Subinertial Flow Research Articles

Observed Oceanic Surface Modes in the Northern South China Sea

Abstract Using observations and theoretical models, a substantial topographic modulation on the quasigeostrophic (QG) dynamics, which results in a primary surface mode distinct from the classic first baroclinic (BC1) mode with a flat bottom, is revealed in the northern South China Sea (NSCS). In contrast to open oceans, the surface-intensified modes decay downward more rapidly over the continental slope of the NSCS, with a mean e-folding scale of approximately 1/5 of water depth. The subinertial flow variability appears to be vertically incoherent, with planetary and topographic Rossby waves dominating in the upper and deep layers, respectively. With a larger deformation radius (Rd), the surface-mode Rossby waves propagate at a phase speed ∼1.5 times of that of the BC1 mode. Moreover, the modal structures can be substantially modified by seasonal NSCS circulation, which is significantly enhanced over continental slopes. Analysis of the triad interactions further implies that the short waves tend to transfer energy to larger scales via the inverse cascade and only those with wavelengths larger than Rd ≈ 70 km in the NSCS can persist because of a slower unstable growth rate but a higher fraction of upscale energy transfer. The present theory excludes the bottom-trapped mode, which is closely associated with topographic Rossby waves and is observed to be significant in the abyssal NSCS. Hence, a complete normal-mode basis for any QG state is required for a study that focuses on flow variability throughout the water column. Our findings provide an insight into the vertical partition of horizontal kinetic energy for QG motions, as well as the relevant oceanic variation in marginal seas.

Observation of Near-Inertial Waves in the Bottom Boundary Layer of an Abyssal Seamount

Abstract The bottom boundary layer (BBL) contributes significantly to the global energy dissipation of low-frequency flows in the abyssal ocean, but how this dissipation occurs remains poorly understood. Using in situ data collected near the BBL at an abyssal seamount in the western Pacific Ocean, we demonstrate that strong bottom-trapped flows over sloping topography can lose their energy to near-inertial waves (NIWs) generated via the adjustment of the bottom Ekman layer. The NIWs with near-resonant frequencies corresponding to internal waves with propagation direction parallel to the topographic slope are observed. These waves are strongest in the BBL and have a correlation with the off-seamount subinertial flows largely attributed to the Ekman transport driven by the bottom-trapped anticyclonic circulation over the seamount. The bottom-intensified NIWs are observed to have dominant upward-propagating energy and hypothesized to be generated via Ekman flow–topography interactions in the BBL. Energy loss from the near-bottom flows to radiating NIWs (∼8 × 10−4 W m−2) is estimated to be substantially larger than that due to bottom drag dissipation (∼2 × 10−4 W m−2), suggesting the important role of internal-wave generation via the Ekman transport adjustment in damping the subinertial flows over the sloping seafloor. Significance Statement Dissipation of geostrophic currents and eddies via the oceanic bottom boundary layer (BBL) plays an important role in modulating the global oceanic mechanical energy budget. The bottom drag has long been considered a key process in inducing such dissipation, but it is recently suggested to be less effective at a sloping bottom. This study suggests another potentially important mechanism that can remove energy from the geostrophic flows at the sloping bottom. This mechanism is depicted as the resonant generation of near-inertial internal waves by near-bottom flows. Generation of internal waves due to near-bottom flows over sloping topography should be common in the global ocean and therefore have significant implications for the oceanic mechanical energy budget.

Subinertial flow patterns in a tropical coral reef system of the southwestern gulf of Mexico

One-year subinertial flow profiles and near-bottom temperatures were recorded in a tropical coral reef of the southwestern Gulf of Mexico to determine whether baroclinicity affects flow distributions. The subinertial flow at four sites was predominantly influenced by three empirical modes of variability. The first mode (explaining 78% of the variance) was mostly associated to wind stress (r = 0.62). Its temporal variability had greatest energy during the northerly wind season (October–May), related to atmospheric cold fronts, at periods of 2–10 days and at 20–30 days. Mode 2 (with 6% of the variance) was related to sea-level slopes, linked also to northerly wind events; this mode induced an anticyclonic circulation around the reefs, although a less intense cyclonic circulation was also observed. Mode 3 (with 5% of the variance) was associated to a baroclinic pressure gradient that drove bidirectional flow profiles: Near-surface flows moving in opposite direction to near-bottom flows. Mode 3 dominated in the vertically stratified summer (June–September), the rainy season, but it was also occasionally observed during northerly winds. Near-bottom summer intrusions of relative cold water were baroclinically driven, linked to Mode 3, and unrelated to Ekman dynamics. Modes 2 and 3 of variability described additional flow distributions relative to the behavior already proposed for the reefs on the southwestern continental shelf of the Gulf of Mexico.

Non‐Local Energy Dissipation of Lee Waves and Turbulence in the South China Sea

AbstractBreaking of internal lee waves has been shown to drive enhanced turbulence and mixing in regions where strong bottom flows interact with rough topography. However, theoretical predictions for the energy conversion from the bottom flows into lee waves differ from the corresponding turbulent energy dissipation rates observed locally at the wave generation sites. Recent idealized numerical simulations suggest that the discrepancy may be attributed to non‐local wave breaking and dissipation effects: when a mean flow impinges on rough topography and generates lee waves and turbulence, a significant fraction of the generated energy gets advected downstream of the generation site and dissipates remotely. Here, we present a case study for the non‐local lee wave energy dissipation in the South China Sea by using a combination of in situ mooring observations of the bottom flow interacting with two topographic features and corresponding numerical simulations. The results show that most of the near‐inertial and high‐frequency response observed at the mooring site is not locally generated, but rather is produced by the interaction of the subinertial flow with the topographic feature upstream. The wave and turbulent energy detected at the mooring site can be enhanced by an order of magnitude compared to the energy of the locally generated motions. The simulations confirm that up to 70% of the enhanced energy is advected into the region by the subinertial flow, while the rest is radiated into the region as waves. Implication of our results for mixing observations and ocean model parameterizations are discussed.

Broadband Submesoscale Vorticity Generated by Flow around an Island

AbstractAn array of moorings deployed off the coast of Palau is used to characterize submesoscale vorticity generated by broadband upper-ocean flows around the island. Palau is a steep-sided archipelago lying in the path of strong zonal geostrophic currents, but tides and inertial oscillations are energetic as well. Vorticity is correspondingly broadband, with both mean and variance O(f) in a surface and subsurface layer (where f is the local Coriolis frequency). However, while subinertial vorticity is linearly related to the incident subinertial current, the relationship between superinertial velocity and superinertial vorticity is weak. Instead, there is a strong nonlinear relationship between subinertial velocity and superinertial vorticity. A key observation of this study is that during periods of strong westward flow, vorticity in the tidal bands increases by an order of magnitude. Empirical orthogonal functions (EOFs) of velocity show this nonstationary, superinertial vorticity variance is due to eddy motion at the scale of the array. Comparison of kinetic energy and vorticity time series suggest that lateral shear against the island varies with the subinertial flow, while tidal currents lead to flow reversals inshore of the recirculating wake and possibly eddy shedding. This is a departure from the idealized analog typically drawn on in island wake studies: a cylinder in a steady flow. In that case, eddy formation occurs at a frequency dependent on the scale of the obstacle and strength of the flow alone. The observed tidal formation frequency likely modulates the strength of submesoscale wake eddies and thus their dynamic relationship to the mesoscale wake downstream of Palau.

Subinertial hydrodynamics around a cape influenced by a western boundary current

Subinertial hydrodynamics were investigated at an inner-shelf around a cape on the Florida Atlantic coast during spring 2014. The area to the northeast and southeast of Cape Canaveral was instrumented with four moored Acoustic Doppler Current Profilers (ADCPs) within the swales on either side of shoals. The purpose was to examine the influence of a western boundary current (WBC) on inner-shelf subinertial flow, which was mainly along the shelf. A Concatenated Hilbert Empirical Orthogonal Function (CHEOF) analysis showed that Mode 1, which accounted for 95.7% of flow variance, described variations in the along-shelf current. Wavelet coherence analysis was used to examine relationships between CHEOF Mode 1 and potential environmental forcings. Analysis of the cross-shelf momentum terms showed geostrophic balance throughout the deployment with occasional contribution of the gradient of the wave radiation stress Sxx. Along-shelf momentum was beyond frictional, as other terms in the balance were also influential. When along-shelf winds and the Florida current are oriented in the same direction, an increase in Florida current transport is observed and vice versa. Enhancement of the Florida current is associated with an increased positive across-shelf sea surface slope in accordance with geostrophy, driving the inner-shelf subinertial flow in the same direction as the western boundary current (WBC). Conversely, southward along-shelf winds coincided with weakening of the Florida current and a negative inner-shelf sea surface slope that maintained geostrophy, driving a southward inner-shelf flow.

Time Scales of Submesoscale Flow Inferred from a Mooring Array

AbstractWhile the distribution of kinetic energy across spatial scales in the submesoscale range (1–100 km) has been estimated from observations, the associated time scales are largely unconstrained. These time scales can provide important insight into the dynamics of submesoscale turbulence because they help quantify to what degree the flow is subinertial and thus constrained by Earth’s rotation. Here a mooring array is used to estimate these time scales in the northeast Atlantic. Frequency-resolved structure functions indicate that energetic wintertime submesoscale turbulence at spatial scales around 10 km evolves on time scales of about 1 day. While these time scales are comparable to the inertial period, the observed flow also displays characteristics of subinertial flow that is geostrophically balanced to leading order. An approximate Helmholtz decomposition shows the order 10-km flow to be dominated by its rotational component, and the root-mean-square Rossby number at these scales is estimated to be 0.3. This rotational dominance and Rossby numbers below one persist down to 2.6 km, the smallest spatial scale accessible by the mooring array, despite substantially superinertial Eulerian evolution. This indicates that the Lagrangian evolution of submesoscale turbulence is slower than the Eulerian time scale estimated from the moorings. The observations therefore suggest that, on average, submesoscale turbulence largely follows subinertial dynamics in the 1–100-km range, even if Doppler shifting produces superinertial Eulerian evolution. Ageostrophic motions become increasingly important for the evolution of submesoscale turbulence as the scale is reduced—the root-mean-square Rossby number reaches 0.5 at a spatial scale of 2.6 km.

Seasonal variability of tides in the deep northern South China Sea

Tides are the major energy source for ocean mixing, regulating the variation of oceanic circulation and sediment transport in the deep sea. Here twenty months of high-resolution current profiles, which were observed via a mooring system at a water depth of 2100 m in the northern South China Sea (SCS), are used to investigate seasonal variability in deep-sea tides. Spectral analysis shows that tides in this region are dominated by diurnal tide, and both diurnal and semidiurnal tide are vertical mode-1 dominant. Baroclinic diurnal tidal current exhibits pronounced seasonal variability, showing its kinetic energy was the strongest in summer, and the maximum depth-averaged value was up to 86.7 cm2 s−2, which was about 1.5 times of that in winter and twice that in spring and autumn. In contrast, baroclinic semidiurnal tide displays no evident seasonal variability. Such seasonal variability in baroclinic tide was mainly modulated by the barotropic forcing from the Luzon Strait. On the other hand, two anticyclonic eddies and one cyclonic eddy, which originated off southwestern Taiwan in winter, crossed the mooring system. The cyclonic eddy had weak impact on current velocity in the deep sea, but the two deep-reaching anticyclonic eddies enhanced the current velocity through the full-water column by inducing strong subinertial flows. Consequently, the kinetic energy of tides was strengthened and the incoherent variance of baroclinic diurnal tide increased in winter, which contributed ~85% of the variability in diurnal tide. Meanwhile, the velocity of baroclinic diurnal tide was reduced in winter, which was attributed to the weakened stratification induced by the passage of anticyclonic eddies in the deep sea. The seasonal variability of tides in the deep northern SCS can provide a dynamic mechanism for interpreting sediment transport processes in the deep sea on different time scales.

A conceptual view on inertial internal waves in relation to the subinertial flow on the central west Florida shelf

The study reported here focuses on inertial internal wave currents on the west Florida midshelf in 50 m depth. In situ observations showed that the seasonal shifts in stratification change both the frequency range of inertial internal waves and their modulation time scales. According to the analysis, the subinertial flow evolution time scales also undergo compatible seasonal variations, and the inertial internal wave currents appear to be temporally and spatially related to the subinertial flow. Specifically, the subinertial flow evolving on frontal-/quasi-geostrophic time scales appears to be accompanied by the near-inertial oscillations/inertia-gravity waves in corresponding small/finite Burger number regimes, respectively. The quasi-geostrophic subinertial currents on the west Florida shelf are probably associated with the synoptic wind-forced flow, whereas the frontal-geostrophic currents are related to the evolution of density fronts. Further details of this conceptual view should, however, be elucidated in the future.

Observations of Shelf Exchange and High‐Frequency Variability in an Alaskan Fjord

AbstractHigh‐frequency observations collected over 3 years are used to describe upper‐ocean variability in Behm Canal, a nonglacial fjord in Southeast Alaska. The fjord is sheltered by surrounding topography and connected to the outer continental shelf by a 400‐m‐deep strait. Summer conditions are characterized by strong near‐surface stratification, a sea breeze wind regime, and tidally dominated flows. In nonsummer months, baroclinic subinertial flows exceeding 0.5 m/s dominate the velocity record. The flow events represent a wind‐driven response to low‐pressure systems that impact the coast as they propagate across the Gulf of Alaska. The observations suggest that the storm systems generate downwelling events that propagate into the fjord with a mode‐one‐like vertical structure in the upper 60 m. Following the initial up‐fjord current pulse lasting approximately a day, a down‐fjord flow occurs lasting several days, the duration of the downwelling anomaly. These subinertial downwelling events likely are the dominant mechanism of shelf‐fjord exchange, with estimated fjord‐flushing times of 50 days. The downwelling events are accompanied by enhanced near‐inertial shear, which exhibits downward energy propagation. Evidence for enhanced energy near the maximum buoyancy frequency in the thermocline suggests high‐frequency internal wave trapping, with the primary energy source in the semidiurnal band associated with tidal and wind forcing. The observations highlight the importance of shelf‐fjord coupling through meteorological forcing and the existence of internal wave energy at a range of frequencies, which have implications for mixing and transport within the fjord.

Turbulence in the Sicily Channel from microstructure measurements

Small scale turbulence in the two main deep passages of the Sicily Channel was characterised for the first time with microstructure measurements collected during four cruises spanning a two year period. Large turbulent kinetic energy dissipation rates (ε) were observed, with averaged values below the mixed layer reaching 10−7 W kg−1, confirming that the Sicily Channel is a hotspot for turbulence. Contrasted depth-averaged ε were observed between the two passages below the mixed layer: enhanced ε in the northeastern passage ranging from 1.3 × 10−8 - 2.7 × 10−7 W kg−1 over the different cruises, and much weaker ε in the southwestern passage ranging from 3.5 × 10−9 to 7.7 × 10−9 W kg−1. This contrast in ε occurs due to a stronger deep flow at the northeastern passage, resulting in larger shear and stronger turbulence. Internal tides act as another important source of turbulence in both passages, modulating the subinertial flow and inducing shear instabilities. Enhanced turbulence was also revealed by additional measurements made downstream (with respect to the deep flow) in the northeastern passage towards the deeper Tyrrhenian Sea, as dense waters overflow above steep topography.A wave-wave parameterisation was tested for ε, which showed a reasonable consistency for the less turbulent southwestern passage, but not for the more turbulent northeastern passage, suggesting a difference in the mechanism of turbulence.

Effect of surface mesoscale eddies on deep-sea currents and mixing in the northeastern South China Sea

Recent studies suggest that deep-reaching surface-generated eddies result in anomalous current velocities in the deep sea, and ultimately lead to energy transfer from mesoscale to small-scale motions. Here we examine the influence of mesoscale eddies on deep-sea subinertial and near-inertial currents, and on possible enhanced oceanic mixing in the deep South China Sea (SCS). We analyzed current velocity data for nearly a full water column. Data were obtained using acoustic Doppler current profilers and recording current meters on a deep-sea mooring system at a depth of 2100m in the northeastern SCS from October 2012 to May 2013. A highly nonlinear southwestward-propagating anticyclonic eddy was detected via a resolved sea-surface-level anomaly. This eddy induced pronounced subinertial currents with a characteristic time scale of 1–2 months and a maximum velocity of up to 0.2ms–1 at the subsurface and 0.1ms–1 at great depth. Near-inertial energy co-occurring with subinertial flows showed a distinctive vertical propagation trend during strong subinertial oscillations in the deep sea. During periods of strong subinertial and near-inertial kinetic energy, estimates of diapycnal diffusivity in the deep ocean showed approximately 10-fold enhancement, with a mean value of 1.2×10–3m2s–1 compared to the background value of 1.4×10–4m2s–1. The results provide observational evidence of the effect of surface-observed mesoscale motions on benthic currents and ocean mixing in the deep SCS.

Eddy-Modulated Internal Waves and Mixing on a Midocean Ridge

Abstract Mesoscale eddies are ubiquitous in the World Ocean and dominate the energy content on subinertial time scales. Recent theoretical and numerical studies suggest a connection between mesoscale eddies and diapycnal mixing in the deep ocean, especially near rough topography in regions of strong geostrophic flow. However, unambiguous observational evidence for such a connection has not yet been found, and it is still unclear what physical processes are responsible for transferring energy from mesoscale to small-scale processes. Here, the authors present observations demonstrating that finescale variability near the crest of the East Pacific Rise is strongly modulated by low-frequency geostrophic flows, including those due to mesoscale eddies. During times of strong subinertial flows, the authors observed elevated kinetic energy on vertical scales <50 m and in the near-inertial band, predominantly upward-propagating near-inertial waves, and increased incidence of layers with Richardson number . In contrast, during times of weak subinertial flows, kinetic energy in the finescale and near-inertial bands is lower, Ri values are higher, and near-inertial waves propagate predominantly downward through the water column. Diapycnal diffusivities estimated indirectly from a simple Ri-based parameterization are consistent with results from a tracer-release experiment and a microstructure survey bracketing the mooring measurements. These observations are consistent with energy transfer (a “cascade”) from subinertial flows, including mesoscale eddies, to near-inertial oscillations, turbulence, and mixing. This interpretation suggests that, in addition to topographic roughness and tidal forcing, parameterization of deep-ocean mixing should also take subinertial flows into account. The findings presented here are expected to be useful for validating and improving numerical-model parameterizations of turbulence and mixing in the ocean.

Spatial and temporal variability of tidal flow in the Strait of Gibraltar

Recent observations collected at different places in the Strait of Gibraltar are used to investigate the temporal and spatial variability of tidal currents in this region. The analysis of a five-year long time series of velocity observations at the Espartel sill (western end of the strait) shows that harmonic constants fluctuate seasonally exhibiting smaller amplitude in winter. This fact, along with an increased subinertial flow during the winter induces a marked decrease in the relative importance of the tide to the total flow compared to the summer. New computations of tidal transport at the key sections of Espartel and Camarinal, together with historical information reported for the Eastern Exit of the strait, have been analyzed jointly to highlight the internal along-strait divergence of the tidal transport in each layer and the transfer of the tidal signal from one layer to the other. This study covers the whole length of the strait, thus extending previous results reported for the central-eastern strait. Most of the topographically forced divergence is accounted for by large vertical displacements of the interface, although velocity observations collected on the continental shelf of the northern strait suggest that coastal recirculation plays some role in the volume conservation at tidal frequencies.

Analyzing the tidal‐related origin of subinertial flows through the Strait of Gibraltar

The effects of tidal dynamics on subinertial flows through the Strait of Gibraltar are analyzed. As found in previous studies, an empirical orthogonal function analysis of subinertial currents at the Camarinal Sill yields two dominant oscillation modes. The first mode presents a barotropic character and rather irregular fluctuations and it has been related to meteorological forcing. The second mode is baroclinic and presents a clear deterministic behavior with time that seems to be related to tidal forcing. Against the hypothesis proposed in previous studies stating that tidal mixing cycles explain the second mode, we show, by using a one‐dimensional numerical model of two‐layer immiscible shallow water, that the origin of this mode may basically be related to nonlinear interactions among the main semidiurnal tidal constituents through the advective terms in the momentum balance and other nonlinear terms in the volume conservation equations. That mode is also crucial to understanding the vertical shear time variations of the horizontal currents. In particular, it minimizes the differences in the maximum shear between neap and spring tides.

Dispersion of a tracer on the East Pacific Rise (9°N to 10°N), including the influence of hydrothermal plumes

On 12 November 2006, 3 kg of sulfur hexafluoride were released in a 1.2 km long streak in the axial summit trough of the East Pacific Rise at 9°30′N to study how circulation and mixing affect larval dispersion. The first half of a tracer survey performed approximately 40 days after the injection found a small percentage of the tracer on the ridge axis between 9°30′N and 10°10′N, with the main concentration near 9°50′N, a site of many active hydrothermal vents. These observations provide evidence of larval connectivity between vent sites on the ridge. The latter half of the survey detected the primary patch of tracer west of the ridge and just south of the Lamont Seamounts, as a majority of the tracer had been transported off the ridge. However, by the end of the survey, the eastern edge of this patch was transported back to within 10 km of the ridge crest at 9°50′N by a reversal in the subinertial flow, suggesting another pathway for larvae between points along the ridge. Both the horizontal and vertical distributions of the tracer were complex and were likely heavily influenced by topography and vents in the area. Elevated tracer concentrations within the axial summit trough and an adjacent depression on the upper ridge flank suggest that tracers may be detained in such depressions. Correlated tracer/turbidity profiles provide direct evidence of entrainment of the tracer into vent plumes from 9°30′N to 10°N. A comparison of the vertical tracer inventory with neutral density vent-plume observations suggests that on the order of 10% of the tracer injected was entrained into vent plumes near the injection site. The results imply that effluent from diffuse hydrothermal sources and larvae of hydrothermal vent fauna can be entrained in significant quantities into plumes from discrete sources and dispersed in the neutrally buoyant plumes.

Meteorologically forced subinertial flows and internal wave generation at the main sill of the Strait of Gibraltar

The generation of large-amplitude internal waves in the Strait of Gibraltar is a widely known phenomenon. Those waves are produced by the interaction of barotropic tidal flow with the main sill (Camarinal Sill) topography and the stratified water column. That interaction primarily causes internal tides that evolve, by non-linear processes, into large-amplitude (more than 100 m) internal waves exhibiting much shorter oscillation periods than those related to the basic tidal variability. Recent observations have shown that on many occasions large-amplitude internal wave generation is dependent on the state of the subinertial flows, which are basically driven by the atmospheric pressure fluctuations over the Mediterranean. Therefore, depending on the meteorological situation over the Mediterranean, internal wave events may be inhibited or activated.

Observation of Luzon Strait transport

Using recently collected current and hydrographic data, we provide a high resolution picture of the subinertial flow and estimate the volume transport through the Luzon Strait. The distribution of the subinertial flow shows a strong westward flow around 100 m in the northern part of the Luzon Strait, while the eastward flow is confined to the deeper layers, mostly at depths around 1000 m. The total volume transport is estimated to be 6 ± 3 Sv during the period of observations from October 4 to 16, 2005. The observations also confirm that the Luzon Strait transport has a sandwiched vertical structure. The net westward volume transport in the deep (>1500 m) layer of the Luzon Strait reaches 2 Sv.

Vertical structure of time-dependent currents in a mid-ocean ridge axial valley

Velocity measurements with high vertical resolution, and a two-dimensional linear quasianalytic model for subinertial oscillatory flows, are used to analyze the vertical structure of flow in the axial valley at Endeavour Segment, Juan de Fuca Ridge. At a site away from hydrothermal vents, observed semidiurnal flows are independent of depth, rectilinear and parallel to the valley axis, while subinertial flows are intensified and re-aligned along-valley toward the bottom. This behavior is consistent with solutions from the model, which show attenuation of subinertial across-valley flow with depth. This cross-flow attenuation is most pronounced for valleys with widths less than the internal Rossby radius of deformation. Reduction of across-valley flow with depth results in a weakened Coriolis force that cannot fully balance the along-valley pressure-gradient force. The resulting force imbalance yields a directly accelerated bottom-intensified along-valley flow. The importance of this physical process in other submarine valleys depends on their geometry, stratification and latitude. If active, this mechanism provides a dynamic background environment for the axial valley to which hydrothermal venting would add complexity. The strong vertical shears and spiraling flows observed within the axial valley for diurnal tidal and lower-frequency flows have important implications in the transport of hydrothermal vent fluid and the dispersal of larvae of vent organisms by bottom currents.

Ageostrophic fluctuations in Cozumel Channel

The Cayman Current flows to the west, and most of it turns north as it approaches the Yucatan coast, producing a persistent northwesterly flow on both sides of Cozumel Island. The transport between the Yucatan Peninsula and Cozumel Island (i.e., through the Cozumel Channel) is close to 5 Sverdrups in the mean, with velocities at midchannel ranging from 20 to 180 cm/s. A recent study of the subinertial flow and pressure difference across Cozumel Channel by Chávez et al. (2003) showed the existence of periods lasting over 1 month with large 3‐day to 1‐week ageostrophic fluctuations. The flow was measured again for a year, now at four locations around Cozumel Island, including two instruments along the axis of the channel 8.6 km apart, thus allowing estimations of the along‐channel velocity gradients. The new measurements reveal that, as suggested in the previous study, the centripetal or curvature acceleration of the current is the most significant contribution in the departure from geostrophy. Indeed, the curvature is, at times, so large that the pressure difference implies a geostrophic flow in the direction opposite to that of the actual flow; that is, the curvature is anticyclonic with amplitude in Rossby number larger than unity. Measures of the intensity of suprainertial variations, in pressure differences and velocity, show that periods of ageostrophic fluctuations are consistently much richer in high‐frequency fluctuations than periods of nearly geostrophic behavior. Nonetheless, the large‐scale Reynolds stresses play an insignificant role throughout.