During the 2010–2011 La Niña and Ningaloo Niño, excessive precipitations in the Maritime Continent and Indonesian‐Australian Basin caused surface waters to freshen by 0.3 practical salinity unit in ...the southeast Indian Ocean. The low‐salinity anomalies are observed to be carried westward by the Indonesian throughflow and the South Equatorial Current and transmitted into the poleward flowing eastern boundary current, the Leeuwin Current, along the Western Australian coast. Low‐salinity anomalies contribute to about 30% of the anomalous increase of the southward Leeuwin Current transport during the evolution of the 2010–2011 Ningaloo Niño, resulting in unprecedented warming off the coast of Western Australia. Episodical freshening of the Leeuwin Current has been observed at the Rottnest coastal reference station of Western Australia during extended La Niña conditions over the past several decades; low‐salinity anomalies at the station during the 2010–2011 Ningaloo Niño are comparable with strong historical events.
Key Points
Excessive precipitation drives freshening of the Indonesian throughflow
Low‐salinity anomalies are carried southward by the Leeuwin Current
Low‐salinity anomaly contributes to intensify the Leeuwin Current transport
Global carbon emissions continue to acidify the oceans, motivating growing concern for the ability of coral reefs to maintain net positive calcification rates. Efforts to develop robust relationships ...between coral reef calcification and carbonate parameters such as aragonite saturation state (Ωarag) aim to facilitate meaningful predictions of how reef calcification will change in the face of ocean acidification. Here we investigate natural trends in carbonate chemistry of a coral reef flat over diel cycles and relate these trends to benthic carbon fluxes by quantifying net community calcification and net community production. We find that, despite an apparent dependence of calcification on Ωarag seen in a simple pairwise relationship, if the dependence of net calcification on net photosynthesis is accounted for, knowing Ωarag does not add substantial explanatory value. This suggests that, over short time scales, the control of Ωarag on net calcification is weak relative to factors governing net photosynthesis.
Key Points
Net production is the primary driver of daily variation in reef calcification
Little evidence for saturation state control of calcification on daily scale
Abstract
Model analyses of an alongshelf flow over a continental shelf and slope reveal upwelling near the shelf break. A stratified, initially uniform, alongshelf flow undergoes a rapid adjustment ...with notable differences onshore and offshore of the shelf break. Over the shelf, a bottom boundary layer and an offshore bottom Ekman transport develop within an inertial period. Over the slope, the bottom offshore transport is reduced from the shelf’s bottom transport by two processes. First, advection of buoyancy downslope induces vertical mixing, destratifying, and thickening the bottom boundary layer. The downward-tilting isopycnals reduce the geostrophic speed near the bottom. The reduced bottom stress weakens the offshore Ekman transport, a process known as buoyancy shutdown of the Ekman transport. Second, the thickening bottom boundary layer and weakening near-bottom speeds are balanced by an upslope ageostrophic transport. The convergence in the bottom transport induces adiabatic upwelling offshore of the shelf break. For a time period after the initial adjustment, scalings are identified for the upwelling speed and the length scale over which it occurs. Numerical experiments are used to test the scalings for a range of initial speeds and stratifications. Upwelling occurs within an inertial period, reaching values of up to 10 m day
−1
within 2 to 7 km offshore of the shelf break. Upwelling drives an interior secondary circulation that accelerates the alongshelf flow over the slope, forming a shelfbreak jet. The model results are compared with upwelling estimates from other models and observations near the Middle Atlantic Bight shelf break.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
Although atmospheric forcing by wind stress or buoyancy flux is known to change the ocean’s potential vorticity (PV) at the surface, less is understood about PV modification in the bottom ...boundary layer. The adjustment of a geostrophic current over a sloped bottom in a stratified ocean generates PV sources and sinks through friction and diapycnal mixing. The time-dependent problem is solved analytically for a no-slip boundary condition, and scalings are identified for the change in PV that arises during the adjustment to steady state. Numerical experiments are run to test the scalings with different turbulent closure schemes. The key parameters that control whether PV is injected into or extracted from the fluid are the direction of the geostrophic current and the ratio of its initial speed to its steady-state speed. When the current is in the direction of Kelvin wave propagation, downslope Ekman flow advects lighter water under denser water, driving diabatic mixing and extracting PV. For a current in the opposite direction, Ekman advection tends to restratify the bottom boundary layer and increase the PV. Mixing near the bottom counteracts this restratification, however, and an increase in PV will only occur for current speeds exceeding a critical value. Consequently, the change in PV is asymmetric for currents of the opposite sign but the same speed, with a bias toward PV removal. In the limit of a large speed ratio, the change in PV is independent of diapycnal mixing.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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•An inviscid two-layer model produces a slope current driven by a density gradient.•A surface onshore flow downwells within the slope current and returns offshore.•The flow is determined entirely by ...Rossby-wave propagation from the coast.•The strength of the flow is sensitive to the upper-layer (thermocline) thickness.•The flow transport is insensitive to the profile of the slope.
In this paper, we investigate basic dynamics of the Leeuwin Current (LC) using a dynamically “minimal” model, one that lies at the bottom of a planned hierarchy of LC models. The model is a steady-state, inviscid, 2-layer system, in which the upper-layer density is fixed to ρ1(y), all mixing and advection are ignored, and β≠0. As a result, solutions can be obtained analytically. Our model both simplifies and extends prior models of this sort, which include bottom drag in a fundamental way and adopt the f-plane.
Solutions are obtained in a semi-infinite domain, x≤xe, y≤yn, in the southern hemisphere (yn<0), and there is a continental slope along the eastern boundary with the depth profile D(x). Generally, the interface beneath layer 1 intersects the continental slope along a “grounding” line, x=xg(y)≤xe, in which case the basin is divided into offshore and coastal regimes, and the model reduces to a 1-layer system in the latter. Solutions are forced by the density gradient ∂ρ1/∂y, by alongshore winds τy, and by the thermocline depth along the northern boundary H1, where H1 simulates the impact of the Indonesian Throughflow on the density structure in the northeastern basin. The flow field can be divided into depth-integrated and shear (thermal-wind) parts, and, because density advection is neglected, the former is independent of the latter. The depth-integrated equations are hyperbolic, their solution determined by the offshore propagation of boundary values along Rossby-wave characteristics.
Even though there is no Rossby-wave damping, a coastal jet is trapped over the slope. Both the coastal jet and the offshore flow field are completely determined by the offshore propagation of signals from the coast; moreover, the offshore circulation depends only on the depth of the continental shelf at the coast, D(xe), and is independent of the slope profile farther offshore. For density-driven solutions, the grounding line shifts offshore and hence the LC deepens poleward; there is downwelling over the shelf, as well as westward flow at the bottom of the upper layer, both associated with the thermal-wind circulation and existing only when β≠0; the speed of the coastal jet is proportional to ∂D/∂x; and its transport is proportional to H12, so that it is strongest farther offshore and is very sensitive to the specified thermocline thickness in the northern basin. When equatorward wind stress is included, an equatorward jet can develop very nearshore provided that the wind stress is strong enough to overcome the density forcing.
•Shelf-slope topography traps Rossby waves and a poleward eastern boundary current.•Transport is controlled by the position where the pycnocline intersects the slope.•Mixing processes and bottom drag ...modify the current's speed and structure.•Buoyancy advection generates a shelfbreak front and jet and an undercurrent below.
The boundary currents over the Western Australian continental shelf and slope consist of the poleward flowing Leeuwin Current (LC) and the equatorward flowing Leeuwin Undercurrent (LUC). Key properties of the LC are its poleward strengthening, deepening to the south, and shelfbreak intensification. The alongshore flow reverses direction below about 300m, forming the LUC at greater depths. To investigate the processes that cause these features, we obtain solutions to an idealized, regional ocean model of the South Indian Ocean. Solutions are forced by relaxing surface density to a prescribed, meridionally varying density profile ρ*(y) with a timescale of δt. In addition, vertical diffusion is intensified near the ocean surface. This diffusion establishes the minimum thickness over which density is well-mixed. We define this thickness as the “upper layer”. Solutions are obtained with and without a continental shelf and slope off Western Australia and for a range of values of δt and mixing parameters. Within this upper layer, there is a meridional density gradient that balances a near-surface, eastward geostrophic flow. The eastward current downwells near the eastern boundary, leading to westward flow at depth. The upper layer's meridional structure and zonal currents crucially depend on coastal processes, including the presence of topography near the eastern boundary. Kelvin waves inhibit the upper layer from deepening at the coast. Rossby waves propagate the coastal density structure offshore, hence modifying the interior currents. A comparison of the solutions with or without a continental shelf and slope demonstrate that topographic trapping of Rossby waves is a necessary process for maintaining realistic eastern boundary current speeds. Significant poleward speeds occur only onshore of where the upper layer intersects the slope, that is, at a grounding line. Its poleward transport increases when surface-enhanced vertical mixing is applied over a greater depth. When the timescale δt is sufficiently short, the poleward current is nearly barotropic. The current's spatial structure over the shelf is controlled by horizontal mixing, having the structure of a Munk layer. Increasing vertical diffusion deepens the upper layer thickness and strengthens the alongshore current speed. Bottom drag leads to an offshore flow along the bottom, reducing the net onshore transport and weakening the current's poleward acceleration. When δt is long, poleward advection of buoyancy forms a density front near the shelf break, intensifying poleward speeds near the surface. With bottom drag, a bottom Ekman flow advects density offshore, shifting the jet core offshore of the shelf break. The resulting cross-shelf density gradient reverses the meridional current's direction at depth, leading to an equatorward undercurrent.
Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole ...Oceanographic Institution), 2010.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 199-205).
In a stratified rotating fluid, frictionally driven circulations couple with the buoyancy field over sloping topography. Analytical and numerical methods are used to quantify the impact of this coupling on the vertical circulation, spindown of geostrophic flows, and the formation of a shelfbreak jet. Over a stratified. slope, linear spindown of a geostrophic along-isobath flow induces cross-isobath Ekman flows. Ekman advection of buoyancy weakens the vertical circulation and slows spindown. Upslope (downslope) Ekman flows tend to inject (remove) potential vorticity into (from) the ocean. Momentum advection and nonlinear buoyancy advection are examined in setting asymmetries in the vertical circulation and the vertical relative vorticity field. During nonlinear homogeneous spindown over a flat bottom, momentum advection weakens Ekman pumping and strengthens Ekman suction, while cyclonic vorticity decays faster than anticyclonic vorticity. During nonlinear stratified spindown over a slope, nonlinear advection of buoyancy enhances the asymmetry in Ekman pumping and suction, whereas anticyclonic vorticity can decay faster than cyclonic vorticity outside of the boundary layers. During the adjustment of a spatially uniform geostrophic current over a shelfbreak, coupling between the Ekman flow and the buoyancy field generates Ekman pumping near the shelfbreak, which leads to the formation of a jet. Scalings are presented for the upwelling strength, the length scale over which it occurs, and the timescale for jet formation. The results are applied to the Middle Atlantic Bight shelfbreak.
by Jessica A. Benthuysen.
Ph.D.
PDF
