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Simulated inter-annual to decadal variability and trends in the North Atlantic for the 1958–2007 period from twenty global ocean – sea-ice coupled models are presented. These simulations are performed as contributions to the second phase of the Coordinated Ocean-ice Reference Experiments (CORE-II). The study is Part II of our companion paper (Danabasoglu et al., 2014) which documented the mean states in the North Atlantic from the same models. A major focus of the present study is the representation of Atlantic meridional overturning circulation (AMOC) variability in the participating models. Relationships between AMOC variability and those of some other related variables, such as subpolar mixed layer depths, the North Atlantic Oscillation (NAO), and the Labrador Sea upper-ocean hydrographic properties, are also investigated. In general, AMOC variability shows three distinct stages. During the first stage that lasts until the mid- to late-1970s, AMOC is relatively steady, remaining lower than its long-term (1958–2007) mean. Thereafter, AMOC intensifies with maximum transports achieved in the mid- to late-1990s. This enhancement is then followed by a weakening trend until the end of our integration period. This sequence of low frequency AMOC variability is consistent with previous studies. Regarding strengthening of AMOC between about the mid-1970s and the mid-1990s, our results support a previously identified variability mechanism where AMOC intensification is connected to increased deep water formation in the subpolar North Atlantic, driven by NAO-related surface fluxes. The simulations tend to show general agreement in their temporal representations of, for example, AMOC, sea surface temperature (SST), and subpolar mixed layer depth variabilities. In particular, the observed variability of the North Atlantic SSTs is captured well by all models. These findings indicate that simulated variability and trends are primarily dictated by the atmospheric datasets which include the influence of ocean dynamics from nature superimposed onto anthropogenic effects. Despite these general agreements, there are many differences among the model solutions, particularly in the spatial structures of variability patterns. For example, the location of the maximum AMOC variability differs among the models between Northern and Southern Hemispheres. [ABSTRACT FROM AUTHOR]

Highlights: [•] Phase II of the Coordinated Ocean-ice Reference Experiments (CORE-II) is introduced. [•] Solutions from CORE-II simulations from eighteen participating models are presented. [•] Mean states in the North Atlantic with a focus on AMOC are examined. [•] The North Atlantic solutions differ substantially among the models. [•] Many factors, including parameterization choices, contribute to these differences. [ABSTRACT FROM AUTHOR]

Abstract: Simulating the oceanic circulation in Eastern Boundary Upwelling Systems (EBUS) is a challenging issue due to the paucity of wind stress products of a sufficiently high spatial resolution to simulate the observed upwelling dynamics. In this study, we present the results of regional simulations of the Humboldt current system (Peru and Chile coasts) to assess the value of a statistical downscaling model of surface forcing. Twin experiments that differ only from the momentum flux forcing are carried out over the 1992–2000 period that encompasses the major 1997/98 El Niño/La Niña event. It is shown that the mean biases of the oceanic circulation can be drastically reduced simply substituting the mean wind field of NCEP reanalysis by a higher resolution mean product (QuikSCAT). The statistical downscaling model improves further the simulations allowing more realistic intraseasonal and interannual coastal undercurrent variability, which is notoriously strong off Central Peru and Central Chile. Despite some limitations, our results suggest that the statistical approach may be useful to regional oceanic studies of present and future climates. [Copyright &y& Elsevier]

Abstract: The Mediterranean basin features a semi-enclosed sea, where interactions and feedbacks between the atmosphere and the Sea at various temporal and spatial scales play a predominant role in the regional climate. This study analyzes the Mediterranean Sea response in sensitivity experiments conducted by driving the NEMO-MED12 oceanic model in perpetual mode with various atmospheric forcings, all produced by the WRF non-hydrostatic mesoscale atmospheric model, but differing by their resolutions: two horizontal resolutions (20km at basin scale and 6.7km in the North-Western [NWE] area) and two temporal resolutions (daily and three-hourly). The atmospheric fields available from August 1998 to July 1999 are in good agreement with estimates derived from satellite data. The heat budget of the Mediterranean Sea represents an heat loss of 5W/m2 and the annual freshwater budget is −1.04m, in agreement with climatologies. An increase in the spatial resolution in the NWE area modifies the modeled circulation from −10% to +15% for the SST, from −30% to +50% for the SSS, from −10% to +30% for the MLD and from −10% to +30% for the EKE in surface. The increase in the wind speed with a better chanelling by the land orography enhances in particular the oceanic convection process in the NWE area. On the other hand, the increase in the temporal resolution reduces the convection process, because of the diurnal restratification of the oceanic upper layer. It also reduces the surface parameters high-frequency variability, whereas it increases the EKE values in surface, due to the rapid response to the wind. [Copyright &y& Elsevier]

Abstract: We develop, calibrate and test a dataset intended to drive global ocean hindcasts simulations of the last five decades. This dataset provides surface meteorological variables needed to estimate air-sea fluxes and is built from 6-hourly surface atmospheric state variables of ERA40. We first compare the raw fields of ERA40 to the CORE.v1 dataset of , used here as a reference, and discuss our choice to use daily radiative fluxes and monthly precipitation products extracted from satellite data rather than their ERA40 counterparts. Both datasets lead to excessively high global imbalances of heat and freshwater fluxes when tested with a prescribed climatological sea surface temperature. After identifying unrealistic time discontinuities (induced by changes in the nature of assimilated observations) and obvious global and regional biases in ERA40 fields (by comparison to high quality observations), we propose a set of corrections. Tropical surface air humidity is decreased from 1979 onward, representation of Arctic surface air temperature is improved using recent observations and the wind is globally increased. These corrections lead to a significant decrease of the excessive positive global imbalance of heat. Radiation and precipitation fields are then submitted to a small adjustment (in zonal mean) that yields a near-zero global imbalance of heat and freshwater. A set of 47-year-long simulations is carried out with the coarse-resolution (2°×2°) version of the NEMO OGCM to assess the sensitivity of the model to the proposed corrections. Model results show that each of the proposed correction contributes to improve the representation of central features of the global ocean circulation. [Copyright &y& Elsevier]

Abstract: The relationship between various atmospheric variables at the sea surface and climatological monthly means of sea surface temperature (SST) is investigated over the global ocean. The goal is to quantify the change in SST that results solely from variations in a particular atmospheric variable. This is accomplished using a series of numerical simulations from an atmospherically-forced ocean general circulation model (OGCM). It is first demonstrated that SST variations at all latitudes are generally strongly and positively correlated with increases in near-surface air temperature, and vapor mixing ratio and net shortwave radiation at the sea surface, while they are often moderately and negatively correlated with increases in near-surface wind speed. There is only a weak and negative relationship between variations in SST and those in net longwave radiation at the sea surface. Variations in the net shortwave radiation and vapor mixing ratio are found to have more influence in driving the seasonal cycle of SST than other atmospheric variables. Global averages of slope values from the least squares fit indicate that a 1°C change in air temperature results in ≈0.2°C change in SST. Similarly, a 1gkg−1 change in vapor mixing ratio gives ≈0.4°C change in SST, and 10Wm−2 change in shortwave (longwave) radiation results in ≈0.13°C (≈0.07°C) change in SST. All these values vary regionally, and are neither constant nor in the same direction everywhere. In addition, some atmospheric variables are already correlated to each other. Therefore, a fractional factorial design which involves the joint effects of all atmospheric variables on SST at the same time is further applied. Results from the factorial design are somewhat consistent with the simple linear regression analysis, in that a 2°C increase in air temperature can typically give an increase in SST, generally ranging between 0.5 and 0.8°C over the global ocean. [Copyright &y& Elsevier]

Abstract: Coordinated Ocean-ice Reference Experiments (COREs) are presented as a tool to explore the behaviour of global ocean-ice models under forcing from a common atmospheric dataset. We highlight issues arising when designing coupled global ocean and sea ice experiments, such as difficulties formulating a consistent forcing methodology and experimental protocol. Particular focus is given to the hydrological forcing, the details of which are key to realizing simulations with stable meridional overturning circulations. The atmospheric forcing from [Large, W., Yeager, S., 2004. Diurnal to decadal global forcing for ocean and sea-ice models: the data sets and flux climatologies. NCAR Technical Note: NCAR/TN-460+STR. CGD Division of the National Center for Atmospheric Research] was developed for coupled-ocean and sea ice models. We found it to be suitable for our purposes, even though its evaluation originally focussed more on the ocean than on the sea-ice. Simulations with this atmospheric forcing are presented from seven global ocean-ice models using the CORE-I design (repeating annual cycle of atmospheric forcing for 500 years). These simulations test the hypothesis that global ocean-ice models run under the same atmospheric state produce qualitatively similar simulations. The validity of this hypothesis is shown to depend on the chosen diagnostic. The CORE simulations provide feedback to the fidelity of the atmospheric forcing and model configuration, with identification of biases promoting avenues for forcing dataset and/or model development. [Copyright &y& Elsevier]

Abstract: The surface salinity response to changes of various external forcings and model parameters is investigated in a regional 1/3° configuration of the NEMO-OPA model implemented over the eastern North-Atlantic subtropical Ocean. Fourteen realistic climatological simulations are run. By default, neither relaxation to climatological surface salinity nor temperature is included. Forcing fields and parameters expected to impact the surface salinity are modified. These include: the wind stress, the surface temperature, wind speed and relative humidity entering the bulk calculation for sensible and latent heat fluxes, the precipitation, the data specified at open boundaries, the lateral viscosity operator, the salt lateral diffusivity, the vertical mixing scheme, the on/off switch of the double diffusion parameterization, the river runoffs and the SSS/SST restoring terms. The SSS standard deviation over this ensemble of model simulations is of the order of 0.1psu, which is also the order of the annual cycle of the surface salinity field in this area. In this experimental framework, the wind stress is found to have the largest impact on the model SSS, both in terms of mean field and annual variability. The sensitivity to the precipitation, atmospheric temperature, open boundary external data, and to the relaxation to climatological SSS and SST, is also significant. Of all the model parameterizations, the lateral salt diffusivity is the one associated to the strongest surface salinity model response. This work allows the identification of the main sources of error for model surface salinity. It paves the way for a more comprehensive investigation of SSS model error statistics, which is needed for future data assimilation experiments motivated by the close launch of the new SSS-observing SMOS and Aquarius satellite missions. [Copyright &y& Elsevier]

The response of the Antarctic circumpolar circulation to wind stress variability is studied in a simple model of the Southern Ocean. The model consists of a zonally reentrant channel with mid-ocean barrier, forced by surface heat flux and stochastic wind stress. The MITgcm code is used to solve for temperature, sea level, and the velocity field.The channel transport responds actively to the stochastic wind forcing through the excitation of several eigenmodes. The fundamental basin mode is excited, which has a period of about 20 days. However, the response is dominated by a topographic mode with a period of 3 days. This barotropic mode is related primarily to topographic Rossby waves propagating on the submarine sill, and Kelvin waves propagating westward on the southern boundary. The mode shares characteristics with numerical predictions of normal modes in the Southern Ocean, as well as with recently observed modes of variability around Antarctica, with periods between 1 and 2 days. These results show that both topographic normal modes and Rossby basin modes can play a role in the adjustment of the ACC to high-frequency wind stress variability.The presence of the resonance in the system leads to interesting phase behavior between the wind stress forcing and the ocean transport: for a considerable frequency range, the transport seems to lead the wind stress variability. This shows that the classical notion of transport passively lagging wind forcing is inadequate when eigenmodes are excited. [Copyright &y& Elsevier]

Climate change and increased coastal urbanization are causing low-lying coastlines to become increasingly susceptible to the threat of extreme water levels and coastal flooding. Robust decision-making on adaptation in the coastal zone, based on reliable ocean-modelling tools, is therefore crucially contingent on accurate assessments of current and future storm surge hazards. This accuracy relies considerably on the quality of the wind forcing used in the ocean models. In this paper, we use a high-resolution, regional 3D ocean model (HBM) covering the North Sea and Baltic Sea to simulate extreme water levels during three extreme storm surge events with different dynamics and patterns, in order to assess their impacts along Denmark's coastlines, which are of varying levels of complexity. We demonstrate that the model is able to reproduce the observed extreme high-water levels accurately, indicating that the system is well suited for producing simulations of present and future projections of extreme storm surges with high resulting impacts and damage potentials. Additionally, we quantify the level at which acknowledged deficiencies in the otherwise most suitable atmospheric forcing data set influence the results of the storm surge simulations. We found that reducing the temporal resolution of the forcing data – that is, replacing two out of every six time stamps with linearly interpolated values – is preferable to using the original forcing data set when recurring noise is present in these time stamps. As a result, for given storm surge events, and depending on the stage reached in the storm's evolution, mean absolute errors can be reduced by 4.5 cm. This emphasizes the importance of considering such model fluctuations when coupling high-resolution atmosphere and ocean models. [Display omitted] • 3D ocean model HBM skillfully reproduces extreme water levels along Danish coastlines. • Performance improves when two out of every six forcing time stamps are removed. • Slightly reduced temporal resolution tops higher resolution but noisy data. • Based on these results, we find the HBM model validated for future projection runs. [ABSTRACT FROM AUTHOR]