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Matthew H. England

Abstract

A series of coarse-resolution models were integrated with a view to determining the most appropriate representation of the largest-scale water masses formed in the Southern Ocean. In particular, it was hoped that the models could realistically simulate Antarctic Bottom and Intermediate Water. The ocean model employed has a global domain with a realistic approximation of the continental outlines and bottom bathymetry. The subgrid-scale variation of bottom bathymetry is removed by spatial averaging over each grid box. The annual mean forcing at the sea surface is derived from climatological fields of temperature, salinity, and wind stress. It is found that the salinity of shelf water in the Weddell and Ross seas is critical if the model is to appropriately simulate the world's intermediate and bottom water masses. If the surface layer is too fresh in the Weddell and Ross seas, any bottom water formed adjacent to Antarctica is significantly less dense than in the real ocean. Furthermore, surface water at about 60°S (normally the region of intermediate water formation) strongly contributes to the model ocean's bottom water. This leaves the simulated bottom water too fresh and warm. On the other hand, with sufficiently salty bottom-water formed in the extreme Southern Ocean, a low-salinity tongue of intermediate water develops at 60°S. It is suggested that the sea-ice component of climate models is critical if the simulation is to capture the high-salinity shelf water and bottom-water formation adjacent to Antarctica and, in turn, allow for a realistic tongue of low-salinity Antarctic Intermediate Water (AAIW). The bathymetry of the Drake Passage is shown to determine the shape and strength of an intense meridional overturning cell in the Southern Ocean. By properly representing the northward extent of the Drake Passage, the formation and equatorward spreading of AAIW is simulated realistically. The scheme of AAIW formation obtained is quite different from the classical notion of circumpolar subduction of surface water at the polar front.

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Matthew H. England

Abstract

A hierarchy of coarse-resolution World Ocean experiments were integrated with a view to determining the most appropriate representation of the global-scale water masses in ocean general circulation models. The largest-scale response of the simulated ocean to the prescribed forcing in each model run is described. The World Ocean model eventually has a realistic approximation of continental outlines and bottom bathymetry. The model forcing at the sea surface is derived from climatological fields of temperature, salinity, and wind stress. The first experiment begins with a quite unrealistic and idealized World Ocean. Subsequent experiments then employ more realistic surface boundary conditions, model geometry, and internal physical processes. In all, 16 changes to the model configuration are investigated.

A fundamental dynamical constraint in the Drake Passage gap appears to limit the outflow rate of bottom water in the Antarctic region. This constraint acts to decouple the extreme Antarctic waters from the rest of the World Ocean. In a similar manner, including a surface wind stress acts to decouple the two hemispheres by limiting near-surface meridional flows across the equator. In the Atlantic basin, this decoupling becomes negligible when North Atlantic Deep Water (NADW) production is simulated. It is found that the representation of low salinity Antarctic Intermediate Water (AAIW) is sensitive to the level of horizontal diffusion employed by the model, as well as the chosen geometry of the Drake Passage gap and the amount of buoyancy provided by the model's deep water. For example, provided that lateral diffusion rates are not too excessive, a fresh tongue of AAIW is simulated if either sufficiently dense bottom water is formed off Antarctica, or if enough NADW outflows into the Southern Ocean. The inclusion of an isopycnal mixing scheme is shown to improve the representation of AAIW in coarse-resolution models.

The rate of horizontal diffusion and the relative location of the Drake Passage gap to the polar westerlies determines the shape and strength of an intense meridional overturning cell in the Southern Ocean. The inclusion of an isopycnal mixing scheme does not affect this circulation pattern significantly. On the other hand, the intensification of NADW production can substantially weaken the downwelling component of this cell by drawing more water of Southern Ocean origin northward. Accurately simulating NADW production and outflow requires a complete seasonal cycle in thermohaline forcing in the North Atlantic. The return path of NADW is primarily via a “cold water route” (i.e., the Drake Passage), although sufficiently strong NADW formation sees some return flow via the Agulhas leakage (i.e., the “warm water route”). By the last experiment of the present study, the model reproduces the subtle vertical layering of deep and intermediate water masses quite accurately. This represents a major success for the coarse-resolution multilevel ocean model.

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Matthew H. England

Abstract

The age of water in the World Ocean is studied using a passive age tracer introduced into a global ocean model. Additional information is derived from a transient “dye” tracer that tracks the time-dependent spreading of surface waters into the model ocean interior. Of particular interest is the nature of ocean ventilation over the 10–100-yr timescale, as well as the simulated age of deep and bottom water masses. In the upper model levels young water is found to correspond with regions of convergence (and downwelling) in the surface Ekman layer. Upwelling and convection are both shown to age the upper ocean by entraining older waters into the surface mixed layer. In the deep model levels, water age varies greatly between oceans, with young water found in convectively active regions (in the North Atlantic and in the Ross and Weddell Seas), and old water found in the deep North Pacific. The oldest water mass mixture (located at 2228-m depth in the western Pacific Ocean) is dated at 1494 years, made up of a combination of sources of water whose age varies between 500 and 5000 years. In the bottom layers of the model, Antarctic Bottom Water ventilates the extreme Southern Ocean over a 50–100-yr timescale, whereas the age approaches 1000 years in the northern limit of the Pacific basin. An analysis of age on the σt = 27.4 kg m−3 isopycnal surface shows North Atlantic Deep Water (NADW) leaving the Atlantic Ocean with an average age of 300 years, although part of this water mass mixture is as young as 60 years. The young signal of NADW penetrates the Indian Ocean and the South Pacific via the circumpolar current over a timescale as short as 15 years, although water penetrating the far deep North Pacific is not detected in significant quantities (using a 10% concentration criterion) until about 500 years after the NADW formation time. A volumetric census of age in the World Ocean model shows relative maxima at 2°–3°C, 1200 years (corresponding to water in the deep North Pacific), and at 3°C, 300–500 years (corresponding to water in the deep Atlantic Ocean). The parameterization of mixing in the ocean model partly determines age, with an isopycnal mixing scheme reducing the deep and bottom water ventilation timescale by about 30%. By monitoring the gradual penetration of surface dye into the most remote ocean grid boxes, the time taken to ventilate the entire World Ocean model can be estimated to be around 5000 years.

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Agus Santoso and Matthew H. England

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The natural variability of the Weddell Sea variety of Antarctic Bottom Water (AABW) is examined in a long-term integration of a coupled climate model. Examination of passive tracer concentrations suggests that the model AABW is predominantly sourced in the Weddell Sea. The maximum rate of the Atlantic sector Antarctic overturning (ψ atl) is shown to effectively represent the outflow of Weddell Sea deep and bottom waters and the compensating inflow of Warm Deep Water (WDW). The variability of ψ atl is found to be driven by surface density variability, which is in turn controlled by sea surface salinity (SSS). This suggests that SSS is a better proxy than SST for post-Holocene paleoclimate reconstructions of the AABW overturning rate. Heat–salt budget and composite analyses reveal that during years of high Weddell Sea salinity, there is an increased removal of summertime sea ice by enhanced wind-driven ice drift, resulting in increased solar radiation absorbed into the ocean. The larger ice-free region in summer then leads to enhanced air–sea heat loss, more rapid ice growth, and therefore greater brine rejection during winter. Together with a negative feedback mechanism involving anomalous WDW inflow and sea ice melting, this results in positively correlated θS anomalies that in turn drive anomalous convection, impacting AABW variability. Analysis of the propagation of θS anomalies is conducted along an isopycnal surface marking the separation boundary between AABW and the overlying Circumpolar Deep Water. Empirical orthogonal function analyses reveal propagation of θS anomalies from the Weddell Sea into the Atlantic interior with the dominant modes characterized by fluctuations on interannual to centennial time scales. Although salinity variability is dominated by along-isopycnal propagation, θ variability is dominated by isopycnal heaving, which implies propagation of density anomalies with the speed of baroclinic waves.

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Matthew H. England and Stefan Rahmstorf

Abstract

The sensitivity of ventilation timescales and radiocarbon (14C) uptake to subgrid-scale mixing parameterization is studied in a global ocean model. Seven experiments are examined that are identical in every manner except their representation of subgrid-scale mixing of tracers. The cases include (i) two runs with traditional Cartesian mixing (HOR), (ii) a run with enhanced isopycnal mixing (ISO), and (iii) four runs in which the effects of eddies on the mean ocean flow are parameterized following Gent and McWilliams (GM). Horizontal, isopycnal, and isopycnal-thickness diffusion coefficients are varied sequentially in the model runs. Of particular interest is the role of the tracer mixing schemes in influencing longer timescale ventilation processes—centennial and beyond—such as deep water mass renewal and circulation.

Simulated ventilation timescales and 14C vary greatly between the three mixing schemes. The isopycnal mixing run exhibits the most rapid water mass renewal due to strong diffusion effects and excessive surface convective overturn, particularly in the Southern Ocean. In contrast, the GM cases show much more gradual renewal of deep and bottom waters, with limited vertical convection of surface waters and slower abyssal currents. Under GM, a background horizontal diffusion or altered isopycnal mixing do not significantly change interior ocean ventilation rates. This means modelers can adjust these background diffusion coefficients under GM (for numerical purposes) without significantly changing model ventilation rates. Reducing the GM isopycnal thickness diffusivity, on the other hand, noticeably increases simulated deep water ventilation rates. In comparison with the HOR runs, deep and bottom water ventilation timescales are reduced by about 30% in ISO, and increased by 30%–40% under GM. Comparison is made between model simulated and observed 14C. The GM runs appear to be the least successful in the North Atlantic Ocean, exhibiting very gradual and only shallow water-mass renewal compared to observations. In the Pacific and Indian Oceans, the HOR and ISO runs are ventilated too rapidly due to strong convection and water-mass contribution from the Southern Ocean. In contrast, the GM runs simulate spuriously old and 14C-depleted bottom and middepth water. The GM cases do, however, capture realistic 14C in the upper 1500 m of the Indian and Pacific Oceans. Overall, none of the model cases reproduce global ocean ventilation rates over centennial timescales (under the chosen set of parameter values). Higher horizontal resolution and a spatially varying GM thickness diffusivity may be required before global models capture long timescale ocean renewal processes with some degree of fidelity.

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Véronique Lago and Matthew H. England

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The sinking and recirculation of Antarctic Bottom Water (AABW) are a major regulator of the storage of heat, carbon, and nutrients in the ocean. This sinking is sensitive to changes in surface buoyancy, in particular because of freshening since salinity plays a greater role in determining density at cold temperatures. Acceleration in Antarctic ice-shelf and land-ice melt could thus significantly impact the ventilation of the world’s oceans, yet future projections do not usually include this effect in models. Here we use an ocean–sea ice model to investigate the potential long-term impact of Antarctic meltwater on ocean circulation and heat storage. The freshwater forcing is derived from present-day estimates of meltwater input from drifting icebergs and basal melt, combined with RCP2.6, RCP4.5, and RCP8.5 scenarios of projected amplification of Antarctic meltwater. We find that the additional freshwater induces a substantial slowdown in the formation rate of AABW, reducing ventilation of the abyssal ocean. Under both the RCP4.5 and RCP8.5 meltwater scenarios, there is a near-complete shutdown of AABW formation within just 50 years, something that is not captured by climate model projections. The abyssal overturning at ~30°S also weakens, with an ~20-yr delay relative to the onset of AABW slowdown. After 200 years, up to ~50% of the original volume of AABW has disappeared as a result of abyssal warming, induced by vertical mixing in the absence of AABW ventilation. This result suggests that climate change could induce the disappearance of present-day abyssal water masses, with implications for the global distribution of heat, carbon, and nutrients.

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Stefan Rahmstorf and Matthew H. England

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A series of experiments with a hybrid model (ocean circulation model with simple atmospheric feedback model) and an ocean-only model is used to study the sensitivity of the ocean’s deep overturning circulation to Southern Hemisphere winds. In particular, the “Drake Passage effect” is examined.

The results show that two factors weaken the control that the Drake Passage effect exerts over the flow of North Atlantic Deep Water (NADW). The first is that thermohaline forcing alone can generate about 75% of the NADW flow found in our model; this ability is lost if atmospheric feedback is neglected. The second is that about two-thirds of the downwelling induced by Ekman transport across Drake Passage occurs in the Southern Hemisphere just north of Drake Passage; only one-third occurs in the North Atlantic and enhances NADW flow. For these two reasons, the influence of Southern Ocean winds on NADW flow is only moderate and not as strong as previously suggested. However, the authors find that the formation rate of Antarctic Bottom Water depends strongly on the winds over the Southern Ocean.

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Matthew H. England and Fei Huang

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The Indonesian Throughflow (ITF) variability is assessed using a retrospective analysis of the global ocean based on the Simple Ocean Data Assimilation (SODA) experiment spanning the period 1950–99. A comparison between the 1983–95 observed ITF, and the simulated ITF suggests a reasonably accurate reconstruction of ocean circulation in the vicinity of the ITF during the available measurement record. A wavelet analysis shows that once the seasonal cycle is removed, the dominant variation of the ITF anomaly is an interannual oscillation with a period of about 4–7 yr. This interannual variability is significantly correlated with the El Niño–Southern Oscillation (ENSO) pattern, with the ITF lagging the ENSO cycle by 8–9 months. This suggests that large-scale tropical ocean–atmosphere interaction plays an important role in the interannual variability of the ITF. Regional upper-ocean heat content variability might also play a role in controlling interannual fluctuations of the ITF transport via geostrophic flows, though it could equally be ITF variations that establish heat content anomalies downstream of the Indonesian archipelago. The model heat transport associated with the ITF is in good agreement with the limited observational record available. Resultant variability in annual mean ITF heat transport is in the range 0.4–1.2 PW, which is significantly correlated with ITF and ENSO indices.

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Agus Santoso and Matthew H. England

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The variability of Antarctic Intermediate Water (AAIW) in a long-term natural integration of a coupled climate model is examined. The mean state of the climate model includes a realistic representation of AAIW, which appears centered on the σ θ = 27.2 kg m−3 density surface (hereinafter σ 27.2) both in observations and the model. An assessment of ventilation rates on the σ 27.2 surface suggests that this particular climate model forms AAIW in a mostly circumpolar fashion, with a significant contribution from Antarctic Surface Water. This motivates the assessment of oceanic variability along this core AAIW isopycnal surface. Complex empirical orthogonal function analyses decompose the variability into three dominant modes showing circumpolar patterns of zonal wavenumber-1, -2, and -3 on the σ 27.2 density surface. The modes contain eastward-propagating signals at interannual to centennial time scales. Mechanisms forcing this variability are investigated using heat and salt budget analyses at the wintertime outcrop of the σ 27.2 surface. Such an approach ignores the mechanism of AAIW variability sourced by Subantarctic Mode Water variations, which has been examined previously and is, for the most part, beyond the present study. Variability in meltwater rates and atmospheric heat and freshwater fluxes are found to dominate the intermediate water variability at the outcrop region. In contrast, northward Ekman transport of heat and salt plays a significant but localized role in AAIW temperature–salinity variability. There is also an important contribution from the Antarctic Circumpolar Current to the variability at the outcrop region via zonal transport of heat and salt. While the magnitude of AAIW natural variability can be large near the outcrop of the salinity minimum layer, recent observations of cooling and freshening at depth are suggested to be beyond that of the unperturbed system.

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Christopher M. Aiken and Matthew H. England

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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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