The Effect of Gulf Stream-induced Baroclinicity on U.S. East Coast Winter Cyclones
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Midlatitude cyclones develop off the Carolinas during winters and move north producing gale-force winds, ice, and heavy snow. It is believed that boundary-layer and air-sea interaction processes are very important during the development stages of these East Coast storms. The marine boundary layer (MBL) off the mid- Atlantic coastline is highly baroclinic due to the proximity of the Gulf Stream just offshore. Typical horizontal distances between the Wilmington coastline and the western edge of the Gulf Stream vary between 90 and 250 km annually, and this distance can deviate by over 30 km within a single week. While similar weekly Gulf Stream position standard deviations also exist at Cape Hatteras, the average annual distance to the Gulf Stream frontal zone is much smaller off Cape Hatteras, normally ranging between 30 and 100 km. This research investigates the low-level baroclinic conditions present prior to observed storm events. The examination of nine years of data on the Gulf Stream position and East Coast winter storms seems to indicate that the degree of low-level baroclinicity and modification existing prior to a cyclonic event may significantly affect the rate of cyclonic deepening off the mid-Atlantic coastline. Statistical analyses linking the observed surface-pressure decrease with both the Gulf Stream frontal location and the prestorm coastal baroclinic conditions are presented. These results quantitatively indicate that Gulf Stream-induced wintertime baroclinicity may significantly affect the regional intensification of East Coast winter cyclones.Keywords:
Gulf Stream
Extratropical cyclone
Winter storm
Boundary current
Abstract : We investigate the impact of 1/8 deg, 1/16 deg, 1/32 deg, and 1/64 deg ocean model resolution on model-data comparisons for the Gulf Stream system between the Florida Straits and the Grand Banks. This includes mean flow and variability, the Gulf Stream pathway, the associated nonlinear recirculation gyres, the large-scale C-shape of the subtropical gyre, and the abyssal circulation. A nonlinear isopycnal, free surface model covering the Atlantic from 9 deg N to 47 deg N or 51 deg N, including the Caribbean and Gulf of Mexico, and a similar 1/16 deg global model are used. The models are forced by winds and by a global thermohaline component via ports in the model boundaries. When calculated using realistic wind forcing and Atlantic model boundaries, linear simulations with Munk western boundary layers and a Sverdrup interior show two unrealistic mean Gulf Stream pathways between Cape Hatteras and the Grand Banks, one proceeding due east from Cape Hatteras, and a second one continuing northward along the western boundary until forced eastward by the regional northern boundary. The northern pathway is augmented when a linear version of the upper ocean global thermohaline contribution to the Gulf Stream is added as a Munk western boundary layer. A major change is required to obtain a realistic pathway in nonlinear models. Resolution of 1/8 deg is eddy-resolving, but mainly gives a wiggly version of the linear model Gulf Stream pathway and weak abyssal flows except for the deep western boundary current forced by ports in the model boundaries. All of the higher resolution simulations show major improvement over the linear and 1/8 deg nonlinear simulations. Additional major improvement is seen with the increase from 1/16 deg to 1/32 deg resolution and modest improvement with a further increase to 1/64 deg.
Gulf Stream
Boundary current
Isopycnal
Eddy
Forcing (mathematics)
Oceanic basin
Sverdrup
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A model of abyssal, subthermocline flow is presented for a basin in which a peninsula intrudes into the basin. The effect of the peninsula is to provide two “eastern” boundaries for the total basin. The latitude-independent baroclinic pressure and density anomalies on both these boundaries are determined by integral conditions, which generalize earlier work. The peninsula also produces a zonal jet between the tip of the peninsula and the western boundary. The baroclinic transport of this current is related to the blocking of westward propagating Rossby waves from the basin's eastern boundary. The baroclinic structure of this current, as well as the interior flow and the western boundary currents, are determined in terms of the distribution of deep-water sources on the perimeter of the basin as well as by the spatial distribution of upwelling into the thermocline. For the zonal jet, boundary currants and interior the flow is strongly baroclinic. Especially in the interior, the baroclinic Stommel-Arons flow is only a small residual obtained by vertically averaging the motion. The upper part of the water column responds strongly to the upwelling forcing, and the lower part of the water column is more strongly influenced by the structure and distribution of the sources. The strong recirculation, which is only weakly coupled to the structure of the sources, renders the total vertical structure of the predicted flow more complex than that of the sources.
Boundary current
Barotropic fluid
Peninsula
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Abstract Contrary to the general notion that extratropical cyclones reduce baroclinicity, the baroclinicity is found to be enhanced in the wake of the extreme winter storm Dagmar. Thus, individual storms can increase baroclinicity, yielding a pathway to secondary cyclogenesis and cyclone clustering. We use a recently introduced diagnostic for baroclinicity—the tendency equation for the isentropic slope—and found that strong diabatic heating due to moisture supply from the subtropical Atlantic led to the enhanced baroclinicity in the rear of Dagmar. Storms ensuing Dagmar benefited from this increased baroclinicity. In contrast to previous studies on the mechanisms of cyclone clustering, we only find weak evidence for Rossby wave breaking and thus propose diabatic heating as an alternative pathway to cyclone clustering.
Extratropical cyclone
Cyclogenesis
Diabatic
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Gulf Stream
Boundary current
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Abstract Baroclinic waves drive both regional variations in weather and large-scale variability in the extratropical general circulation. They generally do not exist in isolation, but rather often form into coherent wave packets that propagate to the east via a mechanism called downstream development. Downstream development has been widely documented and explored. Here we document a novel but also key aspect of baroclinic waves: the downstream suppression of baroclinic activity that occurs in the wake of eastward propagating disturbances. Downstream suppression is apparent not only in the Southern Hemisphere storm track as shown in previous work, but also in the North Pacific and North Atlantic storm tracks. It plays an essential role in driving subseasonal periodicity in extratropical eddy activity in both hemispheres, and gives rise to the observed quiescence of the North Atlantic storm track 1–2 weeks following pronounced eddy activity in the North Pacific sector. It is argued that downstream suppression results from the anomalously low baroclinicity that arises as eastward propagating wave packets convert potential to kinetic energy. In contrast to baroclinic wave packets, which propagate to the east at roughly the group velocity in the upper troposphere, the suppression of baroclinic activity propagates eastward at a slower rate that is comparable to that of the lower to midtropospheric flow. The results have implications for understanding subseasonal variability in the extratropical troposphere of both hemispheres.
Extratropical cyclone
Storm track
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Gulf Stream
Boundary current
Pycnocline
Drift current
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Gulf Stream
Boundary current
Isopycnal
Atlantic hurricane
Sverdrup
Forcing (mathematics)
Oceanic basin
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In this study, the authors analyze the trajectories of 18 RAFOS floats, launched in the deep western boundary current (DWBC) between the Grand Banks and Cape Hatteras to investigate the kinematics and dynamics in the region where the DWBC crosses under the Gulf Stream, near 36°N (the “crossover region”). Floats deployed in the chlorofluorocarbon (CFC) maximum associated with upper Labrador Sea Water (depth ∼800 m) illustrate the entrainment process of this water mass into the Gulf Stream. The behavior of the floats (and fluid parcels) in the crossover region is strongly dependent on the meandering of the Gulf Stream. When the stream is close to its mean position, fluid parcels entrained from the upper DWBC travel along the northern edge of the stream. When a meander trough is present downstream of the entrainment location, DWBC fluid parcels cross the Gulf Stream and sometimes are expelled on the south side. This represents a previously unrecognized mechanism for transporting upper Labrador Sea Water properties across the Gulf Stream. Floats deployed in the DWBC near the deep CFC maximum that identifies overflow water from the Nordic seas (depth ∼3000 m) show a bifurcation in fluid parcel trajectories in the crossover region: fluid parcels that intersect the stream farther west tend to cross more directly and smoothly under the stream, while fluid parcels that hit the stream farther east exhibit more eddy motion and are more likely to be diverted into the interior along the Gulf Stream path. The deep float observations also reveal directly that the deep DWBC crosses under the Gulf Stream while conserving potential vorticity by sliding down the continental slope, as first conceptualized in a steady, two-layer model of the crossover. While potential vorticity is conserved along the deep float tracks on the short timescales associated with crossing under the Gulf Stream (up to a month), potential vorticity decreases over the longer timescales required for fluid parcels to transit the entire crossover region (several months to a year), consistent with what would be expected from eddy mixing.
Gulf Stream
Boundary current
Eddy
Entrainment (biomusicology)
Drifter
Meander (mathematics)
Trough (economics)
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