Showing papers by "Patrick Heimbach published in 2014"

Low-Frequency SST and Upper-Ocean Heat Content Variability in the North Atlantic

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 ano...

Simulation of subice shelf melt rates in a general circulation model: Velocity‐dependent transfer and the role of friction

Abstract: Two parameterizations of turbulent boundary layer processes at the interface between an ice shelf and the ocean beneath are investigated in terms of their impact on simulated melt rates and feedbacks. The parameterizations differ in the transfer coefficients for heat and freshwater fluxes. In their simplest form, they are assumed constant and hence are independent of the velocity of ocean currents at the ice shelf base. An augmented melt rate parameterization accounts for frictional turbulence via transfer coefficients that do depend on boundary layer current velocities via a drag law. In simulations with both parameterizations for idealized as well as realistic cavity geometries under Pine Island Ice Shelf, West Antarctica, significant differences in melt rate patterns between the velocity-independent and velocity-dependent formulations are found. While patterns are strongly correlated to those of thermal forcing for velocity-independent transfer coefficients, melting in the case of velocity-dependent coefficients is collocated with regions of high boundary layer currents, in particular where rapid plume outflow occurs. Both positive and negative feedbacks between melt rates, boundary layer temperature, velocities, and buoyancy fluxes are identified. Melt rates are found to increase with increasing drag coefficient inline image, in agreement with plume model simulations, but optimal values of Cd inferred from plume models are not easily transferable. Uncertainties therefore remain, both regarding simulated melt rate spatial distributions and magnitudes.

Bidecadal Thermal Changes in the Abyssal Ocean

Abstract: 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 infe...

A Hessian-Based Method for Uncertainty Quantification in Global Ocean State Estimation

Abstract: Derivative-based methods are developed for uncertainty quantification (UQ) in large-scale ocean state estimation. The estimation system is based on the adjoint method for solving a least-squares op...

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

Abstract: 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.

Attributing Variability in Atlantic Meridional Overturning to Wind and Buoyancy Forcing

Abstract: An adjoint model is used to attribute variability in the Atlantic Meridional Overturning Circulation (AMOC) to wind and buoyancy forcing over the preceding 15 years. AMOC variability of magnitude ±5 Sv is excited by local wind forcing. In contrast, interannual to decadal AMOC variability of similar amplitude is excited by heat fluxes in the subpolar North Atlantic, with freshwater fluxes playing a very minor role. The magnitude of the reconstructed AMOC variability does not converge as the time window over which past forcing is accounted for is increased to 15 years. Beyond this point the assumption of linearity may break down, suggesting not all of the observed AMOC variability can be attributed in a linear manner to surface forcing. Nevertheless, the reconstructed AMOC variability is broadly consistent with RAPID observations at 26.5◦N, especially in periods dominated by wind forcing.

International Workshop on Understanding the Responses of Greenland's Marine-Terminating Glaciers to Oceanic and Atmospheric Forcing: Challenges to improving observations, process understanding, and modeling

Abstract: following an exceptionally early onset of melting and two months of anomalously high surface temperatures in the Arctic.