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