Mechanisms affecting the overturning response in global warming simulations

Climate models used to produce global warming scenarios exhibit widely diverging responses of the thermohaline circulation (THC). To investigate the mechanisms responsible for this variability, a regional Atlantic Ocean model driven with forcing diagnosed from two coupled greenhouse gas simulations has been employed. One of the coupled models (MPI) shows an almost constant THC, the other (GFDL) shows a declining THC in the twenty-first century. The THC evolution in the regional model corresponds rather closely to that of the respective coupled simulation, that is, it remains constant when driven with the forcing from the MPI model, and declines when driven with the GFDL forcing. These findings indicate that a detailed representation of ocean processes in the region covered by the Atlantic model may not be critical for the simulation of the overall THC changes in a global warming scenario, and specifically that the coupled model’s rather coarse representation of water mass formation processes in the subpolar North Atlantic is unlikely to be the primary cause for the large differences in the THC evolution. Sensitivity experiments have confirmed that a main parameter governing the THC response to global warming is the density of the intermediate waters in the Greenland–Iceland–Norwegian Seas, which in turn influences the density of the North Atlantic Deep Water, whereas changes in the air–sea heat and freshwater fluxes over the subpolar North Atlantic are only of moderate importance, and mainly influence the interannual–decadal variability of THC. Finally, as a consequence of changing surface fluxes, the Labrador Sea convection ceases by about 2030 under both forcings (i.e., even in a situation where the overall THC is stable) indicating that the eventual breakdown of the convection is likely but need not coincide with substantial THC changes

Temporal and spatial dynamics of willow grouse Lagopus lagopus

This study is a first attempt to analyze large-scale willow grouse (Lagopus lagopus) population dynamics. In this thesis I studied the population dynamics of willow grouse in the Swedish mountain range. I estimated the scale of synchrony in both breeding success, and the adult segment between populations throughout the mountain range by using time-series of harvest data and population densities estimated from line transect counts. Little evidence of regular fluctuations/cycles in juveniles in autumn was found, and chick production in willow grouse appears to fluctuate more irregularly than was previously believed. Breeding success estimated as number of juveniles per two adults in autumn, showed a spatial correlation between sites up to 200 km apart, but there was no spatial correlation in the adult segment of the populations. When analyzing the presence and strength of time-lags in density dependent processes, a first order process was detected in the southernmost region, and a combination of weak first and second order processes in the northern part of the mountain range. There was a positive relationship between breeding success the previous year and the intrinsic rate of increase in the northernmost region where first and second order density dependence was weak and of equal strength. No such relationship was found in the southern region where density dependence was stronger and dominated by a first order process. Natal dispersal resulted in 80% of the juvenile females dispersing more than 5 km from their summer area, with a mean dispersal distance of 10.2 km, three times further than for juvenile males (3.4 km). Adult females were migratory between wintering areas used as a juvenile and their first breeding site. At landscape scales, only juvenile females and migratory adult females are important in exchange of individuals between populations. Hunters’ effort provided a useful relationship with harvest rate. Setting limits to the totally allowable effort within an area has a stronger potential for controlling harvest than daily bag limits or adjusting the length of the open season. Another simple harvest strategy would be to prohibit harvest in parts of the total area hunted. By setting aside a part of the total area as a buffer, and placing limits on the harvest effort in the open parts of the hunting area, it would be possible to achieve a cost-efficient system with a small risk of over-harvesting

Transmission Dynamics Of Covid-19 Under Differential Levels Of Masking And Vaccination

Background: Widespread use of cloth or medical masks by the public contributed to decreased transmission of SARS-CoV-2 in the United States throughout the COVID-19 pandemic in 2020 and 2021. Vaccinations granted Emergency Use Authorization in late 2020 and early 2021 offered another measure for preventing morbidity and mortality from COVID-19. Combining pharmaceutical and nonpharmaceutical interventions was estimated to further decrease transmission. However, pressure to repeal mask mandates is growing as more and more people are vaccinated. Therefore, it is vital to understand how changing masking behaviors and increasing proportion of individuals vaccinated impact the transmission dynamics of SARS-CoV-2. Methods: I evaluated how changes in masking and vaccinated proportions over time impacted cases, hospitalizations, and deaths. Outcomes were measured under different constant masking levels for the duration of the epidemic. Outcomes were also measured as masking rate decreased continuously in the vaccinated and unvaccinated subpopulations as vaccinated proportion increased in the full population. These scenarios were both tested in the presence of variants of concern with a 50% higher R0 value. Results: Higher masking proportions were consistent with lower cases, hospitalizations, and deaths. Decreasing masking continuously over time led to significantly higher cases than maintaining high masking throughout. This was seen more strongly when making decreased in the unvaccinated subpopulation, as compared to the vaccinated subpopulation. Presence of variants of concern exacerbated these trends, leading to high cases, hospitalizations, and deaths when masking was relaxed. Conclusion: Reducing masking in any group leads to an increase in cases, hospitalizations, and deaths, particularly in the presence of highly transmissible variants

Temporal and spatial correlation in chick production of willow grouse Lagopus lagopus in Sweden and Norway

We analysed the spatial and temporal dynamics of chick production of willow grouse "Lagopus lagopus" in the Swedish and Norwegian mountain region using harvest data covering 24-38 years and line-transect counts covering 6-10 years from the period 1960-2003. Juvenile and adult grouse were counted in the bag of harvested willow grouse in late August and throughout September at six sites in Sweden and Norway. In addition, numbers of juvenile and adult willow grouse were obtained from ine-transect counts at 21 sites in Sweden just before the hunting season started on 25 August. The juvenile:adult ratio from the harvest data, estimated as number of juveniles per two adults, showed similar long-term averages and distributions between all six sites. The results from the line-transect counts revealed an overall higher average and a greater range of production between sites. We suggest that the difference between estimates based on line-transect counts and harvest data are caused by hunter behaviour. Hunters did not sample the juvenile and adult grouse in proportion to what was present in the population, e.g., productivity estimates derived from harvest samples will underestimate the proportion of juveniles in autumn in most years. We suggest that it is the harvest process that acts to adjust the juvenile to old bird ratio in the harvest data to a similar distribution in different sites. We found little evidence of regular fluctuations/cycles in annual production of juveniles from either harvest data or line-transect data. We conclude that chick production in willow grouse appears to fluctuate more irregularly than was previously believed based on harvest data and can not be used as a potential forecasting tool in willow grouse management as suggested earlier. Spatial correlation was weak and investigating only a limited number of sites to predict the proportion of juveniles in autumn should not be considered as a management tool for large areas such as the state-owned land in Sweden. Line-transect counts are costly, but do provide a more accurate estimate of the proportion of juveniles in the fall population than is revealed by harvest data. A more detailed understanding of hunter behaviour is obviously needed for better interpretation of harvest data

On the determination of internal-wave directional spectra from moored instruments

Presented here is a model for determining the characteristic parameters of an internalwave field using cross spectra of data obtained from vertically separated and/or horizontally separated moored instruments. The model is based on the assumption that the motion consists mainly of linear free internal waves having random phase relationships. The internalwave energy is supposed to be distributed among a finite number of modes having continuous directional distribution...

A technique for the determination of surface heat and freshwater fluxes from hydrographic observations, using an approximate adjoint ocean circulation model

A technique to determine the large-scale time-averaged ocean circulation from hydrographic observations and surface flux estimates is described. It is based on an inversion of the Bryan-Cox ocean general circulation model. We have constructed an approximate adjoint to that model which is computationally simpler and more economic than the exact adjoint. The optimization algorithm, although not optimal in a statistical sense, allows calculation of all state variables such that they are consistent with the equilibrium dynamics of the circulation model and agree as closely as possible with the observed data. To verify the technique, we have performed identical twin experiments with the circulation model in an idealized geometry. It is found that in principle the true model state including surface fluxes can be recovered with acceptable accuracy, even if no information on the surface fluxes is available. Under ideal conditions, the resulting rms errors of the surface fluxes were as low as 3 W/m2 and 0.2 m/year for heat and freshwater, respectively. Regions of deep-water formation due to convection show however larger errors on a small spatial scale, depending on the nonlinear, threshold-like nature of convective adjustment. The optimized solutions are distinctly sensitive to the integration time interval, with optimal values around three years. The results suggest that the procedure is suitable to obtain a consistent description of the oceanic state, and in particular more accurate estimates of the air-sea heat and freshwater fluxes

Recent advances in modelling the ocean circulation and its effects on climate

The authors aim to acquaint the reader with the current state of ocean circulation models, their ability to model the present climate state and its variability, and their major shortcomings and uncertainties. They limit the discussion to three-dimensional models of the physical system. They begin by describing the basic structure of circulation models, and discussing various problems with their implementation. They give a brief overview of the types of observational data in oceanography, and the ways in which the data are used. Some results from models of the wind-driven circulation are discussed, with particular emphasis on the dynamics of mesoscale eddies. Considerable progress has been made in understanding short-term variability associated with ENSO, and the authors describe ocean-atmosphere interactions in the tropics as well as results from coupled ocean-atmosphere models for ENSO variability. Models of the thermohaline circulation are described and some emerging ideas regarding long-term changes are given

The influence of numerical advection schemes on the results of ocean general circulation models

The dependence of results from coarse-resolution models of the North Atlantic circulation on the numerical advection algorithm is studied. In particular, the sensitivity of parameters relevant for climate simulations as e.g., meridional transport of mass and heat and main thermocline thickness is investigated. Three algorithms were considered: (a) a central difference scheme with constant values for horizontal and vertical diffusion, (b) an upstream scheme with no explicit diffusion, and (c) a flux-corrected transport (FCT) scheme with constant and strictly isopycnal diffusion. The temporal evolution of the three models on time scales of centuries is markedly different, the upstream scheme resulting in much shorter adjustment time whereas the central difference scheme is slower and controlled by vertical diffusion rather than advection. In the steady state, the main thermocline structure is much less diffusive in the FCT calculation which also has much lower heat transport. Both horizontal circulation and overturning in the meridional-vertical plane are strongest in the upstream-model. The results are discussed in terms of the effective vertical (diapycnal) mixing in the different models. A significant increase in vertical resolution would be required to eliminate the high sensitivity due to the numerical algorithms, and allow physically motivated mixing formulations to become effective