Search

Oceanic reanalyses are powerful products to reconstruct the historical 3D-state of the ocean and related circulation. At present a challenge is to have oceanic reanalyses covering the whole 20th century. This study describes the exercise of comparing available datasets to force Mediterranean Sea and global oceanic reanalyses from 1901 to present. In particular, we compared available atmospheric reanalyses with a set of experiments performed with an atmospheric general circulation model where sea surface temperature (SST) and sea-ice concentration are prescribed. These types of experiments have the advantage of covering long tune records, at least for the period for which global SST is available. and they can be performed at relatively high horizontal resolutions, a very important requisite for regional oceanic reanalyses. However, they are limited by the intrinsic model biases in representing the mean atmospheric state and its variability. In this study, we show that, within some limits, the atmospheric model performance in representing the basic variables needed for the bulk-formulae to force oceanic data assimilation systems can be comparable to the differences among available atmospheric reanalyses. In the case of the Mediterranean Sea the high horizontal resolution of the set of SST-prescribed experiments combined with their good performance in representing the surface winds in the area made them the most appropriate atmospheric forcing. On the other hand, in the case of the global ocean, atmospheric reanalyses have been proven to be still preferable due to the better representation of spatial and temporal variability of surface winds and radiative fluxes. Because of their intrinsic limitations AMIP experiments cannot provide atmospheric fields alternative to atmospheric reanalyses. Nevertheless, here we show how in the specific case of the Mediterranean Sea, they can be of use, if not preferable, to available atmospheric reanalyses.

In order to be able to forecast the weather and estimate future climate changes in the ocean, it is crucial to understand the past and the mechanisms responsible for the ocean variability. This is particularly true in a complex area such as the Mediterranean Sea with diverse dynamics like deep convection and overturning circulation. To this end, effective tools are ocean reanalyses or reconstructions of the past ocean state. Here we present a new physical reanalysis of the Mediterranean Sea at high resolution, developed in the Copernicus Marine Environment Monitoring Service (CMEMS) framework. The hydrodynamic model is based on the Nucleus for European Modelling of the Ocean (NEMO) combined with a variational data assimilation scheme (OceanVar). The model has a horizontal resolution of 1/24° and 141 unevenly distributed vertical z* levels. It provides daily and monthly temperature, salinity, current, sea level and mixed layer depth as well as hourly fields for surface velocities and sea level. ECMWF ERA-5 atmospheric fields force the model and daily boundary conditions in the Atlantic are taken from a global reanalysis. The reanalysis covers the 33 years from 1987 to 2019. Initialized from SeaDataNet climatology in January 1985, it reaches a nominal state after a 2-years spin-up. In-situ data from CTD, ARGO floats and XBT are assimilated into the model in combination with satellite altimetry observations. This reanalysis has been validated and assessed through comparison to in-situ and satellite observations as well as literature climatologies. The results show an overall improvement of the comparison with observations and a better representation of the main dynamics of the region compared to a previous, lower resolution (1/16°), reanalysis. Temperature and salinity RMSD are decreased by respectively 14 and 18%. The salinity biases at depth of the previous version are corrected. Climate signals show continuous increase of the temperature and salinity, confirming estimates from observations and other reanalysis. The new reanalysis will allow the study of physical processes at multi-scales, from the large scale to the transient small mesoscale structures and the selection of climate indicators for the basin.

Through tropical cyclone (TC) activity the ocean and the atmosphere exchange a large amount of energy. In this work possible improvements introduced by a higher coupling frequency are tested between the two components of a climate model in the representation of TC intensity and TC–ocean feedbacks. The analysis is based on the new Centro Euro-Mediterraneo per I Cambiamenti Climatici Climate Model (CMCC-CM2-VHR), capable of representing realistic TCs up to category-5 storms. A significant role of the negative sea surface temperature (SST) feedback, leading to a weakening of the cyclone intensity, is made apparent by the improved representation of high-frequency coupled processes. The first part of this study demonstrates that a more realistic representation of strong TC count is obtained by coupling atmosphere and ocean components at hourly instead of daily frequency. Coherently, the positive bias of the annually averaged power dissipation index associated with TCs is reduced by one order of magnitude when coupling at the hourly frequency, compared to both forced mode and daily coupling frequency results. The second part of this work shows a case study (a modeled category-5 typhoon) analysis to verify the impact of a more realistic representation of the high-frequency coupling in representing the TC effect on the ocean; the theoretical subsurface warming induced by TCs is well represented when coupling the two components at the higher frequency. This work demonstrates that an increased horizontal resolution of model components is not sufficient to ensure a realistic representation of intense and fast-moving systems, such as tropical and extratropical cyclones, but a concurrent increase in coupling frequency is required.

Euro‐Mediterranean Centre on Climate Change coupled climate model (CMCC‐CM2) represents the new family of the global coupled climate models developed and used at CMCC. It is based on the atmospheric, land and sea ice components from the Community Earth System Model coupled with the global ocean model Nucleus for European Modeling of the Ocean. This study documents the model components, the coupling strategy, particularly for the oceanic, atmospheric, and sea ice components, and the overall model ability in reproducing the observed mean climate and main patterns of interannual variability. As a first step toward a more comprehensive, process‐oriented, validation of the model, this work analyzes a 200‐year simulation performed under constant forcing corresponding to present‐day climate conditions. In terms of mean climate, the model is able to realistically reproduce the main patterns of temperature, precipitation, and winds. Specifically, we report improvements in the representation of the sea surface temperature with respect to the previous version of the model. In terms of mean atmospheric circulation features, we notice a realistic simulation of upper tropospheric winds and midtroposphere geopotential eddies. The oceanic heat transport and the Atlantic meridional overturning circulation satisfactorily compare with present‐day observations and estimates from global ocean reanalyses. The sea ice patterns and associated seasonal variations are realistically reproduced in both hemispheres, with a better skill in winter. Main weaknesses of the simulated climate are related with the precipitation patterns, specifically in the tropical regions with large dry biases over the Amazon basin. Similarly, the seasonal precipitation associated with the monsoons, mostly over Asia, is weaker than observed. The main patterns of interannual variability in terms of dominant empirical orthogonal functions are faithfully reproduced, mostly in the Northern Hemisphere winter. In the tropics the main teleconnection patterns associated with El Niño–Southern Oscillation and with the Indian Ocean Dipole are also in good agreement with observations.

The term "monsoon-desert mechanism" indicates the relationship between the diabatic heating associated with the South Asian summer monsoon rainfall and the remote response in the western sub-tropics where long Rossby waves anchor strong descent with high subsidence. In CMIP5 twenty-first century climate scenarios, the precipitation over South Asia is projected to increase. This study investigates how this change could affect the summer climate projections in the Mediterranean region. In a linear framework the monsoon-desert mechanism in the context of climate change would imply that the change in subsidence over the Mediterranean should be strongly linked with the changes in South Asian monsoon precipitation. The steady-state solution from a linear model forced with CMIP5 model projected precipitation change over South Asia shows a broad region of descent in the Mediterranean, while the results from CMIP5 projections differ having increased descent mostly in the western sector but also decreased descent in parts of the eastern sector. Local changes in circulation, particularly the meridional wind, promote cold air advection that anchors the descent but the barotropic Rossby wave nature of the wind anomalies consisting of alternating northerlies/southerlies favors alternating descent/ascent locations. In fact, the local mid-tropospheric meridional wind changes have the strongest correlation with the regions where the difference in subsidence is largest. There decreased rainfall is mostly balanced by changes in moisture, omega and in the horizontal advection of moisture.

Predicting North Atlantic hurricane activity months in advance is of great potential societal significance. The ocean temperature, both in terms of North Atlantic/tropical averages and upper ocean heat content, is demonstrated to be a significant predictor. To investigate the relationship between the thermal state of the Atlantic Ocean and the tropical cyclone (TC) activity in terms of accumulated cyclone energy (ACE), we use observed 1980-2015 TC records and a 1/4 degrees resolution global ocean reanalysis. This paper highlights the nonlocal effect associated with eastern Atlantic Ocean temperature, via a reduction of wind shear, and provides additional predictive skill of TC activity, when considering subsurface temperature instead of sea surface temperature (SST) only. The most active TC seasons occur for lower than normal wind shear conditions over the main development region, which is also driven by reduced trade wind strength. A significant step toward operationally reliable TC activity predictions is gained after including upper ocean mean temperatures over the eastern Atlantic domain. Remote effects are found to provide potential skill of ACE up to 3 months in advance. These results indicate that consideration of the upper 40-m ocean average temperature improves upon a prediction of September Atlantic hurricane activity using only SST.

In the framework of the second phase of the Coordinated Ocean-ice Reference Experiments (CORE-II), we present an analysis of the representation of the Antarctic Circumpolar Current (ACC) and Southern Ocean meridional overturning circulation (MOC) in a suite of seventeen global ocean-sea ice models. We focus on the mean, variability and trends of both the ACC and MOC over the 1958-2007 period, and discuss their relationship with the surface forcing. We aim to quantify the degree of eddy saturation and eddy compensation in the models participating in CORE-II, and compare our results with available observations, previous fineresolution numerical studies and theoretical constraints. Most models show weak ACC transport sensitivity to changes in forcing during the past five decades, and they can be considered to be in an eddy saturated regime. Larger contrasts arise when considering MOC trends, with a majority of models exhibiting significant strengthening of the MOC during the late 20th and early 21st century. Only a few models show a relatively small sensitivity to forcing changes, responding with an intensified eddy-induced circulation that provides some degree of eddy compensation, while still showing considerable decadal trends. Both ACC and MOC interannual variabilities are largely controlled by the Southern Annular Mode (SAM). Based on these results, models are clustered into two groups. Models with constant or two-dimensional (horizontal) specification of the eddy-induced advection coefficient K show larger ocean interior decadal trends, larger ACC transport decadal trends and no eddy compensation in the MOC. Eddy-permitting models or models with a threedimensional time varying K show smaller changes in isopycnal slopes and associated ACC trends, and partial eddy compensation. As previously argued, a constant in time or space lc is responsible for a poor representation of mesoscale eddy effects and cannot properly simulate the sensitivity of the ACC and MOC to changing surface forcing. Evidence is given for a larger sensitivity of the MOC as compared to the ACC transport, even when approaching eddy saturation. Future process studies designed for disentangling the role of momentum and buoyancy forcing in driving the ACC and MOC are proposed. (C) 2015 Elsevier Ltd. All rights reserved.

Dry summers over the eastern Mediterranean are characterized by strong descent anchored by long Rossby waves, which are forced by diabatic heating associated with summer monsoon rainfall over South Asia. The large-scale teleconnection between rising and subsiding air masses is referred to as the “monsoon–desert mechanism.” This study evaluates the ability of the phase 5 of the Coupled Model Intercomparison Project (CMIP5) models in representing the physical processes involved in this mechanism. An evaluation of statistics between summer climatologies of monsoon diabatic heating and that of vertical velocity over the eastern Mediterranean suggests a linear relationship. Despite large spatial diversity in monsoon heating, descent over the Mediterranean is coherently located and realistic in intensity. To measure the sensitivity of descent to the diversity in the horizontal and vertical distribution of monsoon heating, a series of linear atmosphere model experiments are performed. It is shown that column-integrated heating over both the Bay of Bengal and the Arabian Sea provides the largest descent with a more realistic spatial pattern. In the vertical, CMIP5 models underestimate the diabatic heating at upper levels, while they overestimate it at lower levels, resulting in a weaker forced response and weaker associated descent over the Mediterranean. A moist static energy budget analysis applied to CMIP5 suggests that most models capture the dominant role of horizontal temperature advection and radiative fluxes in balancing descent over the Mediterranean. Based on the objective analysis herein, a subset of models is identified that captures the teleconnection for reasons consistent with observations. The recognized processes vary at interannual time scales as well, with imprints of severe weak/strong monsoons noticeable over the Mediterranean.

The present manuscript compares Marine Isotope Stage 5 (MIS 5, 125–115 kyr BP) and MIS 7 (236–229 kyr BP) with the aim to investigate the origin of the difference in ice-sheet growth over the Northern Hemisphere high latitudes between these last two inceptions. Our approach combines a low resolution coupled atmosphere–ocean–sea-ice general circulation model and a 3-D thermo-mechanical ice-sheet model to simulate the state of the ice sheets associated with the inception climate states of MIS 5 and MIS 7. Our results show that external forcing (orbitals and GHG) and sea-ice albedo feedbacks are the main factors responsible for the difference in the land-ice initial state between MIS 5 and MIS 7 and that our cold climate model bias impacts more during a cold inception, such as MIS 7, than during a warm inception, such as MIS 5. In addition, if proper ice-elevation and albedo feedbacks are not taken into consideration, the evolution towards glacial inception is hardly simulated, especially for MIS 7. Finally, results highlight that while simulated ice volumes for MIS 5 glacial inception almost fit with paleo-reconstructions, the lack of precipitation over high latitudes, identified as a bias of our climate model, does not allow for a proper simulation of MIS 7 glacial inception.

Ocean reanalyses are data assimilative simulations, aimed at estimating the four-dimensional state of the ocean over long periods, in a way as consistent over time as possible. They are designed for a wide range of climate applications, such as climate monitoring and low-frequency variability studies, along with several downstream applications (e.g. biogeochemical and fishery modelling, initial conditions for long-range coupled predictions, regional model nesting). An upgraded version of the Euro-Mediterranean Center for Climate Change (CMCC) eddy-permitting global ocean reanalysis, named CMCC Global Ocean Reanalysis System (C-GLORS) version 4, was recently released. The reanalysis covers the meteorological satellite era (1982-2012). This article details the configuration of the reanalysis system and provides an extensive validation, focusing on the evaluation of main indexes related to climate monitoring. Cumulative denial experiments are also conducted, in order to understand the relative impact of assimilation components included in C-GLORS (i.e. altimetric data, variational assimilation, bias correction and surface nudging). Results indicate that C-GLORS proves reliable in simulating long-term means, heat and freshwater trends, sea-level variability, mean surface circulation and transports, eddy variability and meridional overturning circulation and its associated heat transport, except for a few specific issues (overestimation of volume transports in the Southern Ocean and slight underestimation of the Atlantic ocean meridional overturning circulation and associated heat transport, the latter mostly linked to underestimation of western boundary northward transports). The results also demonstrate the complementarity of the assimilation components, all improving verification skill scores, for example the importance of the variational assimilation for the simulation of the reanalysis small-scale variability, the importance of the bias-correction scheme for correcting subsurface salinity errors or the role of surface nudging in driving the North Atlantic ocean circulation. The analyses presented here offer ideas for improving C-GLORS further and for the requirements of next-generation ocean reanalysis systems.

This study investigates the climate sensitivity to a strong CO2 atmospheric forcing focusing on the North Atlantic Ocean (NA). The analysis is based on a set of 600 years long experiments performed with a state-of-the-art coupled general circulation model (CGCM) with the 1990 reference value of atmospheric CO2 multiplied by 4, 8 and 16. Extreme increases in atmospheric CO2 concentration have been applied to force the climate system towards stable states with different thermo-dynamical properties and analyze how the different resulting oceanic stratification and diffusion affect the Atlantic Meridional Overturning Circulation (AMOC). The AMOC weakens in response to the induced warming with distinctive features in the extreme case: a southward shift of convective sites and the formation of a density front at mid-latitudes. The analysis of the density fluxes reveals that NA loses density at high latitudes and gains it southward of 40°N mainly due to the haline contribution. Our results indicate that the most important processes that control the AMOC are active in the high latitudes and are related to the stability of the water column. The increased ocean stratification stabilizes the ocean interior leading to a decreased vertical diffusivity, a reduction in the formation of deep water and a weaker circulation. In particular, the deep convection collapses mainly in the Labrador Sea as a consequence of the water column stratification under high latitudes freshening.

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. (C) 2015 Elsevier Ltd. All rights reserved.

The connection between Tropical Pacific and North Pacific variability is investigated in a state-of-the-art coupled ocean-atmosphere model, comparing two 20th century simulations at T30 and T106 atmospheric horizontal resolutions. Despite a better simulation of the frequency and the spatial distribution of the Tropical Pacific anomalies associated with the El Nino Southern Oscillation (ENSO) in the high-resolution experiment, the response in the North Pacific is scarcely different from the low-resolution experiment where the ENSO variability is weaker and at higher than observed frequency. In the North Pacific, the response of surface atmospheric fields to the variability in the Tropical Pacific appears to be affected by local coupling processes significantly different in the two experiments. The coupling between sea level pressure (SLP) and sea surface temperature (SST) in the North Pacific as well as the influence of the Tropical Pacific SST has been measured here by means of the coupled manifold technique. In the low-resolution case the SLP variances linked to the fraction of North Pacific SST not influenced by the Tropical Pacific are weak suggesting that the remote influence is strong, consistently with the observations. On the contrary, in the high-resolution experiment the fractions and the patterns of the SLP variances due to the Tropical Pacific SST and those linked to the North Pacific SST are comparable. In the latter case, model systematic errors in the northwestern Pacific influences the local coupling processes thus triggering the remote response. We conclude that an increased atmospheric horizontal resolution does not reduce the coupled model systematic errors in the representation of the teleconnection between the North and the Tropical Pacific and that the validation of coupled models has to consider both remote and local processes. Copyright (c) 2011 Royal Meteorological Society

Simulation characteristics from eighteen global ocean-sea-ice coupled models are presented with a focus on the mean Atlantic meridional overturning circulation (AMOC) and other related fields in the North Atlantic. These experiments use inter-annually varying atmospheric forcing data sets for the 60-year period from 1948 to 2007 and are performed as contributions to the second phase of the Coordinated Oceanice Reference Experiments (CORE-II). The protocol for conducting such CORE-II experiments is summarized. Despite using the same atmospheric forcing, the solutions show significant differences. As most models also differ from available observations, biases in the Labrador Sea region in upper-ocean potential temperature and salinity distributions, mixed layer depths, and sea-ice cover are identified as contributors to differences in AMOC. These differences in the solutions do not suggest an obvious grouping of the models based on their ocean model lineage, their vertical coordinate representations, or surface salinity restoring strengths. Thus, the solution differences among the models are attributed primarily to use of different subgrid scale parameterizations and parameter choices as well as to differences in vertical and horizontal grid resolutions in the ocean models. Use of a wide variety of sea-ice models with diverse snow and sea-ice albedo treatments also contributes to these differences. Based on the diagnostics considered, the majority of the models appear suitable for use in studies involving the North Atlantic, but some models require dedicated development effort. (C) 2013 Elsevier Ltd. All rights reserved.

El Nino-Southern Oscillation (ENSO) is an important air-sea coupled phenomenon that plays a dominant role in the variability of the tropical regions. Observations, atmospheric and oceanic reanalysis datasets are used to classify ENSO and non-ENSO years to investigate the typical features of its periodicity and atmospheric circulation patterns. Among non-ENSO years, we have analyzed a group, called type-II years, with very small SST anomalies in summer that tend to weaken the correlation between ENSO and precipitation in the equatorial regions. A unique character of ENSO is studied in terms of the quasi-biennial periodicity of SST and heat content (HC) fields over the Pacific-Indian Oceans. While the SST tends to have higher biennial frequency along the Equator, the HC maximizes it into two centers in the western Pacific sector. The north-western center, located east of Mindanao, is strongly correlated with SST in the NINO3 region. The classification of El Nino and La Nina years, based on NINO3 SST and north-western Pacific HC respectively, has been used to identify and describe temperature and wind patterns over an extended-ENSO region that includes the tropical Pacific and Indian Oceans. The description of the spatial patterns within the atmospheric ENSO circulation has been extended to tropospheric moisture fields and low-level moisture divergence during November-December-January, differentiating the role of El Nino, when lame amounts of condensational heat are concentrated in the central Pacific, from La Nina that tends to mainly redistribute heat to Maritime Continents and higher latitudes. The influence of the described mechanisms on equatorial convection in the context of the variability of ENSO on longer timescales for the end of the 20th century is questioned. However, the inaccuracy of the atmospheric reanalysis products in terms of precipitation and the shorter time length of more reliable datasets hamper a final conclusion on this issue.

We investigate the effects of realistic oceanic initial conditions on a set of decadal climate predictions performed with a state-of-the-art coupled ocean-atmosphere general circulation model. The decadal predictions are performed in both retrospective (hindcast) and forecast modes. Specifically, the full set of prediction experiments consists of 3-member ensembles of 30-year simulations, starting at 5-year intervals from 1960 to 2005, using historical radiative forcing conditions for the 1960-2005 period, followed by RCP4. 5 scenario settings for the 2006-2035 period. The ocean initial states are provided by ocean reanalyses differing by assimilation methods and assimilated data, but obtained with the same ocean model. The use of alternative ocean reanalyses yields the required perturbation of the full three-dimensional ocean state aimed at generating the ensemble members spread. A full-value initialization technique is adopted. The predictive skill of the system appears to be driven to large extent by trends in the radiative forcing. However, after detrending, a residual skill over specific regions of the ocean emerges in the near-term. Specifically, natural fluctuations in the North Atlantic sea-surface temperature (SST) associated with large-scale multi-decadal variability modes are predictable in the 2-5 year range. This is consistent with significant predictive skill found in the Atlantic meridional overturning circulation over a similar timescale. The dependency of forecast skill on ocean initialization is analysed, revealing a strong impact of details of ocean data assimilation products on the system predictive skill. This points to the need of reducing the large uncertainties that currently affect global ocean reanalyses, in the perspective of providing reliable near-term climate predictions. © 2012 Springer-Verlag.

Under future scenarios of business-as-usual emissions, the ocean storage of anthropogenic carbon is anticipated to decrease because of ocean chemistry constraints and positive feedbacks in the carbon-climate dynamics, whereas it is still unknown how the oceanic carbon cycle will respond to more substantial mitigation scenarios. To evaluate the natural system response to prescribed atmospheric “target” concentrations and assess the response of the ocean carbon pool to these values, 2 centennial projection simulations have been performed with an Earth System Model that includes a fully coupled carbon cycle, forced in one case with a mitigation scenario and the other with the SRES A1B scenario. End of century ocean uptake with the mitigation scenario is projected to return to the same magnitude of carbon fluxes as simulated in 1960 in the Pacific Ocean and to lower values in the Atlantic. With A1B, the major ocean basins are instead projected to decrease the capacity for carbon uptake globally as found with simpler carbon cycle models, while at the regional level the response is contrasting. The model indicates that the equatorial Pacific may increase the carbon uptake rates in both scenarios, owing to enhancement of the biological carbon pump evidenced by an increase in Net Community Production (NCP) following changes in the subsurface equatorial circulation and enhanced iron availability from extratropical regions. NCP is a proxy of the bulk organic carbon made available to the higher trophic levels and potentially exportable from the surface layers. The model results indicate that, besides the localized increase in the equatorial Pacific, the NCP of lower trophic levels in the northern Pacific and Atlantic oceans is projected to be halved with respect to the current climate under a substantial mitigation scenario at the end of the twenty-first century. It is thus suggested that changes due to cumulative carbon emissions up to present and the projected concentration pathways of aerosol in the next decades control the evolution of surface ocean biogeochemistry in the second half of this century more than the specific pathways of atmospheric CO2 concentrations.

One of the main objectives of the global ocean modelling activities at Centro Euro-Mediterraneo per i Cambiamenti Climatici (CMCC) is the production of global ocean re-analyses over multidecadal periods to reconstruct the state of the ocean and the large scale circulation over the recent past. The re-analyses are used for climate applications and for the assessment of the benefits of assimilating ocean observations on seasonal and longer predictions. Here we present the main characteristics of an optimal interpolation based assimilation system used to produce a set of global ocean re-analyses validated against a set of high quality in situ observations and independent data. Differences among the experiments of the set are analyzed in terms of improvements in the method used to assimilate the data and the quality of observations themselves. For example, the integrated ocean heat content, which can be taken as an indicator of climate changes, is examined to detect possible sources of uncertainty of its long-term changes. Global and basin scale upper ocean heat content exhibits warming trends over the last few decades that still depend in a significant way on the assimilated observations and the formulation of the background covariances. However, all the re-analyses show a global warming trend of the oceanic uppermost 700 m over the last five decades that falls within the range of the most recent observation-based estimates. The largest discrepancies between our estimates and observational based ones are confined in the upwelling regions of the Pacific and Atlantic Oceans. Finally, the results show that the climatological heat and salt transports as a function of latitude also fall within the range of the estimates based on observations and atmospheric re-analyses. (C) 2011 Elsevier B.V. All rights reserved.

The development of the Istituto Nazionale di Geofisica e Vulcanologia (INGV)–Centro Euro-Mediterraneo per i Cambiamenti Climatici (CMCC) Seasonal Prediction System (SPS) is documented. In this SPS the ocean initial-conditions estimation includes a reduced-order optimal interpolation procedure for the assimilation of temperature and salinity profiles at the global scale. Nine-member ensemble forecasts have been produced for the period 1991–2003 for two starting dates per year in order to assess the impact of the subsurface assimilation in the ocean for initialization. Comparing the results with control simulations (i.e., without assimilation of subsurface profiles during ocean initialization), it is shown that the improved ocean initialization increases the skill in the prediction of tropical Pacific sea surface temperatures of the system for boreal winter forecasts. Considering the forecast of the 1997/98 El Nin˜o, the data assimilation in the ocean initial conditions leads to a considerable improvement in the representation of its onset and development. The results presented in this paper indicate a better prediction of global-scale surface climate anomalies for the forecasts started in November, probably because of the improvement in the tropical Pacific. For boreal winter, significant increases in the capability of the system to discriminate above-normal and below-normal temperature anomalies are shown in both the tropics and extratropics. Fil: Alessandri, Andrea. Centro Euro-Mediterraneo per i Cambiamenti Climatici; Italia Fil: Borrelli, Andrea. Centro Euro-Mediterraneo per i Cambiamenti Climatici; Italia Fil: Masina, Simona. Centro Euro-Mediterraneo per i Cambiamenti Climatici; Italia. Istituto Nazionale Di Geofisica E Vulcanologia; Italia Fil: Cherchi, Annalisa. Centro Euro-Mediterraneo per i Cambiamenti Climatici; Italia. Istituto Nazionale Di Geofisica E Vulcanologia; Italia Fil: Gualdi, Silvio. Centro Euro-Mediterraneo per i Cambiamenti Climatici; Italia. Istituto Nazionale Di Geofisica E Vulcanologia; Italia Fil: Navarra, Antonio. Centro Euro-Mediterraneo per i Cambiamenti Climatici; Italia. Istituto Nazionale Di Geofisica E Vulcanologia; Italia Fil: Di Pietro, Pierluigi. Istituto Nazionale Di Geofisica E Vulcanologia; Italia Fil: Carril, Andrea Fabiana. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmosfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmosfera; Argentina

A coupled general circulation model has been used to perform a set of experiments with high CO2 concentration (2, 4, 16 times the present day mean value). The experiments have been analyzed to study the response of the climate system to strong radiative forcing in terms of the processes involved in the adjustment at the ocean–atmosphere interface. The analysis of the experiments revealed a non-linear response of the mean state of the atmosphere and ocean to the increase in the carbon dioxide concentration. In the 16 × CO2 experiment the equilibrium at the ocean–atmosphere interface is characterized by an atmosphere with a shut off of the convective precipitation in the tropical Pacific sector, associated with air warmer than the ocean below. A cloud feedback mechanism is found to be involved in the increased stability of the troposphere. In this more stable condition the mean total precipitation is mainly due to large-scale moisture flux even in the tropics. In the equatorial Pacific Ocean the zonal temperature gradient of both surface and sub-surface waters is significantly smaller in the 16 × CO2 experiment than in the control experiment. The thermocline slope and the zonal wind stress decrease as well. When the CO2 concentration increases by about two and four times with respect to the control experiment there is an intensification of El Nino. On the other hand, in the experiment with 16 times the present-day value of CO2, the Tropical Pacific variability weakens, suggesting the possibility of the establishment of permanent warm conditions that look like the peak of El Nino.

This paper presents a global ocean implementation of a multi-component model of marine pelagic biogeochemistry coupled online with an ocean general circulation model forced with climatological surface fields (PELAgic biogeochemistry for Global Ocean Simulations, PELAGOS). The final objective is the inclusion of this model as a component in an Earth System model for climate studies. The pelagic model is based on a functional stoichiometric representation of marine biogeochemical cycles and allows simulating the dynamics of C, N, P, Si, O and Fe taking into account the variation of their elemental ratios in the functional groups. The model also includes a parameterization of variable chlorophyll/carbon ratio in phytoplankton, carrying chl as a prognostic variable. The first part of the paper analyzes the contribution of non-local advective-diffusive terms and local vertical processes to the simulated chl distributions. The comparison of the three experiments shows that the mean chl distribution at higher latitudes is largely determined by mixing processes, while vertical advection controls the distribution in the equatorial upwelling regions. Horizontal advective and diffusive processes are necessary mechanisms for the shape of chl distribution in the sub-tropical Pacific. In the second part, the results have been compared with existing datasets of satellite-derived chlorophyll, surface nutrients, estimates of phytoplankton community composition and primary production data. The agreement is reasonable both in terms of the spatial distribution of annual means and of the seasonal variability in different dynamical oceanographic regions. Results indicate that some of the model biases in chl and surface nutrients distributions can be related to deficiencies in the simulation of physical processes such as advection and mixing. Other discrepancies are attributed to inadequate parameterizations of phytoplankton functional groups. The model has skill in reproducing the overall distribution of large and small phytoplankton but tends to underestimate diatoms in the northern higher latitudes and overestimate nanophytoplankton with respect to picoautotrophs in oligotrophic regions. The performance of the model is discussed in the context of its use in climate studies and an approach for improving the parameterization of functional groups in deterministic models is outlined. (c) 2006 Elsevier B.V. All rights reserved.

In this paper results from the application of an ocean data assimilation (ODA) system, combining a multivariate reduced-order optimal interpolator (OI) scheme with a global ocean general circulation model (OGCM), are described. The present ODA system, designed to assimilate in situ temperature and salinity observations, has been used to produce ocean reanalyses for the 1962–2001 period. The impact of assimilating observed hydrographic data on the ocean mean state and temporal variability is evaluated. A special focus of this work is on the ODA system skill in reproducing a realistic ocean salinity state. Results from a hierarchy of different salinity reanalyses, using varying combinations of assimilated data and background error covariance structures, are described. The impact of the space and time resolution of the background error covariance parameterization on salinity is addressed.

The effect of atmospheric horizontal resolution on tropical variability is investigated within the modified Scale Interaction Experiment (SINTEX) coupled model, SINTEX-Frontier (SINTEX-F), developed jointly at Istituto Nazionale di Geofisica e Vulcanologia (INGV), L’Institut Pierre-Simon Laplace (IPSL), and the Frontier Research System. The ocean resolution is not changed as the atmospheric model resolution is modified from spectral resolution 30 (T30) to spectral resolution 106 (T106). The horizontal resolutions of the atmospheric model T30 and T106 are investigated in terms of the coupling characteristics, frequency, and variability of the tropical ocean–atmosphere interactions. It appears that the T106 resolution is generally beneficial even if it does not eliminate all the major systematic errors of the coupled model. There is an excessive shift west of the cold tongue and ENSO variability, and high resolution also has a somewhat negative impact on the variability in the east Indian Ocean. A dominant 2-yr peak for the Niño-3 variability in the T30 model is moderated in the T106 as it shifts to a longer time scale. At high resolution, new processes come into play, such as the coupling of tropical instability waves, the resolution of coastal flows at the Pacific–Mexican coasts, and improved coastal forcing along the coast of South America. The delayed oscillator seems to be the main mechanism that generates the interannual variability in both models, but the models realize it in different ways. In the T30 model it is confined close to the equator, involving relatively fast equatorial and near-equatorial modes, and in the high-resolution model, it involves a wider latitudinal region and slower waves. It is speculated that the extent of the region that is involved in the interannual variability may be linked to the time scale of the variability itself.