Ice and ocean velocity in the Arctic marginal ice zone: Ice roughness and momentum transfer

The interplay between sea ice concentration, sea ice roughness, ocean stratification, and momentum transfer to the ice and ocean is subject to seasonal and decadal variations that are crucial to understanding the present and future air-ice-ocean system in the Arctic. In this study, continuous observations in the Canada Basin from March through December 2014 were used to investigate spatial differences and temporal changes in under-ice roughness and momentum transfer as the ice cover evolved seasonally. Observations of wind, ice, and ocean properties from four clusters of drifting instrument systems were complemented by direct drill-hole measurements and instrumented overhead flights by NASA operation IceBridge in March, as well as satellite remote sensing imagery about the instrument clusters. Spatially, directly estimated ice-ocean drag coefficients varied by a factor of three with rougher ice associated with smaller multi-year ice floe sizes embedded within the first-year-ice/multi-year-ice conglomerate. Temporal differences in the ice-ocean drag coefficient of 20–30% were observed prior to the mixed layer shoaling in summer and were associated with ice concentrations falling below 100%. The ice-ocean drag coefficient parameterization was found to be invalid in September with low ice concentrations and small ice floe sizes. Maximum momentum transfer to the ice occurred for moderate ice concentrations, and transfer to the ocean for the lowest ice concentrations and shallowest stratification. Wind work and ocean work on the ice were the dominant terms in the kinetic energy budget of the ice throughout the melt season, consistent with free drift conditions. Overall, ice topography, ice concentration, and the shallow summer mixed layer all influenced mixed layer currents and the transfer of momentum within the air-ice-ocean system. The observed changes in momentum transfer show that care must be taken to determine appropriate parameterizations of momentum transfer, and imply that the future Arctic system could become increasingly seasonal

Into the Abyss: Assessing Meridional Heat Transport, Turbulent Mixing and the Effects of Warming in the Deep Ocean

The ocean's overturning circulation is a large-scale conveyor belt responsible for transporting mass, heat and tracers around the global oceans driven primarily by heat and density gradients between different water masses. Two distinct cells of the global MOC have been proposed based upon observations guided by physical constraints, the upper cell (u-MOC) associated with waters sinking to lower to mid-depth in the northern reaches of the North Atlantic Ocean, and the lower or bottom cell (b-MOC) which is linked to the sinking of waters formed around Antarctica to abyssal depths. The deep and abyssal oceans are responsible for absorbing a significant fraction of the global heat budget. Processes that govern the sequestration of heat and carbon in the deep ocean and its redistribution into the interior ocean have huge consequences for the large scale circulation, sea level rise, and the global climate system as a whole. Studying the abyssal ocean depths below 2000 m has historically been limited due to the paucity of high-quality observational data. Only in recent decades have advances in autonomous float technologies, satellite remote sensing, and regular ship-based observational programs begun to reduce the existing data deficit. This thesis uses data from decades of ship-based observations, thousands of profiles from autonomous Argo floats worldwide, and other novel instrumentation to understand and characterize some of the fundamental questions regarding the contemporaneous changes in the abyssal ocean and its impact on climate.In Chapter 2, we construct a heat budget in the Southwest Pacific Basin and utilize ship-based observations gathered over three decades to understand the changes in the large-scale abyssal circulation in the basin. We further calculate the estimates of turbulent mixing in the basin, reconciling them using three different techniques of backing out the turbulent diffusivities in the basin. In Chapter 3, we demonstrate a methodology deploying a novel turbulence profiler called x-Pod and develop a method to reduce spikes in the error-prone data. In Chapter 4, we use a novel unsupervised machine learning technique to characterize different internal wave spectra observed in the ocean, using observations from 15 repeat hydrographic sections around the globe. Lastly, in Chapter 5 we quantify the rate of sea level rise and the contribution of the warming in the abyssal ocean in the Southwest Pacific Basin using data from 4954 profiles from Deep Argo floats. These chapters provide a detailed view of critical processes in the abyssal ocean measured by novel instrumentation to better understand the role of the oceans in a changing global climate

Understanding Full‐Depth Steric Sea Level Change in the Southwest Pacific Basin Using Deep Argo

Abstract Using 9  years of full‐depth profiles from 55 Deep Argo floats in the Southwest Pacific Basin collected between 2014 and 2023, we find consistent warm anomalies compared to a long‐term climatology below 2,000 m ranging between 11 ± 2 to 34 ± 2 m°C, most pronounced between 3,500 and 5,000 m. Over this period, a cooling trend is found between 2,000 and 4,000 m and a significant warming trend below 4,000 m with a maximum rate of 4.1 ± 0.31 m°C yr−1 near 5,000 m, with a possible acceleration over the second half of the period. The integrated Steric Sea Level expansion below 2,000 m was 7.9 ± 1 mm compared to the climatology with a trend of 1.3 ± 1.6 mm dec−1 over the Deep Argo era, contributing significantly to the local sea level budget. We assess the ability to close a full Sea Level Budget, further demonstrating the value of a full‐depth Argo array