Low-Frequency Variability in the Midlatitude Baroclinic Atmosphere Induced by an Oceanic Thermal Front

Yizhak Feliks, Michael Ghil, Eric Simonnet
2007 Journal of the Atmospheric Sciences  
This study examines the flow induced by an east-west-oriented oceanic thermal front in a highly idealized baroclinic model. Previous work showed that thermal fronts could produce energetic midlatitude jets in an equivalent-barotropic atmosphere and that barotropic instabilities of this jet had dominant periods of 25-30 and 65-75 days. The present study extends this work to a two-mode baroclinic free atmosphere. The baroclinic jet produced in this case is subject to both barotropic and
more » ... ropic and baroclinic instabilities. A barotropic symmetric instability propagates westward with periods of roughly 30 days and is similar to those found in the equivalent-barotropic model. A baroclinic instability results in standing-dipole anomalies and oscillates with a period of 6-8 months. A mixed barotropic-baroclinic instability results in anomalies that propagate northward, perpendicular to the jet, with a period of 2-3 months. The later anomalies are reminiscent of the 70-day oscillation found over the North Atlantic in observed fields. The atmospheric flow has two distinct states: the flow in the high-energy state exhibits two large gyres and a strong eastward jet; its antisymmetric component is dominant. The low-energy flow is characterized by small gyres and a weak jet. The model's dynamics depends on the layer-depth ratio. When the model is nearly equivalent-barotropic, symmetric oscillatory modes dominate. As the two layers become nearly equal, antisymmetric oscillatory modes become significant and the mean energy of the flow increases. When the oceanic thermal front's strength T * is weak (T * Յ 1.5°C), the flow is steady. For intermediate values of the strength (1.5°C Ͻ T * Ͻ 3°C), several oscillatory instabilities set in. As the frontal strength increases further (T * Ն 3°C), the flow becomes more turbulent. These results all depend on the atmospheric model's horizontal resolution being sufficiently high.
doi:10.1175/jas3780.1 fatcat:cz2pueulmjcrzp2jkrutp53tum