The influence of high-frequency atmospheric forcing on the circulation of the North Atlantic Ocean with emphasis on the deep convection of the Labrador Sea was investigated by comparing simulations of a coupled ocean-ice model with hourly atmospheric data to simulations in which the high-frequency phenomena were filtered from the air temperature and wind fields. In the absence of high-frequency atmospheric forcing, the strength of the Atlantic meridional overturning circulation and subpolar gyres was found to decrease by 25%. In the Labrador Sea, the eddy kinetic energy decreased by 75% and the average maximum mixed layer depth decreased by between 20% and 110% depending on the climatology. In particular, high-frequency forcing was found to have a greater impact on mixed layer deepening in moderate to warm years whereas in relatively cold years the temperatures alone were enough to facilitate deep convection. Additional simulations in which either the wind or temperature was filtered revealed that the wind, through its impact on the bulk formulas for latent and sensible heat, had a greater impact on deep convection than the temperature.
Model projections of ocean circulation and biogeochemistry are used to investigate large scale climate changes under moderate mitigation (RCP 4.5) and high emissions (RCP 8.5) scenarios along the continental shelf of the Canadian Pacific Coast. To reduce computational cost, an approach for dynamical downscaling of climate projections was developed that uses atmospheric climatologies with augmented winds to simulate historical (1986–2005) and future (2046–2065) periods separately. The two simulations differ in initial and lateral open boundary conditions. For each simulation, the daily climatology of surface winds in the driving model was augmented with high-frequency variability from an atmospheric reanalysis product. The “time-slice” approach was able to reproduce the observed climate state for the historical period. Sensitivity tests confirmed that the high frequency wind variability plays an essential role in freshwater distribution in this region. Projections suggest that sea surface temperature will increase by 1.8–2.4°C and surface salinity will decrease between −0.08 and −0.23 depending on whether a moderate or high emissions scenario is used. Stratification increases throughout the region and there is some evidence of nutrient limitation near the surface. Primary production and phytoplankton productivity (chlorophyll) also increase. Density surfaces are relocated deeper in the water column and this change is mainly driven by surface heating and freshening. Changes in saturation state are mainly due to anthropogenic CO2 with minor contributions from solubility, remineralization and advection. There is little difference between RCP 4.5 and RCP 8.5 with regard to projections of deoxygenation and acidification. The depths of the aragonite saturation state and the oxygen minimum zone are projected to become shallower by ≃ 100 and ≃ 75 m respectively. Extreme states of temperature, oxygen and acidification are projected to become more frequent and more extreme, with the frequency of occurrence of [O2]<60 mmolm-3 expected to approximately double under either scenario.
Abstract. Process-based numerical models are a useful tool for studying marine ecosystems and associated biogeochemical processes in ice-covered regions where observations are scarce. To this end, CSIB v1 (Canadian Sea-ice Biogeochemistry version 1), a new sea-ice biogeochemical model, has been developed and embedded into the Nucleus for European Modelling of the Ocean (NEMO) modelling system. This model consists of a three-compartment (ice algae, nitrate, and ammonium) sea-ice ecosystem and a two-compartment (dimethylsulfoniopropionate and dimethylsulfide) sea-ice sulfur cycle which are coupled to pelagic ecosystem and sulfur-cycle models at the sea-ice–ocean interface. In addition to biological and chemical sources and sinks, the model simulates the horizontal transport of biogeochemical state variables within sea ice through a one-way coupling to a dynamic-thermodynamic sea-ice model (LIM2; the Louvain-la-Neuve Sea Ice Model version 2). The model results for 1979 (after a decadal spin-up) are presented and compared to observations and previous model studies for a brief discussion on the model performance. Furthermore, this paper provides discussion on technical aspects of implementing the sea-ice biogeochemistry and assesses the model sensitivity to (1) the temporal resolution of the snowfall forcing data, (2) the representation of light penetration through snow, (3) the horizontal transport of sea-ice biogeochemical state variables, and (4) light attenuation by ice algae. The sea-ice biogeochemical model has been developed within the generic framework of NEMO to facilitate its use within different configurations and domains, and can be adapted for use with other NEMO-based sub-models such as LIM3 (the Louvain-la-Neuve Sea Ice Model version 3) and PISCES (Pelagic Interactions Scheme for Carbon and Ecosystem Studies).
Lock-release laboratory experiments have been performed to examine the collapse of a localized cylindrical mixed patch of fluid in both two-layer and uniformly stratified ambients. The experiments were performed with and without rotation. Unlike bottom propagating gravity currents, non-rotating intrusions typically propagated many lock radii at constant speed, sometimes stopping abruptly due to interactions with internal waves generated by the return flow into the lock. The initial front speeds of the resulting approximately axisymmetric intrusions emanating from locks of comparable depth and radius were found to be 30%–35% less than the predictions of rectilinear theory. Rotation had no impact on the intrusion's initial speed. However, it limited the maximum distance propagated by the intrusion and caused the patch to expand and contract as it approached geostrophic balance.
The evolution of the stable boundary layer is simulated using an idealized single-column model of pressure-driven flow coupled to a surface energy budget. Several commonly used parameterizations of turbulence are examined. The agreement between the simulated wind and temperature profiles and tower observations from the Cabauw tower is generally good given the simplicity of the model. The collapse and recovery of turbulence is explored in the presence of a large-scale pressure gradient, but excluding transient submesoscale atmospheric forcings such as internal waves and density-driven currents. The sensitivity tests presented here clarify the role of both rotation and the surface energy budget in the collapse and recovery of turbulence for the pressure-driven dry stable boundary layer (SBL). Conditions of stability are affected strongly by the geostrophic winds, the cloud cover, and the thermal conductivity of the surface. Inertial oscillations and the subsurface temperature have a weaker influence. Particularly noteworthy is the relationship between SBL regime and the relative importance of the terms in the surface energy budget.
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