The temperature time series from 4000 m depth in the central Labrador Sea spans two years and is characterized by small and big waves and swirls. It is full of mysteries. As a postdoc from GEOMAR it‘s my second research expedition from St. John‘s to Reykjavik and my task on board is to assess the quality of the data to decide if the instruments can be re-deployed. This means that I am one of the first to look at the data after we recover the instruments from the water – in this case from 4000 m depth. Considering that we can hardly enter this region ourselves, we send measuring instruments there every two years to have them record what we cannot see.
Already a first glance at the data reveals that it‘s not as calm down there as one may think. Numerous oscillations in pressure, temperature and salinity reflect a colorful chaos of many different signals and only through careful analyses we may have the chance to decipher the causes underlying this rich variability. Where does the water originate from? When and where has it been at the surface? Which known or yet undetermined processes have changed it? And which time scales of variability are dominant? This deep, there is no clearly distinguishable seasonal cycle because the distance to the surface is too large. Instead, other mechanisms have left their imprint on the water near the sea floor, most of which cannot be identified by this initial glance. In fact, they may remain elusive even after rigorous analyses.
At any rate, this first look at the data tells me that the instrument has gone through an exciting two years – and that an engaging analysis is waiting for us. Bearing in mind that measurements can drift and have offsets, I always remember that no instrument is perfect and that it only shows us one small fraction of what is happening 4000 m below sea level. Much remains a mystery.
There are many fantastic people from all corners of the world on board of our cruise and in the next few entries I like to introduce some of these people I have the pleasure dealing with every day. One of our scientists, Penny Holliday is an ocean going oceanographer from NOC (National Oceanography Centre), who joined our cruise from Southampton, UK. Penny has worked in Ocean science for over two decades and leads the UK OSNAP program.
Penny told me what really got her interested in Oceanography was the possibility to work on so many projects and how connected everything about the ocean is. Penny herself calls her starting point in oceanography a coincidence. Penny was analysing a hydrographic time series when she noticed that her data set could not be explained without digging deeper into the wider Atlantic circulation. This was her starting point in studying the currents in the North Atlantic, which is the focus of the OSNAP program.
But why does this transport matter? It may not be obvious to think about the cold North Atlantic as an important driver of our pleasant weather in Europe. Penny told me that without the currents in the North Atlantic transporting water South-North which is called the Meridional Overturning Circulation, the weather in Europe would be much colder than it is now. Indeed, understanding the way this circulation changes and what drives it are in Penny’s view the key to adapting better to climate change.
I also asked Penny what she has most enjoyed about Oceanography and she admits it’s the travel and the opportunity to do lots of different things. To Penny one of the best things about oceanography are its interdisciplinary focus, with opportunity to work with lots of people. Penny is frequently on ships and her number one advice is to take the time to meet as many people as possible, make friends and be open to learn about topics that may not be directly related to your research focus.
Penny working on the mooring spool together with GEOMAR student Ilmar Leinmann (Photo credit: Penny Holliday)
On our cruise the last few days were filled with extracting and deploying moorings (K7-K10) and taking CTDs along the 53 North Array. The array stretches from the Newfoundland-Labrador shelf between depths of few hundreds to over 3000 meters deep. Most of the instruments we recover have been sampling for over two years and our task is to extract the data and ready them for the next mooring deployment with the OSNAP program. For us as young scientists we get to learn a lot of practical and technical skills changing batteries and calibrating the sensors and we feel the responsibility of doing a good job. After all the continued success of the program depends on new and accurate data.
Map of the 53 North Array and the total cruise plan for MSM 74.
And we are off! MSM 74 – the oceanographic cruise from St. John’s to Reykjavik is making its way to the Labrador Sea. In these 5 weeks a group of 21 scientists and 24 crew on board the German research vessel “Maria S Merian”, will brace the tough North Atlantic in the name of science. The mission: we will recover and re-deploy German, British and US moorings with measurement devices as well as take 80 CTD casts from the surface to the ocean floor to assist the scientists from the OSNAP program with their effort to estimate the Subpolar Overturning circulation and associated transport of heat and freshwater. In this blog we will detail some of the work being done, why it matters to oceanography and what life on board feels like. We hope you will enjoy reading this blog.
Day 1 – Day 3: Our journey begins in St. John’s – the capital of the easternmost province of Canada: Newfoundland and Labrador. St. John’s is a vibrant little city on the Atlantic coast inside a natural harbour of ragged rocks towering around the fjord. The night before putting out to sea snow fell and for a moment coated much of the city in a fine layer of fine white dust. It must have been an unusual sight to see snow in the end of May for most of our team arriving from places where summer already started. Our team consists of 21 scientists, 6 of which are bio-geochemists from Dalhousie University (Canada) and 15 physical oceanographers from GEOMAR (Germany), Memorial University (Canada), NOC (UK) and EPPS (France).
St. John’s Harbour view from Signal Hill
St. John’s Harbour view from Signal Hill
Last night in St. John’s (May 24/25) and snow
After re-fuelling or bunkering as they say in nautical language, the ship’s ready for the voyage. The pilot boards the ship and helps the captain navigate safely into the open ocean. We all say goodbye to mainland until the end of cruise and are greeted by 2-4 meter swells. We began the journey with safety drills and figuring our safety equipment like entering the free-falling lifeboat and lifejackets.
After this was done it was time for our CTD test station (Station 27), just outside St. John’s. CTD stands for: Conductivity, Temperature and Depth sensor and is used to measure very precisely the temperature of the water. From conductivity, temperature and pressure, the salinity and density can be worked out using standardized equations for sea water. These variables are the most important physical variables captured on all ocean research cruises. However, our CTD cast has many more instruments attached such as an ADCP to measure current speeds, an oxygen sensor to measure dissolved oxygen, a flourometer to measure plant life activity, an altimeter to detect the distance to the seabed, and a UVP (underwater vision profiler) that takes high resolution pictures of living beings under water like plants, marine life and other particles. The CTD is attached to the so called rosette, which includes water bottles that can be closed at different depths and collect samples of sea water. These are then analyzed by the bio-geochemists for gasses such as oxygen or carbon dioxide as well as nutrients.
Everyone was present to learn about the different steps in taking a CTD cast and the necessary water sample preparations that needed to be done. Unfortunately, at this point, some of our scientific crew started to battle the effects of sea sickness and the ship’s doctor was doing his best encouraging us and gave us patches to counteract the sickness feelings. Now we are on our way to our first real mooring and CTD station site and we should arrive there this evening. Looking forward to what the day will bring.
The eastern subpolar North Atlantic has a special role in the Atlantic Overturning Circulation (AMOC) and global climate change. The waters in this regions are subtropical-origin warm and salty water masses, which are carried by the North Atlantic Current. They either move further northward into the Nordic Seas or cyclonically circulate to the Irminger Sea and Labrador Sea and then transform into dense waters. Substantial heat is released along their pathways, which is pivotal to maintaining a relatively warm climate in Northern Europe.
As a component of the Overturning in the Subpolar North Atlantic Program (OSNAP), WHOI-OUC jointly deployed gliders (underwater autonomous vehicles) in the Iceland Basin. The Iceland Basin lies east of the Reykjanes Ridge and west of the Rockall Plateau. The battery in each glider can afford continuous scientific sampling up to 6 months. Therefore, a new mission needs be triggered every half year. This is very challenging because it is not easy to find suitable ships, especially in winter, to recover and deploy vehicles. We had to rent small boats in Iceland and launch gliders near the coast. The glider section is about 260 nautical miles (more than 425 km) south of Iceland. With an averaged horizontal flying speed of 0.2 m/s, the glider takes three to four weeks to navigate to the working site. If the glider needs to swim back to the coast, the vehicle had to stop scientific samplings one month before the battery run out. In other words, about one third (two months over 6 months) of energy will be spent on the non-scientific task!
Despite the various logistics we have to deal with, 6 glider missions have been successfully completed between June 2015 and December 2017. More than 3000 hydrographic profiles have been collected. Moving at approximately 0.2 m/s, gliders ‘fly’ through the ocean from surface to 1000 m. In each dive-climb cycle, they navigate along a sawtooth trajectory and measure temperature, conductivity (salinity), pressure and oxygen. The horizontal sample-spacing averages 3 km, but near the surface and 1000-m turnaround points distance ranges from hundreds of meters to 6 km. The surveyed section is along 58°N with endpoints at 24.5°W and 21°W, respectively. The section is about 200 km in length and a one-way transect is usually completed in 7–10 days.
Hydrographic properties in the Iceland Basin for mesoscale eddy and frontal circulation patterns near 58°N. The left panels show the ocean state on 3 -13 August, 2015, for absolute dynamic topography (a), glider potential temperature (c), and glider salinity (e). The corresponding ocean state on 14-20 December 2016 is displayed in the right panels (b, absolute dynamic topography; d, potential temperature; f, salinity). Glider transect is marked by black lines in a) and b). The isobaths in panel a) and b) are represented by gray lines. The gray contour lines from panel c) to f) display the relative potential density.
The In-situ observations indicate two circulation regimes in the Iceland Basin: a mesoscale eddy like pattern and northward flowing NAC pattern. When a mesoscale eddy is generated, the rotational currents associated with the eddy lead to both northward and southward flow in the Iceland basin. This is quite different from the broad northward flow associated with the NAC when there is no eddy. The transition between the two regimes coupled with the strong temperature front in the Iceland basin can modify the meridional temperature flux on the order of 0.3PW. The dramatic variability induced by alternating eddy and frontal patterns is also found in high-resolution (1/12°) HYCOM simulations. In addition, a separation of large scale and mesoscale processes in the model results suggests that eddies in the Iceland Basin make significant contributions to the variability of the total basinwide poleward heat flux on time scales from subseasonal to interannual.
Detailed examinations of satellite altimetry data suggest that the alternative occurrence of eddy and front are quite common in the Iceland Basin. Therefore, the observed two circulation regimes detected from glider data are generally representative of long-term conditions. The velocity change associated with the eddy and front scenarios contribute to high Eddy Kinetic Energy (EKE). The surface EKE from altimetry data suggest that enhanced EKE is located in the eastern part of the subpolar region, especially in the Iceland Basin and Rockall Trough, coincident with the branches of the NAC. Similar EKE map is also reproduced by the eddy-resolving (1/12°) HYCOM simulations. In addition, the model results reveal that EKE along the OSNAP East line has nice correspondence with the meridional heat transport variability, with the highest values located in the Iceland Basin. As a result, the eddy and front structures captured by the gliders are the dominant processes to generate the temperature transport variability in the Iceland Basin.
a) Mean surface Eddy Kinetic Energy (EKE) from 1993 to 2015 from the satellite data. Unit: m^2/s^2. Magenta dash line represents the OSNAP East. Black diamonds denote the end points for the glider transect. The isobaths are illustrated by white contour lines. b) Standard deviation of the meridional heat transport at each longitude in numerical simulations (red). The mean surface geostrophic EKE from altimeter observations (1992-2015) and numerical model (1992-2014) are displayed in blue and black, respectively. The vertical black dashed lines mark the endpoints of the glider transect, where the meridional heat transport has largest variability.
The results from the glider observations provide a fresh perspective on the dynamics responsible for the poleward heat transport in the subpolar North Atlantic Ocean, revealing that the alternating eddy and front patterns contributes significantly to the total poleward heat transport variability on time scales from subseasonal to interannual. This is different from our understanding about the mechanisms for oceanic heat transport variability, where large-scale circulation changes are believed to be the main driver. Our results emphasize the importance of resolving mesoscale processes in observations and numerical simulations to realistically capture their roles in modulating heat transport variability in the northern North Atlantic. High-resolution observational arrays capable of capturing both large scale and mesoscale variability, such as the OSNAP observing system (which includes moorings, gliders, Argo floats and satellite altimetry), are needed to measure the basinwide ocean MHT in the subpolar North Atlantic.
Results were recently published in Nature Communications:
Zhao, J., A. Bower, J. Yang, and X. Lin, 2018. Meridional heat transport variability induced by mesoscale processes in the subpolar North Atlantic. Nature Communications, 9, 1124, doi: 10.1038/s41467-018-03134-x
Science has many aspects. The collection of data on the process of region you are studying; the actual digging through the data to find out what is going on (which is what most people seem to associate with science); writing down the results so they are documented and others can learn what you learned; discussing the results with colleagues in your fields and hearing about new methods and results; and the training of new scientists. Sometime all of these happen within a very short time frame.
After finishing a proposal and a paper draft at the end of January, February started with a short research cruise. This may not have been an OSNAP cruise, but all hydrographic cruises share very similar aspects. The run-up to the cruise is a lot about getting the logistics arranged and preparing a cruise plan. Does everyone have
The RV Pelagia, our home for the duration of this cruise.
the right paperwork to go on board? How are we going to divide the tasks on board? How many measurements stations will be able to do and where? And most importantly… will all the instruments work and the data be good? Once on board things start to fall into place. Those who are back in their familiar environment show the ropes to the newcomers. Instruments are prepared and plans discussed with the ship’s crew. During the cruise, while data collection progresses, we process the initial results and get an idea of whether our science goals will be fulfilled. Once land is in sight on the other end of the cruise everyone is tired and ready to go home.
Unless, due to some haphazard planning, you need to proceed directly to a scientific conference. While cruise departure and arrival day are never fixed until you actually departed or arrived, dates of meetings with several thousand participants tend to be quite fixed. The Ocean Sciences Meeting is a biannual meeting with all fields of oceanography. About 5000 people met in Portland, OR, this week to discuss their work. This happens in “sessions”, submeetings organized by topic, of which there were nearly 500. The new OSNAP results were presented in
Closing remarks at the OSM18.
the AMOC (Atlantic Meridional Overturning Circulation) session. Several of us had oral talks. Susan Lozier presented the (near) final numbers of the overturning over the whole OSNAP line. Penny Holliday shows us the circulation estimates from the OSNAP hydrographic sections in 2014 and 2016. Bill Johns detailed the Iceland Scotland Overflow Water flow along the east flank of the Reykjanes Ridge. I presented our investigation of variability west of the Reykjanes Ridge. Feili Li compared Labrador Sea Water formation with Labrador Sea overturning. More OSNAP talks were held in other sessions (see OSM blog post). Since there are too many of us to all get talks (even if they only last 15 min) some are requested to present posters. Poster sessions are held during the last two hours of the day. Presenters stand next to their posters and the rest of use walk around and either browse poster titles to find something that interests us or seek out poster titles we identified beforehand in the program. Often the most interesting discussions are held at posters we unintendedly come across and these are great chances to meet new people. Between the talks and posters we catch up with old friends or meeting with co-authors on papers or potential new collaborators.
Now that I’ve returned from the Ocean Sciences Meeting I’m starting a new chapter. My first PhD student, Roos Bol, has started her position at NIOZ this week. She recently finished her MSc thesis at the NOC in Southampton and I’m very excited to be working with her. She’ll be investigating our OSNAP data from the Irminger Current array and will be coming along on this summer’s OSNAP cruise. Over the next four year I’ll expect to be teaching her all about data collection, data processing, writing down results and giving presentations at conferences.
Katherine Kornei captures an overview of the Atlantic Meridional Overturning Circulation, and OSNAP’s role in observing overturning in the North Atlantic. The summary “Ocean array alters view of Atlantic ‘conveyor belt’” can be found here.
Earlier this week many of the researchers and scientists involved in OSNAP presented their work, based on the first two years of continuous monitoring in the North Atlantic, at the Ocean Sciences Meeting in Portland, OR. While there are still a lot of implications and details forthcoming, Nature – News reported on these findings in a short summary linked below.
Abstract: Changes in freshwater transport into the subpolar North Atlantic have the potential to disrupt or enhance the formation of dense water with subsequent impact on the meridional overturning circulation and associated ocean heat transport. Freshwater budget components in the subpolar North Atlantic include input from the atmosphere (precipitation vs evaporation, and river run-off), Greenland ice-sheet melt, saline subtropical water carried by the MOC, the dense overflow waters, and Arctic-origin freshwater carried by the shallow boundary currents that follow pathways west and east of Greenland. In this analysis we use a multi-decadal data set from the Labrador Shelf to characterise long-term variability in the transport of Arctic freshwater in the Labrador current. We first present evidence from an eddy-permitting global ocean circulation model to determine the origins of the water sampled by our time series. In particular we examine the dynamics of the currents on the Labrador Shelf in order to isolate the Arctic-origin water masses. We describe how we derive a 65-year record of changing Arctic freshwater transport from the observational data set. We will show that the multi-year changes in freshwater transport in the Labrador Current are consistent with independently-observed changes in subpolar freshwater storage.
Abstract: An international effort, Overturning in the Subpolar North Atlantic Program (OSNAP), is a partnership among oceanographers from the US, UK, Germany, the Netherlands, Canada and China whose goal is to measure and understand what drives the Atlantic Meridional Overturning Circulation (AMOC) and its variability. With high-resolution mooring arrays from the Labrador coast to the Scottish shelf, OSNAP provides a continuous record of the full water column, trans-basin fluxes of heat, mass and freshwater in the subpolar North Atlantic and has been operational since 2014. Data from the first 21 months of the full OSNAP observing system has been used to produce the first continuous time series of these variables. In addition to these time series, time mean estimates for all fluxes and attendant uncertainties will be presented, along with comparisons with other contemporaneous AMOC measurements and a discussion of subpolar overturning variability.
Abstract: The international observational program, OSNAP (Overturning in the Subpolar North Atlantic Program) began in the summer of 2014 for the purpose of recording continuous trans-basin observations of volume, heat and freshwater. OSNAP will investigate the complex interplay between AMOC and gyre circulation, air-sea fluxes and ocean heat and freshwater transport convergence which presently lack observational evidence. The OSNAP array uses moored instruments, gliders and floats to measure velocity, temperature and salinity along a section from Canada to Greenland to Scotland. Here we present detailed views of the full-depth properties and velocity field from two high resolution hydrographic sections along the OSNAP line taken at the start of programme in June-July 2014 and during mooring turnaround cruises in May-August 2016. We derive estimates of the meridional overturning and gyre circulation and their components of heat and freshwater flux, finding that while the overturning dominates the heat flux, the freshwater flux is predominantly carried by the gyre. We show a notable difference in the magnitude of the overturning circulation and the heat and freshwater fluxes as measured by the two synoptic sections, and discuss how this relates to the associated differences in temperature, salinity and density fields.
Abstract: Since 2014, an array of current meters deployed as part of the OSNAP trans-basin observing system has provided new measurements of the southward flow of Iceland-Scotland Overflow water (ISOW) along the eastern flank of the Reykjanes Ridge in the Iceland Basin. The location of the array, near 58°N, captures the ISOW Deep Western Boundary Current at the farthest downstream location in the Iceland Basin before significant amounts of ISOW can flow into the Irminger Basin through deep fractures in the Reykjanes Ridge. The transport of the ISOW plume at this location – based on the first two years of OSNAP observations (July 2014 to July 2016) – is 5.8 ± 0.9 Sv for ?? >27.8. Most of this transport is carried in a main branch of the plume along the upper ridge crest in depths from 1400-2200 m. A secondary branch in depths of 2400-2700 m along the lower ridge crest carries about 1 Sv. The transport of the ISOW plume varies over a considerable range, from about 2-10 Sv on weekly to monthly time scales (std. dev. = 2.4 Sv); however the mean currents from two individual year-long deployments are very similar and indicate a robust mean flow structure. Watermass analysis of the plume from continuous temperature/salinity measurements shows that about 50% of the plume transport (2.6-3.0 Sv) is derived from pure Norwegian Sea Overflow waters (NSOW) – consistent with the amount of NSOW known to be flowing over the northern sills into the Iceland Basin – while the remainder is made up of approximately equal parts of entrained Labrador Sea Water and modified Atlantic thermocline waters. The observed ISOW transport at this location is larger by almost 2 Sv than previous values obtained farther north in the Iceland Basin, suggesting that either additional entrainment into the ISOW plume occurs as it approaches the southern tip of the Reykjanes Ridge, or that the previous measurements did not fully capture the plume transport.
Abstract: The “null-hypothesis” for sea surface temperature (SST) variability is that the ocean mixed layer integrates stochastic atmospheric forcing, leading to red SST spectra. According to this hypothesis, decorrelation timescales (e.g., e-folding timescales) of SST are a function of the mixed-layer depth (MLD) and the damping parameter. In this work we evaluate the ability of the null-hypothesis to explain interannual SST variations in the extra-tropical North Atlantic and North Pacific. First, we develop an idealized red-noise model of the mixed layer heat balance in the North Atlantic, in which the oceanic contribution is neglected in order to isolate the effects of atmospheric forcing. We evaluate the e-folding timescale in this model using observational datasets. Results suggest that in both the North Atlantic and the North Pacific, e-folding timescales depend strongly on the mixed layer depth, but the relationship is stronger in the North Atlantic. Then, we use gridded ocean temperature observations to directly calculate the decorrelation timescales for both SST and upper-ocean heat content and compare these timescales to those predicted by our theoretical model. Regions where decorrelation timescales differ significantly from those predicted by our theoretical model indicate the importance of processes other than local atmospheric forcing, including reemergence of SST anomalies, ocean dynamics, and/or external forcing.
Abstract: The North Atlantic undergoes swings in sea-surface temperature (SST) on multidecadal timescales, with consequent impacts on the climate of adjacent land areas. Proposed mechanisms behind this Atlantic Multidecadal Variability (AMV) fall into two main categories: external forcing e.g. due to anthropogenic aerosols; or internal modes of variability e.g. involving the Atlantic Meridional Overturning Circulation (AMOC). In either case the relationship between the changes in oceanic heat transport and the SST is not well understood. Here we develop a framework to investigate which physical processes determine SST variability on decadal to multidecadal timescales by evaluating contributions from the net ocean-atmosphere heat flux, the divergence of the temperature transport, and entrainment between the mixed layer and the layer beneath. We analyse the 300-year present-day control simulation of the HADGEM3-GC2 coupled climate model, which shows a 20-30 year AMV variability similar to that observed.
We find that the AMOC leads the AMV by ~5 years. The model suggests that a key process connecting the AMOC to the AMV is heat transport divergence into/out of the mixed layer. AMOC changes themselves are preceded by changes in the eddy heat transport divergence in the deep ocean on times scales of ~12 years.
Abstract: Recent studies have shown that a thermohaline coordinate system can be used to simplify the complex spatial structure of the global ocean circulation with minimal loss of information (e.g. Zika et al 2012, Groeskamp et al 2014). This thermohaline framework is particularly useful in studying the fluxes of heat and freshwater within the ocean, such as those associated with the AMOC.
In contribution to OSNAP we have developed a novel inverse method in thermohaline coordinates called the Regional Thermohaline Inverse Method (RTHIM). For a control volume, RTHIM invokes a balance between advection into the volume, fluxes of heat and freshwater through the surface, and interior mixing within the volume. Taking known surface fluxes and temperature-salinity distributions, RTHIM determines unknown section velocities and rates of interior mixing.
Using a 20-year mean of NEMO model data from 1988-2007, we have validated RTHIM for an Arctic control volume bounded to the south by a section at around 60°N by comparing section transports and interior mixing rates from the inverse solution with those diagnosed from the model. We find that the RTHIM solutions are robust to various model parameters and initial conditions. The MOC, heat and freshwater transports calculated from the RTHIM solutions are within 15%, 11% and 8%, respectively, of the NEMO ‘truth’. We also see good agreement between mixing rates obtained from the RTHIM solution and those diagnosed from the model.
Our aim is to construct a domain bounded by the OSNAP line and Bering Strait, and apply RTHIM to observations from satellite altimetry, gridded Argo and a selection of surface flux products. From this we can obtain independent estimates of the AMOC at the array, and mixing rates within the Arctic and Subpolar North Atlantic basins. Since these products extend 20 years before the OSNAP observations, our analysis will help contextualise the AMOC variability measured by the array and assess the significance of trends.
Tuesday, February 13, 2018; 4:00 PM – 6:00 PM
Oregon Convention Center; Poster Hall
Abstract: While it has generally been understood that the amount of deep water formed in the Labrador Sea (LSW) impacts the meridional overturning circulation (MOC), this relationship has not been validated against observations. A current observational program (Overturning in the Subpolar North Atlantic Program: OSNAP) is aimed at ascertaining this linkage, but it will be a few years before this observational time series has sufficient degrees of freedom to evaluate the necessary correlations on time scales exceeding the annual. For now, we turn to a suite of global ocean and ocean–sea-ice models, varying in resolution from non-eddy-permitting to eddy-permitting (1°–¼°), to investigate the local and downstream relationships between the LSW volume and the MOC on interannual to decadal time scales. Simulated measures of the LSW volume changes and MOC variability are compared to available observational measures. In this presentation, we show that all models display a strong relationship between the LSW volume changes and the local overturning variability within the Labrador Sea, but this relationship degrades downstream. However, there are some differences among the models in their representations of these relationships.
Abstract: The meridional heat flux in the subpolar North Atlantic is pivotal to maintaining a relatively warm climate in Northern Europe. Much of the variability in the basin-wide northward heat flux between Greenland and Scotland occurs in the Iceland Basin (east of the Reykjanes Ridge and west of the Rockall Plateau), where the North Atlantic Current (NAC) carries relatively warm and salty water northward. As a component of the Overturning in the Subpolar North Atlantic Program (OSNAP), WHOI-OUC jointly deployed gliders in the Iceland Basin to continuously monitor the circulation and corresponding temperature flux associated with the NAC. In-situ observations indicate two circulation regimes in the Iceland Basin: a mesoscale eddy like pattern and northward flowing NAC pattern. When a mesoscale eddy is generated, the rotational currents associated with the eddy lead to both northward and southward flow in the Iceland basin. This is quite different from the broad northward flow associated with the NAC when there is no eddy. The transition between the two regimes coupled with the strong temperature front in the Iceland basin can modify the meridional temperature flux on the order of 0.3PW. The dramatic variability induced by alternating eddy and frontal patterns is also found in high-resolution (1/12°) HYCOM simulations. In addition, a separation of large scale and mesoscale processes in the model results suggests that eddies in the Iceland Basin make significant contributions to the variability of the total basinwide poleward heat flux on time scales from subseasonal to interannual.
Wednesday, February 14, 2018 – Location: A107-A109
Abstract: The Gulf Stream has been characterized as either a barrier or blender to fluid transfer, a duality relevant to gyre-scale climate adjustment. However, previous characterization depended on relatively sparse, Lagrangian in-situ observations. The finite-time Lyapunov exponent (FTLE) is calculated from satellite altimetry to identify Lagrangian coherent structures (LCS) in the Gulf Stream region. The focus here is on the transient and intermittent behavior associated with eddy propagation and eddy-jet interaction over timescales of a few days, in contrast to other studies characterized by longer integration times. These LCS provide dense sampling of flow, capture dynamically-distinct regions associated with transport and mixing, and even represent some flow structure at finer spatial scale than the observational grid. Independent satellite observations of ocean color contain similar flow-dependent structures, providing verification of the method and highlighting transport and mixing processes that influence sea surface temperature and chlorophyll, amongst other water properties.
Diagnosed LCS support the existing Bower (1991) kinematic model of the Gulf Stream, but also highlight many new processes of comparable importance. These include vortex pinch-off and formation of spiral eddies, clearly identified by LCS, and which may be explained by considering changes to flow topology and the dynamics of shear-flow instability at both small and large Rossby number. Such processes, seen though LCS, may enhance validation of climate models.
The spatial distribution of these intermittent processes is characterized in terms of the criticality of jet dynamics with respect to Rossby wave propagation, and whether the jet is in an unstable or wave-maker regime. The generation and connectivity of hyperbolic fixed points in the flow appear to play an important role in governing large-scale transport and mixing across the Gulf Stream.
Wednesday, February 14, 2018; 4:00 PM – 6:00 PM, Oregon Convention Center; Poster Hall
by Doug Wallace, Dalhousie University and Brad deYoung, Memorial University
The Labrador Sea, off the east coast of Canada (see figure), is one of the few places where the deep ocean exchanges gases such as oxygen and carbon dioxide (CO2) directly with the atmosphere. Localized deep convection releases large amounts of heat to the atmosphere and the resulting Labrador Sea Water contributes to the global ocean thermohaline circulation that redistributes heat from low latitudes to the poles. Transport out of the Labrador Sea carries oxygen and anthropogenic CO2 into the North Atlantic interior, oxygenating subsurface layers and slowing the accumulation of CO2 in the atmosphere, but exacerbating ocean acidification along Canada’s sensitive eastern continental margin. The combined action of convection and horizontal circulation redistributes nutrients and contaminants (e.g. from future deepwater oil production along the deep Labrador slope) potentially affecting ocean productivity and marine ecosystem health.
It seems to be a quiet year for OSNAP – just one OSNAP cruise took place last summer and so most of the moorings have stayed in the water for a year and a half. But OSNAP scientists have been working intensively on analyzing the first two years of data and we have started to obtain some very interesting results based on measurements from individual arrays. Some of those results were presented and discussed at a workshop held in Southampton, UK in early November (blog post). Much more is coming and will be presented at the Ocean Sciences Meeting 2018 in Portland, OR. Please stay tuned as we will soon publish a dedicated blog post with a list of all OSNAP-related presentations at OSM 2018.
In addition to analyses by individual groups, all OSNAP scientists have been working closely on the first data products from the full array. We are now in the process analyzing preliminary results and finalizing the flux estimates. Final products are expected to be delivered in spring of 2018. These final products include the overturning volume and associated heat and freshwater transport time series along with the cross-sections of velocity, temperature, and salinity (Figure 1 below shows the mean velocity and property fields at OSNAP). It is always worth mentioning that OSNAP is not an isolated program as our results are based on many existing observational efforts in the region (e.g., Argo, AVISO) and the results will be analyzed in coordination with ongoing programs (e.g., OVIDE, RAPID).
During the first two years of the OSNAP deployment, the subpolar North Atlantic experienced a widespread cooling with two successive intense winters (2014/15 and 2015/16). Strong air-sea heat fluxes during those two winters led to intensified deep convection with an enormously large production of Labrador Sea Water. All the changes make us wonder about any concurrent changes in circulations (overturning and gyre) at the subpolar latitudes. Once we have firmed up the flux estimates at OSNAP, we will soon begin the task of investigating those observed changes in the region and linking them to local and/or remote forcing mechanisms.
Next year sure will be a productive year for OSNAP – be prepared for exciting news!