Ocean Dynamics Research at KNMI
Sybren Drijfhout, Wilco Hazeleger, Caroline Katsman, Andreas Sterl.
The heat capacity and the inertia of the ocean are large compared to the
atmosphere. This means that changes in the ocean occur slowly. Therefore low
frequency climate variations (interannual time scales and longer) can be
coupled to changes in the ocean circulation. We study fundamental aspects of
this ocean circulation and its coupling to the atmosphere. The focus is on
oceanic processes that contribute to variability of climate. Oceanic features
on different spatial and time scales are studied, ranging from tens to
thousands kilometers, from months to millennia. A distinct and relevant effect
of climate change is sea level rise. We study ocean mixing processes that
contribute to sea level rise and try to reduce the uncertainty in projections
of future sea level rise. The main tools for investigation are state-of-the-art
numerical models of the ocean circulation (model-support).
Some of these models are coupled to models of the atmosphere to study coupled
ocean-atmosphere processes. We work on the following subjects:
1) Large scale ocean circulation
2) Coupled
ocean-atmosphere processes
3) Sea level rise
4) Eddies and internal
waves
Large scale ocean circulation
There is a circulation in the
world's oceans that connects all ocean basins. In the Nordic seas large amounts
of water sink to the ocean bottom as water gets very dense due to cooling. In
the deep ocean this water spreads southward through the Atlantic basin and
propagates into the Pacific and Indian Ocean. Around Antarctica cool water
sinks to the bottom of the ocean as well. In the different ocean basins water
rises slowly to the surface and flows back into the Atlantic. The global
recirculation is called the conveyor belt and encompasses a global
transport of heat and salinity. Within the respective ocean basins water
masses subduct in the subtropics and upwell in the equator, forced by the
winds. These so-called subtropical cells are embedded in the global
conveyor belt. We study pathways of the conveyor and subtropical
cells with a Lagrangian trajectory technique, the impact of eddies on the
structure of the conveyor and mechanisms of variability of these overturning
cells.
contact information: Sybren Drijfhout or Wilco Hazeleger
Coupled ocean-atmosphere processes (see also PATCH)
Large scale climate patterns can change due to internal and forced variability.
It is studied by what mechanisms variability in the tropical oceans and
atmosphere (e.g. ENSO,TAV) induce changes in extratropical climate patterns
(e.g. NAO) and whether that feeds back on the tropics. These oceanic and
atmospheric teleconnections are studied under varying external forcing (solar
and antropogenic) and it is studied whether extratropics-tropics interaction
can lead to predictability at low frequencies. This research is done in the PATCH
project.
A
coupled ocean-atmosphere model nicknamed SPEEDO is developed for this purpose. The
atmosphere model (SPEEDY )
solves the primitive equations, it has relatively coarse resolution and simple
parameterization schemes. The ocean model is the isopycnic state-of-the-art
model. This coupled model is very flexible (SST forced, coupled to mixed layer,
coupled to full ocean, different basins) and allows for many and long
integrations. The model is developed in cooperation with CKO and ICTP in
Trieste (Italy). This is a joint project of the and group and is an
interdivisional theme of the Climate and Seismology department at KNMI. Also,
data from a large ensemble of a coupled ocean-atmosphere model is used to study
different aspects of the coupled ocean-atmosphere system (see ).
contact information: Wilco Hazeleger
Sea level rise
Obviously, for a low-lying
country like the Netherlands, future changes in sea level are of great
importance. For the 21st century, climate models predict a globally averaged
sea level rise of 9 to 88 cm (IPCC, 2001). About 75% of the predicted
increase is due to the thermal expansion of the ocean in response to a warming
atmosphere. The large spread in the projections is partly due to use of
different emission scenarios for the 21st century which affect the atmopheric
temperature rise. In addition, the various climate models predict different
responses to a given atmospheric warming. In particular, the models differ in
the effectiveness of heat uptake by the (deep) ocean, which has large
consequences for the thermal expansion.
Modelling the ocean heat uptake is a challenging task, as it involves
small-scale mixing processes (that are not yet fully understood) which are no
explicitly solved in the models and need to be parameterized. In our work, we
study the impacts of various proposed mixing parameterizations on the (deep)
ocean heat uptake and the resulting thermal expansion, using idealized ocean
models as well as realistic, global ocean-atmosphere models. In addition, we
develop and test new mixing schemes that represent the lastest physical
insights on mixing in the deep ocean.
contact information: Caroline Katsman
Eddies and internal waves
contact information: Sybren Drijfhout
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