Abstract A time series of the physical and biogeochemical properties of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the Drake Passage between 1969 and 2005 is constructed using 24 transects of measurements across the passage. Both water masses have experienced substantial variability on interannual to interdecadal time scales. SAMW is formed by winter overturning on the equatorward flank of the Antarctic Circumpolar Current (ACC) in and to the west of the Drake Passage. Its interannual variability is primarily driven by variations in wintertime air–sea turbulent heat fluxes and net evaporation modulated by the El Niño–Southern Oscillation (ENSO). Despite their spatial proximity, the AAIW in the Drake Passage has a very different source than that of the SAMW because it is ventilated by the northward subduction of Winter Water originating in the Bellingshausen Sea. Changes in AAIW are mainly forced by variability in Winter Water properties resulting from fluctuations in wintertime air–sea turbulent heat fluxes and spring sea ice melting, both of which are linked to predominantly ENSO-driven variations in the intensity of meridional winds to the west of the Antarctic Peninsula. A prominent exception to the prevalent modes of SAMW and AAIW formation occurred in 1998, when strong wind forcing associated with constructive interference between ENSO and the southern annular mode (SAM) triggered a transitory shift to an Ekman-dominated mode of SAMW ventilation and a 1–2-yr shutdown of AAIW production. The interdecadal evolutions of SAMW and AAIW in the Drake Passage are distinct and driven by different processes. SAMW warmed (by ∼0.3°C) and salinified (by ∼0.04) during the 1970s and experienced the reverse trends between 1990 and 2005, when the coldest and freshest SAMW on record was observed. In contrast, AAIW underwent a net freshening (by ∼0.05) between the 1970s and the twenty-first century. Although the reversing changes in SAMW were chiefly forced by a ∼30-yr oscillation in regional air–sea turbulent heat fluxes and precipitation associated with the interdecadal Pacific oscillation, with a SAM-driven intensification of the Ekman supply of Antarctic surface waters from the south contributing significantly too, the freshening of AAIW was linked to the extreme climate change that occurred to the west of the Antarctic Peninsula in recent decades. There, a freshening of the Winter Water ventilating AAIW was brought about by increased precipitation and a retreat of the winter sea ice edge, which were seemingly forced by an interdecadal trend in the SAM and regional positive feedbacks in the air–sea ice coupled climate system. All in all, these findings highlight the role of the major modes of Southern Hemisphere climate variability in driving the evolution of SAMW and AAIW in the Drake Passage region and the wider South Atlantic and suggest that these modes may have contributed significantly to the hemispheric-scale changes undergone by those waters in recent decades.
Most climate models predict a slowing down of the Atlantic Meridional Overturning Circulation during the 21st century. Using a 100 year climate change integration of a high resolution coupled climate model, we show that a 5.3 Sv reduction in the deep southward transport in the subtropical North Atlantic is balanced solely by a weakening of the northward surface western boundary current, and not by an increase in the southward transport integrated across the interior ocean away from the western boundary. This is consistent with Sverdrup balance holding to a good approximation outside of the western boundary region on decadal time scales, and may help to spatially constrain past and future change in the overturning circulation. The subtropical gyre weakens by 3.4 Sv over the same period due to a weakened wind stress curl. These changes combine to give a net 8.7 Sv reduction in upper western boundary transport.
It has been argued that diapycnal mixing has a strongly stabilizing role in the global thermohaline circulation (THC). Negative feedback between THC transport and low-latitude buoyancy distribution is present in theory based on thermocline scaling, but is absent from Stommel’s classical model. Here, it is demonstrated that these two models can be viewed as opposite limits of a single theory. Stommel’s model represents unlimited diapycnal mixing, whereas the thermocline scaling represents weak mixing. The latter limit is more applicable to the modern ocean, and previous studies suggest that it is associated with a more stable THC. A new box model, which can operate near either limit, is developed to enable explicit analysis of the transient behaviour. The model is perturbed from equilibrium with an increase in surface freshwater forcing, and initially behaves as if the only feedbacks are those present in Stommel’s model. The response is buffered by any upper ocean horizontal mixing, then by propagation of salinity anomalies, each of which are stabilizing mechanisms. However, negative feedback associated with limited diapycnal mixing only prevents thermohaline catastrophe in a modest parameter domain. This is because the time-scale associated with vertical advective-diffusive balance is much longer than the time required for the THC to change mode. The model is then tuned to allow equilibrium THC transport to be independent of the rate of mixing. The equilibrium surface salinity difference controls the classical THC-transport/salinity positive feedback, whereas the equilibrium interior density difference controls the mean-flow negative feedback. When mixing is strong, unrealistic vertical homogenization occurs, causing a convergence in surface and interior meridional gradients. This reduces positive feedback, and increases stability, in the tuned model. Therefore, Stommel’s model appears to overestimate, rather than underestimate, THC stability to high-frequency changes in forcing.
The extratropical response to the Madden-Julian Oscillation (MJO) is modulated by two prominent modes of low-frequency sea surface temperature (SST) variability: the Atlantic Multidecadal Variability (AMV) and the Pacific Decadal Oscillation (PDO). Utilizing the UK Earth System Model (UKESM) 1100 year pre-industrial control simulation from CMIP6, this study offers a unique opportunity to explore decadal variability with an extensive dataset, surpassing the limitations of previous studies which focussed on reanalysis products. The results underscore a statistically significant influence of both AMV and PDO on the extratropical response across all MJO phases. Non-linear interactions between the MJO teleconnection and SST forcing are observed prominently in the modification of the response to MJO phase 6 (enhanced convection over the western Pacific), with AMV+ and PDO+ background states amplifying distinct teleconnection patterns, notably the negative North Atlantic Oscillation (NAO-) and the deepened Aleutian Low responses, respectively. These changes are greater in magnitude than would be expected from the linear superposition of the individual atmospheric responses to the SST mode and the MJO. The amplification of the MJO phase 6 teleconnection to the North Atlantic aligns with prior research based on ERA5 reanalysis data. While modulation of the response to MJO phase 3 (enhanced convection over the eastern Indian Ocean) is evident, it is less pronounced compared to phase 6, and the mechanisms via which it acts are less clear. Intriguingly, alterations in the teleconnection, such as a weaker Aleutian Low during PDO+, contradict the anticipated modulation. Since MJO phase 3 and PDO+ tend to weaken and strengthen the Aleutian Low, respectively, it would be reasonable to expect that these effects would cancel. Instead, the weakening of the Low after MJO phase 3 is increased during PDO+. A possible mechanism for the modulation of the teleconnections is a linear superposition of Rossby wave modes excited by the MJO, contingent upon the SST state. In the case of MJO phase 6, this corresponds to an amplification of the existing modes, and hence of the expected response. For MJO phase 3, however, there is an indication that other Rossby wave modes may also be excited in certain SST states, leading to interference which is out of phase with the primary response. Acknowledging the limitations of observational and reanalysis datasets, this study underscores the pivotal role of climate models in the effective study of decadal and multi-decadal variability. Importantly, the study has significant implications for extratropical forecasting over the coming decades. The modulation of the MJO teleconnection by AMV and PDO suggests modifications in predictability, crucial for refining forecasting techniques. Furthermore, these results provide a contextual foundation for studies examining MJO teleconnections in future climates, enabling a more accurate dissection of responses influenced by internal and anthropogenically forced variability.
The United Kingdom Fine-Resolution Antartic Model (FRAM project) it a community program to study the Southern Ocean. Central to this is an eddy-resolving three-dimensional primitive-equation ocean general circulation model. The open boundary condition at the northern boundary is described here. The boundary condition is based on that of Stevens (1990).
Abstract An open boundary condition is constructed for three dimensional primitive equation ocean circulation models. The boundary condition utilises dominant balances in the governing equations to assist calculations of variables at the boundary. The boundary condition can be used in two forms. Firstly as a passive one in which there is no forcing at the boundary and phenomena generated within the domain of interest can propagate outwards without distorting the interior. Secondly as an active condition where a model is forced by the boundary condition. Three simple idealised tests are performed to verify the open boundary condition, (1) a passive condition to test the outflow of free Kelvin waves, (2) an active condition during the spin up phase of an ocean, (3) finally an example of the use of the condition in a tropical ocean.
Abstract The confluence between the Brazil Current and the Malvinas Current [the Brazil–Malvinas Confluence (BMC)] in the Argentine Basin is characterized by a complicated thermohaline structure favoring the exchanges of mass, heat, and salt between the subtropical gyre and the Antarctic Circumpolar Current (ACC). Analysis of thermohaline properties of hydrographic sections in the BMC reveals strong interactions between the ACC and subtropical fronts. In the Subantarctic Front, Subantarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW), and Circumpolar Deep Water (CDW) warm (become saltier) by 0.4° (0.08), 0.3° (0.02), and 0.6°C (0.1), respectively. In the subtropical gyre, AAIW and North Atlantic Deep Water have cooled (freshened) by 0.4° (0.07) and 0.7°C (0.11), respectively. To quantify those ACC–subtropical gyre interactions, a box inverse model surrounding the confluence is built. The model diagnoses a subduction of 16 ± 4 Sv (1 Sv ≡ 106 m3 s−1) of newly formed SAMW and AAIW under the subtropical gyre corresponding to about half of the total subduction rate of the South Atlantic found in previous studies. Cross-frontal heat (0.06 PW) and salt (2.4 × 1012 kg s−1) gains by the ACC in the BMC contribute to the meridional poleward heat and salt fluxes across the ACC. These estimates correspond to perhaps half of the total cross-ACC poleward heat flux. The authors’ results highlight the BMC as a key region in the subtropical–ACC exchanges.
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