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 Large and Yeager (2004), 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.
Highlights
Simulating the evolution of the global ocean over the last few decades using Ocean General Circulation models (OGCMs) has been made possible since globally gridded interannual weather reanalysis products have become available
It is important to note that the corrections applied are such that their impact on fluxes lies within the range of usual flux uncertainties
Surface atmospheric state variables of ERA40, and to a lesser extent those of NCEP, suffer from time discontinuities related to the evolution of the origin of data used in their respective assimilation process
Summary
Simulating the evolution of the global ocean over the last few decades using Ocean General Circulation models (OGCMs) has been made possible since globally gridded interannual weather reanalysis products have become available. Large and Yeager (2004), hereafter referred to as LY04, introduced a dataset for the “Coordinated Ocean Reference Experiments” carried out in the framework of the Working Group on Ocean Model Development (WGOMD) of WCRP (COREs, Griffies et al, 2009) This dataset provides the ocean modeling community with a complete long-term ocean and sea-ice forcing, intended to drive interannual OGCM inter-comparisons and ocean hindcast experiments of the last 5 decades (1958 to present). This dataset, on referred to as LYDS (Large and Yeager Data Set), is based on the NCEP reanalysis (Kalnay et al, 1996) and implements recent reconstructed flux products as a replacement for traditionally weak components of reanalyzes, such as radiation and precipitation (Table 1).
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