2012 Annual Report

Physical Oceanography

» Physical Oceanography Department Website

Our branch of ocean science focuses on the fluid-dynamic processes that shape ocean currents, the role of those flows in Earth’s climate system, and their interaction with the ocean geochemical systems and ecosystems. Scientists use a mix of approaches to address physical oceanographic research questions, including directly observing using instruments—operated in the field by scientists and technicians or sampling autonomously; designing and carrying out laboratory experiments; and applying analytical and numerical methods for solving the relevant dynamical equations. As of this writing, the WHOI Physical Oceanography department encompasses a scientific staff of 32 individuals and 17 postdoctoral scholars and investigators. These researchers are aided by a support staff of 60, with guidance from 15 scientist and oceanographer emeriti. There are currently 17 MIT/WHOI Joint Program students enrolled in a physical oceanography curriculum. The Department added one part-time employee in 2012 and lost 1 to retirement.

In 2012, PO Department staff members were engaged in approximately 225 active research projects. The focuses of these studies ranged geographically from the North Pole to the Southern Ocean and from the ocean abyss through the air-sea interface and into the atmosphere. The processes being examined ranged in spatial scale from millimeter to basin size, and in time scale from seconds to multi-century. PO researchers participated in approximately 26 major oceanographic cruises lasting up to 50 days and numerous 1-2 day trips, and contributed to countless workshops and scientific conferences.

A sampling of research highlights from 2012

SPURS

SPURS surface mooring buoy on deck of R/V Knorr

After deployment, with Associate Scientist Tom Farrar in foreground

Several department members including Ray Schmitt, Dave Fratantoni, Lou St. Laurent and Tom Farrar are engaged in the Salinity Processes in the Upper Ocean Regional Study, a program investigating the ocean's role in the global water cycle. SPURS' initial focus is the 'sea surface salinity maximum' region in the eastern North Atlantic Ocean—the place where highest salinity surface water in the open Atlantic is believed to result from excessive evaporation associated with dry air originating over the North African deserts blowing out over the ocean. A variety of autonomous and human-supported ocean sensors are being utilized in SPURS, as well as information from a new NASA satellite that can "see" variations in sea surface salinity. The goal of SPURS is improve our understanding of the water cycle over the oceans and its ties to climate.

During the initial SPURS instrument deployment cruise in October aboard R/V Knorr, a voice call was arranged for Chief Scientist Ray Schmitt, NASA SPURS Program manager Eric Lindstrom and Knorr's Captain Adam Seamans to talk with Sunita "Suni" Williams, Commander of the International Space Station, who congratulated the team aboard Knorr on their SPURS expedition. 

DynAMITE

Still image from the video taken during the ROV Jason operation aboard R/V Knorr to recover the stuck DYNAMITE mooring. The knife held by Jason's manipulator arm sawed through the nylon rope mooring segment immediately above the anchor, allowing all of the instrumentation and other equipment on the mooring to be recovered. (See video above "Go Down Jason, Let My Mooring Go")

The Dynamics of Abyssal Mixing and Interior Transports Experiment, led by Ruth Curry and Kurt Polzin, is investigating the processes by which the densest waters in the Atlantic are transformed by vertical mixing into warmer, less-dense water classes (termed Lower North Atlantic Deep Water), and the circulation through the abyssal western North Atlantic basin that results from that mixing. The dense waters originate as stratified inflows from the south and north (called Antarctic Bottom Water and Denmark Strait Overflow Water, respectively). Along their flow paths, turbulent mixing causes these dense waters to entrain overlying warmer waters, changing the characteristics of the bottom flows, weakening their stratification, and making them more buoyant. The resulting upward transfer of mass has consequences for the ocean's abyssal circulation and for ocean budgets of heat, mass and tracers that are important to Earth's climate system. Ruth and Kurt conducted a field program focused on the region where most of the water mass transformation appears to take place: between 20° – 40° N. They found that turbulent mixing is enhanced over the rugged topography along the Mid Atlantic Ridge and Bermuda Rise.

The DynAMITE field program had two major elements.The first was a moored instrument array extending southeast away from Bermuda, whose purpose was to sample an abyssal flow that Ruth had inferred from historical observations. The second was a regional survey using traditional water sampling techniques and a specialized untethered profiler to sample the ocean mixing processes. Both program elements generated stressful periods: The profiler became stuck in the bottom for a week during its 2011 cruise before a dragging operation dislodged it; and one of the moorings failed to release after its 18-month mission. The mooring was successfully recovered in 2012 with a 2-ship operation in which ROV Jason aboard Knorr was used to cut the mooring free, and it was subsequently brought aboard R/V Atlantic Explorer.

BGOS

Photograph of an Ice-Tethered Profiler shortly after deployment in open water, because there were no sufficiently large and strong ice floes in the area to allow normal deployment atop the ice.

Andrey Proshutinsky is lead investigator of the Beaufort Gyre Observing System in the Arctic Ocean, ably assisted by Rick Krishfield. Ice, ocean, atmosphere: these three components constitute the Arctic climate system. At its heart is one of the least studied bodies of water on the planet—the Beaufort Gyre, a clockwise swirling lens of icy water north of Alaska ten times the size of Lake Michigan. Recent observations suggest that because of global warming, the rhythms of the Beaufort Gyre have altered. To investigate what this means for the future of the Arctic climate, scientists from the United States, Canada, and Japan conduct annual, month-long expeditions to the region in summer to sample the water properties and deploy instruments that measure them throughout rest of the year. The 2012 cruise took place during of a record minimum in the late-summer horizontal extent of the sea ice pack, which complicated deployment of ice-based instrument systems. BGOS and other programs contributing to the Arctic Observing Network are returning observations that shed light on the processes driving Arctic change. A leading explanation is the "ice-albedo-feedback" mechanism, in which the ocean absorbs heat in leads between ice floes, causing the ice to melt, and making wider leads. Scientist Lisan Yu is studying an increasingly important additional factor, the role of low clouds lessening the loss of heat to space.

DIMES

An albatross investigates a free-fall profiler used in the DIMES study to sample ocean mixing processes.

The study of turbulent mixing in the Southern Ocean (DIMES: the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean) continued in 2012 with a major expedition on the U.K. research vessel James Cook to Drake Passage and the Scotia Sea. Lead WHOI investigators in DIMES include Lou St. Laurent and John Toole from the PO Department and Jim Ledwell from the AOPE Department. During the cruise there were two medical evacuations to Port Stanley, Falkland Islands,  and one of the WHOI free-fall profilers was lost (the same instrument almost lost in the DynAMITE program). These events constrained the measurement program, but the data that was collected buttressed initial findings that indicated the intensity of ocean mixing varies widely within the Southern Ocean. Regions where the ocean bottom is relatively smooth have relative weak mixing, whereas in the areas where the deep-reaching Antarctic Circumpolar Current and associated eddies encounter tall, rough bathymetry—such as along the Phoenix Ridge in the Drake Passage, the mixing is greatly enhanced. As the field program continues, DIMES investigators are exploring how this pattern of mixing influences ocean circulation.

SeaFlux

Estimates of the air-sea exchange of latent heat (left) and its uncertainty (right) from the newly-available SeaFlux climatology. The positive values
denote a transfer of heat from the ocean to the atmosphere across the air-sea interface associated with ocean evaporation.

The goal of the now-decade-long international SeaFlux research program, led by Carol Anne Clayson, is to produce high-resolution (in space and time) estimates of the turbulent air-sea heat and moisture exchanges globally, using satellite remote sensing data. 2012 saw the release of Version 1.0 of the SeaFlux product: initial estimates of turbulent surface heat fluxes and associated near-surface variables, including a diurnally (daily)-varying sea surface temperature. Data are available at 0.25 x 0.25 degree spatial and 3-hourly temporal resolution, covering the period 1998-2007. This new data set joins several other products based on marine-deck observations, weather model re-analyses and blended analyses (such as one from Lisan Yu and Bob Weller called the OAFlux product) that attempt to quantify air-sea exchange. Each product has strengths and weaknesses; the continuing research effort seeks to improve estimates of air-sea exchange and better quantify and understand the earth's evolving climate system.

Saqardliup Glacier, West Greenland

Associate Scientist Fiamma Straneo and colleagues conducting small-boat operations adjacent to Saqardliup Glacier in West Greenland (left),
including use of a REMUS AUV (right).

A collaboration between PO scientists (Fiamma Straneo and Al Plueddemann) together with Sarah Das (G&G), supported by the Arctic Research Initiative, led to the first successful survey of a tidewater glacier face in Greenland by an AUV. The research focuses on understanding why many of Greenland's tidewater glaciers accelerated in recent decades, leading to a rise in sea level and a freshening of the downstream ocean. Key to addressing these questions is collecting data where the glaciers meet the ocean - a dangerous and inaccessible region because of frequent iceberg calving. To acquire these data, WHOI scientists utilized a REMUS 200 AUV and a system of buoys to guide the AUV along the glacier face - collecting temperature, salinity and velocity observations. Amy Kukulya and Robin Littlefield (AOPE) conducted the REMUS field operations, together with Straneo and Das, in July of 2012. The science party (including Jeff Pietro and JP student Rebecca Jackson) camped for 9 days at the edge of Saqardliup Glacier in West Greenland - surveying the glacier edge and catching the occasional fish for dinner.

Surface Currents at MVCO

MVCO Surface Currents

Maps of surface current south of Martha's Vineyard obtained by Associate Scientist Anthony Kirincich with his high-frequency radar installation.

A main research focus for Anthony Kirincich since joining the PO Scientific Staff has been the ocean circulation and associated lateral dispersion in inner-shelf domains, the coastal waters just beyond the surf zone. Observing the highly-variable inner-shelf flows presents a host of technical difficulties. Anthony has tackled this challenge by installing and perfecting a land-based high-frequency radar system in conjunction with Martha's Vineyard Coastal Observatory that is capable of returning surface maps of horizontal currents over a 20 x 20 km region at 400-m resolution every 5 minutes. The system infers the surface current by precisely measuring the speed of surface gravity waves and relating the Doppler shift of the wave speed to the underlying currents. The MVCO HF-radar system celebrated an anniversary in 2012 after measuring surface currents for a full two-year period. Anthony is using the acquired data to investigate what influence circulation features such as squirts, jets and eddies exert on the exchange of water masses, nutrients, and pollutants across the shallow inner part of the continental shelf.

Fluid parcel trajectories

Example of a flow geometry in a simplified model of an ocean eddy.

Irina Rypina and Larry Pratt are investigating three-dimensional chaotic advection from a theoretical standpoint, to develop better understanding of mixing and stirring in ocean flows. The Lagrangian dynamics of fluid particle trajectories in time-dependent, three-dimensional fluid flows can be exceptionally complex. Irina and Larry are using dynamical systems theory to simplify the dynamics and identify some special relationships between variables characterizing a given circulation that shape the geometry of the flow. The image below shows an example of one such object arising in a simplified model of an ocean eddy. Each colored surface is produced from tracing the evolution of one fluid particle trajectory as it moves inside the eddy, and marking its position at discrete intervals of time. The outer blue and the inner red surfaces are topologically different from each other, and the black surface separates these two classes of trajectories. That black surface may be considered a barrier to the exchange of fluid particles across it. Predicting such surfaces for ocean flows will allow better quantification of the dispersion of, for example, pollutants. In a related study, Irina is collaborating with Anthony Kirincich to apply dynamical systems ideas to help interpret the MVCO ocean velocity data. Images from Larry and Irina's research are also contributing to an exhibit at the Boston Museum of Science entitled "Ocean Stories: A Synergy of Art and Science."

These highlights represent a small fraction of the research conducted by PO investigators in 2012. Deriving from these programs, approximately 70 peer-reviewed papers (co-) authored by PO Department investigators appeared in print in 2012, with a comparable number in press.

Continuing superior performance was recognized this year by these promotions: J. Thomas Farrar to Associate Scientist without Tenure, Daniel Torres to Research Specialist, Larry George to Engineer II and Andrew Davies to Engineering Assistant III. Congratulations all. Particular kudos go to Ray Schmitt, who was made Fellow of the American Geophysical Union this year.

John Toole, Department Chair

Last updated: September 6, 2013