Summertime cooling of the shallow continental shelf

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 116 (2011): C07015, doi:10.1029/2010JC006744.In summer on the shallow New England continental shelf, near the coast the water temperature is much cooler than the observed surface heat flux suggests. Using depth-integrated heat budgets in 12 and 27 m water depth calculated from observed surface heat flux, water temperature, and velocity, we demonstrate that on time scales of weeks to months the water is persistently cooled due to a mean upwelling circulation. Because the mean wind is weak, that mean circulation is likely not wind driven; it is partly a tidal residual circulation. A feedback exists between the cross-shelf and surface heat fluxes: the two fluxes remain nearly in balance for months, so the water temperature is nearly constant in spite of strong surface heating (the heat budget is two-dimensional). A conceptual model explains the feedback mechanism: the short flushing time of the shallow shelf produces a near steady state heat balance, regardless of the exact form of the circulation, and the feedback is via the influence of surface heating on temperature stratification. Along-shelf heat flux divergence is apparently small compared to the surface and cross-shelf heat flux divergences on time scales of weeks to months. Heat transport due to Stokes drift from surface gravity waves is substantial, warms the shallow shelf in summer, and was previously ignored. In winter, the surface heat flux dominates and the observed water temperature is close to the temperature predicted from surface cooling (the heat budget is one-dimensional); weak winter stratification makes the cross-shelf heat flux small even during strong cross-shelf circulation.This research was funded by National Aeronautics and Space Administration Headquarters grant NNG04GL03G and Earth System Science Fellowship Grant NNG04GQ14H; Woods Hole Oceanographic Institution through Academic Programs Fellowship Funds and MVCO; National Science Foundation grants OCE‐0241292, OCE‐0548961, and OCE‐0337892; the Jewett/ EDUC/Harrison Foundation; and Office of Naval Research contracts N00014‐01‐1‐0029 and N00014‐05‐10090 for the Low‐Wind Component of the Coupled Boundary Layers Air‐Sea Transfer Experiment

A Survey of Ocean Simulation and Rendering Techniques in Computer Graphics

This paper presents a survey of ocean simulation and rendering methods in computer graphics. To model and animate the ocean's surface, these methods mainly rely on two main approaches: on the one hand, those which approximate ocean dynamics with parametric, spectral or hybrid models and use empirical laws from oceanographic research. We will see that this type of methods essentially allows the simulation of ocean scenes in the deep water domain, without breaking waves. On the other hand, physically-based methods use Navier-Stokes Equations (NSE) to represent breaking waves and more generally ocean surface near the shore. We also describe ocean rendering methods in computer graphics, with a special interest in the simulation of phenomena such as foam and spray, and light's interaction with the ocean surface

Effects of integration time on in-water radiometric profiles

This work investigates the effects of integration time on in-water downward irradiance E-d, upward irradiance E-u and upwelling radiance L-u profile data acquired with free-fall hyperspectral systems. Analyzed quantities are the subsurface value and the diffuse attenuation coefficient derived by applying linear and non-linear regression schemes. Case studies include oligotrophic waters (Case-1), as well as waters dominated by colored dissolved organic matter (CDOM) and non-algal particles (NAP). Assuming a 24-bit digitization, measurements resulting from the accumulation of photons over integration times varying between 8 and 2048ms are evaluated at depths corresponding to: 1) the beginning of each integration interval (FST); 2) the end of each integration interval (LST); 3) the averages of FST and LST values (AVG); and finally 4) the values weighted accounting for the diffuse attenuation coefficient of water (WGT). Statistical figures show that the effects of integration time can bias results well above 5% as a function of the depth definition. Results indicate the validity of the WGT depth definition and the fair applicability of the AVG one. Instead, both the FST and LST depths should not be adopted since they may introduce pronounced biases in E-u and L-u regression products for highly absorbing waters. Finally, the study reconfirms the relevance of combining multiple radiometric casts into a single profile to increase precision of regression products. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Application of the Beer–Lambert Model to Attenuation of Photosynthetically Active Radiation in a Shallow, Eutrophic Lake

Models of primary production in aquatic systems must include a means to estimate subsurface light. Such models often use the Beer–Lambert law, assuming exponential attenuation of light with depth. It is further assumed that the diffuse attenuation coefficient may be estimated as a summation of scattering/absorbing constituent concentrations multiplied by their respective specific attenuation coefficients. While theoretical deviations from these assumptions have been documented, it is useful to consider the empirical performance of this common approach. Photosynthetically active radiation (PAR) levels and water quality conditions were recorded weekly from six to eight monitoring stations in western Lake Erie between 2012 and 2016. Exponential PAR extinction models yielded a mean attenuation coefficient of 1.55 m (interquartile range = 0.74–1.90 m). While more complex light attenuation models are available, analysis of residuals indicated that the simple Beer–Lambert model is adequate for shallow, eutrophic waters similar to western Lake Erie (R2 > 0.9 for 96% of samples). Three groups of water quality variables were predictive of PAR attenuation: total and nonvolatile suspended particles, dissolved organic substances (dissolved organic carbon and chromophoric dissolved organic matter), and organic solids (volatile suspended solids and chlorophyll). Multiple regression models using these variables predicted 3–90% of the variability in PAR attenuation, with a median adjusted R2 = 0.86. Explanatory variables within these groups may substitute for each other while maintaining similar model performance, indicating that various combinations of water quality variables may be useful to predict PAR attenuation, depending on availability within a model framework or monitoring program.Key PointsThe Beer–Lambert law effectively models photosynthetically active radiation in western Lake Erie, despite some systematic deviationsField‐obtained water quality parameters can predict photosynthetically active radiation attenuation with a high degree of confidenceSuspended particle concentration is most predictive of photosynthetically active radiation attenuation in this turbid, eutrophic basinPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/147097/1/wrcr23654_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/147097/2/wrcr23654-sup-0001-2018WR023024-SI.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/147097/3/wrcr23654.pd

Vision and Foraging in Cormorants: More like Herons than Hawks?

Background Great cormorants (Phalacrocorax carbo L.) show the highest known foraging yield for a marine predator and they are often perceived to be in conflict with human economic interests. They are generally regarded as visually-guided, pursuit-dive foragers, so it would be expected that cormorants have excellent vision much like aerial predators, such as hawks which detect and pursue prey from a distance. Indeed cormorant eyes appear to show some specific adaptations to the amphibious life style. They are reported to have a highly pliable lens and powerful intraocular muscles which are thought to accommodate for the loss of corneal refractive power that accompanies immersion and ensures a well focussed image on the retina. However, nothing is known of the visual performance of these birds and how this might influence their prey capture technique. Methodology/Principal Findings We measured the aquatic visual acuity of great cormorants under a range of viewing conditions (illuminance, target contrast, viewing distance) and found it to be unexpectedly poor. Cormorant visual acuity under a range of viewing conditions is in fact comparable to unaided humans under water, and very inferior to that of aerial predators. We present a prey detectability model based upon the known acuity of cormorants at different illuminances, target contrasts and viewing distances. This shows that cormorants are able to detect individual prey only at close range (less than 1 m). Conclusions/Significance We conclude that cormorants are not the aquatic equivalent of hawks. Their efficient hunting involves the use of specialised foraging techniques which employ brief short-distance pursuit and/or rapid neck extension to capture prey that is visually detected or flushed only at short range. This technique appears to be driven proximately by the cormorant's limited visual capacities, and is analogous to the foraging techniques employed by herons

Atmosphere-ocean coupled processes in the Madden-Julian oscillation

The Madden-Julian oscillation (MJO) is a convectively coupled 30-70 day (intraseasonal) tropical atmospheric mode that drives variations in global weather, but which is poorly simulated in most atmospheric general circulation models. Over the past two decades, field campaigns and modeling experiments have suggested that tropical atmosphere-ocean interactions may sustain or amplify the pattern of enhanced and suppressed atmospheric convection that defines the MJO, and encourage its eastward propagation through the Indian and Pacific Oceans. New observations collected during the past decade have advanced our understand of the ocean response to atmospheric MJO forcing and the resulting intraseasonal sea surface temperature (SST) fluctuations. Numerous modeling studies have revealed a considerable impact of the mean state on MJO ocean-atmosphere coupled processes, as well as the importance of resolving the diurnal cycle of atmosphere--upper-ocean interactions. New diagnostic methods provide insight to atmospheric variability and physical processes associated with the MJO, but offer limited insight on the role of ocean feedbacks. Consequently, uncertainty remains concerning the role of the ocean in MJO theory. Our understanding of how atmosphere-ocean coupled processes affect the MJO can be improved by collecting observations in poorly sampled regions of MJO activity, assessing oceanic and atmospheric drivers of surface fluxes, improving the representation of upper-ocean mixing in coupled-model simulations, designing model experiments that minimize mean-state differences, and developing diagnostic tools to evaluate the nature and role of coupled ocean-atmosphere processes over the MJO cycle

Surface shear stress dependence of gas transfer velocity parameterizations using DNS

Air-water gas-exchange is studied in direct numerical simulations (DNS) of free-surface flows driven by natural convection and weak winds. The wind is modeled as a constant surface-shear-stress and the gas-transfer is modeled via a passive scalar. The simulations are characterized via a Richardson number Ri=Bν/u*4 where B, ν, and u* are the buoyancy flux, kinematic viscosity, and friction velocity respectively. The simulations comprise 0<Ri<∞ ranging from convection-dominated to shear-dominated cases. The results are used to: (i) evaluate parameterizations of the air-water gas-exchange, (ii) determine, for a given buoyancy flux, the wind speed at which gas transfer becomes primarily shear driven, and (iii) find an expression for the gas-transfer velocity for flows driven by both convection and shear. The evaluated gas transfer-velocity parametrizations are based on either the rate of turbulent kinetic energy dissipation, the surface flow-divergence, the surface heat-flux, or the wind-speed. The parametrizations based on dissipation or divergence show an unfavorable Ri dependence for flows with combined forcing whereas the parametrization based on heat-flux only shows a limited Ri dependence. The two parametrizations based on wind speed give reasonable estimates for the transfer-velocity, depending however on the surface heat-flux. The transition from convection- to shear-dominated gas-transfer-velocity is shown to be at Ri≈0.004. Furthermore, the gas-transfer is shown to be well represented by two different approaches: (i) additive forcing expressed as kg,sum=AShearu*Ri/Ric+11/4Sc-n where Ric=AShear/ABuoy4, and (ii) either buoyancy or shear dominated expressed as, kg=ABuoyBν1/4Sc-n, Ri>Ric or kg=Ashearu*Sc-n, Ri<Ric. Here ABuoy=0.4 and AShear=0.1 are constants, and n is an exponent that depends on the water surface-characteristics

Solar heating of the oceans-diurnal, seasonal and meridional variation

Solar heating is an important factor in modelling the upper boundary layer of the ocean. It influences not only the temperature, but also the depth of the mixed layer and must be taken into account in circulation dynamics. The study reported in this paper was designed to reveal the principal features of the global climatology of solar heating in the ocean, with such applications in mind. The meridional, seasonal and diurnal variations of the vertical distribution of solar heating inside the ocean, expressed in terms of I(z), the rate of heat accumulation below depth z, and †(z) = (1/c). dzI(z), the rate of temperature rise, are calculated for given values of cloud cover and seawater turbidity (expressed in terms of Jerlov's water types) using a model that incorporates a new parametrization of I(z)/I(0), which is shown to be more accurate than previous versions. At present there exist no reliable global climatologies of cloud cover and seawater turbidity, so the values of the corresponding parameters are held constant in each computation, which is then repeated using parameter sets covering the full ranges from clear to overcast sky, clear to turbid ocean water. It is found that uncertainty in cloud cover is more important in the mixed layer, and uncertainty in seawater turbidity is more important below. The results presented in this paper are mainly concerned with solar heating below the mixed layer. It is calculated that the annual temperature rise can exceed 1 K and the annual heat accumulation can exceed 100 MJ/m2 below the mixed layer in the tropics. At higher latitudes solar heating produces similar heating rates in summer, but the stored heat is extracted locally in winter when the mixed layer depth exceeds the maximum depth of solar heating, defined here by a daily temperature rise of 1 mK or a heat flux of 86.4 KJ/m2d (=1 W/m2). The sensitivity of the seasonal and meridional variations of the maximum depth of solar heating to cloud cover and seawater turbidity is investigated. The actual change of temperature due to solar heating in the seasonal thermocline at Ocean Weather Station ‘C’ is calculated using Bunker's monthly mean cloud cover and Jerlov's seawater turbidity. Extension of such calculations to the whole of the World Ocean must await the publication of global climatologies of cloud cover and seawater turbidity, which are expected to be derived from satellite observations during the next decade. A solar heating climatology is a prerequisite for computation of the thermal response of the ocean to CO2 pollution of the atmosphere. The implications of the results obtained from the present study are discussed. An early rise in tropical sea surface temperature seems likely, but exact prediction will be hindered by uncertainty in the turbidity of the tropical ocean