Thierry Penduff

DRAKKAR is a consortium of European ocean modelling teams. It was “created to take up the challenges of developing realistic global eddy-resolving/ permitting ocean/sea-ice models, and of building an ensemble of high resolution model hindcasts representing the ocean circulation from the 1960s to present” (quoting the DRAKKAR Group, 2007, in a CLIVAR Exchanges paper where the DRAKKAR strategy was presented for the first time). Now in the second decade of its existence, the DRAKKAR Group is active and thriving, and it is now timely to present recent developments and future plans in this special issue of CLIVAR Exchanges.

A global 1•4 ° ocean model simulation, performed during the DRAKKAR project, is used to investigate the interseasonal to interannual variability of thermohaline properties in subsurface waters across the Antarctic Circumpolar Current (ACC) South of Australia over the last decades. In the model simulation, subsurface variability appears to be dominated by a mode whose main characteristics are the following: a maximum of variability located at the interface between the Subantarctic Mode Water (SAMW) and the Antarctic Intermediate Water (AAIW), and an intermittent interseasonal period. This dominant mode appears to be consistent with previous studies based on hydrographic sections along the WOCE-SR3 line where the mode was observed and named the Pulsation Mode. Further investigations show that the Pulsation Mode is associated with ACC frontal variability constrained by topography. In particular, our detailed study shows that the Pulsation Mode corresponds to a baroclinic adjustment of the ACC to changes in atmospheric winds. Indeed, this mode exhibits a close correlation both with zonal wind stress South of Australia and with the Southern Annular Mode (SAM), the dominant atmospheric mode of Southern Hemisphere variability. On interseasonal to interannual periods, the regional ocean circulation is thus shown to switch between two typical states depending on the phase of the SAM. Although it is still unclear whether the Pulsation Mode induces changes in water masses formation, we emphasize that the structure of hydrographic sections South of Australia depends strongly on the SAM phase, which should therefore be considered when analysing in-situ data. In this study, model data are used to provide a detailed description of ocean variability and to analyse the associated physical processes. This study thus examplifies how ocean model simulations driven by atmospheric reanalyses can compensate for the scarcity of observational data in the Southern Ocean and offer a complementary view in order to get a better understanding of ocean variability.

ABSTRACT This global study evaluates how the varying geometry of the Argo array of profiling floats has affected the actual distributions of mixed layer depth (MLD), temperature (MLT) and heat content (MLHC) annual cycles between 2004 and 2009. These quantities' monthly distributions are computed regionally from a global 1/4° simulation with and without Argo-like subsampling, and the subsequent medians are compared. Argo-like subsampling is shown to bias the medians of MLD, MLT and MLHC distributions by about ±10m, ±1°C, ±1GJ/m2, respectively, with maximum values reaching ±100m, ±5°C, ±5GJ/m2 in certain regions and months. MLD distributions are most distorted where and when the array geometry is irregular, and where MLD distributions are far from Gaussian. The differ- ences between medians of subsampled and fully-sampled distributions are also compared to the actual width of fully-sampled MLHC distributions in every monthly regional bin to evaluate the intrinsic accuracy of the array. Comparing results from several periods (2004–2005, 2006–2007 and 2008–2009), it is shown that Argo-based estimates of mixed layer statistics have improved when the array reached its target density at the end of 2007.

Estimates of meridional heat transport (MHT) at oceanic cross-sections simulated by ocean general circulation models show only few similarities, for both climate means and temporal variability characteristics, with MHT estimates derived from hydrographic observations. The discrepancies noticed between these MHT estimates mainly reflect i) the differences in the methods used for computing the MHT with model outputs and with observations

... Penduff 1 , J.-M. Molines 1 , Arne Biastoch 2 , Sybren Drijfhout. (2008). 1 : Laboratoire des écoulements géophysiques et industriels (LEGI). CNRS : UMR5519 – Université Joseph Fourier - Grenoble I – Institut National Polytechnique de Grenoble - INPG. 2 : IfM-GEOMAR. ...

One of the primary objectives of the ARGO array is to monitor the evolution of the global ocean heat content over a wide range of timescales. Our two-step OSSE study makes use of a 50-year ocean/sea-ice gobal simulation performed by the DRAKKAR consortium to evaluate the accuracy of the ARGO array in monitoring 3 important climate indexes over the years 2000-2006: the linear trend, the seasonal, and the interannual variabilities of the global ocean. The impacts of the floats' limited vertical extent and of the 2000-2006 array's geometry are evaluated sucessively as follows. First, the model simulation is used to globally map the 3D distribution of these three 3 climate indexes, and to further estimate how ARGO's restrictions in terms of depth ranges (and thus in terms of geographical locations) may prevent the array from accessing regions of significant contributions: the aforementioned 3 indexes are evaluated both in the area potentially accessible to ARGO ("A&quot...

The DRAKKAR Group is developping a hierarchy of global ocean/sea-ice models and building en ensemble of 50-year simulations for climate- and process-oriented studies. These simulations mostly differ by their forcing, numerical parameters, and resolution. The four simulations considered here were driven by the same surface forcing but performed at 2°, 1°, 1/2° and 1/4° horizontal resolution. These simulations are collocated onto AVISO sea-level anomaly (SLA) observations, then quantitatively compared to AVISO and among themselves with respect to the first four statistical moments of SLA (mean, variance, skewness, kurtosis) within three frequency ranges. We quantify how increased model resolution progressively improves the observation-simulation agreement, not only in terms of mean flow and mesoscale activity as already known, but also in terms of e.g. spatial distribution and magnitude of large-scale interannual variabilities, distribution and statistical structure of extreme events,...

Mercator Ocean Quarterly Newsletter# 22–July 2006–Page 29 A study of model errors in surface layers due to uncertainties in the atmospheric forcing fields A study of model errors in surface layers due to uncertainties in the atmospheric forcing fields By Nadia Ayoub1, Marc ...

The penetration of atmospheric forcing into the ocean is a fundamental issue in physical oceanography and climate change science. In this study, we use a ensemble method with fifty 7-months (September to March) primitive equation ° North Atlantic integrations to investigate the processes that control the vertical transmission of small atmospheric perturbations into the ocean. We focus principally on its impact on the upper oceanic temperature field. The ensemble is generated by perturbing the wind, atmospheric temperature and incoming solar radiation of the ERA40 reanalysis. The perturbations are estimated from the differences between the ERA40 and CORE reanalysis data sets. The ensemble standard deviation of various oceanic quantities is then examined in the upper 200 metres of 3 distinct regions of the North Atlantic located in the Gulf Stream region, in the northern equatorial band and in the North East Atlantic. These show that even a very small perturbation on atmospheric varia...