Abstract: Abstract The opening and constriction of oceanic gateways played an essential role in shaping the global climate throughout Earth's history. In this review, we provide an overview of the best-documented feedbacks between gateway dynamics and climate change throughout the Cenozoic. The discussed tectonically induced events comprise: (i) the opening of the Tasmanian Gateway and the glaciation of Antarctica during the Eocene–Oligocene; (ii) the water-mass exchange between the Atlantic and the Mediterranean via the Strait of Gibraltar that has occurred since the Miocene; and (iii) the closure of the American Seaway and (iv) the constriction of the Indonesian Throughflow, both argued to have been instrumental in the intensification of Northern Hemisphere Glaciation during the late Pliocene and early Pleistocene. Lastly, we look at (v) the climatic impact of the flooding and submergence of the Bering Strait during the Plio-Pleistocene and its influence on the Atlantic Meridional Overturning Circulation. While different in their underlying mechanisms, geographical scale and temporal evolution, these case studies demonstrate that even seemingly small-scale changes in the configuration of ocean seaways fundamentally altered the global climate system via their impact on oceanic currents, global heat transfer and carbon storage.

Abstract: Abstract Since 2000, the Indian Ocean has warmed more rapidly than the Atlantic or Pacific Oceans. Air–sea fluxes alone cannot explain the rapid Indian Ocean warming, which has so far been linked to an increase in temperature transport into the basin through the Indonesian Throughflow (ITF). Here, we investigate the role that the heat transport out of the basin at 36°S plays in the warming. Adding the heat transport out of the basin to the ITF temperature transport into the basin, we calculate the decadal mean Indian Ocean heat budget over the 2010s. We find that heat convergence increased within the Indian Ocean over 2000–19. The heat convergence over the 2010s is of the same order as the warming rate, and thus the net air–sea fluxes are near zero. This is a significant change from previous analyses using transbasin hydrographic sections from 1987, 2002, and 2009, which all found divergences of heat. A 2-yr time series shows that seasonal aliasing is not responsible for the decadal change. The anomalous ocean heat convergence over the 2010s in comparison with previous estimates is due to changes in ocean currents at both the southern boundary (33%) and the ITF (67%). We hypothesize that the changes at the southern boundary are linked to an observed broadening of the Agulhas Current, implying that temperature and velocity data at the western boundary are crucial to constrain heat budget changes.

Abstract: The aim of this paper is to analyse the effect of a downward vertical net mass flow on the type of instability that occurs in a horizontal fluid-saturated porous layer that is heated from below. The strength of the downflow is modelled by Pe, the Péclet number. First, we prove the validity of the principle of exchange of stabilities, consequently, we perform a linear instability analysis of the basic steady flow to determine the critical Darcy-Rayleigh number for the onset of steady convective instability as a function of Pe. Then, a weakly nonlinear stability analysis is performed to determine the smallest value of the Péclet number for which the onset of convection corresponds to a subcritical instability.

Abstract: Abstract This paper presents a systematic study of flow and heat transfer mechanisms in a compressor disk cavity with an axial throughflow under centrifugal buoyancy-driven convection, comparing with previously published experimental data. Wall-modeled large-eddy simulations (WMLES) are conducted for six operating conditions, covering a range of rotational Reynolds number (3.2×105−2.2×106), buoyancy parameter (0.11–0.26), and Rossby number (0.4–0.8). Numerical accuracy and computational efficiency of the simulations are considered. Wall heat transfer predictions are compared with measured data with a good level of agreement. A constant rothalpy core occurs at high Eckert number, appearing to reduce the driving buoyancy force. The flow in the cavity is turbulent with unsteady laminar Ekman layers observed on both disks except in the bore flow affected region on the downstream disk cob. The shroud heat transfer Nusselt number–Rayleigh number scaling agrees with that of natural convection under gravity for high Rayleigh numbers. Disk heat transfer is dominated by conduction across unsteady Ekman layers, except on the downstream disk cob. The disk bore heat transfer is close to a pipe flow forced convection correlation. The unsteady flow structure is investigated showing strong unsteadiness in the cavity that extends into the axial throughflow.

Abstract: ABSTRACT Multiple proxies in three sediment cores from Northwestern Baffin Bay document the timing of Lancaster Sound Ice Stream (LSIS) retreat that led to Arctic–Atlantic throughflow in Parry Channel, an important source of freshwater that can impact the Atlantic Meridional Overturning Circulation. The Late Glacial to Holocene timing of ice retreat and channel opening and the responses of the regional ocean environment to these events are presented. We use quantitative mineral composition, foraminiferal assemblages, biogenic silica, ice-rafted debris (IRD), and 14C-based age models to document and date the events and environmental changes occurring during deglaciation of this major marine channel. Findings show that retreat of the LSIS into Lancaster Sound occurred before ~15.3 cal ka BP, about 800 years before the onset of major iceberg calving events from the LSIS, named the Baffin Bay Detrital Carbonate events (BBDC 1 and BBDC 0). The end of BBDC 0 occurred at ~10.6 cal ka BP, which coincides with the opening of Parry Channel. A marine environment productive of calcareous benthic and planktic foraminifera, with diminished meltwater, seasonal sea ice, warmer summer temperatures, and inflowing, nutrient-rich Arctic surface water characterizes the interval between the opening of Parry Channel and the opening of Nares Strait. Paired planktic and benthic 14C ages over this 2,200-year interval show diminishing age offsets suggesting progressive mixing of the upper ~850–900 m of the water column. The opening of Nares Strait by ~8.2 cal ka BP coincides with increased biogenic silica in the form of abundant, large centric diatoms and dissolution of CaCO3. The paucity of calcareous organisms after 8.2 cal ka BP resulted in poor chronological control in the cores to interpret changing environments after 8.2 cal ka BP.

Abstract: Abstract. The subtropical southern Indian Ocean (SIO) has been described as one of the world's largest heat accumulators due to its remarkable warming during the past 2 decades. However, the relative contributions of remote (of Pacific origin) forcing and local wind forcing to the variability of heat content and sea level in the SIO have not been fully attributed. Here, we combine a general circulation model, an analytic linear reduced-gravity model, and observations to disentangle the spatial and temporal inputs of each forcing component on interannual to decadal timescales. A sensitivity experiment is conducted with artificially closed Indonesian straits to physically isolate the Indian Ocean and Pacific Ocean, intentionally removing the Indonesian Throughflow (ITF) influence on the Indian Ocean heat content and sea level variability. We show that the relative contribution of the signals originating in the equatorial Pacific vs. signals caused by local wind forcing to the interannual variability of sea level and heat content in the SIO is dependent on location within the basin (low latitude vs. midlatitude and western side vs. eastern side of the basin). The closure of the ITF in the numerical experiment reduces the amplitude of interannual-to-decadal sea level changes compared to the simulation with a realistic ITF. However, the spatial and temporal evolution of sea level patterns in the two simulations remain similar and correlated with El Niño–Southern Oscillation (ENSO). This suggests that these patterns are mostly determined by local wind forcing and oceanic processes, linked to ENSO via the “atmospheric bridge” effect. We conclude that local wind forcing is an important driver for the interannual changes of sea level, heat content, and meridional transports in the SIO subtropical gyre, while oceanic signals originating in the Pacific amplify locally forced signals.

Abstract: The Indonesian Throughflow (ITF) is projected to slow down under anthropogenic warming. Several mechanisms—some mutually conflicting—have been proposed but the detailed processes causing this slowdown remain unclear. By turning on/off buoyancy and wind forcings globally and in key regions, this study investigates the dynamical adjustments underlying the centennial ITF slowdown in the global oceans and climate models. Our results show that the projected weakened ITF transport in the top 1500 m is dominated by remote anomalous buoyancy forcing in the North Atlantic Ocean. Specifically, surface freshening and warming over the North Atlantic Ocean slow the Atlantic meridional overturning circulation (AMOC), and the resultant dynamic signals propagate through the coastal-equatorial waveguide into the southeastern Indian Ocean and western Pacific Ocean, causing the reduction of ITF transport over a deep layer. In contrast, the anomalous surface buoyancy flux in the Indo-Pacific affects the ocean temperature and salinity in a shallow upper layer, resulting in ITF changes in forms of high baroclinic mode structure with negligible impacts on the net ITF transport. A vertical partitioning index is proposed to distinguish the remote forcing via the AMOC and regional forcing in the Indo-Pacific Ocean, which could be useful for monitoring, attributing and predicting the changing ITF transport under global warming.

Abstract: Flow and heat transfer in axial compressor disc cavities involve strong interaction of axial throughflow at the disc bores with centrifugal buoyant flow in the cavities. This paper presents large eddy simulation (LES) of flow and heat transfer in rotating cavities with a heated shroud and a relatively weak axial cooling throughflow. The conditions considered for a single cavity configuration correspond to Rossby numbers Ro=0.2 and 0.3, rotational Reynolds numbers ReΩ=3.2×105 and 7.7×105, and buoyancy parameters βΔT=0.24 and 0.26. Reasonable agreement of the results with shroud heat transfer measurements was confirmed for the Ro=0.2 condition for which test data were available. A dual cavity configuration for Ro=0.3 and ReΩ=3.2×105 is also modelled. The simulations show that, at low Ro conditions, flow reversals occur along the length of the bore flow path, upstream and downstream of the rotating cavities. With the dual cavity strong, unsteady interactions between the flows in the two cavities occur. These flow interactions result in less stable flow structures, higher air temperatures within the cavities and lower shroud and disc heat transfer compared to the single cavity case. FFT analysis reveals a complex phase-locking mechanism between flows in the two cavities.

Abstract: Abstract The Indonesian throughflow (ITF) transports a significant amount of warm freshwater from the Pacific to the Indian Ocean, making it critical to the global climate system. This study examines decadal ITF variations using ocean reanalysis data as well as climate model simulations from the Coupled Model Inter-comparison Project Phase 5 (CMIP5). While the observed annual cycle of ITF transport is known to be correlated with the annual cycle of sea surface height (SSH) difference between the Pacific and Indian Oceans, ocean reanalysis data (1959–2015) show that the Pacific Ocean SSH variability controls more than 85% of ITF variation on decadal timescales. In contrast, the Indian Ocean SSH variability contributes less than 15%. While those observed contributions are mostly reproduced in the CMIP5 historical simulations, an analysis of future climate projections shows a 25–30% increase in the Indian Ocean SSH variability to decadal ITF variations and a corresponding decrease in the Pacific contribution. These projected changes in the Indian Ocean SSH variability are associated with a 23% increase in the amplitudes of negative zonal wind stress anomalies over the equatorial Indian Ocean, along with a 12º eastward shift in the center of action in these anomalies. This combined effect of the increased amplitude and eastward shift in the zonal wind stress increases the SSHA variance over the Indian Ocean, increasing its contribution to the ITF variation. The decadal ITF changes discussed in this study will be crucial in understanding the future global climate variability, strongly coupled to Indo-Pacific interactions.

Abstract: Internal tides (ITs) in the Indonesian seas were largely investigated and hotspots of intensified mixing identified in the straits in regional models and observations. Both of them indicate strong mixing up to 10⁻⁴cm/s even close to the surface and show that tides at spring-neap cycle cool by 0.2°C the surface water at ITs’ generation sites.These findings supported the idea of strong and surfaced mixing capable of providing cold and nutrient-rich water favorable for the whole ecosystem. However, it has never been assessed through an ad-hoc study. Our aim is to provide a quantification of ITs impact on chlorophyll-a through a coupled model, whose physical part was validated against the INDOMIX data in precedent studies and the biogeochemical part is compared to in-situ samples and satellite products. In particular, explicit tides’ inclusion within the model improves the representation of chlorophyll and of the analyzed nutrients. Results from harmonic analysis of chlorophyll-a demonstrate that tidal forcing modify spring/neap tides’ variability on the regions of maximum concentration in correspondence to ITs’ génération areas and to plateau sites where barotropic tides produce large friction reaching the surface. The adoption of measured vertical diffusivities explains the biogéochemical tracers’ transformation within the Halmahera Sea and used to estimate the nutrients’ turbulent flux, with an associated increase in new production of ~25% of the total and a growth in mean chlorophyll of ~30%. Hence, we confirm the key role of ITs in shaping vertical distribution and variability of chlorophyll as well as nutrients in the maritime continent.

Abstract: Abstract. In this study, we have compared the ocean heat content (OHC), estimated using two eddy-resolving hindcast simulations based on Ocean General Circulation Model for the Earth Simulator version 1 (OFES1) and version 2 (OFES2). Results from a global objective analysis of subsurface temperature (EN4) were taken as a reference. Both EN4 and OFES1 suggest that OHC has increased in most regions of the top 2000 m during 1960–2016, which is mainly associated with the deepening of neutral density surfaces and variations along the neutral density surfaces of regional importance. Upon comparing the results obtained from the two OFES hindcasts, we found substantial differences in the temporal and spatial distributions of the OHC, especially in the Atlantic Ocean. A basin-wide heat budget analysis showed that there was less surface heating for the major basins in OFES2. The horizontal heat advection was mostly similar; however, OFES2 had a significantly stronger meridional heat advection associated with the Indonesian Throughflow (ITF) above 300 m. Additionally, large discrepancies in the vertical heat advection were also evinced when the two OFES results were compared, especially at a depth of 300 m in the Indian Ocean. We inferred that there are large discrepancies in the vertical heat diffusion (those that cannot be directly evaluated in this study due to data unavailability), which, along with the different magnitudes of sea surface heat flux and vertical heat advection, were the major factors responsible for the examined differences in OHC. This work suggests that OFES1 provides a reasonable multi-decadal estimate of global and basin-integrated warming trends above 700 m, except for the top 300 m for the Pacific Ocean and between 300–700 m for the Indian Ocean. Although the estimates of the global OHC during 1960–2016 are consistent with observations between 700–2000 m, caution is warranted while examining the basin-wide multi-decadal OHC variations using OFES1. The seemingly suboptimal OHC estimate based on OFES2 suggests that any conclusions on long-term climate variations derived from OFES2 might suffer from large drifts, necessitating audits.

Abstract: The meridional distribution of the flow parameters inside the centrifugal compressor is of great importance to its overall performance, as well as its matching performance under a thermal cycle. A time-marching throughflow method for the off-design performance analysis of the centrifugal compressor is described. The method is based on the strictly conservative throughflow-governing equations, and an improved method of boundary-condition enforcement is developed based on Newton’s method to achieve a robust and fast throughflow simulation. An inviscid blade force model was adopted to obtain the flow deflection inside the blade passage. Empirical loss models were integrated into the throughflow model to simulate the viscous force effects in the real three-dimensional flow. Two test cases are presented to validate the throughflow method by comparisons with the experimental data or CFD results, including the NASA low-speed centrifugal compressor (LSCC) and the Allison high-performance centrifugal compressor (HPCC). The simulation indicated that the developed enforcement method for the inlet and outlet boundary conditions significantly improves the computational robustness. For both the LSCC and HPCC cases, reasonable flow-parameter distribution was obtained and accurate overall characteristics were also predicted under the off-design conditions. The results indicated that the developed time-marching throughflow method is effective and efficient for the performance analysis of centrifugal compressors.

Abstract: Different types of rotating disk boundary layers were previously shown to be absolutely unstable. The present study checks if this is also true for the flow between narrowly spaced co-rotating disks with merged boundary layers and imposed throughflow. The described method is able to reproduce the absolute instability of the von Karman boundary layer and of radially outward flow between co-rotating disks with separated boundary layers. Nevertheless, no absolute instability is found for the case of narrow disk spacing with merged boundary layers for both radially inward and outward flow. The data does however show convective instability. Stability maps are provided for the analysed parameter space.

Abstract: The flow and heat transfer within rotating cavities is often discussed as a conjugate problem: the temperature distribution within the cavity disks drives the large-scale flow structure within the cavity, and the cavity aerodynamics influence the heat transfer to the disks. However, most simulations of rotating cavities only consider the fluid domain in isolation. This is particularly true for turbulence resolving approaches such as large eddy simulation (LES). The large timescale disparity between the fluid time steps used in LES and the characteristic solid time-scale complicates the use of LES with conjugate heat transfer (CHT). A further issue is that an under-resolved solid mesh artificially amplifies higher frequency temperature fluctuations from the fluid. This paper addresses these challenges with a new method for LES-CHT where the low-frequency temperature fluctuation caused by the large-scale flow structure is accounted for using a multi-scale frequency domain approach. We investigate two cases: axially heated disks made of a low conductivity material, and disks made from a higher conductivity material with a temperature set by radial conduction from the shroud. The formation of small-scale flow structures on both the disk and shroud is dependent on the heating configuration of the cavity - indicating that high-fidelity thermal boundary conditions should be used when simulating rotating cavities. The formation of heating induced vortical flow structures near the disk is particularly interesting, as this is unexpected from the laminar Ekman layer modelling argument usually used to consider this region.

Abstract: In this study, the start of thermosolutal penetrative convection in a horizontal fluid layer heated and salted from below was investigated analytically and graphically in the presence of throughflow and changeable gravity field effects. The normal mode technique is used to test the system's stability properties. The mathematical expressions for stationary and oscillatory Rayleigh numbers are obtained as a function of the governing parameters. The analysis demonstrates that the vertical throughflow, gravity parameter, and heat source effect all have a considerable influence on the beginning of convective motion in double‐diffusive flow. The impacts of variable gravity, vertical throughflow, and heat source parameters, as well as other physical parameters such as Solutal Rayleigh numbers and Lewis numbers, on stationary and oscillatory convection, are studied and illustrated graphically, and some previously published results are recovered in the limiting cases.

Abstract: A regional ocean model simulation for the period from 2000 through 2016 is used to study oceanic wave propagation in the Indonesian seas. The model simulation compares favorably with the interannual variability of the satellite derived sea level anomaly and the observed currents in the entrance passages of ITF. Lag correlation analysis suggests that the interannual sea level anomaly and opposite responses of the velocity above and below the thermocline in the western (the Sulawesi Sea and the Makassar Strait) and eastern (the Halmahera Sea) pathways of ITF are associated with the ENSO induced Rossby waves from the equatorial Pacific rather than the local wind forcing. In particular, during La Niña years ENSO induced anomalies traveling through the western pathway are comparable to those through the traditional eastern waveguide, suggesting the importance of both pathways in the propagation of ENSO signals in the Indonesian seas. Different roles of the western and eastern pathways in transmitting the ENSO signals into the Indonesian seas are associated with the nonlinear processes in the Sulawesi Sea, which appears to be linked with different states of the western boundary currents during the warm and cold ENSO phases.

Abstract: The South Equatorial Current (SEC) in the south Indian Ocean (SIO) contributes to mass and heat exchanges among the Pacific, Indian, and Atlantic Oceans. By analyzing satellite and in situ observations, this study examines the seasonal structure and the interannual variability of the SEC. The SEC is mainly part of the Seychelles-Chagos thermocline ridge (SCTR) circulation during December to April, and is composed of the internal SCTR circulation and the intrusive Indonesian Throughflow during May to November. The SEC thus presents straight and meandering routes in these two periods, respectively. Furthermore, observations and model experiments show that the SEC's meandering route has obvious interannual variability, with exceptional northwestward extension in 1994, 1997, and 2019 during the past three decades. These exceptional routes are caused primarily by extreme positive Indian Ocean Dipole (IOD) events, which induce a strengthened westward transport of the fresh water. These exceptional routes thus reflect the abnormal freshening in the SIO. Mooring observations reveal that a significant long-term freshening event occurred in the SIO from August 2019 to at least July 2020. The persistent low salinity confirms that there is substantial water exchange in the SIO during the extreme positive IOD years.

Abstract: The present research examines the joint influence of throughflow and Coriolis force on the onset of double-diffusive convection with an internal heat source modelled by Darcy’s law. The BVP4C routine in MATLAB R2020a is used to solve the eigenvalue problem numerically. Critical Rayleigh numbers are obtained for designated values of governing parameters. The effect of the internal heat source parameter, Taylor number, Darcy number, and Peclet number on the system’s stability is investigated. The internal heat source parameter has a destabilizing influence on the system, according to our findings. The reason behind this observation is that the presence of an internal heat source in the porous medium may cause more molecular diffusion inside the medium. The Taylor number, on the other hand, stabilizes the system for both upward and downward throughflow because rotation introduces vorticity into the fluid. Thus, the fluid moves with higher velocity in horizontal planes. The velocity of the fluid perpendicular to the planes reduces as a result of this motion. Thus, the onset of convection is delayed.

Abstract: A fast computational method of the aerodynamic characteristics of fan and booster with inlet distortion is developed in this paper. This method is based on a time-marching throughflow model, and the governing equations are circumferentially averaged Navier-Stokes equations. A model of distributed inviscid blade force and viscid blade force is adopted to reproduce the flow deflection and loss. The deviation angle and loss parameters are interpolated from a database which is extracted from 3-D simulation results of compressor with uniform inlet. After a validation case is performed, the aerodynamical performances of a multistage fan and booster with inlet radial and circumferential distortion are computed. The results show the predictive ability of this method, and the acceptable computational time cost indicates the future application potential of this method during the design stage of a new turbomachinery.

Abstract: Purpose Novel aircraft propulsion configurations require a greater integration of the propulsive system with the airframe. As a consequence of the closer integration of the propulsive system, higher levels of flow distortion at the fan face are expected. This distortion will propagate through the fan and penalize the system performance. This will also modify the exhaust design requirements. This paper aims to propose a methodology for the aerodynamic optimization of the exhaust for novel embedded propulsive systems. To model the distortion transfer, a low order throughflow fan model is included. Design/methodology/approach As the case study a 2D axisymmetric aft-mounted annular boundary layer ingestion (BLI) propulsor is used. An automated computational fluid dynamics approach is applied with a parametric definition of the design space. A throughflow body force model for the fan is implemented and validated for 2D axisymmetric and 3D flows. A multi-objective optimization based on evolutionary algorithms is used for the exhaust design. Findings By the application of the optimization methodology, a maximum benefit of approximately 0.32% of the total aircraft required thrust was observed by the application of compact exhaust designs. Furthermore, for the embedded system, it is observed that the design of the compact exhaust and the nacelle afterbody have a considerable impact on the aerodynamic performance. Originality/value This paper presents a novel approach for the exhaust design of embedded propulsive systems in novel aircraft configurations. To the best of the authors’ knowledge, this is the first detailed optimization of the exhaust system on an annular aft-mounted BLI propulsor.