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.
Abstract Significant heat is transported from the atmosphere through the upper ocean in the equatorial Pacific cold tongue (PCT), primarily a result of turbulence-enhanced mixing, but with a significant contribution from penetrating solar radiation in especially the eastern extent of the PCT. We consider whether diathermal heat transport in the PCT is sufficiently large to influence global-scale heating from Earth’s energy imbalance by evaluating its magnitude and variability through the 21.5°C isotherm. Here, multiyear observations of mixing at 0°, 140°W are augmented by neural network predictions to fill in data gaps and extend the existing record of mixing to a 30-yr period. Spatial extrapolation of these estimates to a larger area is guided by a high-resolution model. We find that the average heat transport through 21.5°C in the PCT, which comprises 0.2%–0.6% of the World Ocean’s surface area, is many times larger than the global atmosphere’s heating rate, 24%–116% of the global ocean’s heating rate between 100- and 300-m depth and 7.9%–24% of the global ocean’s heating rate below 100 m. Further, decadal variations of mixing in the PCT, driven by changes in El Niño–Southern Oscillation (ENSO) phase, are large enough and of the correct sign to account for transient global-scale variations in atmosphere and subsurface ocean heating during and after the 1998–2012 atmospheric warming slowdown. These results provide observation-based evidence that the PCT is a globally significant pathway for heat transport from the atmosphere to the subsurface ocean. Significance Statement A combination of strong turbulence and stratification above the Equatorial Undercurrent leads to large vertical heat transport in the equatorial Pacific cold tongue (PCT). A multiyear time series of mixing measurements is used to train a neural network to fill data gaps and extend the record to 30 years. From this record, we find that the rate of heating of cooler waters by mixing in the PCT is comparable to global heating rates of the atmosphere and subsurface ocean. In addition, decadal variations are of the correct sign and magnitude to account for decadal variations in air–sea heat exchanges, pointing to the PCT as a conduit for global-scale heat transfers between different components of the Earth system.
