Baroclinic Component Research Articles

Critical Latitude in Tidal Dynamics Using the Kara Sea as an Example

It is well known that, within the linear nonviscous equations of tidal dynamics, the amplitudes of oscillations of the barotropic and baroclinic tidal velocity components unlimitedly increase when approaching the critical latitude. It is also known that the linear equations of tidal dynamics with a constant and specified vertical eddy viscosity indicate the occurrence of significant tidal velocity shears in the near-bottom layer, which are responsible for increasing the baroclinic tidal energy dissipation, the turbulent kinetic energy, and the thickness of the bottom boundary layer. The first circumstance—the growth of the amplitudes of oscillations of the barotropic and baroclinic tidal velocity components—is due to the elimination in the original equations of small terms, which are small everywhere except for the critical latitude zone. The second circumstance—the occurrence of significant tidal velocity shears—is due to the fact that internal tidal waves, which induce the dissipation of the baroclinic tidal energy and the diapycnal diffusion, are either not taken into account or described inadequately. It is suggested that diapycnal diffusion can lead to the degeneration (complete or partial) of tidal velocity shears, with all the ensuing consequences. The aforesaid is confirmed by simulation results obtained using the QUODDY-4 high-resolution three-dimensional finite-element hydrostatic model along the 66.25° E section, which passes in the Kara Sea across the critical latitude.

An experimental investigation of blocking by partial barriers in a rotating baroclinic annulus

We present a series of experimental investigations in which a differentially-heated annulus was used to investigate the effects of topography on rotating, stratified flows with similarities to the Earth’s atmospheric or oceanic circulation. In particular, we compare and investigate blocking effects via partial mechanical barriers to previous experiments by the authors utilising azimuthally-periodic topography. The mechanical obstacle used was an isolated ridge, forming a partial barrier, employed to study the difference between partially blocked and fully unblocked flow. The topography was found to lead to the formation of bottom-trapped waves, as well as impacting the circulation at a level much higher than the top of the ridge. This produced a unique flow structure when the drifting flow and the topography interacted in the form of an “interference” regime at low Taylor number, but forming an erratic “irregular” regime at higher Taylor number. The results also showed evidence of resonant wave-triads, similar to those noted with periodic wavenumber-3 topography by Marshall and Read (Geophys. Astrophys. Fluid Dyn., 2015, 109), though the component wavenumbers of the wave-triads and their impact on the flow were found to depend on the topography in question. With periodic topography, wave-triads were found to occur between both the baroclinic and barotropic components of the zonal wavenumber-3 mode and the wavenumber-6 baroclinic component, whereas with the partial barrier two nonlinear resonant wave-triads were noted, each sharing a common wavenumber-1 mode.

Volume, Heat, and Salt Transports through the Soya Strait and Their Seasonal and Interannual Variations

AbstractVolume, heat, and salt transports through the Soya Strait are estimated based on measurements from high-frequency ocean radars during 2003–15 and all available hydrographic data. The baroclinic velocity structure derived from the climatological geopotential anomaly is combined with the sea surface gradient obtained from radar-derived surface velocities to estimate the absolute velocity structure. The annual-mean volume, heat, and salt transports are 0.91 Sv (1 Sv ≡ 106 m3 s−1), 25.5 TW, and 31.15 × 106 kg s−1, respectively. The volume transport exhibits strong seasonal variations, with a maximum of 1.41 Sv in August and a minimum of 0.23 Sv in January. The seasonal amplitude and phase roughly correspond to those of the Tsushima–Korea Strait. Time series of the monthly transport is presented for the 12 yr, assuming that the baroclinic components are the monthly climatological values. In cold seasons (November to April), the monthly volume transport is strongly correlated with the sea level difference between the Japan and Okhotsk Seas, and an empirical formula to estimate the transport from the sea level difference is introduced. It is likely that the sea level setup by the wind stress along the east coast of Sakhalin determines the sea level difference, which explains the seasonal and interannual wintertime variations of transport through the strait. The annual flux of water through the Soya Strait with a density greater than 26.8σθ, a potential source of Okhotsk Sea Intermediate Waters, is estimated to be 0.18 Sv.

Baroclinic-to-Barotropic Pathway in El Niño–Southern Oscillation Teleconnections from the Viewpoint of a Barotropic Rossby Wave Source

Abstract The baroclinic-to-barotropic pathway in ENSO teleconnections is examined from the viewpoint of a barotropic Rossby wave source that results from decomposition into barotropic and baroclinic components. Diagnoses using the NCEP–NCAR reanalysis are supplemented by analysis of the response of a tropical atmospheric model of intermediate complexity to the NCEP–NCAR barotropic Rossby wave source. Among the three barotropic Rossby wave source contributions (shear advection, vertical advection, and surface drag), the leading contribution is from shear advection and, more specifically, the mean baroclinic zonal wind advecting the anomalous baroclinic zonal wind. Vertical advection is the smallest term, while surface drag tends to cancel and reinforce the shear advection in different regions through damping on the baroclinic mode, which spins up a barotropic response. There are also nontrivial impacts of transients in the barotropic wind response to ENSO. Both tropical and subtropical baroclinic vorticity advection contribute to the barotropic component of the Pacific subtropical jet near the coast of North America, where the resulting barotropic wind contribution approximately doubles the zonal jet anomaly at upper levels, relative to the baroclinic anomalies alone. In this view, the barotropic Rossby wave source in the subtropics simply arises from the basic-state baroclinic flow acting on the well-known baroclinic ENSO flow pattern that spreads from the deep tropics into the subtropics over a scale of equatorial radius of deformation. This is inseparably connected to the leading deep tropical Rossby wave source that arises from eastern Pacific climatological baroclinic winds advecting the tropical portion of the same ENSO flow pattern.

Variability of the Antarctic Coastal Current in the Amundsen Sea

The nature of the Antarctic Coastal Current (AACC) and its seasonal and non-seasonal (several-day) variability were investigated using long-term mooring data obtained near the Dotson Ice Shelf in the Amundsen Sea. The moored instruments were operated from February 2012 to January 2014 in the Dotson Trough, which is one of the deep troughs on the Amundsen Sea shelf. The hydrographic structure of sigma density distribution at the ice shelf (27.25 kg m−3 at the surface) was found to be higher than at the continental shelf (27.00 kg m−3), whereas in the middle layer, 27.45 kg m−3 isopycnal extended to 300 m depth near the ice shelf but below 200 m depth in the offshore. The surface mixed layer thickened from the shelf break toward the ice shelf. This thickening was caused partially by coastal downwelling resulting from onshore Ekman transport and by Ekman pumping in the coastal area. The baroclinic component of the AACC near the Dotson Ice Shelf increased from July to October and decreased from January to June in 2013. In comparison with other potential driving forces, the seasonal and non-seasonal (short-term) variation of the AACC correlated well with density variability, determined principally by salinity variation. Therefore, it is suggested that the variability of the isopycnocline by Ekman pumping in the coastal area is one of the important factors controlling the seasonality and non-seasonal variability of the AACC.

Open boundary conditions for tidally and subtidally forced circulation in a limited‐area coastal model using the Regional Ocean Modeling System (ROMS)

AbstractIn limited‐area ocean models, open boundary conditions (OBCs) often create dynamic inconsistencies and perform poorly in resolving tidal or subtidal flow when both forces exist. Orlanski‐type radiation OBCs are reasonably efficient at treating the subtidally forced flow, and Flather‐type OBCs are commonly adapted for the tidally forced flow. However, neither of them performs well when tidal and subtidal forces simultaneously drive the flows. We have developed a novel OBC that integrates the active OBC in Gan and Allen (2005) and a Flather‐type OBC. This new OBC accommodates the concurrent Tidal and Subtidal (TST) forcing, and the respective tidal or subtidal forcing, at the open boundary of a limited‐area model. This new TST‐OBC treats the tidal component with a Flather‐type OBC, and it separates subtidal barotropic and baroclinic components into local (forced) and global (unforced) components. Then an unforced Orlanski‐type OBC can be applied to the global part. We applied the TST‐OBC to all model variables to reduce dynamic inconsistence. Using the Regional Ocean Modeling System, we applied the TST‐OBC to the shallow East China Sea shelf where strong tidal and subtidal forces over complex topography govern the circulation. Our numerical experiments and analyses suggest that the TST‐OBC was robust for both concurrent tidal‐subtidal forcing and solely tidal or subtidal forcing at the open boundary. It reduced spurious energy reflection, and, overall, it performed better than an Orlanski‐type or Flather‐type OBC in reproducing realistic tidal and subtidal shelf circulation.

Evolution of instability before and during a torrential rainstorm in North China

NCEP-NCAR reanalysis data were used to analyze the characteristics and evolution mechanism of convective and symmetric instability before and during a heavy rainfall event that occurred in Beijing on 21 July 2012. Approximately twelve hours before the rainstorm, the atmosphere was mainly dominated by convective instability in the lower level of 900–800 hPa. The strong southwesterly low-level jet conveyed the moist and warm airflow continuously to the area of torrential rain, maintaining and enhancing the unstable energy. When the precipitation occurred, unstable energy was released and the convective instability weakened. Meanwhile, due to the baroclinicity enhancement in the atmosphere, the symmetric instability strengthened, maintaining and promoting the subsequent torrential rain. Deriving the convective instability tendency equation demonstrated that the barotropic component of potential divergence and the advection term played a major role in enhancing the convective instability before the rainstorm. Analysis of the tendency equation of moist potential vorticity showed that the coupled term of vertical vorticity and the baroclinic component of potential divergence was the primary factor influencing the development of symmetric instability during the precipitation. Comparing the effects of these factors on convective instability and symmetric instability showed some correlation.

El Niño–Southern Oscillation Sea Level Pressure Anomalies in the Western Pacific: Why Are They There?*

Abstract Although sea level pressure (SLP) anomalies in the western Pacific have long been recognized as an integral part of the classic Southern Oscillation pattern associated with El Niño–Southern Oscillation (ENSO), there is an unresolved question regarding the dynamics that maintain these anomalies. Traditional studies of the ENSO response in the tropics assume a single deep baroclinic mode associated with the tropospheric temperature anomalies. However, the SLP anomalies in the western Pacific are spatially separated from the baroclinic signal in the NCEP–NCAR reanalysis, CMIP5 models, and an intermediate complexity model [a quasi-equilibrium tropical circulation model (QTCM)]. Separation of ENSO SLP anomalies in the tropical Pacific into baroclinic and barotropic components indicates that the barotropic component contributes throughout the tropics and constitutes the primary contribution in the western Pacific. To demonstrate the roles of baroclinic and barotropic modes in ENSO teleconnections within the tropics, a series of QTCM experiments is performed, where anomalies in the interactions between baroclinic and barotropic modes are suppressed over increasingly wider latitudinal bands in the tropical Pacific. If this suppression is done in the 15°N–15°S band, the pressure signals in the western Pacific are only partly removed, whereas if it is done in the 30°N–30°S band, the anomalies in the western Pacific are almost entirely removed. This suggests the following pathway: interactions with SST anomalies create the baroclinic response in the central and eastern Pacific, but baroclinic–barotropic interactions, arising substantially in the subtropical Pacific, generate a barotropic response that yields the SLP anomalies in the western Pacific.

Eulerian Volume Transport Induced by Spatially Damped Internal Equatorial Kelvin Waves

AbstractThe Eulerian volume transport in internal equatorial Kelvin waves subject to viscous attenuation is investigated theoretically by integrating the horizontal momentum equations in the vertical. In terms of small perturbations, the time-averaged horizontal transports are determined to second order in wave steepness. The total Lagrangian volume transport in this problem consists of a Stokes transport plus an Eulerian transport. It is known that the Stokes transport, that is, the vertically integrated Stokes drift, in inviscid internal equatorial Kelvin waves vanishes identically in the rigid-lid approximation for arbitrary vertical variation of the Brunt–Väisälä frequency. The present study considers spatial wave damping due to viscosity. The Stokes transport still becomes zero, but now the radiation stresses due to decaying waves become source terms for the Eulerian mean transport. Calculations of the wave-induced Eulerian transport yield a jetlike symmetric mean flow along the equator from west to east for each baroclinic component, with compensating westward flows on both sides. The flow system scales as the internal equatorial Rossby radius in the north–south direction. The total eastward part of the Eulerian volume flux centered about the equator is estimated to about 0.2 Sv (1 Sv ≡ 106 m3 s−1) for the first baroclinic mode.

Ekman Spiral in a Horizontally Inhomogeneous Ocean with Varying Eddy Viscosity

The classical Ekman spiral is generated by surface wind stress with constant eddy viscosity in a homogeneous ocean. In real oceans, the eddy viscosity varies due to turbulent mixing caused by surface wind and buoyancy forcing. Horizontally inhomogeneous density produces vertical geostrophic shear which contributes to current shear that also affects the Ekman spiral. Based on similar theoretical framework as the classical Ekman spiral, the baroclinic components of the Ekman spiral caused by the horizontally inhomogeneous density are obtained analytically with the varying eddy viscosity calculated from surface wind and buoyancy forcing using the K-profile parameterization (KPP). Along with the three existing types of eddy viscosity due to pure wind forcing (zero surface buoyancy flux), such an effect is evaluated using the climatological monthly mean data of surface wind stress, buoyancy flux, ocean temperature and salinity, and mixed layer depth.

Vertical Structure of Ocean Pressure Variations with Application to Satellite-Gravimetric Observations

AbstractThe nature of ocean bottom pressure () variability is considered on large spatial scales and long temporal scales. Monthly gridded estimates from the Gravity Recovery and Climate Experiment (GRACE) Release-05 and the new version 4 bidecadal ocean state estimate of the Consortium for Estimating the Circulation and Climate of the Ocean (ECCO) are used. Estimates of from GRACE and ECCO are generally in good agreement, providing an independent measure of the quality of both products. Diagnostic fields from the state estimate are used to compute barotropic (depth independent) and baroclinic (depth dependent) components. The relative roles of baroclinic and barotropic processes are found to vary with latitude and time scale: variations in at higher latitudes and shorter periods are affected by barotropic processes, whereas fluctuations at lower latitudes and longer periods can be influenced by baroclinic effects, broadly consistent with theoretical scaling arguments. Wind-driven Rossby waves and coupling of baroclinic and barotropic modes due to flow–topography interactions appear to be important influences on the baroclinic variability. Decadal simulations of monthly variability based on purely barotropic frameworks are expected to be in error by about 30% on average ( in the tropical ocean and at higher latitudes). Results have implications for applying GRACE observations to problems such as estimating transports of the Antarctic Circumpolar Current.

Influence of baroclinicity in the atmospheric boundary layer and Ekman friction on the surface wind speed during cold-air outbreaks in the Arctic

Extreme cases of cold-air outbreaks in the Arctic during spring 2013 are identified using MODIS images from Terra and Aqua satellites. Spatial variability of the surface wind speed during considered cases of cold-air outbreaks is quantified using the ERA Interim reanalysis and data retrieved from the satellite microwave radiometer AMSR2. To explain the observed variability of wind speed in the atmospheric boundary layer (ABL) the contributions of baroclinicity in the ABL and Ekman friction are quantified. For this purpose diagnostic relationships based on the concept of a mixed-layer model are used. It is demonstrated that baroclinic component of the geostrophic wind caused by the horizontal temperature gradients in the ABL over the open water has a strong effect on the spatial variability of wind speed during considered cases of cold-air outbreaks.

A dynamical framework for the origin of the diagonal South Pacific and South Atlantic Convergence Zones

The South Pacific Convergence Zone (SPCZ) and South Atlantic Convergence Zone (SACZ) are diagonal bands of precipitation that extend from the Equator southeastward into the Southern Hemisphere over the western Pacific and Atlantic Oceans, respectively. With mean precipitation rates over 5 mm day−1, they are a major component of the tropical and global climate in austral summer. However, their basic formation mechanism is not fully understood. Here, a conceptual framework for the diagonal convergence zones is developed, based on calculations of the vorticity budget from reanalysis and Rossby wave theory.Wave trains propagate eastward along the Southern Hemisphere subtropical jet, with initially quasi‐circular vorticity centres. In the zonally sheared environment on the equatorward flank of the jet, these vorticity centres become elongated and develop a northwest–southeast tilt. Ray‐tracing diagnostics in a non‐divergent, barotropic Rossby wave framework then explain the observed equatorward propagation of these diagonal vorticity structures toward the westerly ducts over the equatorial Pacific and Atlantic. The baroclinic component of these circulations leads to destabilisation and ascent ahead of the cyclonic vorticity anomaly in the wave, triggering deep convection because of the high sea surface temperatures in this region. Latent heat release then forces additional ascent and strong upper‐tropospheric divergence, with an associated anticyclonic vorticity tendency. A vorticity budget shows that this cancels out the advective cyclonic vorticity tendency in the wave train over the SPCZ, and dissipates the wave within a day. The mean SPCZ is consequently comprised of the sum of these pulses of diagonal bands of precipitation.Similar mechanisms also operate in the SACZ. However, the vorticity anomalies in the wave trains are stronger, and the precipitation and negative feedback from the divergence and anticyclonic vorticity tendency are weaker, resulting in continued propagation of the wave and a more diffuse diagonal convergence zone.

Argo data assimilation into HYCOM with an EnOI method in the Atlantic Ocean

Abstract. An ocean data assimilation system to assimilate Argo temperature (T) and salinity (S) profiles into the HYbrid Coordinate Ocean Model (HYCOM) was constructed, implemented and evaluated for the first time in the Atlantic Ocean (78° S to 50° N and 98° W to 20° E). The system is based on the ensemble optimal interpolation (EnOI) algorithm proposed by Xie and Zhu (2010), especially made to deal with the hybrid nature of the HYCOM vertical coordinate system with multiple steps. The Argo T–S profiles were projected to the model vertical space to create pseudo-observed layer thicknesses (Δ pobs), which correspond to the model target densities. The first step was to assimilate Δ pobs considering the sub-state vector composed by the model layer thickness (Δ p) and the baroclinic velocity components. After that, T and S were assimilated separately. Finally, T was diagnosed below the mixed layer to preserve the density of the model isopycnal layers. Five experiments were performed from 1 January 2010 to 31 December 2012: a control run without assimilation, and four assimilation runs considering the different vertical localizations of T, S and Δ p. The assimilation experiments were able to significantly improve the thermohaline structure produced by the control run. They reduced the root mean square deviation (RMSD) of T and S calculated with respect to Argo independent data in 34 and 44%, respectively, in comparison to the control run. In some regions, such as the western North Atlantic, substantial corrections in the 20 °C isotherm depth and the upper ocean heat content towards climatological states were achieved. The runs with a vertical localization of Δ p showed positive impacts in the correction of the thermohaline structure and reduced the RMSD of T (S) from 0.993 °C (0.149 psu) to 0.905 °C (0.138 psu) for the whole domain with respect to the other assimilation runs.

Seasonal and interannual variability of the West Greenland Current System in the Labrador Sea in 1993–2008

AbstractThe West Greenland Current System (WGCS) transports heat and freshwater into the Labrador Sea, influencing the formation of Labrador Sea Water, a key component of the Atlantic Meridional Overturning Circulation. Notwithstanding its importance, relatively little is known about the structure and transport of this current system and its seasonal and interannual variability. Here we use historical hydrographic data from 1992 to 2008, combined with AVISO satellite altimetry, to diagnose the mean properties as well as seasonal and interannual variability of the boundary current system. We find that while the surface, fresh, cold West Greenland Current is amplified in summer, the subsurface warm, salty Irminger Current has maximum transport in winter, when its waters are also warmer and saltier. Seasonal changes in the total transport are thus mostly due to changes in the baroclinic structure of the current. By contrast, we find a trend toward warmer/saltier waters and a slowdown of the WGCS, within the period studied. The latter is attributed to changes in the barotropic component of the current. Superimposed on this trend, warm and salty anomalies transit through the system in 1997 and 2003 and are associated with a rapid increase in the transport of the boundary current due to changes in the baroclinic component. The boundary current changes precede similar changes in the interior with a 1–2 year lag, indicating that anomalies advected into the region by the boundary current can play an important role in the modulation of convection in the Labrador Sea.

Mass exchange at the Strait of Gibraltar in response to tidal and lower frequency forcing as simulated by a Mediterranean Sea model

The exchange between the Atlantic and the Mediterranean at the Strait of Gibraltar is studied based on numerical simulations of the Mediterranean Sea compared to two sets of observations. The model used has a varying horizontal resolution, highest at the Strait of Gibraltar. Numerical simulations forced by tide, by the subinertial variability, by both and by increasing the diffusion at the Strait are performed and compared to each other. The model successfully reproduces the main observed features of the variability at the tidal and at the lower frequency time scales including the phasing between the barotropic and baroclinic flow components and density variations. The model also simulates the strong mixing at the strait by tide and the resulting fortnightly modulation of the flow, with exchange reduction during spring tides and outflowing waters and acceleration during neap tides and inflowing waters. It is shown that tidal oscillations reduce the two-way exchange by interaction with the subinertial variability. The effects of tide on the Mediterranean Sea thermohaline circulation are also examined using multi-decadal simulations. It is shown that the model reproduces the cooling and saltening of waters crossing the strait in the upper layer and the warming and freshening of waters crossing the strait in the deeper layer, as previously shown by high resolution models of the Strait of Gibraltar. These changes are shown to cool and increase the salinity of the Mediterranean waters especially in the upper and intermediate layers. The water-cooling is shown to lead to a reduction of the heat loss at the sea surface. Based on model results, it is concluded that tide may have an effect on the Mediterranean Sea heat budget and hence on the atmosphere above. A validation of this conclusion is however needed, in particular using higher resolution models.

Wave turbulence in the two-layer ocean model

AbstractThis paper looks at the two-layer ocean model from a wave-turbulence (WT) perspective. A symmetric form of the two-layer kinetic equation for Rossby waves is derived using canonical variables, allowing the turbulent cascade of energy between the barotropic and baroclinic modes to be studied. It is already well known that in two-layers, energy is transferred via triad interactions from the large-scale baroclinic modes to the baroclinic and barotropic modes at the Rossby deformation scale, where barotropization takes place, and from there to the large-scale barotropic modes via an inverse transfer. However, by applying WT theory, we find that energy is transferred via dominant$\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}\{+--\}$triads with one barotropic component and two baroclinic components, and that the direct transfer of energy is local and the inverse energy transfer is non-local. We study this non-locality using scale separation and obtain a system of coupled equations for the small-scale baroclinic component and the large-scale barotropic component. Since the total energy of the small-scale component is not conserved, but the total barotropic plus baroclinic energy is conserved, the baroclinic energy loss at small scales will be compensated by the growth of the barotropic energy at large scales. Using the frequency resonance condition, we show that in the presence of the beta-effect this transfer is mostly anisotropic and mostly to the zonal component.

Observed wintertime tidal and subtidal currents over the continental shelf in the northern South China Sea

AbstractSynthesis analyses were performed to examine characteristics of tidal and subtidal currents at eight mooring sites deployed over the northern South China Sea (NSCS) continental shelf in the 2006–2007 and 2009–2010 winters. Rotary spectra and harmonic analysis results showed that observed tidal currents in the NSCS were dominated by baroclinic diurnal tides with phases varying both vertically and horizontally. This feature was supported by the CC‐FVCOM results, which demonstrated that the diurnal tidal flow over this shelf was characterized by baroclinic Kelvin waves with vertical phase differences varying in different flow zones. The northeasterly wind‐induced southwestward flow prevailed over the NSCS shelf during winter, with episodic appearances of mesoscale eddies and a bottom‐intensified buoyancy‐driven slope water intrusion. The moored current records captured a warm‐core anticyclonic eddy, which originated from the southwestern coast of Taiwan and propagated southwestward along the slope consistent with a combination of β‐plane and topographic Rossby waves. The eddy was surface‐intensified with a swirl speed of >50 cm/s and a vertical scale of ∼400 m. In absence of eddies and onshore deep slope water intrusion, the observed southwestward flow was highly coherent with the northeasterly wind stress. Observations did not support the existence of the permanent wintertime South China Sea Warm Current (SCSWC). The definition of SCSWC, which was based mainly on thermal wind calculations with assumed level of no motion at the bottom, needs to be interpreted with caution since the observed circulation over the NSCS shelf in winter included both barotropic and baroclinic components.

Semidiurnal tidal currents in the deep ocean near the East Pacific Rise between 9° and 10°N

AbstractSpatial and temporal variations of the semidiurnal tidal currents, especially the baroclinic components, in the deep ocean near a segment of the East Pacific Rise (EPR) are examined using observations from a set of current meter moorings. Over the ridge crest, the baroclinic semidiurnal tidal current is dominated by the zonal component and is coherent along the observed ridge segment. In the direction across the ridge crest, the baroclinic semidiurnal tidal current is not coherent and its magnitude decreases away from the ridge crest. Vertically, the baroclinic semidiurnal tidal current covaries in a few hundred meters near the seafloor. Both the semidiurnal kinetic energy (KE) and shear show significant subinertial temporal variations. By comparing with background forcings, the author found that the subinertial variation of the semidiurnal tides over the ridge crest is related to both the spring‐neap cycles and the eddy‐induced low‐frequency flows in the deep ocean, particularly their cross‐ridge components. The observation that the baroclinic tidal currents at the axial stations flowing perpendicularly to the ridge crest suggests that the baroclinic semidiurnal tides at the axial stations are generated locally. It is therefore probable that the subinertial variation of the baroclinic semidiurnal tides near the EPR is due to the eddy‐modulated internal tide generation. Since the mesoscale eddies in the eastern tropical Pacific vary with the state of the climate, the observed eddy‐modulated internal tide generation connects climate change and variability to the physical and biogeochemical dynamics in the deep ocean and implies an unexplored feedback mechanism potentially affecting the climate system.

Annual Cycle in Southern Tropical Indian Ocean Bottom Pressure

Abstract The seasonal monsoon drives a dynamic response in the southern tropical Indian Ocean, previously observed in baroclinic Rossby wave signatures in annual sea level and thermocline depth anomalies. In this paper, monthly mass grids based on Release-05 Gravity Recovery and Climate Experiment (GRACE) data are used to study the annual cycle in southern tropical Indian Ocean bottom pressure. To interpret the satellite data, a linear model of the ocean’s response to wind forcing—based on the theory of vertical normal modes and comprising baroclinic and barotropic components—is considered. The model is evaluated using stratification from an ocean atlas and winds from an atmospheric reanalysis. Good correspondence between model and data is found over the southern tropical Indian Ocean: the model explains 81% of the annual variance in the data on average between 10° and 25°S. Model solutions suggest that, while the annual baroclinic Rossby wave has a seafloor signature, the annual cycle in the deep sea generally involves important barotropic dynamics, in contrast to the response in the upper ocean, which is largely baroclinic.