Michael C. Gregg

Surface mixed and mixing layer depths

Abstract In order to understand the daily cycle of heat storage within the surface mixed layer it is necessary to distinguish between the mixed layer, the zone of relatively homogeneous water formed by the history of mixing, and the mixing layer, the zone in which mixing is currently active. We compare surface layer definitions based on density (or temperature) with turbulence measurements to evaluate their skill in finding mixed and mixing layer depths, using definitions based on density increase from the surface, and on density gradients. Both types of definition are capable of finding the mixed layer depth, with some tuning for local conditions. Neither definition, however, gives mixing layer depths consistently matching the turbulence measurements, although density differences give more stable results. Measurements of turbulent dissipation rates or overturning length scales often yield consistent estimates of mixing layer depths, but there are cases where overturning lengths give distinctly better results. We conclude that overturning length scales give the most reliable measure of mixing layer depth, although conventional shipborne CTDs are seldom capable of sufficiently resolving the overturns.

Reduced mixing from the breaking of internal waves in equatorial waters

In the oceans, heat, salt and nutrients are redistributed much more easily within water masses of uniform density than across surfaces separating waters of different densities. But the magnitude and distribution of mixing across density surfaces are also important for the Earths climate as well as the concentrations of organisms. Most of this mixing occurs where internal waves break, overturning the density stratification of the ocean and creating patches of turbulence. Predictions of the rate at which internal waves dissipate were confirmed earlier at mid-latitudes. Here we present observations of temperature and velocity fluctuations in the Pacific and Atlantic oceans between 42° N and 2° S to extend that result to equatorial regions. We find a strong latitude dependence of dissipation in accordance with the predictions. In our observations, dissipation rates and accompanying mixing across density surfaces near the Equator are less than 10% of those at mid-latitudes for a similar background of internal waves. Reduced mixing close to the Equator will have to be taken into account in numerical simulations of ocean dynamics—for example, in climate change experiments.

Comparison of Turbulence Kinetic Energy Dissipation Rate Estimates from Two Ocean Microstructure Profilers

Abstract Almost 1000 microstructure profiles from two separate groups on two separate ships using different instrumentation, signal processing, and calibration procedures were compared for a 3.5-day time period at 0°, 140°W and within 11 km of each other. Systematic bias in the estimates of ϵ is less than a factor of 2, which is within estimates of the cumulative uncertainties in the measurement of ϵ. Although there is no evidence for strong gradients in mean currents, water properties, or surface meteorology, occasional hourly averages of ϵ differ by several factors of 10. Both groups observed periods where ϵ estimates exceeded those of the other group by large factors. The authors believe that the primary reason for these large differences is natural variability, which appears to be greater in the meridional direction than in the zonal direction.

Mixing on the Late-Summer New England Shelf—Solibores, Shear, and Stratification

Observations are presented of microstructure and velocity measurements made on the outer New England shelf in the late summer of 1996 as part of the Coastal Mixing and Optics Experiment. The depth- and timeaveraged turbulent dissipation rate was 5‐50 (3 1029 Wk g 21). The associated average diapycnal diffusivity in stratified water was 5‐20 ( 1026 m2 s 21), comparable to observed open-ocean thermocline values and too low to explain the strong variability observed in local water properties. Dissipation rates and diffusivity were both highly episodic. Turbulent boundary layers grew down from the surface and up from the bottom. The dissipation rate within the bottom boundary layer had an average of 1.2 3 1027 Wk g 21 and varied in magnitude with the strength of near-bottom flow from the barotropic tide, an along-shelf flow, and low-frequency internal waves. The average dissipation rate in the peak thermocline was 5 3 1028 Wk g21; one-half of the thermocline dissipation was due to the strong shear and strain within six solibores that cumulatively lasted less than a day but contained 100-fold elevated dissipation and diffusivity. Nonsolibore, midcolumn dissipation was strongly correlated with shear from low-frequency internal waves. Dissipation was not well parameterized by Gregg‐Henyey-type scaling. An alternate scaling, modified to account for observed coastal internal wave properties, was in good agreement with measured dissipation rates. At the end of the observational period Hurricane Edouard passed by, producing strong dissipation rates (4 3 1026 Wk g 21) and consequent mixing during and for several days following the peak winds.

Genomic insights into the origin of farming in the ancient Near East

We report genome-wide ancient DNA from 44 ancient Near Easterners ranging in time between ~12,000 and 1,400 bc, from Natufian hunter–gatherers to Bronze Age farmers. We show that the earliest populations of the Near East derived around half their ancestry from a ‘Basal Eurasian’ lineage that had little if any Neanderthal admixture and that separated from other non-African lineages before their separation from each other. The first farmers of the southern Levant (Israel and Jordan) and Zagros Mountains (Iran) were strongly genetically differentiated, and each descended from local hunter–gatherers. By the time of the Bronze Age, these two populations and Anatolian-related farmers had mixed with each other and with the hunter–gatherers of Europe to greatly reduce genetic differentiation. The impact of the Near Eastern farmers extended beyond the Near East: farmers related to those of Anatolia spread westward into Europe; farmers related to those of the Levant spread southward into East Africa; farmers related to those of Iran spread northward into the Eurasian steppe; and people related to both the early farmers of Iran and to the pastoralists of the Eurasian steppe spread eastward into South Asia.

Observations of turbulence in a tidal beam and across a coastal ridge

During a microstructure survey off California in Monterey Bay, we found a midwater beam of strong turbulence emanating from the shelf break along the ray path of the semidiurnal M2 internal tide. Within the 50-m-thick beam the turbulence kinetic energy dissipation rate e exceeded 10−6 W kg−1, and the diapycnal eddy diffusivity Kρ was >0.01 m2 s−1. The beam extended 4 km off the shelf break. Several factors suggest that this beam of strong turbulence resulted from the breaking of semidiurnal internal tides: the beam appeared to originate from the shelf break, which is a potential generation site for semidiurnal internal tides; the beam closely followed the ray path of the semidiurnal internal tide; the average e off the shelf break varied by a factor of 100 with a semidiurnal tidal periodicity; the isopycnal displacement confirmed the presence of semidiurnal internal tides. Processes associated with the breaking of internal tides are intermittent and sporadic. At the same location we also observed equally intense turbulence in a ∼100-m-thick layer of stratified water across the ridge of a sea fan. This layer of strong turbulence was separated from the bottom and was clearly not generated by bottom friction. Although less well resolved in time, the strong turbulence above the bottom seemed to vary with the semidiurnal tide and existed at the lee of the ridge, where the isopycnal surface dipped and rebounded in a pattern resembling that of internal hydraulic jumps. On the basis of the behavior of the density field, we believe that the deep mixing was most likely produced by the across-ridge current of internal tides. The breaking of internal tides at middepth, where the Richardson number is close to the critical value, is likely due to shear instability. The presence of the coastal ridge provides an alternative pathway for converting energy from internal tides to turbulence via internal hydraulics. Multiplying the average e in the midwater beam by the length of the global coastline gives 31 GW, only a small fraction of the estimated 360 GW dissipated globally by M2 internal tides. Our observations suggest that either most internal tides are generated away from shelf breaks or most internal tides generated at shelf breaks propagate away from their generation sites, rather than dissipate locally, and eventually contribute to pelagic mixing.

Energetics of M2 Barotropic-to-Baroclinic Tidal Conversion at the Hawaiian Islands

Abstract A high-resolution primitive equation model simulation is used to form an energy budget for the principal semidiurnal tide (M2) over a region of the Hawaiian Ridge from Niihau to Maui. This region includes the Kaena Ridge, one of the three main internal tide generation sites along the Hawaiian Ridge and the main study site of the Hawaii Ocean Mixing Experiment. The 0.01°–horizontal resolution simulation has a high level of skill when compared to satellite and in situ sea level observations, moored ADCP currents, and notably reasonable agreement with microstructure data. Barotropic and baroclinic energy equations are derived from the model’s sigma coordinate governing equations and are evaluated from the model simulation to form an energy budget. The M2 barotropic tide loses 2.7 GW of energy over the study region. Of this, 163 MW (6%) is dissipated by bottom friction and 2.3 GW (85%) is converted into internal tides. Internal tide generation primarily occurs along the flanks of the Kaena Ridge and ...

Observations of Persistent Mixing and Near-Inertial Internal Waves

Abstract Repeated profiles of microstructure and shear alongside a drogued buoy show a 10 m thick mixing zone at the same depth as a near-inertial feature. Because the profile was diffusively stable and free of thermohaline intrusions, internal wave breakdown is the only mechanism capable of producing mixing. Both the near-inertial feature and the mixing patch were observed for over three days and then faded out. It is not possible to determine whether they disappeared because the near-inertial feature was dissipated by the mixing or because the drogue drifted away; both are plausible. Kinematical models of mixing use a standard internal wave spectrum to predict the frequency of occurrence and persistence of shear instabilities. Observed distributions of ϵ and χ patches thinner than 2 m are similar to the model predictions, although the dissipation rates are low. Most are just at or below the transition dissipation rate ϵtr. Laboratory experiments have established that if ϵ

Intense, Variable Mixing near the Head of Monterey Submarine Canyon

Abstract A microstructure survey near the head of Monterey Submarine Canyon, the first in a canyon, confirmed earlier inferences that coastal submarine canyons are sites of intense mixing. The data collected during two weeks in August 1997 showed turbulent kinetic energy dissipation and diapycnal diffusivity up to 103 times higher than in the open ocean. Dissipation and diapycnal diffusivity within 10 km of the canyon head were among the highest observed anywhere (e = 1.1 × 10−6 W kg−1; Kρ = 1.0 × 10−2 m2 s−1). Mixing occurred mainly in an on-axis stratified turbulent layer, with thickness and intensity increasing from neap to spring tide. Strain spectra showed a gentler than k−1z rolloff, suggesting that critical reflection and scattering may push energy into high wavenumbers. Dissipation dependence on shear appears to be much weaker in the canyon than in the open ocean, with indications that the dependence maybe as low as e ∝ S. Coastal canyons may account for a small but significant fraction of the int...

EPIC2001 and the Coupled Ocean–Atmosphere System of the Tropical East Pacific

Abstract Coupled global ocean–atmosphere models currently do a poor job of predicting conditions in the tropical east Pacific, and have a particularly hard time reproducing the annual cycle in this region. This poor performance is probably due to the sensitivity of the east Pacific to the inadequate representation of certain physical processes in the modeled ocean and atmosphere. The representations of deep cumulus convection, ocean mixing, and stratus region energetics are known to be problematic in such models. The U.S. Climate Variability and Predictability (CLIVAR) program sponsored the field experiment East Pacific Investigation of Climate Processes in the Coupled Ocean–Atmosphere System 2001 (EPIC2001), which has the goal of providing the observational basis needed to improve the representation of certain key physical processes in models. In addition to physical processes, EPIC2001 research is directed toward a better understanding and simulation of the effects of short-term variability in the east ...