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Amit Tandon and Chris Garrett

Abstract

The restratification in the surface mixed layer driven by a horizontal density gradient following a storm is examined. For a constant layer depth H and constant buoyancy gradient |bx| = M 2, geostrophic adjustment leads to new stratification with N 2 = M 4/ f 2 and Richardson number Ri = 1. With the inclusion of time dependence, inertial oscillations result and give Ri = ½. If the horizontal buoyancy gradient is confined, the minimum Ri for an initial distribution of buoyancy b(x) is given by 1 − ½ H|bxx|max/f 2. The resulting maximum restratification is N 2 = M 4/ (f 2− ½ H|bxx). This restratification can be significant in coastal oceans and possibly in some frontal areas of the open ocean.

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Amit Tandon and Chris Garrett

Abstract

The restratification of a mixed layer with horizontal density gradients above a stratified layer is considered. Solutions are obtained on the assumption that the width across this front is much larger than the local radius of deformation Δb/|f| based on the buoyancy change across the front Δb, mean mixed layer depth Rdh̄, and the Coriolis parameter f, where b is defined as Rdg(ρ−ρ00, but the fractional change in the mixed layer depth is not required to be small. For an initially quiescent mixed layer, created by homogenizing a fluid of constant stratification to a depth that varies horizontally, the isopycnals in the mixed layer tilt about their intersections with the top surface in the adjusted state, and the base of the mixed layer flattens slightly in the frontal region. Other cases considered include mixed layer fronts with initial momentum out of geostrophic balance, created by vertical mixing of a layer with horizontal gradients previously in thermal wind balance. For a wide front, the isopycnals pivot about the middepth for this case. In all cases, for a wide front, the new vertical buoyancy gradient is Rd M 4/f 2h̄, where Rd M 2=|b|h̄ is the magnitude of the horizontal buoyancy gradient, and the Richardson number of the adjusted state is 1, as in an earlier constant depth case.

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Amit Tandon and Chris Garrett

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A recent parameterization of mesoscale eddies by Gent and McWilliams (GMc) represents their effects as advective and diffusive fluxes along isopycnals. The form chosen for the added transport velocity due to eddies flattens isopycnals as in baroclinic instability but implicitly assumes purely viscous dissipation of the available potential energy released. If, however, the energy dissipation occurs in the ocean interior due to a process such as internal wave breaking, it is likely to cause diapycnal mixing. The implied diffusivity is large in a frontal situation, but the analysis of the spindown equation for a quasigeostrophic front shows that it causes only small changes in the frontal evolution. The spindown equation also permits analysis of the relative importance of various terms describing subgrid-scale fluxes of momentum and buoyancy, and may be interpreted in terms of Eliassen–Palm fluxes. Another possibility for the dissipation of the eddy energy that is generated from the mean available potential energy in the GMc mechanism involves air–sea interaction and subsequent water mass modification, but this is also clearly diabatic across mean isopycnals. The GMc parameterization does accomplish diabatic transfer across mean isopycnals near the surface due to the boundary conditions on the advective eddy flux, though it is not clear that this is the same as if the effect air–sea interaction on the eddies were treated explicitly. The cross-frontal volume flux must be compatible with the buoyancy budget. In the case of the Southern Ocean, this may require the net meridional circulation cell to be weak if the air–sea buoyancy flux is small.

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Amit Tandon and Sidney Leibovich

Abstract

Finite-amplitude Langmuir circulation in the form of rolls parallel to the wind direction is shown to be subject to three-dimensional instability under certain circumstances. Density stratification is not required for instability to manifest. The preferred form of this secondary instability appears to be traveling waves propagating in the direction of the wind. These cause the rolls, and their surface windrows, to deviate from the wind direction by a small angle for which estimates are given. The results of the paper show the value of secondary stability results for the design of numerical experiments to simulate Langmuir circulation.

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Amit Tandon and Konstantin Zahariev

Abstract

Forcing by wind stress and air–sea buoyancy flux climatologies between σ t = 23.5 and 26.5 results in differing water mass transformations in the North Atlantic, reflecting the opposing tendencies of wind stress and air–sea fluxes. This difference needs to be reconciled in terms of various processes that lead to diapycnal advection and mixing. This study attempts to quantify the contribution of one such process to water mass transformation—the small-scale but ubiquitous process of mixed layer entrainment and deepening. An estimate is computed using formulas developed earlier, the Levitus hydrography, and a mixed layer model forced by observed fluxes. The mixed layer and forcing data are taken from the Marine Light Mixed Layer Experiment mooring, which includes both spring and fall mixed layer transitions. The sensitivity to averaging of synoptic events is also explored. Calculations presented here indicate that, if hourly winds are used, the water mass transformation due to mixed layer entrainment has annual peak contributions of about O(4) Sv for σ t = 24.0 (Sv ≡ 106 m3 s−1). This is comparable to the annual transformation attained by diapycnal mixing in the upper-ocean water masses. However, with daily averaged winds and without diurnal variation in buoyancy forcing, this contribution is up to an order of magnitude smaller. Another set of mixed layer simulations includes an annual cycle with a shallow and strong summer thermocline. Inclusion of synoptic summer forcing for this scenario leads to transformation values several times larger than above, about O(14) Sv at σ t = 24.0. The peak contribution in this case is almost two orders of magnitude smaller if the synoptic forcing is averaged daily and the diurnal cycle is not resolved. These results suggest that the numerical diagnostics using general circulation models may significantly underestimate entrainment mixing if the combination of diurnal variation and synoptic wind events is not resolved or explicitly parameterized.

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Robert A. Weller, Sudip Majumder, and Amit Tandon

Abstract

This paper describes the occurrence of diurnal restratification events found in the southeast trade wind regime off northern Chile. This is a region where persistent marine stratus clouds are found and where there is a less than complete understanding of the dynamics that govern the maintenance of the sea surface temperature. A surface mooring deployed in the region provides surface meteorological, air–sea flux, and upper-ocean temperature, salinity, and velocity data. In the presence of steady southeast trade winds and strong evaporation, a warm, salty surface mixed layer is found in the upper ocean. During the year, these trade winds, at times, drop dramatically and surface heating leads to the formation of shallow, warm diurnal mixed layers over one to several days. At the end of such a low wind period, mean sea surface temperature is warmer. Though magnitudes of the individual diurnal warming events are consistent with local forcing, as judged by running a one-dimensional model, the net warming at the end of a low wind event is more difficult to predict. This is found to stem from differences between the observed and predicted near-inertial shear and the depths over which the warmed water is distributed. As a result, the evolution of SST has a dependency on these diurnal restratification events and on near-surface processes that govern the depth over which the heat gained during such events is distributed.

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Gualtiero Badin, Amit Tandon, and Amala Mahadevan

Abstract

Using a process study model, the effect of mixed layer submesoscale instabilities on the lateral mixing of passive tracers in the pycnocline is explored. Mixed layer eddies that are generated from the baroclinic instability of a front within the mixed layer are found to penetrate into the pycnocline leading to an eddying flow field that acts to mix properties laterally along isopycnal surfaces. The mixing of passive tracers released on such isopycnal surfaces is quantified by estimating the variance of the tracer distribution over time. The evolution of the tracer variance reveals that the flow undergoes three different turbulent regimes. The first regime, lasting about 3–4 days (about 5 inertial periods) exhibits near-diffusive behavior; dispersion of the tracer grows nearly linearly with time. In the second regime, which lasts for about 10 days (about 14 inertial periods), tracer dispersion exhibits exponential growth because of the integrated action of high strain rates created by the instabilities. In the third regime, tracer dispersion follows Richardson’s power law. The Nakamura effective diffusivity is used to study the role of individual dynamical filaments in lateral mixing. The filaments, which carry a high concentration of tracer, are characterized by the coincidence of large horizontal strain rate with large vertical vorticity. Within filaments, tracer is sheared without being dispersed, and consequently the effective diffusivity is small in filaments. While the filament centers act as barriers to transport, eddy fluxes are enhanced at the filament edges where gradients are large.

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Takeyoshi Nagai, Amit Tandon, Eric Kunze, and Amala Mahadevan

Abstract

While near-inertial waves are known to be generated by atmospheric storms, recent observations in the Kuroshio Front find intense near-inertial internal-wave shear along sloping isopycnals, even during calm weather. Recent literature suggests that spontaneous generation of near-inertial waves by frontal instabilities could represent a major sink for the subinertial quasigeostrophic circulation. An unforced three-dimensional 1-km-resolution model, initialized with the observed cross-Kuroshio structure, is used to explore this mechanism. After several weeks, the model exhibits growth of 10–100-km-scale frontal meanders, accompanied by O(10) mW m−2 spontaneous generation of near-inertial waves associated with readjustment of submesoscale fronts forced out of balance by mesoscale confluent flows. These waves have properties resembling those in the observations. However, they are reabsorbed into the model Kuroshio Front with no more than 15% dissipating or radiating away. Thus, spontaneous generation of near-inertial waves represents a redistribution of quasigeostrophic energy rather than a significant sink.

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K. Jossia Joseph, Amit Tandon, R. Venkatesan, J. Thomas Farrar, and Robert A. Weller

Abstract

The inception of a moored buoy network in the northern Indian Ocean in 1997 paved the way for systematic collection of long-term time series observations of meteorological and oceanographic parameters. This buoy network was revamped in 2011 with Ocean Moored buoy Network for north Indian Ocean (OMNI) buoys fitted with additional sensors to better quantify the air–sea fluxes. An intercomparison of OMNI buoy measurements with the nearby Woods Hole Oceanographic Institution (WHOI) mooring during the year 2015 revealed an overestimation of downwelling longwave radiation (LWR↓). Analysis of the OMNI and WHOI radiation sensors at a test station at National Institute of Ocean Technology (NIOT) during 2019 revealed that the accurate and stable amplification of the thermopile voltage records along with the customized datalogger in the WHOI system results in better estimations of LWR↓. The offset in NIOT measured LWR↓ is estimated first by segregating the LWR↓ during clear-sky conditions identified using the downwelling shortwave radiation measurements from the same test station, and second, finding the offset by taking the difference with expected theoretical clear-sky LWR↓. The corrected LWR↓ exhibited good agreement with that of collocated WHOI measurements, with a correlation of 0.93. This method is applied to the OMNI field measurements and again compared with the nearby WHOI mooring measurements, exhibiting a better correlation of 0.95. This work has led to the revamping of radiation measurements in OMNI buoys and provides a reliable method to correct past measurements and improve estimation of air–sea fluxes in the Indian Ocean.

Significance Statement

Downwelling longwave radiation (LWR↓) is an important climate variable for calculating air–sea heat exchange and quantifying Earth’s energy budget. An intercomparison of LWR↓ measurements between ocean observing platforms in the north Indian Ocean revealed a systematic offset in National Institute of Ocean Technology (NIOT) Ocean Moored buoy Network for north Indian Ocean (OMNI) buoys. The observed offset limited our capability to accurately estimate air–sea fluxes in the Indian Ocean. The sensor measurements were compared with a standard reference system, which revealed problems in thermopile amplifier as the root cause of the offset. This work led to the development of a reliable method to correct the offset in LWR↓ and revamping of radiation measurements in NIOT-OMNI buoys. The correction is being applied to the past measurements from 12 OMNI buoys over 8 years to improve the estimation of air–sea fluxes in the Indian Ocean.

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Sanjiv Ramachandran, Amit Tandon, Jennifer Mackinnon, Andrew J. Lucas, Robert Pinkel, Amy F. Waterhouse, Jonathan Nash, Emily Shroyer, Amala Mahadevan, Robert A. Weller, and J. Thomas Farrar

Abstract

Lateral submesoscale processes and their influence on vertical stratification at shallow salinity fronts in the central Bay of Bengal during the winter monsoon are explored using high-resolution data from a cruise in November 2013. The observations are from a radiator survey centered at a salinity-controlled density front, embedded in a zone of moderate mesoscale strain (0.15 times the Coriolis parameter) and forced by winds with a downfront orientation. Below a thin mixed layer, often ≤10 m, the analysis shows several dynamical signatures indicative of submesoscale processes: (i) negative Ertel potential vorticity (PV); (ii) low-PV anomalies with O(1–10) km lateral extent, where the vorticity estimated on isopycnals and the isopycnal thickness are tightly coupled, varying in lockstep to yield low PV; (iii) flow conditions susceptible to forced symmetric instability (FSI) or bearing the imprint of earlier FSI events; (iv) negative lateral gradients in the absolute momentum field (inertial instability); and (v) strong contribution from differential sheared advection at O(1) km scales to the growth rate of the depth-averaged stratification. The findings here show one-dimensional vertical processes alone cannot explain the vertical stratification and its lateral variability over O(1–10) km scales at the radiator survey.

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