Bidecadal Thermal Changes in the Abyssal Ocean

A dynamically consistent state estimate is used for the period 1992–2011 to describe the changes in oceanic temperatures and heat content, with an emphasis on determining the noise background in the abyssal (below 2000 m) depths. Interpretation requires close attention to the long memory of the deep ocean, implying that meteorological forcing of decades to thousands of years ago should still be producing trendlike changes in abyssal heat content. Much of the deep-ocean volume remained unobserved. At the present time, warming is seen in the deep western Atlantic and Southern Oceans, roughly consistent with those regions of the ocean expected to display the earliest responses to surface disturbances. Parts of the deeper ocean, below 3600 m, show cooling. Most of the variation in the abyssal Pacific Ocean is comparatively featureless, consistent with the slow, diffusive approach to a steady state expected there. In the global average, changes in heat content below 2000 m are roughly 10% of those inferred for the upper ocean over the 20-yr period. A useful global observing strategy for detecting future change has to be designed to account for the different time and spatial scales manifested in the observed changes. If the precision estimates of heat content change are independent of systematic errors, determining oceanic heat uptake values equivalent to 0.1 W m−2 is possibly attainable over future bidecadal periods.Earth and Planetary Science

Satellite-derived submarine melt rates and mass balance (2011–2015) for Greenland's largest remaining ice tongues

© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in The Cryosphere 11 (2017): 2773-2782, doi:10.5194/tc-11-2773-2017.Ice-shelf-like floating extensions at the termini of Greenland glaciers are undergoing rapid changes with potential implications for the stability of upstream glaciers and the ice sheet as a whole. While submarine melting is recognized as a major contributor to mass loss, the spatial distribution of submarine melting and its contribution to the total mass balance of these floating extensions is incompletely known and understood. Here, we use high-resolution WorldView satellite imagery collected between 2011 and 2015 to infer the magnitude and spatial variability of melt rates under Greenland's largest remaining ice tongues – Nioghalvfjerdsbræ (79 North Glacier, 79N), Ryder Glacier (RG), and Petermann Glacier (PG). Submarine melt rates under the ice tongues vary considerably, exceeding 50 m a−1 near the grounding zone and decaying rapidly downstream. Channels, likely originating from upstream subglacial channels, give rise to large melt variations across the ice tongues. We compare the total melt rates to the influx of ice to the ice tongue to assess their contribution to the current mass balance. At Petermann Glacier and Ryder Glacier, we find that the combined submarine and aerial melt approximately balances the ice flux from the grounded ice sheet. At Nioghalvfjerdsbræ the total melt flux (14.2 ± 0.96 km3 a−1 w.e., water equivalent) exceeds the inflow of ice (10.2 ± 0.59 km3 a−1 w.e.), indicating present thinning of the ice tongue.Nat Wilson, Fiammetta Straneo, and Patrick Heimbach were supported by NASA NNX13AK88G and NSF OCE 1434041

Seasonal variability of submarine melt rate and circulation in an East Greenland fjord

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 118 (2013): 2492–2506, doi:10.1002/jgrc.20142.The circulation in a glacial fjord driven by a large tidewater glacier is investigated using a nonhydrostatic ocean general circulation model with a melt rate parameterization at the vertical glacier front. The model configuration and water properties are based on data collected in Sermilik Fjord near Helheim Glacier, a major Greenland outlet glacier. The approximately two-layer stratification of the fjord's ambient waters causes the meltwater plume at the glacier front to drive a “double cell” circulation with two distinct outflows, one at the free surface and one at the layers' interface. In summer, the discharge of surface runoff at the base of the glacier (subglacial discharge) causes the circulation to be much more vigorous and associated with a larger melt rate than in winter. The simulated “double cell” circulation is consistent, in both seasons, with observations from Sermilik Fjord. Seasonal differences are also present in the vertical structure of the melt rate, which is maximum at the base of the glacier in summer and at the layers' interface in winter. Simulated submarine melt rates are strongly sensitive to the amount of subglacial discharge, to changes in water temperature, and to the height of the layers. They are also consistent with those inferred from simplified one-dimensional models based on the theory of buoyant plumes. Our results also indicate that to correctly represent the dynamics of the meltwater plume, care must be taken in the choice of viscosity and diffusivity values in the model.Support to CC and FS was given by the National Science Foundation project OCE-1130008. CC received support also from the WHOI Arctic Research Initiative. RS and PH are supported in part by NSF project OCE-1129746. Additional funding for RS comes through ISAC-CNR U.O.S. Torino as part of the projects SHARE PAPRIKA and EU FP7 ACQWA, and for PH through NASA/MAP project NNX11AQ12G (ECCO-ICES).2013-11-1

2012 Project Summary Sensitivity Patterns of Atlantic Meridional Overturning and Related Climate Diagnostics over the Instrumental Period

The long-‐term goals are to understand, with a comprehensive data set and a state-‐of-‐the-‐art ocean model, the nature of the North Atlantic Ocean circulation, with a particular emphasis on its decadal variability and climate consequences. The so-‐called meridional overturning circulation (MOC) is a simplified schematic of the complex North Atlantic Ocean circulation that is believed important to the climate system. As such, it is a useful shorthand for the description of circulation changes (past, ongoing, and possibly in the future) that can have serious climate implications and consequences for society in general. Adjoint models, which provide comprehensive sensitivities, are used to study the MOC in four distinct, but nonetheless, overlapping ways. In one approach, the adjoint is used as a numerical tool for fitting a general circulation model to a great variety of oceanic observations. Approach 2 exploits explicitly the mathematical result that the adjoint solution (the Lagrange multipliers) are the sensitivity of an arbitrarily chosen scalar-‐ function, for example, climate metrics that capture Atlantic transport and heat content variability, to almost any perturbation in the model or its external constraints (initial and boundary conditions). Approach 3 extends the adjoint application through formulating a

Optimal Excitation of Interannual Atlantic Meridional Overturning Circulation Variability

The optimal excitation of Atlantic meridional overturning circulation (MOC) anomalies is investigated in an ocean general circulation model with an idealized configuration. The optimal three-dimensional spatial structure of temperature and salinity perturbations, defined as the leading singular vector and generating the maximum amplification of MOC anomalies, is evaluated by solving a generalized eigenvalue problem using tangent linear and adjoint models. Despite the stable linearized dynamics, a large amplification of MOC anomalies, mostly due to the interference of nonnormal modes, is initiated by the optimal perturbations. The largest amplification of MOC anomalies, found to be excited by high-latitude deep density perturbations in the northern part of the basin, is achieved after about 7.5 years. The anomalies grow as a result of a conversion of mean available potential energy into potential and kinetic energy of the perturbations, reminiscent of baroclinic instability. The time scale of growth of MOC anomalies can be understood by examining the time evolution of deep zonal density gradients, which are related to the MOC via the thermal wind relation. The velocity of propagation of the density anomalies, found to depend on the horizontal component of the mean flow velocity and the mean density gradient, determines the growth time scale of the MOC anomalies and therefore provides an upper bound on the MOC predictability time. The results suggest that the nonnormal linearized ocean dynamics can give rise to enhanced MOC variability if, for instance, overflows, eddies, and/or deep convection can excite high-latitude density anomalies in the ocean interior with a structure resembling that of the optimal perturbations found in this study. The findings also indicate that errors in ocean initial conditions or in model parameterizations or processes, particularly at depth, may significantly reduce the Atlantic MOC predictability time to less than a decade.Earth and Planetary Science

Parameter and state estimation with a time-dependent adjoint marine ice sheet model

To date, assimilation of observations into large-scale ice models has consisted predominantly of time-independent inversions of surface velocities for basal traction, bed elevation, or ice stiffness, and has relied primarily on analytically derived adjoints of glaciological stress balance models. To overcome limitations of such "snapshot" inversions – i.e., their inability to assimilate time-dependent data for the purpose of constraining transient flow states, or to produce initial states with minimum artificial drift and suitable for time-dependent simulations – we have developed an adjoint of a time-dependent parallel glaciological flow model. The model implements a hybrid shallow shelf–shallow ice stress balance, solves the continuity equation for ice thickness evolution, and can represent the floating, fast-sliding, and frozen bed regimes of a marine ice sheet. The adjoint is generated by a combination of analytic methods and the use of algorithmic differentiation (AD) software. Several experiments are carried out with idealized geometries and synthetic observations, including inversion of time-dependent surface elevations for past thicknesses, and simultaneous retrieval of basal traction and topography from surface data. Flexible generation of the adjoint for a range of independent uncertain variables is exemplified through sensitivity calculations of grounded ice volume to changes in basal melting of floating and basal sliding of grounded ice. The results are encouraging and suggest the feasibility, using real observations, of improved ice sheet state estimation and comprehensive transient sensitivity assessments

Impact of periodic intermediary flows on submarine melting of a Greenland glacier

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 119 (2014): 7078–7098, doi:10.1002/2014JC009953.The submarine melting of a vertical glacier front, induced by an intermediary circulation forced by periodic density variations at the mouth of a fjord, is investigated using a nonhydrostatic ocean general circulation model and idealized laboratory experiments. The idealized configurations broadly match that of Sermilik Fjord, southeast Greenland, a largely two layers system characterized by strong seasonal variability of subglacial discharge. Consistent with observations, the numerical results suggest that the intermediary circulation is an effective mechanism for the advection of shelf anomalies inside the fjord. In the numerical simulations, the advection mechanism is a density intrusion with a velocity which is an order of magnitude larger than the velocities associated with a glacier-driven circulation. In summer, submarine melting is mostly influenced by the discharge of surface runoff at the base of the glacier and the intermediary circulation induces small changes in submarine melting. In winter, on the other hand, submarine melting depends only on the water properties and velocity distribution at the glacier front. Hence, the properties of the waters advected by the intermediary circulation to the glacier front are found to be the primary control of the submarine melting. When the density of the intrusion is intermediate between those found in the fjord's two layers, there is a significant reduction in submarine melting. On the other hand, when the density is close to that of the bottom layer, only a slight reduction in submarine melting is observed. The numerical results compare favorably to idealized laboratory experiments with a similar setup.Support to C. Cenedese and F. Straneo was given by the National Science Foundation project OCE-1130008. C. Cenedese received support also from the WHOI Arctic Research Initiative. R. Sciascia and P. Heimbach are supported in part by NSF project OCE-1129746. Additional funding for P. Heimbach comes through NASA's project NNH11ZDA001N-IDS A.28.2015-04-2

Global reconstruction of historical ocean heat storage and transport

Most of the excess energy stored in the climate system due to anthropogenic greenhouse gas emissions has been taken up by the oceans, leading to thermal expansion and sea level rise. The oceans thus have an important role in the Earth’s energy imbalance. Observational constraints on future anthropogenic warming critically depend on accurate estimates of past ocean heat content (OHC) change. We present a novel reconstruction of OHC since 1871, with global coverage of the full ocean depth. Our estimates combine timeseries of observed sea surface temperatures, with much longer historical coverage than those in the ocean interior, together with a representation (a Green’s function) of time-independent ocean transport processes. For 1955-2017, our estimates are comparable to direct estimates made by infilling the available 3D time-dependent ocean temperature observations. We find that the global ocean absorbed heat during this period at a rate of 0.30 ± 0.06 W/m2 in the upper 2000 m and 0.028 ± 0.026 W/m2 below 2000 m, with large decadal fluctuations. The total OHC change since 1871 is estimated at 436 ±91 × 1021 J, with an increase during 1921-1946 (145 ± 62× 1021 J) that is as large as during 1990-2015. By comparing with direct estimates, we also infer that, during 1955-2017, up to half of the Atlantic Ocean warming and thermosteric sea level rise at low-to-mid latitudes emerged due to heat convergence from changes in ocean transport

Global General Circulation of the Ocean Estimated by the ECCO-Consortium

Following on the heels of the World Ocean Circulation Experiment, the Estimating the Circulation and Climate of the Ocean (ECCO) consortium has been directed at making the best possible estimates of ocean circulation and its role in climate. ECCO is combining state-of-the-art ocean general circulation models with the nearly complete global ocean data sets for 1992 to present. Solutions are now available that adequately fit almost all types of ocean observations and that are, simultaneously, consistent with the model. These solutions are being applied to understanding ocean variability, biological cycles, coastal physics, geodesy, and many other areas.National Oceanographic Partnership Program (U.S.)United States. National Aeronautics and Space AdministrationNational Science Foundation (U.S.)United States. National Oceanic and Atmospheric AdministrationNational Center for Atmospheric Research (U.S.)San Diego Supercomputer CenterGeophysical Fluid Dynamics Laboratory (U.S.)Jet Propulsion Laboratory (U.S.)