Momentum governors of California Undercurrent transport

2021 ◽  
Author(s):  
Ru Chen ◽  
James C. McWilliams ◽  
Lionel Renault
Keyword(s):  
Spatial Variability ◽  
Pressure Gradient ◽  
Wind Stress ◽  
Temporal Variability ◽  
Current System ◽  
California Current ◽  
Regional Model ◽  
California Current System ◽  
Significant Variability ◽  
Pressure Anomaly

AbstractThe California Undercurrent (CUC) transport, with significant variability ranging from weeks to decades, has consequences for both the climate and biogeochemistry of the California Current system. This study evaluates the governors of the CUC transport and its temporal variability from a momentum perspective, using a mesoscale-resolving regional model. From a 16-year mean perspective, the along-isobath pressure gradient acts to accelerate the CUC, whereas eddy advection retards it. The topographic form stress, which is part of the volume integrated along-isobath pressure gradient, not only acts in the direction of the time-mean CUC, but also greatly modulates the temporal variability of the CUC transport. This temporal variability is also correlated with the eddy momentum advection. The eddy stress plays a role in transferring both the equatorward wind stress and poleward CUC momentum downward. A theory is formulated to show that, in addition to the conventional vertical redistribution of momentum, the eddy stress can also redistribute momentum horizontally in the area where the correlation between the pressure anomaly and isopycnal fluctuations has large spatial variability.

scholarly journals Surface Stress and Atmospheric Boundary Layer Response to Mesoscale SST Structure in Coupled Simulations of the Northern California Current System

2019 ◽  
Vol 148 (1) ◽  
pp. 259-287
Author(s):  
R. M. Samelson ◽  
L. W. O’Neill ◽  
D. B. Chelton ◽  
E. D. Skyllingstad ◽  
P. L. Barbour ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Wind Stress ◽  
Current System ◽  
California Current ◽  
Heat Fluxes ◽  
Neutral Stability ◽  
Northern California ◽  
Coupling Coefficients ◽  
California Current System ◽  
Northern California Current

Abstract The influence of mesoscale sea surface temperature (SST) variations on wind stress and boundary layer winds is examined from coupled ocean–atmosphere numerical simulations and satellite observations of the northern California Current System. Model coupling coefficients relating the divergence and curl of wind stress and wind to downwind and crosswind SST gradients are generally smaller than observed values and vary by a factor of 2 depending on planetary boundary layer (PBL) scheme, with values larger for smoothed fields on the 0.25° observational grid than for unsmoothed fields on the 12-km model grid. Divergence coefficients are larger than curl coefficients on the 0.25° grid but not on the model grid, consistent with stronger scale dependence for the divergence response than for curl in a spatial cross-spectral analysis. Coupling coefficients for 10-m equivalent neutral stability winds are 30%–50% larger than those for 10-m wind, implying a correlated effect of surface-layer stability variations. Crosswind surface air temperature and SST gradients are more strongly coupled than downwind gradients, while the opposite is true for downwind and crosswind heat flux and SST gradients. Midlevel boundary layer wind coupling coefficients show a reversed response relative to the surface that is predicted by an analytical model; a predicted second reversal with height is not seen in the simulations. The relative values of coupling coefficients are consistent with previous results for the same PBL schemes in the Agulhas Return Current region, but their magnitudes are smaller, likely because of the effect of mean wind on perturbation heat fluxes.

scholarly journals Eddy–Wind Interaction in the California Current System: Dynamics and Impacts

2016 ◽  
Vol 46 (2) ◽  
pp. 439-459 ◽  
Author(s):  
Hyodae Seo ◽  
Arthur J. Miller ◽  
Joel R. Norris
Keyword(s):  
Wind Stress ◽  
Current System ◽  
California Current ◽  
Surface Current ◽  
Small Scale ◽  
Propagation Time ◽  
Ekman Pumping ◽  
Induced Current ◽  
California Current System ◽  
The Mean

AbstractThe summertime California Current System (CCS) is characterized by energetic mesoscale eddies, whose sea surface temperature (SST) and surface current can significantly modify the wind stress and Ekman pumping. Relative importance of the eddy–wind interactions via SST and surface current in the CCS is examined using a high-resolution (7 km) regional coupled model with a novel coupling approach to isolate the small-scale air–sea coupling by SST and surface current. Results show that when the eddy-induced surface current is allowed to modify the wind stress, the spatially averaged surface eddy kinetic energy (EKE) is reduced by 42%, and this is primarily due to enhanced surface eddy drag and reduced wind energy transfer. In contrast, the eddy-induced SST–wind coupling has no significant impact on the EKE. Furthermore, eddy-induced SST and surface current modify the Ekman pumping via their crosswind SST gradient and surface vorticity gradient, respectively. The resultant magnitudes of the Ekman pumping velocity are comparable, but the implied feedback effects on the eddy statistics are different. The surface current-induced Ekman pumping mainly attenuates the amplitude of cyclonic and anticyclonic eddies, acting to reduce the eddy activity, while the SST-induced Ekman pumping primarily affects the propagation. Time mean–rectified change in SST is determined by the altered offshore temperature advection by the mean and eddy currents, but the magnitude of the mean SST change is greater with the eddy-induced current effect. The demonstrated remarkably strong dynamical response in the CCS system to the eddy-induced current–wind coupling indicates that eddy-induced current should play an important role in the regional coupled ocean–atmosphere system.

scholarly journals Coastal Trapped Waves, Alongshore Pressure Gradients, and the California Undercurrent*

2014 ◽  
Vol 44 (1) ◽  
pp. 319-342 ◽  
Author(s):  
Thomas P. Connolly ◽  
Barbara M. Hickey ◽  
Igor Shulman ◽  
Richard E. Thomson
Keyword(s):  
Pressure Gradient ◽  
Wind Stress ◽  
West Coast ◽  
Current System ◽  
Ocean Model ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Pressure Gradients ◽  
Transport Pathway ◽  
California Current System

Abstract The California Undercurrent (CUC), a poleward-flowing feature over the continental slope, is a key transport pathway along the west coast of North America and an important component of regional upwelling dynamics. This study examines the poleward undercurrent and alongshore pressure gradients in the northern California Current System (CCS), where local wind stress forcing is relatively weak. The dynamics of the undercurrent are compared in the primitive equation Navy Coastal Ocean Model and a linear coastal trapped wave model. Both models are validated using hydrographic data and current-meter observations in the core of the undercurrent in the northern CCS. In the linear model, variability in the predominantly equatorward wind stress along the U.S. West Coast produces episodic reversals to poleward flow over the northern CCS slope during summer. However, reproducing the persistence of the undercurrent during late summer requires additional incoming energy from sea level variability applied south of the region of the strongest wind forcing. The relative importance of the barotropic and baroclinic components of the modeled alongshore pressure gradient changes with latitude. In contrast to the southern and central portions of the CCS, the baroclinic component of the alongshore pressure gradient provides the primary poleward force at CUC depths over the northern CCS slope. At time scales from weeks to months, the alongshore pressure gradient force is primarily balanced by the Coriolis force associated with onshore flow.

An Adjoint Sensitivity Analysis of the Southern California Current Circulation and Ecosystem

2009 ◽  
Vol 39 (3) ◽  
pp. 702-720 ◽  
Author(s):  
Andrew M. Moore ◽  
Hernan G. Arango ◽  
Emanuele Di Lorenzo ◽  
Arthur J. Miller ◽  
Bruce D. Cornuelle
Keyword(s):  
Sensitivity Analysis ◽  
Kinetic Energy ◽  
Wind Stress ◽  
Current System ◽  
Baroclinic Instability ◽  
California Current ◽  
Eddy Kinetic Energy ◽  
Ocean Modeling ◽  
Complex Flow ◽  
California Current System

Abstract Adjoint methods of sensitivity analysis were applied to the California Current using the Regional Ocean Modeling Systems (ROMS) with medium resolution, aimed at diagnosing the circulation sensitivity to variations in surface forcing. The sensitivities of coastal variations in SST, eddy kinetic energy, and baroclinic instability of complex time-evolving flows were quantified. Each aspect of the circulation exhibits significant interannual and seasonal variations in sensitivity controlled by mesoscale circulation features. Central California SST is equally sensitive to wind stress and surface heat flux, but less so to wind stress curl, displaying the greatest sensitivity when upwelling-favorable winds are relaxing and the least sensitivity during the peak of upwelling. SST sensitivity is typically 2–4 times larger during summer than during spring, although larger variations occur during some years. The sensitivity of central coast eddy kinetic energy to surface forcing is constant on average throughout the year. Perturbations in the wind that align with mesoscale eddies to enhance the strength of the circulation by local Ekman pumping yield the greatest sensitivities. The sensitivity of the potential for baroclinic instability is greatest when nearshore horizontal temperature gradients are largest, and it is associated with variations in wind stress concentrated along the core of the California Current. The sensitivity varies by a factor of ∼1.5 throughout the year. A new and important aspect of this work is identification of the complex flow dependence and seasonal dependence of the sensitivity of the ROMS California Current System (CCS) circulation to variations in surface forcing that was hitherto not previously appreciated.

Summertime Coupling between Sea Surface Temperature and Wind Stress in the California Current System

2007 ◽  
Vol 37 (3) ◽  
pp. 495-517 ◽  
Author(s):  
Dudley B. Chelton ◽  
Michael G. Schlax ◽  
Roger M. Samelson
Keyword(s):  
Wind Stress ◽  
Current System ◽  
California Current ◽  
Sea Surface ◽  
Wind Stress Curl ◽  
California Current System ◽  
The Poor ◽  
Ocean Atmosphere ◽  
Ocean Atmosphere Interaction ◽  
Atmosphere Interaction

Abstract Satellite observations of wind stress and sea surface temperature (SST) are analyzed to investigate ocean–atmosphere interaction in the California Current System (CCS). As in regions of strong SST fronts elsewhere in the World Ocean, SST in the CCS region is positively correlated with surface wind stress when SST fronts are strong, which occurs during the summertime in the CCS region. This ocean influence on the atmosphere is apparently due to SST modification of stability and mixing in the atmospheric boundary layer and is most clearly manifest in the derivative wind stress fields: wind stress curl and divergence are linearly related to, respectively, the crosswind and downwind components of the local SST gradient. The dynamic range of the Ekman upwelling velocities associated with the summertime SST-induced perturbations of the wind stress curl is larger than that of the upwelling velocities associated with the mean summertime wind stress curl. This suggests significant feedback effects on the ocean, which likely modify the SST distribution that perturbed the wind stress curl field. The atmosphere and ocean off the west coast of North America must therefore be considered a fully coupled system. It is shown that the observed summertime ocean–atmosphere interaction is poorly represented in the NOAA North American Mesoscale Model (formerly called the Eta Model). This is due, at least in part, to the poor resolution and accuracy of the SST boundary condition used in the model. The sparse distribution of meteorological observations available over the CCS for data assimilation may also contribute to the poor model performance.

Sign in / Sign up

Export Citation Format

Share Document