The research on ocean circulation and hydrography focusses on the regional processes related with the wind-driven and thermohaline ocean circulation in order to improve our understanding of climate variability. The observational work, by mooring deployments and repeated surveys (some yearly), is aimed at identifying variability over time scales from seasonal to decadal. Much of this research has been carried out in multi-national programmes (e.g. WOCE, OMEX, INSTANT, CLIVAR), ensuring extensive coverage of the regions under study. Repeat hydrographic surveys have been carried out and moored current meters have been in place in Indonesia, the Mozambique Channel, the East Madagascar Current, the Agulhas Current, and the Irminger Sea. These mooring sites have been realized through several projects:

• LOCO (Long-Term Ocean Climate Observations) funded by the Netherlands Organisation for Scientific Research NWO (ALW)
• THOR (THermohaline Ocean circulation at Risk?) financed by the European Commission through the 7th Framework Programme for Research
• INATEX (INdian-ATlantic EXchange in present and past climate) funded by the Netherlands Organisation for Scientific Research NWO (ALW)
• NACLIM (North Atlantic CLIMate), financed by the European Commission through the 7th Framework Programme for Research. 

These Eulerian observatories have delivered continuous time series of e.g. ocean temperature, salinity and volume transports since 2003 and contribute to the OceanSITES worldwide system of long-term, deepwater reference stations. Current areas of focus are the Irminger Sea, the Mozambique Channel, and the East Greenland Current. In NACLIM the mooring program in the Irminger Sea will be continued and extended with current meter moorings on the Reykjanes Ridge.

• Irminger Sea mooring program 
• Mozambique Channel mooring program 

A major activity of the department is the bi-annual hydrographic survey of the former WOCE AR7E section between Greenland and Ireland. This regular survey forms a contribution to the monitoring of the North Atlantic Ocean within e.g. projects THOR and NACLIM. It also gives an opportunity to train oceanography students from Utrecht University in observational techniques, and for scientists from other disciplines to carry out basin-wide observations. The department of Physical Oceanography also participates in programmes of other departments (e.g. TRANSAT, Archimedes, and GEOTRACES).

For the study on internal waves and mixing, observational projects are carried out in the Atlantic Ocean and Mediterranean Sea, exploiting a newly developed fast and accurate thermistor ('NIOZ4'). The overarching theme is to unravel the interplay between internal waves and stratification. Numerical and theoretical models are used to study internal-wave propagation in an idealized context; for this purpose, too, laboratory experiments are carried out at NIOZ and at other European facilities.

Tides and oceanic turbulence

The stably-stratified ocean supports internal gravity waves. Depending on the nature of the ocean's stratification (the rate of change of the density field), these waves propagate obliquely through the ocean (when the stratification is more or less uniform) or horizontally (when the stratification is concentrated in a thin layer). In either case they transport energy and momentum "elsewhere". This energy may become remotely available for mixing. Such processes have been observed in the deep ocean, as well as in the shallow Wadden Sea, and even in the very shallow water (1.5 m) in front of the Texel beach.

Breaking internal wave, recorded with thermistor string

It is speculated that these waves may thus play an important role in maintaining the large-scale, deep circulation, by providing downward mixing of heat. Their mixing may also be relevant for sustaining marine ecosystems, by providing nutrients to the water column. Because of their fairly persistent nature this applies in particular to the two most important contributions: internal tides (internal waves of tidal period) and near-inertial waves (internal waves of period close to the local inertial period).

Internal waves are studied by means of observations, numerical and hydraulic models, and theoretical research. For the in situ study of internal waves and observations of deep solibores (violent short-duration events) occurring over a sloping bottom that dominate near bottom mixing and sediment-re-suspension, fast sampling high accuracy thermistor strings have been developed in cooperation with the technical departments. Such observations are carried out at oceanic sites as well as in the estuarine waters of the south-east Netherlands Delta. For laboratory studies a wave tank has been developed, where different types of stratification can be simulated. Video observations of that tank are digitized to produce the 2-D velocity field. For the detailed simulation of the generation and propagation of internal waves, de department also relies on the use of numerical models.

Model simulation of an internal wave reflecting at a slope

Coastal research has always been one of the natural focus areas in the FYS department, but the focus has strongly increased in recent years. For over ten years the Marsdiep inlet has been monitored by the TESO ferry equipped with instruments (an ADCP, thermo-salinograph and optical sensors). The monitoring of physical parameters in the Wadden Sea has been continued (T and S since 1860), and even is extended in the multi-disciplinary IN PLACE monitoring programme, with a sensor mast in the open Wadden Sea. An important step has been the introduction of a numerical-model tool (GETM/GOTM) for the study of hydrodynamic processes and SPM transport in the western Wadden Sea and Southern Bight.Dynamica van de kustzone en van ondiepe zeeën.

Dynamics of coastal zones and shallow seas

This research theme consists of studies in the relatively shallow coastal zone and estuaries using both field and laboratory observations and theoretical and numerical modelling. This subject was initiated at NIOZ in the early seventies, inspired by the importance of the tidal currents for the ecosystem of the nearby Wadden Sea. But the department is also involved in projects, carried out in the coastal zone of the North Sea, and in the estuarine waters of Zeeland. In principle the coastal research focuses on observational programmes, but in the last years a system of numerical models, based on the GETM/GOTM code has been established.

The erosion, transport, and deposition of suspended matter are focal points of the coastal research. A detailed analysis of the exchange of water and sediments from the coastal zone to the Wadden Sea is done based on ongoing ferry observations of currents and suspended sediments in the Marsdiep inlet. Similar research on silt transport in the Dutch coastal zone is carried out in the framework of the ECOSHAPE programme "Building with Nature". The modelling experience, built up with this research, is now also used for the multi-disciplinary study of fish and larvae in the Wadden Sea and the North Sea.

From a current survey for the development of a floating tidal power plant a programme has been developed to study the spatially and temporally varying current and salinity structure in the Marsdiep area. Observations are carried out with moored instruments, anchored and cruising research vessels, remote sensing, and free falling turbulence probes. The observations, made daily from the Ferry between Den Helder and Texel, add to the data set.

The daily observations of temperature and salinity (since 1860) from the Marsdiep jetty in front of NIOZ contribute to a developing multi-disciplinary time series of environmental factors. The automatic colour observations add to that data set, and give information on suspended matter and Chlorophyll concentrations. Similar sensors have been placed at an observational tower on a tidal flat as port of the multi-disciplinary IN PLACE monitoring programme.