THE NORTH ATLANTIC OSCILLATION (NAO) JENNY KAFKA

WHAT IS THE NAO? -Discovered by Sir Gilbert Walker in early 20 th century -Based upon the difference in surface sea-level pressure between the Subtropical High (Azores) and the Subpolar Low (Iceland) ONE OF MANY TELECONNECTION INDICES -Describes a recurring and persistent large-scale pattern of pressure and circulation anomalies -Gives us an understanding of climate variability

IMPORTANCE Positive Phase vs. Negative Phase The NAO shapes the weather patterns over much of the Northern Hemisphere (especially in winter) e.g. Negative phase associated with cold & snowy winters in Northeast

TIME SCALE Can switch between modes in a matter of weeks, but one mode tends to dominate over a given year Multi-decadal trends tend to persist ~ 20 years

THE NAO AND THE OCEAN HIGH FREQUENCY RESPONSE (Seasonal to interannual) -Correlation between NAO phase and SST temperature  “NAO SST tripole” + NAO: - SST anomaly by Greenland; + SST anomaly off US coast - NAO: + SST anomaly by Greenland; - SST anomaly off US coast Visbeck et al. 2003

THE NAO AND THE OCEAN HIGH FREQUENCY RESPONSE (Seasonal to interannual) - Observed response of wind stress pattern changes + NAO: Stronger westerlies; slightly stronger easterlies - NAO: Weaker westerlies; slightly weaker easterlies Visbeck et al. 2003

THE NAO AND THE OCEAN LOW FREQUENCY RESPONSE (SEVERAL YEARS TO DECADES) - Strength/position of boundary currents  0-3 years to adjust - Meridional Overturning Circulation (MOC)  10+ years to adjust - Downstream movement of +SST anomaly - Loss of -SST anomaly over sub polar gyre Visbeck et al. 2003

RESOURCES "Climate Prediction Center - North Atlantic Oscillation (NAO)." Climate Prediction Center - North Atlantic Oscillation (NAO). National Weather Service, n.d. Web. 02 Dec Henson, Bob. "Twisters and Teleconnections." AtmosNews. NCAR UCAR, n.d. Web. 03 Dec "North Atlantic Oscillation." Columbia University, n.d. Web. 02 Dec "North Atlantic Oscillation (NAO)." National Climatic Data Center (NCDC). N.p., n.d. Web. 03 Dec Visbeck, M., Chassignet, E. P., Curry, R. G., Delworth, T. L., Dickson, R. R. and Krahmann, G. (2013) The Ocean's Response to North Atlantic Oscillation Variability, in The North Atlantic Oscillation: Climatic Significance and Environmental Impact (eds J. W. Hurrell, Y. Kushnir, G. Ottersen and M. Visbeck), American Geophysical Union, Washington, D. C.. doi: /134GM06