Gael Forget - Academia.edu (original) (raw)

Papers by Gael Forget

Research paper thumbnail of 4d-var assimilation of profiling floats in an ogcm of the North Atlantic: an observing system simulation experiment

In the context of the ARGO project, we study how an idealised network of profiling floats can con... more In the context of the ARGO project, we study how an idealised network of profiling floats can constrain the thermohaline structure of the upper ocean in a primitive equation model of the North Atlantic. We use a 4D-variational assimilation formalism (strong constraint adjoint method) with a model resolution of 1° and an assimilation window of one-year. The synthetic data are

Research paper thumbnail of Combining Argo profiles with a general circulation model in the North Atlantic. Part 1: Estimation of hydrographic and circulation anomalies from synthetic profiles, over a year

Ocean Modelling, 2008

A set of Argo profiles collected in the North Atlantic between May 2002 and April 2003 is combine... more A set of Argo profiles collected in the North Atlantic between May 2002 and April 2003 is combined with a low-resolution general circulation model (GCM) using the adjoint method. Fitting the real hydrographic observations leads to vast improvements in the model circulation, including the sea surface height and the meridional heat transport. We find striking differences in basin-scale transports compared with previous assimilation experiments that use the same GCM and a similar spatial resolution. Based on forward modeling studies, it is argued that these differences are due to different assimilation experiment durations. Over 1 year, the hydrography interpolated with the GCM from Argo profiles better represents the contemporary structures than does a long-term averaged climatology. The GCM dynamics are robust enough to distinguish between contemporary hydrography and climatological hydrography.

Research paper thumbnail of Mapping Ocean Observations in a Dynamical Framework: A 2004–06 Ocean Atlas

Journal of Physical Oceanography, 2010

This paper exploits a new observational atlas for the near-global ocean for the best-observed 3-y... more This paper exploits a new observational atlas for the near-global ocean for the best-observed 3-yr period from December 2003 through November 2006. The atlas consists of mapped observations and derived quantities. Together they form a full representation of the ocean state and its seasonal cycle. The mapped observations are primarily altimeter data, satellite SST, and Argo profiles. GCM interpolation is used to synthesize these datasets, and the resulting atlas is a fairly close fit to each one of them. For observed quantities especially, the atlas is a practical means to evaluate free-running GCM simulations and to put field experiments into a broader context. The atlas-derived quantities include the middepth dynamic topography, as well as ocean fluxes of heat and salt-freshwater. The atlas is publicly available online (www.ecco-group. org). This paper provides insight into two oceanographic problems that are the subject of vigorous ongoing research. First, regarding ocean circulation estimates, it can be inferred that the RMS uncertainty in modern surface dynamic topography (SDT) estimates is only on the order of 3.5 cm at scales beyond 300 km. In that context, it is found that assumptions of ''reference-level'' dynamic topography may yield significant errors (of order 2.2 cm or more) in SDT estimates using in situ data. Second, in the perspective of mode water investigations, it is estimated that ocean fluxes (advection plus mixing) largely contribute to the seasonal fluctuation in heat content and freshwater/salt content. Hence, representing the seasonal cycle as a simple interplay of air-sea flux and ocean storage would not yield a meaningful approximation. For the salt-freshwater seasonal cycle especially, contributions from ocean fluxes usually exceed direct air-sea flux contributions.

Research paper thumbnail of Determining the Origins of Advective Heat Transport Convergence Variability in the North Atlantic

Research paper thumbnail of Low-frequency SST and upper-ocean heat content variability in the North Atlantic

Journal of Climate, 2014

ABSTRACT A recent state estimate covering the period 1992-2010 from the Estimating the Circulatio... more ABSTRACT A recent state estimate covering the period 1992-2010 from the Estimating the Circulation and Climate of the Ocean (ECCO) project is utilized to quantify the upper-ocean heat budget in the North Atlantic on monthly to interannual time scales (seasonal cycle removed). Three novel techniques are introduced: 1) the heat budget is integrated over the maximum climatological mixed layer depth (integral denoted as H), which gives results that are relevant for explaining SST while avoiding strong contributions from vertical diffusion and entrainment; 2) advective convergences are separated into Ekman and geostrophic parts, a technique that is successful away from ocean boundaries; and 3) air sea heat fluxes and Ekman advection are combined into one local forcing term. The central results of our analysis are as follows: 1) In the interior of subtropical gyre, local forcing explains the majority of H variance on all time scales resolved by the ECCO estimate. 2) In the Gulf Stream region, low-frequency H anomalies are forced by geostrophic convergences and damped by air sea heat fluxes. 3) In the interior of the subpolar gyre, diffusion and bolus transports play a leading order role in H variability, and these transports are correlated with low-frequency variability in wintertime mixed layer depths.

Research paper thumbnail of ECCO version 4: an integrated framework for non-linear inverse modeling and global ocean state estimation

Geoscientific Model Development Discussions, 2015

This paper presents the ECCO v4 non-linear inverse modeling framework and its baseline solution f... more This paper presents the ECCO v4 non-linear inverse modeling framework and its baseline solution for the evolving ocean state over the period 1992-2011. Both components are publicly available and highly integrated with the MITgcm. They are both subjected to regular, automated regression tests. The modeling framework includes sets of global 5 conformal grids, a global model setup, implementations of model-data constraints and adjustable control parameters, an interface to algorithmic differentiation, as well as a grid-independent, fully capable Matlab toolbox. The reference ECCO v4 solution is a dynamically consistent ocean state estimate (ECCO-Production, release 1) without un-identified sources of heat and buoyancy, which any interested user will be able to 10 reproduce accurately. The solution is an acceptable fit to most data and has been found physically plausible in many respects, as documented here and in related publications. Users are being provided with capabilities to assess model-data misfits for themselves. The synergy between modeling and data synthesis is asserted through the joint presentation of the modeling framework and the state estimate. In particular, 15 the inverse estimate of parameterized physics was instrumental in improving the fit to the observed hydrography, and becomes an integral part of the ocean model setup available for general use. More generally, a first assessment of the relative importance of external, parametric and structural model errors is presented. Parametric and external model uncertainties appear to be of comparable importance and dominate over 20 structural model uncertainty. The results generally underline the importance of including turbulent transport parameters in the inverse problem.

Research paper thumbnail of A Comparison of Atmospheric Reanalysis Surface Products over the Ocean and Implications for Uncertainties in Air–Sea Boundary Forcing

Journal of Climate, 2013

ABSTRACT This paper investigates the uncertainties related to atmospheric fields from reanalysis ... more ABSTRACT This paper investigates the uncertainties related to atmospheric fields from reanalysis products used in forcing ocean models. Four reanalysis products, namely from 1) the interim ECMWF Re-Analysis (ERA-Interim), 2) version 2 of the Common Reference Ocean–Ice Experiments (CORE2), 3) the 25-Year Japanese Reanalysis Project (JRA-25), and 4) NCEP–NCAR, are evaluated against satellite-derived observations for eight different fields (zonal and meridional winds, precipitation, specific humidity, continental discharge, surface air temperature, and downwelling longwave and shortwave radiation fluxes). No single product is found to agree better in all fields with satellite-derived observations. Reanalysis products are mostly comparable to each other because of their similar physical assumptions and assimilation of common observations. Adjusted atmospheric fields from the Estimating the Circulation and Climate of the Ocean (ECCO) optimizations are also in agreement with other reanalysis products. Time-mean and time-variable errors are estimated separately and mapped globally in space, based on 14-day average fields to focus on monthly to interannual periods. Time-variable errors are larger in comparison to the signal than time-mean errors for most fields, thus justifying the need to separate them for studying uncertainties as well as formulating optimization procedures. Precipitation and wind stress fields show significant time-mean and time-variable errors whereas downwelling radiation, air temperature, and humidity fields show small time-mean errors but large time-variable errors, particularly in the tropics. Uncertainties based on evaluating multiple products presented here are considerably larger than uncertainties based on single product pairs.

Research paper thumbnail of Combining altimetric and all other data with a general circulation model

Proceedings of the 15 Years of Progress in Radar Altimetry Symposium, Venice, Mar 1, 2006

The TOPEX/POSEIDON mission was formulated in conjunction with the World Ocean Circulation Experim... more The TOPEX/POSEIDON mission was formulated in conjunction with the World Ocean Circulation Experiment (WOCE). Although altimetric data are by far the largest ocean data set that emerged from that experiment, it was recognized from the outset that the best estimate of the ocean circulation and its variability would be made by combining all the data with a good general circulation model. The US ECCO-GODAE consortium has now produced useful estimates of the threedimensional time evolving ocean circulation at 1 ...

Research paper thumbnail of Observational Requirements for Global-Scale Ocean Climate Analysis: Lessons from Ocean State Estimation

Proceedings of OceanObs'09: Sustained Ocean Observations and Information for Society, 2010

Research paper thumbnail of Studies of North Atlantic Eighteen Degree Water formation and dissipation during the CLIMODE experiment using a data-model synthesis

A data-model synthesis is used to quantify the cycle of Eighteen Degree Water (EDW) formation and... more A data-model synthesis is used to quantify the cycle of Eighteen Degree Water (EDW) formation and dissipation and the underlying driving mechanisms. The MITgcm and its adjoint is used to dynamically interpolate Argo profiles, SST and altimetric observations during the CLIMODE experiment. Air sea fluxes and the initial T/S fields are used as controls to minimize the misfit between the

Research paper thumbnail of ECCOc: The ECCO-GODAE Ocean State Estimation Effort at MIT/AER

Research paper thumbnail of Potential of the ARGO network to produce an oceanic synthesis of the hydrology and the circulation in the North Atlantic using a 4D-variational method

Research paper thumbnail of Vertical Redistribution of Oceanic Heat Content

Journal of Climate, 2015

Estimated values of recent oceanic heat uptake are on the order of a few tenths of a W m 22 , and... more Estimated values of recent oceanic heat uptake are on the order of a few tenths of a W m 22 , and are a very small residual of air-sea exchanges, with annual average regional magnitudes of hundreds of W m 22 . Using a dynamically consistent state estimate, the redistribution of heat within the ocean is calculated over a 20-yr period. The 20-yr mean vertical heat flux shows strong variations in both the lateral and vertical directions, consistent with the ocean being a dynamically active and spatially complex heat exchanger. Between mixing and advection, the two processes determining the vertical heat transport in the deep ocean, advection plays a more important role in setting the spatial patterns of vertical heat exchange and its temporal variations. The global integral of vertical heat flux shows an upward heat transport in the deep ocean, suggesting a cooling trend in the deep ocean. These results support an inference that the near-surface thermal properties of the ocean are a consequence, at least in part, of internal redistributions of heat, some of which must reflect water that has undergone long trajectories since last exposure to the atmosphere. The small residual heat exchange with the atmosphere today is unlikely to represent the interaction with an ocean that was in thermal equilibrium at the start of global warming. An analogy is drawn with carbon-14 ''reservoir ages,'' which range from over hundreds to a thousand years.

Research paper thumbnail of Synthesis and Assimilation Systems - Essential Adjuncts to the Global Ocean Observing System

Proceedings of OceanObs'09: Sustained Ocean Observations and Information for Society, 2010

Research paper thumbnail of North Atlantic simulations in Coordinated Ocean-ice Reference Experiments phase II (CORE-II). Part I: Mean states

Research paper thumbnail of Estimated Seasonal Cycle of North Atlantic Eighteen Degree Water Volume

Journal of Physical Oceanography, 2011

Research paper thumbnail of Estimated Global Hydrographic Variability

Journal of Physical Oceanography, 2007

An estimate is made of the three-dimensional global oceanic temperature and salinity variability,... more An estimate is made of the three-dimensional global oceanic temperature and salinity variability, omitting the seasonal cycle, both as a major descriptive element of the ocean circulation and for use in the error estimates of state estimation. Historical hydrography, recent data from the World Ocean Circulation Experiment, and Argo profile data are all used. Root-mean-square vertical displacements in the upper 300 m of the ocean are generally smaller than 50 m, except in energetic boundary currents and in the North Atlantic subpolar gyre. Variability in temperature and salinity is strongly correlated below the top 100 m. Salinity contributions to sea surface height variability appear more significant at low latitudes than expected, possibly resulting from advective and diffusive processes. Results are generally consistent with altimetric variability under two simple kinematic hypotheses, and much of the observed structure coincides with known dynamical features. A large fraction of the sea surface height variability is consistent with the hypothesis of dominance of the first baroclinic mode.

Research paper thumbnail of Using Transformation and Formation Maps to Study the Role of Air–Sea Heat Fluxes in North Atlantic Eighteen Degree Water Formation

Journal of Physical Oceanography, 2009

The Walin water mass framework quantifies the rate at which water is transformed from one tempera... more The Walin water mass framework quantifies the rate at which water is transformed from one temperature class to another by air-sea heat fluxes (transformation). The divergence of the transformation rate yields the rate at which a given temperature range is created or destroyed by air-sea heat fluxes (formation). Walin's framework provides a precise integral statement at the expense of losing spatial information. In this study the integrand of Walin's expression to yield transformation and formation maps is plotted and used to study the role of air-sea heat fluxes in the cycle of formation-destruction of the 188 6 18C layer in the North Atlantic.

Research paper thumbnail of Estimates of the Southern Ocean general circulation improved by animal-borne instruments

Geophysical Research Letters, 2013

Research paper thumbnail of Assessing 4D-VAR for dynamical mapping of coastal high-frequency radar in San Diego

Dynamics of Atmospheres and Oceans, 2009

The problem of dynamically mapping high-frequency (HF) radar radial velocity observations is inve... more The problem of dynamically mapping high-frequency (HF) radar radial velocity observations is investigated using a threedimensional hydrodynamic model of the San Diego coastal region and an adjoint-based assimilation method. The HF radar provides near-real-time radial velocities from three sites covering the region offshore of San Diego Bay. The hydrodynamical model is the Massachusetts Institute of Technology general circulation model (MITgcm) with 1 km horizontal resolution and 40 vertical layers. The domain is centered on Point Loma, extending 117 km offshore and 120 km alongshore. The reference run (before adjustment) is initialized from a single profile of T and S and is forced with wind data from a single shore station and with zero heat and fresh water fluxes. The adjoint of the model is used to adjust initial temperature, salinity, and velocity, hourly temperature, salinity and horizontal velocities at the open boundaries, and hourly surface fluxes of momentum, heat and freshwater so that the model reproduces hourly HF radar radial velocity observations. Results from a small number of experiments suggest that the adjoint method can be successfully used over 10-day windows at coastal model resolution. It produces a dynamically consistent model run that fits HF radar data with errors near the specified uncertainties. In a test of the forecasting capability of the San Diego model after adjustment, the forecast skill was shown to exceed persistence for up to 20 h.

Research paper thumbnail of 4d-var assimilation of profiling floats in an ogcm of the North Atlantic: an observing system simulation experiment

In the context of the ARGO project, we study how an idealised network of profiling floats can con... more In the context of the ARGO project, we study how an idealised network of profiling floats can constrain the thermohaline structure of the upper ocean in a primitive equation model of the North Atlantic. We use a 4D-variational assimilation formalism (strong constraint adjoint method) with a model resolution of 1° and an assimilation window of one-year. The synthetic data are

Research paper thumbnail of Combining Argo profiles with a general circulation model in the North Atlantic. Part 1: Estimation of hydrographic and circulation anomalies from synthetic profiles, over a year

Ocean Modelling, 2008

A set of Argo profiles collected in the North Atlantic between May 2002 and April 2003 is combine... more A set of Argo profiles collected in the North Atlantic between May 2002 and April 2003 is combined with a low-resolution general circulation model (GCM) using the adjoint method. Fitting the real hydrographic observations leads to vast improvements in the model circulation, including the sea surface height and the meridional heat transport. We find striking differences in basin-scale transports compared with previous assimilation experiments that use the same GCM and a similar spatial resolution. Based on forward modeling studies, it is argued that these differences are due to different assimilation experiment durations. Over 1 year, the hydrography interpolated with the GCM from Argo profiles better represents the contemporary structures than does a long-term averaged climatology. The GCM dynamics are robust enough to distinguish between contemporary hydrography and climatological hydrography.

Research paper thumbnail of Mapping Ocean Observations in a Dynamical Framework: A 2004–06 Ocean Atlas

Journal of Physical Oceanography, 2010

This paper exploits a new observational atlas for the near-global ocean for the best-observed 3-y... more This paper exploits a new observational atlas for the near-global ocean for the best-observed 3-yr period from December 2003 through November 2006. The atlas consists of mapped observations and derived quantities. Together they form a full representation of the ocean state and its seasonal cycle. The mapped observations are primarily altimeter data, satellite SST, and Argo profiles. GCM interpolation is used to synthesize these datasets, and the resulting atlas is a fairly close fit to each one of them. For observed quantities especially, the atlas is a practical means to evaluate free-running GCM simulations and to put field experiments into a broader context. The atlas-derived quantities include the middepth dynamic topography, as well as ocean fluxes of heat and salt-freshwater. The atlas is publicly available online (www.ecco-group. org). This paper provides insight into two oceanographic problems that are the subject of vigorous ongoing research. First, regarding ocean circulation estimates, it can be inferred that the RMS uncertainty in modern surface dynamic topography (SDT) estimates is only on the order of 3.5 cm at scales beyond 300 km. In that context, it is found that assumptions of ''reference-level'' dynamic topography may yield significant errors (of order 2.2 cm or more) in SDT estimates using in situ data. Second, in the perspective of mode water investigations, it is estimated that ocean fluxes (advection plus mixing) largely contribute to the seasonal fluctuation in heat content and freshwater/salt content. Hence, representing the seasonal cycle as a simple interplay of air-sea flux and ocean storage would not yield a meaningful approximation. For the salt-freshwater seasonal cycle especially, contributions from ocean fluxes usually exceed direct air-sea flux contributions.

Research paper thumbnail of Determining the Origins of Advective Heat Transport Convergence Variability in the North Atlantic

Research paper thumbnail of Low-frequency SST and upper-ocean heat content variability in the North Atlantic

Journal of Climate, 2014

ABSTRACT A recent state estimate covering the period 1992-2010 from the Estimating the Circulatio... more ABSTRACT A recent state estimate covering the period 1992-2010 from the Estimating the Circulation and Climate of the Ocean (ECCO) project is utilized to quantify the upper-ocean heat budget in the North Atlantic on monthly to interannual time scales (seasonal cycle removed). Three novel techniques are introduced: 1) the heat budget is integrated over the maximum climatological mixed layer depth (integral denoted as H), which gives results that are relevant for explaining SST while avoiding strong contributions from vertical diffusion and entrainment; 2) advective convergences are separated into Ekman and geostrophic parts, a technique that is successful away from ocean boundaries; and 3) air sea heat fluxes and Ekman advection are combined into one local forcing term. The central results of our analysis are as follows: 1) In the interior of subtropical gyre, local forcing explains the majority of H variance on all time scales resolved by the ECCO estimate. 2) In the Gulf Stream region, low-frequency H anomalies are forced by geostrophic convergences and damped by air sea heat fluxes. 3) In the interior of the subpolar gyre, diffusion and bolus transports play a leading order role in H variability, and these transports are correlated with low-frequency variability in wintertime mixed layer depths.

Research paper thumbnail of ECCO version 4: an integrated framework for non-linear inverse modeling and global ocean state estimation

Geoscientific Model Development Discussions, 2015

This paper presents the ECCO v4 non-linear inverse modeling framework and its baseline solution f... more This paper presents the ECCO v4 non-linear inverse modeling framework and its baseline solution for the evolving ocean state over the period 1992-2011. Both components are publicly available and highly integrated with the MITgcm. They are both subjected to regular, automated regression tests. The modeling framework includes sets of global 5 conformal grids, a global model setup, implementations of model-data constraints and adjustable control parameters, an interface to algorithmic differentiation, as well as a grid-independent, fully capable Matlab toolbox. The reference ECCO v4 solution is a dynamically consistent ocean state estimate (ECCO-Production, release 1) without un-identified sources of heat and buoyancy, which any interested user will be able to 10 reproduce accurately. The solution is an acceptable fit to most data and has been found physically plausible in many respects, as documented here and in related publications. Users are being provided with capabilities to assess model-data misfits for themselves. The synergy between modeling and data synthesis is asserted through the joint presentation of the modeling framework and the state estimate. In particular, 15 the inverse estimate of parameterized physics was instrumental in improving the fit to the observed hydrography, and becomes an integral part of the ocean model setup available for general use. More generally, a first assessment of the relative importance of external, parametric and structural model errors is presented. Parametric and external model uncertainties appear to be of comparable importance and dominate over 20 structural model uncertainty. The results generally underline the importance of including turbulent transport parameters in the inverse problem.

Research paper thumbnail of A Comparison of Atmospheric Reanalysis Surface Products over the Ocean and Implications for Uncertainties in Air–Sea Boundary Forcing

Journal of Climate, 2013

ABSTRACT This paper investigates the uncertainties related to atmospheric fields from reanalysis ... more ABSTRACT This paper investigates the uncertainties related to atmospheric fields from reanalysis products used in forcing ocean models. Four reanalysis products, namely from 1) the interim ECMWF Re-Analysis (ERA-Interim), 2) version 2 of the Common Reference Ocean–Ice Experiments (CORE2), 3) the 25-Year Japanese Reanalysis Project (JRA-25), and 4) NCEP–NCAR, are evaluated against satellite-derived observations for eight different fields (zonal and meridional winds, precipitation, specific humidity, continental discharge, surface air temperature, and downwelling longwave and shortwave radiation fluxes). No single product is found to agree better in all fields with satellite-derived observations. Reanalysis products are mostly comparable to each other because of their similar physical assumptions and assimilation of common observations. Adjusted atmospheric fields from the Estimating the Circulation and Climate of the Ocean (ECCO) optimizations are also in agreement with other reanalysis products. Time-mean and time-variable errors are estimated separately and mapped globally in space, based on 14-day average fields to focus on monthly to interannual periods. Time-variable errors are larger in comparison to the signal than time-mean errors for most fields, thus justifying the need to separate them for studying uncertainties as well as formulating optimization procedures. Precipitation and wind stress fields show significant time-mean and time-variable errors whereas downwelling radiation, air temperature, and humidity fields show small time-mean errors but large time-variable errors, particularly in the tropics. Uncertainties based on evaluating multiple products presented here are considerably larger than uncertainties based on single product pairs.

Research paper thumbnail of Combining altimetric and all other data with a general circulation model

Proceedings of the 15 Years of Progress in Radar Altimetry Symposium, Venice, Mar 1, 2006

The TOPEX/POSEIDON mission was formulated in conjunction with the World Ocean Circulation Experim... more The TOPEX/POSEIDON mission was formulated in conjunction with the World Ocean Circulation Experiment (WOCE). Although altimetric data are by far the largest ocean data set that emerged from that experiment, it was recognized from the outset that the best estimate of the ocean circulation and its variability would be made by combining all the data with a good general circulation model. The US ECCO-GODAE consortium has now produced useful estimates of the threedimensional time evolving ocean circulation at 1 ...

Research paper thumbnail of Observational Requirements for Global-Scale Ocean Climate Analysis: Lessons from Ocean State Estimation

Proceedings of OceanObs'09: Sustained Ocean Observations and Information for Society, 2010

Research paper thumbnail of Studies of North Atlantic Eighteen Degree Water formation and dissipation during the CLIMODE experiment using a data-model synthesis

A data-model synthesis is used to quantify the cycle of Eighteen Degree Water (EDW) formation and... more A data-model synthesis is used to quantify the cycle of Eighteen Degree Water (EDW) formation and dissipation and the underlying driving mechanisms. The MITgcm and its adjoint is used to dynamically interpolate Argo profiles, SST and altimetric observations during the CLIMODE experiment. Air sea fluxes and the initial T/S fields are used as controls to minimize the misfit between the

Research paper thumbnail of ECCOc: The ECCO-GODAE Ocean State Estimation Effort at MIT/AER

Research paper thumbnail of Potential of the ARGO network to produce an oceanic synthesis of the hydrology and the circulation in the North Atlantic using a 4D-variational method

Research paper thumbnail of Vertical Redistribution of Oceanic Heat Content

Journal of Climate, 2015

Estimated values of recent oceanic heat uptake are on the order of a few tenths of a W m 22 , and... more Estimated values of recent oceanic heat uptake are on the order of a few tenths of a W m 22 , and are a very small residual of air-sea exchanges, with annual average regional magnitudes of hundreds of W m 22 . Using a dynamically consistent state estimate, the redistribution of heat within the ocean is calculated over a 20-yr period. The 20-yr mean vertical heat flux shows strong variations in both the lateral and vertical directions, consistent with the ocean being a dynamically active and spatially complex heat exchanger. Between mixing and advection, the two processes determining the vertical heat transport in the deep ocean, advection plays a more important role in setting the spatial patterns of vertical heat exchange and its temporal variations. The global integral of vertical heat flux shows an upward heat transport in the deep ocean, suggesting a cooling trend in the deep ocean. These results support an inference that the near-surface thermal properties of the ocean are a consequence, at least in part, of internal redistributions of heat, some of which must reflect water that has undergone long trajectories since last exposure to the atmosphere. The small residual heat exchange with the atmosphere today is unlikely to represent the interaction with an ocean that was in thermal equilibrium at the start of global warming. An analogy is drawn with carbon-14 ''reservoir ages,'' which range from over hundreds to a thousand years.

Research paper thumbnail of Synthesis and Assimilation Systems - Essential Adjuncts to the Global Ocean Observing System

Proceedings of OceanObs'09: Sustained Ocean Observations and Information for Society, 2010

Research paper thumbnail of North Atlantic simulations in Coordinated Ocean-ice Reference Experiments phase II (CORE-II). Part I: Mean states

Research paper thumbnail of Estimated Seasonal Cycle of North Atlantic Eighteen Degree Water Volume

Journal of Physical Oceanography, 2011

Research paper thumbnail of Estimated Global Hydrographic Variability

Journal of Physical Oceanography, 2007

An estimate is made of the three-dimensional global oceanic temperature and salinity variability,... more An estimate is made of the three-dimensional global oceanic temperature and salinity variability, omitting the seasonal cycle, both as a major descriptive element of the ocean circulation and for use in the error estimates of state estimation. Historical hydrography, recent data from the World Ocean Circulation Experiment, and Argo profile data are all used. Root-mean-square vertical displacements in the upper 300 m of the ocean are generally smaller than 50 m, except in energetic boundary currents and in the North Atlantic subpolar gyre. Variability in temperature and salinity is strongly correlated below the top 100 m. Salinity contributions to sea surface height variability appear more significant at low latitudes than expected, possibly resulting from advective and diffusive processes. Results are generally consistent with altimetric variability under two simple kinematic hypotheses, and much of the observed structure coincides with known dynamical features. A large fraction of the sea surface height variability is consistent with the hypothesis of dominance of the first baroclinic mode.

Research paper thumbnail of Using Transformation and Formation Maps to Study the Role of Air–Sea Heat Fluxes in North Atlantic Eighteen Degree Water Formation

Journal of Physical Oceanography, 2009

The Walin water mass framework quantifies the rate at which water is transformed from one tempera... more The Walin water mass framework quantifies the rate at which water is transformed from one temperature class to another by air-sea heat fluxes (transformation). The divergence of the transformation rate yields the rate at which a given temperature range is created or destroyed by air-sea heat fluxes (formation). Walin's framework provides a precise integral statement at the expense of losing spatial information. In this study the integrand of Walin's expression to yield transformation and formation maps is plotted and used to study the role of air-sea heat fluxes in the cycle of formation-destruction of the 188 6 18C layer in the North Atlantic.

Research paper thumbnail of Estimates of the Southern Ocean general circulation improved by animal-borne instruments

Geophysical Research Letters, 2013

Research paper thumbnail of Assessing 4D-VAR for dynamical mapping of coastal high-frequency radar in San Diego

Dynamics of Atmospheres and Oceans, 2009

The problem of dynamically mapping high-frequency (HF) radar radial velocity observations is inve... more The problem of dynamically mapping high-frequency (HF) radar radial velocity observations is investigated using a threedimensional hydrodynamic model of the San Diego coastal region and an adjoint-based assimilation method. The HF radar provides near-real-time radial velocities from three sites covering the region offshore of San Diego Bay. The hydrodynamical model is the Massachusetts Institute of Technology general circulation model (MITgcm) with 1 km horizontal resolution and 40 vertical layers. The domain is centered on Point Loma, extending 117 km offshore and 120 km alongshore. The reference run (before adjustment) is initialized from a single profile of T and S and is forced with wind data from a single shore station and with zero heat and fresh water fluxes. The adjoint of the model is used to adjust initial temperature, salinity, and velocity, hourly temperature, salinity and horizontal velocities at the open boundaries, and hourly surface fluxes of momentum, heat and freshwater so that the model reproduces hourly HF radar radial velocity observations. Results from a small number of experiments suggest that the adjoint method can be successfully used over 10-day windows at coastal model resolution. It produces a dynamically consistent model run that fits HF radar data with errors near the specified uncertainties. In a test of the forecasting capability of the San Diego model after adjustment, the forecast skill was shown to exceed persistence for up to 20 h.