Krasting, John P., Stephen M Griffies, Jan-Erik Tesdal, Graeme A MacGilchrist, Rebecca L Beadling, and Christopher M Little, December 2024: Steric sea level rise and relationships with model drift and water mass representation in GFDL CM4 and ESM4. Journal of Climate, 37(24), DOI:10.1175/JCLI-D-23-0591.1. Abstract
Density-driven steric seawater changes are a leading-order contributor to global mean sea level rise. However, intermodel differences in the magnitude and spatial patterns of steric sea level rise exist at regional scales and often emerge during the spinup and preindustrial control integrations of climate models. Steric sea level results from an eddy-permitting climate model, GFDL CM4, are compared with a lower-resolution counterpart, GFDL-ESM4. The results from both models are examined through basin-scale heat budgets and watermass analysis, and we compare the patterns of ocean heat uptake, redistribution, and sea level differ in ocean-only [i.e., Ocean Model Intercomparison Project (OMIP)] and coupled climate configurations. After correcting for model drift, both GFDL CM4 and GFDL-ESM4 simulate nearly equivalent ocean heat content change and global sea level rise during the historical period. However, the GFDL CM4 model exhibits as much as a 40% increase in surface ocean heat uptake in the Southern Ocean and subsequent increases in horizontal export to other ocean basins after bias correction. The results suggest regional differences in the processes governing Southern Ocean heat export, such as the formation of Antarctic Intermediate Water (AAIW), Subpolar Mode Water (SPMW), and gyre transport between the two models, and that sea level changes in these models cannot be fully bias-corrected. Since the process-level differences between the two models are evident in the preindustrial control simulations of both models, these results suggest that the control simulations are important for identifying and correcting sea level–related model biases.
Little, Christopher M., R Horton, Robert E Kopp, Michael Oppenheimer, Gabriel A Vecchi, and Gabriele Villarini, December 2015: Joint projections of US East Coast sea level and storm surge. Nature Climate Change, 5(12), DOI:10.1038/nclimate2801. Abstract
Future coastal flood risk will be strongly influenced by sea-level rise (SLR) and changes in the frequency and intensity of tropical cyclones. These two factors are generally considered independently. Here, we assess twenty-first century changes in the coastal hazard for the US East Coast using a flood index (FI) that accounts for changes in flood duration and magnitude driven by SLR and changes in power dissipation index (PDI, an integrated measure of tropical cyclone intensity, frequency and duration). Sea-level rise and PDI are derived from representative concentration pathway (RCP) simulations of 15 atmosphere–ocean general circulation models (AOGCMs). By 2080–2099, projected changes in the FI relative to 1986–2005 are substantial and positively skewed: a 10th–90th percentile range 4–75 times higher for RCP 2.6 and 35–350 times higher for RCP 8.5. High-end FI projections are driven by three AOGCMs that project the largest increases in SLR, PDI and upper ocean temperatures. Changes in PDI are particularly influential if their intra-model correlation with SLR is included, increasing the RCP 8.5 90th percentile FI by a further 25%. Sea-level rise from other, possibly correlated, climate processes (for example, ice sheet and glacier mass changes) will further increase coastal flood risk and should be accounted for in comprehensive assessments.
Sergienko, Olga V., D N Goldberg, and Christopher M Little, June 2013: Alternative ice shelf equilibria determined by ocean environment. Journal of Geophysical Research, 118(2), DOI:10.1002/jgrf.20054. Abstract
Dynamic and thermodynamic regimes of ice shelves experiencing weak (
≲
1 m year
1
)
to strong (~10myear
1
) basal melting in cold (bottom temperature close to the in situ
freezing point) and warm oceans (bottom temperature more than half of a degree warmer
than the in situ freezing point) are investigated using a 1-D coupled ice/ocean model
complemented with a newly derived analytic expression for the steady state temperature
distribution in ice shelves. This expression suggests the existence of a basal thermal
boundary layer with thickness inversely proportional to the basal melt rate. Model
simulations show that ice shelves a
fl
oat in warm ocean waters have signi
fi
cantly colder
internal ice temperatures than those that
fl
oat in cold waters. Our results indicate that in
steady states, the mass balance of ice shelves experiencing strong and weak melting is
controlled by different processes: in ice shelves with strong melting, it is a balance between
ice advection and basal melting, and in ice shelves with weak melting, it is a balance
between ice advection and deformation. Sensitivity simulations show that ice shelves in
cold and warm oceans respond differently to increase of the ocean heat content. Ice shelves
in cold waters are more sensitive to warming of the ocean bottom waters, while ice shelves
in warm waters are more sensitive to shallowing of the depth of the thermocline.
Goldberg, D N., Christopher M Little, Olga V Sergienko, Anand Gnanadesikan, Robert Hallberg, and Michael Oppenheimer, June 2012: Investigation of land ice-ocean interaction with a fully coupled ice-ocean model, Part 2: Sensitivity to external forcings. Journal of Geophysical Research: Earth Surface, 117, F02038, DOI:10.1029/2011JF002247. Abstract
A coupled ice stream-ice shelf-ocean cavity model is used to assess the sensitivity of the coupled system to far-field ocean temperatures, varying from 0.0 to 1.80C, as well as sensitivity to the parameters controlling grounded ice flow. A response to warming is seen in grounding line retreat and grounded ice loss that cannot be inferred from the response of integrated melt rates alone. This is due to concentrated thinning at the ice shelf lateral margin, and to processes that contribute to this thinning. Parameters controlling the flow of grounded ice have a strong influence on the response to sub-ice shelf melting, but this influence is not seen until several years after an initial perturbation in temperatures. The simulated melt rates are on the order of that observed for Pine Island Glacier in the 1990s. However, retreat rates are much slower, possibly due to unrepresented bedrock features.
Goldberg, D N., Christopher M Little, Olga V Sergienko, Anand Gnanadesikan, Robert Hallberg, and Michael Oppenheimer, June 2012: Investigation of land ice-ocean interaction with a fully coupled ice-ocean model, Part 1: Model description and behavior. Journal of Geophysical Research: Earth Surface, 117, F02037, DOI:10.1029/2011JF002246. Abstract
Antarctic ice shelves interact closely with the ocean cavities beneath them, with ice shelf geometry influencing ocean cavity circulation, and heat from the ocean driving changes in the ice shelves, as well as the grounded ice streams that feed them. We present a new coupled model of an ice stream-ice shelf-ocean system that is used to study this interaction. The model is capable of representing a moving grounding line and dynamically responding ocean circulation within the ice shelf cavity. Idealized experiments designed to investigate the response of the coupled system to instantaneous increases in ocean temperature show ice-ocean system responses on multiple timescales. Melt rates and ice shelf basal slopes near the grounding line adjust in 1-2 years, and downstream advection of the resulting ice shelf thinning takes place on decadal timescales. Retreat of the grounding line and adjustment of grounded ice takes place on a much longer timescale, and the system takes several centuries to reach a new steady state. During this slow retreat, and in the absence of either an upward-or downward-sloping bed or long-term trends in ocean heat content, the ice shelf and melt rates maintain a characteristic pattern relative to the grounding line.
We overview problems and prospects in ocean circulation models, with emphasis on certain developments aiming to
enhance the physical integrity and flexibility of large-scale models used to study global climate. We also consider elements
of observational measures rendering information to help evaluate simulations and to guide development priorities.
http://www.oceanobs09.net/blog/?p=88
Little, Christopher M., Anand Gnanadesikan, and Michael Oppenheimer, December 2009: How ice shelf morphology controls basal melting. Journal of Geophysical Research: Oceans, 114, C12007, DOI:10.1029/2008JC005197. Abstract
The response of ice shelf basal melting to climate is a function of ocean temperature, circulation, and mixing in the open ocean and the coupling of this external forcing to the sub–ice shelf circulation. Because slope strongly influences the properties of buoyancy-driven flow near the ice shelf base, ice shelf morphology plays a critical role in linking external, subsurface heat sources to the ice. In this paper, the slope-driven dynamic control of local and area-integrated melting rates is examined under a wide range of ocean temperatures and ice shelf shapes, with an emphasis on smaller, steeper ice shelves. A 3-D numerical ocean model is used to simulate the circulation underneath five idealized ice shelves, forced with subsurface ocean temperatures ranging from −2.0°C to 1.5°C. In the sub–ice shelf mixed layer, three spatially distinct dynamic regimes are present. Entrainment of heat occurs predominately under deeper sections of the ice shelf; local and area-integrated melting rates are most sensitive to changes in slope in this “initiation” region. Some entrained heat is advected upslope and used to melt ice in the “maintenance” region; however, flow convergence in the “outflow” region limits heat loss in flatter portions of the ice shelf. Heat flux to the ice exhibits (1) a spatially nonuniform, superlinear dependence on slope and (2) a shape- and temperature-dependent, internally controlled efficiency. Because the efficiency of heat flux through the mixed layer decreases with increasing ocean temperature, numerical simulations diverge from a simple quadratic scaling law.
Little, Christopher M., Anand Gnanadesikan, and Robert Hallberg, October 2008: Large-scale oceanographic constraints on the distribution of melting and freezing under ice shelves. Journal of Physical Oceanography, 38(10), DOI:10.1175/2008JPO3928.1. Abstract
Previous studies suggest that ice shelves experience asymmetric melting and freezing. Topography may constrain oceanic circulation (and thus basal melt–freeze patterns) through its influence on the potential vorticity (PV) field. However, melting and freezing induce a local circulation that may modify locations of heat transport to the ice shelf. This paper investigates the influence of buoyancy fluxes on locations of melting and freezing under different bathymetric conditions. An idealized set of numerical simulations (the “decoupled” simulations) employs spatially and temporally fixed diapycnal fluxes. These experiments, in combination with scaling considerations, indicate that while flow in the interior is governed by large-scale topographic gradients, recirculation plumes dominate near buoyancy fluxes. Thermodynamically decoupled models are then compared to those in which ice–ocean heat and freshwater fluxes are driven by the interior flow (the “coupled” simulations). Near the southern boundary, strong cyclonic flow forced by melt-induced upwelling drives inflow and melting to the east. Recirculation is less evident in the upper water column, as shoaling of meltwater-freshened layers dissipates the dynamic influence of buoyancy forcing, yet freezing remains intensified in the west. In coupled simulations, the flow throughout the cavity is relatively insensitive to bathymetry; stratification, the slope of the ice shelf, and strong, meridionally distributed buoyancy fluxes weaken its influence.
