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Although the westerly winds that drive the Antarctic Circumpolar Current (ACC) have increased over the past several decades, the ACC response remains an open question. Here we use a 15-year time series of concurrent upper-ocean temperature, salinity, and ocean velocity with high spatial resolution across Drake Passage to analyze whether the net Drake Passage transport has accelerated in the last 15 years. We find that, although the net Drake Passage transport relative to 760 m shows insignificant acceleration, the net transport trend comprises compensating trends across the ACC frontal regions. Our results show an increase in the mesoscale eddy activity between the fronts consistent with buoyancy changes in the fronts and with an eddy saturation state. Furthermore, the increased eddy activity may play a role in redistributing momentum across the ACC frontal regions. The increase in eddy activity is expected to intensify the eddy-driven upwelling of deep warm waters around Antarctica, which has significant implications for ice-melting, sea level rise, and global climate.

 

The Makassar Strait throughflow (MST) constitutes a significant component of the Indonesian throughflow (ITF) and plays a pivotal role in the interbasin exchange between the Indian and Pacific Oceans. While previous studies have suggested that the buoyancy forcing plays a role in influencing the seasonality of the MST, the quantitative contribution of salinity effect on MST seasonality remains unclear. Here we use the measurements from the Monitoring ITF program and the Global Ocean Physics Reanalysis product to investigate the seasonality of MST and quantify the impact of the salinity effect. We find that the halosteric variability due to the salinity effect contributes to approximately (69.6 ± 11.7) % of the total seasonal variability of surface dynamic height gradient along the Makassar Strait, and dominates the seasonality of the upper layer MST. The primary drivers for freshwater forcing are horizontal advection through the Karimata Strait and precipitation in the Java Sea.

 

Abstract Since the inception of the international South Atlantic Meridional Overturning Circulation initiative in the 21st century, substantial advances have been made in observing and understanding the Southern Hemisphere component of the Atlantic Meridional Overturning Circulation (AMOC). Here we synthesize insights gained into overturning flows, interocean exchanges, and water mass distributions and pathways in the South Atlantic. The overturning circulation in the South Atlantic uniquely carries heat equatorward and exports freshwater poleward and consists of two strong overturning cells. Density and pressure gradients, winds, eddies, boundary currents, and interocean exchanges create an energetic circulation in the subtropical and tropical South Atlantic Ocean. The relative importance of these drivers varies with the observed latitude and time scale. AMOC, interocean exchanges, and climate changes drive ocean warming at all depths, upper ocean salinification, and freshening in the deep and abyssal ocean in the South Atlantic. Long-term sustained observations are critical to detect and understand these changes and their impacts. 

Turbulence-enhanced mixing of upper ocean heat allows interaction between the tropical atmosphere and cold water masses that impact climate at higher latitudes thereby regulating air–sea coupling and poleward heat transport. Tropical cyclones (TCs) can drastically enhance upper ocean mixing and generate powerful near-inertial internal waves (NIWs) that propagate down into the deep ocean. Globally, downward mixing of heat during TC passage causes warming in the seasonal thermocline and pumps 0.15 to 0.6 PW of heat into the unventilated ocean. The final distribution of excess heat contributed by TCs is needed to understand subsequent consequences for climate; however, it is not well constrained by current observations. Notably, whether or not excess heat supplied by TCs penetrates deep enough to be kept in the ocean beyond the winter season is a matter of debate. Here, we show that NIWs generated by TCs drive thermocline mixing weeks after TC passage and thus greatly deepen the extent of downward heat transfer induced by TCs. Microstructure measurements of the turbulent diffusivity ( κ ) and turbulent heat flux ( J q ) in the Western Pacific before and after the passage of three TCs indicate that mean thermocline values of κ and J q increased by factors of 2 to 7 and 2 to 4 (95% confidence level), respectively, after TC passage. Excess mixing is shown to be associated with the vertical shear of NIWs, demonstrating that studies of TC–climate interactions ought to represent NIWs and their mixing to accurately capture TC effects on background ocean stratification and climate. 

Multidecadal variability of the Indonesian Throughflow (ITF) is crucial for the Indo-Pacific and global climate due to significant interbasin exchanges of heat and freshwater. Previous studies suggest that both wind and buoyancy forcing may drive ITF variability, but the role of precipitation and salinity effect in the variability of ITF on multidecadal time scales remains largely unexplored. Here, we investigate the multidecadal changes and long-term trend of the ITF transport during the past six decades, with a focus on the role of precipitation and salinity effect. The diverse datasets consistently indicate a substantial upward trend in the halosteric component of geostrophic transport of ITF in the outflow region at 114°E during the six decades. We find that the meridional differences of the salinity trend in the outflow region explain the increasing trend of the halosteric component of ITF transport. On a larger scale, the tropical western Pacific Ocean and Indonesian seas have experienced significant freshening, which has strengthened the Indo-Pacific pressure gradient and thus enhanced the ITF. In contrast, the equatorial trade wind in the western Pacific Ocean has weakened over recent decades, implying that changes in wind forcing have contributed to weakening the ITF. The combined effect of strengthened halosteric and weakened thermosteric components has resulted in a weak strengthening for the total ITF with large uncertainties. Although both the thermosteric and halosteric components are associated with natural climate modes, our results suggest that the importance of salinity effect is likely increasing given the enhanced water cycle under global warming.

 

Abstract Eddy heat flux plays a fundamental role in the Southern Ocean meridional overturning circulation, providing the only mechanism for poleward heat transport above the topography and below the Ekman layer at the latitudes of Drake Passage. Models and observations identify Drake Passage as one of a handful of hot spots in the Southern Ocean where eddy heat transport across the Antarctic Circumpolar Current (ACC) is enhanced. Quantifying this transport, however, together with its spatial distribution and temporal variability, remains an open question. This study quantifies eddy heat flux as a function of ACC streamlines using a unique 20-yr time series of upper-ocean temperature and velocity transects with unprecedented horizontal resolution. Eddy heat flux is calculated using both time-mean and time-varying streamlines to isolate the dynamically important across-ACC heat flux component. The time-varying streamlines provide the best estimate of the across-ACC component because they track the shifting and meandering of the ACC fronts. The depth-integrated (0–900 m) across-stream eddy heat flux is maximum poleward in the south flank of the Subantarctic Front (−0.10 ± 0.05 GW m −1 ) and decreases toward the south, becoming statistically insignificant in the Polar Front, indicating heat convergence south of the Subantarctic Front. The time series provides an uncommon opportunity to explore the seasonal cycle of eddy heat flux. Poleward eddy heat flux in the Polar Front Zone is enhanced during austral autumn–winter, suggesting a seasonal variation in eddy-driven upwelling and thus the meridional overturning circulation. 

Drake Passage is a key region for transport between the surface and interior ocean, but a mechanistic understanding of this exchange remains immature. Here, we present wintertime, submesoscale‐resolving hydrographic transects spanning the southern boundary of the Antarctic Circumpolar Current and the Polar Front (PF). Despite the strong surface wind and buoyancy forcing, a freshwater lens suppresses surface‐interior exchange south of the PF; ventilation is instead localized to the PF. Multiple lines of the analysis suggest submesoscale processes contribute to ventilation at the PF, including small‐scale, O(10 km), frontal structure in water mass properties below the mixed layer and modulation of a surface eddy diffusivity at sub‐50 km scales. These results show that ventilation is sensitive to both submesoscale properties near fronts and non‐local processes, for example, sea‐ice melt, that set stratification and mixed layer properties. This highlights the need for adaptive observing strategies to constrain Southern Ocean heat and carbon budgets.

 

Abstract The Propagation of Intraseasonal Tropical Oscillations (PISTON) experiment conducted a field campaign inAugust-October 2018. The R/V Thomas G. Thompson made two cruises in thewestern North Pacific region north of Palau and east of the Philippines. Using select field observations and global observational and reanalysis data sets, this study describes the large-scale state and evolution of the atmosphere and ocean during these cruises. Intraseasonal variability was weak during the field program, except for a period of suppressed convection in October. Tropical cyclone activity, on the other hand, was strong. Variability at the ship location was characterized by periods of low-level easterly atmospheric flow with embedded westward propagating synoptic-scale atmospheric disturbances, punctuated by periods of strong low-level westerly winds that were both connected to the Asian monsoon westerlies and associated with tropical cyclones. In the most dramatic case, westerlies persisted for days during and after tropical cyclone Jebi had passed to the north of the ship. In these periods, the sea surface temperature was reduced by a couple of degrees by both wind mixing and net surface heat fluxes that were strongly (~200 Wm −2 ) out of the ocean, due to both large latent heat flux and cloud shading associated with widespread deep convection. Underway conductivity-temperature transects showed dramatic cooling and deepening of the ocean mixed layer and erosion of the barrier layer after the passage of Typhoon Mangkhut due to entrainment of cooler water from below. Strong zonal currents observed over at least the upper 400 meters were likely related to the generation and propagation of near-inertial currents.