Heating and Cooling or Ebbing and Flowing? Oceanic Change from a Thermohaline Perspective

This thesis develops and applies novel methods to understand water mass variability and change in the global ocean. A water mass framework is developed that determines the diathermal and diahaline transformations of water between water mass classes from the temporal variations in their volumetric distribution. Water masses are defined in terms of their temperature and salinity. This reveals the roles of air--sea fluxes, mixing and transport changes. The second chapter explores the drivers of interannual variability in the overturning circulation in the North Atlantic subtropical gyre using the water mass framework. Variations in the volumetric distribution of water masses reveal that transport anomalies at the gyre boundaries predominantly set the volume and heat budget and that these transport anomalies are governed by Ekman pumping over the gyre. In the third and fourth chapters of this thesis the water mass framework is applied to observations of temperature and salinity in the Southern Ocean. Seasonal variations in the distribution of water masses reveal the imprint of the Southern Ocean overturning. This highlights the importance of seasonally varying air-sea fluxes in the formation of intermediate water at the expense of deep water, winter water and surface water. This reveals a diabatic pathway for the upwelling and conversion of deep water into intermediate water. Deep water is first cooled and freshened during the winter by mixing with overlying winter water triggered by a cabbeling instability. Sea ice-melt and surface heating then warm and freshen this seasonally formed water mass to create intermediate water during the summer months. These results suggest that the process of cabbeling could be a rate determining step in the global overturning circulation and the upwelling of deep waters. The fifth chapter of this thesis explores an alternative method to determine a volumetric distribution using individual Argo profiles. The volumetric distribution determined using this profile based estimate is compared to the distribution calculated using a geographically interpolated dataset. This comparison reveals that the interpolation scheme used to geographically grid Argo appears to artificially mix water masses toward the centre of the distribution.<br/

The imprint of Southern Ocean overturning on seasonal water mass variability in Drake Passage

Seasonal changes in water mass properties are discussed in thermohaline coordinates from a seasonal climatology and repeat hydrographic sections. The SR1b CTD transects along Drake Passage are used as a case study. The amount of water within temperature and salinity classes and changes therein are used to estimate dia-thermal and dia-haline transformations. These transformations are considered in combination with climatologies of surface buoyancy flux to determine the relative contributions of surface buoyancy fluxes and subsurface mixing to changes in the distribution of water in thermohaline coordinates. The framework developed provides unique insights into the thermohaline circulation of the water masses that are present within Drake Passage, including the erosion of Antarctic Winter Water (AAWW) during the summer months and the interaction between the Circumpolar Deep Waters (CDW) and Antarctic Intermediate Water (AAIW). The results presented are consistent with summertime wind-driven inflation of the CDW layer and deflation of the AAIW layer, and with new AAIW produced in the winter as a mixture of CDW, remnant AAWW, and surface waters. This analysis therefore highlights the role of surface buoyancy fluxes in the Southern Ocean overturning

High-latitude ocean ventilation and its role in Earth's climate transitions

The processes regulating ocean ventilation at high latitudes are re-examined based on a range of observations spanning all scales of ocean circulation, from the centimetre scales of turbulence to the basin scales of gyres. It is argued that high-latitude ocean ventilation is controlled by mechanisms that differ in fundamental ways from those that set the overturning circulation. This is contrary to the assumption of broad equivalence between the two that is commonly adopted in interpreting the role of the high-latitude oceans in Earth's climate transitions. Illustrations of how recognizing this distinction may change our view of the ocean's role in the climate system are offered

Recent wind-driven variability in Atlantic water mass distribution and meridional overturning circulation

Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 47 (2017): 633-647, doi:10.1175/JPO-D-16-0089.1.Interannual variability in the volumetric water mass distribution within the North Atlantic Subtropical Gyre is described in relation to variability in the Atlantic meridional overturning circulation. The relative roles of diabatic and adiabatic processes in the volume and heat budgets of the subtropical gyre are investigated by projecting data into temperature coordinates as volumes of water using an Argo-based climatology and an ocean state estimate (ECCO version 4). This highlights that variations in the subtropical gyre volume budget are predominantly set by transport divergence in the gyre. A strong correlation between the volume anomaly due to transport divergence and the variability of both thermocline depth and Ekman pumping over the gyre suggests that wind-driven heave drives transport anomalies at the gyre boundaries. This wind-driven heaving contributes significantly to variations in the heat content of the gyre, as do anomalies in the air–sea fluxes. The analysis presented suggests that wind forcing plays an important role in driving interannual variability in the Atlantic meridional overturning circulation and that this variability can be unraveled from spatially distributed hydrographic observations using the framework presented here.DGE was supported by a Natural Environment Research Council studentship award at the University of Southampton. JMT’s contribution was supported by the U.S. National Science Foundation (Grant OCE-1332667). GF’s contribution was supported by the U.S. National Science Foundation through Grant OCE-0961713 and by the U.S. National Oceanic and Atmospheric Administration through Grant NA10OAR4310135. The contributions of JDZ and AJGN were supported by the NERC Grant ‘‘Climate scale analysis of air and water masses’’ (NE/ K012932/1). ACNG gratefully acknowledges support from the Leverhulme Trust, the Royal Society, and the Wolfson Foundation. LY was supported by NASA Ocean Vector Wind Science Team (OVWST) activities under Grant NNA10AO86G

Super Residual Circulation : A New Perspective on Ocean Vertical Heat Transport

Ocean circulation and mixing regulate Earth's climate by moving heat vertically within the ocean. We present a new formalism to diagnose the role of ocean circulation and diabatic processes in setting vertical heat transport in ocean models. In this formalism we use temperature tendencies, rather than explicit vertical velocities, to diagnose circulation. Using quasi-steady-state simulations from the Australian Community Climate and Earth-System Simulator Ocean Model (ACCESS-OM2), we diagnose a diathermal overturning circulation in temperature-depth space. Furthermore, projection of tendencies due to diabatic processes onto this coordinate permits us to represent these as apparent overturning circulations. Our framework permits us to extend the concept of "Super Residual Transport,'' which combines mean and eddy advection terms with subgridscale isopycnal mixing due to mesoscale eddies but excludes small-scale threedimensional turbulent mixing effect, to construct a new overturning circulation-the "Super Residual Circulation'' (SRC). We find that in the coarse-resolution version of ACCESS-OM2 (nominally 1 degrees horizontal resolution) the SRC is dominated by an similar to 11-Sv (1 Sv [10(6) m(3) s(-1)) circulation that transports heat upward. The SRC's upward heat transport is;2 times as large in a finer-horizontal-resolution (0.1 degrees) version of ACCESS, suggesting that a differing balance of super-residual and parameterized small-scale processes may emerge as eddies are resolved. Our analysis adds new insight into superresidual processes, because the SRC elucidates the pathways in temperature and depth space along which water mass transformation occurs.Peer reviewe

The cold transit of Southern Ocean upwelling

The upwelling of deep waters in the Southern Ocean is a critical component of the climate system. The time and zonal mean dynamics of this circulation describe the upwelling of Circumpolar Deep Water and the downwelling of Antarctic Intermediate Water. The thermodynamic drivers of the circulation and their seasonal cycle play a potentially key regulatory role. Here an observationally constrained ocean model and an observation‐based seasonal climatology are analyzed from a thermodynamic perspective, to assess the diabatic processes controlling overturning in the Southern Ocean. This reveals a seasonal two‐stage cold transit in the formation of intermediate water from upwelled deep water. First, relatively warm and saline deep water is transformed into colder and fresher near‐surface winter water via wintertime mixing. Second, winter water warms to form intermediate water through summertime surface heat fluxes. The mixing‐driven pathway from deep water to winter water follows mixing lines in thermohaline coordinates indicative of nonlinear processes

Drivers of exceptionally cold North Atlantic Ocean temperatures and their link to the 2015 European heat wave

The North Atlantic and Europe experienced two extreme climate events in 2015: exceptionally cold ocean surface temperatures and a summer heat wave ranked in the top ten over the past 65 years. Here, we show that the cold ocean temperatures were the most extreme in the modern record over much of the mid-high latitude North-East Atlantic. Further, by considering surface heat loss, ocean heat content and wind driven upwelling we explain for the first time the genesis of this cold ocean anomaly. We find that it is primarily due to extreme ocean heat loss driven by atmospheric circulation changes in the preceding two winters combined with the re-emergence of cold ocean water masses. Furthermore, we reveal that a similar cold Atlantic anomaly was also present prior to the most extreme European heat waves since the 1980s indicating that it is a common factor in the development of these events. For the specific case of 2015, we show that the ocean anomaly is linked to a stationary position of the Jet Stream that favours the development of high surface temperatures over Central Europe during the heat wave. Our study calls for an urgent assessment of the impact of ocean drivers on major European summer temperature extremes in order to provide better advance warning measures of these high societal impact events

Annual Cycle of Turbulent Dissipation Estimated from Seagliders

The rate of dissipation of turbulent kinetic energy is estimated using Seaglider observations of vertical water velocity in the midlatitude North Atlantic. This estimate is based on the large‐eddy method, allowing the use of measurements of turbulent energy at large scales O(1–10 m) to diagnose the rate of energy dissipated through viscous processes at scales O(1 mm). The Seaglider data considered here were obtained in a region of high stratification (1 × 10−4<N < 1×10−2s−1), where previous implementations of this method fail. The large‐eddy method is generalized to high‐stratification by high‐pass filtering vertical velocity with a cutoff dependent on the local buoyancy frequency, producing a year‐long time series of dissipation rate spanning the uppermost 1,000 m with subdaily resolution. This is compared to the dissipation rate estimated from a moored 600 kHz acoustic Doppler current profiler. The variability of the Seaglider‐based dissipation correlates with one‐dimensional scalings of wind‐ and buoyancy‐driven mixed‐layer turbulence. Plain Language Summary Measuring ocean turbulence is crucial for understanding how heat and carbon dioxide are transferred from the atmosphere to the deep ocean. However, measurements of ocean turbulence are sparse. Here autonomous Seagliders are used to estimate turbulence in the surface kilometer of the North Atlantic Ocean. Using an estimate of the vertical water velocity from the flight of the Seaglider through the water, we estimate turbulence by assuming the energy of the largest turbulent fluctuations is representative of the energy dissipated at molecular scales. This approach has been used previously in an ocean region where the vertical gradient of density is small. Our results show that this previous approach fails when the vertical density gradient increases, as it does not account for other processes that are unrelated to turbulence. We introduce a generalized method that isolates only the turbulent processes by accounting for the strength of the vertical density gradient. We show that this new estimate agrees with other turbulence measurements. Our estimate also agrees well with a simple estimates of turbulence from atmospheric processes. This study therefore presents method that can be applied to existing and new Seaglider data to greatly increase our measurements of ocean turbulence

Buoyancy forcing and the subpolar Atlantic meridional overturning circulation

The North Atlantic meridional overturning circulation and its variability are examined in terms of the overturning in density space and diapycnal water mass transformation. The magnitude of the mean overturning is similar to the surface water mass transformation, but the density and properties of these waters are modified by diapycnal mixing. Surface waters are progressively densified while circulating cyclonically around the subpolar gyre, with the densest waters and deepest convection occurring in the Labrador Sea and Nordic Seas. The eddy-driven interaction between the convective interior and boundary currents is a key to the export of dense waters from marginal seas. Due to the multitude of pathways of dense waters within the subpolar gyre, as well as mixing with older waters, waters exiting the subpolar gyre have a wide range of ages, with a mean age on the order of a decade. As a result, interannual changes in water mass transformation are mostly balanced locally and do not result in changes in export to the subtropics. Only persistent changes in water mass transformation result in changes in export to the subtropics. The dilution of signals from upstream water mass transformation suggests that variability in export of dense waters to the subtropics may be controlled by other processes, including interaction of dense waters with the energetic upper ocean. This article is part of a discussion meeting issue ‘Atlantic overturning: new observations and challenges’

Wind‐forced symmetric instability at a transient mid‐ocean front

Mooring and glider observations and a high‐resolution satellite sea surface temperature image reveal features of a transient submesoscale front in a typical mid‐ocean region of the Northeast Atlantic. Analysis of the observations suggests that the front is forced by downfront winds and undergoes symmetric instability, resulting in elevated upper‐ocean kinetic energy, re‐stratification and turbulent dissipation. The instability is triggered as downfront winds act on weak upper‐ocean vertical stratification and strong lateral stratification produced by mesoscale frontogenesis. The instability's estimated rate of kinetic energy extraction from the front accounts for the difference between the measured rate of turbulent dissipation and the predicted contribution from one‐dimensional scalings of buoyancy‐ and wind‐driven turbulence, indicating that the instability underpins the enhanced dissipation. These results provide direct evidence of the occurrence of symmetric instability in a quiescent open‐ocean environment, and highlight the need to represent the instability's re‐stratification and dissipative effects in climate‐scale ocean models