IOCEAN-ATMOSPHERE COUPLED MODEL SIMULATION OF NORTH ATLANTIC INTERANNUAL VARIABILITY I: LOCAL AIR-SEA INTERACTION

The North Atlantic inter annual variability associated with the North Atlantic Oscillation (NAO) is examined in a 300 years integration of Bergen Clima te Model (BCM). The physical processes dominating the interannual scale tripolarpattern variability mode of the North Atlantic sea surface temperature (SST) are analyzed. Possible impacts of the tripolar SST anomalies on the decadal scale variability of the North Atlantic climate are also discussed. The dominant mod e of the North Atlantic wintertime SST variability exhibits a meridional tri-polar pattern, with a colder subpolar region, a warmer midlatitudes, and a colder region between the equator and 30°N. Validations on the model performance show that the BCM reproduces the observed NAO pattern and the tri-polar like SST vari ability mode successfully. The atmospheric circulation change associated with the tri-polar SST anomalies exhibits as NAO and has a barotropic structure. The power spectrum of the simulated NAO index is white and the interannual scale varia bility stands out, as the observational data reveals. The tripolar structure SST anomalies over the North Atlantic, which also appears at the interannual scale with a spectral peak at 3—4 years, are mainly resulted from the barotropic driving of the NAO like atmospheric forcing, and thermal process plays a dominantrole in this process. When the NAO is strong, the westerly weakens over the midlat itudes; both the sensible and the latent heat flux release from the ocean tend to decrease, which warms the ocean and leads to positive SST anomalies. Over the sub polar region including the Labrador Sea, a stronger NAO is associated with a deeper Icelandic Low; the high latitudes originated colder air then blows over the ocean surface, the air-sea temperature difference become larger, and the se n sible heat flux loss from the ocean increases, which subsequently leads to colder SST anomalies. The intensified easterly over the tropical area is also an impo rtant mechanism dominating the colder SST. The feedback of the tripolar SST anomalies on the atmosphere is weak. Further analysis reveals no evidence supportin g the possible feedback of the tripolar structure SSTA on the regional climate at multiannual and decadal scales.