Winter Convective Mixing in the northern Arabian Sea during Contrasting Monsoons

Along-track Argo observations in the northern Arabian Sea during 2017 – 19 showed by far the most contrasting winter convective mixing; 2017 – 18 was characterized by less intense convective mixing resulting in a mixed layer depth of 110 m, while 2018 – 19 experienced strong and prolonged convective mixing with the mixed layer deepening to 150 m. The response of the mixed layer to contrasting atmospheric forcing and the associated formation of Arabian Sea High Salinity Water (ASHSW) in the northeastern Arabian Sea are studied using a combination of Argo float observations, gridded observations, a data assimilative general circulation model and a series of 1-D model simulations. The 1-D model experiments show that the response of winter mixed layer to atmospheric forcing is not only influenced by winter surface buoyancy loss, but also by a preconditioned response to freshwater fluxes and associated buoyancy gain by the ocean during the summer that is preceding the following winter. A shallower and short-lived winter mixed layer occurred during 2017 – 18 following the exceptionally high precipitation over evaporation during the summer monsoon in 2017. The precipitation induced salinity stratification (a salinity anomaly of -0.7 psu) during summer inhibited convective mixing in the following winter resulting in a shallow winter mixed layer (103 m). Combined with weak buoyancy loss due to weaker surface heat loss in the northeastern Arabian Sea, this caused an early termination of the convective mixing (February 26, 2018). In contrast, the winter convective mixing during 2018 – 19 was deeper (143 m) and long-lived. The 2018 summer, by comparison, was characterized by normal or below normal precipitation which generated a weakly stratified ocean pre-conditioned to winter mixing. This combined with colder and drier air from the land mass to the north with low specific humidity lead to strong buoyancy loss, and resulted in prolonged winter convective mixing through March 25, 2019.

Untangling the Mistral and seasonal atmospheric forcing driving deep convection in the Gulf of Lion: 2012–2013

Deep convection in the Gulf of Lion is believed to be primarily driven by the Mistral winds. However, our findings show that the seasonal atmospheric change provides roughly 2/3 of the buoyancy loss required for deep convection to occur, for the 2012 to 2013 year, with the Mistral supplying the final 1/3. Two NEMOMED12 ocean simulations of the Mediterranean Sea were run for the Aug. 1st, 2012 to July 31st, 2013 year, forced with two sets of atmospheric forcing data from a RegIPSL coupled run within the Med-CORDEX framework. One set of atmospheric forcing data was left unmodified, while the other was filtered to remove the signal of the Mistral. The Control simulation featured deep convection, while the Seasonal did not. A simple model was derived, relating the anomaly scale forcing (the difference between the Control and Seasonal runs) and the seasonal scale forcing to the ocean response through the Stratification Index. This simple model revealed that the Mistral's effect on buoyancy loss depends more on its strength rather than its frequency or duration. The simple model also revealed that the seasonal cycle of the Stratification Index is equal to the net surface heat flux over the course of the year, with the stratification maximum and minimum occurring roughly at the fall and spring equinoxes.

Distinct controls on the strength of the abyssal overturning circulation: channel versus basin dynamics

Although the reconfiguration of the abyssal overturning circulation has been argued to be a salient feature of Earth’s past climate changes, our understanding of the physical mechanisms controlling its strength remains limited. In particular, existing scaling theories disagree on the relative importance of the dynamics in the Southern Ocean versus the dynamics in the basins to the north. In this study, we systematically investigate these theories and compare them with a set of numerical simulations generated from an ocean general circulation model with idealized geometry, designed to capture only the basic ingredients considered by the theories. It is shown that the disagreement between existing theories can be partially explained by the fact that the overturning strengths measured in the channel and in the basin scale distinctly with the external parameters, including surface buoyancy loss, diapycnal diffusivity, wind stress, and eddy diffusivity. The overturning in the re-entrant channel, which represents the Southern Ocean, is found to be sensitive to all these parameters, in addition to a strong dependence on bottom topography. By contrast, the basin overturning varies with the integrated surface buoyancy loss rate and diapycnal diffusivity but is mostly unaffected by winds and channel topography. The simulated parameter dependence of the basin overturning can be described by a scaling theory that is based only on basin dynamics.

Why did deep convection persist over four consecutive winters (2015–2018) southeast of Cape Farewell?

After more than a decade of shallow convection, deep convection returned to the Irminger Sea in 2008 and occurred several times since then to reach exceptional convection depths (> 1500 m) in 2015 and 2016. Additionally, deep mixed layers deeper than 1600 m were also reported southeast of Cape Farewell in 2015. In this context, we used Argo data to show that deep convection occurred southeast of Cape Farewell (SECF) in 2016 and persisted during two additional years in 2017 and 2018 with a maximum convection depth deeper than 1300 m. In this article, we investigate the respective roles of air–sea buoyancy flux and preconditioning of the water column (ocean interior buoyancy content) to explain this 4-year persistence of deep convection SECF. We analyzed the respective contributions of the heat and freshwater components. Contrary to the very negative air–sea buoyancy flux that was observed during winter 2015, the buoyancy fluxes over the SECF region during the winters of 2016, 2017 and 2018 were close to the climatological average. We estimated the preconditioning of the water column as the buoyancy that needs to be removed (B) from the end-of-summer water column to homogenize it down to a given depth. B was lower for the winters of 2016–2018 than for the 2008–2015 winter mean, especially due to a vanishing stratification from 600 down to ∼1300 m. This means that less air–sea buoyancy loss was necessary to reach a given convection depth than in the mean, and once convection reached 600 m little additional buoyancy loss was needed to homogenize the water column down to 1300 m. We show that the decrease in B was due to the combined effects of the local cooling of the intermediate water (200–800 m) and the advection of a negative S anomaly in the 1200–1400 m layer. This favorable preconditioning permitted the very deep convection observed in 2016–2018 despite the atmospheric forcing being close to the climatological average.

Two superimposed cold and fresh anomalies enhanced Irminger Sea deep convection in 2016–2018

While Earth system models project a reduction, or even a shut-down, of deep convection in the North Atlantic Ocean in response to anthropogenic forcing, deep convection returned to the Irminger Sea in 2008 and occurred several times since then to reach exceptional depths > 1,500 m in 2015 and 2016. In this context, we used Argo data to show that deep convection persisted in the Irminger Sea during two additional years in 2017 and 2018 with maximum convection depth > 1,300 m. In this article, we investigate the respective roles of air-sea flux and preconditioning of the water column to explain this exceptional 4-year persistence of deep convection; we quantified them in terms of buoyancy and analyzed both the heat and freshwater components. Contrary to the very negative air-sea buoyancy flux that was observed during winter 2015, the buoyancy fluxes over the Irminger Sea during winters 2016, 2017 and 2018 were close to climatological average. We estimated the preconditioning of the water column as the buoyancy that needs to be removed (B) from the end of summer water column to homogenize the water column down to a given depth. B was lower for winters 2016–2018 than for the mean 2008–2015, including a vanishing stratification from 600 m down to ~1,300 m. It means that less air-sea buoyancy loss was necessary to reach a given convection depth than in the mean and once convection reached 600 m little additional buoyancy loss was needed to homogenize the water column down to 1,300 m. We showed that the decrease in B was due to the combined effects of a cooling of the intermediate water (200–800 m) and a decrease in salinity in the 1,200–1,400 m layer. This favorable preconditioning permitted the very deep convection observed in 2016–2018 despite the atmospheric forcing was close to the climatological average.

The Effect of Southern Ocean Surface Buoyancy Loss on the Deep-Ocean Circulation and Stratification

The deep-ocean circulation and stratification have likely undergone major changes during past climates, which may have played an important role in the modulation of atmospheric CO2 concentrations. The mechanisms by which the deep-ocean circulation changed, however, are still poorly understood and represent a major challenge to the understanding of past and future climates. This study highlights the importance of the integrated buoyancy loss rate around Antarctica in modulating the abyssal circulation and stratification. Theoretical arguments and idealized numerical simulations suggest that enhanced buoyancy loss around Antarctica leads to a strong increase in the abyssal stratification, consistent with proxy observations for the last glacial maximum. Enhanced buoyancy loss moreover leads to a contraction of the middepth overturning cell and thus upward shift of North Atlantic Deep Water (NADW). The abyssal overturning cell initially expands to fill the void. However, if the buoyancy loss rate further increases, the abyssal cell also contracts, leaving a “dead zone” with vanishing meridional flow at middepth.

Tracking of Buoyancy Flux of Underwater Plumes for Identification, Close Visual Inspection and Repair of Leaking Underwater Pipelines in Muddy Waters

The importance of close visual inspection of leaking sections of pipelines prior to repair activities cannot be over-emphasized. Underwater optical cameras are important gadgets for most underwater inspection vehicles and submarines. However, the optical camera ordinarily immersed in a gaseous or liquid plume in muddy water condition will see no reliable trace but nearly blank or the scatter of diffused light of illuminating lamps. The implication is that underwater water tools that could be used in clear water to inspect and identify point of leaks on pressure containment structures would not be useful when such structure is installed in muddy or poor underwater visibility conditions. Recent developments have demonstrated a diver assisted technique of close visual inspection of leaking containments structures installed in muddy water using clean water injection. This present paper demonstrates a technique of tracking and identifying leaking points on pipelines installed in unclear/muddy water conditions using optical cameras installed in a novel manner. The method leads a remotely operated or hyperbaric system to the point of leak in muddy water conditions for close visual inspection and subsequent repair. The tool performance is validated in a muddy water of Secchi measure of less than 1 cm and in a number of trials, the tool is found sitting at the leak point. Secchi measure is the visual depth into the water column. Forces that could be found in the plume and the consequences of buoyancy loss to floating or submarine equipment are also examined. Some techniques using remotely operated vehicles and manned hyperbaric bells for leak identification, close visual inspection and repair of pipelines installed in muddy water using the benefit of this presented methodology are proposed and discussed.

Downwelling in Basins Subject to Buoyancy Loss

Recent observational, theoretical, and modeling studies all suggest that the upper part of the downwelling limb of the thermohaline circulation is concentrated in strong currents subject to buoyancy loss near lateral boundaries. This is fundamentally different from the traditional view that downwelling takes place in regions of deep convection. Even when resolving the buoyant boundary currents, coarse-resolution global circulation and climate models rely on parameterizations of poorly known turbulent mixing processes. In this study, the first direct measurements of downwelling occurring within a basin subject to buoyancy loss are obtained. Downwelling is observed near the basin’s vertical wall within the buoyant boundary current flowing cyclonically around the basin. Although the entire basin is cooled, large-scale mean downwelling is absent in the basin interior. Laboratory rotating experiments are conducted to explicitly resolve the turbulent mixing due to convective plumes and the baroclinic eddies generated by the boundary current, and to identify where downwelling takes place. Small vertical velocities can be measured more reliably in the laboratory than in many numerical calculations, whereas the measurement of these small vertical velocities is still a challenge for field experiments. Downwelling is observed near the vertical wall within a boundary layer with a thickness that scales with the baroclinic Rossby radius of deformation, consistent with the dynamical balance proposed by a previous numerical study. Hence, downwelling in the Labrador Sea and Lofoten Basin cyclonic boundary currents may be concentrated in a baroclinic Rossby radius of deformation thick boundary layer in regions with large eddy generation.

Effects of Wind on Convection in Strongly and Weakly Baroclinic Flows with Application to the Labrador Sea*

Large buoyancy loss driving deep convection is often associated with a large wind stress that is typically omitted in simulations of convection. Here it is shown that this omission is not justified when overturning occurs in a horizontally inhomogeneous ocean. In strongly baroclinic flows, convective mixing is influenced both by the background horizontal density gradient and by the across-front advection of buoyancy due to wind. The former process—known as slantwise convection—results in deeper convection, while the effect of wind depends on the relative orientation of wind with respect to the baroclinic front. For the case of the Labrador Sea, wintertime winds act to destabilize the baroclinic Labrador Current causing a buoyancy removal roughly one-third as large as the air–sea buoyancy loss. Simulations using a nonhydrostatic numerical model, initialized and forced with observed fields from the Labrador Sea, show how the combination of wind and lateral gradients can result in significant convection within the current, in contrast with previous ideas. Though the advection of buoyancy due to wind in weakly baroclinic flows is negligible compared to the surface buoyancy removal typical of convective conditions, convective plumes are substantially deformed by wind. This deformation, and the associated across-front secondary circulation, are explained in terms of the vertical advection of wind-generated vorticity from the surface boundary layer to deeper depths. This mechanism generates vertical structure within the convective layer, contradicting the historical notion that properties become vertically homogenized during convection. For the interior Labrador Sea, this mechanism may be partly responsible for the vertical variability observed during convection, which modeling studies have until now failed to reproduce.