2024 Theses Doctoral

Water mass transformation through the lens of numerical models and observations

Bailey, Shanice Tseng

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.