John A. T. Bye

Identification and Climatology of Southern Hemisphere Mobile Fronts in a Modern Reanalysis

AbstractPresented here is an objective approach to identify, characterize, and track Southern Hemisphere mobile fronts in hemispheric analyses of relatively modest resolution, such as reanalyses. Among the principles in its design were that it should be based on broadscale synoptic considerations and be as simple and easily understood as possible. The resulting Eulerian scheme has been applied to the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA)–Interim and a climatology of frontal characteristics, at both the 10-m and 850-hPa levels, derived for the period 1 January 1989–28 February 2009. The knowledge of the character of these features is central to understanding weather and climate over the hemisphere.In both summer and winter the latitude belt 40°–60°S hosts the highest frequency of frontal points, but there are significant zonal asymmetries within this band. The climatology reveals that the longest fronts are in the Indian Ocean where mean lengths exceed 2000 km. The me...

A theoretical model of the flow in the mouth of Spencer Gulf, South Australia

Abstract The salinity distribution in Spencer Gulf, South Australia, indicates an inflow of low salinity water on the West side of the Gulf, modification of the water mass at the northern end and an outflow of water of increased salinity on the East side. This system appears to be an excellent example of a theoretical model recently proposed by Whitehead, Leetmaa & Knox (1974) for flushing controlled by a buoyancy-inertial current.

Inertial coupling of fluids with large density contrast

Abstract The coupling of two fluids of large density contrast is considered, and it is shown that the presence of a discontinuity of Lagrangian velocity across the interface between the two fluids, due to gravity waves, leads to an inertially coupled drag law of the form τ s = K|ϱ 1 1 2 u 1 − ϱ 2 1 2 u 2 |(ϱ 1 1 2 u 1 ϱ 2 1 2 u 2 ), where ϱ1 and ϱ2 are the densities of the fluids, u 1 and u 2 are the tangential fluid velocities relative to the interfacial non-wave induced velocity, K is a drag coefficient and τs is the interfacial shearing stress.

The General Circulation in a Dissipative Ocean Basin with Longshore Wind Stresses

Abstract Solutions of the Stommel equation are presented which take the form of free waves in the interior of the ocean basin, driven by convergences and divergences in coastal transports brought about by the variation of the longshore wind stress around the coast. These waves have been termed “coastal waves” and result from the beta-effect in the presence of a uniform frictional process, such as the loss of momentum by the ocean to the atmosphere. The coastal waves which typically transport 5 Sv extend significantly into the interior of the ocean from all boundaries except the western boundary, and also drive a westward nonlinear current, appear to be an important feature in the general circulation. A good example of a quasi-steady wavefield induced by intermediate-scale coastline geography occurs in the Flinders Current off the south coast of Australia. The western boundary current, of course, compensates for imbalances in interior transport. Its structure results from forcing, both by this transport an...

Tropical Transition of the 2001 Australian Duck

Abstract In March 2001, a hybrid low pressure system, unofficially referred to as Donald (or the Duck), developed in the Tasman Sea under tropical–extratropical influence, making landfall on the southeastern Australian coast. Here, it is shown that atmospheric blocking in the Tasman Sea produced a split in the subtropical jet, allowing persistent weak vertical wind shear to manifest in the vicinity of the developing low. It is hypothesized that this occurred through sustained injections of potential vorticity originating from higher latitudes. Hours before landfall near Byron Bay, the system developed an eye with a short-lived warm core at 500 hPa. Cyclone tracking revealed an erratic track before the system decayed and produced heavy rains and flash flooding. A three-dimensional air parcel backward-trajectory scheme showed that the air parcels arriving in the vicinity of the mature cyclone originated from tropical sources at lower levels and from the far extratropics at higher levels, confirming the hybr...

Inertially coupled Ekman layers

Abstract The inertial coupling model of the surface shear stress at the sea surface ( Bye, 1995 ) which takes account of the surface wavefield, has been applied to couple the Ekman layers of the ocean and atmosphere. We determine the surface shear stress and geostrophic drag coefficient, under barotropic conditions. The results are expressed in terms of the shear between the inertially weighted (i.e. velocity×square root of the density) relative geostrophic velocities in the two fluids, in which the reference velocity need not be specified, a priori. We find, in particular, that the deflection of the relative surface geostrophic wind to the surface shear stress in naturally occurring seastates, is about 9°. In the application of the analysis to general circulation models, it is argued that, since the inertially weighted relative geostrophic velocities in air and water are of similar magnitude, this implies that the surface shear stress can be significantly reduced by the current component of the inertially weighted geostrophic shear, with a corresponding reduction in importance of the Ekman transport.

Momentum and heat balances in steady coastal currents

Abstract The momentum balance for coastal currents forced by a longshore wind stress and a longshore density gradient is examined for coastlines of various orientations. We isolate the coastal circulation by requiring that the longshore wind stress and density gradient become uniform in the adjacent deep ocean in which the longshore bottom pressure gradient is assumed to be zero, and derive a vorticity equation for the longshore current for a prescribed cross-shelf variation of wind stress and density structure. Solutions of this equation are obtained for a uniform longshore wind stress along a coastline with an exponential shelf in two situations: (A) For a uniform cross-shelf density on the inner shelf and a zero longshore bottom pressure gradient on the outer shelf, the division between the two regions occurring approximately at the depth of the main thermocline, and (B) for a thermohaline structure which is independent of the cross-shelf co-ordinate such that the coastal current is barotropic. It is shown that on a zonal coast the coastal transports due to the wind stress forcing are of O (σ τ Wl ϱ 0 β ) , and due to the density gradient forcing of O(− σ ϱ 0 gh T 2 / β∂δ 0 / ∂l ) where τ Wl and − ϱ 0 ∂δ 0 / ∂l are respectively, the longshore wind stress and surface density gradient, ϱ 0 is a reference density for seawater, g is the acceleration of gravity, h T is the scale depth for the main thermocline, β= d f d y where f=2Ω sin ϕ in which Ω is the angular speed of rotation of Earth, and ϕ is the latitude, and σ= W T W is a shelf parameter in which W T is the exponential shelf scale depth and W is an eddy scale. On eastern boundaries the transports are less [especially in (A)] and are identically zero on western boundaries on which the transport is determined by external (non-local) processes. The longshore velocity structure on the western boundary depends strongly on σ and incorporates a system of currents and counter-currents. The heat balance for the coastal circulation is investigated by considering the solution of the heat conservation equation for the surface temperature with the cross-shelf boundary conditions of zero heat flux through the coast and into deep ocean. It is found that the sea surface temperature anomaly is zero for wind stress forcing and positive for density gradient forcing (at constant salinity). Mixed forcing however may lead to upwelling which is characterized by a negative sea surface anomaly. Upwelling occurs on non-western boundaries on which there is a longshore wind stress of sufficient intensity or on western boundaries where there is an external transport of sufficient magnitude to generate a net transport up the longshore surface density gradient. This occurs on eastern coasts where there is poleward advection of warm water and sufficiently strong equatorward meridional winds, and on western coasts in a polar gyre where there is a strong equatorward external transport and a poleward temperature gradient. The Ekman circulation maintains the upwelling anomalies.

Atmosphere–Ocean Momentum Exchange in General Circulation Models

A series of two-layer quasigeostrophic solutions for the ocean circulation driven by a steady wind in a channel with topography and in a flat bottom rectangular basin are presented in which the atmosphere and ocean are inertially coupled through the surface stress relation. The only other frictional processes are biharmonic lateral friction (under free-slip boundary conditions) and topographic form stress; there is no bottom friction involved. The results indicate that realistic momentum balances can be obtained on this physical basis. Two types of solutions are obtained, which are called (i) the I series in which the inertial coupling relation is applied directly in the earth reference frame with no current averaging and almost steady stream fields occur and (ii) the S series in which the inertial coupling relation is applied for long current averaging periods, of the order 100 days, rather than instantaneously. The solutions for the longer current averaging periods produce vigorous eddy fields, but their time-mean is very similar to the corresponding solution with no current averaging. Surface Stokes drift streamfields are also generated by the inertial coupling mechanism. Some implications of the results for general circulation modeling are discussed.

Transfer of inland salts to the marine environment at the head of Spencer Gulf, South Australia

Upper Spencer Gulf is linked by a narrow tidal channel to an extensive system of salt marshes, channels and salt pans overlying Cainozoic sediments of the Pirie-Torrens Basin in arid South Australia. The gulf here is characterised by a vigorous tidal regime with a spring range of 3–4 m, current velocities reaching 1.0 m s−1, and salinities greater than that estimated from evaporation alone. We suggest that the anomalously high salinities are maintained by input from interstitial brines from the northern salt pan corridor, and by the “scavenging” effect of surface water after occasional heavy rains. These processes are particularly important in buffering the salinity of tidal waters in winter, when evaporation is reduced. Our hypothesis is based on the distribution of total salts, major ions, selected trace elements and the 87Sr/86Sr isotopic ratio in samples of surface and interstitial water collected during 1986–1988, and on oceanographic measurements of the tidal regime. A removal rate of the order of 10 kg s−1 of total salts from inland brines to tidal water has been derived from hydrodynamical simulations of the system.

A new ‘bio-comfort’ perspective for Melbourne based on heat stress, air pollution and pollen

Humans are at risk from exposure to extremes in their environment, yet there is no consistent way to fully quantify and understand the risk when considering more than just meteorological variables. An outdoor ‘bio-comfort’ threshold is defined for Melbourne, Australia using a combination of heat stress, air particulate concentration and grass pollen count, where comfortable conditions imply an ideal range of temperature, humidity and wind speed, acceptable levels of air particulates and a low pollen count. This is a new approach to defining the comfort of human populations. While other works have looked into the separate impacts of different variables, this is the first time that a unified bio-comfort threshold is suggested. Composite maps of surface pressure are used to illustrate the genesis and evolution of the atmospheric structures conducive to an uncomfortable day. When there is an uncomfortable day due to heat stress conditions in Melbourne, there is a high pressure anomaly to the east bringing warm air from the northern interior of Australia. This anomaly is part of a slow moving blocking high originating over the Indian Ocean. Uncomfortable days due to high particulate levels have an approaching cold front. However, for air particulate cases during the cold season there are stable atmospheric conditions enhanced by a blocking high emanating from Australia and linking with the Antarctic continent. Finally, when grass pollen levels are high, there are northerly winds carrying the pollen from rural grass lands to Melbourne, due to a stationary trough of low pressure inland. Analysis into days with multiple types of stress revealed that the atmospheric signals associated with each type of discomfort are present regardless of whether the day is uncomfortable due to one or multiple variables. Therefore, these bio-comfort results are significant because they offer a degree of predictability for future uncomfortable days in Melbourne.