Category Archives: Student/Postdoc Blog

by Xianmin Hu

I have been in Edmonton for almost ten years (PhD study and then postdoc of University of Alberta). When I tell people I am studying (physical) oceanography, they always laugh at me. I wish I had a perfect answer when they ask me “Why are you studying the ocean here? There is no ocean in Edmonton.” Physical Oceanography sounds strange to most, so I always “hide” the physical part in the name.

Well, there are too many things I can’t explain to them well enough. I count myself as an oceanographer but I don’t swim. Actually I prefer to move upward as I do climbing …. My friends joked that that I am preparing for the sea level changes. We know the sea level changes slowly, so one step higher might be the most efficient way (selfish though) to step away from the sea level rising problem.

With very little experience at sea, I am working with computers most of the time. Yes, I am one of them, the mysterious numerical ocean modellers. I have been working with the NEMO model (same name of the famous little fish) for years. To be more specific, I mainly do simulations with a regional configuration called ANHA. Someone once asked, “Is ANHA your girlfriend?” Of course, he was joking. ANHA stands for the Arctic Ocean and Northern Hemisphere Atlantic configuration. However, he was also right. To me, when you opt to sacrifice your personal time on one thing, it could be love; otherwise, it is just a one-way “benefit friendship”.

Sorry for drifting too far away, which is also a common problem in numerical modelling. Let’s pull it back to the ocean. My concern of the ocean is the salt. Life needs more salt, however, an ocean modeller may not agree because more salt leads to the model drift, particularly in the high latitude, which makes people feel bad for models.

However, sometimes, it is not necessarily the fault of the model itself. The ocean is thirsty. If there is little river discharge and rain, the Arctic Ocean and Atlantic Ocean salinity could look something like figure 1.

Well, let’s feed the ocean with a reasonable amount of river discharge and precipitation, and the ocean looks much better (figure 2).

Look (figure 2), can you see the freshwater plumes from the the big rivers? I was very excited to “see” these rivers showing up in the simulation. Meanwhile, a picky person like you may also notice the fresh water around southern Greenland. Yes, we can assume Greenland is just a rock island in the model. But we know the Greenland Ice Sheet (GrIS) is melting and feeding the surrounding ocean with a large amount of freshwater. Here we must give Jonathan Bamber credit for his freshwater estimates from the Greenland Ice Sheet.

The ocean is pretty happy with the freshwater from Greenland, thus, it is willing to tell us more about her, even the secret pathway of the Greenland meltwater (figure 3).

Not a big surprise, it agrees with the general ocean circulation in this region as we know it. However, did you ever think about the spatial distribution, e.g., how much is accumulated in Baffin Bay and how far it can go down to south along the east coast of North America? The models do tell people something that is hidden behind what we see.

In the end, a nice story was made, but still, there is no ocean in Edmonton….

By Elizabeth Comer

In my last blog a year ago I mentioned I would be going to sea for the Extended Ellett Line (fig. 1) annual research cruise, which is part of the OSNAP array.  I boarded the RRS Discovery during June and we steamed towards Iceland, where we started sampling at each CDT station whilst heading back to Scotland. During this cruise I focussed mainly on the ADCP measurements and processing, as I was going to start using it within my research and this was the perfect way to get familiar with it. I had a brilliant time on this cruise, as the photos hopefully depict (fig. 2, 3 and 4), and it was a great insight to Oceanography in the field.

Now onto how this data will aid my research. The aim of my current research is to investigate the long-term mean and variability in volume, freshwater and heat transport of the Subpolar North Atlantic. All the hydrographic and velocity measurements collected annually are used to compute these transports, providing a near 20-year timeseries. It is important that we take continuous measurements at this location and compute these transports as 90% of the Atlantic inflow into the Nordica Sea and Arctic takes place between Iceland and Scotland, as well as roughly half the returning buy levitra online dense water. This research will add to our knowledge of how the Meridional Overturning Circulation in the Subpolar Atlantic is varying on a decadal to seasonal timescale, and add insight to the mechanisms controlling it. This observational data also enables the enhancement of oceanic models to produce transport variabilities on longer timescales.

The research into the long-term transport that has been carried out with my Supervisors Dr Penny Holliday and Prof Sheldon Bacon will be published in the near future, but below is a taster of the 18-year (1997-2015) mean Lowered-ADCP velocity across the section (fig. 5). This data is used to produce the long-term absolute velocity by providing a reference velocity for the geostrophic velocity field, which the transports are computed from. In this study I investigate how the transport varies across the region, highlighting the key routes for transport and why it is important we know this. Furthermore, the Scottish Continental Slope Current that runs northward along the Scottish shelf edge and the Rockall-Hatton Bank have the highest temperatures and salinities of the region. Hence, I hypothesis that these are fundamental routes for heat transport northward.

by Patricia Handmann

In October 2014 I started my PhD at GEOMAR and Kiel University. I was coming from KIT (Karlsruhe Institute of Technology ), where I studied experimental and theoretical physics and worked on cloud microphysics for a year as part of my diploma (masters) thesis. Before I started my diploma thesis I got the chance of an internship at Alfred-Wegener Institute and worked on quality control of some old cruise data from RV POLARSIRKEL in 1980/81, which was a great experience. I had a lot of fun during these three months at AWI while getting to know this new field of ocean physics. So when I was looking for a PhD after this internship I was intrigued by the general topic of deep-water formation in the northern and southern hemisphere. When I started at GEOMAR this idea quickly evolved into a more focused study comparing a high-resolution ocean model (VIKING20) with observations from the 53°N array. This boundary current observatory is maintained by Kiel scientists since 1997 to document the changes of the deep-water circulation in the Subpolar North Atlantic (SPNA).

My main research question is: What are processes imprinting variability to the deep-water masses in the SPNA, where do they occur and what are the physics behind them.

But lets start with some background information …

The exit of the Labrador Sea is a key location in the subpolar North Atlantic concerning the integral quantities of the Deep Western Boundary Current (DWBC). It is the place where deep water masses from different origins and pathways meet. The combination of these is collectively called North Atlantic Deep Water (NADW).

To evaluate the high resolution model VIKING20 by means of integral quantities at the exit of the Labrador Sea, and to interpret the observed hydrographic and dynamic DWBC features with consideration of the underlying physical processes and forcing is the aim of my work.

I found that the VIKING20 model, which is driven by CORE2 atmospheric forcing, can be nicely compared to more than decade-long observations at the exit of the Labrador Sea near 53°N. VIKING20 is a high resolution (1/20°) nest, based on the global configuration of the NEMO-LIM2 ocean-sea ice model ORCA25 in the North Atlantic and implemented by two-way nesting (Behrens [2013];Böning et al. [2016]). The average flow field, being one of the integral quantities of the boundary current at 53°N including the bottom flow-intensification, is reproduced by VIKING20 (figure 1).

Although circulation and recirculation is stronger and more barotropic in the model than in the observations the overall transport at 53°N including both circulation and recirculation coincides with the observed transport of NADW of ~30 Sv (Zantopp et al. [2017]). Is the model, apart from its challenges in hydrography and hence different baroclinicity, still reproducing variability imprinting processes on Labrador Sea Water (LSW) and the lower North Atlantic Deep Water (LNADW)?

In both model and observations the low pass filtered time series are less correlated than the high frequency containing raw transport signal. This could be interpreted as reproduction of low frequency variability imprinting processes that are reproduced by the model. 

But pursuing a PhD in Physical Oceanography does not only include programming and working on data in an office, it is also hands on ship work. Hence I was also able to gain some subpolar and Labrador Sea experience during the cruise MSM54 from St. Johns to Reykjavik from mid May to June 2016. On this cruise I could see and experience the Labrador Sea. We exchanged the mooring array at 53°N and did a high resolution hydrographic survey to maintain the high quality and dense coverage of data in the Labrador Sea. Furthermore moorings in the central Labrador Sea, Irminger Sea and near the west Greenland shelf break where renewed. Knowing what the problems and powers of observational oceanography in this region are is helping me a lot in understanding challenges in my model – observation comparison process.

My current work focuses on the low frequency variability in the model and the observations. Some of the exciting findings will be published soon.

The processes causing this variability are still subject to ongoing research.

Bibliography

Behrens, E. (2013), The oceanic response to Greenland melting: the effect of increasing model resolution, Kiel, Christian-Albrechts-Universität, Diss., 2013.

Böning, C. W., E. Behrens, A. Biastoch, K. Getzlaff, and J. L. Bamber (2016), Emerging impact of Greenland meltwater on deepwater formation in the North Atlantic Ocean, Nature Geoscience.

Fischer, J., M. Visbeck, R. Zantopp, and N. Nunes (2010), Interannual to decadal variability of outflow from the Labrador Sea, Geophysical Research Letters, 37(24).

Zantopp, R., J. Fischer, M. Visbeck, and J. Karstensen (2017), From interannual to decadal—17 years of boundary current transports at the exit of the Labrador Sea, Journal of Geophysical Research: Oceans.

By Neill Mackay

One of the challenges in oceanography is that taking observations is expensive and time-consuming, and while the observations we do make are essential to our understanding, it is only possible to observe a tiny fraction of the vast history of the ocean in time and space. We turn to models to try to fill in the blanks, and my work as a post-doc at the National Oceanography Centre in Liverpool makes use of a particular type of model called an inverse model.

With the usual sort of ocean model, or ‘forward model’, we take an initial guess at the state of the ocean and set of physical laws and press ‘play’, and the model runs forwards in time, producing outputs for some later point which can be compared with the real ocean observations. The difficulty is in choosing the initial state which results in a good fit between the model output and the observations. Various parameters, such as mixing rate – how fast or slowly water with different properties becomes homogenised in the ocean – might be adjusted within the model to improve the fit. An inverse model is kind of a backwards model, as we start with the observations, apply the same set (or a subset) of physical laws and obtain the desired parameters as outputs. These parameters can then tell us something useful about a particular region of the ocean we are studying.

In OSNAP, the mooring arrays will provide a set of continuous observations along a section over a period of years. The inverse model we are developing, called the Regional Thermohaline Inverse Model or ‘RTHIM’ for short, will help to provide some context for the mooring observations by telling us something about what happens in the region to the north of the section (i.e. within the Subpolar Gyre), and also what may have happened over a longer time period before the observations started. RTHIM makes use of all the available historical observations of the region from satellites, automated floats and oceanographic surveys, going back more than 25 years. It works on the principle that the temperature and salinity (saltiness) of the water in the region (the ‘Thermohaline’ bit) changes either when water flows in or out of the region from the rest of the ocean; or when heat or freshwater is transferred through the ocean surface (e.g. through cooling by the winds or rainfall, respectively); or when different types of water are mixed together in the interior. Making use of this balance, RTHIM allows us to work out the flow of different waters into and out of the Subpolar Gyre, and the rates of mixing within it, without using the OSNAP array observations. We can then compare our inverse solution with the observations, and in addition we can find solutions for a time before the OSNAP mooring arrays were deployed. This means that variability measured by the OSNAP array over time, for example of the flow into and out of the Subpolar Gyre, can be put in context. Importantly then we can start to tease out where observed changes are due to natural variability, and where they are part of a trend – such as one related to man-made climate change.

 

by Loïc Houpert

So today, I decided to contribute to the blog by telling you what I like in my work as an observational physical oceanographer.

I am working as a postdoc at the Scottish Association for Marine Science in the beautiful and “occasionally” wet town of Oban. Being a physical oceanographer, I am generally interested in understanding the ocean’s circulation, but right now my interest is more focused: how is the ocean’s heat carried by the currents in the North Atlantic and where is this heat going? The transports of heat and also freshwater by the North Atlantic current system are particularly important for the temperature, precipitation, and wind patterns and strength over the European continent. In my research, I am using autonomous instruments (underwater glider and fixed instrumented lines) that continuously record the state of the ocean.

Going at sea to take measurements and deploy/recover instruments is definitely the best part of my work, although stressful at times. But after coming to shore, the most interesting part of the job is to actually unravel all these big datasets and try to identify physical signals that are associated to the dynamic of the ocean. In addition to having good knowledge of physical oceanography, several factors are important when working on observations: curiosity (being interested in seeing buy ambien online what is in the data), imagination/intuition (finding a way to put together the different pieces of the puzzle) and of course a little bit of skepticism (test the results’ robustness again and again …!).

I really choose to become an observational oceanographer in the second year of my Master, during my research project. It’s true that I had some second thoughts after my first sea-going experience as I was very sick for most of the time of this 7-day cruise… However, 8 months later, during my PhD, I took part in my second cruise and everything went (surprisingly) well. Of course some days were more bumpy than others, but at the same time, this 3-week cruise was in the middle of the Gulf of Lions in winter to sample the impacts of strong storm and deep (2000m) vertical mixing on the marine ecosystems…. All of this to say that you should never stay on a bad first experience when going at sea. Tenaciousness… this is also a good quality if you want to analyze ocean observations!

 

Tillys Petit, PhD student (who also enjoy the view from my office, figure 2)

Nowadays, numerical models are increasingly used to understand and predict climatic issues such as global warming, rising sea level, or shift of oceanic circulations. To answer those questions, numerical models compute a collection of data from an initial setup, allowing us to first visualize the actual state and then the evolution of the temperature, sea level or oceanic circulation around the world. But how can we know if the output is/will be in agreement with the reality? To validate a numerical model, we still have to compare the actual state given by the model with its in-situ observations. But in-situ data are still too often lacking, and cruises are thus carried out. The new set of data is firstly analysed to document the general circulation and to identify new mechanical processes, and secondly used as benchmark for models.

Currently my work is to document the oceanic circulation across the Reykjanes Ridge (South of Iceland) where very little data is available. A strong current-bathymetry interaction could impact the circulation, hence the need for better understanding of this process. To fill this gap, a cruise (RREX) was carried out in June 2015 and another is planned in July 2017. During the 2015 RREX cruise, a lot of new in-situ data were obtained along 4 sections (figure 1), such as velocity of the flow and salinity-temperature-oxygen profiles. Moorings were also deployed and will be recovered during the second cruise. Up to now, I have studied the data of the first cruise, which are of good quality, allowing us to fully address our scientific objectives. Because I was not on board in 2015 I cannot tell you how the cruise was, but I will certainly keep you inform of the general ambiance during the second!