The overturning circulation of the ocean plays a governing role in the earth's climate because of the enormous capacity of the ocean to hold heat and carbon dioxide. The Southern Ocean, which surrounds Antarctica, plays a disproportionate role in this overturning circulation because this is one of the main areas where deep waters rise to the surface to exchange heat and carbon dioxide with the atmosphere. Although the Antarctic Circumpolar Current (ACC) system brings deep water to the surface, dynamical constraints inhibit meridional exchanges. Ocean eddies are believed to play a dominant role in transporting water south across the ACC above deep ridges, feeding water driven northward by the intense winds. The extent to which this Isopycnal circulation is "short-circuited" by mixing across density layers is important to climate models but is unknown.
Intellectual Merit: Conceptual models of global meridional overturning and numerical predictions for future climate are strongly sensitive to the methods used to represent mixingalong and across the Antarctic Circumpolar Current (ACC), where isopycnals are steeply tilted. Neither diapycnal nor isopycnal mixing has been measured in the Southern Ocean in a systematic way. The goals of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) are to measure eddy mixing along density surfaces in the subsurface ocean (isopycnal mixing), and across those density layers (diapycnal mixing), and to determine how those processes depend on the larger scale dynamics of the ocean, so that they can be properly represented in numerical models of ocean circulation and of climate. To reveal these processes at work in the ACC, a chemical tracer and 75 floats that follow the water along isopycnal surfaces will be released in the ACC near 1300 m depth, 60 S, and 110 W, early in 2008. Floats that measure fine-structure T, S, and velocity within and above the tracer cloud will be released at the same time. The floats and tracer will be carried by the ACC over the relatively smooth bottom of the SE Pacific, spreading both across and along the current as they travel. After a year, the leading edge of the tracer will just start to pass over the ridges of Drake Passage into the Scotia Sea. Another 75 isopycnal floats will be released near the center of the tracer patch at this time. Trajectories of the floats, measured acoustically with an array of sound sources, will be used to study and to measure isopycnal dispersion. Spreading of the tracer will give integrated measures of both isopycnal and diapycnal dispersion. The eddy field, and its vertical structure, will be studied with sea surface height measured by satellite altimeters, and with hydrographic profiles taken from research vessels and from autonomous instruments drifting with the tracer. Turbulent dissipation, from which diapycnal mixing can be estimated, will be measured with ship-based free-falling profilers to study the spatial and temporal scales of the mixing and to examine suspected hot spots of mixing. Shear driving this mixing will be measured with the free-falling profilers and with special floats drifting with the tracer and floats that profile between the surface and the tracer layer.
Broader Impact: DIMES will deploy a variety of instruments including microstructure and finestructure profilers and and isopycnal-following autonomous floats, some for the first time in Southern Ocean. The mixing results will be made available to aid in improving representations of mixing in climate models. In addition, profiling DIMES floats will augment the Argo database for the Southern Ocean. The project will involve a postdoctoral investigator, graduate students at Florida State University and Scripps Institution of Oceanography and will offer research opportunities to one to two undergraduates per year.
This project is a contribution to the U.S. CLIVAR (CLImate VARiability and predictability) program.
1 Introduction 1.1 The Southern Ocean The Antarctic Circumpolar Current (ACC) is a continuous eastward current that circles the continent of Antarctica. Collectively, the oceanic regions through which it flows (the southern portions of the Pacific, Atlantic, and Indian Oceans) are known as the "Southern Ocean". Because it connects the major oceans, the ACC is a hub of the large-scale overturning circulation that ventilates the deep ocean and transports heat between the equator and the poles. For this reason, and the role it plays in regulating the flow of heat to the Antarctic continental margin, the Southern Ocean is an important part of many computer simulations of climate change. Understanding the processes at work in the ACC, then, is critical to improving predictions of the ocean’s response to climate change, including greenhouse gas forcing. 1.2 Diapycnal Mixing Processes Many of the processes least able to be simulated by computer models of the ocean involve "small-scale" (i.e., not well-resolved by the model’s discrete grid) fluid motions ranging from 100 km eddies down to centimeter-scale turbulence. The part of these motions that stirs and ultimately mixes fluids of different densities (i. e., primarily vertical turbulent motions) is known as "diapycnal" (or across density surfaces). Much of this mixing occurs near the surface and bottom of the ocean, where direct wind forcing or drag on bottom topography is able to generate turbulence. In the ocean interior, mixing is much reduced and water is able to move for long distances with very little modification. However, because of the long residence time of water in the ocean interior (up to many hundreds of years), the mixing that does occur there can have far-reaching consequences. For the most part, it is believed that mid-ocean mixing occurs through the breaking of internal waves, which propagate both vertically and horizontally throughout the ocean’s stratified interior. The unique aspects of the ACC, including strong winds and cold air temperatures, a strong current that extends to the bottom of the ocean, and complex bottom topography, suggest that information about "typical" deep ocean conditions may not be applicable. 1.3 DIMES The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean, was conceived to measure the processes and rates at which fluid is mixed across and along density surfaces. The experiment has been underway since early 2009, with major components including (a) the release of a patch of chemical tracer (CF3SF5) that can be detected in minute quantities over the span of many years; (b) ship surveys of hydrography (temperature, salinity, currents) and microstructure (turbulence); (c) acoustically-tracked neutrally-buoyant floats to follow horizontal water motions; and (d) autonomous floats (EM-APEX) measuring verti- cal profiles temperature, salinity, and currents over the course of the tracer spreading and sending data back via satellite phone link. This project report is primarily about part (d). 2 Outcomes The principal outcomes of the first 3 years of tracer and profiling float measurements in the DIMES experiment (Figure 1) can be summarized as follows: • The initial hypothesis that subsurface mixing in the ACC is relatively weak in the Southeast Pacific has been confirmed. The region is still one of persistent strong fronts and an active eddy field, causing mixing levels to be moderately elevated above those typically found in the mid-latitude open ocean [Ledwell et al., 2011]. • In addition, the hypothesis that mixing is dramatically elevated in the Drake Passage and Scotia Sea can be confirmed (Figure 2) by both the measurements of tracer spread the the shear from the EM-APEX. • The moderately-enhanced mixing in the Pacific sector can be related to the downward propagation of near-inertial internal waves driven by surface wind forcing. • The increase of finestructure energy (and presumably turbulence) with depth in the Drake Passage and Scotia Sea suggests that bottom-generated internal waves are a major source of energy for mixing in this region. Thus, not only the level, but also the dominant mechanisms of mixing contrast between the Pacific and Atlantic sectors. • In addition to integral mixing estimates from the tracer, DIMES has begun to quantify the statistical intermittency in Southern Ocean mixing processes (Figure 1). The measurements to date suggest that the temporal intermittency can be connected to statistics of the wind field, while spatial intermittency is connected to particular features of the topography. 3 Future Plans Both the tracer and the profiling float components of the DIMES experiment are continuing through follow-on NSF grants. Additional EM-APEX will enhance the sparse coverage of the internal wave measurements in the Drake Passage and Scotia Sea and additional tracer cruises will allow sampling of the continued evolution of the patch as it continues eastward. A graduate student has begun to work on the EM-APEX dataset for his Ph.D. dissertation and will continue under the new grant.
