The North Atlantic Oscillation (NAO)

The NAO is a modulation in the strength and position of the surface westerlies over the Atlantic. It exerts a dominant influence on temperatures, precipitation and storms, fisheries and eco-systems of the Atlantic sector and surrounding continents. (Fig. 1). The NAO is receiving particular attention because it explains a significant fraction of the positive trend seen in hemispheric averaged temperature over the last 30 yrs. Understanding of the NAO and its time-dependence appear to be central to three of the main questions in the global change debate: has the climate warmed? and if so why? and how?

Figure 1: Schematic diagram showing some of the climatic consequences of (left) positive and (right) negative phases of the NAO. Click on the images to see a the detailed figures.

John Marshall are David Ferreria are leading a research effort attempting to identify and study mechanisms that drive the NAO. The governing physics is highly complex but the primary excitation mechanism is the interaction between synoptic eddies and the atmospheric jet-stream. Changes in the phase of the NAO are associated with large fluctuations in air-sea heat and momentum fluxes driving sea surface temperature (SST) anomalies and variability in the underlying ocean. On decadal timescales the ocean, through its effect on SST anomalies, may be a player in modulating the phase and amplitude of the NAO.


Fig.2. The principal component (PC) time series of the leading EOF of seasonal (December through March) SLP anomalies over the Atlantic sector (20-70N, 90W-40E) is shown in color.

You can link to Jim Hurrell's NAO index here.

Lecture on The interaction of the North Atlantic Oscillation with ocean circulation by John Marshall.

You can read some of our papers on the subject here.

Marshall, J., Kushnir, Y.,  Battisti, D., Chang, P., Czaja, A., Dickson, R., McCartney, M., Saravanan, R., Visbeck, M. (2001)  North Atlantic Climate Variability: phenomena, impacts and mechanisms. Inter. Jour. Climatology, vol.21, No.15, 1863-1898
Czaja A. and C. Frankignoul (2002), Observed impact of Atlantic SST anomalies on the North Atlantic Oscillation , J. of Climate, vol 15, No 6, 606-623.
Czaja, A. and J. Marshall (2001)  Observations of Atmosphere-ocean coupling in the North Atlantic
QJRMS,  127, 1893-1916
Marshall, J., Johnson, H. and J. Goodman, (2001) Interaction of the North Atlantic Oscillation with ocean circulation. J. of Climate, vol 14, no 7, 1399-142
Ferreira, D., Frankignoul, C. and J. Marshall (2001) Coupled ocean-atmosphere dynamics in a simple mid-latitude climate model J. of Climate, vol 14, 3704-3723
Czaja, A. and J. Marshall (2000) On the interpretation of the response of atmospheric models to prescribed time-varying SST anomalies Geo Phys Lett, Vol 27, No 13, pgs 1927-1930
Goodman, J. and J. Marshall (1999) A model of decadal middle-latitude atmosphere-ocean coupled modes.
Journal of Climate Vol 12, No. 2, pages 621-641