Kinematics and Dynamics of Oceanic Overflows: Application to the Denmark Strait and Faroe Bank Channel

An important mechanism in redistribution of heat and salt in the ocean is known as the global meridional overturning circulation. Oceanic overflows contribute to the deep branches of the conveyor belt. Determining overflow dynamics and kinematics is critical to estimating the changes in overflow production and composition as it ultimately impacts global ocean circulation. In this work, we first present the kinematics of a specific sea-strait, and then study some aspects of overflow dynamics. We study the overflow of dense water from Denmark Strait (known as Denmark Strait Overflow, DSO) which feeds the lower limb of the conveyor belt in the northern extremity of the Atlantic Ocean. We investigate the upstream pathways of the DSO through the application of backward Lagrangian particle tracking in a realistic ocean model. The Lagrangian analysis confirms the existence of previously known branches from the North and it also reveals an additional pathway emerging from south of Iceland. The southern pathways supply over 25% of the DSO during winter of 2008 when the North Atlantic Oscillation (NAO) index was positive and can potentially change depending on the phase of the NAO. The southern pathways mark a more direct route from the near-surface subpolar North Atlantic to the NADW. The second part of this study involves the dynamics of overflow pathway partitioning and the effect of upstream reservoir on overflow production for an idealized sea-strait geometry with a continuously varying (parabolic) cross-section. We use rotating hydraulic theory and idealized modeling to reveal the relation between reservoir conditions, strait geometry, and overflow transport. The results reveal that the basin circulation intrudes more into the channel for a wide parabola with low curvature than a narrow parabola causing large variations in the interface height near the channel. Far enough from the channel entrance, the hydraulically controlled flow in the strait is nearly independent of the basin circulation regardless of the parabolic curvature. Comparing the model to theory, we find that the measurement of the wetted edges of the flow at the critical section can be used for prediction of the volume flux. Based on this finding, we suggest three monitoring strategies for transport estimation and compare the estimates with the observed values at the Faroe Bank Channel. The results show that the estimated transports are within the range of observed values. The third part of this work is about the effect of hydraulic control on the variability in transport observed in some sea-straits on timescales such as the seasonal cycle. We force our numerical model with periodic inflow in the upstream basin for subcritical and hydraulically controlled flow to see the effect of hydraulic control on the suppression of time variability. Results reveal that although the narrowing and shallowing of topography lead to a local suppression of time dependence, the hydraulic control at the sill causes a further suppression of time variability

Lagrangian perspective on the origins of Denmark Strait Overflow

Author Posting. © American Meteorological Society, 2020. 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 50(8), (2020): 2393-2414, doi:10.1175/JPO-D-19-0210.1.The Denmark Strait Overflow (DSO) is an important contributor to the lower limb of the Atlantic meridional overturning circulation (AMOC). Determining DSO formation and its pathways is not only important for local oceanography but also critical to estimating the state and variability of the AMOC. Despite prior attempts to understand the DSO sources, its upstream pathways and circulation remain uncertain due to short-term (3–5 days) variability. This makes it challenging to study the DSO from observations. Given this complexity, this study maps the upstream pathways and along-pathway changes in its water properties, using Lagrangian backtracking of the DSO sources in a realistic numerical ocean simulation. The Lagrangian pathways confirm that several branches contribute to the DSO from the north such as the East Greenland Current (EGC), the separated EGC (sEGC), and the North Icelandic Jet (NIJ). Moreover, the model results reveal additional pathways from south of Iceland, which supplied over 16% of the DSO annually and over 25% of the DSO during winter of 2008, when the NAO index was positive. The southern contribution is about 34% by the end of March. The southern pathways mark a more direct route from the near-surface subpolar North Atlantic to the North Atlantic Deep Water (NADW), and needs to be explored further, with in situ observations.This work was financially supported by the U.S. National Science Foundation under Grant Numbers OAC-1835640, OCE-1633124, OCE-1433448, and OCE-1259210

OceanSpy: A Python package to facilitate ocean model data analysis and visualization

Simulations of ocean currents using numerical circulation models are becoming increasingly realistic. At the same time, these models generate increasingly large volumes of model output data. These trends make analysis of the model data harder for two reasons. First, researchers must use high-performance data-analysis clusters to access these large data sets. Second, they must post-process the data to extract oceanographically-useful information. Moreover, the increasing model realism encourages researchers to compare simulations to observations of the natural ocean. To achieve this task model data must be analyzed in the way observational oceanographers analyze field measurements; and, ideally, by the observational oceanographers themselves. The OceanSpy package addresses these needs