Mesoscale eddies modulate mixed layer depth globally.

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 46(3), (2019):1505-1512, doi:10.1029/2018GL080006.Mesoscale eddies, energetic vortices covering nearly a third of the ocean surface at any one time, modulate the spatial and temporal evolution of the mixed layer. We present a global analysis of concurrent satellite observations of mesoscale eddies with hydrographic profiles by autonomous Argo floats, revealing rich geographic and seasonal variability in the influence of eddies on mixed layer depth. Anticyclones deepen the mixed layer depth, whereas cyclones thin it, with the magnitude of these eddy‐induced mixed layer depth anomalies being largest in winter. Eddy‐centric composite averages reveal that the largest anomalies occur at the eddy center and decrease with distance from the center. Furthermore, the extent to which eddies modulate mixed layer depth is linearly related to the sea surface height amplitude of the eddies. Finally, large eddy‐mediated mixed layer depth anomalies are more common in anticyclones when compared to cyclones. We present candidate mechanisms for this observed asymmetry.This project was supported by NASA grants NNX13AE47G and NNX16AH9G. This manuscript was improved as a result of helpful discussions with Jeffery Early, Johnathan Lilly, and Eric Kunze of Northwest Research Associates. D. J. M. also gratefully acknowledges support of the National Science Foundation. The eddy data set used here is distributed by AVISO at https://www.aviso.altimetry.fr/en/data/products/value-added-products/global-mesoscale-eddy-trajectoryproduct.html. The MLD data can be accessed at http://mixedlayer.ucsd.edu.2019-06-0

Evolution of Turbulence, Heat Content, and Freshwater Lenses in the Diurnal Warm Layer

Thorough understanding of the mechanisms controlling the temperature structure in the surface mixed layer of the ocean and, in particular, accurate values of sea surface temperature are critical for properly parameterizing air-sea heat exchange and quantifying the amount of heat redistributed below the surface. It is however difficult to obtain routine in-situ measurements of the sea surface temperature from oceanographic moorings or research vessels, and even more difficult to measure the detailed evolution of the temperature structure. Oceanographers instead rely on parameterizations of a diurnal warm layer forced by temperature profiles or time series to estimate the time-varying surface temperature structure. For the first time, the time-varying near-surface temperature structure, turbulence and surface heat fluxes were measured at the same time in the Indian Ocean during the DYNAMO field experiment. These measurements showed the abrupt termination of nighttime mixing at sunrise and subsequent decay during approximately one hour, they showed a rapid growth of turbulence thereafter as a balance of shear and buoyancy production and turbulent kinetic energy dissipation, and they showed an equilibrium state in the afternoon. Elevated turbulence were attributed to shear instabilities from the observation of temperature ramps in low-moderate wind conditions, but could not be distinguished from Langmuir circulations in higher winds. Distinct relationships of the vertical temperature gradient, wind speed and turbulence dissipation emerged when classifying data by presence of temperature ramps. These measurements also permitted a re-assessment of the vertical structure and physics of the diurnal warm layer with implications for heat budget assessment, therefore helping to identify weaknesses in current parameterizations. The shape of temperature profiles results from the ability of turbulence to export downward the heat deposited near the surface by exponentially attenuated subsurface solar radiation. When stratification was weak in the early morning surface heat was distributed over the top eight meters resulting in heat in excess of local solar radiation divergence. After complete restratification, surface heat was trapped above the mixed layer depth where it was both input from local divergence of the absorbed solar radiation and from the downwelling of surface heat through mixing. Below the mixed layer, the divergence of attenuated solar radiations was the only heat source. Shear instabilities at the base of the mixed layer entrain cooler fluid from below thereby deepening the mixed layer depth and distributing heat and momentum over a thicker layer. In late afternoon when net surface cooling exceeded the net heating from the divergence of penetrating solar radiation, the temperature structure was destabilized from above, mixing heat downward. The heat accumulated over the previous hours and stored in the mixed layer was then eroded both from above through convection and from below through shear instabilities. Our observations also permitted a detailed look at freshwater lenses deposited by strong localized precipitation, which can affect the heat content directly from the addition of cooler rainwater, but also indirectly by modifying the stratification of the upper ocean. Twenty-six lenses were identified, ten of which propagated at the internal wave speed and featured buoyant gravity current characteristics. The temperature and salinity anomalies of lenses were related to their age and rain volume precipitated, and they were either cooler or warmer than ambient water. This propensity to retain heat created a patchy temperature environment both at the surface and in the near-surface as pockets of warm and cool water were observed within lenses. Thermohaline anomalies were estimated to dissipate in three days on average, but up to 25 days, if the turbulent mixing of ambient water was the only source of heat and salt

Air–Sea Interactions from Westerly Wind Bursts During the November 2011 MJO in the Indian Ocean

The life cycles of three Madden–Julian oscillation (MJO) events were observed over the Indian Ocean as part of the Dynamics of the MJO (DYNAMO) experiment. During November 2011 near 0°, 80°E, the site of the research vessel Roger Revelle, the authors observed intense multiscale interactions within an MJO convective envelope, including exchanges between synoptic, meso, convective, and turbulence scales in both atmosphere and ocean and complicated by a developing tropical cyclone. Embedded within the MJO event, two bursts of sustained westerly wind (>10 m s−1; 0–8-km height) and enhanced precipitation passed over the ship, each propagating eastward as convectively coupled Kelvin waves at an average speed of 8.6 m s−1. The ocean response was rapid, energetic, and complex. The Yoshida–Wyrtki jet at the equator accelerated from less than 0.5 m s−1 to more than 1.5 m s−1 in 2 days. This doubled the eastward transport along the ocean's equatorial waveguide. Oceanic (subsurface) turbulent heat fluxes were comparable to atmospheric surface fluxes, thus playing a comparable role in cooling the sea surface. The sustained eastward surface jet continued to energize shear-driven entrainment at its base (near 100-m depth) after the MJO wind bursts subsided, thereby further modifying sea surface temperature for a period of several weeks after the storms had passed

Air-Sea Interactions from Westerly Wind Bursts During the November 2011 MJO in the Indian Ocean

The life cycles of three Madden–Julian oscillation (MJO) events were observed over the Indian Ocean as part of the Dynamics of the MJO (DYNAMO) experiment. During November 2011 near 0°, 80°E, the site of the research vessel Roger Revelle, the authors observed intense multiscale interactions within an MJO convective envelope, including exchanges between synoptic, meso, convective, and turbulence scales in both atmosphere and ocean and complicated by a developing tropical cyclone. Embedded within the MJO event, two bursts of sustained westerly wind (>10 m s⁻¹; 0–8-km height) and enhanced precipitation passed over the ship, each propagating eastward as convectively coupled Kelvin waves at an average speed of 8.6 m s⁻¹. The ocean response was rapid, energetic, and complex. The Yoshida–Wyrtki jet at the equator accelerated from less than 0.5 m s⁻¹ to more than 1.5 m s⁻¹ in 2 days. This doubled the eastward transport along the ocean's equatorial waveguide. Oceanic (subsurface) turbulent heat fluxes were comparable to atmospheric surface fluxes, thus playing a comparable role in cooling the sea surface. The sustained eastward surface jet continued to energize shear-driven entrainment at its base (near 100-m depth) after the MJO wind bursts subsided, thereby further modifying sea surface temperature for a period of several weeks after the storms had passed

New Insights into the Bacterial Fitness-Associated Mechanisms Revealed by the Characterization of Large Plasmids of an Avian Pathogenic E. coli

Extra-intestinal pathogenic E. coli (ExPEC), including avian pathogenic E. coli (APEC), pose a considerable threat to both human and animal health, with illness causing substantial economic loss. APEC strain χ7122 (O78∶K80∶H9), containing three large plasmids [pChi7122-1 (IncFIB/FIIA-FIC), pChi7122-2 (IncFII), and pChi7122-3 (IncI(2))]; and a small plasmid pChi7122-4 (ColE2-like), has been used for many years as a model strain to study the molecular mechanisms of ExPEC pathogenicity and zoonotic potential. We previously sequenced and characterized the plasmid pChi7122-1 and determined its importance in systemic APEC infection; however the roles of the other pChi7122 plasmids were still ambiguous. Herein we present the sequence of the remaining pChi7122 plasmids, confirming that pChi7122-2 and pChi7122-3 encode an ABC iron transport system (eitABCD) and a putative type IV fimbriae respectively, whereas pChi7122-4 is a cryptic plasmid. New features were also identified, including a gene cluster on pChi7122-2 that is not present in other E. coli strains but is found in Salmonella serovars and is predicted to encode the sugars catabolic pathways. In vitro evaluation of the APEC χ7122 derivative strains with the three large plasmids, either individually or in combinations, provided new insights into the role of plasmids in biofilm formation, bile and acid tolerance, and the interaction of E. coli strains with 3-D cultures of intestinal epithelial cells. In this study, we show that the nature and combinations of plasmids, as well as the background of the host strains, have an effect on these phenomena. Our data reveal new insights into the role of extra-chromosomal sequences in fitness and diversity of ExPEC in their phenotypes

Variation of atmospheric volatile organic compounds over the Southern Indian Ocean (30–49°S)

International audienc

Turbulent diapycnal fluxes as a pilot Essential Ocean Variable

Peer reviewed: TrueAcknowledgements: All co-authors, and especially the lead authors acknowledge the tremendous input from J. Moum who helped re-oriented the main topic of this white paper toward what the ocean community is now considering the best option for turbulence data to become an EOV. We are also in debt of all the pioneers who participated in the development of today’s turbulence technology.We contend that ocean turbulent fluxes should be included in the list of Essential Ocean Variables (EOVs) created by the Global Ocean Observing System. This list aims to identify variables that are essential to observe to inform policy and maintain a healthy and resilient ocean. Diapycnal turbulent fluxes quantify the rates of exchange of tracers (such as temperature, salinity, density or nutrients, all of which are already EOVs) across a density layer. Measuring them is necessary to close the tracer concentration budgets of these quantities. Measuring turbulent fluxes of buoyancy (Jb ), heat (Jq ), salinity (JS ) or any other tracer requires either synchronous microscale (a few centimeters) measurements of both the vector velocity and the scalar (e.g., temperature) to produce time series of the highly correlated perturbations of the two variables, or microscale measurements of turbulent dissipation rates of kinetic energy (ϵ) and of thermal/salinity/tracer variance (χ), from which fluxes can be derived. Unlike isopycnal turbulent fluxes, which are dominated by the mesoscale (tens of kilometers), microscale diapycnal fluxes cannot be derived as the product of existing EOVs, but rather require observations at the appropriate scales. The instrumentation, standardization of measurement practices, and data coordination of turbulence observations have advanced greatly in the past decade and are becoming increasingly robust. With more routine measurements, we can begin to unravel the relationships between physical mixing processes and ecosystem health. In addition to laying out the scientific relevance of the turbulent diapycnal fluxes, this review also compiles the current developments steering the community toward such routine measurements, strengthening the case for registering the turbulent diapycnal fluxes as an pilot Essential Ocean Variable

Corrigendum: Turbulent diapycnal fluxes as a pilot Essential Ocean Variable

Peer reviewed: Tru