The framework of this dissertation work relies heavily on the water mass transformation theory (WMT). The theory conceptualizes the explicit relationship between mechanical and thermodynamic processes on water masses, and subsequently, on ocean circulation due to surface fluxes, advective transport, and diffusive mixing. Through high-resolution model and reanalyses data, computation of WMT budgets were made possible to study the physical drivers of water mass variability using ocean and climate models. More specifically, I have applied WMT to study: 1) the interannual variability of Weddell-Sea-derived Antarctic Bottom Water; 2) the transformation of North Atlantic Subtropical Mode Water due to eddy-induced lateral mixing in the near surface; and 3) the physical drivers behind the latest marine heatwave (MHW) that occurred in the Gulf of Mexico in summer 2023. The study in Chapter 1 investigates the variability of WMT within the Weddell Gyre (WG). The WG serves as a pivotal site for the meridional overturning circulation (MOC) and ocean ventilation because it is the primary origin of the largest volume of water mass in the global ocean, Antarctic Bottom Water (AABW). Recent mooring data suggest substantial seasonal and interannual variability of AABW properties exiting the WG, and studies have linked the variability to the large-scale climate forcings affecting wind stress in the WG region. However, the specific thermodynamic mechanisms that link variability in surface forcings to variability in water mass transformations and AABW export remain unclear. This study explores how current state of the art data-assimilating ocean reanalyses can help fill the gaps in our understanding of the thermodynamic drivers of AABW variability in the WG via WMT volume budgets derived from Walin’s classic WMT framework. The three ocean reanalyses used are: Estimating the Circulation and Climate of the Ocean state estimate (ECCOv4), Southern Ocean State Estimate (SOSE) and Simple Ocean Data Assimilation (SODA). From the model outputs, we diagnose a closed form of the water mass budget for AABW that explicitly accounts for transport across the WG boundary, surface forcing, interior mixing, and numerical mixing. We examine the annual mean climatology of the WMT budget terms, the seasonal climatology, and finally the interannual variability. Our finding suggests that the relatively coarse resolution of these models did not realistically capture AABW formation, export and variability. In ECCO and SOSE, we see strong interannual variability in AABW volume budget. In SOSE, we find an accelerating loss of AABW during 2005-2010, driven largely by interior mixing and changes in surface salt fluxes. ECCO shows a similar trend during a 4-yr time period starting in late 2007, but also reveals such trends to be part of interannual variability over a much longer time period. Overall, ECCO provides the most useful timeseries for understanding the processes and mechanisms that drive WMT and export variability in the WG. SODA, in contrast, displays unphysically large variability in AABW volume, which we attribute to its data assimilation scheme. We also examine correlations between the WMT budgets and large-scale climate indices, including ENSO and SAM, and find no strong relationships. The goal of Chapter 2 was to gain novel insight to the mechanisms and thermodynamics of North Atlantic Subtropical Mode Water (NASTMW) creation, destruction and transformation in the North Atlantic through the lens of two high-resolution ocean models. This mode water is found throughout the northwestern part of the subtropical gyre, and its formation area is south of the Gulf Stream Extension. Though studies have looked at the variability of NASTMW, the mechanisms for their variations have not been fully explored. Thanks to the eddy-resolving nature of the two datasets from CESM and CM2.6 control runs, and the water mass transformation framework, we were able to quantify the contributions of NASTMW transformations due to surface eddies in the mixed layer of the North Atlantic. Using these models, we confirm previous findings that air-sea fluxes are the main cause of the formation and destruction of surface water masses over the whole basin. We find that in both models, the haline component of lateral mixing at the surface in the Gulf Stream region is a driver of mode water transformation. Chapter 3 aims to understand the mechanisms of the activation and evolution of the marine heatwave (MHW) that occurred in the Gulf of Mexico (GOM) during summer 2023. We quantified contributions of the thermodynamic processes that transformed surface waters in the GOM into an unprecedented large volume of extremely warm water (> 31.8). Through water mass transforma- tion analysis of reanalyses data, we find that the genesis of this MHW was due to the compounding effect of anomalously warm winter surface water priming the region for a MHW, coupled with greater exposure to strong solar radiation. Transformation due to total surface fluxes (sensible and latent heat, solar and longwave radiation) contributed to the MHW volume at a peak rate of 17.7 Sv (106 m3 s−1 = Sv), while mixing countered the effect by 14.6 Sv at its peak. Total transformation during this 2023 MHW peaked at 4.9 Sv.

The upper ocean mediates the transfer of heat and carbon between the atmosphere and ocean interior. The study of this dynamic environment, made possible in part by long-term time series gathered from oceanographic moorings, is therefore crucial to our understanding of Earth’s climate. In this thesis, we use moored datasets from the Southeast Pacific and Southern Oceans to explore two upper-ocean processes relevant to the transfer and eventual sequestration of atmospheric heat and carbon into the deep ocean: wind-, wave-, and buoyancy-forced turbulence and the release of brine in Antarctic polynyas that drives the formation of Antarctic Bottom Water (AABW). In Chapter 1, we use measurements of turbulence kinetic energy (TKE) dissipation rate (ε) collected at 8.4 m depth on the long-established Stratus Mooring in the Southeast Pacific (20° S, 85° W) to assess the applicability of Monin-Obukhov similarity theory (MOST), Law of the Wall (LOW), and other boundary layer similarity scalings to turbulence in the upper ocean. TKE facilitates the mixing of heat, momentum, and solutes within and between the ocean and atmosphere and is generated in the upper ocean primarily by wind, waves, and buoyancy fluxes. Its production can generally be assumed to equal its dissipation, and measurements of ε therefore serve as a means for quantifying turbulence in a system. We present 9 months of ε measurements, a remarkably long time series made possible by the use of a moored pulse-coherent Acoustic Doppler Current Profiler (ADCP), a new methodology for measuring ε that uniquely allows for concurrent surface flux and wave measurements across an extensive length of time and range of conditions. We find that turbulence regimes are quantified similarly using the classic Obukhov length scale (L_M=(u_*³)/(κ𝐵ₒ), where u_* is ocean-side friction velocity, κ is the von Kármán constant, and B_0 is surface buoyancy flux) and the newer Langmuir stability length scale (L_L=(〖u_s u〗_*²)/B_0 , where u_s is surface Stokes drift velocity), suggesting that u_* implicitly captures the influence of Langmuir turbulence at this site. This is consistent with the strong correlation observed between u_s and u_*, likely promoted by the steady southeast trade winds, and suggests that classic wind and buoyancy-based boundary layer scalings sufficiently describe turbulence in this this region. Accordingly, we find the LOW (ε=(u_*³)/κz, where z is instrument depth) and surface buoyancy scaling (ε=B_0, where B_0 is destabilizing surface buoyancy flux) used in classic turbulence scaling studies, such as Lombardo and Gregg (1989), to describe our measurements well, and a newer scaling for Langmuir turbulence scaling based on u_s and u_* to scale ε well at times but to be overall less consistent than (u_*³)/κz. The performance of MOST relationships from prior studies in a variety of aquatic and atmospheric settings are also examined, and we find them to largely agree with our data in conditions where both convection and wind-driven current shear act as significant sources of TKE (-1<z/L_M <0). The apparent redundancy of Langmuir turbulence scaling and the sufficiency of LOW and MOST observed in this study may help inform the development of general circulation models (GCMs), which rely on boundary layer scaling to parametrize turbulent mixing in the upper ocean. In Chapters 2 and 3, we focus on the Terra Nova Bay Polynya in the western Ross Sea of Antarctica, where High Salinity Shelf Water (HSSW) forms as a result of the cooling and salinification of the surface ocean by an intense katabatic wind regime and its associated ice production. HSSW is a precursor to AABW, a vital water mass that feeds the bottom limb of the meridional overturning circulation (MOC) and facilitates the sequestration of atmospheric heat and carbon into the abyss. A decades-long freshening trend in the salinity of Ross Sea HSSW resulting from increased glacial meltwater fluxes, and more recently, its abrupt reversal associated with the occurrence of a climate anomaly, have highlighted the complexity of this system and its sensitivity to changes in climate. Because the density of HSSW has a direct impact on the density of downstream AABW, and therefore the strength of the MOC, it is imperative to better understand the variability and mechanisms of HSSW formation. However, inhospitable wintertime conditions in this region severely restrict the collection of in-situ data in the presence of active brine rejection and HSSW formation. Here, we present an unprecedented set of upper-ocean salinity, temperature, turbulence, current velocity, and acoustic surface tracking time series collected from a mooring in Terra Nova Bay during austral winter 2017. One poorly constrained aspect of HSSW in Terra Nova Bay is its rate of production, and in Chapter 2 we endeavor to produce the first production rate estimates to be based on in-situ salinity data. We find an average production rate of ~0.6 Sverdrups (10⁶ m³ s⁻¹), which allows us to improve on and validate an existing approach for estimating rates using parametrized net surface heat fluxes out of the polynya. We use this approach to examine interannual variability in production across the decade and find estimates of HSSW production in Terra Nova Bay to be largely increasing from 2015 onward. As higher production rates of Terra Nova Bay HSSW, the saltiest variety of HSSW across Antarctica, could increase the salinity of downstream AABW, this apparent increase may have played a previously unrecognized role in the recently observed recovery of AABW salinity in this region. In Chapter 3, we examine a number of interconnected processes surrounding HSSW formation, including the coupling of salinity to winds, the breakdown of summer stratification that primes the water column for HSSW formation in the winter, wind-driven turbulence that facilitates the breakdown of stratification and mixing of HSSW to depth, and potential circulation pathways for HSSW formed at the mooring site. We find that salinity at the shallowest depth on the mooring line, 47 m, couples strongly to wind speeds measured at the nearby Automatic Weather Station (AWS) Manuela from April onward, demonstrating the dependence of polynya formation, ice production, and brine rejection on winds at the mooring site. Salinity at the deepest depth on the mooring line, 360 m, couples to salinity at 47 m beginning in June, following the progressive breakdown of lingering summertime water column stratification that previous studies have established as a prerequisite for HSSW formation in the winter. We incorporate concepts from Chapter 1 to explore the scaling of turbulence in a polynya environment, finding that daily-averages of ε are sufficiently approximated according to the classic LOW scaling, despite visible evidence of Langmuir circulation in the polynya. To the best of our knowledge, this represents the first examination of turbulence scaling using in-situ time series measurements in an Antarctic polynya, an environment that connects the turbulent mixing of heat and solutes in the upper ocean to the properties of the deepest layer of the ocean. Lastly, we infer from current velocities and a late-winter coupling of salinity measured at our mooring to that measured by a second mooring within the Drygalski Basin that HSSW may travel one of two pathways following its formation at our mooring site: Directly southeastward into the Drygalski Basin or northeastward along with the cyclonic gyre of Terra Nova Bay. More mooring deployments across space and time within the bay are needed in order to further elucidate the variability and mechanisms surrounding HSSW formation, critical foci of study in the context of a rapidly changing Antarctic environment.