scholarly journals Hysteresis and Dynamics of a Western Boundary Current Flowing by a Gap Forced by Impingement of Mesoscale Eddies

2011 ◽  
Vol 41 (5) ◽  
pp. 878-888 ◽  
Author(s):  
Dongliang Yuan ◽  
Zheng Wang
Keyword(s):  
Hopf Bifurcation ◽  
Mesoscale Eddies ◽  
Small Deformation ◽  
Ocean Model ◽  
Western Boundary Current ◽  
Western Boundary ◽  
Periodic Regime ◽  
Boundary Current ◽  
Nonlinear Hysteresis ◽  
Vorticity Balance

Abstract Hysteresis of a western boundary current (WBC) flowing by a wide gap of a western boundary and the dynamics of the WBC variations associated with the impingement of mesoscale eddies from the eastern side of the gap are studied using a 1.5-layer reduced-gravity quasigeostrophic ocean model. The study focuses on two issues not covered by existing studies: the effects of finite baroclinic deformation radii and time dependence perturbed by mesoscale eddies. The results of the study show that the hysteresis of the WBC of finite baroclinic deformation radii is not controlled by multiple steady-state balances of the quasigeostrophic vorticity equation. Instead, the hysteresis is controlled by the periodic penetrating and the leaping regimes of the vorticity balance. The regime of the vorticity balance inside the gap is dependent on the history of the WBC evolution, which gives rise to the hysteresis of the WBC path. Numerical experiments have shown that the parameter domain of the hysteresis is not sensitive to the baroclinic deformation radius. However, the domain of the periodic solution, which is determined by the lower Hopf bifurcation of the nonlinear system, is found to be sensitive to the magnitude of the baroclinic deformation radius. The lower Hopf bifurcation from steady penetration to periodic penetration is found to occur at lower Reynolds numbers for larger deformation radii. In general, the lower Hopf bifurcation stays outside the hysteresis domain of the Reynolds number. However, for very small deformation radii, the lower Hopf bifurcation falls inside the hysteresis domain, which results in the transition from the leaping to the penetrating regimes of the WBC to skip the periodic regime and hence the disappearance of the upper Hopf bifurcation. Mesoscale eddies approaching the gap from the eastern basin are found to have significant impact on the WBC path inside the gap when the WBC is at a critical state along the hysteresis loop. Cyclonic (anticyclonic) eddies play the role of reducing (enhancing) the inertial advection of vorticity in the vicinity of the gap so that transitions of the WBC path from the leaping (periodic penetrating) to the periodic penetrating (leaping) regimes are induced. In addition, cyclonic eddies are able to induce transitions of the WBC from the periodic penetrating to the leaping regimes through enhancing the meridional advection by its right fling. The transitions are irreversible because of the nonlinear hysteresis and are found to be sensitive to the strength, size, and approaching path of the eddy.

Evaluation of global ocean model on simulating deep western boundary current in the Pacific

2020 ◽  
Author(s):  
Huichang Jiang ◽  
Hongzhou Xu
Keyword(s):  
Spatial Structure ◽  
Water Mass ◽  
Volume Transport ◽  
Ocean Model ◽  
Western Boundary Current ◽  
Global Ocean ◽  
Western Boundary ◽  
Boundary Current ◽  
Deep Western Boundary Current ◽  
The Pacific

<p>As an important branch of the global overturning circulation, the deep western boundary current (DWBC) in the Pacific was poorly understood due to sparse observations. Six state-of-the-art global ocean model outputs were used herein to evaluate their performance for simulating the DWBC in the Melanesian Basin (MB) and Central Pacific Basin (CPB). These model outputs were compared to the historical observations, in aspects of water-mass characteristics, spatial structure and meridional volume transport of the DWBC, and seasonal variation. The results showed that most of the models reproduced the DWBC in the two basins well. Besides OFES with obvious cold and salty biases, the other models had minor deviations of the temperature and salinity in the deep layer. These models can reconstruct the spatial structure of the DWBC in detail and simulate appropriate transports of the eastern branch DWBC, ranging from 6.36 Sv to 8.55 Sv. But the western branch DWBC was underestimated in the models except HYCOM (4.48 Sv). HYCOM performed best for the DWBC with a whole transport of 12.84 Sv. Analysis of the temperature and salinity from Levitus data demonstrates the existence of annual and semi-annual cycles in the deep water of the MB and CPB, respectively, with warmer and saltier water mass in summer and autumn. Overall, the six models have good abilities to simulate the seasonal variations of temperature and volume transport of the DWBC in the Pacific. The seasonal signals probably originated from the DWBC upstream and propagated along its pathway.</p>

scholarly journals Influence of an Island on Hysteresis of a Western Boundary Current Flowing across a Gap

2019 ◽  
Vol 49 (5) ◽  
pp. 1353-1366
Author(s):  
Huan Mei ◽  
Yiquan Qi ◽  
Bo Qiu ◽  
Xuhua Cheng ◽  
Xiangbai Wu
Keyword(s):  
Reynolds Number ◽  
Critical Reynolds Number ◽  
Ocean Model ◽  
Western Boundary Current ◽  
Western Boundary ◽  
Boundary Current ◽  
Balance Analysis ◽  
Meridional Advection ◽  
Gap Width ◽  
Eddy Shedding

Abstract We investigate impact of an island on hysteresis of a western boundary current (WBC) flowing across a gap using a nonlinear 1.5-layer ocean model. The results of hysteresis curves show the island in the middle of the gap facilitates the WBC intrusion. The inserted (removed) island in the middle of the gap promotes the WBC to shed eddy (leap across the gap) when the WBC path transits from the periodic penetrating (leaping) to the leaping (periodic penetrating) regime without (with) an island. Vorticity balance analysis reveals that the WBC transition from the eddy-shedding (leaping) to the leaping (eddy-shedding) regime is induced by increased (decreased) meridional advection. Moreover, the critical Reynolds number of the WBC at the Hopf bifurcation is not sensitive to the size and location of the island when the total gap width is fixed. The critical Reynolds number of the WBC translating from the eddy shedding to the leaping regime increases when either the total gap width increases or the island’s meridional length increases; however, the critical Reynolds number is inversely proportional to the width of the southern gap with fixed total gap width and enlarged island length. The island promotes the WBC to shed eddy except when the island is near the northern barrier. The influence of an eastward-shifted island on the WBC transition from the eddy-shedding to the leaping regime is gradually reduced when the island is east of the Munk layer.

scholarly journals Dynamics of Mesoscale Eddies Interacting With a Western Boundary Current Flowing by a Gap

2019 ◽  
Vol 124 (6) ◽  
pp. 4117-4132
Author(s):  
Dongliang Yuan ◽  
Xinxin Song ◽  
Ya Yang ◽  
William K. Dewar
Keyword(s):  
Mesoscale Eddies ◽  
Western Boundary Current ◽  
Western Boundary ◽  
Boundary Current

scholarly journals Full vorticity budget of the Arabian Sea from a 0.1° ocean model: Sverdrup dynamics, Rossby waves and nonlinear eddy effects

2021 ◽  
Author(s):  
He Wang ◽  
Julie L. McClean ◽  
Lynne D. Talley
Keyword(s):  
General Circulation ◽  
Arabian Sea ◽  
Rossby Waves ◽  
Open Ocean ◽  
Western Boundary Current ◽  
Western Boundary ◽  
Boundary Current ◽  
Basin Scale ◽  
Vorticity Budget ◽  
Vorticity Balance

AbstractThe Arabian Sea, influenced by the Indian monsoon, has many unique features including its basin scale seasonally reversing surface circulation and the Great Whirl, a seasonal anti-cyclonic system appearing during the southwest monsoon close to the western boundary. To establish a comprehensive dynamical picture of the Arabian Sea, we utilize numerical model output and design a full vorticity budget that includes a fully-decomposed nonlinear term. The ocean general circulation model has 0.1° resolution and is mesoscale eddy-resolving in the region. In the western boundary current system, we highlight the role of nonlinear eddies in the life cycle of the Great Whirl. The nonlinear eddy term is of leading order importance in this feature’s vorticity balance. Specifically, it contributes to the Great Whirl’s persistence in boreal fall after the weakening of the southwesterly winds. In the open ocean, Sverdrup dynamics and annual Rossby waves are found to dominate the vorticity balance; the latter is considered as a key factor in the formation of the Great Whirl and the sea-sonal reversal of the western boundary current. In addition, we discuss different forms of vertically-integrated vorticity equations in the model and argue that the bottom pressure torque term can be interpreted analogously as friction in the western boundary and vortex stretching in the open ocean.

scholarly journals Variability and drivers of ocean temperature extremes in a warming Western Boundary Current

2021 ◽  
pp. 1-30
Keyword(s):  
Climate Models ◽  
Heat Content ◽  
Ocean Model ◽  
Western Boundary Current ◽  
Ecological Impacts ◽  
Temperature Extremes ◽  
Western Boundary ◽  
Spatial And Temporal Variability ◽  
Boundary Current ◽  
Anticyclonic Eddies

Abstract Western Boundary Current (WBC) extensions such as the East Australian Current (EAC) southern extension are warming 2-3 times faster than the global average. However, there are nuances in the spatial and temporal variability of the warming that are not well resolved in climate models. In addition, the physical drivers of ocean heat content (OHC) extremes are not well understood. Here, using a high-resolution ocean model run for multiple decades, we show nonuniform warming trends in OHC in the EAC, with strong positive trends in the southern extension region (~36°S-38°S) but negative OHC trends equatorward of 33°S. The OHC variability in the EAC is associated with the formation of anticyclonic eddies, which is modulated by transport ~880 km upstream (EAC-mode) and the westward propagation of Rossby waves (Eddy-mode). Diagnosing the drivers of temperature extremes has implications for predictability both in the EAC and in WBCs more broadly, where ocean warming is already having considerable ecological impacts.

scholarly journals Response of the Separated Western Boundary Current to Harmonic and Stochastic Wind Stress Variations in a 1.5-Layer Ocean Model

2005 ◽  
Vol 35 (8) ◽  
pp. 1341-1358 ◽  
Author(s):  
J. Sirven
Keyword(s):  
Wind Stress ◽  
Ocean Model ◽  
Western Boundary Current ◽  
Interior Solution ◽  
Western Boundary ◽  
Thermocline Depth ◽  
Delayed Response ◽  
Boundary Current ◽  
Stress Changes ◽  
Stress Variations

Abstract A time-dependent version of the Parsons model (geostrophic 1.5-layer model of the ventilated thermocline) has been developed to investigate the response of the midlatitude ocean to wind stress variations in a simple configuration. In this model, the total amount of water is kept constant and the eastern boundary thermocline depth can vary in time so as to maintain mass balance. Here, basin modes are not investigated, in contrast to many recent studies, but the emphasis is on the line where the motionless second layer outcrops, which represents the separated western boundary current. It is shown that the position of this line only depends on the wind stress, the earth rotation, and the thermocline interior solution. The position is not influenced by the parameterization of the dissipative processes. This generalizes previous results established in the stationary case. The displacement of the outcrop line in the case of harmonic or stochastic wind stress variations is computed numerically, showing a lag of 0–4 yr that results from a combination of the instantaneous Ekman response and the delayed response due to Rossby wave propagation. Such delay is in satisfactory agreement with observations of Gulf Stream adjustment to wind stress changes, considering the limitations of the model, and is in good agreement with intermediate-resolution OGCM models. Although inertial effects and buoyancy forcing also need to be considered, this suggests that the outcropping mechanism plays a role in the variability of the separated boundary currents and may be dominant in non-eddy-resolving ocean models.

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