Decay Mechanisms of Near-Inertial Mixed Layer Oscillations in the Bay of Bengal (original) (raw)
Related papers
Near-inertial motions in the coastal ocean
1995
Internal-inertial waves are frequently observed in the upper ocean and below the thermocline. A three-dimensional general circulation model with turbulence-closure mixed layer is used to study the generation and propagation of near-inertial motion below the mixed layer. In particular, the problem of effect of a coastal wall on the wind induced inertial-internal wave field is re-examined using a fully nonlinear model. Responding to a wind pulse, a sharp wavefront propagates offshore. After the wavefront passage, strong near-inertial internal waves, marked by the tilting velocity isolines and the interface oscillations, are generated. The predicted near-inertial motion is consistent with the wave dispersion relation. Downward energy propagation occurs after the wavefront passage, and both kinetic and potential energy are strongly modified. After several inertial periods, the kinetic energy in the upper layer can be completely removed. The theoretical results, which are supported by observations, indicate that internal-inertial wave are important for mixing in the upper coastal ocean. 0924-7963/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO924-7963(94)00030-l
Generation, propagation and dissipation of near-inertial waves in the tropical ocean
2015
Several aspects of the generation, propagation and dissipation of near-inertial waves were examined for tropical regions using a suite of numerical models and observational data. The primary goals were to investigate how the wind input of inertial kinetic energy partitions between loss by turbulent dissipation at the base of the mixed layer versus downward radiation of near-inertial waves, how deep the near-inertial energy penetrates into the interior, and how the background circulation and stratification impact the radiation of near-inertial waves. Results from a 1D model for upper ocean turbulence indicate that, in the eastern tropical Pacific, approximately 50% of the energy input by the wind is radiated downwards into the thermocline as near-inertial waves, despite displaying significant variability between forcing events. Estimates of the vertical energy flux for near-inertial wave packets observed in data collected in the tropical Indian Ocean are in general agreement with the...
The Impact of Oceanic Near-Inertial Waves on Climate
Journal of Climate, 2013
The Community Climate System Model, version 4 (CCSM4) is used to assess the climate impact of wind-generated near-inertial waves (NIWs). Even with high-frequency coupling, CCSM4 underestimates the strength of NIWs, so that a parameterization for NIWs is developed and included into CCSM4. Numerous assumptions enter this parameterization, the core of which is that the NIW velocity signal is detected during the model integration, and amplified in the shear computation of the ocean surface boundary layer module. It is found that NIWs deepen the ocean mixed layer by up to 30%, but they contribute little to the ventilation and mixing of the ocean below the thermocline. However, the deepening of the tropical mixed layer by NIWs leads to a change in tropical sea surface temperature and precipitation. Atmospheric teleconnections then change the global sea level pressure fields so that the midlatitude westerlies become weaker. Unfortunately, the magnitude of the real air-sea flux of NIW energ...
A strong October storm generated 0.35-0.7 m/s inertial frequency currents in the 35 m deep mixed layer of a 300x300 km region of the northeast Pacific Ocean. The evolution of these currents were observed for a 23 day period of weak winds following the storm using a combination of 36 surface drifters drogued at 15 m and 3 moorings with acoustic Doppler velocity profilers. These observation, plus a CTD survey, were used to describe the subinertial, geostrophic flow in this same region. Then it was tested whether the linear internal wave theory combined with advection by the measured subinertial currents can explain the observed evolution of the inertial frequency currents.
Shear Flow Instabilities and Unstable Events Over the North Bay of Bengal
Journal of Geophysical Research: Oceans, 2018
A year-long mooring data are used to study the upper ocean unstable events and instabilities at 18 ∘ N 89 ∘ E, which is a climatologically important region in the North Bay of Bengal. Near-surface stability is studied from the context of the buoyancy frequency normalized shear (V z ∕N) and reduced shear (S 2 − 4N 2) which are convenient measures to quantify flow stability, compared to the more widely used Richardson number (Ri). The analysis is carried out across three contrasting time periods, the monsoon, postmonsoon, and the winter of year 2012. Although it is well known that the flow stability changes from stable to unstable at Ri = Ri cr = 0.25, the relative importance of the perturbations of shear and buoyancy frequency in driving the unstable events is not well studied over the open oceans and more particularly over the Bay of Bengal. At 18 ∘ N, 89 ∘ E both higher than average shear and lower than average buoyancy frequency perturbations are crucial in driving the unstable events during the summer and premonsoon period. However, at increasing depths, the influence of shear perturbations becomes more dominant. Invoking the Miles-Howard criteria for flow instability, it is seen that during the postmonsoon period, the buoyancy frequency perturbations are more critical than shear perturbations in driving the unstable events. In winter, the unstable events are influenced by both the buoyancy frequency and shear perturbations. Plain Language Summary This study is about upper ocean mixing in the Bay of Bengal. The Bay of Bengal is one of the least explored when compared with other oceans and seas. It is peculiar in the sense that it is landlocked from three directions. The Indian subcontinent is the home for more than a billion people and the economy crucially depends on the monsoons. Understanding near-surface air-sea interaction processes would help give fundamental insight into the weather over the subcontinent. The last decade was witness to a number of buoys that were deployed in the Bay of Bengal to measure key meteorological and ocean parameters. The Bay of Bengal is characterized by a strong and shallow stratification. This inhibits mixing. Lack of mixing has strong ramifications. Some of the mechanisms that inhibit or drive the near-surface mixing are studied. This study is of fundamental importance and has relevance to weather modeling and air-sea interaction across a broad scales of motion.
Radiation of Mixed Layer Near-Inertial Oscillations into the Ocean Interior
Journal of Physical Oceanography, 2001
The radiation from the mixed layer into the interior of the ocean of near-inertial oscillations in the presence of the beta effect is reconsidered as an initial-value problem. Making use of the fact that the mixed layer depth is much smaller than the total depth of the ocean, the solution is obtained in the limit of an ocean that is effectively infinitely deep. For a uniform initial condition, analytical results for the velocity, horizontal kinetic energy density, and fluxes are obtained. This is the canonical solution for the radiation of near-inertial oscillations in the vertical, which captures the basic mechanisms due to the beta effect, and leads to the formation of small scales in the vertical. By superposing events, an average vertical wavenumber spectrum is constructed. The predicted decay of near-inertial mixed layer energy in the presence of the beta effect occurs on a timescale similar to that observed.
A snapshot of internal waves and hydrodynamic instabilities in the southern Bay of Bengal
Journal of Geophysical Research: Oceans, 2016
Measurements conducted in the southern Bay of Bengal (BoB) as a part of the ASIRI-EBoB Program portray the characteristics of high-frequency internal waves in the upper pycnocline as well as the velocity structure with episodic events of shear instability. A 20 h time series of CTD, ADCP, and acoustic backscatter profiles down to 150 m as well as temporal CTD measurements in the pycnocline at z 5 54 m were taken to the east of Sri Lanka. Internal waves of periods 10−40minwererecordedatalldepthsbelowashallow(10-40 min were recorded at all depths below a shallow (10−40minwererecordedatalldepthsbelowashallow(20-30 m) surface mixed layer in the background of an 8 m amplitude internal tide. The absolute values of vertical displacements associated with high-frequency waves followed the Nakagami distribution with a median value of 2.1 m and a 95% quintile 6.5 m. The internal wave amplitudes are normally distributed. The tails of the distribution deviate from normality due to episodic high-amplitude displacements. The sporadic appearance of internal waves with amplitudes exceeding 5musuallycoincidedwithpatchesoflowRichardsonnumbers,pointingtolocalshearinstabilityasapossiblemechanismofinternalwave−inducedturbulence.TheprobabilityofshearinstabilityinthesummerBoBpycnoclinebasedonanexponentialdistributionoftheinverseRichardsonnumber,however,appearstoberelativelylow,notexceeding45 m usually coincided with patches of low Richardson numbers, pointing to local shear instability as a possible mechanism of internalwave-induced turbulence. The probability of shear instability in the summer BoB pycnocline based on an exponential distribution of the inverse Richardson number, however, appears to be relatively low, not exceeding 4% for Ri < 0.25 and about 10% for Ri < 0.36 (K-H billows). The probability of the generation of asymmetric breaking internal waves and Holmboe instabilities is above 5musuallycoincidedwithpatchesoflowRichardsonnumbers,pointingtolocalshearinstabilityasapossiblemechanismofinternalwave−inducedturbulence.TheprobabilityofshearinstabilityinthesummerBoBpycnoclinebasedonanexponentialdistributionoftheinverseRichardsonnumber,however,appearstoberelativelylow,notexceeding425%. To fill this gap, an array of six moorings was deployed in December 2013 to collect long-term (more than a year and a half) oceanographic data throughout the upper 450 m of the water column, supplemented by the first detailed ADCP survey in the southern BoB [Wijesekera et al., 2015]. In July 2014, a series of highspatial-resolution sections of CTD (ScanFish) and ADCP measurements were carried out in the mooring region, between 5815 0-8812 0 N and 85815 0-86825 0 E, to quantify advection and mixing associated with the Summer Monsoon Current, freshwater-saltwater exchanges, and internal wave forcing. Allusion to internal waves in the Indian Ocean dates back to the mid-nineteenth century [Maury, 1861]. Internal waves in the BoB are affected by the rough bathymetry, seasonal wind forcing, and strong stratification, which govern both the generation and breakdown of internal waves. In particular, the generation of internal waves due to interaction of barotropic tide with bathymetry has been reported over shallow gaps between Andaman and
Deep Sea Research Part I: Oceanographic Research Papers, 2014
An autonomous upwardly-moving microstructure profiler was used to collect measurements of the rate of dissipation of turbulent kinetic energy (ε) in the tropical Indian Ocean during a single diurnal cycle, from about 50 m depth to the sea surface. This dataset is one of only a few to resolve upper ocean ε over a diurnal cycle from below the active mixing layer up to the air-sea interface. Wind speed was weak with an average value of $ 5 m s À 1 and the wave field was swell-dominated. Within the wind and wave affected surface layer (WWSL), ε values were on the order of 10 À 7 -10 À 6 W kg À 1 at a depth of 0.75 m and when averaged, were almost a factor of two above classical law of the wall theory, possibly indicative of an additional source of energy from the wave field. Below this depth, ε values were closer to wall layer scaling, suggesting that the work of the Reynolds stress on the wind-induced vertical shear was the major source of turbulence within this layer. No evidence of persistent elevated near-surface ε characteristic of wave-breaking conditions was found. Profiles collected during night-time displayed relatively constant ε values at depths between the WWSL and the base of the mixing layer, characteristic of mixing by convective overturning. Within the remnant layer, depth-averaged values of ε started decaying exponentially with an e-folding time of 47 min, about 30 min after the reversal of the total surface net heat flux from oceanic loss to gain.
Near-inertial waves and deep ocean mixing
Physica Scripta, 2013
For the existing pattern of global oceanic circulation to exist, there should be sufficiently strong turbulent mixing in the abyssal ocean, the mechanisms of which are not well understood as yet. The review discusses a plausible mechanism of deep ocean mixing caused by near-inertial waves in the abyssal ocean. It is well known how winds in the atmosphere generate near-inertial waves in the upper ocean, which then propagate downwards losing their energy in the process; only a fraction of the energy at the surface reaches the abyssal ocean. An open question is whether and, if yes, how these weakened inertial motions could cause mixing in the deep. We review the progress in the mathematical description of a mechanism that results in an intense breaking of near-inertial waves near the bottom of the ocean and thus enhances the mixing. We give an overview of the present state of understanding of the problem covering both the published and the unpublished results; we also outline the key open questions. For typical ocean stratification, the account of the horizontal component of the Earth's rotation leads to the existence of near-bottom wide waveguides for near-inertial waves. Due to the β-effect these waveguides are narrowing in the poleward direction. Near-inertial waves propagating poleward get trapped in the waveguides; we describe how in the process these waves are focusing more and more in the vertical direction, while simultaneously their group velocity tends to zero and wave-induced vertical shear significantly increases. This causes the development of shear instability, which is interpreted as wave breaking. Remarkably, this mechanism of local intensification of turbulent mixing in the abyssal ocean can be adequately described within the framework of linear theory. The qualitative picture is similar to wind wave breaking on a beach: the abyssal ocean always acts as a surf zone for near-inertial waves.
Journal of Geophysical Research, 1998
Patterns in sea surface temperature (SST) on 5-km scales were observed from low-flying research aircraft on a light wind day during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. An inverse trend was observed between the SST and the sea surface mean square slope (mss). However, low correlation coefficients indicate that the dominant process causing the spatial variation of SST under these light wind conditions is neither well controlled by the wind speed nor well monitored by the mss. The SST spatial pattern persisted for at least 1 hour and propagated toward the NE at about 1 m s -•, a factor of 1.6 faster than the speed of the surface current. Coupling between internal gravity waves propagating on the seasonal thermocline and the diurnal surface layer is examined as a possible explanation for the observed SST variability in space and time.