Towed thermister chain observations of fronts in the subtropical North Pacific

A thermistor chain was towed 1400 km through the eastern North Pacific subtropical frontal zone in January 1980. The observations resolve surface layer temperature features with horizontal wavelengths of 0.2-200 km and vertical scales of 10-70 m. The dominant features, which have horizontal wavelengths of 10-100 km, amplitudes of 0.2°-1.0°C, and random orientation, likely arise from baroclinic instability. Associated with them is a plateau below 0.1 cpkm in the horizontal temperature gradient spectrum. Strong temperature fronts O(1°-2°C/3-10 km) are observed near 33°N, 31°N, and 27°N. Temperature variability is partially density compensated by salinity, with the fraction of compensation increasing northward. There is evidence of vertical mixing during high winds. Temperature at 15-m depth is roughly normally distributed around the climatological surface mean, with a standard deviation of approximately 0.5°C. The standard deviation would correspond to an adiabatic meridional displacement of 80-100 km in the mean gradient. Horizontal temperature gradient at 15-m depth has maximum values in excess of 0.25°C/100 m and kurtosis near 80. In the band 0.10-1 cpkm, the 15-m gradient spectrum is inversely proportional to wave number, consistent with predictions from geostrophic turbulence theory, while the spectrum at 70-m depth has additional variance that is consistent with Garrett-Munk internal wave displacements.Keywords: upper ocean processes, eddies and mesoscale processes, Fronts and jets, Pacific OceanKeywords: upper ocean processes, eddies and mesoscale processes, Fronts and jets, Pacific Ocea

Mixed layer observations during the NORPAX POLE experiment : a data report

This data report contains observations made from R/P FLIP as part of the first process-oriented NORPAX (North Pacific Experiment) experiment. The experiment was named POLE to indicate that the horizontal extent of sampling was small compared to the largest scale investigated in NORPAX. The part of the experiment reported here was conducted during the period 28 January 74 through 14 February 74. During this time, FLIP occupied a station approximately 800 miles north of the Hawaiian Island Chain (35°N. lat, 155° W. lonq) under free drift conditions. Direct measurements of the incident solar, reflected solar, net all-wave and net long-wave fluxes were made from R/P FLIP during the period 2 to 14 Febru­ary 74. The sea surface temperature was also observed with a radiation thermometer. Standard meteorological observations, from which the latent and sensible heat fluxes from the sea surface to the atmosphere-were derived, also were made during this period. Vertical profiles of temperature and salinity were taken from R/P FLIP throughout the period 30 January through 14 February 74. Profiling was con­centrated in the mixed layer and thermocline. The maximum depth reached was 325 meters. On average, 8 profiles were measured per day. On occasion, more intensive sampling was maintained

A numerical study of melt ponds

High-resolution turbulence simulations are used to examine the importance of melt pond geometry in setting pond growth rates and albedo. Modeling the circulation of water in melt ponds using large-eddy simulation shows that both convective and windforced conditions generate well-mixed ponds, suggesting that stratification is not a significant factor in pond circulation. Simulations with a variety of pond shapes and sizes indicate that the basic ratio of sidewall area to bottom area, R, can be used to characterize melting rates for ponds with simple shapes. Ponds with large values of R will generally melt more rapidly in the horizontal direction at the expense of bottom melting. Consequently, small and elongated ponds will have a relatively larger lateral growth rate in comparison with large, symmetric ponds, assuming minimal lateral flux of meltwater. Simulations also show that pond shape can affect the sidewall and bottom turbulence transfer rates. Ponds with large R tend to have reduced transfer rates because of weaker circulations. A bulk pond model is developed on the basis of a rectangular geometry and an assumption of uniform mixing as suggested by the turbulence model and pond scaling using R. Comparison of the bulk model with results from the large-eddy simulation cases shows good agreement

Towed thermistor chain observations in JASIN

Observations of temperature and pressure between 10 and 70 m depth were taken with a towed thermistor chain during late August and early September, 1978, about 400 km northwest of Scotland as a part of the JASIN Experiment. The chain was usually towed at a speed of 3 mis around a 15-km square centered at 59°N, 12°30W. On two occasions tows were made around five-km squares as part of coordinated observations involving several ships. The observations were averaged over sequential 30-second intervals and isotherm depths were interpolated from the averaged observations. Cross-sections of temperature and isotherm depth are presented. Spectra of the depth of the lowest and highest isotherm of each cross-section are also presented

Simulation of turbulent exchange processes in summertime leads

Ice-ocean heat exchange in polar leads was examined using a large-eddy simulation model coupled to a slab ice model. Simulations were performed using an idealized square domain for a range of lead sizes, surface wind stress (0.05–0.1 N/m²), and lead temperature/salinity profiles. Particular emphasis was placed on understanding the role of fresh water in leads and how stratification controls the heat budget and ice edge melting rate. With uniform initial conditions we found that solar heating was not strong enough to develop lead freshening via ice edge melting; even weak winds (0.02 N/m²) generated circulations that maintained a well-mixed lead. In the weak wind case, adding a fresh water flux representative of surface melt runoff provided enough additional stratification so that the lead water became isolated from the rest of the simulated ocean boundary layer. However, stronger winds (0.1 N/m²) prevented the fresh water layer from forming. Experiments initialized with temperature/salinity profiles similar to observed cases (fresh water layer capping the lead) demonstrated that lateral melting rates increase with expanding lead size, agreeing with simple heat balance calculations for a square lead without vertical mixing. However, with stronger winds, lateral melting rates decreased because of greater turbulent mixing of cold water from beneath the fresh layer. Inspection of the lead circulation indicated that the strongest melting occurred where the ice edge currents were the largest. Overall, melting fluxes for a 24 m² lead ranged from 200 to 400 W m², depending on the wind speed. Without the fresh layer, fluxes ranged from 50 to 60 W m², suggesting that fresh water stratification can have a dominate role in controlling ice edge melting

Mixing in the equatorial surface layer and thermocline

Twelve days of microstructure measurements at the equator (140°W) in November 1984 showed a surprisingly strong effect of both the daily cycle of solar heating and wind on mixing in the upper ocean. Because of limited variations in atmospheric forcing and currents during the experiment, processes in the daily mixing cycle were similar from day to day. Only the intensity of mixing varied. The lower boundary of the diurnal surface layer separated two distinct mixing regimes, the diurnal surface layer and the thermocline. Within the diurnal surface layer (which extended to 10- to 35-m depth), turbulent kinetic energy dissipation rates ε varied relatively little. Although variations in surface layer depth coincided with the daily change in direction of air-sea surface buoyancy production of turbulent kinetic energy (or simply, the surface buoyancy flux), ε was significantly greater relative to the buoyancy flux than was expected for a simple convective layer. In the thermocline below the diurnal surface layer, ε was highly intermittent; the day-night cycle was stronger, and variability was enhanced by turbulent "bursts" of 2-3 hours duration, which may be related to internal wave breaking events. The turbulent heat flux crossing 20-m depth was almost equal to the surface heat flux less the irradiance penetrating below 20 m. Seventy percent of the surface heat flux was transported vertically to the water below 30 m by turbulent mixing. Only a negligible amount penetrated to the core of the Equatorial Undercurrent. The gradient Richardson number Ri distinguishes between statistically different mixing environments. However, ε cannot be predicted from the value of Ri, since the intensity of mixing depends on the intensity of forcing in a way not specified by the value of Ri alone

Horizontal wave number spectra of temperature in the unstably stratified oceanic surface layer

Horizontal wave number spectra of temperature in the unstably stratified oceanic surface layer were determined from measurements on a bow boom at a depth of 2 m. Spectra were estimated in the wavelength band from 2 m to 2 km, normalized in accordance with Monin-Obukhov similarity theory, and averaged in groups with similar stability parameter and fractional mixed layer depth. The shapes of the wave number-weighted oceanic spectra agree qualitatively with observed and modeled atmospheric spectra, including the wavelength of the peaks and the variation of peak wave number with stability. However, the peak spectral levels disagree by as much as a factor of two and the variation of spectral level with stability is in the opposite sense for the oceanic and atmospheric spectra. The wave number of the peak in the near neutral oceanic spectrum is similar to the wave number of the peak in the longitudinal velocity spectrum observed in the atmospheric surface layer, which is consistent with temperature acting as a passive tracer in near neutral conditions. The wave number of the peak in the free convection oceanic spectrum is similar to the wave number of the peak in the spectrum of vertical velocity observed in the atmospheric surface layer during free convection, which reflects the dynamical role played by temperature in a freely convecting boundary layer. The difference between oceanic and modeled near-neutral spectral levels at a wavelength of 2 m suggests that dissipation could be enhanced (u p to a factor of three) by surface wave breaking

Simulation of melt pond evolution on level ice

A melt pond model is presented that predicts pond size and depth changes, given an initial ice thickness field and representative surface fluxes. The model is based on the assumption that as sea ice melts, fresh water builds up in the ice pore space and eventually saturates the ice. Under these conditions, a water table is defined equal to the draft of the ice or sea level, and ponds are produced in ice surface depressions, much like lakes in a watershed. Pond evolution is forced by applying fluxes of heat at the pond surface and a radiative transfer model for solar radiation that penetrates the pond. Results from the model using forcing data from the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment and representative pond parameters indicate that the model accurately simulates pond depth and fractional area over the summer melt season, with fractional area increasing linearly. Overall, ice albedo is affected primarily by the increase in pond coverage. Decrease in pond albedo from pond deepening has a much lower influence on the total albedo. Cases with predominately sunny conditions are shown to produce more rapid pond expansion than overcast cases. In both sunny and cloudy cases the fractional area increases linearly

Observations of internal gravity waves under the Arctic pack ice

Internal gravity waves measured under the Arctic pack ice were strikingly different from measurements at lower latitudes. The total wave energy, integrated over the internal wave frequency band, was lower by a factor of 0.03-0.07, and the spectral slope at high frequency was nearly -1 in contrast to the -2 observed at lower latitudes. This result has implications for theoretical investigations of the generation, evolution, and destruction of internal waves and is also important for other processes, such as the propagation of sound, and the wave-induced turbulent diffusion of heat, plankton, and chemical tracers

Individual-specific changes in the human gut microbiota after challenge with enterotoxigenic Escherichia coli and subsequent ciprofloxacin treatment

Acknowledgements The authors wish to thank Mark Stares, Richard Rance, and other members of the Wellcome Trust Sanger Institute’s 454 sequencing team for generating the 16S rRNA gene data. Lili Fox Vélez provided editorial support. Funding IA, JNP, and MP were partly supported by the NIH, grants R01-AI-100947 to MP, and R21-GM-107683 to Matthias Chung, subcontract to MP. JNP was partly supported by an NSF graduate fellowship number DGE750616. IA, JNP, BRL, OCS and MP were supported in part by the Bill and Melinda Gates Foundation, award number 42917 to OCS. JP and AWW received core funding support from The Wellcome Trust (grant number 098051). AWW, and the Rowett Institute of Nutrition and Health, University of Aberdeen, receive core funding support from the Scottish Government Rural and Environmental Science and Analysis Service (RESAS).Peer reviewedPublisher PD