The ocean circulation as we know it is a result of processes that happen on the time scales, ranging from milliseconds to millenia and lengths scales ranging from molecular scales to that of the circumpherence of the planet. The combined (possibly nonlinear) interaction of all these processes lead to a complex ocean circulation that is highly variable in both space and time. As a physical oceanographer it is my duty to understand this.
My research interest spans from global scale ocean circulation on time scales of centuries, to small-scale mixing proesses on time scales of seconds.
Mixing in the ocean is important for the global ocean circulation and therefore for the circulation and uptake of tracers such as salt, heat and anthropogenic carbon. Evidently, mixing is not the same everywhere (think of the difference between calm waters and the surf zone). However, the changes in space and time of the mixing strengths are not well known. As a result, the implementation of mixing in numerical ocean circulation models is highly simplified. This can lead to uncertainties in prediction of the future's climate. As part of my research I have developed new inverse methods that uses measurements of temperature and salinity (and other tracers in the future) to obtain more accurate estimates of the distribution and magnitude of mixing in the ocean.
Global Ocean Circulation and the Distribution of Tracers
The 3-dimensional time varying ocean circulation can be represented as a 2-dimensional time averaged circulation, using a streamfunction. Although streamfunctions have been calculated for many combinations of two coordinates, historically one of the coordinates has always been a fixed x, y, or z coordinate. In my work I developed methods to calculate streamfunctions for coordinates defined by combinations of two ocean tracers, such as Salinity, Heat, Anthropogenic carbon, density, and many others. Each such combination can lead to new insight in the general ocean circulation and (re-) distribution of the analyzed tracers in the ocean.
Estuaries and Resonantly Generated Internal Waves
I have studied the Marsdiep Tidal inlet (Netherlands). This is a shallow estuary with very strong tidal currents and is subject to freshwater outflow from Lake IJssel. As a result there is a constant shift between vertically well-mixed conditions during strong tidal flows, and vertically stratified condition during slack tide. During the transition from strong currents to slack tide, we repeatedly obtained one of the first observations of resonantly generated internal solitons. Solitons are a special kind of internal wave. These solitons and other internal wave types, that may be present in the Marsdiep, are expected to contribute to net transport of nutrients and sediments.
I grew up in the coastal regions of The Netherlands, which is as flat as a pancake. Having to commute to work by bike, I required to battle wind, rather then hills. However, when I moved to Hobart (Australia), commuting to work included battling hills rather then wind. I couldn't help myself thinking how wind and hills relate, and found myself calculating an incline of a hill, as an "Incline Equivalent Windspeed". This study is published in the Journal of Science and is open access. I wrote a short piece on this (in Dutch and English) regarding the last mountain that needed to be climbed for the 2017 Tour de France.
A sketch by Malou Zuidema Illustrations (website), illustrating a combination of external ingredients and ocean mixing, will lead to the creation of different types of water (fresher Vs Saltier, Warmer Vs Cooler, etc).
The global ocean circulation represented as a streamfunction in Salinity (S) and Temperature (T) coordinates. This particular example is obtained from observations of the ocean's S and T distribution in combination with air-sea fluxes of fresh water and heat.
A Photo taken by an Astronaut on board of the International Space Station, showing the effects of ocean waves underneath the surface, on the surface of the ocean. Such "Internal waves" are important for transport of tracers and sediment. Breaking internal waves also lead to mixing, a little bit like breaking waves at the beach. More details can be found in Groeskamp et al. (2011, pdf)