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A Numerical Study of Linkage Between the North Atlantic Oscillation (NAO) and Climate Changes in the Tropical and Subtropical Atlantic Ocean

Principal Investigator: Jiayan Yang

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Final Summary Report CICOR Cooperative Agreement 1998-2002

In the past year (1999-200} the PI has worked closely with Dr. T. Joyce on a project to examine heat content variability in the tropical and subtropical Atlantic Ocean by using both models and observed data. We have investigated several mechanisms that may contribute to the variability in the tropics. Heat content in the upper layer (0-200m} in the equatorial Atlantic Ocean has considerable variations on interannual to decadal time scales (using two sets of gridded temperature data; one from T. Joyce [WHOI] and the other from W. White [Scripps]}. We studied its forcing mechanisms by examining the atmospheric variability, using both NCEP and COADS data, and responses from a model to atmospheric forcing. It was found that the change of the trade wind along the equator is a main factor that gives rise to heat content variations.

We have also investigated the role of Rossby and Kelvin waves in linking variability in the tropics and extratropics. The center of the action associated with NAO is outside the tropics. To understand how the NAO affects the tropics, one must examine how the extratropical and tropical oceans interact and communicate with each other. One of the most efficient ways is through Kelvin wave propagation along side boundaries. Rossby waves can be generated in response to NAO-induced changes in wind-stress curl. When a Rossby wave propagates westward and reflects at the western boundary, part of its energy will be reflected back to the ocean interior via short Rossby waves, but the remaining part will go to coastal Kelvin waves which propagate along the western boundary to the tropics and may affect the thermocline depth and heat content variability there.

We have examined this problem in three steps. First, we solved the Rossby-wave reflection problem analytically, and found that the magnitude of Kelvin-wave response depends a number of factors, such as the amplitude and wavelength of incoming Rossby waves, the latitude at which the reflection occurs. In high latitude, such as in Labrador Sea, the Kelvin wave response from a Rosdby-wave reflection is small. However, if Kelvin waves are directly forced by either convection or atmosphere forcing along the western boundary, their amplitude waves are much greater and the response in the tropics is significant. Second, we have used a numerical model to conduct several sensitivity tests to examine a few types of forcing associated with NAO, e.g., boundary versus interior, subtropics versus subpolar, etc., and found that model results are highly consistent with the analytical solution. Third, we have analy2ed sea-level data from many tidal stations along both eastern and western boundaries to examine the Kelvin-wave connection in the Atlantic Ocean. It was found that sea-level variations along both boundaries and in almost all latitudes in the northern hemisphere are highly correlated with the NAO index. The phases of sea-level variations in these stations suggest that boundary Kelvin waves forced by surface wind stress associated with NAO play a major role.

Yan J. 1999: A Linka a Between Decadal Climate Variations In The Labrador Sea And The Tropical Atlantic Ocean. geophysical Research Letters, Vol. 26, No. 8,1023-1026.

Yang J. an,d TY. Jo ce: Kelvin and Rossby waves and a connection between the tropics and extratropics (tentative title), in preparation.

Last updated: August 19, 2008

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