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Christopher M. Aiken and Matthew H. England

Abstract

A simple linearized transport model of anomalous Southern Ocean sea surface temperature (SST) is studied to determine whether it can sustain anomalies of realistic amplitudes under a physically based stochastic forcing. As noted in previous studies, eigenmodes of this system with zonal wavenumbers 2 and 3 share key propagation characteristics with the SST anomalies associated with the Antarctic Circumpolar Wave (ACW). The system is solved on a grid that follows the path of the Antarctic Circumpolar Current (ACC) and is forced by a stochastic heat flux. The forcing is white in space and time and represents the advection of the mean SST gradient by high-frequency variations in the cross-ACC velocity, due to mesoscale eddy variability. The magnitude of the stochastic forcing is determined from a global eddy-permitting ocean model. Anomalous ocean surface velocity variability (8 cm s−1) coupled to a mean cross-ACC SST gradient of 0.8°C (°latitude)−1 sustains anomalous interannual SST variability at low wavenumbers and amplitudes of the order of 1°C, consistent with those associated with the ACW. In the long-term mean, variance is broadly spread among low wavenumbers, in contrast to the dominance of one or two zonal wavenumbers in the ACW observations. It is found, however, that the model produces single dominant wavenumbers over individual periods of decades, suggesting that the apparent unimodal nature of the ACW may be an artifact of the short observational record used to infer it. Alternatively, it is shown that a nonisotropic forcing may also result in a stronger preference for particular zonal wavenumbers. It is shown that if the atmosphere at mid to high southern latitudes has an equivalent barotropic response to heating, then the resulting sea level pressure anomalies reproduce the phase relationship of the observed ACW. These results are consistent with the notion that a simple stochastically forced advection of SST anomalies can explain SST variability associated with the ACW to leading order.

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Christopher M. Aiken and Matthew H. England

Abstract

The role played by Southern Hemisphere sea ice in the global climate system is explored using an earth system climate model of intermediate complexity. An ensemble of experiments is analyzed in which freshwater forcing equivalent to a complete 100-yr meltback of Southern Hemisphere sea ice is applied to a model run that simulates the present climate. This freshwater forcing acts to mildy subdue Southern Ocean deep overturning, reducing mean Antarctic Bottom Water (AABW) export by 0.5 Sv (1 Sv ≡ 106 m3 s−1) in the ensemble average. The decreased convective overturning cools the surface waters, thereby increasing sea ice volume and thus forming a negative feedback that stabilizes Antarctic sea ice. In contrast, the reduced convective overturn warms subsurface waters in the Southern Ocean, which, combined with the imposed freshening, results in a reduction in the meridional steric height gradient and hence a slowdown of the Antarctic Circumpolar Current (ACC). The reduction in ACC strength is, however, only modest at 1.5 Sv. These responses are thus of only weak magnitude, and the system recovers to its original state over time scales of decades. An extreme scenario experiment with essentially instantaneous addition of this meltwater load shows similar results, indicating the limited response of the climate system to the freshening implied by Antarctic sea ice melt. An additional experiment in which a much larger freshwater forcing of approximately 0.4 Sv is applied over 100 yr confirms the relatively weak response of the model’s climate state to such forcing, relative to the well-documented climatic effects of freshwater forcing added to the North Atlantic.

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Christopher M. Aiken, Matthew H. England, and Christopher J. C. Reason

Abstract

Generalized stability theory is applied to a simple dynamical model of interannual ocean–atmosphere variability in the southern midlatitudes to determine the perturbations that create the most rapid growth of energy in the system. The model is composed of a barotropic quasigeostrophic atmosphere coupled to a 1.5-layer quasigeostrophic ocean, each linearized about a zonally invariant mean state, and with atmospheric and ocean surface temperature obeying a simple heat balance. Eigenanalysis of the system reveals modes of interannual variability that resemble the so-called Antarctic Circumpolar Wave (ACW), consistent with an earlier analytical study of the system. The optimal excitation of these modes relative to an energy norm is found to be a perturbation almost entirely restricted to the ocean momentum field and is shown to resemble strongly the optimal perturbations in energy for the system. Over interannual time scales most rapid growth is seen in zonal wavenumbers 4–6, despite the fact that the least-damped eigenmodes of the system are of a lower zonal wavenumber. The rapid transient growth in energy occurs by extracting perturbation energy from the mean state through advection of the mean meridional oceanic temperature gradient. This transient growth of high-zonal-wavenumber modes dominates the model’s variability when it is forced by noise that is white in space or time. A dominant low-zonal-wavenumber response, consistent with the observed and modeled ACW, occurs only when the forcing is red in space or time, with decorrelation scales greater than 3 yr or 10 000 km. It is concluded that, if the ACW is a coupled mode analogous to that supported in this simple model, then it is excited by other large-scale phenomena such as ENSO rather than by sources of higher-frequency forcing.

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Christopher M. Aiken, Andrew M. Moore, and Jason H. Middleton

Abstract

The effect of stochastic variability upon the wake downstream of a real headland is investigated. From observations taken at Bass Point, Australia, three sources of stochastic forcing are identified and quantified, these being variability in the wind stress, in the flow incident on the headland, and that generated in the flow by complex reef topography at the headland's tip. These sources of stochastic forcing are found to induce a significant degree of unsteadiness in a nonlinear numerical model of flow around Bass Point that, in the absence of stochastic forcing, simulates steady recirculating flow. The observed stochastic variability of the flow incident on the headland is found to be larger than is necessary to support unsteadiness in the simulated straining zone. The dynamics of the perturbation growth observed in the model are understood by applying the techniques of generalized stability theory to the associated tangent linear model (TLM). The eigenmodes of the TLM confirm that the system is asymptotically stable, but analysis reveals that these eigenmodes are nonorthogonal and hence that the dynamical system is nonnormal. It is found that the model's response to stochastic forcing is not controlled by the least damped eigenmodes of the TLM, but rather by the least damped of its most nonnormal modes. The pseudospectra of the system suggest that the nonnormal modes are excited and sustained by the process of pseudoresonance. The nonnormality of the system permits transient growth of disturbances, because this allows a linear sum of exponentially decaying nonorthogonal modes to grow for a finite period because of eigenmode interference. Only in the absence of forcing do the least damped modes dictate the character of the flow. It is concluded that the subcritical transition to unsteady flow observed in the model may occur in straining coastal ocean circulations in general because of the ubiquity of environmental noise. As a result, nondimensional parameters such as the Reynolds number may only give a probability of transition occurring in nonnormal geophysical systems.

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Christopher M. Aiken, Wilhelm Petersen, Friedhelm Schroeder, Martina Gehrung, and Paola A Ramírez von Holle

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Results from two field campaigns in the Chilean fjords region are presented to demonstrate the benefits and limitations of the “pocket FerryBox” for monitoring from ships of opportunity. The October 2009 (spring) campaign covered the region of the Chilean coast between 41.5° and 46.7°S, and that in March 2010 (autumn) covered the region between 41.5° and 51.8°S. In the campaigns the pocket FerryBox—a portable flow-through system for underway multiparametric monitoring—was installed temporarily on board the vessel M/V Ro-Ro Evangelistas. The taking of water samples allowed posterior calibration of the sensors and analyses for nutrients and plankton. The pocket FerryBox may be configured with multiple sensors [in this case temperature, salinity, dissolved oxygen, chlorophyll-a fluorescence, pH, turbidity, and colored dissolved organic matter (CDOM)] and includes the hardware and software for data acquisition and real-time presentation. In the Chilean campaigns multiple transects of up to 1700 km in length were obtained, which provided a unique and highly valuable dataset at a very low cost. The data uncovered a number of previously unreported results, including a tidally driven low dissolved oxygen zone in the Corcovado Gulf, a high level of spatial and temporal variability of, and a complex relationship between, dissolved oxygen and chlorophyll-a fluorescence, and the detection of high concentrations of CDOM in the vicinity of the Laguna San Rafael. The campaigns confirm that the pocket FerryBox may be easily installed on board ships of opportunity to obtain rapid, low-cost, and spatially extensive surveys of highly relevant surface water properties.

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