Martin R. Wadley

Modelling the dynamics and thermodynamics of icebergs

Abstract Icebergs are a significant hazard for polar shipping, and, geophysically, are significant components of the mass balance of continental ice sheets while providing major freshwater inputs to the polar oceans. Some modelling of iceberg trajectories has been undertaken in the past, principally in the Labrador Sea, but here we present a hemispheric-wide attempt to model iceberg motion in the Arctic and North Atlantic Oceans. We show that the basic force balance in iceberg motion is between water drag and water advection, but with the pure geostrophic balance being only a minor component of the latter. Iceberg density maps essentially demonstrate the effect of the major boundary currents but we show that the time and size of calving from individual tidewater glaciers are important variables in determining the ultimate fate of bergs. The biggest bergs never leave the Arctic Ocean. All modelled icebergs have melted after about 5 years from their release date, although most melt over the first year. During their lifetime most, but not all bergs, overturn several times. Our model shows good agreement with the limited observational data. We therefore suggest that icebergs, both modelled and observed, may be exploited as previously little-used geophysical tracers.

Ice-age survival of Atlantic cod: agreement between palaeoecology models and genetics

Scant scientific attention has been given to the abundance and distribution of marine biota in the face of the lower sea level, and steeper latitudinal gradient in climate, during the ice-age conditions that have dominated the past million years. Here we examine the glacial persistence of Atlantic cod (Gadus morhua) populations using two ecological-niche-models (ENM) and the first broad synthesis of multi-locus gene sequence data for this species. One ENM uses a maximum entropy approach (Maxent); the other is a new ENM for Atlantic cod, using ecophysiological parameters based on observed reproductive events rather than adult distribution. Both the ENMs were tested for present-day conditions, then used to hindcast ranges at the last glacial maximum (LGM) ca 21 kyr ago, employing climate model data. Although the LGM range of Atlantic cod was much smaller, and fragmented, both the ENMs agreed that populations should have been able to persist in suitable habitat on both sides of the Atlantic. The genetic results showed a degree of trans-Atlantic divergence consistent with genealogically continuous populations on both sides of the North Atlantic since long before the LGM, confirming the ENM results. In contrast, both the ENMs and the genetic data suggest that the Greenland G. morhua population post-dates the LGM.

Prediction of iceberg trajectories for the North Atlantic and Arctic Oceans

Icebergs are a well-known hazard for shipping. Their study also provides information about diverse geophysical processes, as varied as ocean circulation, air-sea fluxes, calving rates of glaciers or the mass balance of ice sheets. As a first step to obtaining this information from iceberg data we have developed a model of iceberg drift driven by ocean and atmospheric forcing derived from general circulation models. We have applied the drift model to a distribution of typical icebergs released from the main tidewater glaciers of the North Atlantic and Arctic Oceans. We demonstrate that the main driving force of iceberg motion is rooted in the unsteady component of oceanic advection. From simulated trajectories we are able to reproduce the observed southwards limit of iceberg penetration and demonstrate sometimes surprising geographical links between iceberg origin and ultimate melting zones.

Simulations of two Last Glacial Maximum ocean states

Two stable thermohaline circulation states were produced in simulations of the last glacial maximum (LGM) ocean, both forced by the same prescribed atmosphere. Direct application of this atmosphere led to an equilibrium state with strong North Atlantic Deep Water formation (northern sinking state (NSS)). Upon applying a weak freshwater flux anomaly to the North Atlantic for a short time a state was entered with little North Atlantic, but significant Southern Ocean, Deep Water formation (southern sinking state (SSS)). A fully dynamic and thermodynamic iceberg trajectory model was developed, and icebergs were seeded into both states. Distinctly different pathways were found, particularly in the eastern Atlantic where the SSS icebergs tend to move south along the European coast while they move north in the NSS. While neither ocean state is claimed to be a definitive model for the LGM ocean, paleoclimate data are more consistent with the characteristics of the NSS.

Implementation of variable time stepping in an ocean general circulation model

Abstract Ocean general circulation models (OGCMs) which represent the governing equations on a finite difference grid require shorter time steps with increasing resolution. Thus, until now, in the absence of filtering, the time step length has been determined by the smallest grid spacing within the model domain. Here we present a method for reducing the time step length (and increasing the number of time steps taken) at selected points in the grid, so as to minimise the computational cost of integrating the OGCM, whilst achieving numerical stability throughout the model domain without filtering. This variable time stepping method can be used to overcome numerical constraints associated with the convergence of longitude–latitude grids at the poles, and also to allow efficient integration of model domains with variable resolution. Examples of the computational saving are given.

Ocean processes at the Antarctic continental slope

The Antarctic continental shelves and slopes occupy relatively small areas, but, nevertheless, are important for global climate, biogeochemical cycling and ecosystem functioning. Processes of water mass transformation through sea ice formation/melting and ocean–atmosphere interaction are key to the formation of deep and bottom waters as well as determining the heat flux beneath ice shelves. Climate models, however, struggle to capture these physical processes and are unable to reproduce water mass properties of the region. Dynamics at the continental slope are key for correctly modelling climate, yet their small spatial scale presents challenges both for ocean modelling and for observational studies. Cross-slope exchange processes are also vital for the flux of nutrients such as iron from the continental shelf into the mixed layer of the Southern Ocean. An iron-cycling model embedded in an eddy-permitting ocean model reveals the importance of sedimentary iron in fertilizing parts of the Southern Ocean. Ocean gliders play a key role in improving our ability to observe and understand these small-scale processes at the continental shelf break. The Gliders: Excellent New Tools for Observing the Ocean (GENTOO) project deployed three Seagliders for up to two months in early 2012 to sample the water to the east of the Antarctic Peninsula in unprecedented temporal and spatial detail. The glider data resolve small-scale exchange processes across the shelf-break front (the Antarctic Slope Front) and the fronts biogeochemical signature. GENTOO demonstrated the capability of ocean gliders to play a key role in a future multi-disciplinary Southern Ocean observing system.

Arctic pathways of Pacific Water: Arctic Ocean Model Intercomparison experiments

Abstract Pacific Water (PW) enters the Arctic Ocean through Bering Strait and brings in heat, fresh water, and nutrients from the northern Bering Sea. The circulation of PW in the central Arctic Ocean is only partially understood due to the lack of observations. In this paper, pathways of PW are investigated using simulations with six state‐of‐the art regional and global Ocean General Circulation Models (OGCMs). In the simulations, PW is tracked by a passive tracer, released in Bering Strait. Simulated PW spreads from the Bering Strait region in three major branches. One of them starts in the Barrow Canyon, bringing PW along the continental slope of Alaska into the Canadian Straits and then into Baffin Bay. The second begins in the vicinity of the Herald Canyon and transports PW along the continental slope of the East Siberian Sea into the Transpolar Drift, and then through Fram Strait and the Greenland Sea. The third branch begins near the Herald Shoal and the central Chukchi shelf and brings PW into the Beaufort Gyre. In the models, the wind, acting via Ekman pumping, drives the seasonal and interannual variability of PW in the Canadian Basin of the Arctic Ocean. The wind affects the simulated PW pathways by changing the vertical shear of the relative vorticity of the ocean flow in the Canada Basin.

On modelling present day and last glacial maximum oceanic δ18O distributions

Present-day (PD) and Last Glacial Maximum (LGM) simulations of the global ocean are presented, with the oxygen-18 isotope included as a passive tracer. The gradient of the PD North Atlantic surface y 18 O:salinity relationship is found to result from different processes at low and high latitudes. At low latitudes, the balance between surface 18 O flux and oceanic advection and mixing sets the surface y 18 O:salinity gradient, whereas at high latitudes, mixing between 18 O-depleted runoff and precipitation to the Arctic, Bering Strait inflow, and waters from lower latitudes, controls the y 18 O:salinity gradient. The importance of the Bering Strait contribution has not previously been recognised. These gradients change significantly at the LGM, and are found to be sensitive to both Arctic runoff y 18 O concentrations and changes in oceanic advection, particularly the rate of exchange of North Atlantic deep water with the global ocean. It is concluded that reconstructions of past climates from records of sea surface y 18 O based on analogues of the PD y 18 O:salinity relationship are likely to be in error. D 2002 Elsevier

Are “Great Salinity Anomalies” Advective?

Abstract “Great Salinity Anomalies” (GSAs) have been observed to propagate around the North Atlantic subpolar gyre. Similar anomalies occur in the Third Hadley Centre Coupled Atmosphere–Ocean GCM (HadCM3) of preindustrial climate. It has been hypothesized that these salinity anomalies result from the advection of anomalously low salinity waters around the subpolar gyre. Here, the consequences of using passive tracers in the HadCM3 climate model to tag the anomalously low salinity water associated with a GSA in the Greenland and Labrador Seas are reported. Rather than predominantly advecting around the modeled subpolar gyre in accordance with the upper-ocean salinity anomaly, the tracers mix to intermediate depths, before becoming incorporated into the models North Atlantic Deep Water. Horizontal advection of the tracer in the upper ocean is limited to around 1000 km, compared with the gyre-scale propagation of the salinity anomalies. It is concluded that GSAs are unlikely to be caused by the advection of...

Glacial thermohaline circulation states of the northern Atlantic: the compatibility of modelling and observations

Observational evidence from deep‐sea cores suggests that the ocean circulation during the last glacial cycle was highly variable, at times occupying states very different from those found today. Modelling can be used to dynamically constrain the possible circulation states compatible with observations, thus guiding both understanding of past climate but also the geographical and scientific thrust of future palaeoceanographic research. The Last Glacial Maximum has been extensively studied and here, using carbon isotopes, among other variables, the most likely thermohaline state consistent with palaeoclimatic data is constructed. Past and new modelling efforts for the Last Glacial Maximum are then examined, to contrast questions resolvable by the modelling/data comparison with those that remain unanswered. This shows modelling evidence to confirm the prevailing view of intermediate‐depth North Atlantic Deep Water being produced at the Last Glacial Maximum, in combination with deep water production around Antarctica. The potential sites for this deep and intermediate water production are defined by the basic state of the thermohaline circulation. However, their relative importance is a function of small perturbations in the surface temperature and salinity fields brought about by active coupling between the ocean and atmosphere. Regions where these water masses may have been produced at the Last Glacial Maximum are suggested.