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Kate Snow
,
Andrew McC. Hogg
,
Bernadette M. Sloyan
, and
Stephanie M. Downes

Abstract

The influence of freshwater and heat flux changes on Antarctic Bottom Water (AABW) properties are investigated within a realistic bathymetry coupled ocean–ice sector model of the Atlantic Ocean. The model simulations are conducted at eddy-permitting resolution where dense shelf water production dominates over open ocean convection in forming AABW. Freshwater and heat flux perturbations are applied independently and have contradictory surface responses, with increased upper-ocean temperature and reduced ice formation under heating and the opposite under increased freshwater fluxes. AABW transport into the abyssal ocean reduces under both flux changes, with the reduction in transport being proportional to the net buoyancy flux anomaly south of 60°S.

Through inclusion of shelf-sourced AABW, a process absent from most current generation climate models, cooling and freshening of dense source water is facilitated via reduced on-shelf/off-shelf exchange flow. Such cooling is propagated to the abyssal ocean, while compensating warming in the deep ocean under heating introduces a decadal-scale variability of the abyssal water masses. This study emphasizes the fundamental role buoyancy plays in controlling AABW, as well as the importance of the inclusion of shelf-sourced AABW within climate models in order to attain the complete spectrum of possible climate change responses.

Full access
Dhruv Bhagtani
,
Andrew McC. Hogg
,
Ryan M. Holmes
, and
Navid C. Constantinou

Abstract

Gyres are central features of large-scale ocean circulation and are involved in transporting tracers such as heat, nutrients, and carbon dioxide within and across ocean basins. Traditionally, the gyre circulation is thought to be driven by surface winds and quantified via Sverdrup balance, but it has been proposed that surface buoyancy fluxes may also contribute to gyre forcing. Through a series of eddy-permitting global ocean model simulations with perturbed surface forcing, the relative contribution of wind stress and surface heat flux forcing to the large-scale ocean circulation is investigated, focusing on the subtropical gyres. In addition to gyre strength being linearly proportional to wind stress, it is shown that the gyre circulation is strongly impacted by variations in the surface heat flux (specifically, its meridional gradient) through a rearrangement of the ocean’s buoyancy structure. On shorter time scales (∼10 years), the gyre circulation anomalies are proportional to the magnitude of the surface heat flux gradient perturbation, with up to ∼0.15 Sv (1 Sv ≡ 106 m3 s−1) anomaly induced per watt per square meter change in the surface heat flux. On time scales longer than a decade, the gyre response to surface buoyancy flux gradient perturbations becomes nonlinear as ocean circulation anomalies feed back onto the buoyancy structure induced by the surface buoyancy fluxes. These interactions complicate the development of a buoyancy-driven theory for the gyres to complement the Sverdrup relation. The flux-forced simulations underscore the importance of surface buoyancy forcing in steering the large-scale ocean circulation.

Significance Statement

Ocean gyres are large swirling circulation features that redistribute heat across ocean basins. It is commonly believed that surface winds are the sole driver of ocean gyres, but recent literature suggests that other mechanisms could also be influential. We perform a series of numerical simulations in which we artificially change either the winds or the heating at the ocean’s surface and investigate how each factor independently affects the ocean gyres. We find that gyres are steered by both winds and surface heating, and that the ocean circulation responds differently to heating on short and long time scales. In addition, the circulation depends on where the heating is applied at the ocean’s surface. Through these simulations, we argue that a complete theory about ocean gyres must consider heating at the ocean’s surface as a possible driver, in addition to the winds.

Restricted access
Julia Neme
,
Matthew H. England
,
Andrew McC. Hogg
,
Hemant Khatri
, and
Stephen M. Griffies

Abstract

The Weddell Gyre is one of the dominant features of the Southern Ocean circulation and its dynamics have been linked to processes of climatic relevance. Variability in the strength of the gyre’s horizontal transport has been linked to heat transport toward the Antarctic margins and changes in the properties and rates of export of bottom waters from the Weddell Sea region to the abyssal global ocean. However, the precise physical mechanisms that force variability in the Weddell’s lateral circulation across different time scales remain unknown. In this study, we use a barotropic vorticity budget from a mesoscale eddy active model simulation to attribute changes in gyre strength to variability in possible driving processes. We find that the Weddell Gyre’s circulation is sensitive to bottom friction associated with the overflowing dense waters at its western boundary. In particular, an increase in the production of dense waters at the southwestern continental shelf strengthens the bottom flow at the gyre’s western boundary, yet this drives a weakening of the depth-integrated barotropic circulation via increased bottom friction. Strengthening surface winds initially accelerate the gyre, but within a few years the response reverses once dense water production and export increases. These results reveal that the gyre can weaken in response to stronger surface winds, putting into question the traditional assumption of a direct relationship between surface stress and gyre strength in regions where overflowing dense water forms part of the depth-integrated flow.

Restricted access
Darryn W. Waugh
,
Andrew McC. Hogg
,
Paul Spence
,
Matthew H. England
, and
Thomas W. N. Haine

ABSTRACT

Changes in ventilation of the Southern Hemisphere oceans in response to changes in midlatitude westerly winds are examined by analyzing the ideal age tracer from global eddy-permitting ocean–ice model simulations in which there is an abrupt increase and/or a meridional shift in the winds. The age response in mode and intermediate waters is found to be close to linear; the response of a combined increase and shift of peak winds is similar to the sum of the individual responses to an increase and a shift. Further, a barotropic response, following Sverdrup balance, can explain much of the age response to the changes in wind stress. There are similar peak decreases (of around 50 years) in the ideal age for a 40% increase or 2.5° poleward shift in the wind stress. However, while the age decreases throughout the thermocline for an increase in the winds, for a poleward shift in the winds the age increases in the north part of the thermocline and there are decreases in age only south of 35°S. As a consequence, the change in the volume of young water differs, with a 15% increase in the volume of water with ages younger than 50 years for a 40% increase in the winds but essentially no change in this volume for a 2.5° shift. As ventilation plays a critical role in the uptake of carbon and heat, these results suggest that the storage of anthropogenic carbon and heat in mode and intermediate waters will likely increase with a strengthening of the winds, but will be much less sensitive to a meridional shift in the peak wind stress.

Full access
Paige E. Martin
,
Brian K. Arbic
,
Andrew McC. Hogg
,
Andrew E. Kiss
,
James R. Munroe
, and
Jeffrey R. Blundell

Abstract

Climate variability is investigated by identifying the energy sources and sinks in an idealized, coupled, ocean–atmosphere model, tuned to mimic the North Atlantic region. The spectral energy budget is calculated in the frequency domain to determine the processes that either deposit energy into or extract energy from each fluid, over time scales from one day up to 100 years. Nonlinear advection of kinetic energy is found to be the dominant source of low-frequency variability in both the ocean and the atmosphere, albeit in differing layers in each fluid. To understand the spatial patterns of the spectral energy budget, spatial maps of certain terms in the spectral energy budget are plotted, averaged over various frequency bands. These maps reveal three dynamically distinct regions: along the western boundary, the western boundary current separation, and the remainder of the domain. The western boundary current separation is found to be a preferred region to energize oceanic variability across a broad range of time scales (from monthly to decadal), while the western boundary itself acts as the dominant sink of energy in the domain at time scales longer than 50 days. This study paves the way for future work, using the same spectral methods, to address the question of forced versus intrinsic variability in a coupled climate system.

Free access
Adele K. Morrison
,
Wilma G. C. Huneke
,
Julia Neme
,
Paul Spence
,
Andrew McC. Hogg
,
Matthew H. England
, and
Stephen M. Griffies

Abstract

Winds around the Antarctic continental margin are known to exert a strong control on the local ocean stratification and circulation. However, past work has largely focused on the ocean response to changing winds in limited regional sectors and the circumpolar dynamical response to polar wind change remains uncertain. In this work, we use a high-resolution global ocean–sea ice model to investigate how dense shelf water formation and the temperature of continental shelf waters respond to changes in the zonal and meridional components of the polar surface winds. Increasing the zonal easterly wind component drives an enhanced southward Ekman transport in the surface layer, raising sea level over the continental shelf and deepening coastal isopycnals. The downward isopycnal movement cools the continental shelf, as colder surface waters replace warmer waters below. However, in this model the zonal easterly winds do not impact the strength of the abyssal overturning circulation, in contrast to past idealized model studies. Instead, increasing the meridional wind speed strengthens the abyssal overturning circulation via a sea ice advection mechanism. Enhanced offshore meridional wind speed increases the northward export of sea ice, resulting in decreased sea ice thickness over the continental shelf. The reduction in sea ice coverage leads to increased air–sea heat loss, sea ice formation, brine rejection, dense shelf water formation, and abyssal overturning circulation. Increasing the meridional winds causes warming at depth over most of the continental shelf, due to a heat advection feedback associated with the enhanced overturning circulation.

Restricted access
Abhishek Savita
,
Jan D. Zika
,
Catia M. Domingues
,
Simon J. Marsland
,
Gwyn Dafydd Evans
,
Fabio Boeira Dias
,
Ryan M. Holmes
, and
Andrew McC. Hogg

Abstract

Ocean circulation and mixing regulate Earth’s climate by moving heat vertically within the ocean. We present a new formalism to diagnose the role of ocean circulation and diabatic processes in setting vertical heat transport in ocean models. In this formalism we use temperature tendencies, rather than explicit vertical velocities, to diagnose circulation. Using quasi-steady-state simulations from the Australian Community Climate and Earth-System Simulator Ocean Model (ACCESS-OM2), we diagnose a diathermal overturning circulation in temperature–depth space. Furthermore, projection of tendencies due to diabatic processes onto this coordinate permits us to represent these as apparent overturning circulations. Our framework permits us to extend the concept of “Super Residual Transport,” which combines mean and eddy advection terms with subgridscale isopycnal mixing due to mesoscale eddies but excludes small-scale three-dimensional turbulent mixing effect, to construct a new overturning circulation—the “Super Residual Circulation” (SRC). We find that in the coarse-resolution version of ACCESS-OM2 (nominally 1° horizontal resolution) the SRC is dominated by an ~11-Sv (1 Sv ≡ 106 m3 s−1) circulation that transports heat upward. The SRC’s upward heat transport is ~2 times as large in a finer-horizontal-resolution (0.1°) version of ACCESS, suggesting that a differing balance of super-residual and parameterized small-scale processes may emerge as eddies are resolved. Our analysis adds new insight into superresidual processes, because the SRC elucidates the pathways in temperature and depth space along which water mass transformation occurs.

Open access