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The Agulhas Current (AC) is a critical component of global ocean circulation. However, due to a lack of multidecadal observations, it is not clear how the AC has changed in response to anthropogenic forcing. A recent observational study suggests a broadening and slight weakening of the AC in the past few decades, while others suggest a strengthening of the AC during the historical period. In this paper, we find substantial internal variability of the AC on decadal to multidecadal time scales in high-resolution models. We show that the AC consistently exhibits two modes of decadal and multidecadal (i.e., low frequency) variability in a series of high-resolution climate models: a uniform mode that is largely associated with changes in the AC strength and a dipole mode that is mainly related to width changes of the AC. We demonstrate that the uniform mode is mainly forced externally by the decadal variations of the wind field and presents a decline under global warming, suggesting a weakening of the AC in response to anthropogenic forcing. The dipole mode, on the other hand, is mainly due to internal dynamics and does not show a trend during the historical period. Using a quasigeostrophic model that captures the dipole mode, we attribute the dipole mode to low-frequency potential vorticity changes in the western boundary, driven by a divergence of relative potential vorticity due to eddy activity. Thus, our results present further context for the interpretation of the AC responses in a changing climate based on a short observational record.

Category-level 6D object pose tracking is very challenging in the field of 3D computer vision. Keypoint-based object pose estimation has demonstrated its effectiveness in dealing with it. However, current approaches first estimate the keypoints through a neural network and further compute the inter-frame pose change via least-squares optimization. They estimate rotation and translation in the same way, ignoring the differences between them. In this work, we propose a keypoint-based disentangled pose network (KDPNet), which disentangles the 6D object pose change to 3D rotation and 3D translation. Specifically, the translation is directly estimated by the network and the rotation is indirectly calculated by Singular Value Decomposition (SVD) according to the keypoints. Extensive experiments on NOCS-REAL275 dataset demonstrate the superiority of our method.

The long-term response of the Atlantic meridional overturning circulation (AMOC) to climate change remains poorly understood, in part due to the computational expense associated with running atmosphere–ocean general circulation models (GCMs) to equilibrium. Here, we use a collection of millennial-length GCM simulations to examine the transient and equilibrium responses of the AMOC to an abrupt quadrupling of atmospheric carbon dioxide. We find that GCMs consistently simulate an AMOC weakening during the first century but exhibit diverse behaviors over longer time scales, showing different recovery levels. To explain the AMOC behavior, we use a thermal-wind expression, which links the overturning circulation to the meridional density difference between deep-water formation regions and the Atlantic basin. Using this expression, we attribute the evolution of the AMOC on different time scales to changes in temperature and salinity in distinct regions. The initial AMOC shoaling and weakening occurs on centennial time scales and is attributed to a warming of the deep-water formation region. A partial recovery of the AMOC occurs over the next few centuries, and is linked to a simultaneous warming of the Atlantic basin and a positive high-latitude salinity anomaly. The latter reduces the subsurface stratification and reinvigorates deep-water formation. GCMs that exhibit a prolonged AMOC weakening tend to have smaller high-latitude salinity anomalies and increased Arctic sea ice loss. After multiple millennia, the AMOC in some GCMs is stronger than the initial state due to warming of the low-latitude Atlantic. These results highlight the importance of considering high-latitude freshwater changes when examining the past and future evolution of the AMOC evolution on long time scales. Significance Statement The long-term response of the ocean’s global overturning circulation to warming remains poorly understood largely because it is expensive to run state-of-the-art climate models. This study makes use of a unique collection of millennial-length climate simulations from different climate models to examine the response of the Atlantic overturning circulation to warming on long time scales. We find that climate models consistently simulate a weakening of the Atlantic overturning circulation during the first century after warming, but disagree on long-term changes, showing different recovery levels of the Atlantic overturning circulation. Using a simple expression, which emulates the evolution of the Atlantic overturning circulation in climate models, we show that climate models with little to no recovery tend to have a small North Atlantic salinity anomaly while climate models with a stronger recovery tend to have a large North Atlantic salinity anomaly. These results highlight the importance of monitoring high-latitude freshwater sources throughout the twenty-first century and considering the relative role of temperature and salinity changes when examining the future and past evolution of the Atlantic overturning circulation on long time scales.

The reorganization of the Atlantic meridional overturning circulation (AMOC) is often associated with changes in Earth’s climate. These AMOC changes are communicated to the Indo-Pacific basins via wave processes and induce an overturning circulation anomaly that opposes the Atlantic changes on decadal to centennial time scales. We examine the role of this transient, interbasin overturning response, driven by an AMOC weakening, both in an ocean-only model with idealized geometry and in a coupled CO2 quadrupling experiment, in which the ocean warms on two distinct time scales: a fast decadal surface warming and a slow centennial subsurface warming. We show that the transient interbasin overturning produces a zonal heat redistribution between the Atlantic and Indo-Pacific basins. Following a weakened AMOC, an anomalous northward heat transport emerges in the Indo-Pacific, which substantially compensates for the Atlantic southward heat transport anomaly. This zonal heat redistribution manifests as a thermal interbasin seesaw between the high-latitude North Atlantic and the subsurface Indo-Pacific and helps to explain why Antarctic temperature records generally show more gradual changes than the Northern Hemisphere during the last glacial period. In the coupled CO2 quadrupling experiment, we find that the interbasin heat transport due to a weakened AMOC contributes substantially to the slow centennial subsurface warming in the Indo-Pacific, accounting for more than half of the heat content increase and sea level rise. Thus, our results suggest that the transient interbasin overturning circulation is a key component of the global ocean heat budget in a changing climate.

Most category-level object pose estimation methods are multi-tasking, including instance segmentation, Normalized Object Coordinate Space (NOCS) map estimation and classification. However, previous approaches overlooked the connection between multiple tasks. In this work, we propose an efficient network to make better use of the complementarity between different tasks. Specifically, we propose an external sharing unit (ESU) to promote instance segmentation and NOCS map estimation. In addition, we propose an internal sharing unit (ISU) to improve the NOCS map estimation. The NOCS map head has three branches. And the estimated coordinates of each branch have strong correlation. Extensive experiments on the CAMERA and REAL dataset demonstrate the effectiveness of joint optimization in multi-tasking category-level object estimation. Experimental results also show that the proposed method can improve not only accuracy but also efficiency on several benchmarks.

Depth map plays an important role in our daily life including automatic driving, intelligent robots, and commercial manufacture. However, existing depth estimation methods are still facing some difficulties in real-life applications. Firstly, using merely point-wise metrics during training usually causes surface distortion due to the lack of geometric restrictions. To tackle this problem, we introduce a normal restriction module to the network to improve the method performance with regards to surface integrity. Secondly, although 3D methods usually achieve higher accuracy by directly concatenating features as cost aggregation, they sacrifice the time and memory efficiency. To make the disparity estimation network light enough for adding the normal restriction, we propose a 2D aggregation method to replace the 3D ones. Experiments prove the effectiveness of our method and we show that our results are competitive to 3D methods under popular benchmarks including KITTI, SceneFlow and Midddlebury.

Recent research offers new insights on Antarctic sea ice, which, despite global warming, has increased in overall extent over the past 40 years.

The subpolar gyres of the Southern Ocean form an important dynamical link between the Antarctic Circumpolar Current (ACC) and the coastline of Antarctica. Despite their key involvement in the production and export of bottom water and the poleward transport of oceanic heat, these gyres are rarely acknowledged in conceptual models of the Southern Ocean circulation, which tend to focus on the zonally averaged overturning across the ACC. To isolate the effect of these gyres on the regional circulation, we carried out a set of numerical simulations with idealized representations of the Weddell Sea sector in the Southern Ocean. A key result is that the zonally oriented submarine ridge along the northern periphery of the subpolar gyre plays a fundamental role in setting the stratification and circulation across the entire region. In addition to sharpening and strengthening the horizontal circulation of the gyre, the zonal ridge establishes a strong meridional density front that separates the weakly stratified subpolar gyre from the more stratified circumpolar flow. Critically, the formation of this front shifts the latitudinal outcrop position of certain deep isopycnals such that they experience different buoyancy forcing at the surface. Additionally, the zonal ridge modifies the mechanisms by which heat is transported poleward by the ocean, favoring heat transport by transient eddies while suppressing that by stationary eddies. This study highlights the need to characterize how bathymetry at the subpolar gyre–ACC boundary may constrain the transient response of the regional circulation to changes in surface forcing. Significance Statement This study explores the impact of seafloor bathymetry on the dynamics of subpolar gyres in the Southern Ocean. The subpolar gyres are major circulation features that connect the Antarctic Circumpolar Current (ACC) and the coastline of Antarctica. This work provides deeper insight for how the submarine ridges that exist along the northern periphery of these gyres shape the vertical distribution of tracers and overturning circulation in these regions. These findings highlight an underappreciated yet fundamentally important topographical constraint on the three-dimensional cycling of heat and carbon in the Southern Ocean—processes that have far-reaching implications for the global climate. Future work should explore how the presence of these ridges affect the time-evolving response of the Southern Ocean to changes in surface conditions.

Interactions between the open ocean and marginal seas have been suggested to be critical to the redistribution and dissipation of global energy. Here, we propose a mechanism for the upper open ocean influencing the deep flow in marginal seas that hinges on the formation and propagation of topographic Rossby waves (TRWs). Observations and high-resolution simulations suggest substantial intraseasonal variability with periods of 5–60 days associated with the deep flow over continental slopes of the northern South China Sea (NSCS). These fluctuations generally account for over 40% of the total deep kinetic energy variability and the number can reach 70% over the slopes to the west of the Luzon Strait, southwest of the Dongsha Islands, and northeast of the Xisha Islands. By examining the spatiotemporal features of the fluctuations, we demonstrate that the intraseasonal variability of the deep flow in the NSCS is closely associated with TRWs. Using a recently developed multiscale energetics analysis in combination with a ray tracing model, we show that the energy sources of TRWs can be traced back to the east of the Dongsha Islands, where the Kuroshio intrusion and related eddies energize the TRWs primarily through pressure work. These waves propagate westward across the NSCS and drive the intraseasonal variability of the deep flow over continental slopes. Our findings highlight an energy pathway from the open ocean western boundary current to the abyssal marginal sea that could modulate regional circulation as well as exchanges between major ocean basins.

The long-term response of the Atlantic meridional overturning circulation (AMOC) to anthropogenic climate change remains poorly understood in part, due to the computational expenses associated with running fully-coupled climate models to equilibrium. Here, we use a collection of millennial-length simulations from multiple state-of-the-art climate models to examine the transient and equilibrium responses of the AMOC to an abrupt quadrupling of atmospheric carbon-dioxide. All climate models exhibit a weakening of the AMOC on centennial timescales, but they disagree on the recovery of the AMOC over next millennia, despite the same greenhouse-gas forcing. In some models, the AMOC recovers after approximately 200 years, while in others the AMOC does not fully recover even after approximately 1000 years. To explain the behavior of the AMOC we relate the overturning circulation in the North Atlantic to the meridional density difference between the basin interior and the region of deep-water formation. This scaling both reproduces the initial decline and gradual recovery of the AMOC, and explains the inter-model spread of the AMOC responses. The initial shoaling and weakening occurs on centennial timescales and is attributed to the warming of the northern convection region. We argue that the AMOC weakens on a timescale linked to a combination of its initial depth and the global surface heat flux sensitivity. The recovery of the AMOC results from a pile-up of salinity in the Atlantic basin, when the AMOC is weakened, that propagates northward and reinvigorates convection. A weaker AMOC recovery is associated with a smaller salinity anomaly. We further show through surface water mass transformation that Southern Ocean processes may impact the salinity anomaly in the Atlantic basin. These results highlight the importance of considering the evolution of the AMOC and ocean heat transport beyond the 21st century as short-term changes are not indicative of long-term changes.

Climate models consistently project a robust weakening of the Indonesian Throughflow (ITF) and the Atlantic meridional overturning circulation (AMOC) in response to greenhouse gas forcing. Previous studies of ITF variability have largely focused on local processes in the Indo‐Pacific Basin. Here, we propose that much of the centennial‐scale ITF weakening is dynamically linked to changes in the Atlantic Basin and communicated between basins via wave processes. In response to an AMOC slowdown, the Indian Ocean develops a northward surface transport anomaly that converges mass and modifies sea surface height in the Indian Ocean, which weakens the ITF. We illustrate these dynamic interbasin connections using a 1.5‐layer reduced gravity model and then validate the responses in a comprehensive general circulation model. Our results highlight the importance of transient volume exchanges between the Atlantic and Indo‐Pacific basins in regulating the global ocean circulation in a changing climate.

Author(s): Sun, Shantong; Thompson, Andrew F; Eisenman, Ian | Abstract: AbstractClimate models consistently project (i) a decline in the formation of North Atlantic Deep Water (NADW) and (ii) a strengthening of the Southern Hemisphere westerly winds in response to anthropogenic greenhouse gas forcing. These two processes suggest potentially conflicting tendencies of the Atlantic meridional overturning circulation (AMOC): a weakening AMOC due to changes in the North Atlantic but a strengthening AMOC due to changes in the Southern Ocean. Here we focus on the transient evolution of the global ocean overturning circulation in response to a perturbation to the NADW formation rate. We propose that the adjustment of the Indo-Pacific overturning circulation is a critical component in mediating AMOC changes. Using a hierarchy of ocean and climate models, we show that the Indo-Pacific overturning circulation provides the first response to AMOC changes through wave processes, whereas the Southern Ocean overturning circulation responds on longer (centennial to millennial) time scales that are determined by eddy diffusion processes. Changes in the Indo-Pacific overturning circulation compensate AMOC changes, which allows the Southern Ocean overturning circulation to evolve independently of the AMOC, at least over time scales up to many decades. In a warming climate, the Indo-Pacific develops an overturning circulation anomaly associated with the weakening AMOC that is characterized by a northward transport close to the surface and a southward transport in the deep ocean, which could effectively redistribute heat between the basins. Our results highlight the importance of interbasin exchange in the response of the global ocean overturning circulation to a changing climate.

The sensitivity of the Antarctic Circumpolar Current (ACC) transport to surface buoyancy conditions in the North Atlantic is investigated using a sector configuration of an ocean general circulation model. We find that the sensitivity of the ACC transport is significantly weaker than previous studies. We attribute this difference to the different depth of the simulated Atlantic Meridional Overturning Circulation. Because a fast restoring buoyancy boundary condition is used that strongly constrains the surface buoyancy structure at the Southern Ocean surface, the ACC transport is determined by the isopycnal slope that is coupled to the overturning circulation in the Southern Ocean. By changing the surface buoyancy in the North Atlantic, the shared buoyancy contour between the North Atlantic and the Southern Ocean is varied, and consequently the strength of the overturning circulation is modified. For different depth of the simulated overturning circulation, the response of the ACC transport to changes in the strength of the overturning circulation varies substantially. This is illustrated in two conceptual models based on the residual-mean theory of overturning circulation. Our results imply that the sensitivity of the ACC transport to surface forcing in the North Atlantic could vary substantially in different models depending on the simulated vertical structure of the overturning circulation.

The role of the South China Sea (SCS) in regulating the North Pacific circulation is investigated using a two-layer quasigeostrophic (QG) model. The double-gyre circulations in the North Pacific with and without the SCS are compared and analyzed. It is found that the SCS acts as a sink of both potential vorticity (PV) and energy in the Pacific–SCS system for the mean state. Consequently, the Kuroshio and the upstream Kuroshio Extension (KE) are weaker in the presence of the SCS. Moreover, the eddy activity is also lower in the North Pacific Ocean because the barotropic instability is suppressed for a weaker circulation. In terms of low-frequency variations at interannual to decadal time scale, the presence of the SCS is found to enhance the variability of the latitudinal position and intensity of the KE jet. This is explained by a positive feedback process that is associated with the negative correlation between the inertia of the Kuroshio and its intrusion into the SCS.

The response of the South China Sea (SCS) circulation to intraseasonal variability of the summer monsoon is studied with both observations and a 1.5-layer reduced-gravity model. Intraseasonal variability of the SCS summer monsoon is characterized by evolution of the wind jet intensity in the midbasin with typical amplitude of 6 m s−1 and several peaks on its power spectrum between 10 and 60 days. However, this study finds that intraseasonal variability of the sea surface height (SSH) in the SCS presents significant variability to the southeast of Vietnam with amplitude of 6 cm and a period only between 40 and 60 days. This implicates the frequency selectivity of oceanic response to wind forcing. Numerical experiments suggest that the intrinsic variability of the SCS circulation accounts for this phenomenon. Based on the Rossby basin mode theory, this is explained by the interaction between the long, westward-propagating Rossby waves and the short, eastward-propagating Rossby waves.

Depth estimation has widely demands in autopilot and scene reconstruction. Although depth estimation has been greatly improved by deep learning, there is still some room for improvement. Usually, stereo-matching-based depth estimation matches low-resolution features and then up-samples depth map to full-resolution. Such methods suffer from low accuracy because of information loss in low-resolution features. To solve this problem, a depth estimation method via multi-resolution gradual-refining stereo matching is proposed. As with classic methods, this method first extracts pyramid features by convolution network and estimates low-resolution depth maps. Then, as an innovation, stereo-matching at each pyramid feature is successively executed using the low-resolution maps as initial depth, which limits search range of stereo-matching for accuracy and efficiency. Results of stereo-matching are used as residuals to refine the depth maps. Experimental results demonstrate that accuracy of depth estimation by the proposed method is significantly improved, but computational complexity does not much increase compared with classic methods.

A high precision method for position and attitude determination is presented. One of the difficulties in pose determination is the lack of reliable visual features. As a result, we exploit a novel convolutional neural network for stereo matching, so as to solve the problem of image feature extraction. The network is based on a siamese network and incorporates dense block into each branch. There is occlusion problem in the close-range phase of measuring object. In order to solve the occlusion, a multi-view system is established. The measurement results meet the accuracy requirements of pose determination and verify the feasibility of the method.

Author(s): Sun, S; Eisenman, I; Stewart, AL | Abstract: Paleoclimate proxy data suggest that the Atlantic Meridional Overturning Circulation (AMOC) was shallower at the Last Glacial Maximum (LGM) than its preindustrial (PI) depth. Previous studies have suggested that this shoaling necessarily accompanies Antarctic sea ice expansion at the LGM. Here the influence of Southern Ocean surface forcing on the AMOC depth is investigated using ocean-only simulations from a state-of-the-art climate model with surface forcing specified from the output of previous coupled PI and LGM simulations. In contrast to previous expectations, we find that applying LGM surface forcing in the Southern Ocean and PI surface forcing elsewhere causes the AMOC to shoal only about half as much as when LGM surface forcing is applied globally. We show that this occurs because diapycnal mixing renders the Southern Ocean overturning circulation more diabatic than previously assumed, which diminishes the influence of Southern Ocean surface buoyancy forcing on the depth of the AMOC.

In this paper, seasonal variation of the South Equatorial Current (SEC) bifurcation off the Madagascar coast in the upper south Indian Ocean (SIO) is investigated based on a new climatology derived from the World Ocean Database and 19-year satellite altimeter observations. The mean bifurcation integrated over the upper thermocline is around 18°S and reaches the southernmost position in June/July and the northernmost position in November/December, with a north–south amplitude of about 1°. It is demonstrated that the linear, reduced gravity, long Rossby model, which works well for the North Equatorial Current (NEC) bifurcation in the North Pacific, is insufficient to reproduce the seasonal cycle and the mean position of the SEC bifurcation off the Madagascar coast. This suggests the importance of Madagascar in regulating the SEC bifurcation. Application of Godfrey’s island rule reveals that compared to the zero Sverdrup transport latitude, the mean SEC bifurcation is shifted poleward by over 0.8° because of the meridional transport of about 5 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) between Madagascar and Australia. A time-dependent linear model that extends the Godfrey’s island rule is adopted to examine the seasonal variation of the SEC bifurcation. This time-dependent island rule model simulates the seasonal SEC bifurcation well both in terms of its mean position and peak seasons. It provides a dynamic framework to clarify the baroclinic adjustment processes involved in the presence of an island.

Previous observation and model studies show that the upper-ocean stratification is enhanced under global warming (Capotondi et al.; Cravatte et al.; Deser et al., etc.). The response of the recirculation, which is associated with the western boundary current (WBC) jet extension and significantly increases its transport, to the intensified stratification, is studied in a two-layer quasigeostrophic ocean circulation model. It is found that the barotropic transport of the circulation first increases with stratification but then decreases as a result of saturation of the surface-layer circulation intensity when the stratification exceeds a threshold. PV budget analysis indicates that the saturation is caused by the increased intergyre transport of relative potential vorticity resulting from the intensified variability of the jet location. Both the barotropic instability and bifurcation mechanisms contribute to the intensified variability of the jet location. Because of barotropic instability, eddies are generated in the confluence region of the WBCs and advected eastward, causing the variability of the jet location. With increased stratification, the surface-layer circulation is strengthened and the barotropic instability is intensified. As a result, the surface flow becomes more variable with excessive eddies and intense variability of the jet. With the increasing stratification, three regimes, each marked by its own variation of the jet location, emerge owing to the successive system bifurcations. In the last two regimes, variability of the jet location is further enhanced by frequent switches among the different dynamic states on multidecadal time scales.

©2016. American Geophysical Union. All Rights Reserved. Previous studies have suggested that the global ocean density stratification below ∼3000 m is approximately set by its direct connection to the Southern Ocean surface density, which in turn is constrained by the atmosphere. Here the role of Southern Ocean surface forcing in glacial-interglacial stratification changes is investigated using a comprehensive climate model and an idealized conceptual model. Southern Ocean surface forcing is found to control the global deep ocean stratification up to ∼2000 m, which is much shallower than previously thought and contrary to the expectation that the North Atlantic surface forcing should strongly influence the ocean at intermediate depths. We show that this is due to the approximately fixed surface freshwater fluxes, rather than a fixed surface density distribution in the Southern Ocean as was previously assumed. These results suggest that Southern Ocean surface freshwater forcing controls glacial-interglacial stratification changes in much of the deep ocean.