Browsing by Author "Lozier, Mary Susan"

The Atlantic Meridional Overturning Circulation (AMOC) is characterized as a northward upper limb that carries warm near-surface waters from southern latitudes to the subpolar North Atlantic, and a southward lower limb that transports cold deep waters back to the southern latitudes. Due to its special role in distributing heat, carbon and water masses globally, AMOC as an essential part in the climate system, has long been a strong focus within the ocean community. For decades, AMOC variability has been attributed to changes in deep water production at high latitudes in the North Atlantic through a geostrophic response in the Deep Western Boundary Current (DWBC), the assumed major export pathway of the deep waters. However, recent Lagrangian studies have revealed the importance of eddy-driven interior pathways, challenging the traditional DWBC-dominated spreading pattern, hence the linkage between deep water formation and AMOC. Under the new spreading scheme of the deep waters, this dissertation provides an extended Lagrangian analysis on the spreading pathways of two major deep waters, Labrador Sea Water (LSW) and Iceland Scotland Overflow Water (ISOW), and re-examines the relationships among deep water production, deep water export and the strength of AMOC.

A Lagrangian simulation of newly-formed LSW in an ocean/sea ice model (1/4°) reveals strong recirculation of the water mass in the subpolar gyre, with a small portion exported to the subtropical gyre through an advective-diffusive pathway. Furthermore, no significant correlation between LSW production and its Lagrangian export to the subtropical gyre is found on interannual to decadal time scales, suggesting a negligible or at best modest impact of LSW production on the subtropical AMOC.

In a combined Lagrangian and Eulerian frame, a first comprehensive description of ISOW spreading branches in the eastern North Atlantic is presented with observational data and output from an eddy-resolving ocean model. The major export pathway for ISOW is shown to be the southward branch into the Western European Basin, instead of the branch through the Charlie Gibbs Fracture Zone, the traditional DWBC pathway. Interestingly, these two branches show compensating relationships in the model, a result assumed to reflect these pathways’ interactions with the North Atlantic Current in magnitude and/or position shift.

Finally, with output from ocean circulation models and an ocean reanalysis dataset, the meridional connection of the deep water transport anomalies and their relationships with AMOC are assessed. It is shown that deep water transport anomalies in the subpolar gyre do not propagate coherently to the subtropical gyre in general, particularly so for Upper North Atlantic Deep Water (UNADW, containing LSW). Furthermore, while UNADW and Lower North Atlantic Deep Water (LNADW, containing overflow waters) transports in the subpolar gyre are linked to local AMOC strength on interannual to decadal time scales, in the subtropical gyre only LNADW transport variability shows this linkage, the latter results consistent with observations. These analyses suggest a dominance of gyre-specific, rather than basin-wide, mechanisms for water mass transport variability. Thus, latitudinal coherence in AMOC is likely unrelated to continuity in water mass transport anomalies. An exception to this generalization is possible with strong LNADW transport events.

Taken together, this dissertation emphasizes the importance of interior pathways to the export of deep waters from the subpolar to the subtropical gyre. It also reveals a tenuous linkage between deep water production and AMOC strength at subtropical latitudes on interannual to decadal time scales.

The South Atlantic has been of special interest to the overturning circulation as its source waters may have the ability to influence deep water convection in the subpolar North Atlantic, and hence affect the overturning variability or strength. As such, it is important to have a thorough understand of the pathways that regulate the upper-ocean flow in the South Atlantic. This dissertation revisits previously studied aspects of the circulation from an observational perspective focusing on the use of Lagrangian data and statistical tools that complement the observational data. More specifically, surface drifter trajectories, Argo float trajectories, and satellite altimetry products are used to investigate the cold and warm water routes, the subtropical gyre circulation, and the connection between the North Brazil Current and the tropical North Atlantic. The statistical tools used throughout this dissertation are rooted in Markov chain theory, which allows for the construction of probability distribution maps that represent mean pathways and artificially extend the lifetime of observational trajectories. In addition, a new method derived from transition path theory is used to specifically identify pathways that connect a desired source and target region. The results reveal that the cold and warm water routes share multiple pathways throughout the South Atlantic and contribute comparable amounts to the Benguela Current waters. The cold water route follows internal pathways suggesting a significant role in the subtropical gyre circulation in setting pathways. The analysis of sea surface height data shows no significant trends in the subtropical gyre size and strength over the past 25 years. Finally, this work highlights the importance of pathways following the Atlantic interior to the tropical North Atlantic from the North Brazil Current. Future work will focus on understanding the differences between the two-dimensional pathways revealed from the observational trajectories and three-dimensional pathways simulated by model trajectories.

The North Atlantic Ocean transports on the order of 1 petawatt of heat poleward from the Equator toward the high-latitude Arctic via an exchange of warm, upper-layer water for cold bottom water, reducing the Equator-to-pole temperature gradient and providing an opportunity for regional climate predictability at seasonal to decadal time scales. As more high-quality observations and numerical models of the ocean have become available in the past decade, oceanographers have realized that the variability in this oceanic meridional heat transport is not coherent across latitudes, with a prominent break in the meridional coherence between the subtropical gyre and the subpolar gyre. In this dissertation, I demonstrate the progress I have made on understanding how heat is conveyed from the subtropical gyre to the subpolar gyre in the North Atlantic at these critical inter-gyre latitudes (35°N-50°N).

I use a suite of data including databases of in situ measurements of oceanic temperature and salinity, satellite observations of sea-surface temperature and height, and ocean model output of ocean current velocities, temperature and salinity. I conclude that a majority of the inter-gyre heat transport in the North Atlantic can be explained by variability in the strength of the sub-surface transport between the gyres. In this work, I also test whether pathways for propagating sea-surface temperature anomalies exist between the gyres as has been previously suggested and find that there is no evidence for this pathway in more modern satellite measurements. In addition, I show that variability in the sub-surface pathway cannot be explained by dynamics in the size and strength of the subpolar gyre as has long been assumed. Finally, I do a detailed analysis within two ocean circulation models and conclude that the oceanic heat fluxes are as important or more important as the surface atmospheric forcing to the temperature variability in the northeastern Atlantic, even at high frequencies. I then find that the origin of these oceanic heat fluxes stem from variability in the upper limb of the overturning circulation at inter-gyre latitudes. These results impact how we might expect to track ocean heat fluxes and thus where to look for climate predictability in the North Atlantic sector.

Strengthened stratification of the upper ocean, associated with either anthropogenic warming trends or natural climate oscillations, is generally expected to inhibit marine primary productivity at low and mid latitudes, based on the supposition that increased water column stability will decrease vertical mixing and consequently the upward entrainment of deep nutrients into the euphotic zone. Herein, we examine the local stratification control of productivity over the subtropical and equatorial Pacific by directly comparing a wide range of contemporaneous metrics, drawn from the modern observational record, for interannual stratification and productivity variability. We find no correlation between the two in the subtropical North Pacific. In the equatorial Pacific we do observe a correlation, but find no evidence of a strong causal connection between the two- instead, our analysis suggest that both biomass and stratification in this region are impacted by changes in the westward transport, via surface currents, of relatively cold, nutrient-rich waters that have been upwelled in the eastern Equatorial Pacific. The importance of horizontal nutrient supply is further evidenced by an analysis of seasonal variability in the subtropical North Atlantic, where the annual contraction and expansion of the oligotrophic region appears to be strongly influenced by the waxing and waning, respectively, of lateral nutrient transfers from neighboring, nutrient rich waters of the subpolar gyre and the West African upwelling zone.

The timing of the spring phytoplankton bloom in the subpolar North Atlantic Ocean has important consequences for the marine carbon cycle and ecosystems. There are currently several proposed mechanisms to explain the timing of this bloom. The conventional theory holds that the bloom begins when the ocean warms and the seasonal mixed layer shoals in the spring, decreasing the depth to which phytoplankton are mixed and increasing the light available to the population. Recent work has attributed the beginning of the bloom to decreases in turbulence within the upper ocean, driven by the onset of positive heat fluxes or decreases in the strength of local winds. Other studies have focused on the increase in the seasonal mixed layer in the winter as a driver of changes in ecosystem interactions and a control on the spring bloom. Finally, submesoscale eddies, occurring as a result of lateral density gradients, have been proposed as a stratification mechanism that can create phytoplankton blooms prior to the onset of ocean surface warming.

This dissertation critically examines and compares the proposed theories for the initiation of the spring bloom and draws on these theories to propose a new framework: that blooms begin when the active mixing depth shoals, a process generally driven by a weakening of surface heat fluxes and consequent shift from convective mixing to wind-driven mixing. Using surface forcing data, we develop a parameterization for the active mixing depth from estimates of the largest energy-containing eddies in the upper ocean.

Using in situ records of turbulent mixing and biomass, we find that the spring phytoplankton bloom occurs after mixing shifts from being driven by convection to being driven by wind, and that biomass increases as the active mixing depth shoals. Using remote sensing data, we examine patterns of bloom initiation in the North Atlantic at the basin scale, compare current theories of bloom initiation, and find that the shoaling of the active mixing depth better predicts the onset of the bloom across the North Atlantic subpolar basin and over multiple years than do other current theories. Additionally, using a process study model, we evaluate the importance of submesoscale eddy-driven stratification as a control on the initiation of the spring bloom, determining that this mechanism has a relatively minor effect on alleviation of phytoplankton light limitation. Finally, we describe potential techniques and tools to examine whether interannual variability in the active mixing depth acts as a control on variability in the timing of the spring bloom.

This dissertation identifies and quantifies the impact of processes driving two biogeochemical phenomena of interest by considering them in the context of the large-scale circulation in which they occur. Both processes occur in the North Pacific transition zone (NPTZ), a basin-wide region near the subpolar-subtropical gyre boundary.

First, this work investigates the physical forcing behind the large seasonal variability in the location of the Transition zone chlorophyll front (TZCF). The TZCF is a persistent gradient in sea surface color that separates high chlorophyll waters to the north from low chlorophyll waters to the south. The chlorophyll front shifts seasonally by about 1000 km, oscillating between its southernmost winter latitude and its northernmost summer latitude. The forcing behind this seasonal signal is of interest because a number of migratory marine animals, both commercial and endangered, appear to track it.

This first study finds that vertical processes, traditionally viewed as controlling the dynamical supply of nutrients to surface waters, are insufficient to explain seasonal variations in nutrient supply to the transition zone. Instead, the wind-driven horizontal Ekman transport of nutrients, moving southward from the subpolar gyre into the northern reaches of the subtropical gyre, drives the southward migration of the TZCF. Such lateral transport of nitrate supports up to 40% of new primary productivity in the region annually, and nearly all of new primary productivity in the winter.

Second, this work investigates why the North Pacific transition zone waters are a notably strong sink for atmospheric carbon dioxide (CO2) on a mean annual basis, while seasonally they vary from a sink in the winter to a neutral to weak source in the summer. As the partial pressure of carbon dioxide (pCO2) in the surface mixed layer exerts primary control on the direction and magnitude of air-sea carbon exchange, this study quantifies the impact of processes regulating seawater pCO2: temperature, salinity, advection of dissolved inorganic carbon (DIC) and alkalinity (ALK), mixing of DIC and ALK, biology, and air-sea carbon flux.

Seasonal controls on pCO2 in the North Pacific transition zone differ from annual controls. Temperature effects dominate the seasonal signal, but are partially countered throughout the year by opposing processes. In spring and summer, biological drawdown partially offsets the increase in pCO2 due to warming waters; in fall and winter, the vertical entrainment of carbon moderates the decrease in pCO2 due to cooling waters. On a mean annual basis, air-sea carbon flux, biology, mixing, and advection all have a net impact on seawater pCO2. Though important seasonally, temperature has a small impact on pCO2 and air-sea carbon flux annually, accounting for only about 15-20% of oceanic carbon uptake through temperature-driven solubility changes.

This second study again finds an important role for lateral processes to play in regulating biogeochemical phenomena in the North Pacific transition zone. The ability of the region to uptake atmospheric carbon year after year is maintained by those processes exporting carbon from its surface waters: the vertical export of organic carbon to depth, and the lateral geostrophic advection of carbon out of the region. This lateral advection alone determines the location of the sink region: of the processes impacting seawater pCO2 on a mean annual basis, only the geostrophic divergence of DIC disproportionately lowers pCO2 in the transition zone latitudes, supporting greater atmospheric CO2 uptake here than in surrounding regions.

This dissertation identifies and quantifies processes driving biogeochemical features in the North Pacific transition zone, finding the large-scale circulation in the region plays a significant role in regulating these processes. The unique physical oceanographic characteristics of the NPTZ, and in particular the lateral transport, support biological and chemical attributes notably distinct from adjacent waters.

In response to the differential heating of the earth, atmospheric and oceanic flows constantly act to carry surplus energy from low to high latitudes. In the ocean, this poleward energy flux occurs as part of the large scale meridional overturning circulation: warm, shallow waters are transported to high latitudes where they cool and sink, then follow subsurface pathways equatorward until they are once again upwelled to the surface and reheated. In the North Atlantic, the upper limb of this circulation has always been explained in simplistic terms: the Gulf Stream/North Atlantic Current system carries surface waters directly to high latitudes, resulting in elevated sea surface temperatures in the eastern subpolar gyre, and, because the prevailing winds sweeping across the Atlantic are warmed by these waters, anomalously warm temperatures in Western Europe. This view has long been supported by Eulerian measurements of North Atlantic sea surface temperature and surface velocities, which imply a direct and continuous transport of surface waters between the two gyres. However, though the importance of this redistribution of heat from low to high latitudes has been broadly recognized, few studies have focused on this transport within the Lagrangian frame.

The three studies included in this dissertation use data from the observational record and from a high resolution model of ocean circulation to re-examine our understanding of upper limb transport between the subtropical and subpolar gyres. Specifically, each chapter explores intergyre Lagrangian pathways and investigates the impact of those pathways on subpolar property fields. The findings from the studies suggest that intergyre transport pathways are primarily located beneath the surface and that subtropical surface waters are largely absent from the intergyre exchange process, a very different image of intergyre transport than that compiled from Eulerian data alone. As such, these studies also highlight the importance of including 3d Lagrangian information in examinations of transport pathways.

Ekman suction and pumping are often invoked to explain the observed difference in primary production and chlorophyll a between the North Atlantic subpolar and subtropical gyres. Between the gyres, the intergyre region can be loosely defined by its lack of a strong Ekman suction or pumping of nutrients. Despite the lack of a strong Ekman supply of nutrients, the mean seasonal cycle in chlorophyll a in the intergyre is remarkably similar to that seen in the subpolar gyre. In this thesis, we present research on mechanisms for nutrient supply to the intergyre that might support its high production. Using biogeochemical and physical reanalysis ocean data products, a nutrient budget is constructed for a region in the eastern North Atlantic, within the intergyre. Analysis of this budget shows that the seasonal entrainment flux, resultant from the movement of the mixed layer across vertical nutrient gradients, is responsible for the majority of the nutrient supply to the region. Hydrographic ocean data and particle trajectories run in a numerical model are then used to show that the waters seasonally entrained into the intergyre mixed layer likely originate in the Gulf Stream, linking nutrient supply in the North Atlantic intergyre to high downstream nutrient fluxes observed in the Gulf Stream.