ALBERT

Description: The abyssal ocean circulation is a key component of the global meridional overturning circulation, cycling heat, carbon, oxygen and nutrients throughout the world ocean. The strongest historical trend observed in the abyssal ocean is warming at high southern latitudes, yet it is unclear what processes have driven this warming, and whether it is linked to a slowdown in the ocean's overturning circulation. Furthermore, future change in the abyssal overturning remains uncertain, with the latest CMIP6 projections not accounting for dynamic ice-sheet melt. In this talk I will present new transient forced high-resolution coupled ocean – sea-ice model simulations to show that under a high emissions scenario, abyssal warming is set to accelerate over the next 30 years. We find that meltwater input around Antarctica drives a contraction of Antarctic Bottom Water (AABW), opening a pathway that allows warm Circumpolar Deep Water greater access to the continental shelf. The reduction in AABW formation results in warming and ageing of the abyssal ocean, consistent with recent measurements. In contrast, projected wind and thermal forcing has little impact on the properties, age, and volume of AABW. These results highlight the critical importance of Antarctic meltwater in setting the abyssal ocean overturning, with implications for global ocean biogeochemistry and climate that could last for centuries.

Description: We deployed a propulsion system-aided glider in a high current environment off the Florida shelf fitted with an ADCP, a CTD, and optics channel sensors to obtain measurements of current velocities, salinity, temperature, pressure, dissolved organic matter (DOM), chlorophyll, and backscatter. We also used a wave-powered profiling platform, a Wirewalker, fitted with both an ADCP and a CTD to obtain measurements at a 120-m isobath. ADCP velocities from both devices were validated using glider coordinates and overlapping temporal windows for comparisons between dives. Processing of the datasets motion-corrected velocity measurements from both devices. Additionally, the glider optics channels were used to assess differences in particle distributions through time. Wirewalker velocity measurements qualitatively coincided with the glider’s ADCP overall, albeit not perfectly quantitatively. This was partly dependent on whether the upcast or downcast for each dive was compared, as well as distance from the glider. Both ADCPs’ velocity measurements show clear evidence of a southward flowing intermittent undercurrent jet previously reported by Soloviev et al. (2017). The glider’s optics channels also show evidence of the undercurrent, and a possible influx of water from Port Everglades. The volume transport by the southward flow is relatively small compared to the Florida Current’s transport. Nevertheless, the processes that maintain and account for the variability of the southward flow are important for a number of practical applications including the propagation of pollution and genetic information against the Florida Current.

Description: The understanding of cumulus mixing has long been challenged by difficulties in quantifying entrainment, detrainment, and dilution (EDD). Although recent advancements have been made in this area, they have not led to robust causal explanations of the mechanisms controlling EDD. Both large-eddy simulations (LES) and observational studies have identified correlations between different aspects of EDD and environmental or cloud-related parameters, but it is unclear whether these correlations reflect causes or effects of EDD on the cloud. To gain further insights, this study conducts LES of the shallow-to-deep convective transition. Two numerical configurations are used, one a classical LES where an ensemble of cumuli develop in an otherwise horizontally homogeneous flow and another a locally forced cumulus over a surface heat source. To quantify EDD, a new “semi-direct” diagnosis technique is developed that sidesteps the detailed and time-consuming calculations required for direct quantification. This method is explained and verified against a corresponding direct calculation. Then, the sensitivities of EDD within buoyant convective cores to core properties (area, updraft speed, buoyancy) are examined. While EDD varies strongly and inversely with all three core properties over all clouds, analyses that control for individual properties reveal that updraft speed and buoyancy are more strongly correlated with EDD than cloud cross-sectional area. Also, EDD is nearly twice as large for cumuli in the LES configuration than for the locally forced cumulus configuration. Additional investigation is performed to interpret the above trends as well as the sharp difference in EDD between the two configurations.

Description: Antarctic Bottom Water (AABW) is a major component of the global overturning circulation, originating around the Antarctic continental margin. In recent decades AABW has both warmed and freshened, but there is also evidence of large interannual variability. The causes of this underlying variability are not yet fully understood, in part due to a lack of ocean and air-sea-ice flux measurements in the region. Here, we simulate the formation and export of AABW from 1958 to 2018 using a global, eddying ocean–sea-ice model in which the four AABW formation regions and transports agree reasonably well with observations. The simulated formation and export of AABW exhibits strong interannual variability which is not correlated between the different formation regions. Reservoirs of very dense waters at depth in the Weddell and Ross Seas following 1-2 years of strong surface water mass transformation can lead to higher AABW export for up to a decade. In Prydz Bay and at the Adélie Coast in contrast, dense water reservoirs do not persist beyond 1 year which we attribute to the narrower shelf extent in the East Antarctic AABW formation regions. The main factor controlling years of high AABW formation are weaker easterly winds, which reduce sea ice import into the AABW formation region, leaving increased areas of open water primed for air-sea buoyancy loss and convective overturning. Our study highlights the variability of simulated AABW formation in all four formation regions, with potential implications for interpreting trends in observational data using only limited duration and coverage.

Description: The Antarctic Slope Current is guided by the topographic gradient of the continental slope and acts as a dynamical barrier between the continental shelf and the open ocean. Exchanges of water masses across the shelf break are important for future predictions of the Antarctic ice shelves and the ventilation of the deep ocean. The Antarctic Slope Current is surface-intensified in many regions but bottom-intensified in regions of dense overflows. This study investigates the role of dense overflows in contributing to the dynamics of the bottom-intensified flow using a 0.1° global ocean-sea ice model. The occurrence of the bottom-intensification is tightly linked to dense overflows and bottom speeds correlate with dense overflows on interannual time scales. A lack of vertical connectivity between the bottom and surface flow, however, suggests that the along-slope bottom water flows are coincidentally co-located with the Antarctic Slope Current, rather than dynamically a part of the current.

Description: Ocean heat transport towards Antarctica directly drives the melting of Antarctic ice shelves, modulating sea level rise and the formation of Antarctic Bottom Water. A common dynamical assumption is that heat transport across the Antarctic continental slope is modulated by the strength of the Antarctic Slope Current (ASC), which is thought to act as a barrier to cross-slope heat transport. However, observations of the ASC are too scarce to investigate its relationship to poleward heat transport across large circumpolar spatial scales, or over long temporal scales. Also, until recently, ocean models lacked the spatial resolution required to accurately represent the ASC or the eddy heat transport onto the Antarctic shelf. In this study, we analyze the relationship between the ASC and the cross-slope heat transport in a circumpolar, eddy-rich ocean and sea ice simulation. We find that the local strength of the time-mean ASC is not a good predictor of local cross-slope heat transport, i.e., spatial variability in the ASC is not related to spatial variability in poleward heat transport. However, there is a relationship between ASC strength and cross-slope heat transport in the temporal domain. We quantify the strength of the relationship across different time scales (sub-seasonal, seasonal and interannual) and across varying model resolution (from 1/10º to 1/20º to 1/40º).

Description: Primary ice nucleation mechanisms are rarely sufficient for producing observed concentrations of ice crystals, meaning secondary ice production processes (SIP) must occur under some conditions. Data from the High Altitude Ice Crystals-High Ice Water Content (HAIC-HIWC) field campaigns and related mesoscale model simulations with a property-based microphysics scheme (P3) are used to show multiple different SIP processes must be considered to explain observed ice crystal number concentrations. In particular, a multi-modal gamma fitting routine is used to automatically determine the number of modes and fit parameters of each mode for measured size distributions. These are combined with measured total water contents (TWCs) to characterize how cloud microphysical properties vary with temperature, TWC, and convective and meteorological characteristics (e.g., strength, proximity to convection, surface characteristics, updraft velocities) to assess under what conditions SIP processes might act. Comparison against WRF simulations using existing cloud microphysical parameterization schemes show that the intensity and spatial extent of the observed airborne X-band reflectivity and measured size distributions are not well replicated. Sensitivity tests showed that without modeling a variety of SIP processes, total ice number concentrations were 2 to 3 orders of magnitude less than observed between -10C and -45C. Including only one of three SIP mechanisms separately (i.e., Hallett-Mossop mechanism, fragmentation during ice-ice collisions, and shattering of freezing droplets) cannot replicate observed concentrations, but including all three SIP processes can. An effect of SIP on convective invigoration is also exhibited. Implications for understanding SIP processes and for their representations in models are discussed.

Description: Meltwater from the Greenland Ice Sheet has the potential to freshen the surface of the Labrador Sea and limit deep convection. Typically, meltwater is added to ocean models at the surface, neglecting the observed mixing of meltwater within fjords. We investigate how a more realistic representation of meltwater amplifies the transport of freshwater across the southwest Greenland shelf break. Results from four coupled ocean/sea-ice simulations forced with atmospheric reanalysis are compared for the years 1992-1993. We compare eddy permitting and mesoscale eddy resolving (horizontal resolutions of approx. 5km and 3km in the study region, respectively) simulations with no meltwater to examine the importance of mesoscale processes in off shelf transport. Two different meltwater forcing schemes are implemented in the mesoscale eddy resolving configuration: meltwater added to the surface layer and meltwater vertically distributed over the upper 200m. When meltwater is added, there is an increase in off shelf freshwater transport (S〈u〉ref〈/u〉=34.7) compared to the no-meltwater case (total transport of 45.2 ~mSv). Transport off the shelf increases by 12mSv in the surface meltwater case and by 23.1mSv in the vertically distributed meltwater case. In the mesoscale eddy resolving simulations, when meltwater is vertically distributed, there is enhanced baroclinic conversion at the shelf break compared to the simulation with no meltwater. This suggests that meltwater from the Greenland Ice Sheet could have a disproportionate impact on freshwater transport into the Labrador Sea by modifying cross shelf exchange.

Description: A glider equipped with a propulsion system was deployed in the Straits of Florida to study a strong western boundary current. The glider carried an ADCP and a CTD to collect current velocity, temperature, and density measurements. The velocity data were analyzed using LADCP techniques to correct for the glider's movement using an onboard GPS and bottom tracking. The mooring array, consisting of CTD chains and bottom-mounted ADCPs, was also deployed in the Straits of Florida. The glider's CTD and LADCP profiles from these case studies were consistent with the mooring array. These results confirmed the effectiveness of gliders with propulsion systems for oceanographic measurements in strong western boundary current environments, such as the Gulf Stream. However, the propulsion system increases the risk of entrapment. Hence an acoustic beacon was added for search and rescue in case of entanglement with fishing lines or debris commonly found in populated coastal areas.

Description: Numerous studies based on observations and large-eddy simulation (LES) have shown that cumulus clouds are typically composed of one or more ascending large thermals (“large” meaning a thermal size similar to the width of the cloud as a whole). Here, mechanisms driving entrainment in cumulus thermals are examined and contrasted with entrainment in dry thermals. Dry thermals entrain mainly by a process of baroclinic vorticity generation which results from their initial buoyancy becoming concentrated near the thermals’ rotation centers, while they undergo little detrainment. As a result, net entrainment of environmental fluid is driven by an organized flow into the thermals, and they grow in volume as they rise. A similar picture holds for both laminar and turbulent dry thermals, suggesting that smaller-scale turbulent eddies are relatively unimportant in driving entrainment. The nature of entrainment for moist cumulus thermals is substantially different mainly because of condensation and latent heating in their cores. This limits the spreading induced by baroclinic generation of vorticity and thus entrainment efficiency is only about one-half that for dry thermals, all else being equal. This result is consistent with LES showing that cumulus thermals undergo little increase in size as they ascend. Thus, the dilution that does occur in cumulus thermals is mainly associated with entrainment (inflow) balanced by detrainment (outflow), presumably driven by smaller-scale turbulent eddies, in contrast to the situation for dry thermals. A conceptual model based on these ideas will be presented, as well as implications for representing entrainment in cumulus parameterizations.