Jochen Kämpf

Shallow, brine-driven free convection in polar oceans: Nonhydrostatic numerical process studies

A three-dimensional nonhydrostatic convection model, which accounts for small-scale ice-ocean interactions, is used to study convection in shallow sea (coastal) ice formation regions which contribute significantly to water mass formation in both the Arctic and Antarctic Ocean. For certain conditions the results presented in this paper are also transferable to shallow open ocean convection. The model is applied to an initial well-mixed ocean at rest with a temperature close to the freezing point. The ocean, initially free of ice, is exposed to cold and dry polar air. We consider situations in which the mean wind stress is negligible but wind fluctuations result in (small) sensible and latent heat fluxes corresponding to a wind speed of 2 m s−1. Cellular convection patterns develop in the ocean, finally occupying a mean aspect ratio of 2. Convection is driven by salt release during frazil ice formation due to supercooling. Newly forming sea ice is collected along convergent (downwelling) regions at the surface, thus showing also cellular structures. Because the area of insulating sea ice remains small, new ice can be formed continuously, and the surface buoyancy forcing remains large. This collection of ice in small fractions of the sea surface results in a latent heat polynya type, which is very effective in terms of dense water mass formation. A comparison of the three-dimensional model and a two-dimensional (slice) model shows that key results can be reproduced with the slice model. In summary, the results of the process studies indicate that cellular features in the sea ice cover, which may be detectable by remote sensing techniques, are closely related to active brine-driven convection.

Sediment-Driven Downslope Flow in Submarine Canyons and Channels: Three-Dimensional Numerical Experiments

Abstract The role of submarine canyons and channels in sediment-driven downslope flow (sediment plumes) is examined, using a three-dimensional, rotational numerical model that couples the hydrodynamics and sediment transport. The model domain consists of a bottom ocean layer of constant height coupled with an essentially inert upper ocean. The model equations are cast in a rotated, bottom-following coordinate system in which vertical grid spacing is independent of the ocean depth and bathymetry can be resolved accurately. This allows for tracing bottom-attached sediment plumes (∼decameters in height) from shallow water into great depths of the ocean. The calculations reproduce morphologic features related to the occurrance of sediment plumes, such as the formations of 1) localized deposition areas of sediment off the mouth of submarine canyons and 2) levees at both sides of submarine channels. Furthermore, it is demonstrated that sediment plumes are not only important for the transport of littoral sedimen...

Marine Connectivity in a Large Inverse Estuary

Abstract With the addition of several thousand passive virtual particles, a well calibrated hydrodynamic model is employed to explore marine connectivity in Spencer Gulf, South Australia, which is a large inverse estuary, on time scales of months to years. Based on a new method of “cumulative flushing time,” findings reveal that Spencer Gulf consists of two distinct regimes. Lower Spencer Gulf is advectively flushed every winter on a seasonal basis. In contrast to this, Upper Spencer Gulf is dominated by diffusive rather than advective processes and experiences flushing over much longer time scales (200–400 days). The physical uniqueness of Upper Spencer Gulf might explain why this region accommodates the largest known giant cuttlefish spawning aggregation in the world. Using a simple approach to mimic the bottom-dwelling behaviour of this species, we were able to reproduce some observed migratory features, but the model fails to predict the return of cuttlefish to their spawning grounds, which remains a puzzle for future studies.

Sediment‐induced slope convection: Two‐dimensional numerical case studies

The potential role of sediment in oceanic slope convection is examined by means of a rotational numerical model applied in a vertical ocean slice. The model couples the hydrodynamics with transport, settling, deposition, and resuspension of fine-grained silty muds. Sediment plumes (turbidity currents), descending on an idealized continental slope with constant bottom slope, are driven from an initial density anomaly caused by an assumed suspension of sediment in shelf water. A number of case studies were conducted in order to understand the effects of (1) different suspended sediment concentrations in shelf water as compared to an equivalent salinity anomaly (salt brine release), (2) different oceanic density stratifications, and (3) resuspension of bed sediment. It is demonstrated that sediment plumes may account for a downslope transport of water, which, once void of its sediment load, becomes lighter than water above. Then, sedimentation along the slope, with a maximum adjacent to the foot of the slope, drives vigorous upward convection (parameterized in the model), stirring slope water over a depth range of several hundred meters. This is in agreement with field observations from a tropical ocean. Detrainment associated with sediment settling constitutes an important mechanism inherent in sediment plumes. It not only induces upward convection but also prevents the rapid increase in plume thickness caused by entrainment as compared to “water mass plumes.” Owing to a balance between entrainment and detrainment, the sediment plume, while descending on the slope, attains constant height and bed shear velocities. In order to facilitate the detection of sediment plumes in (historical or future) field data, we describe their simulated traces in terms of water mass properties and flow anomalies.

On the interaction of time-variable flows with a shelfbreak canyon

Abstract Process-oriented hydrodynamic modeling is employed to study the interaction of along-slope flows with an idealized submarine shelfbreak canyon. The model is forced via prescription of oscillatory flows superposed on steady background flows of various strength and direction. Findings suggest that purely oscillatory flow does not produce significant net onshore transport of dense water. It is rather the steady component of the flow that creates substantial up-canyon flows of ∼0.05 Sv (1 Sv = 106 m3 s−1) in volume transport. This takes place exclusively for flows running on average against the propagation direction of coastal Kelvin waves, whereas flows of the opposite direction operate to suppress cross-shelf density fluxes.

Impact of multiple submarine channels on the descent of dense water at high latitudes

A three-dimensional numerical hydrodynamic model is applied to examine the impact of multiple submarine channels (<10 km across, <100 m deep), common to most continental margins of the ocean, on the descent of dense water at high latitudes. The model consists of an ocean bottom layer of constant height that follows variable bottom topography under constant vertical grid spacing. An idealized continental slope of constant bottom slope is considered, including parallel channels that run perpendicular to main isobaths. The ocean is initially homogeneous and at rest. Forcing is due to a layer of dense water prescribed along the upslope boundary. When the channel aspect ratio (ratio of width to depth) exceeds the main bottom slope, dense water is carried downslope by narrow channel plumes centered along the channel axes. Owing to a small internal Rossby radius less than the channel width the plume dynamics are governed by a geostrophic balance across the channels. Interaction of adjacent channel plumes leads to complex bottom-parallel circulations. The net downslope density flux resulting from these circulations exceeds that of viscous (ageostrophic) flow of dense water developing without channels. When the channel aspect ratio is less than the main bottom slope, the descent of dense water is dominated by viscous flow. Narrow geostrophic circulation patterns along channels, superimposed on the mean flow, however, induce advective entrainment of lighter ambient water across the leading density front of descending water. As a result of this, the net downslope density flux is reduced as compared to that without channels. Sensitivity studies reveal that the channel-modified dynamics are independent of the magnitude of the eddy viscosity.

Hydrodynamics and Flushing of Coffin Bay, South Australia: A Small Tidal Inverse Estuary of Interconnected Bays

ABSTRACT Kämpf, J. and Ellis, H., 2015. Hydrodynamics and flushing of Coffin Bay, South Australia: A small tidal inverse estuary of interconnected bays. Using a three-dimensional hydrodynamic model and the concept of water age, this study explores the hydrodynamics and flushing characteristics of Coffin Bay, South Australia, which is a small inverse estuary of interconnected bays. Model findings suggest that the estuary is mainly tidally flushed throughout the year. Despite the existence of strong tidal flows >1 m/s in passages between bays of the estuary, the resultant tidal stirring zones are largely disconnected from each other. This disconnection induces a relatively slow flushing of the estuarys inner bays, where maximum water ages are 80–100 days. Given the tidal dominance in the flushing dynamics, a simple diffusion equation based on a total effective transverse diffusivity D (estimated from the model predictions) can be used to describe the flushing behaviour of the estuary. This equation is applied to derive the thermal response of Coffin Bay to either temperature variations in ambient shelf water or changes in surface heat fluxes. Findings indicate that, on timescales of 30 days, the inner bays of Coffin Bay respond predominantly to changes in surface heat fluxes, whereas the outer bay is more responsive to temperature variations in ambient shelf water. We quantify these individual responses in terms of spatially variable “thermal response factors,” which are fundamental properties inherent in the estuarys flushing dynamics.

Undercurrent‐driven upwelling in the northwestern Arafura Sea

A three-dimensional process-oriented model is applied to explore the circulation in the Arafura Sea during the southeasterly monsoon. During this period, widespread phytoplankton blooms develop in a large area (300 km × 300 km) in the northwestern Arafura Sea. The model findings indicate that undercurrents are the principal source of nutrient-rich Banda Sea slope water for the region. It is demonstrated that these undercurrents operate to flush the northwestern Arafura Sea with Banda Sea slope water on a time scale of 1–3 months, which is consistent with observational evidence. The simulated undercurrents are the signature of the classical lee effect, frequently observed in lakes, that comes into play in the Arafura Sea given its bay-like geometry, shallow-water depth (40–50 m), and close vicinity to the equator. It is also shown that density stratification and rotational effects have important influences on the pathway and intensity of the overturning circulation in the northwestern Arafura Sea, which needs to be further explored in the future.

On the “hidden” phytoplankton blooms on Australia's southern shelves

Phytoplankton blooms on Australias southern shelves are revisited using satellite-derived monthly data of chlorophyll a concentrations for the period 2003–2015. It is known that the region hosts a seasonal coastal upwelling system that develops in austral summer (January–March) with chlorophyll a concentrations of >2 mg/m3. While this summer upwelling is spatially limited to a few hot spots, here we show that widespread phytoplankton blooms of moderate (~1 mg/m3) chlorophyll a concentrations develop during autumn and early winter on most of Australias extensive southern shelves—from the vast shelves of the Great Australian Bight (GAB) in the west to Bass Strait in the east. This surprising finding disproves the widespread belief that shelf waters of the GAB are generally oligotrophic and may explain the relatively high abundance of both forage fish (sardines) and upper trophic-level predators (e.g., tuna and whales) in the region.

South Australia’s Large Inverse Estuaries: On the Road to Ruin

This chapter provides an overview of the past, present and likely future of South Australian gulfs – Spencer Gulf and Gulf St. Vincent. It describes the distinct physical factors shaping these inverse estuaries, their unique ecology, past environmental degradation and future threats. Rather than direct climate-change impacts, the reader will learn that traditional industrialization poses the biggest threat to the gulfs’ ecosystem health, despite recent enhanced efforts of protection and conservation of natural habitat.