Abstract Climate forcing, sensitivity, and feedback metrics are evaluated in both the United Kingdom's physical climate model HadGEM3‐GC3.1 at low (‐LL) and medium (‐MM) resolution and the United Kingdom's Earth System Model UKESM1. The effective climate sensitivity (EffCS) to a doubling of CO 2 is 5.5 K for HadGEM3.1‐GC3.1‐LL and 5.4 K for UKESM1. The transient climate response is 2.5 and 2.8 K, respectively. While the EffCS is larger than that seen in the previous generation of models, none of the model's forcing or feedback processes are found to be atypical of models, though the cloud feedback is at the high end. The relatively large EffCS results from an unusual combination of a typical CO 2 forcing with a relatively small feedback parameter. Compared to the previous U.K. climate model, HadGEM3‐GC2.0, the EffCS has increased from 3.2 to 5.5 K due to an increase in CO 2 forcing, surface albedo feedback, and midlatitude cloud feedback. All changes are well understood and due to physical improvements in the model. At higher atmospheric and ocean resolution (HadGEM3‐GC3.1‐MM), there is a compensation between increased marine stratocumulus cloud feedback and reduced Antarctic sea‐ice feedback. In UKESM1, a CO 2 fertilization effect induces a land surface vegetation change and albedo radiative effect. Historical aerosol forcing in HadGEM3‐GC3.1‐LL is −1.1 W m −2 . In HadGEM3‐GC3.1‐LL historical simulations, cloud feedback is found to be less positive than in abrupt‐4xCO 2 , in agreement with atmosphere‐only experiments forced with observed historical sea surface temperature and sea‐ice variations. However, variability in the coupled model's historical sea‐ice trends hampers accurate diagnosis of the model's total historical feedback.
The UK has a long maritime heritage and the marine and coastal environment continues to play an important role in the national culture and economy. United Kingdom waters cover an area approximately three times greater than its land and the UKs coastline is the longest in the EU. Over half a million people are directly employed in maritime activities (e.g. shipping, tourism, fisheries)
and 95% of international trade into and out of the UK passes through its sea ports (EU Maritime Policy Facts and Figures United Kingdom). In 2004 sea-fish with an initial value of £513 million were landed by the UK fishing fleet. It has been estimated that the total turnover of the marine sector in 1999–2000 was just under £70 billion, of which almost £40 billion was due to Oil & Gas and Leisure. Beyond the direct maritime economy the
UKs marine environment provides a number of important goods and services to the UK. Along the coast, more than £150 billion of assets are estimated to be at risk from flooding by the sea, with an excess of £75 billion at risk in London alone (estimated from Halcrow, 2001).
Abstract The Pre‐Industrial climate model simulation is intended as an equilibrium control experiment that uses constant external forcing, yet in the Coupled Model Intercomparison Project Phase 6 (CMIP6) model HadGEM3‐GC31‐LL a distinct change in climate state occurs around year 500 of the 2000 year simulation. The global mean near surface air temperature increases by almost 0.5 K associated with a reduction in southern hemisphere sea ice area of almost 20%. Here we show this step change in the state of the climate to be a consequence of the onset of deep convection in the Weddell and Ross Sea gyres. The delayed onset of convection in the gyres is a consequence of a positive, downward, top‐of‐atmosphere radiative balance, and continual ocean heat up‐take during the model spin‐up and the initial pre‐industrial control simulation. Consequently, model spin‐up strategy should be revised to initialize pre‐industrial simulations that are in energy balance.
Replication data for: Future response of Antarctic continental shelf temperatures to ice shelf basal melting and calving This manuscript is in press at Geophysical Research Letters. SSP585FW_ensemble.tar.gz contains data needed to replicate the figures and analysis in the manuscript.
Current studies of the impact of climate change mitigation options tend to scale patterns of precipitation change linearly with surface temperature. Using climate model simulations, we show a nonlinear hydrological response to transient global warming and a substantial side effect of climate mitigation. In an idealised representation of mitigation action, where we reverse the trend of global warming, the precipitation response shows significant hysteresis behaviour due to heat previously accumulated in the ocean. Stabilising or reducing CO 2 concentrations in the atmosphere is found temporarily to strengthen the global hydrological cycle, while reducing rainfall over some tropical and subtropical regions. The drying trend under global warming over The Amazon, Australia and western Africa may intensify for decades after CO 2 reductions. The inertia due to accumulated heat in the ocean implies a commitment to hydrological cycle changes long after stabilisation or reduction of atmospheric CO 2 concentration.
The European Space Agency's ERS‐1 radar altimeter is the first to include separate operating modes to optimise performance over both ocean and non‐ocean surfaces. As part of the ERS‐1 commissioning activities, we have carried out a study of the tracking performance of this instrument over non‐ocean surfaces. Statistics for land ice, sea ice, arid lands, and inland water are presented. Performance in both operating modes is shown to be better than that of previous missions.
<p><br>Around Greenland, the transport of heat and fresh meltwater between the ocean and Greenland's Ice Sheet is mediated by circulation in several hundred proglacial fjords. These fjords are long and narrow, with circulation controlled by a variety of processes. This circulation, and the resultant heat transported to the ice sheet has global implications. However, the spatial scales of these fjords means that they cannot be directly represented in global scale climate models, as currently achievable horizontal resolutions are too coarse to resolve fjords directly. Therefore, a subgrid-scale parameterization scheme is required, to include the impact of fjord circulation on Greenland's Ice Sheet in these models. The development of such a scheme requires increased theoretical understanding, with the aim of capturing the circulation response simply, over a relevant range of the parameter space.</p><p>Current climate models add freshwater runoff from Greenland's Ice Sheet into the ocean model in the surface grid cell, and do not account for the impacts of fjord circulation on melt rates at glacial termini. Therefore, we focus on predicting the depth at which fresh meltwater enters the wider ocean, and the flow structure at the ice face itself, to understand the feedback on ice melt rates. We consider a subglacial discharge driven regime, with a localised source of subglacial discharge into the fjord at the glacial grounding line. We employ a combination of computational modelling using idealised configurations in MITgcm, and theoretical explorations, to capture this circulation as simply as possible. For fjords without sills, we find that the cross-fjord integrated velocity profile at the fjord mouth echoes that at the ice face. Further, we find that a horizontal recirculation cell develops at the ice face, as the fjord responds to horizontal velocities driven by the plume itself, generating flow across the entire ice face. We use scaling laws previously developed for turbulent plumes to provide a simple prediction of the cross-fjord integrated velocity structure at the fjord mouth, predicting the depth level at which meltwater enters the wider ocean. We develop theoretical predictions for the cross-fjord flow at the ice face, as a consequence of the flow directly induced by a buoyant plume and the circulation response in the fjord, allowing prediction of the pattern of melt across the ice face.</p>
<p>The Arctic sea ice cover is not a continuous expanse of ice but is instead composed of individual sea ice floes. These floes can range in size from just a few metres to tens of kilometres. Floe size can influence a variety of processes, including lateral melt rates, momentum transfer within the sea ice-ocean-atmosphere system, surface moisture flux, and sea ice rheology. Sea ice models have traditionally defined floe size using a single parameter, if floe size is explicitly treated at all. There have been several recent efforts to incorporate models of the Floe Size Distribution (FSD) into sea ice models in order to explore both how the shape of the FSD emerges and evolves and its impact on the sea ice cover, including the seasonal retreat. Existing models have generally focused on ocean surface wave-floe interactions and thermodynamic melting and growth processes. However, in-situ observations have indicated the presence of mechanisms other than wave fracture involved in the fragmentation of floes, including brittle failure and melt-induced break up.</p><p>In this study we consider two alternative FSD models within the CICE sea ice model: the first assumes the FSD follows a power law with a fixed exponent, with parameterisations of individual processes characterised using a variable FSD tracer; the second uses a prognostic approach, with the shape of the FSD an emergent characteristic of the model rather than imposed. We firstly use case studies to understand how similarities and differences in the impacts of the two FSD models on the sea ice emerge, including the different spatial and temporal variability of these impacts. We also consider whether the inclusion of FSD processes in sea ice models can enhance seasonal predictability. We will also demonstrate the need to include in-plane brittle fracture processes in FSD models and discuss the requirements needed within any parameterisation of the brittle failure mechanism.</p>
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