cools in the subpolar gyre between England, Newfoundland and Iceland. The Gulf stream transports those temperature anomalies downstream towards Europe. Martin Visbeck 25 April, 2017

Atlantic Ocean SSTs and the NAO Some scientist 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. Martin Visbeck 25 April, 2017

The ocean is simulated by the Lamont Ocean model (LOAM) NAO movie Animation 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. Martin Visbeck 25 April, 2017

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? Martin Visbeck 25 April, 2017

Conclusions 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 scientist argue that the NAO is strongly coupled to the stratosphere and will be significantly influenced by "global warming". Other scientist 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. Martin Visbeck 25 April, 2017

Experiment Martin Visbeck 25 April, 2017