Description: The CMCC Global Ocean Physical Reanalysis System (C-GLORS) is used to simulate the state of the ocean in the last decades. It consists of a variational data assimilation system (OceanVar), capable of assimilating all in-situ observations along with altimetry data, and a forecast step performed by the ocean model NEMO coupled with the LIM2 sea-ice model. KEY STRENGTHS: - Data are available for a large number of ocean parameters - An extensive validation has been conducted and is freely available - The reanalysis is performed at high resolution (1/4 degree) and spans the last 30 years KEY LIMITATIONS: - Quality may be discontinuos and depend on observation coverage - Uncertainty estimates are simply derived through verification skill scores Note (2017-02-09): C-GLORSv5 is dissemniated on PANGAEA at reduced resolution, i.e. a regular 0.5x0.5 degree grid. For full resolution data sets see "CMCC Global Ocean Reanalysis System (C-GLORS), external website" (Related to:)

Description: The Euro-Mediterranean Center on Climate Change Global Ocean Physical Reanalysis (CGOLRS) v7 is a homogeneous three-dimensional reconstruction of the global ocean state over the last decades constrained by observations. It spans the altimeter era (1993-2019) and is produced by the global ocean circulation model NEMO coupled with the LIM2 sea-ice model with an overall working resolution of 1/4°. The data assimilation system consists of a three-dimensional variational scheme (called OceanVar) capable of assimilating all in-situ observations along with altimetry data. The data assimilation system is performed every 7 days, using a 10-day assimilation time-window. On-line surface nudging schemes, during model integration, assimilate satellite observations of sea surface temperature, sea ice concentration and sea ice thickness. An older version (v5) of the same Reanalysis family but spanning more years 1980-2014 is available at doi:10.1594/PANGAEA.857995. In the v7 we increased the vertical resolution (75 depth levels) and included an observational quality control (VarQC) with non-gaussian error that allows to ingest more observations. An upgraded ocean model version (NEMO-LIM2 3.6) is also used together with a general retuning of the DA scheme. Data are available for a large number of ocean parameters and an extensive validation has been conducted together with a comparison with other Reanalyses. The quality may depend on observation coverage and uncertainty estimates are simply derived through verification skill scores. A more detailed description can be found at http://c-glors.cmcc.it and related references. To access other variables please refer to https://dds.cmcc.it/

Description: The Atlantic meridional overturning circulation and meridional heat transport (hereafter the AMOC and MHT) at 34°S as simulated by global 1/16° eddy-rich (henceforth GLOB16) and ¼° eddy-permitting (GLOB4) models are compared with observational estimates. Three different methods are used for calculating the modelled AMOC: the first method (MOCmod) is based on simulated velocity fields, while the second (MOCob) relies on the same assumptions as available observed-derived estimates. The third method (MOCob2) is also based on hydrostatic and geostrophic relationships, but relative to a barotropic circulation instead of the definition of velocity at a specific reference depth. All methods correctly reproduce the time-mean GLOB16 AMOC strength, but the value of the non-Ekman component of the GLOB16 AMOC is only about 75% of the observed-derived estimate. The GLOB16 MHT is also significantly less than observation value (slightly more than 60% of the observed). However, the mean AMOC and MHT values at 34°S obtained with coarser resolution GLOB4 model are comparable with the observed-derived estimates. Possible causes for the differences between the eddy-rich model and observational data are studied. It is shown that the density field from the eddy-rich model has high temporal variability along 34°S with spatial scale of about two hundred km that can be due to mesoscale variations, caused by Agulhas “leakage”. This results in the decrease of the mean meridional geostrophic velocity, which leads to smaller values of the AMOC and MHT in GLOB16: subsampling GLOB16 density on ¼° or ½° longitude grid along 34°S for MOCob calculation significantly increases the AMOC values. The findings in this paper provide guidance in understanding AMOC and MHT dissimilarities between observation-based estimates and eddy-rich ocean models at 34°S. This article is protected by copyright. All rights reserved.

Description: The Atlantic meridional overturning circulation (AMOC) and the associated meridional heat transport (MHT) at 26.5°N are investigated in two global ocean models at different resolutions and setup and compared with observational estimates from the Rapid Climate Change programme (RAPID). Three different methods of calculation are used to compute the modelled meridional transports: the first method (MOCmod) is based on simulated velocity fields, the second (MOC_endpoint) relies on the same assumptions as for the RAPID calculations, and the third (MOC_ff_baro) is also based on hydrostatic and geostrophic relationships, but relative to the model barotropic circulation. All methods correctly reproduce the time-mean AMOC strength at 26.5°N, although some differences with observations are present at depth. Similar to other model results, despite the higher AMOC simulated by our eddy-rich global model, the corresponding heat transport is significantly lower than the RAPID estimates. The above described differences between the deep structure of the modelled and observed AMOC impact the heat transport less than the discrepancies between the MOCmod and MOC_endpoint AMOC structure in the upper ∼500m layer. The comparison of the AMOC obtained by the three methods suggests that an inadequate representation of currents near the western boundary by geostrophy leads to underestimate the southward circulation in the upper-mid ocean and largely impact the heat transport calculations. In our analysis, calculation based on RAPID assumptions (applied in MOC_endpoint) result in a higher mean heat transport (〉25%) compared to MOCmod calculation. This article is protected by copyright. All rights reserved.

Description: A set of four eddy-permitting global ocean reanalyses produced in the framework of the MyOcean project have been compared over the altimetry period 1993–2011. The main differences among the reanalyses used here come from the data assimilation scheme implemented to control the ocean state by inserting reprocessed observations of sea surface temperature (SST), in situ temperature and salinity profiles, sea level anomaly and sea-ice concentration. A first objective of this work includes assessing the interannual variability and trends for a series of parameters, usually considered in the community as essential ocean variables: SST, sea surface salinity, temperature and salinity averaged over meaningful layers of the water column, sea level, transports across pre-defined sections, and sea ice parameters. The eddy-permitting nature of the global reanalyses allows also to estimate eddy kinetic energy. The results show that in general there is a good consistency between the different reanalyses. An intercomparison against experiments without data assimilation was done during the MyOcean project and we conclude that data assimilation is crucial for correctly simulating some quantities such as regional trends of sea level as well as the eddy kinetic energy. A second objective is to show that the ensemble mean of reanalyses can be evaluated as one single system regarding its reliability in reproducing the climate signals, where both variability and uncertainties are assessed through the ensemble spread and signal-to-noise ratio. The main advantage of having access to several reanalyses differing in the way data assimilation is performed is that it becomes possible to assess part of the total uncertainty. Given the fact that we use very similar ocean models and atmospheric forcing, we can conclude that the spread of the ensemble of reanalyses is mainly representative of our ability to gauge uncertainty in the assimilation methods. This uncertainty changes a lot from one ocean parameter to another, especially in global indices. However, despite several caveats in the design of the multi-system ensemble, the main conclusion from this study is that an eddy-permitting multi-system ensemble approach has become mature and our results provide a first step towards a systematic comparison of eddy-permitting global ocean reanalyses aimed at providing robust conclusions on the recent evolution of the oceanic state.

Description: Published

Description: 813–841

Description: 4A. Oceanografia e clima

Description: JCR Journal

Description: Since 2016, the Copernicus Marine Environment Monitoring Service (CMEMS) has produced and disseminated an ensemble of four global ocean reanalyses produced at eddy-permitting resolution for the period from 1993 to present, called GREP (Global ocean Reanalysis Ensemble Product). This dataset offers the possibility to investigate the potential benefits of a multi-system approach for ocean reanalyses, since the four reanalyses span by construction the same spatial and temporal scales. In particular, our investigations focus on the added value of the information on the ensemble spread, implicitly contained in the GREP ensemble, for temperature, salinity, and steric sea level studies. It is shown that in spite of the small ensemble size, the spread is capable of estimating the flow-dependent uncertainty in the ensemble mean, although proper re-scaling is needed to achieve reliability. The GREP members also exhibit larger consistency (smaller spread) than their predecessors, suggesting advancement with time of the reanalysis vintage. The uncertainty information is crucial for monitoring the climate of the ocean, even at regional level, as GREP shows consistency with CMEMS high-resolution regional products and complement the regional estimates with uncertainty estimates. Further applications of the spread include the monitoring of the impact of changes in ocean observing networks; the use of multi-model ensemble anomalies in hybrid ensemble-variational retrospective analysis systems, which outperform static covariances and represent a promising application of GREP. Overall, the spread information of the GREP product is found to significantly contribute to the crucial requirement of uncertainty estimates for climatic datasets.

Description: Data from the reanalyses presented in this work are available from the Copernicus Marine Environment Monitoring Service (CMEMS, http://marine.copernicus.eu/). Part of this work was supported by the EOS COST Action (“Evaluation of Ocean Synthesis”, http://eos-cost.eu/) through its Short Term Scientific Missions program. The full C-GLORS dataset is available at http://c-glors.cmcc.it. This work has received funding from the Copernicus Marine Environment Monitoring Service (CMEMS).

Description: Published

Description: 287-312

Description: 4A. Oceanografia e clima

Description: JCR Journal

Description: The interannual-decadal variability of the wintertime mixed layer depths (MLDs) over the North Pacific is investigated from an empirical orthogonal function (EOF) analysis of an ensemble of global ocean reanalyses. The first leading EOF mode represents the interannual MLD anomalies centered in the eastern part of the central mode water formation region in phase opposition with those in the eastern subtropics and the central Alaskan Gyre. This first EOF mode is highly correlated with the Pacific decadal oscillation index on both the interannual and decadal time scales. The second leading EOF mode represents the MLD variability in the subtropical mode water (STMW) formation region and has a good correlation with the wintertime West Pacific (WP) index with time lag of 3 years, suggesting the importance of the oceanic dynamical response to the change in the surface wind field associated with the meridional shifts of the Aleutian Low. The above MLD variabilities are in basic agreement with previous observational and modeling findings. Moreover the reanalysis ensemble provides uncertainty estimates. The interannual MLD anomalies in the first and second EOF modes are consistently represented by the individual reanalyses and the amplitudes of the variabilities generally exceed the ensemble spread of the reanalyses. Besides, the resulting MLD variability indices, spanning the 1948–2012 period, should be helpful for characterizing the North Pacific climate variability. In particular, a 6-year oscillation including the WP teleconnection pattern in the atmosphere and the oceanic MLD variability in the STMW formation region is first detected.

Description: Published

Description: 891–907

Description: 4A. Oceanografia e clima

Description: JCR Journal

Description: Intercomparison and evaluation of the global ocean surface mixed layer depth (MLD) fields estimated from a suite of major ocean syntheses are conducted. Compared with the reference MLDs calculated from individual profiles, MLDs calculated from monthly mean and gridded profiles show negative biases of 10–20 m in early spring related to the re-stratification process of relatively deep mixed layers. Vertical resolution of profiles also influences the MLD estimation. MLDs are underestimated by approximately 5–7 (14–16) m with the vertical resolution of 25 (50) m when the criterion of potential density exceeding the 10-m value by 0.03 kg m−3 is used for the MLD estimation. Using the larger criterion (0.125 kg m−3) generally reduces the underestimations. In addition, positive biases greater than 100 m are found in wintertime subpolar regions when MLD criteria based on temperature are used. Biases of the reanalyses are due to both model errors and errors related to differences between the assimilation methods. The result shows that these errors are partially cancelled out through the ensemble averaging. Moreover, the bias in the ensemble mean field of the reanalyses is smaller than in the observation-only analyses. This is largely attributed to comparably higher resolutions of the reanalyses. The robust reproduction of both the seasonal cycle and interannual variability by the ensemble mean of the reanalyses indicates a great potential of the ensemble mean MLD field for investigating and monitoring upper ocean processes.

Description: Published

Description: 753–773

Description: 4A. Oceanografia e clima

Description: JCR Journal

Description: Decadal climate predictability in the South Atlantic is explored by performing reforecast experiments using a coupled general circulation model with two initialization schemes; one is assimilated with observed sea surface temperature (SST) only, and the other is additionally assimilated with observed subsurface ocean temperature and salinity. The South Atlantic is known to undergo decadal variability exhibiting a meridional dipole of SST anomalies through variations in the subtropical high and ocean heat transport. Decadal reforecast experiments in which only the model SST is initialized with the observation do not predict well the observed decadal SST variability in the South Atlantic, while the other experiments in which the model SST and subsurface ocean are initialized with the observation skillfully predict the observed decadal SST variability, particularly in the Southeast Atlantic. In-depth analysis of upper-ocean heat content reveals that a significant improvement of zonal heat transport in the Southeast Atlantic leads to skillful prediction of decadal SST variability there. These results demonstrate potential roles of subsurface ocean assimilation in the skillful prediction of decadal climate variability over the South Atlantic.

Description: Published

Description: id 8523

Description: 4A. Oceanografia e clima

Description: JCR Journal

Description: Ocean temperature observations are crucial for a host of climate research and forecasting activities, such as climate monitoring, ocean reanalysis and state estimation, seasonal-to-decadal forecasts, and ocean forecasting. For all of these applications, it is crucial to understand the uncertainty attached to each of the observations, accounting for changes in instrument technology and observing practices over time. Here, we describe the rationale behind the uncertainty specification provided for all in situ ocean temperature observations in the International Quality-controlled Ocean Database (IQuOD) v0.1, a value-added data product served alongside the World Ocean Database (WOD). We collected information from manufacturer specifications and other publications, providing the end user with uncertainty estimates based mainly on instrument type, along with extant auxiliary information such as calibration and collection method. The provision of a consistent set of observation uncertainties will provide a more complete understanding of historical ocean observations used to examine the changing environment. Moving forward, IQuOD will continue to work with the ocean observation, data assimilation and ocean climate communities to further refine uncertainty quantification. We encourage submissions of metadata and information about historical practices to the IQuOD project and WOD.

Description: Published

Description: 689695

Description: 4A. Oceanografia e clima

Description: JCR Journal