scholarly journals Overturning response to a surface wind stress doubling in an eddying and a non-eddying ocean

2021 ◽  
Author(s):  
Veit Lüschow ◽  
Jochem Marotzke ◽  
Jin-Song von Storch
Keyword(s):  
Wind Stress ◽  
Surface Wind ◽  
Relative Vorticity ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Boundary Current ◽  
Mean Flow ◽  
Surface Wind Stress ◽  
Stress Changes

AbstractIn this paper, the overturning responses to wind stress changes of an eddying and a non-eddying ocean are compared. Differences are found in the deep overturning cell in the low-latitude North Atlantic with substantial implications for the deep western boundary current (DWBC). In an ocean-only twin experiment with one eddying and one non-eddying configuration of the MPI ocean model, two different forcings are being applied: the standard NCEP forcing and the NCEP forcing with 2x surface wind stress. The response to the wind stress doubling in the Atlantic meridional overturning circulation is similar in the eddying and the non-eddying configuration, showing an increase by about 4 Sv (~25%, 1 Sv = 106 m3s−1). In contrast, the DWBC responds with a speedup in the non-eddying and a slowdown in the eddying configuration. This paper demonstrates that the DWBC slowdown in the eddying configuration is largely balanced by eddy vorticity fluxes. Because those fluxes are not resolved and also not captured by an eddy parameterization in the non-eddying configuration, such a DWBC slowdown is likely not to occur in non-eddying ocean models which therefore might not capture the whole range of overturning responses. Furthermore, evidence is provided that the balancing effect of the eddies is not a passive reaction to a remotely triggered DWBC slowdown. Instead, deep eddies which are sourced from the upper ocean provide an excess input of relative vorticity which then actively forces the DWBC mean flow to slow down.

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 Southern Ocean Wind Stress in CMIP5 Models: Role of Wind Fluctuations

2020 ◽  
Vol 33 (4) ◽  
pp. 1209-1226 ◽  
Author(s):  
Xia Lin ◽  
Xiaoming Zhai ◽  
Zhaomin Wang ◽  
David R. Munday
Keyword(s):  
Southern Ocean ◽  
Time Scales ◽  
Wind Stress ◽  
Surface Wind ◽  
Coupled Model ◽  
Cmip5 Models ◽  
Coupled Models ◽  
Surface Wind Stress ◽  
Stress Changes ◽  
The Mean

AbstractThe Southern Ocean (SO) surface wind stress is a major atmospheric forcing for driving the Antarctic Circumpolar Current and the global overturning circulation. Here the effects of wind fluctuations at different time scales on SO wind stress in 18 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are investigated. It is found that including wind fluctuations, especially on time scales associated with synoptic storms, in the stress calculation strongly enhances the mean strength, modulates the seasonal cycle, and significantly amplifies the trends of SO wind stress. In 11 out of the 18 CMIP5 models, the SO wind stress has strengthened significantly over the period of 1960–2005. Among them, the strengthening trend of SO wind stress in one CMIP5 model is due to the increase in the intensity of wind fluctuations, while in all the other 10 models the strengthening trend is due to the increasing strength of the mean westerly wind. These discrepancies in SO wind stress trend in CMIP5 models may explain some of the diverging behaviors in the model-simulated SO circulation. Our results suggest that to reduce the uncertainty in SO responses to wind stress changes in the coupled models, both the mean wind and wind fluctuations need to be better simulated.

scholarly journals Decadal Response of Global Circulation to Southern Ocean Zonal Wind Stress Perturbation

2009 ◽  
Vol 39 (8) ◽  
pp. 1888-1904 ◽  
Author(s):  
Barry A. Klinger ◽  
Carlos Cruz
Keyword(s):  
Southern Ocean ◽  
Southern Hemisphere ◽  
Time Scales ◽  
Wind Stress ◽  
Volume Transport ◽  
Ocean Model ◽  
Return Flow ◽  
Western Boundary ◽  
Boundary Current ◽  
Wind Variability

Abstract A substantial component of North Atlantic Deep Water formation may be driven by westerly wind stress over the Southern Ocean. Variability of this wind stress on decadal time scales may lead to circulation variability far from the forcing region. The Hybrid Coordinate Ocean Model (HYCOM), a numerical ocean model, is used to investigate the spatial patterns and the time scales associated with such wind variability. The evolution of circulation and density anomalies is observed by comparing one 80-yr simulation, forced in part by relatively strong Southern Hemisphere westerlies, with a simulation driven by climatological wind. The volume transport anomaly takes about 10 yr to reach near-full strength in the entire Southern Hemisphere; however, in the Northern Hemisphere, it grows for the duration of the run. The Southern Hemisphere Indo-Pacific volume transport anomaly is about twice the strength of that found in the Atlantic. In the thermocline, water exits the southern westerlies belt in a broad flow that feeds a western boundary current (WBC) in both the Atlantic and Pacific Oceans. These WBCs in turn feed an Indonesian Throughflow from the Pacific and cyclonic gyres in the far north, which are broadly consistent with the Stommel–Arons theory. The deep return flow in each hemisphere is strongly affected by deep-sea ridges, which leads to a number of midocean “WBCs.” The wind perturbation causes isopycnals to sink over most of the basin. After about 20 yr, this sinking is very roughly uniform with latitude, though it varies by basin.

The impact of topography and eddy parameterization on the simulated Southern Ocean circulation response to changes in surface wind stress

2020 ◽  
Author(s):  
Hailu Kong ◽  
Malte F. Jansen
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
Ocean Circulation ◽  
Surface Wind ◽  
Flat Bottom ◽  
Coarse Resolution ◽  
Surface Wind Stress ◽  
Stress Changes ◽  
The Impact ◽  
Meso Scale

AbstractIt remains uncertain how the Southern Ocean circulation responds to changes in surface wind stress, and whether coarse resolution simulations, where meso-scale eddy fluxes are parameterized, can adequately capture the response. We address this problem using two idealized model setups mimicking the Southern Ocean: a flat bottom channel, and a channel with moderately complex topography. Under each topographic configuration and varying wind stress, we compare several coarse resolution simulations, configured with different eddy parameterizations, against an eddy-resolving simulation. We find that: (1) without topography, sensitivity of the Antarctic Circumpolar Current (ACC) to wind stress is overestimated by coarse resolution simulations, due to an underestimate of the sensitivity of the eddy diffusivity; (2) in the presence of topography, stationary eddies dominate over transient eddies in counteracting the direct response of the ACC and overturning circulation to wind stress changes; (3) coarse resolution simulations with parameterized eddies capture this counteracting effect reasonably well, largely due to their ability to resolve stationary eddies. Our results highlight the importance of topography in modulating the response of the Southern Ocean circulation to changes in surface wind stress. The interaction between meso-scale eddies and stationary meanders induced by topography requires more attention in future development and testing of eddy parameterizations.

scholarly journals Decadal Changes of Wind Stress over the Southern Ocean Associated with Antarctic Ozone Depletion

2007 ◽  
Vol 20 (14) ◽  
pp. 3395-3410 ◽  
Author(s):  
Xiao-Yi Yang ◽  
Rui Xin Huang ◽  
Dong Xiao Wang
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
Zonal Wind ◽  
Ozone Depletion ◽  
Surface Wind ◽  
Southern Annular Mode ◽  
Mean Flow ◽  
Annular Mode ◽  
Surface Wind Stress ◽  
Antarctic Ozone

Abstract Using 40-yr ECMWF Re-Analysis (ERA-40) data and in situ observations, the positive trend of Southern Ocean surface wind stress during two recent decades is detected, and its close linkage with spring Antarctic ozone depletion is established. The spring Antarctic ozone depletion affects the Southern Hemisphere lower-stratospheric circulation in late spring/early summer. The positive feedback involves the strengthening and cooling of the polar vortex, the enhancement of meridional temperature gradients and the meridional and vertical potential vorticity gradients, the acceleration of the circumpolar westerlies, and the reduction of the upward wave flux. This feedback loop, together with the ozone-related photochemical interaction, leads to the upward tendency of lower-stratospheric zonal wind in austral summer. In addition, the stratosphere–troposphere coupling, facilitated by ozone-related dynamics and the Southern Annular Mode, cooperates to relay the zonal wind anomalies to the upper troposphere. The wave–mean flow interaction and the meridional circulation work together in the form of the Southern Annular Mode, which transfers anomalous wind signals downward to the surface, triggering a striking strengthening of surface wind stress over the Southern Ocean.

scholarly journals Test and evaluation of a simple parameterization to enhance air-sea coupling in a global coupled model

2018 ◽  
Vol 3 (3) ◽  
Author(s):  
Fanghua Xu 1
Keyword(s):  
Wind Stress ◽  
Ocean Circulation ◽  
Southern Oscillation ◽  
Current System ◽  
Surface Wind ◽  
Coupled Model ◽  
Global Ocean ◽  
Western Boundary ◽  
Surface Wind Stress ◽  
Gyre Circulation

A simple temperature-dependent wind stress scheme is implemented in National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM), aiming to enhance positive wind stress and sea surface temperature (SST) correlation in SST-frontal regions. A series of three-year coupled experiments are conducted to determine a proper coupling coefficient for the scheme based on the agreement of surface wind stress and SST at oceanic mesoscale between model simulations and observations. Afterwards, 80-year simulations with/without the scheme are conducted to explore its effects on simulated ocean states and variability. The results show that the new scheme indeed improves the positive correlation between SST and wind stress magnitude near the large oceanic fronts. With more realistic surface heat flux and wind stress, the global SST biases are reduced. The global ocean circulation represented by barotropic stream function exhibits a weakened gyre circulation close to the western boundary separation, in agreement with previous studies. The simulation of equatorial Pacific current system is improved as well. The overestimated El Niño Southern Oscillation (ENSO) magnitude in original CESM is reduced by ~30% after using the new scheme with an improved period.

The Antilles Current and wind-driven gyre circulation at 26oN

2021 ◽  
Author(s):  
Eleanor Frajka-Williams ◽  
William E. Johns ◽  
Harry L. Bryden ◽  
David A. Smeed ◽  
Aurelie Duchez ◽  
...  
Keyword(s):  
Interannual Variability ◽  
Wind Stress ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Florida Current ◽  
Boundary Current ◽  
Wind Stress Curl ◽  
The Bahamas ◽  
Western Atlantic ◽  
Gyre Circulation

<p>The Antilles Current is a narrow, northward flowing boundary current in the western Atlantic just east of the Bahamas. Its role in the larger scale circulation has been debated: alternately thought to be part of the western boundary closure of the gyre circulation or the northward flowing limb of the meridional overturning circulation (MOC). From 19 years of moored current meter observations (1987--1991, 2004--2018), we define the strength of the Antilles Current by the net transport between the Bahamas and 76.5°W (spanning about 45 km zonally) and in the thermocline (0–1000 m). We find a mean northward transport of 3.5 Sv, substantial interannual variability, and no discernable trend since 1987. The interannual variability of the AC transport is independent of the variability of the Florida Current (the Gulf Stream through the Florida Straits). Instead, the Antilles Current contributes to the interannual variability of the MOC at 26°N, while the trend in the strength of the gyre circulation (defined as the transbasin thermocline transport minus the AC) is responsible for the trend in the MOC. In particular, the 2009/10 slowdown of the MOC resulted from a weaker northward AC transport, rather than an intensified gyre transport. Using the recent 14 years of in situ transport records, we compare the interannual variability of the gyre circulation to that of wind stress curl forcing via a Sverdrup transport calculation, identifying a potential role for wind stress curl (WSC) forcing at 26°N with a ~2 year lag until 2016.<span>  </span>From 2016, the predicted gyre circulation using WSC diverges from the measured gyre strength.</p>

Sensitivity of Western Boundary Transport at the Mean North Equatorial Current Bifurcation Latitude to Wind Forcing

2012 ◽  
Vol 42 (11) ◽  
pp. 2056-2072 ◽  
Author(s):  
Xuebin Zhang ◽  
Bruce Cornuelle ◽  
Dean Roemmich
Keyword(s):  
Wind Stress ◽  
Time Lag ◽  
Surface Wind ◽  
North Equatorial Current ◽  
Western Boundary ◽  
Adjoint Sensitivity ◽  
Surface Wind Stress ◽  
The Pacific ◽  
The Pacific Ocean ◽  
The Mean

Abstract The bifurcation of the North Equatorial Current (NEC) plays an important role in the heat and water mass exchanges between the tropical and subtropical gyres in the Pacific Ocean. The variability of western boundary transport (WBT) east of the Philippine coast at the mean NEC bifurcation latitude (12°N) is examined here. A tropical Pacific regional model is set up based on the Massachusetts Institute of Technology general circulation model and its adjoint, which calculates the sensitivities of a defined meridional transport to atmospheric forcing fields and ocean state going backward in time. The adjoint-derived sensitivity of the WBT at the mean NEC bifurcation latitude to surface wind stress is dominated by curl-like patterns that are located farther eastward and southward with increasing time lag. The temporal evolution of the adjoint sensitivity of the WBT to wind stress resembles wind-forced Rossby wave dynamics but propagating with speeds determined by the background stratification and current, suggesting that wind-forced Rossby waves are the underlying mechanism. Interannual-to-decadal variations of the WBT can be hindcast well by multiplying the adjoint sensitivity and the time-lagged wind stress over the whole model domain and summing over time lags. The analysis agrees with previous findings that surface wind stress (especially zonal wind stress in the western subtropical Pacific) largely determines the WBT east of the Philippines, and with a time lag based on Rossby wave propagation. This adjoint sensitivity study quantifies the contribution of wind stress at all latitudes and longitudes and provides a novel perspective to understand the relationship between the WBT and wind forcing over the Pacific Ocean.

Eddy-Mean flow oscillations in the Southern Ocean

2020 ◽  
Author(s):  
Sebastiano Roncoroni ◽  
David Ferreira
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
General Circulation ◽  
Meridional Overturning Circulation ◽  
Characteristic Time Scale ◽  
Intrinsic Variability ◽  
Mean Flow ◽  
Flow Interaction ◽  
Stress Changes ◽  
Non Linear Oscillator

<p>Geostrophic eddies have a leading order effect on the dynamics of the Southern Ocean (SO), and numerous studies have shown that they are also key to the response of both the zonal transport and the meridional overturning circulation to wind stress changes. The role played by eddies in setting the intrinsic variability of the SO, however, is less well-understood. Here, inspired by recent work on the atmospheric jet, we investigate whether the eddy-mean flow interaction in the Antarctic Circumpolar Current can be described by a prey-predator nonlinear model.</p><p> </p><p>To this end, we analyse data from a high-resolution eddy-resolving configuration of the MIT general circulation model: an idealised “channel” model with mechanical and thermodynamical forcing at the surface, and plausible zonal and meridional circulations.</p><p> </p><p>Here, we show that a mechanism of eddy-mean flow interaction driving the intrinsic variability of the SO-like model is well described by a stochastic non-linear oscillator with damping. This model is a generalisation of the Ambaum-Novak oscillator, which has been successfully employed to describe the atmospheric storm track variability.</p><p> </p><p>We find that, on length scales similar to that of individual zonal jets, the eddy-mean flow interaction is characterised by a high-frequency oscillatory mode, and that the characteristic time scale of the oscillation is comparable with classical estimates of the baroclinic life-cycle. A Gaussian smoothing of the phase space diagram also reveals the damped oscillatory character of the oscillation: this is in contrast with the atmospheric case, where damping is negligible and orbits are confined to energy surfaces.</p><p> </p><p>This result may help inform the interpretation of the SO intrinsic and forced variability (such as, for example, the response to wind stress changes), and pave the way to further studies featuring more realistic model configurations.</p>

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