OS31C-1284
Regional and Global Mean Sea Level Variability Over the Modern Instrumental Record
The possibility of sea level rise in the context of global climate change has received much attention in recent years. Determination of sea level rise and its causes, either globally or regionally, must however cope with other signals in the sea level record. A comprehensive look at sea level variability over the modern instrumental period (1992-present) is made possible by the 3-dimensional, time-dependent ocean state estimates produced under ECCO-GODAE. Such estimates involve a least-squares optimization that produces a "best" fit of the MITgcm to most available ocean data, including several altimetric missions and all in-situ hydrography. The estimated regional sea level patterns exhibit interannual and longer period variability that can easily mask expected long-term trends in mean sea level. Both steric and mass changes contribute to sea level change at regional and global levels, and thermal and haline effects are evident over the full water column, stressing the need for surface-to-bottom measurements. Spatial patterns of variability are not simply related to a passive ocean response to heating and cooling but involve changes in its 3-dimensional circulation. Uncertainties in mean sea level estimates remain large given the possibility of systematic errors in all datasets, including the atmospheric surface fluxes. Various ways of improving model formulation and implementation of data constraints relevant for determining global mean quantities are examined.
OS31C-1285
Estimating Rapid Large-Scale Variability in Sea Level, With Application to Dealiasing of Satellite Missions
Considerable nontidal sea level variability is found over the deep ocean at monthly and shorter timescales and at spatial scales greater than a few hundred kilometers. Such variability is mostly atmospherically-driven, can be barotropic or baroclinic and thus with a complex relation to bottom pressure, and is not fully resolved by satellite altimetry or gravity measurements. Model simulations of the rapid, large-scale signals can be affected by errors in forcing fields, bottom topography and other uncertain factors. Here we examine estimates of these signals obtained from the ECCO-GODAE solutions, which are constrained to most available oceanic data in a least-squares optimization procedure. Results indicate a considerable impact of the data on the estimated rapid, large-scale variability. Comparisons with independent hourly data from tide gauges reveal improvements in the constrained estimates and initial tests suggest that such estimates should be useful in procedures to dealias current satellite altimetry and gravity measurements.
OS31C-1286
A Kalman filter and 3dVAR inter-comparison with NCEP's new operational forecasting system
The ECCO Kalman filter is adapted to the Modular Ocean Model (MOM4) employed in the latest operational ocean analyses of the National Centers for Environmental Prediction (NCEP), National Oceanic and Atmospheric Administration. The ocean state estimates of the Kalman filter are compared with those of NCEP's 3dVAR method to assess the relative impact of the different approaches on analyzing and forecasting seasonal-to-interannual climate variability. The ECCO filter employs partitioned, reduced-state, and time-asymptotic approximations of the model state error covariance matrix associated with inaccuracies in atmospheric wind forcing. The filter assimilates temporal anomalies of satellite sea level and in situ temperature profile measurements over the world's oceans. New advancements are devised in filter implementation including use of adjoint codes and improvements in spatial interpolation around land masses and in estimating effects of vertical heaving motion. The next operational model of NCEP is based on MOM4 and employs a tripolar grid that spans the entire globe including the Arctic Ocean with a nominal 0.5-degree grid with 40 vertical levels. A 3dVAR method is employed to assimilate temperature and synthetic salinity profiles. The salinity profiles are constructed from the temperature profiles and a local TS relationship. The model error variances are assumed to be proportional to the local temperature and salinity gradients and are computed from the most recent 5-day model average. Utilizing identical models, the inter-comparison provides a unique assessment of the two assimilation methods independent of potential differences in models that are used. The two ocean state estimates will be compared with respect to various observations. Circulation indexes will be analyzed such as strengths of subtropical cells, meridional overturning circulation, and integrated upper ocean heat content changes. The assimilations' impact on subsequent variability of the ocean will be assessed by examining model evolution resulting from data assimilated model initializations.
OS31C-1287
Realizations of a Yearlong Eddy-Permitting Hindcast of the Tropical Pacific With the ECCO Assimilation System
The ECCO assimilation system has been used to estimate the state of an eddy-resolving model of the tropical Pacific Ocean for the year 2000. The initial guess for forcing is from the NCEP-NCAR re-analysis and the boundary conditions come from the ECCO global re-analysis. The model is fit to observations in the tropical Pacific region by controlling the initial temperature and salinity; temperature, salinity and horizontal velocities at the open boundaries; and surface fluxes of momentum, heat and freshwater. The model is constrained with most of the available datasets in the tropical Pacific, including climatologies, TAO, float, XBT, drifter, and satellite SST and SSH data. The iterated adjoint-based descent is able to significantly improve the model consistency with the multivariate data sets, providing a dynamically consistent realization of the tropical Pacific circulation that generally matches the observations to within (admittedly poorly-known) errors. The adjusted control fields are smoothed and applied in model runs without assimilation to check that small changes in the controls do not greatly change the model hindcast and to provide a small ensemble of acceptable model solutions. In addition, the original and smoothed controls are applied to a version of the model with doubled horizontal resolution resulting in a broadly similar "downscaled" hindcast, showing that the adjustments are not tuned to a single configuration. The time-evolving model state and the adjusted controls should be useful for analysis or to supply the forcing, initial, and boundary conditions for runs of other models.
OS31C-1288
A MITgcm/DART Ocean Analysis and Prediction System with Application to the Gulf of Mexico
The ECCO system is a new generation of ocean assimilation systems based on the Massachusetts Institute of Technology general circulation model (MITgcm) and its adjoint. The system has been used to produce the first global 1° ocean state estimates. It is now also used for regional and coastal MITgcm applications. To improve the predictive capabilities of the ECCO system, the Data Assimilation Research Testbed (DART), which is an ensemble Kalman filter (EnKF)-based data assimilation package, has been recently integrated to the ECCO system. DART is a software facility employing different EnKFs and advanced inflation/localization schemes. It has been developed at the National Center of Atmospheric Research (NCAR) and is now used for different operational weather forecasting problems. This contribution describes the integration of DART and the MITgcm, and discusses how this ensemble-based system can complement the existing adjoint-based assimilation system. An example of a 1/10° MITgcm/DART application for predicting the evolution of the loop current in the Gulf of Mexico is presented.
OS31C-1289
Exploring ENSO-like Interannual SST Variation in the Equatorial Central Pacific using GECCO assimilation
German ECCO (GECCO) assimilation dataset is used to understand the properties and dynamics of the interannual sea surface temperature (SST) variability in the equatorial central Pacific (CP), which appears like ENSO but differs from the typical type of ENSO in several aspects. The CP-type SST variability exhibits distinctive spatial pattern, temporal evolution, and teleconnection from the canonical ENSO events. It is also dominated by a quasi-biennial periodicity, rather than a 4-5 year periodicity of the typical ENSO. Results show that the CP-type SST variability has a different subsurface evolution compared to the delayed oscillator mode of ENSO. Ocean heat budget and composite analysis are performed with the GECCO product to identify the physical processes that control the CP-type interannual SST variability. Differences in the response of near surface equatorial currents between the CP-type and canonical-ENSO events are also identified. The five-decade long of GECCO product is also used to examine the decadal variations of the CP- type SST variability and its phase relationship with the canonical ENSO events. The change of the phase relationship is used to explain why the propagation direction of ENSO SST anomalies changes before and after 1976/77. Subsurface ocean indices are proposed to better identify the CP-type SST variability.
OS31C-1290
Mechanisms Controlling Seasonal Mixed Layer Temperature of the Indonesian Seas in an ECCO Assimilation Product
We use an ECCO data assimilation product to examine seasonal mixed layer processes in the Indonesian Seas, focusing on mechanisms controlling mixed layer temperature (MLT) cooling in boreal summer. Surface heat flux dominates seasonal MLT with significant secondary reinforcing contributions by subsurface processes. Spring and fall warming occur when insolation near the equator increases, and surface heat loss and subsurface cooling decrease during monsoon transitions. Boreal summer cooling occurs when the insolation maximum is in the northern hemisphere and monsoon winds are heightened, causing increased surface heat loss and turbulent mixing. When we examine the MLT budgets in the western half (Java Sea) and eastern half (Banda and Arafura Seas) of the Indonesian Seas separately, horizontal advection is found to induce strong compensating effects in the west, but weak reinforcing effects in the east. Vertical and horizontal advective cooling are both small in the east, whereas cooling by vertical mixing is strong. Accordingly, subseasonal-to-seasonal scale winds over the Arafura Sea are relatively variable, especially on shorter timescales, effects of which can rectify into the seasonal cycle. There is negative wind curl near the New Guinea coast in boreal summer, but away from the coast wind curl is small. These factors are consistent with the weak (strong) vertical advective (vertical mixing induced) cooling. Our model resolution is too coarse to address shelf-upwelling effects on cooling. However, we believe vertical mixing plays an important role in MLT variability, with a contribution that is approximately half that of the surface heat flux.
OS31C-1291
Sea Surface Temperature and Heat Budget Variability in ECCO2
Oceanic mixed layer heat budgets are crucial for climate modeling efforts because they govern the evolution of sea surface temperatures (SST), the most important oceanic parameter driving atmospheric circulation. Mixed layer heat budget variability is controlled by surface heat fluxes, entrainment, advection, and diffusion. The respective role of these processes varies as a function of region, spatial scale, and frequency. We present results from an analysis of mixed layer heat budgets in a 1992-2007 ECCO2 ocean state estimate. We quantify the various contributions to changes in mixed layer temperature and their associated errors for three oceanic regions in the North Pacific Ocean: the subtropical gyre, an upwelling region off the US West Coast, and a dynamically active area in the Kuroshio region. This regionalization allows for a simplification of the heat budget and the identification of various regimes and frequency/wavenumber bands that are dominated by one or two of the heat budget terms. An integral part of this investigation is the mixed layer heat budget error analysis that allows us to work towards improving the representation of mixed layer processes in ocean models.
OS31C-1292
ECCO2: High Resolution Global Ocean and Sea Ice Data Synthesis
The Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) project aims to produce a best-
possible, time-evolving synthesis of most available ocean and sea-ice data at a resolution that permits ocean
eddies. ECCO2 analyses are obtained via least squares fit of a global, full-depth-ocean, and sea-ice
configuration of the Massachusetts Institute of Technology general circulation model (MITgcm) to the
available satellite and in-situ data. What sets apart ECCO2 analyses from operational high-resolution ocean
data assimilation products is their physical consistency; the analyses do not contain discontinuities when and
where data are ingested. ECCO2 analyses are intended to help quantify the role of the oceans in the global
carbon cycle, to understand the recent evolution of the polar oceans, to monitor time-evolving term balances
within and between different components of the Earth system, and for many other science applications.
A first ECCO2 analysis for the 1992-2007 period has been obtained using a Green's Function approach to
estimate initial temperature and salinity conditions, surface boundary conditions, and several empirical model
parameters. Data constraints include altimetry, gravity, drifter, hydrographic, and sea-ice data. A large
complement of high-frequency and high-resolution diagnostics has been saved; these diagnostics are made
available to the scientific community via ftp and OPeNDAP servers at
http://ecco2.org. This presentation provides a brief overview of this first ECCO2 analysis, of the estimation
methodology, of the solution characteristics, and of some early science applications.
http://ecco2.org/
OS31C-1293
Variability of Meridional Overturning Circulation and Heat Transport in the 1992-2007 ECCO2 Synthesis
The zonally-integrated meridional velocity and temperature fields of the Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) high-resolution global ocean and sea ice data synthesis are used to investigate the Meridional Overturning Circulation (MOC) and associated heat transport over the period of 1992-2007. The ECCO2 data synthesis is obtained by least squares fit of a global ocean and sea-ice configuration of the Massachusetts Institute of Technology general circulation model (MITgcm) to the available satellite and in-situ data using a Green's function approach. The results show that the mean states of MOC and heat transport are in good agreement with observations and with estimates from other studies. Empirical Orthogonal Function (EOF) analysis shows that the variability in global MOC is dominated by the annual cycle with principal contribution from the Indo-Pacific Basin. The Atlantic component of the MOC is characterized by strong intraseasonal variability. On time scales longer than one year, the ECCO2 estimates show trend-like patterns in both global temperature and MOC fields but no apparent trend in the global heat transport. The heat transport in the Atlantic Sector is characterized by different trends over the first and second half of the estimation period.
OS31C-1294
Meridional heat transports in the ocean from an ECCO2 data synthesis
A global ocean data synthesis product at eddy-permitting resolution from Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) project are used to estimate the total and eddy merdional heat transports in the ocean. These estimates are compared to previous observational and modeling studies. The contributions of the overturning and the horizontal circulations to the total heat transport are evaluated. The eddy heat transport was estimated as the deviation of the zonally integrated heat transport from its 3-month averages. The estimated heat transport, thus, contains signals only with periods shorter than 3 months, which are mainly associated with the eddy variability. We show that in a number of locations the time-mean eddy heat transport constitutes a considerable portion of the total time-mean heat transport, in particular, in the tropics, in the Southern Ocean and in the Kuroshio Current. This research demonstrates that the variability of the eddy heat transport is a significant contributor to the variability of the total heat transport and globally it explains about 1/3 of its variance. Eddies are also found to explain a significant portion of the seasonal-interannual heat transport variance. Using the estimates of the meridional heat transport we investigate the mechanisms of the recent sea level rise in the Subpolar North Atlantic and demonstrate the importance of the oceanic heat convergence.
OS31C-1295
Numerical and Theoretical Investigations of North Pacific Subtropical Mode Water With Implications to Pacific Climate Variability
An investigation using a combined numerical modeling and theoretical approach is followed to better resolve the role of Subtropical Mode Water (STMW) in the exchange of information between the atmosphere and the ocean linked to climate variability in the North Pacific Ocean. In this, a high resolution MIT General Circulation Model (MITgcm) simulation is analyzed to study the formation, isolation and dispersal of STMW and identify correlations between STMW variations and established climatic signals in the Pacific basin. During a 171- month time period (from January 1992 to March 2006), the seasonal variability is the dominant temporal variation observed. From climatological model fields, STMW exhibits distinct features in time and space. In addition to seasonality there is also an interannual signal observed in STMW variability. This interannual variation pattern is connected closely to the climate shifts of North Pacific, with further investigation showing that there is a high correlation between the STMW variability and the Pacific Decadal Oscillation index. To identify the mechanisms responsible for this interannual STMW variability, classical ocean thermocline theories are reviewed and STMW connections to large scale ocean circulation patterns are explored. A planetary geostrophic ocean model (PGOM) is employed as a theoretical platform for this purpose. Specifically, numerical PGOM experiments are performed to isolate and examine in further detail, the influence of variations of the large scale wind stress pattern and large scale air-sea heat flux on STMW variability. It may be gathered from these experiments that variability in this large scale wind stress is seen to affect the variability pattern of model STMW. Yet, results also indicate that the amplitude of seasonal and interannual variability of STMW volume is primarily dominated by the variability in the air-sea heat flux.
OS31C-1296
Incorporation of the RAPID 26N and Florida Strait cable observations into the ECCO- GODAE state estimate
Daily timeseries of the meridional overturning circulation (MOC) estimated from the UK/US RAPID/MOCHA array at 26.5N in the Atlantic and the ECCO-GODAE state estimate show significant correlations for both the MOC and the Ekman transport with their respective counterpart (at the 95 percent confidence interval). However, the time-mean value of the MOC in ECCO-GODAE is several Sverdrups weaker than the RAPID/MOCHA estimate, while the Ekman transport is similar. Here, we test whether the additional incorporation of the temperature/salinity observations from the RAPID/MOCHA mooring array, as well as the cable estimates of volume transport in the Florida Current into ECCO-GODAE change ECCO-GODAE's representation of the MOC in the Atlantic. Initially, we use experimental one year integrations, where the incorporation of the data results in an increased MOC of about 1 Sv for the northward branch of the MOC above 1000 m, and an increased MOC of about 1 Sv for the southward branch of the MOC between about 2000 m and 3000 m, at 26N and adjacent latitudes. The meridional heat transport increases by about 0.05 PW between 26N and about 40N. Subsequently, we investigate how both the RAPID/MOCHA and Florida Current observations affect the full 16 year ECCO-GODAE state estimate (1992-2007).
OS31C-1297
Transient sensitivities of sea ice export through the Canadian Arctic Archipelago inferred from a coupled ocean/sea-ice adjoint model
The sensitivity of sea-ice export through the Canadian Arctic Archipelago (CAA), measured in terms of its solid freshwater export through Lancaster Sound, to changes in various elements of the ocean and sea-ice state, and to elements of the atmospheric forcing fields through time and space is assessed by means of a coupled ocean/sea-ice adjoint model. The adjoint model furnishes full spatial sensitivity maps (also known as Lagrange multipliers) of the export metric to a variety of model variables at any chosen point in time, providing the unique capability to quantify major drivers of sea-ice export variability. The underlying model is the MIT ocean general circulation model (MITgcm), which is coupled to a Hibler-type dynamic/thermodynamic sea-ice model. The configuration is based on the Arctic face of the ECCO3 high-resolution cubed-sphere model, but coarsened to 36-km horizontal grid spacing. The adjoint of the coupled system has been derived by means of automatic differentiation using the software tool TAF. Finite perturbation simulations are performed to check the information provided by the adjoint. The sea-ice model's performance in the presence of narrow straits is assessed with different sea-ice lateral boundary conditions. The adjoint sensitivity clearly exposes the role of the model trajectory and the transient nature of the problem. The complex interplay between forcing, dynamics, and boundary condition is demonstrated in the comparison between the different calculations. The study is a step towards fully coupled adjoint-based ocean/sea-ice state estimation at basin to global scales as part of the ECCO efforts.
OS31C-1298
The Role of Ice Shelf Ocean Interaction in the ECCO2 Data Synthesis
Dense water masses that form on the continental shelves around Antarctica and spread into the global abyss are a major contributor to the global overturning circulation. Untill recently the Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) project did not adequately represent many high latitude processes, including ice shelf ocean interactions. This work concerns the addition of an ice shelf package to the ECCO2 ocean state estimates. The ECCO2 solutions are obtained by fitting a high resolution (18 km horizontal grid spacing and 50 vertical levels) global-ocean and sea-ice configuration of the Massachusetts Institute of Technology general circulation model (MITgcm) to the available ocean and sea-ice data. The ice shelf package included in these studies is based on the ISOMIP parameterisations for heat and freshwater exchanges at the ice-ocean interface. Ice, Cloud,and land Elevation Satellite (IceSAT) data provided the ice shelf thickness and Antarctic Bedrock Mapping Project (BEDMAP) data provided the water column thickness in the cavities. The ice shelves were treated as floating slabs on water without flexural deformation. The results presented here focus on a domain taken from the global solution reaching from the Amundsen Bellingshausen Sea to 30º E. The simulations cover the period 1992 to 2006 and show, e.g., an increase in sea ice thickness in agreement with previous studies. A variety of sensitivity simulations of this configuration are used to optimise model parameters which, in turn, contribute to further improve the global model solution.
OS31C-1299
Assessment of the ECCO2 regional optimized solution in the Arctic
One of the Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) project's objectives is to
realistically estimate the Arctic Ocean circulation and sea-ice distribution during the ocean satellite era
(1978-present). Towards the release of a global optimized ECCO2 solution to the scientific community, an
optimized Arctic solution has been obtained using the Green's function approach to minimize model/data
misfits. This paper provides a comprehensive assessment of this Arctic solution using satellite and in-situ
measurements of sea-ice, freshwater and heat fluxes, and temperature/salinity vertical profiles. Compared to
the baseline, the optimized Arctic solution improved significantly with an overall model-data cost reduction of
54%. For the Arctic Ocean, the optimized solution successfully reproduces the Cold Halocline in the
Canadian Basin and improved significantly Atlantic Water properties in the Arctic Ocean and Greenland
Sea. In addition, transport of freshwater across Fram and Bering Straits are compatible with derived
estimates from previous published results. For sea-ice, both thickness and velocity improve with cost
reductions of 40% and 68%, respectively. In addition to model-data validation, we also compare our Arctic
solution to AOMIP results to identify the solution's strengths and weaknesses and to give recommendations
for model improvements.
http://ecco2.jpl.nasa.gov
OS31C-1300
Assimilation of Sea Ice Observations in a Coupled Ocean Sea Ice State Estimate of the Labrador Sea Using the Adjoint Method
Despite being one of the most abundant high-latitude observations of the world ocean, satellite-derived sea ice concentration (SIC) observations are often excluded in global ocean data assimilation efforts. In efforts where the adjoint method, or method of Lagrange multipliers, is utilized, such as in the Estimating the Climate and Circulation of the Ocean (ECCO) project, this exclusion has been due to the difficulty in generating useful adjoints of highly nonlinear sea ice models. Without a sea ice model adjoint, it is impossible to identify the physical pathways linking the sea ice state to the model control variables, such as the atmospheric state and initial ocean state. Now that a useful sea ice model adjoint has been created, SIC observations can be readily included in state estimation efforts such as ECCO. We demonstrate that additionally including SIC data quantitatively improves a regional one year ocean-sea ice state estimate compared with an estimate made using only observations of in situ ocean temperature and salinity taken from CTDs, profiling floats, and XBTs. The state estimate is made in a regional 1/3 degree coupled sea ice-ocean model of the Labrador Sea using the MIT General Circulation Model.
OS31C-1301
On the influence of terrestrial DOC on the Arctic Ocean Carbon Cycle
It is estimated that riverine sources of terriginous DOC may represent a source of carbon to the Arctic Ocean as large as ca 30 Tg C a-1; a significant flux in the regional budget. We use a numerical model to explore the role of Pan-Arctic riverine fluxes of dissolved organic carbon (DOC) in contributing to air-sea fluxes of carbon dioxide in the Arctic Ocean. The model is based on the Arctic sector of the eddy-permitting ECCO2 configuration of the MIT ocean model, forced by time-varying NCEP re-analysis products with an explicit representation of fresh water run-off in the Arctic region. Open boundary conditions are taken from the ECCO2 global ocean state estimates. The biogeochemical model explicitly represents the cycling of dissolved inorganic carbon, alkalinity, dissolved oxygen, phosphate and dissolved organic matter of marine origin, and riverine DOC. We parametrize riverine DOC fluxes to the Arctic Ocean using a combination of freshwater river runoff and DOC data collected at the mouth of main Arctic rivers. The DOC tracer is given a half-life of 10 years based on estimates from field data and supported by a previous, idealized modeling study. We will discuss the role of terriginous DOC sources in regional air-sea carbon fluxes through a comparison of model integrations, which include and exclude this source.