Abstract While the effects of along-front winds aligned with the frontal jet in submesoscale surface-layer fronts have been extensively studied in the past, the impact of cross-front winds is less well explored at the moment. This study investigates the effects of cross-front winds on a dense submesoscale filament in the central Baltic Sea based on high-resolution turbulence and velocity observations, supplemented by idealized numerical simulations. Our focus on a dense filament, by definition laterally bounded by two fronts, allowed us to systematically distinguish between fronts with upgradient winds (directed from lighter to denser waters) and down-gradient winds (pointing into the opposite direction). Our data show that upgradient winds induce a down-front Ekman transport that augments the geostrophically balanced frontal jet. This type of forcing also drives near-surface Ekman currents that reinforce the secondary cross-front circulation, triggering rapid restratification and a shoaling of the turbulent surface layer. In this case, the combination of enhanced turbulence shear production and strong stratification leads to vigorous and efficient diapycnal mixing. Conversely, down-gradient winds suppress both the frontal jet and the secondary cross-front circulation, induce shear-driven convection, and result in a well-mixed surface layer with weak vertical shear. Idealized numerical simulations further indicate that the observed asymmetry in frontal mixing reflects distinct kinetic energy exchange pathways within the filament, mediated by geostrophic shear production on time scales of hours.
Abstract This study develops a quantitative model for the meridional drift of nonlinear mesoscale eddies in a reduced-gravity, quasi-geostrophic framework. The metric is the drift velocity of an eddy centroid defined and tracked by a closed height-anomaly contour. The analysis considers both single-eddy evolution and eddy interaction. As nonlinearity increases, the evolution of an isolated, axisymmetric eddy transitions from Rossby-wave dispersion with minimal meridional deflection to movement as a compact entity with persistent meridional drift. Using the difference between centroid and height-extremum trajectories as a measure of dispersiveness, a practical criterion for the compact-eddy regime is established. For eddies in this regime, a central result is a quantitative relationship that connects the meridional drift to meridional advection of the eddy boundary. When applied to isolated eddies, this relationship accurately captures the temporal variability of the drift velocity in numerical simulations. Decomposition of eddy circulation reveals a well-known secondary, tilted β -dipole (” β -gyres”) structure. The dipole dominates the boundary advection, thereby leading to a direct and validated relationship in which the temporal variability of centroid drift is controlled by the boundary-averaged meridional dipole velocity, with a proportionality factor depending on eddy size. The size dependence also implies that smaller eddies drift more rapidly. Because the boundary-advection formulation is general and could account for mutual advection, the quantitative relationship remains applicable when eddies interact. This general utility is demonstrated by testing against simulations where realistic eddy interaction is permitted.
The Ross Sea continental shelf plays a critical role in Antarctic Bottom Water formation through the production of High Salinity Shelf Water (HSSW), with implications for the global overturning circulation. The salinity of HSSW exhibits pronounced interannual variability, while its spatiotemporal characteristics and governing mechanisms remain poorly understood. Using a regional ocean-sea ice-ice shelf coupled model, we investigate the nature of this variability and the major driving processes. Here, in Part I, we focus on surface-driven processes, analyzing the factors governing surface salinity variability, including interannual changes in surface salt flux and the role of surface salt advection. Our results show that variability in surface salt flux is primarily driven by southwesterly wind anomalies over the continental shelf, which are influenced by the atmospheric circulation pattern across the Ross Sea. These wind anomalies control the net sea ice transport change across the shelf break, along with sea ice production mainly over the eastern continental shelf and within the Ross Ice Shelf Polynya. Advected westward by surface currents, salinity anomalies originating in the eastern continental shelf region thereby influence surface properties in the western Ross Ice Shelf Polynya and the Terra Nova Bay Polynya, the primary HSSW formation sites. Identifying these surface-driven mechanisms provides a foundation for assessing the contributions of internal oceanic processes to interannual salinity variability, which are investigated in Part II.
The cumulative effects of tropical cyclones (TCs) have a long-lasting impact on the meridional ocean heat transport (MHT), yet the underlying mechanisms remain poorly understood. Using high-resolution numerical simulations, this study investigates how TCs modulate MHT in the North Pacific through two experiments: one with TC wind forcing removed and the other with TC wind forcing reconstructed from observations. Results show that TCs weaken poleward MHT by 1.6% +/- 1.2% between 0 degrees and 16 degrees N but strengthen it by 2.2% +/- 1.6% between 16 degrees and 30 degrees N. By decomposing the MHT anomalies into an overturning component and a gyre component, we separately examine the dynamic and thermodynamic processes at play. The response is found to be dominated by changes in the overturning component through changes in the strength of the subtropical overturning cell. This cell is weakened between 0 degrees and 15 degrees N due to reduced Ekman transport and interior geostrophic return flow but strengthened between 15 degrees and 30 degrees N due to intensified Ekman transport and weakened western boundary transport. In contrast, the gyre component, controlled by both dynamic and thermodynamic processes, offsets nearly half of the overturning-induced changes. TCs also amplify the seasonal amplitude of North Pacific MHT by up to 9.5% +/- 1.5%, primarily through the seasonal modulation of the overturning cell strength. These results demonstrate that TCs modulate both the mean state and seasonal variability of the North Pacific MHT, highlighting the dominant role of the overturning cell strength and the compensating effects of the gyre component, which are often overlooked by previous research.
Abstract The Global Overturning Circulation (GOC) is a key component of the climate system, transporting heat, carbon, and salt through the global ocean. Within ocean basins, this circulation is geostrophic, implying that its strength scales with the zonal buoyancy gradient. The GOC is therefore influenced by buoyancy dynamics along the eastern and western boundaries, which affect, for example, its sensitivity to convective mixing and the latitudinal dependence of its transient adjustment. Here, we build on Callies and Marotzke (2012) and develop a reduced model of the geostrophic GOC that prognostically evolves boundary buoyancy. The model is interhemispheric, includes a re-entrant channel, and incorporates wind-driven upwelling. It demonstrates how zonal pressure differences—and thus the GOC—are maintained by local boundary dynamics, enabling an equilibrium interhemispheric circulation under certain surface forcing conditions. The model provides a transparent framework for advection-mediated interhemispheric overturning adjustment, showing that cross-equatorial propagation of transport anomalies requires the transfer of western boundary buoyancy anomalies to the eastern boundary. The interhemispheric propagation rate of these transport anomalies is controlled by the poleward extension of the eastern boundary buoyancy anomaly, which is depth dependent: in the deep ocean, slow along-boundary spreading controls the rate, whereas in the upper ocean, vertical advection of the background stratification enables rapid extension. This result implies that Southern Ocean wind forcing drives rapid cross-equatorial adjustment, whereas North Atlantic buoyancy forcing produces a slow interhemispheric response that filters the cross-equatorial propagation of signals with periods shorter than several decades. These results are reproduced in an idealized ocean circulation model.
