Armin Köhl | University of Hamburg (original) (raw)
Papers by Armin Köhl
Journal of Physical Oceanography, 2005
Journal of Geophysical Research, 2006
1] A new criterion, based on the shallowest extreme curvature of near surface layer density or te... more 1] A new criterion, based on the shallowest extreme curvature of near surface layer density or temperature profiles, is established for demarking the mixed layer depth, h mix . Using historical global hydrographic profile data, including conductivitytemperature-depth and expendable bathythermograph data obtained during World Ocean Circulation Experiment, its seasonal variability and monthly to interannual anomalies are computed. Unlike the more commonly used D criterion, the new criterion is able to deal with both different vertical resolutions of the data set and a large variety of observed stratification profiles. For about two thirds of the profiles our algorithm produces an h mix/c that is more reliable than the one of the D criterion. The uncertainty for h mix/c is ±5 m for high-(<5 m) and ±8 m for low-(<20 m) resolution profiles. A quality index, QI mix , which compares the variance of a profile above h mix to the variance to a depth of 1.5 Â h mix , shows that for the 70% of the profile data for which a clearly recognizable well-mixed zone exists near the surface, our criterion identifies the depth of the wellmixed zone in all cases. The standard deviation of anomalous monthly h mix/c is typically 20-70% of the long-term mean h mix/c . In the tropical Pacific the monthly mean anomalies of h mix/c are not well correlated with anomalies of sea surface temperature, which indicate that a variety of turbulent processes, other than surface heat fluxes, are important in the upper ocean there. Comparisons between observed h mix/c and Massachusetts Institute of Techonology/ocean general circulation model/Estimating the Circulation and Climate of the Ocean model simulated mixed layer depth indicate that the KPP algorithm captures in general a 30% smaller mixed layer depth than observed.
Journal of Geophysical Research, 2006
1] A new criterion, based on the shallowest extreme curvature of near surface layer density or te... more 1] A new criterion, based on the shallowest extreme curvature of near surface layer density or temperature profiles, is established for demarking the mixed layer depth, h mix . Using historical global hydrographic profile data, including conductivitytemperature-depth and expendable bathythermograph data obtained during World Ocean Circulation Experiment, its seasonal variability and monthly to interannual anomalies are computed. Unlike the more commonly used D criterion, the new criterion is able to deal with both different vertical resolutions of the data set and a large variety of observed stratification profiles. For about two thirds of the profiles our algorithm produces an h mix/c that is more reliable than the one of the D criterion. The uncertainty for h mix/c is ±5 m for high-(<5 m) and ±8 m for low-(<20 m) resolution profiles. A quality index, QI mix , which compares the variance of a profile above h mix to the variance to a depth of 1.5 Â h mix , shows that for the 70% of the profile data for which a clearly recognizable well-mixed zone exists near the surface, our criterion identifies the depth of the wellmixed zone in all cases. The standard deviation of anomalous monthly h mix/c is typically 20-70% of the long-term mean h mix/c . In the tropical Pacific the monthly mean anomalies of h mix/c are not well correlated with anomalies of sea surface temperature, which indicate that a variety of turbulent processes, other than surface heat fluxes, are important in the upper ocean there. Comparisons between observed h mix/c and Massachusetts Institute of Techonology/ocean general circulation model/Estimating the Circulation and Climate of the Ocean model simulated mixed layer depth indicate that the KPP algorithm captures in general a 30% smaller mixed layer depth than observed.
Journal of Geophysical Research, 2003
1] A first step for improving the climatological state of high-resolution general circulation mod... more 1] A first step for improving the climatological state of high-resolution general circulation models by means of data assimilation is presented. A method developed for the assimilation of statistical characteristics into chaotic ocean models is applied to assimilate SSH variability from TOPEX/POSEIDON and ERS1 in association with temperature and salinity from the World Ocean Atlas 1997 in order to estimate the underlying mean circulation. The method requires a parameterization of SSH variability which derives from the approach of Green and Stone. By estimating initial conditions for temperature and salinity, a mean state is achieved which, although not fully consistent with the altimetric and climatological data, is markedly improved on time scales of one year in comparison to the control run. The assimilation of SSH variability data introduces complementary information about the main frontal structures consistent with climatological observations. The state is however not an equilibrium state and returns back to the first guess quasiequilibrium state for longer integration periods.
Tellus Series A-dynamic Meteorology and Oceanography, 2002
The study investigates perspectives of the parameter estimation problem with the adjoint method i... more The study investigates perspectives of the parameter estimation problem with the adjoint method in eddy-resolving models. Sensitivity to initial conditions resulting from the chaotic nature of this type of model limits the direct application of the adjoint method by predictability. Prolonging the period of assimilation is accompanied by the appearance of an increasing number of secondary minima of the cost function that prevents the convergence of this method. In the framework of the Lorenz model it is shown that averaged quantities are suitable for describing invariant properties, and that secondary minima are for this type of data transformed into stochastic deviations. An adjoint method suitable for the assimilation of statistical characteristics of data and applicable on time scales beyond the predictability limit is presented. The approach assumes a greater predictability for averaged quantities. The adjoint to a prognostic model for statistical moments is employed for calculating cost function gradients that ignore the fine structure resulting from secondary minima. Coarse resolution versions of eddy-resolving models are used for this purpose. Identical twin experiments are performed with a quasigeostrophic model to evaluate the performance and limitations of this approach in improving models by estimating parameters. The wind stress curl is estimated from a simulated mean stream function. A very simple parameterization scheme for the assimilation of second-order moments is shown to permit the estimation of gradients that perform efficiently in minimizing cost functions. * Corresponding author. Present affiliation: Scripps estimations with high-resolution models seems to Institution of Oceanography, 8605 La Jolla Shores Drive, be straightforward, there are substantial difficult-
Journal of Oceanography, 2006
Journal of Geophysical Research, 2009
ABSTRACT Interannual changes in simulated flow fields of the Nordic Seas are analyzed with respec... more ABSTRACT Interannual changes in simulated flow fields of the Nordic Seas are analyzed with respect to their dynamic causes and consequences regarding the flow of dense water from the Nordic Seas into the subpolar North Atlantic across the Greenland-Iceland-Scotland Ridge. The simple case of pure density-driven outflow with closed northern boundaries shows that dense water mainly originates in the northern Lofoten Basin and flows southward in three branches, namely along the Norwegian continental slope, along the Mohn and Jan Mayen Ridges, and a weak current along the east Greenland continental slope. Adding variable exchange through Fram Strait shows a strengthening of the most western branch and strong recirculations that may reverse the other two branches. For this case, we find in-phase modulation of the Denmark Strait overflow (DSO) by a changing Fram Strait supply and a Faroe-Shetland transport that is in opposite phase. The scaling of this relation provides a potential explanation of recently observed DSO changes. However, details of the changes in the simulated pathways suggest, in accord with the size of the prescribed varying Fram Strait supply, basin-wide wind stress curl and local convection, which feeds water from different source regions into the outflow pathways, as the primary cause for the upstream flow field reorganizations.
Journal of Geophysical Research, 2009
ABSTRACT Interannual changes in simulated flow fields of the Nordic Seas are analyzed with respec... more ABSTRACT Interannual changes in simulated flow fields of the Nordic Seas are analyzed with respect to their dynamic causes and consequences regarding the flow of dense water from the Nordic Seas into the subpolar North Atlantic across the Greenland-Iceland-Scotland Ridge. The simple case of pure density-driven outflow with closed northern boundaries shows that dense water mainly originates in the northern Lofoten Basin and flows southward in three branches, namely along the Norwegian continental slope, along the Mohn and Jan Mayen Ridges, and a weak current along the east Greenland continental slope. Adding variable exchange through Fram Strait shows a strengthening of the most western branch and strong recirculations that may reverse the other two branches. For this case, we find in-phase modulation of the Denmark Strait overflow (DSO) by a changing Fram Strait supply and a Faroe-Shetland transport that is in opposite phase. The scaling of this relation provides a potential explanation of recently observed DSO changes. However, details of the changes in the simulated pathways suggest, in accord with the size of the prescribed varying Fram Strait supply, basin-wide wind stress curl and local convection, which feeds water from different source regions into the outflow pathways, as the primary cause for the upstream flow field reorganizations.
Surveys in Geophysics, 2011
We investigate the transient response of the global coupled ocean–atmosphere system to enhanced f... more We investigate the transient response of the global coupled ocean–atmosphere system to enhanced freshwater forcing representative of melting of the Greenland ice sheets. A 50-year long simulation by a coupled atmosphere–ocean general circulation model (CGCM) is compared with another of the same length in which Greenland melting is prescribed. To highlight the importance of coupled atmosphere–ocean processes, the CGCM results are compared with those of two other experiments carried out with the oceanic general circulation model (OGCM). In one of these OGCM experiments, the prescribed surface fluxes of heat, momentum and freshwater correspond to the unperturbed simulation by the CGCM; in the other experiment, Greenland melting is added to the freshwater flux. The responses by the CGCM and OGCM to the Greenland melting have similar patterns in the Atlantic, albeit the former having five times larger amplitudes in sea surface height anomalies. The CGCM shows likewise stronger variability in all state variables in all ocean basins because the impact of Greenland melting is quickly communicated to all ocean basins via atmospheric bridges. We conclude that the response of the global climate to Greenland ice melting is highly dependent on coupled atmosphere–ocean processes. These lead to reduced latent heat flux into the atmosphere and an associated increase in net freshwater flux into the ocean, especially in the subpolar North Atlantic. The combined result is a stronger response of the coupled system to Greenland ice sheet melting.
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.
Journal of Physical Oceanography, 2007
... on a vertical grid with 23 levels (as in SEA02 with one additional 500-m-thick level). ... As... more ... on a vertical grid with 23 levels (as in SEA02 with one additional 500-m-thick level). ... As a result, in our runs all sources for heat and freshwater reside in the surface ... b) climatology, thereby constraining water masses while at the same time allowing interannual variability to occur. ...
Journal of Climate, 2009
The intention of this project is to improve the qualitative and quantitative climate prediction c... more The intention of this project is to improve the qualitative and quantitative climate prediction capabilities. For this purpose the coupled model of the Max Planck Institute for Meteorology consisting of the atmosphere model ECHAM5 and the ocean model MPIOM is initialized with oceanic synthesis fields of the German contribution to the Estimating the Circulation and Climate of the Ocean (GECCO)
Journal of Geophysical Research, 2004
1] ECCO state estimation results from 10 years during the World Ocean Circulation Experiment are ... more 1] ECCO state estimation results from 10 years during the World Ocean Circulation Experiment are used to assess the quality of surface flux adjustments made to the initial NCEP re-analysis-1 products. During the state estimation procedure, surface fluxes are adjusted together with initial temperature and salinity conditions so that the model simulation becomes consistent with ocean observations. Independent estimates of the adjustments from bulk formula and regional field observations are also employed to evaluate the results. Buoyancy flux adjustments are found to be within the crude prior error bars on these fields. Outside the boundary current regions, they are consistent with known large-scale deficiencies in the NCEP products. Wind stress adjustments are also everywhere within the prior error bars, but exhibit regional small-scale features that reflect ocean model failures to resolve intense boundary currents. On large scales, the inferred adjustments to NCEP wind stress fields are consistent with inferences made from satellite wind stress measurements. Further improvements in the surface flux estimates obtained through state estimation procedures are anticipated as the estimation procedure becomes more complete by including the use of improved prior error covariance information, and as the ocean model becomes more skillful, for example, in simulating boundary currents by increasing its resolution. INDEX TERMS: 4504
Dynamics of Atmospheres and Oceans, 2010
estimates. The average variation is about 3.6 Sv, which is largely due to the increase of ITF tra... more estimates. The average variation is about 3.6 Sv, which is largely due to the increase of ITF transport from 1993 to 2000. However, the three-year average during the 2004-2006 INSTANT Program period is within 0.5 Sv of the long-term mean for the past few decades.
Quarterly Journal of The Royal Meteorological Society, 2005
A variational data assimilation system has been implemented for the tropical Pacific Ocean using ... more A variational data assimilation system has been implemented for the tropical Pacific Ocean using an eddy-permitting regional implementation of the MITgcm. The adjoint assimilation system was developed by the Estimation of the Circulation and the Climate of the Ocean consortium, and has been extended to deal with open boundaries. This system is used to adjust the model to match observations in the tropical Pacific region using control parameters which include initial conditions, open boundaries and time-dependent surface fluxes. This paper focuses on problems related to strong adjoint sensitivities that may impede the model fit to the observations. A decomposition of the velocities at the open boundaries into barotropic and baroclinic modes is introduced to deal with very strong sensitivities of the model sea surface height to the barotropic component of the inflow. Increased viscosity and diffusivity terms are used in the adjoint model to reduce exponentially growing sensitivities in the backward run associated with nonlinearity of the forward eScholarship provides open access, scholarly publishing services to the University of California and delivers a dynamic research platform to scholars worldwide. model. Simplified experiments in which the model was constrained with Levitus temperature and salinity data, Reynolds sea surface temperature data and TOPEX/POSEIDON altimeter data were performed to demonstrate the controllability of this assimilation system and to study its sensitivity to the starting guesses for forcing and initial conditions.
Tellus Series A-dynamic Meteorology and Oceanography, 2010
Numerical simulations of the Atlantic and Arctic Oceans using an ocean-sea ice model are analysed... more Numerical simulations of the Atlantic and Arctic Oceans using an ocean-sea ice model are analysed with respect to interannual-to-decadal variability (1948-2008) of the dense water overflows east and west of Iceland. Besides the good agreement between observed and simulated Denmark Strait and Faroe-Bank Channel overflows, the model shows an antiphase relation between both transports at time scales of 3-10yr. The analysis of wind stress curl over the Nordic Seas suggests that the phase relation is locally forced. The main mode of atmospheric variability in the Atlantic reinforces the dense overflow west of Iceland during positive phases of the North Atlantic Oscillation (NAO) and decreases it during negative phases. East of Iceland the overflow is decreased during positive phases of the NAO and increased during negative phases. After the mid-1990s, the antiphase relation is less clear due to a reduction of wind stress curl variance and a change in the production of dense water in the Nordic Seas. In that period, the Norwegian Sea experienced a reduction of its cyclonicity, leading to the export of a surplus of dense water across the Faroe-Shetland Channel. The forcing of the gyre variability is attributed to large-scale wind stress curl changes and to shifts of convection regimes linked to Arctic-Nordic Seas dense water exchange modifications at Fram Strait. Simulated sea surface height variability is highly correlated with the dense water volume in the Greenland and Norwegian Seas gyres, implying that altimetric measurements are good indicators of dense water content in the Nordic Seas and therefore provide a potential for monitoring the reservoirs and explain/anticipate changes in the overflows.
Tellus Series A-dynamic Meteorology and Oceanography, 2010
Surface net freshwater flux fields, estimated from the GECCO ocean state estimation effort over t... more Surface net freshwater flux fields, estimated from the GECCO ocean state estimation effort over the 50 yr period 1951–2001, are compared to purely satellite-based HOAPS freshwater flux estimates and to the NCEP atmospheric re-analysis net surface freshwater flux fields to assess the quality of all flux products and to improve our understanding of the time-mean surface freshwater flux distribution as well as its temporal variability. Surface flux fields are adjusted by the GECCO state estimation procedure together with initial temperature and salinity conditions so that the model simulation becomes consistent with ocean observations. The entirely independent HOAPS net surface freshwater flux fields result from the difference between SSM/I based precipitation estimates and fields of evaporation resulting from a bulk aerodynamic approach using SSM/I data and the Pathfinder SST. All three products agree well on a global scale. However, overall GECCO seems to have moved away from the NCEP/NCAR first guess surface fluxes and is often closer to the HOAPS data set. This holds for the time mean as well as for the seasonal cycle.
Journal of Physical Oceanography, 2007
The warming Nordic seas potentially tend to decrease the overflow across the Greenland-Iceland-Sc... more The warming Nordic seas potentially tend to decrease the overflow across the Greenland-Iceland-Scotland Ridge (GISR) system. Recent observations by Macrander et al. document a significant drop in the intensity of outflowing Denmark Strait Overflow Water of more than 20% over 3 yr and a simultaneous increase in the temperature of the bottom layers of more than 0.4°C. A simulation of the exchange across the GISR with a regional ocean circulation model is used here to identify possible mechanisms that control changes in the Denmark Strait overflow and its relations to changed forcing condition. On seasonal and longer time scales, the authors establish links of the overflow anomalies to a decreasing capacity of the dense water reservoir caused by a change of circulation pattern north of the sill. On annual and shorter time scales, the wind stress curl around Iceland determines the barotropic circulation around the island and thus the barotropic flow through Denmark Strait. For the overlapping time scales, the barotropic and overflow component interactively determine transport variations. Last, a relation between sea surface height and reservoir height changes upstream of the sill is used to predict the overflow variability from altimeter data. Estimated changes are in agreement with other recent transport estimates based on current-meter arrays.
Journal of Geophysical Research, 2011
Eos, Transactions American Geophysical Union, 2005
Estimates of ocean circulation constrained by in situ and remotely sensed observations have becom... more Estimates of ocean circulation constrained by in situ and remotely sensed observations have become routinely available during the past five years, and they are being applied to myriad scientific and operational problems . Under the Global Ocean Data Assimilation Experiment (GODAE), several regional and global estimates have evolved for applications in climate research, seasonal forecasting, naval operations, marine safety, fisheries,the offshore oil industry, coastal management, and other areas.
Journal of Physical Oceanography, 2005
Journal of Geophysical Research, 2006
1] A new criterion, based on the shallowest extreme curvature of near surface layer density or te... more 1] A new criterion, based on the shallowest extreme curvature of near surface layer density or temperature profiles, is established for demarking the mixed layer depth, h mix . Using historical global hydrographic profile data, including conductivitytemperature-depth and expendable bathythermograph data obtained during World Ocean Circulation Experiment, its seasonal variability and monthly to interannual anomalies are computed. Unlike the more commonly used D criterion, the new criterion is able to deal with both different vertical resolutions of the data set and a large variety of observed stratification profiles. For about two thirds of the profiles our algorithm produces an h mix/c that is more reliable than the one of the D criterion. The uncertainty for h mix/c is ±5 m for high-(<5 m) and ±8 m for low-(<20 m) resolution profiles. A quality index, QI mix , which compares the variance of a profile above h mix to the variance to a depth of 1.5 Â h mix , shows that for the 70% of the profile data for which a clearly recognizable well-mixed zone exists near the surface, our criterion identifies the depth of the wellmixed zone in all cases. The standard deviation of anomalous monthly h mix/c is typically 20-70% of the long-term mean h mix/c . In the tropical Pacific the monthly mean anomalies of h mix/c are not well correlated with anomalies of sea surface temperature, which indicate that a variety of turbulent processes, other than surface heat fluxes, are important in the upper ocean there. Comparisons between observed h mix/c and Massachusetts Institute of Techonology/ocean general circulation model/Estimating the Circulation and Climate of the Ocean model simulated mixed layer depth indicate that the KPP algorithm captures in general a 30% smaller mixed layer depth than observed.
Journal of Geophysical Research, 2006
1] A new criterion, based on the shallowest extreme curvature of near surface layer density or te... more 1] A new criterion, based on the shallowest extreme curvature of near surface layer density or temperature profiles, is established for demarking the mixed layer depth, h mix . Using historical global hydrographic profile data, including conductivitytemperature-depth and expendable bathythermograph data obtained during World Ocean Circulation Experiment, its seasonal variability and monthly to interannual anomalies are computed. Unlike the more commonly used D criterion, the new criterion is able to deal with both different vertical resolutions of the data set and a large variety of observed stratification profiles. For about two thirds of the profiles our algorithm produces an h mix/c that is more reliable than the one of the D criterion. The uncertainty for h mix/c is ±5 m for high-(<5 m) and ±8 m for low-(<20 m) resolution profiles. A quality index, QI mix , which compares the variance of a profile above h mix to the variance to a depth of 1.5 Â h mix , shows that for the 70% of the profile data for which a clearly recognizable well-mixed zone exists near the surface, our criterion identifies the depth of the wellmixed zone in all cases. The standard deviation of anomalous monthly h mix/c is typically 20-70% of the long-term mean h mix/c . In the tropical Pacific the monthly mean anomalies of h mix/c are not well correlated with anomalies of sea surface temperature, which indicate that a variety of turbulent processes, other than surface heat fluxes, are important in the upper ocean there. Comparisons between observed h mix/c and Massachusetts Institute of Techonology/ocean general circulation model/Estimating the Circulation and Climate of the Ocean model simulated mixed layer depth indicate that the KPP algorithm captures in general a 30% smaller mixed layer depth than observed.
Journal of Geophysical Research, 2003
1] A first step for improving the climatological state of high-resolution general circulation mod... more 1] A first step for improving the climatological state of high-resolution general circulation models by means of data assimilation is presented. A method developed for the assimilation of statistical characteristics into chaotic ocean models is applied to assimilate SSH variability from TOPEX/POSEIDON and ERS1 in association with temperature and salinity from the World Ocean Atlas 1997 in order to estimate the underlying mean circulation. The method requires a parameterization of SSH variability which derives from the approach of Green and Stone. By estimating initial conditions for temperature and salinity, a mean state is achieved which, although not fully consistent with the altimetric and climatological data, is markedly improved on time scales of one year in comparison to the control run. The assimilation of SSH variability data introduces complementary information about the main frontal structures consistent with climatological observations. The state is however not an equilibrium state and returns back to the first guess quasiequilibrium state for longer integration periods.
Tellus Series A-dynamic Meteorology and Oceanography, 2002
The study investigates perspectives of the parameter estimation problem with the adjoint method i... more The study investigates perspectives of the parameter estimation problem with the adjoint method in eddy-resolving models. Sensitivity to initial conditions resulting from the chaotic nature of this type of model limits the direct application of the adjoint method by predictability. Prolonging the period of assimilation is accompanied by the appearance of an increasing number of secondary minima of the cost function that prevents the convergence of this method. In the framework of the Lorenz model it is shown that averaged quantities are suitable for describing invariant properties, and that secondary minima are for this type of data transformed into stochastic deviations. An adjoint method suitable for the assimilation of statistical characteristics of data and applicable on time scales beyond the predictability limit is presented. The approach assumes a greater predictability for averaged quantities. The adjoint to a prognostic model for statistical moments is employed for calculating cost function gradients that ignore the fine structure resulting from secondary minima. Coarse resolution versions of eddy-resolving models are used for this purpose. Identical twin experiments are performed with a quasigeostrophic model to evaluate the performance and limitations of this approach in improving models by estimating parameters. The wind stress curl is estimated from a simulated mean stream function. A very simple parameterization scheme for the assimilation of second-order moments is shown to permit the estimation of gradients that perform efficiently in minimizing cost functions. * Corresponding author. Present affiliation: Scripps estimations with high-resolution models seems to Institution of Oceanography, 8605 La Jolla Shores Drive, be straightforward, there are substantial difficult-
Journal of Oceanography, 2006
Journal of Geophysical Research, 2009
ABSTRACT Interannual changes in simulated flow fields of the Nordic Seas are analyzed with respec... more ABSTRACT Interannual changes in simulated flow fields of the Nordic Seas are analyzed with respect to their dynamic causes and consequences regarding the flow of dense water from the Nordic Seas into the subpolar North Atlantic across the Greenland-Iceland-Scotland Ridge. The simple case of pure density-driven outflow with closed northern boundaries shows that dense water mainly originates in the northern Lofoten Basin and flows southward in three branches, namely along the Norwegian continental slope, along the Mohn and Jan Mayen Ridges, and a weak current along the east Greenland continental slope. Adding variable exchange through Fram Strait shows a strengthening of the most western branch and strong recirculations that may reverse the other two branches. For this case, we find in-phase modulation of the Denmark Strait overflow (DSO) by a changing Fram Strait supply and a Faroe-Shetland transport that is in opposite phase. The scaling of this relation provides a potential explanation of recently observed DSO changes. However, details of the changes in the simulated pathways suggest, in accord with the size of the prescribed varying Fram Strait supply, basin-wide wind stress curl and local convection, which feeds water from different source regions into the outflow pathways, as the primary cause for the upstream flow field reorganizations.
Journal of Geophysical Research, 2009
ABSTRACT Interannual changes in simulated flow fields of the Nordic Seas are analyzed with respec... more ABSTRACT Interannual changes in simulated flow fields of the Nordic Seas are analyzed with respect to their dynamic causes and consequences regarding the flow of dense water from the Nordic Seas into the subpolar North Atlantic across the Greenland-Iceland-Scotland Ridge. The simple case of pure density-driven outflow with closed northern boundaries shows that dense water mainly originates in the northern Lofoten Basin and flows southward in three branches, namely along the Norwegian continental slope, along the Mohn and Jan Mayen Ridges, and a weak current along the east Greenland continental slope. Adding variable exchange through Fram Strait shows a strengthening of the most western branch and strong recirculations that may reverse the other two branches. For this case, we find in-phase modulation of the Denmark Strait overflow (DSO) by a changing Fram Strait supply and a Faroe-Shetland transport that is in opposite phase. The scaling of this relation provides a potential explanation of recently observed DSO changes. However, details of the changes in the simulated pathways suggest, in accord with the size of the prescribed varying Fram Strait supply, basin-wide wind stress curl and local convection, which feeds water from different source regions into the outflow pathways, as the primary cause for the upstream flow field reorganizations.
Surveys in Geophysics, 2011
We investigate the transient response of the global coupled ocean–atmosphere system to enhanced f... more We investigate the transient response of the global coupled ocean–atmosphere system to enhanced freshwater forcing representative of melting of the Greenland ice sheets. A 50-year long simulation by a coupled atmosphere–ocean general circulation model (CGCM) is compared with another of the same length in which Greenland melting is prescribed. To highlight the importance of coupled atmosphere–ocean processes, the CGCM results are compared with those of two other experiments carried out with the oceanic general circulation model (OGCM). In one of these OGCM experiments, the prescribed surface fluxes of heat, momentum and freshwater correspond to the unperturbed simulation by the CGCM; in the other experiment, Greenland melting is added to the freshwater flux. The responses by the CGCM and OGCM to the Greenland melting have similar patterns in the Atlantic, albeit the former having five times larger amplitudes in sea surface height anomalies. The CGCM shows likewise stronger variability in all state variables in all ocean basins because the impact of Greenland melting is quickly communicated to all ocean basins via atmospheric bridges. We conclude that the response of the global climate to Greenland ice melting is highly dependent on coupled atmosphere–ocean processes. These lead to reduced latent heat flux into the atmosphere and an associated increase in net freshwater flux into the ocean, especially in the subpolar North Atlantic. The combined result is a stronger response of the coupled system to Greenland ice sheet melting.
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.
Journal of Physical Oceanography, 2007
... on a vertical grid with 23 levels (as in SEA02 with one additional 500-m-thick level). ... As... more ... on a vertical grid with 23 levels (as in SEA02 with one additional 500-m-thick level). ... As a result, in our runs all sources for heat and freshwater reside in the surface ... b) climatology, thereby constraining water masses while at the same time allowing interannual variability to occur. ...
Journal of Climate, 2009
The intention of this project is to improve the qualitative and quantitative climate prediction c... more The intention of this project is to improve the qualitative and quantitative climate prediction capabilities. For this purpose the coupled model of the Max Planck Institute for Meteorology consisting of the atmosphere model ECHAM5 and the ocean model MPIOM is initialized with oceanic synthesis fields of the German contribution to the Estimating the Circulation and Climate of the Ocean (GECCO)
Journal of Geophysical Research, 2004
1] ECCO state estimation results from 10 years during the World Ocean Circulation Experiment are ... more 1] ECCO state estimation results from 10 years during the World Ocean Circulation Experiment are used to assess the quality of surface flux adjustments made to the initial NCEP re-analysis-1 products. During the state estimation procedure, surface fluxes are adjusted together with initial temperature and salinity conditions so that the model simulation becomes consistent with ocean observations. Independent estimates of the adjustments from bulk formula and regional field observations are also employed to evaluate the results. Buoyancy flux adjustments are found to be within the crude prior error bars on these fields. Outside the boundary current regions, they are consistent with known large-scale deficiencies in the NCEP products. Wind stress adjustments are also everywhere within the prior error bars, but exhibit regional small-scale features that reflect ocean model failures to resolve intense boundary currents. On large scales, the inferred adjustments to NCEP wind stress fields are consistent with inferences made from satellite wind stress measurements. Further improvements in the surface flux estimates obtained through state estimation procedures are anticipated as the estimation procedure becomes more complete by including the use of improved prior error covariance information, and as the ocean model becomes more skillful, for example, in simulating boundary currents by increasing its resolution. INDEX TERMS: 4504
Dynamics of Atmospheres and Oceans, 2010
estimates. The average variation is about 3.6 Sv, which is largely due to the increase of ITF tra... more estimates. The average variation is about 3.6 Sv, which is largely due to the increase of ITF transport from 1993 to 2000. However, the three-year average during the 2004-2006 INSTANT Program period is within 0.5 Sv of the long-term mean for the past few decades.
Quarterly Journal of The Royal Meteorological Society, 2005
A variational data assimilation system has been implemented for the tropical Pacific Ocean using ... more A variational data assimilation system has been implemented for the tropical Pacific Ocean using an eddy-permitting regional implementation of the MITgcm. The adjoint assimilation system was developed by the Estimation of the Circulation and the Climate of the Ocean consortium, and has been extended to deal with open boundaries. This system is used to adjust the model to match observations in the tropical Pacific region using control parameters which include initial conditions, open boundaries and time-dependent surface fluxes. This paper focuses on problems related to strong adjoint sensitivities that may impede the model fit to the observations. A decomposition of the velocities at the open boundaries into barotropic and baroclinic modes is introduced to deal with very strong sensitivities of the model sea surface height to the barotropic component of the inflow. Increased viscosity and diffusivity terms are used in the adjoint model to reduce exponentially growing sensitivities in the backward run associated with nonlinearity of the forward eScholarship provides open access, scholarly publishing services to the University of California and delivers a dynamic research platform to scholars worldwide. model. Simplified experiments in which the model was constrained with Levitus temperature and salinity data, Reynolds sea surface temperature data and TOPEX/POSEIDON altimeter data were performed to demonstrate the controllability of this assimilation system and to study its sensitivity to the starting guesses for forcing and initial conditions.
Tellus Series A-dynamic Meteorology and Oceanography, 2010
Numerical simulations of the Atlantic and Arctic Oceans using an ocean-sea ice model are analysed... more Numerical simulations of the Atlantic and Arctic Oceans using an ocean-sea ice model are analysed with respect to interannual-to-decadal variability (1948-2008) of the dense water overflows east and west of Iceland. Besides the good agreement between observed and simulated Denmark Strait and Faroe-Bank Channel overflows, the model shows an antiphase relation between both transports at time scales of 3-10yr. The analysis of wind stress curl over the Nordic Seas suggests that the phase relation is locally forced. The main mode of atmospheric variability in the Atlantic reinforces the dense overflow west of Iceland during positive phases of the North Atlantic Oscillation (NAO) and decreases it during negative phases. East of Iceland the overflow is decreased during positive phases of the NAO and increased during negative phases. After the mid-1990s, the antiphase relation is less clear due to a reduction of wind stress curl variance and a change in the production of dense water in the Nordic Seas. In that period, the Norwegian Sea experienced a reduction of its cyclonicity, leading to the export of a surplus of dense water across the Faroe-Shetland Channel. The forcing of the gyre variability is attributed to large-scale wind stress curl changes and to shifts of convection regimes linked to Arctic-Nordic Seas dense water exchange modifications at Fram Strait. Simulated sea surface height variability is highly correlated with the dense water volume in the Greenland and Norwegian Seas gyres, implying that altimetric measurements are good indicators of dense water content in the Nordic Seas and therefore provide a potential for monitoring the reservoirs and explain/anticipate changes in the overflows.
Tellus Series A-dynamic Meteorology and Oceanography, 2010
Surface net freshwater flux fields, estimated from the GECCO ocean state estimation effort over t... more Surface net freshwater flux fields, estimated from the GECCO ocean state estimation effort over the 50 yr period 1951–2001, are compared to purely satellite-based HOAPS freshwater flux estimates and to the NCEP atmospheric re-analysis net surface freshwater flux fields to assess the quality of all flux products and to improve our understanding of the time-mean surface freshwater flux distribution as well as its temporal variability. Surface flux fields are adjusted by the GECCO state estimation procedure together with initial temperature and salinity conditions so that the model simulation becomes consistent with ocean observations. The entirely independent HOAPS net surface freshwater flux fields result from the difference between SSM/I based precipitation estimates and fields of evaporation resulting from a bulk aerodynamic approach using SSM/I data and the Pathfinder SST. All three products agree well on a global scale. However, overall GECCO seems to have moved away from the NCEP/NCAR first guess surface fluxes and is often closer to the HOAPS data set. This holds for the time mean as well as for the seasonal cycle.
Journal of Physical Oceanography, 2007
The warming Nordic seas potentially tend to decrease the overflow across the Greenland-Iceland-Sc... more The warming Nordic seas potentially tend to decrease the overflow across the Greenland-Iceland-Scotland Ridge (GISR) system. Recent observations by Macrander et al. document a significant drop in the intensity of outflowing Denmark Strait Overflow Water of more than 20% over 3 yr and a simultaneous increase in the temperature of the bottom layers of more than 0.4°C. A simulation of the exchange across the GISR with a regional ocean circulation model is used here to identify possible mechanisms that control changes in the Denmark Strait overflow and its relations to changed forcing condition. On seasonal and longer time scales, the authors establish links of the overflow anomalies to a decreasing capacity of the dense water reservoir caused by a change of circulation pattern north of the sill. On annual and shorter time scales, the wind stress curl around Iceland determines the barotropic circulation around the island and thus the barotropic flow through Denmark Strait. For the overlapping time scales, the barotropic and overflow component interactively determine transport variations. Last, a relation between sea surface height and reservoir height changes upstream of the sill is used to predict the overflow variability from altimeter data. Estimated changes are in agreement with other recent transport estimates based on current-meter arrays.
Journal of Geophysical Research, 2011
Eos, Transactions American Geophysical Union, 2005
Estimates of ocean circulation constrained by in situ and remotely sensed observations have becom... more Estimates of ocean circulation constrained by in situ and remotely sensed observations have become routinely available during the past five years, and they are being applied to myriad scientific and operational problems . Under the Global Ocean Data Assimilation Experiment (GODAE), several regional and global estimates have evolved for applications in climate research, seasonal forecasting, naval operations, marine safety, fisheries,the offshore oil industry, coastal management, and other areas.