Reduced-gravity Model Research Articles

Dynamics of Eddy Generation in the Southeast Tropical Indian Ocean

AbstractSatellite observations indicate that the southeast tropical Indian Ocean (SETIO) is a region full of eddies, which are generated near 120°E and propagate westward/southwestward. The vertical structure of eddies shows that eddy‐induced temperature anomalies are mainly confined within the upper 200 m. The maximum anomaly is located at ∼100 m depth, near the depth of the thermocline, indicating that eddy generation is related to vertical motions of the thermocline. To further investigate the generation mechanisms of mesoscale eddies, we obtain a suite of solutions of the 1½‐layer reduced gravity model, which can capture the first baroclinic process quite well. The solutions suggest that wind forcing from the tropical Pacific Ocean and nonlinearity play important roles in eddy generation in the SETIO.

Interdecadal variability of the Western Subarctic Gyre in the North Pacific Ocean

Interdecadal variations of the Western Subarctic Gyre (WSAG) in the North Pacific were examined mainly by analysis of absolute dynamic topography (ADT) during 1993–2017. The ADT was based on altimetry-derived sea level anomalies superimposed on state-of-the-art mean dynamic topography. We specified geostrophic surface streamlines of the WSAG associated with closed isolines of the ADT. The WSAG intensity was the strongest in the late 1990s, when the WSAG was zonally contracted, and it decreased to the mid-2010s in a linear manner with time. The WSAG expanded abruptly toward the east around 2001 in a step function–like manner. After this abrupt shift, the WSAG was connected most strongly to the Alaskan Gyre. This condition persisted even into the late 2010s. The shift to the very elongated pattern was also accompanied by a drastic lowering of the ADT around 170°W, 50°N, where the WSAG and Alaskan Gyre had been distinctly separated by a peak of the ADT during the previous period when the shape of the gyre was less elongated. A reduced-gravity model of wind-induced, long, baroclinic Rossby waves with weak dissipation could account for both the interdecadal weakening of the WSAG and the step function–like lowering of the ADT peak at the time of the shift of the WSAG to the very elongated pattern. In addition to the baroclinic response of the subarctic ocean to basin-scale wind stress, mesoscale clockwise eddies contributed to sea level rise around the center of the WSAG. In particular, there was an interdecadal increase in the frequency of appearance of strong, mesoscale, clockwise eddies that were generated south of the Aleutian Islands at longitudes of 170°E−180°. Those eddies propagated approximately to the west-southwest, and they disappeared around the center of the WSAG.

Centennial Changes in the Indonesian Throughflow Connected to the Atlantic Meridional Overturning Circulation: The Ocean's Transient Conveyor Belt

AbstractClimate models consistently project a robust weakening of the Indonesian Throughflow (ITF) and the Atlantic meridional overturning circulation (AMOC) in response to greenhouse gas forcing. Previous studies of ITF variability have largely focused on local processes in the Indo‐Pacific Basin. Here, we propose that much of the centennial‐scale ITF weakening is dynamically linked to changes in the Atlantic Basin and communicated between basins via wave processes. In response to an AMOC slowdown, the Indian Ocean develops a northward surface transport anomaly that converges mass and modifies sea surface height in the Indian Ocean, which weakens the ITF. We illustrate these dynamic interbasin connections using a 1.5‐layer reduced gravity model and then validate the responses in a comprehensive general circulation model. Our results highlight the importance of transient volume exchanges between the Atlantic and Indo‐Pacific basins in regulating the global ocean circulation in a changing climate.

Influence of two inlets of the Luzon overflow on the deep circulation in the northern South China Sea

An inverse reduced-gravity model is used to simulate the deep South China Sea (SCS) circulation. A set of experiments are conducted using this model to study the influence of the Luzon overflow through the two inlets on the deep circulation in the northern SCS. Model results suggest that the relative contribution of these inlets largely depends on the magnitude of the input transport of the overflow, but the northern inlet is more efficient than the southern inlet in driving the deep circulation in the northern SCS. When all of the Luzon overflow occurs through the northern inlet the deep circulation in the northern SCS is enhanced. Conversely, when all of the Luzon overflow occurs through the southern inlet the circulation in the northern SCS is weakened. A Lagrangian trajectory model is also developed and applied to these cases. The Lagrangian results indicate that the location of the Luzon overflow likely has impacts upon the sediment transport into the northern SCS.

Generation and Intensification of Mesoscale Anticyclones by Orographic Wind Jets: The Case of Ierapetra Eddies Forced by the Etesians

AbstractMotivated by the recurrent formation and intensification of the Ierapetra anticyclones in the southeast of Crete, we investigated with a reduced gravity model the response of the oceanic surface layer to a seasonal wind jet that varies slowly (over several weeks or months) and mimics the Etesian winds. Our study answers why the oceanic response to such forcing is mainly a large and intense anticyclone. We build a dimensionless parameter that integrates in time the Ekman pumping and quantifies the relative amplitude of the isopycnal displacement induced by the local wind stress curl. According to the range of the wind forcing parameter, the oceanic response to a symmetric wind jet could be a symmetric dipole or a strongly asymmetric structure dominated by an intense and robust anticyclone. This intrinsic asymmetry is driven by the finite isopycnal displacements that occur when the wind forcing parameter reaches unity. Since the anticyclonic wind shear for the Etesian wind jet is 2 times larger than the cyclonic one, the asymmetry of the oceanic response is enhanced. Several weeks after the wind forcing has stopped, an intense mesoscale anticyclone which satisfies the cyclogeostrophic balance remains. Moreover, if a similar wind jet blows on a preexisting anticyclone, the latter remains robust and coherent and gets reintensified in agreement with the previous analysis of remote sensing observations. Hence, this paper is the first numerical study which provides a dynamical understanding for the formation of the single Ierapetra anticyclones and explains their intensification 1 year after formation.

The characteristics of the mid-depth striations in the North Indian Ocean

Argo float trajectory data has been used to estimate the velocities of mid-depth (1000 db) currents in the North Indian Ocean (NIO). The structure of the upper ocean absolute geostrophic currents can be derived more accurately from the Argo temperature and salinity profiles by referring to the estimated velocities rather than by assuming a level of “no motion”. The derived flow field reveals that the eastward zonal velocities generate striation-like structures in the Arabian Sea, which is most prominent above the layer of 500 db layer, with a meridional scale of ~300 km; however, such structures are barely observed in the Bay of Bengal. This meridional scale as well as the distribution of the mid-depth striations are unique to the NIO, as compared with the other ocean basins. The nonlinear 112-layer reduced-gravity model combined with the triad baroclinic Rossby wave interaction theory have been used to determine the essential factors controlling the characteristics of the quasi-zonal striation structure. Compared with the North Pacific Ocean, the meridional scale in the NIO is narrower, and is caused by the smaller basin size in the equatorial zone, rather than by the semiannual wind stress forcing period or the eastern boundary angle. Furthermore, the coastal trapped Kelvin waves significantly contribute to the generation of the zonal striations in the Arabian Sea.

Seasonal variation of the North Equatorial Current bifurcation in regional model: Role of open boundary conditions

The role of open boundary conditions in modelling the seasonal variation of the North Equatorial Current Bifurcation latitude (NBL) is studied based on a linear Rossby wave model, 1.5-layer reduced-gravity model and Hybrid Coordinate Ocean Model (HYCOM). For a regional ocean model that covers the western Pacific, it is found that the seasonal variation of the NBL can be well reproduced as long as the data at the open boundary is provided by the corresponding basin experiment, no matter where the location is. However, the south–north migration of NBL as well as its peak seasons is not properly simulated if we apply other types of data at the open boundary as its location is close to the west. Sensitivity experiments indicate that the bad performance is attributed to the over-specification error and incomplete adjustment of interaction between wind forcing and westward propagating signals from incompatible open boundary data. The performance gets better as we set the open boundary at proper location and choose suitable open boundary condition. These conclusions can be extended to the simulations of low-latitude western boundary currents.

On the seasonal variability of the Oyashio extension fronts

Previous study reported that the annual-mean eastern Oyashio Extension (OE) front shifts northward while the western OE front has no obvious poleward shift during 1982–2017 by Wu et al. (Geophys Res Lett 45:9042–9048, 2018). Here we revisit this topic and focus on the seasonal variability and shifts of the OE fronts from 1982 to 2018, with observational reanalysis data and a 1.5-layer reduced-gravity model simulation. In winter, both the western and eastern OE fronts demonstrate consistent northward movement. While in summer, the eastern OE front still moves northward but the western OE front has no obvious and even southward shift. It is shown that the trade wind’s expansion during 1982–2018 favours the northward shift of the OE fronts for both winter and summer. However, there is a local cold Ekman heat transport anomaly along the western OE front in summer, which surpasses the effect of trade wind expansion and prohibits northward movement of the front. This cold Ekman advection is due to a westerly wind anomaly induced firstly by the Atlantic Multi-decadal Oscillation (AMO) and secondly by the Pacific Decadal Oscillation (PDO). In winter, the local Ekman heat transport is less effective than in summer in changing the OE front position because of the deep mixed layer. Our study demonstrates the seasonality of the OE front shift and highlights the importance of local Ekman heat transport associated with the AMO. Our results also partly explain the rainfall changes in both winter and summer in the western Pacific Ocean in the past 37 years, since the rainband east of Japan is affected by the sea surface temperature and its front.

Poleward Shift of the Pacific North Equatorial Current Bifurcation

AbstractThe dynamics of the poleward shift of the Pacific North Equatorial Current bifurcation latitude (NBL) is studied using a 5.5‐layer reduced gravity model. It is found that the poleward shift of the NBL is associated with the asymmetric intensity of the wind stress curl input to the Pacific tropical and subtropical gyres. Stronger wind stress curl in the subtropical gyre leads to equatorward transport in the interior upper ocean across the boundary between the two gyres, causing a poleward transport compensation at the western boundary. In the lower layer ocean, in turn, there is poleward (equatorward) transport at the interior (western boundary) due to Sverdrup balance which requires zero transport at the gyre boundary where zonally integrated wind stress curl is zero. Therefore, the NBL exhibits a titling feature, with its position being more equatorward in the upper layer and more poleward in the lower layer. The equatorial currents bifurcations in other basins are also characterized by the poleward titling vertical structure. The wind stress curl over the subtropical gyre is generally stronger than that over the tropical gyre, resulting in the bifurcations shifting poleward with increasing depth.

Hysteresis of a periodic or leaking western boundary current flowing by a gap

A 1.5-layer quasi-geostrophic reduced gravity model is used to study the hysteresis of a periodic or leaking western boundary current (WBC) flowing by a gap. When the periods of the WBC variations are much longer than the Rossby adjustment time scales of the circulation in the vicinity of the gap, the Hopf bifurcations during the Reincrease and Re-decrease loops are delayed to produce a new domain of hysteresis of the Reynolds numbers, and the critical Reynolds numbers of the WBC regime transitions change significantly, with the domain of the hysteresis Reynolds number larger for shorter periodic forcing. When the periods of the WBC variations are comparable to those of the Rossby adjustment time scales of the circulation in the vicinity of the gap, the WBC path inside the gap becomes periodic without hysteresis. The intrusion of the WBC into the western basin generally gets smaller as the period decreases. In addition, the partial leakage of the WBC transport through the gap into the western basin is found to have significant impact on the hysteresis loop of the WBC path when the leaked transport is larger than 1/2 of the WBC. Both the intrusion extent and the critical Reynolds numbers of the WBC regime transition are changed, and the larger the throughflow transport, the larger the change.

Eddy generation mechanism in the eastern South China Sea

Mesoscale eddy generation mechanisms in the eastern South China Sea (ESCS) are investigated using altimetry observations and solutions of a nonlinear, 1½-layer reduced-gravity model. We estimate the relative roles of the wind forcing in the interior South China Sea (SCS) and the remote forcing from the western tropical Pacific (WTP) in eddy generation in the ESCS. Model solutions show that the high-frequency wind in the interior SCS is the primary forcing for eddies, which explains about 54% of the mesoscale eddies generated in the ESCS. Signals from the WTP also play an important role. Wind-driven equatorial signals reach the west coast of Luzon Island through the Sibutu Passage and Mindoro Strait. The reflected Rossby waves from the west coast of Luzon Island propagate westward, become unstable, and turn into eddies. The signals driven by high-frequency wind from the WTP explain about 40% of the mesoscale eddies generated in the ESCS. The high-frequency wind forcing in both the SCS and the WTP is important for eddy generation in the ESCS.

Seasonal Phasing of Agulhas Current Transport Tied to a Baroclinic Adjustment of Near‐Field Winds

AbstractThe Agulhas Current plays a significant role in both local and global ocean circulation and climate regulation, yet the mechanisms that determine the seasonal cycle of the current remain unclear, with discrepancies between ocean models and observations. Observations from moorings across the current and a 22‐year proxy of Agulhas Current volume transport reveal that the current is over 25% stronger in austral summer than in winter. We hypothesize that winds over the Southern Indian Ocean play a critical role in determining this seasonal phasing through barotropic and first baroclinic mode adjustments and communication to the western boundary via Rossby waves. Our hypothesis is explored using single‐layer and one‐and‐a‐half layer models. We find that the barotropic contribution to seasonal phasing is small, with the majority of the seasonal signal deflected offshore and along the Mozambique Ridge. The summertime maximum and wintertime minimum can, however, be reproduced by a one‐and‐a‐half layer reduced gravity model in which adjustment time to wind forcing via Rossby waves is in line with observations from satellite altimetry. Additionally, near‐field winds (to the west of 35°E) are shown to have a controlling influence on the seasonal phasing, as signals from farther afield dissipate through destructive interference with overlying winds before reaching the western boundary. These results suggest a critical role for a baroclinic adjustment to near‐field winds in setting the summertime maximum in Agulhas Current transport.

A Modeling Study of Interannual Variability of Bay of Bengal Mixing and Barrier Layer Formation

AbstractThe year to year variability of surface mixing in the Bay of Bengal (BoB) is examined with the help of an Ocean Dynamic‐Thermodynamic Model (ODTM) and observational data. The model embeds a conventional nonlinear primitive equation based reduced gravity model (RGM) with “N” active layers overlying a 1/2 quiescent layer. The model is coupled to a high‐resolution mixed layer model (MLM) to capture the physics of the mixed layer. The MLM incorporates a level‐2 turbulence closure mixing scheme based on Mellor and Yamada (1982). As a result of the coupling, our model calculates the net hydrostatic pressure as a sum of those due to the thickness of RGM layers with constant densities and due to the dynamic topography of MLM levels with variable densities. At each time step the coupling between RGM and MLM is achieved as follows: (i) a layer‐averaged tendency of momentum and tracers (i.e., temperature and salinity) of the MLM is updated to the RGM, (ii) momentum and tracers of the MLM get advected by large‐scale horizontal dynamics of the RGM, and (iii) horizontal layer divergence of the RGM aides for the vertical advection of the MLM. By virtue of this coupling, the model faithfully reproduces the seasonal cycle of temperature, salinity, sea surface height, thermocline, mixed and barrier layer thickness of the tropical Indian Ocean. A simulation with Coordinated Ocean‐Ice Reference Experiment (CORE) forcing for 15 years (from 1995 to 2009) is used as a test case to study the interannual variability (IAV) of the mixed layer depth and barrier layer thickness (BL) in BoB. The dominant modes of IAV in the BoB mixing are governed by the correspondingly varying surface momentum, heat, and fresh water fluxes with very little contribution from entrainment of heat and/or salt at the base of the mixed layer. Further, these fluxes are controlled by El Niño‐Southern Oscillation (ENSO) variability with very little influence from Indian Ocean Dipole‐Zonal Mode (IODZM). The BL IAV is predominantly controlled by precipitation forcing from ENSO. A stability analysis revealed that the turbulent kinetic energy (TKE) and the stability function (SH) are negatively correlated when ENSO is the dominant forcing. Such a correlation between TKE and SH is expected since unstable stratification conditions exist during positive ENSO, where the kinetic energy production is enhanced by the unstable buoyancy forcing leading to an increased TKE. During the negative phase of ENSO, stably stratified conditions exist, where the kinetic energy production is offset by the stable buoyancy force, reducing the TKE. This is indicative of high (low) turbulent kinetic energy production, low (high) flux Richardson number based stability function (SH), and low (high) dominance of buoyancy‐driven mixing during positive (negative) phases of ENSO. The results highlight that the counteracting influence of TKE and SH is a plausible reason for relatively weaker amplitude of IAV of BoB mixing compared to its normal seasonal cycle.

UPPER LAYER CIRCULATION IN THE BANDA SEA IN RESPONSE TO THE ONSET OF MONSOON WINDS

Aspects of the upper layer circulation in the interior of the Banda Sea, Indonesia, associated with local forcing by monsoon winds are examined numerically through the use of a reduced gravity model. The basin is located between approximately 4°S and 8°S and is partially enclosed by chains of islands. The primary emphasis is an evaluation of the free wave response which contributes to the steady or slowly varying circulation. Basin response appears to be characterized by interacting Kelvin waves and Rossby at low frequencies, and by evanescent Poincare waves of higher frequencies. Passages between islands along the perimeter of the basin appear to be nearly, impermeable to Rossby waves, which contribute to a pattern of westward propagating quasi geostrophic eddies. This pattern would persist during periods of wind transition.

Deep water characteristics and circulation in the South China Sea

This study investigates the deep circulation in the South China Sea (SCS) using oceanographic observations combined with results from a bottom layer reduced gravity model. The SCS water, 2000 m below the surface, is quite different from that in the adjacent Pacific Ocean, and it is characterized by its low dissolved oxygen (DO), high temperature and low salinity. The horizontal distribution of deep water properties indicates a basin-scale cyclonic circulation driven by the Luzon overflow. The results of the bottom layer reduced gravity model are consistent with the existence of the cyclonic circulation in the deep SCS. The circulation is stronger at the northern/western boundary. After overflowing the sill of the Luzon Strait, the deep water moves broadly southwestward, constrained by the 3500 m isobath. The broadening of the southward flow is induced by the downwelling velocity in the interior of the deep basin. The main deep circulation bifurcates into two branches after the Zhongsha Islands. The southward branch continues flowing along the 3500 m isobath, and the eastward branch forms the sub-basin scale cyclonic circulation around the seamounts in the central deep SCS. The returning flow along the east boundary is fairly weak. The numerical experiments of the bottom layer reduced gravity model reveal the important roles of topography, bottom friction, and the upwelling/downwelling pattern in controlling the spatial structure, particularly the strong, deep western boundary current.

Meridional dynamics of grounded abyssal water masses on a sloping bottom in a mid-latitude -plane

Observations, numerical simulations and theoretical scaling arguments suggest that in mid-latitudes, away from the source regions and the equator, the meridional transport of abyssal water masses along a continental slope corresponds to planetary geostrophic flows that are gravity- or density-driven and topographically steered. We investigate these dynamics using a nonlinear reduced-gravity model that can describe grounded abyssal meridional flow over sloping topography that crosses the planetary vorticity gradient. Exact nonlinear steady and time-dependent solutions are obtained. The general steady theory is illustrated for a non-parallel equatorward flow that possesses a single along-slope grounding along the upslope flank of the current (complementing previous work). Four specific nonlinear time-dependent solutions are described. Two initial-value problems are solved exactly. The first initial configuration corresponds to an equatorward abyssal flow that has no cross-slope shear in the along-slope velocity and possesses a single grounding along the upslope flank of the current. The nonlinear time-dependent evolution of this initial current into a non-parallel shear flow is described. The second initial condition corresponds to an isolated radially symmetric grounded abyssal pool or dome. The nonlinear time-dependent evolution of this abyssal dome, which propagates equatorward with unsteady along- and cross-slope velocities while deforming into an elliptically shaped abyssal dome with $\unicode[STIX]{x1D6FD}$-induced diminishing height, is described. Finally, the nonlinear time-dependent boundary-value problem can be solved exactly in which the in-flow boundary condition on the poleward boundary of the mid-latitude domain corresponds to a time-dependent abyssal current with both an upslope and downslope grounding. Two specific time-dependent boundary conditions are examined. The first corresponds to a time-limited surge in the equatorward volume transport in the abyssal current along the poleward boundary. The second configuration corresponds to the nonlinear evolution of a finite-amplitude downslope plume or loop that forms in the abyssal current that is reminiscent of those seen in baroclinic instability simulations.

Selective Response of the South China Sea Circulation to Summer Monsoon

AbstractThe response of the South China Sea (SCS) circulation to intraseasonal variability of the summer monsoon is studied with both observations and a 1.5-layer reduced-gravity model. Intraseasonal variability of the SCS summer monsoon is characterized by evolution of the wind jet intensity in the midbasin with typical amplitude of 6 m s−1 and several peaks on its power spectrum between 10 and 60 days. However, this study finds that intraseasonal variability of the sea surface height (SSH) in the SCS presents significant variability to the southeast of Vietnam with amplitude of 6 cm and a period only between 40 and 60 days. This implicates the frequency selectivity of oceanic response to wind forcing. Numerical experiments suggest that the intrinsic variability of the SCS circulation accounts for this phenomenon. Based on the Rossby basin mode theory, this is explained by the interaction between the long, westward-propagating Rossby waves and the short, eastward-propagating Rossby waves.

Western Boundary Sea Level: A Theory, Rule of Thumb, and Application to Climate Models

AbstractTo better understand coastal sea level variability and changes, a theory that predicts sea levels along a curved western boundary using interior ocean sea level information is proposed. The western boundary sea level at a particular latitude is expressed by the sum of contributions from interior sea levels propagating onto the western boundary by long Rossby waves between that latitude and a higher latitude, and from the western boundary sea level at the higher latitude. This theory is examined by using a linear, reduced gravity model. A comparison between the theory and the model shows good agreement. A simple scaling law (or rule of thumb) derived from the theory provides a measure of the higher-latitude sea level and ocean interior sea level contributions. The theory is then tested using data from 34 climate models in phase 5 of the Coupled Model Intercomparison Project (CMIP5) for dynamic sea level changes between the end of the twentieth and twenty-first centuries. The theory captures the nearly uniform sea level rise from the Labrador Sea to New York City (NYC), with a reduction in the increase of sea level farther south toward the equator, qualitatively consistent with the CMIP5 multimodel ensemble, even though the theory underestimates the equatorward reduction rate. Along the South American east coast, the theory successfully reproduced the spatial pattern of the sea level change. The theory suggests a strong link between a sea level rise hot spot along the northeastern coast of North America and the sea level increase in the Labrador Sea, consistent with the result that rates of NYC sea level rise are highly correlated to those in the Labrador Sea in CMIP5 models.

Variability of the Pacific North Equatorial Current from 1993 to 2012 based on a 1/8° Pacific model simulation

AbstractBased on a multidecadal Pacific basin model simulation, interannual variations of the North Equatorial Current (NEC) are investigated. The model reproduces well the characteristics of the NEC and its interannual variability. The NEC transport is calculated as an integral of the westward velocity from 6°N to 21°N and from the surface to the 1200 m depth. The magnitude and standard deviation of the NEC transport increase from 46.5 and 3.9 Sv at 175°E to 66.7 and 6.5 Sv at 130°E, respectively, and both peak around 132°E prior to entering the bifurcation region. The NEC transport tends to be higher during positive Oceanic Niño Index (ONI) years but lower during negative ONI years with the maximum difference of more than 20 Sv. The interannual variability of the NEC transport is closely related to changes of the sea surface height in the tropical Pacific Ocean, and the increase of the NEC is mostly balanced by the increase in the North Equatorial Counter Current (NECC) on the tropical gyre side. The present study further suggests a long‐term decline of the NEC transport from 1993 to 2012, which is consistent with the patterns in the trend of wind stress curl. Transport anomalies reconstructed from the normal modes of zonal velocity suggest that the first baroclinic mode captures about 95% of the variance in the NEC transport, while the second mode adds only additional 3–4%. A 1.5‐layer reduced gravity model further reveals that the first (second) baroclinic mode is driven primarily by the wind (thermal) forcing. The wind forcing plays a predominant role in determining the interannual variability in the NEC transport while the effect of the thermal forcing is rather limited.

Geometric structure of pseudo-plane quadratic flows

Quadratic flows have the unique property of uniform strain and are commonly used in turbulence modeling and hydrodynamic analysis. While previous applications focused on two-dimensional homogeneous fluid, this study examines the geometric structure of three-dimensional quadratic flows in stratified fluid by solving a steady-state pseudo-plane flow model. The complete set of exact solutions reveals that steady quadratic flows have an invariant conic type in the non-rotating frame and a non-rotatory vertical structure in the rotating frame. Three baroclinic solutions with vertically non-aligned formulation disprove an earlier conjecture. All elliptic and hyperbolic solutions, except for the inertial ones, exhibit vertical concentricity. The rich geometry of quadratic flows stands in contrast to the depleted geometry of high-degree polynomial flows. A paradox in the steady solutions of shallow-water reduced-gravity models is also explained.