H. J. S. Fernando

Observations and scaling of the upper mixed layer in the North Atlantic

[1]xa0The dependence of the mixed layer depth hD on the sea surface fluxes is analyzed based on measurements taken along a cross-Atlantic section 53°N. A linear function hD ≈ 0.44Lf, where Lf = u*/f is the Ekman scale, well represents the influence of the wind stress u* and rotation f on the mixed-layer deepening, thus indicating that the influence of convective mixing in late spring at this latitude is of a lesser importance. Also, data showed reasonable correlation of hD with the stratified Ekman scale LfN = u*/, where Npc is the buoyancy frequency in the pycnocline, according to hD ≈ 1.9LfN. In both cases the highest correlation between hD and the corresponding lengthscales is achieved when u* values taken 12 hours in advance of the mixed layer measurements were used, which may signify the adjustment time of inertial oscillations to produce critical shear at the base of the mixed layer. The vertical profiles of the dissipation rate ɛ(z) are parameterized by two formulae that are based on the law of the wall scaling ɛs(z) = u*3/0.4z and the buoyancy flux Jb: ɛ1(z) = 2.6ɛs(z) + 0.6Jb and ɛ2(z) = ɛs(z) + 3.7Jb. The first parameterization is used to calculate the integrated dissipation int over the mixing layer, which was found to be ∼3–7% (5% on the average) of the wind work E10. The positive correlation between hD and int/E10 suggests that in deeper quasi-homogeneous layers a larger portion of the wind work is consumed by viscous dissipation vis-a-vis that is used for entrainment. As such, the mixing efficiency, which is based on integral quantities, is expected to decrease with the growth of the mixed layer.

Spatial decay of energy density of tidal internal waves

[1]xa0The spatial decay of energy density of tidal internal waves (TIW) was studied using field data taken in the Indian Ocean near the Mascarene Ridge and in the Canary Basin of the eastern Atlantic near the Heyres-Irving-Cruiser chain of seamounts. Several moorings were deployed at distances between 90 and 1745 km east of these topographic features, with instruments located in the depth range 500–2500 m. The energy densities of TIW averaged over the spring-neap cycle were calculated using semidiurnal tidal components of current and temperature time series as well as local vertical gradients of temperature and density. It was found that the horizontal component of TIW, EH, is less depth-dependent compared to the vertical component, Eζ, although both components showed a general decrease of magnitude with the distance from topography. The decrease of total energy density ETW = EH + Eζ with distance from the topography is more rapid than that assumed in the work of Morozov [1995], and followed an inverse power law. At a distance of about x ≈ 10λ from the topography (where λ is the wavelength of the first mode), ETW in the main thermocline becomes equal to the energy density of the forcing barotropic tide, whereas for x/λ < 2, ETW exceeds the energy of the entire range of internal waves of the Garrett-Munk spectrum. A nonhydrostatic, nonlinear, two-dimensional numerical model shows a reasonable agreement with the observations for x/λ < 2–3, but in the far field it predicts a faster spatial decay of ETW than observed, possibly because of topographic generation of TIW along the measurement swath. The turbulent diffusivity estimates based on the McComas and Muller [1981] model exceeded 10−4 m2/s within the main pycnocline at x = 100 km and suggest mixing enhancements due to TIW up to distances of 1000 km from the topography.

Episodes of nonlinear internal waves in the northern East China Sea

[1]xa0Episodes of high-frequency internal waves, which lasted approximately 3 hours, were detected in the northern East China Sea during a specific phase of the barotropic tide (i.e., low tide at 32°N, 125°E). The observed internal waves influenced the whole water column. The wave packets were presumably generated near the ocean shelf break, approximately 200 km to the southeast of the test site. During several internal wave episodes, which coincided with the neap tide, large-amplitude solitary wave-like features emerged preceding higher frequency internal waves. Shear instability of tidal currents is explored as a possible mechanism for sustaining or regenerating internal waves in the packets during the course of their propagation. It is suggested that rotating velocity field of tidal current supports sufficient vertical shear within wave packets to cause outbreaks of K-H instability. These instabilities may gradually transition to more symmetric Holmboe waves, following the increase of the bulk Richardson number.

Intermittency of near‐bottom turbulence in tidal flow on a shallow shelf

U.S. Office of Naval Research [N00014-05-1-0245]; Spanish Ministry of Education and Science [FIS2008-03608]; Major State Program of China for Basic Research [2006CB400602]; Catalan Institute for Water Research (ICRA)

Arctic Ocean mixed‐layer eddy generation under leads in sea ice

[1]xa0Convection in the Arctic Ocean mixed layer occurs under openings in sea ice where new sea ice is formed. The process of brine rejection during ice formation creates negatively buoyant fluid at the surface of ice leads. When the fluid convects downward the ocean mixed layer below the lead is stirred vertically and, through planetary rotational effects, along-lead flows at the lead edges are induced. These vertical and lateral motions have been previously studied using two-dimensional numerical ocean models and observed in field programs such as LeadEx. In this paper the generation of Arctic Ocean eddies through dynamic instabilities is investigated using a three-dimensional, nonhydrostatic ocean model. Results suggest that salt-enriched vortices can develop at the base of the mixed layer as a consequence of brine rejection, but the vortex formation requires a time period of several days, a time long enough that the progenitor lead would likely have closed. The scale of the eddies is found to depend on the magnitude of the buoyancy forcing, the width of the lead, and the duration of the buoyancy forcing. The number of vortices produced depends on lead width and duration of buoyancy forcing but is less sensitive to the magnitude of the buoyancy forcing. Vortex sizes, scaled appropriately, and vortex numbers are compared with those found in recent laboratory studies of convectively driven flows from line segment sources.

Lead-induced convection: a laboratory perspective

Abstract The polar ice cap often cracks and forms long, narrow channels of open water which are known as leads. These openings play an important role in the heat budget and circulation of polar oceans. In the winter opening of a lead is associated with the onset of turbulent convection above and below the lead, because of the refreezing of surface water and rejection of heat into the atmosphere, respectively. Lead-induced convection is associated with a rich variety of fluid-dynamical phenomena, yet studying them in filed situations is not easy due to the unpredictability of their occurrence and short resident times. As such, laboratory experiments and numerical models can play a major role in lead-related studies. The purpose of this paper is to review the results of some previous laboratory experiments, which appear to be of importance in guiding and interpreting field experiments, and to present the results of some new laboratory experiments dealing with lead-induced motions.

Low-Frequency Currents from Deep Moorings in the Southern Bay of Bengal

AbstractHigh-resolution currents and hydrographic fields were measured at six deep-water moorings in the southern Bay of Bengal (BoB) by the Naval Research Laboratory as part of an international effort focused on the dynamics of the Indian Ocean. Currents, temperature, and salinity were sampled over the upper 500 m for 20 months between December 2013 and August 2015. One of the major goals is to understand the space–time scales of the currents and physical processes that contribute to the exchange of water between the BoB and the Arabian Sea. The observations captured Southwest and Northeast Monsoon Currents, seasonally varying large eddies including a cyclonic eddy, the Sri Lanka dome (SLD), and an anticyclonic eddy southeast of the SLD. The observations further showed intraseasonal oscillations with periods of 30–70 days, near-inertial currents, and tides. Monthly averaged velocities commonly exceeded 50 cm s−1 near the surface, and extreme velocities exceeded 150 cm s−1 during the southwest monsoon. Ti...

Probability Distribution of Turbulent Kinetic Energy Dissipation Rate in Ocean: Observations and Approximations

The probability distribution of turbulent kinetic energy dissipation rate in stratified ocean usually deviates from the classic lognormal distribution that has been formulated for and often observed in unstratified homogeneous layers of atmospheric and oceanic turbulence. Our measurements of vertical profiles of micro-scale shear, collected in the East China Sea, northern Bay of Bengal, to the south and east of Sri Lanka, and in the Gulf Stream region, show that the probability distributions of the dissipation rate ɛ∼r in the pycnoclines (r ∼ 1.4 m is the averaging scale) can be successfully modeled by the Burr (type XII) probability distribution. In weakly stratified boundary layers, lognormal distribution of ɛ∼r is preferable, although the Burr is an acceptable alternative. The skewness Skɛ and the kurtosis Kɛ of the dissipation rate appear to be well correlated in a wide range of Skɛ and Kɛ variability.

A snapshot of internal waves and hydrodynamic instabilities in the southern Bay of Bengal

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 10–40 min were recorded at all depths below a shallow ( 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 5 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 25%.