Ocean Sciences Editors' Vox

Observing the Ocean

How measurements from a glider deployed off the coast of Peru are contributing to a much-needed long time-series data set.

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This is one of a series of Editors’ Vox that discovers how our Editors and Associate Editors fill their time when they’re not reviewing manuscripts for AGU Journals. Despite the challenges of getting an internet connection from his research vessel in the Pacific, one of the Associate Editors of Geophysical Research Letters, Toste Tanhua, found a few moments to tell us about his latest oceanographic research.

What is the focus of your current field trip?

As a chemical oceanographer I regularly go to sea on research vessels since the measurements that I specialize in (trace substances to estimate ventilation and circulation) cannot be measured by autonomous platforms (e.g. “robots”). This particular cruise is a 39 day trip in the South Pacific Ocean off the coast of Chile and Peru to a region of “upwelling” where cold, nutrient-rich water rises up towards the surface. We are conducting an experiment where we measure the dispersion of a deliberately released tracer to understand fluxes of nutrients from the water laver directly above the bottom to the interior ocean. The sediments deprived of oxygen in this part of the ocean release particularly large amounts of nutrients that can feed phytoplankton growth and which, by sinking organic particles, contribute to expansion of the oxygen minimum zone.

What kind of data are you collecting, how, and why?

Some characteristics of the ocean, such as sea surface temperature, can be observed from satellites; other features, such as the depth of the thermocline, oxygen concentration, and subsurface ocean currents, need to be observed and measured on-site. Our fieldtrip focuses on the latter. In addition to observations from the research vessel we are deploying a small fleet of autonomous gliders that glide back and forth from the shallow shelf to the deep open ocean. Gliders are roughly two-meter long torpedo-shaped robots that can change their buoyancy slightly which they use to propel themselves forward in a see-saw pattern between the surface ocean close to the bottom (mostly not deeper than 2000 meters) over the shelf recording salinity, temperature and oxygen. This particular glider also features a microstructure sensor to characterize small scale motions that induces mixing. Although I have deployed or recovered a dozen gliders each time is different, this time we had company of a large sea lion.

One of the aspects that I am particularly interested in is oxygen levels. The Tropical Southeast Pacific is known for its very large oxygen minimum zone (OMZ). Several studies in the past have shown that the vertical extent of the OMZ has increased over the last decades and the data from this cruise seems to verify a continuation of that trend. This has implications for marine biology and potentially for fisheries, fish need oxygen for breathing and expanding OMZs reduces the livable room and is likely bad news.

Another focus of my investigations is temperature. We have observed unusually warm sea surface temperatures (SST) off the coast of Peru, up to 5 degrees warmer than climatological temperatures – very high anomalies indeed. Although this warm water is, currently, restricted to the very east of the Tropical Pacific, and one can debate whether or not this is an El Nino phenomena based on various criteria, the inhabitants of Peru and Colombia that are suffering from heavy rain fall and floods certainly feels the connection between ocean heat content of their coasts and precipitation.

Who else is on your research vessel and what are they studying?

We have a “full ship” with 28 scientists from 7 different countries studying chemical tracers, trace metals, physical oceanography, geology and biology, thus covering several processes in the ocean. Many of these complicated, and not fully understood, interactions require interdisciplinary observing systems and research, including modeling on different spatiotemporal scales, in order to understand and predict. I am fortunate to be working in a long-term, interdisciplinary research project where all these aspects, and some more, are represented in order to better understand climate-biogeochemical interaction in the tropical ocean, particularly the oxygen minimum zones. It is enormously powerful to gather a large team of scientists from different disciplines to work on a common question, all with different views and expertise.

How do your observations contribute to oceanographic research more broadly?

Collecting data over long time scales is essential for detecting and quantifying trends. This is important across the Earth sciences where we are trying to determine whether changes are due to natural variability or anthropogenic activities. However, long time-series data is quite rare in the ocean: few fixed-place ocean time-series data sets exist, and results from sparse temporally resolved observations, from ship sections for instance, can be biased by short-time scale variability. Our data will fill geographical gaps in data mainly from the area northern Chile and southern Peru. We also followed a so called repeat hydrography line (P21E) from the Peruvian coast roughly 1,000 nautical miles towards the west; these lines are particularly useful to assess changes on decadal scales.

There are certainly many unresolved questions to be answered about the tropical oceans, and the global ocean for that matter. To find ways of effective cooperation and integration of sustained observations, experiments, process studies and various modeling remains a challenge for the global community of oceanographers. I am hopeful that the earth system science will, again, prove to be up to the task.

—Toste Tanhua, GEOMAR Helmholtz Institute for Ocean Research Kiel, Germany; email: [email protected]