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.

The Deep Western Boundary Current and Adjacent Interior Circulation at 24°–30°N: Mean Structure and Mesoscale Variability

2020 ◽  
Vol 50 (9) ◽  
pp. 2735-2758
Author(s):  
Tiago Carrilho Biló ◽  
William E. Johns
Keyword(s):  
North Atlantic ◽  
Western Boundary Current ◽  
Western Boundary ◽  
Boundary Current ◽  
Deep Western Boundary Current ◽  
Mean Structure ◽  
The Mean ◽  
Theoretical Evidence ◽  
Anticyclonic Eddies ◽  
Geostrophic Velocities

AbstractThe mean North Atlantic Deep Water (NADW, 1000 < z < 5000 m) circulation and deep western boundary current (DWBC) variability offshore of Abaco, Bahamas, at 26.5°N are investigated from nearly two decades of velocity and hydrographic observations, and outputs from a 30-yr-long eddy-resolving global simulation. Observations at 26.5°N and Argo-derived geostrophic velocities show the presence of a mean Abaco Gyre spanning the NADW layer, consisting of a closed cyclonic circulation between approximately 24° and 30°N and 72° and 77°W. The southward-flowing portion of this gyre (the DWBC) is constrained to within ~150 km of the western boundary with a mean transport of ~30 Sv (1 Sv ≡ 106 m3 s−1). Offshore of the DWBC, the data show a consistent northward recirculation with net transports varying from 6.5 to 16 Sv. Current meter records spanning 2008–17 supported by the numerical simulation indicate that the DWBC transport variability is dominated by two distinct types of fluctuations: 1) periods of 250–280 days that occur regularly throughout the time series and 2) energetic oscillations with periods between 400 and 700 days that occur sporadically every 5–6 years and force the DWBC to meander far offshore for several months. The shorter-period variations are related to DWBC meandering caused by eddies propagating southward along the continental slope at 24°–30°N, while the longer-period oscillations appear to be related to large anticyclonic eddies that slowly propagate northwestward counter to the DWBC flow between ~20° and 26.5°N. Observational and theoretical evidence suggest that these two types of variability might be generated, respectively, by DWBC instability processes and Rossby waves reflecting from the western boundary.

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.

scholarly journals Eddy-Train Encounters with a Continental Boundary: A South Atlantic Case Study

2012 ◽  
Vol 42 (9) ◽  
pp. 1548-1565 ◽  
Author(s):  
José L. L. Azevedo ◽  
Doron Nof ◽  
Mauricio M. Mata
Keyword(s):  
Vertical Wall ◽  
South Atlantic ◽  
Western Boundary Current ◽  
Western Boundary ◽  
Boundary Current ◽  
Agulhas Current ◽  
The North ◽  
Anticyclonic Eddies ◽  
Good Agreement

Abstract Satellite altimetry suggests that large anticyclonic eddies (rings) originating from the Agulhas Current retroflection occasionally make their way across the entire South Atlantic Ocean. What happens when these rings encounter a western boundary current? In this work, interactions between a “train” of nonlinear lens-like eddies and a Southern Hemisphere continental boundary are investigated analytically and numerically on a β plane. The train of eddies is modeled as a steady double-frontal zonal current with the same vorticity and transport as the eddies themselves. The continental boundary is represented by a vertical wall, which is purely meridional in one case and is tilted with respect to the north in another case. It is demonstrated analytically that the eddy–wall encounter produces an equatorward flow parallel to the continental wall, thus suggesting a weakening of the transport of the associated (poleward flowing) western boundary current upstream of the encounter zone and unchanged transport downstream. A large stationary eddy is established in the contact zone because its β-induced force is necessary to balance the other forces along the wall. The size of this eddy is directly proportional to the transport of the eddy train and the meridional tilt of the wall. These scenarios are in good agreement with results obtained numerically using an isopycnal Bleck and Boudra model.

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

&lt;p&gt;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.&lt;/p&gt;

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 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.

scholarly journals Frequency-Domain Analysis of the Energy Budget in an Idealized Coupled Ocean–Atmosphere Model

2020 ◽  
Vol 33 (2) ◽  
pp. 707-726 ◽  
Author(s):  
Paige E. Martin ◽  
Brian K. Arbic ◽  
Andrew McC. Hogg ◽  
Andrew E. Kiss ◽  
James R. Munroe ◽  
...  
Keyword(s):  
Frequency Domain ◽  
Time Scales ◽  
Energy Budget ◽  
Western Boundary Current ◽  
Spectral Energy ◽  
Western Boundary ◽  
Intrinsic Variability ◽  
Boundary Current ◽  
Ocean Atmosphere ◽  
Atmosphere Model

AbstractClimate variability is investigated by identifying the energy sources and sinks in an idealized, coupled, ocean–atmosphere model, tuned to mimic the North Atlantic region. The spectral energy budget is calculated in the frequency domain to determine the processes that either deposit energy into or extract energy from each fluid, over time scales from one day up to 100 years. Nonlinear advection of kinetic energy is found to be the dominant source of low-frequency variability in both the ocean and the atmosphere, albeit in differing layers in each fluid. To understand the spatial patterns of the spectral energy budget, spatial maps of certain terms in the spectral energy budget are plotted, averaged over various frequency bands. These maps reveal three dynamically distinct regions: along the western boundary, the western boundary current separation, and the remainder of the domain. The western boundary current separation is found to be a preferred region to energize oceanic variability across a broad range of time scales (from monthly to decadal), while the western boundary itself acts as the dominant sink of energy in the domain at time scales longer than 50 days. This study paves the way for future work, using the same spectral methods, to address the question of forced versus intrinsic variability in a coupled climate system.

scholarly journals Midlatitude–Equatorial Dynamics of a Grounded Deep Western Boundary Current. Part I: Midlatitude Flow and the Transition to the Equatorial Region

2015 ◽  
Vol 45 (10) ◽  
pp. 2457-2469 ◽  
Author(s):  
Gordon E. Swaters
Keyword(s):  
Bottom Topography ◽  
Zonal Flow ◽  
Ocean Basin ◽  
Equatorial Region ◽  
Western Boundary Current ◽  
Western Boundary ◽  
Boundary Current ◽  
Qualitative Properties ◽  
Deep Western Boundary Current ◽  
The Tropics

AbstractA comprehensive theoretical study of the nonlinear hemispheric-scale midlatitude and cross-equatorial steady-state dynamics of a grounded deep western boundary current is given. The domain considered is an idealized differentially rotating, meridionally aligned basin with zonally varying parabolic bottom topography so that the model ocean shallows on both the western and eastern sides of the basin. Away from the equator, the flow is governed by nonlinear planetary geostrophic dynamics on sloping topography in which the potential vorticity equation can be explicitly solved. As the flow enters the equatorial region, it speeds up and becomes increasingly nonlinear and passes through two distinguished inertial layers referred to as the “intermediate” and “inner” inertial equatorial boundary layers, respectively. The flow in the intermediate equatorial region is shown to accelerate and turn eastward, forming a narrow equatorial jet. The qualitative properties of the solution presented are consistent with the known dynamical characteristics of the deep western boundary currents as they flow from the midlatitudes into the tropics. The predominately zonal flow across the ocean basin in the inner equatorial region (and its exit from the equatorial region) is determined in Part II of this study.

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