Intensification of winter cyclones over the ocean and the corresponding oceanic response are some of the most challenging problems in air-sea interaction. Climatologically, the largest transfers of sensible and latent heat from the ocean to the atmosphere occur off the east coast of the United States along the Gulf Stream SST front during winter. A substantial part of these transfers takes place during the cold-air outbreak phase of a passing winter storm. As cold, dry, Arctic air flows off of the continent and over the warm Gulf Stream, instantaneous transfer rates can exceed climatological values by several times (Xue et al., 1995). The significant contribution of these storms to the overall transfer of heat from the ocean to the atmosphere manifests the important role of the western boundary currents of the subtropical gyres in the poleward transport of heat by the ocean-atmosphere system.

• Modification of the Gulf Stream through Strong Air-Sea Interactions in Winter: Observations and Numerical Simulations. (Xue, Bane, and Goodman, 1995: JPO, 533-557)
  • Abstract
    The greatest fluxes of heat and moisture from the ocean to the atmosphere occur off the east coast of North America during winter when the Gulf Stream is vigorously cooled by strong cold air outbreaks that move off the continent. In this paper observational and numerical modeling methods are employed to investigate the response of the Gulf Stream to such strong cooling events. Both methods show that the surface mixed layer can deepen several tens of meters during a strong outbreak and that the heat decrease within the upper layer of the Gulf Stream, 2.9 ¥ 10¹³ J in the model and 3.2 (±0.7) ¥ 10¹³ J in observations (per meter alongstream) for one case study, is balanced closely by the amount of oceanic heat released to the atmosphere. Computations also show that the cross-stream circulation is dominated by Ekman-like, wind-driven motion with velocities on the order of 20 cm s^-1. A vertical circulation cell within the Gulf Stream, with vertical velocities on the order of 0.1 cm s^-1. Is found to be a result of convergence/divergence of the Ekman transport due to the altered inertial frequency caused by the horizontal velocity shear of the Gulf Stream jet.
    
    
• Gulf Stream and its Meanders in Response to Cold Air outbreaks. (Xue and Bane, 1997: JPO, 2606-2629)
  • Abstract
    The three-dimensional Princeton Ocean Model is used to examine the modification of the Gulf Stream and its meanders by cold air outbreaks. Two types of Gulf Stream meanders are found in the model. Meanders on the shoreward side of the Gulf Stream are baroclinically unstable. They are affected little by the atmospheric forcing because their energy source is stored at the permanent thermocline which is well below the influence of the surface forcing. Meanders on the seaward side of the Stream are both barotropically and baroclinically unstable. The energy feeding these meanders is stored at the surface front separating the Gulf Stream and the Sargasso Sea which is greatly reduced in case of cold air outbreaks. Thus meanders there reduce strength and also seem to slow their downstream propagation due to the southward Ekman flow. Heat budget calculations suggest two almost separable processes. The oceanic heat released to the atmosphere during these severe cooling episodes comes almost exclusively from the upper water column. Transport of heat by meanders from the Gulf Stream to the shelf, though it is large, does not disrupt the principal balance. It is balanced nicely with the net heat transport in the downstream direction.
    
    
• A 2D Coupled Atmosphere-Ocean Model Study of Cold Air Advection over the Gulf Stream. (Xue, Pan, and Bane, submitted, MWR)
  • Abstract
    The two-dimensional, Advanced Regional Prediction System (ARPS) has been coupled with the Princeton Ocean Model (POM) to study air-sea interaction processes during an extreme cold air outbreak over the Gulf Stream. Emphases have been placed on the development of the mesoscale front and local winds in the lower atmosphere due to differential fluxes over the land, the cold shelf water, and the warm Gulf Stream, and on how the mesoscale front and the local winds feed back to the ocean and modify the upper ocean temperature and current fields. Model results show that a shallow mesoscale atmospheric front is generated over the Gulf Stream and progresses eastward with the prevailing airflow. Behind the front, the wind intensifies by as much as 75 % and a northerly low-level-wind maximum with speeds near 5 m s-1 appears. The low-level northerly winds remain relatively strong even after the front has progresses past the Gulf Stream. The total surface heat flux in the coupled experiment is about 10 percent less than the total surface heat flux in the experiment with fixed SST, suggesting that the oceanic feedback to the mesoscale atmospheric features might not be of leading importance. On the other hand, the response of the upper ocean velocity field to the local winds is on the order of 20 cm s-1, dominating over the response to the synoptic winds. This suggests the modification in the atmosphere by air-sea fluxes, which induces the locally enhanced winds, has considerable impact on the ocean.
    
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• 3D Coupled Atmosphere-Ocean Model Study of Air-Sea Interaction off the East Coast of North America (Xue and Bane)
  • We propose a coupled atmosphere-ocean model study to continue our investigation of wintertime air-sea interaction off the east coast of North America, emphasizing the Gulf Stream region east of Cape Hatteras and the coastal region in the Middle Atlantic Bight. Analyses of model results include temporal and spatial variation of the air-sea fluxes, evolution of the oceanic mixed layer and the marine atmospheric boundary layer, and momentum, heat and potential vorticity balance in the ocean. The scientific objectives of the study are: 1) to determine the temporal and spatial variations of the air-sea fluxes over the MAB and over the Gulf Stream east of Cape Hatteras during a winter storm ; 2) to examine the differences between the South Atlantic Bight and the Middle Atlantic Bight in response to east coast winter storms ; 3) to quantify the modifications of the Gulf Stream east of Cape Hatteras in terms of heat balance, vorticity balance, and the circulation induced by the atmospheric forcing; 4) to examine whether and how the oceanic response to individual storm is preserved in the swift Gulf Stream; and 5) to determine the sensitivity of the lower atmosphere to the Gulf Stream meanders and the warm core rings. The three-dimensional, coupled atmosphere-ocean model developed for the proposed study can also be used in other studies of coastal air-sea interaction.