Theo Gerkema

Theo Gerkema

ROYAL NETHERLANDS INSTITUTE

FOR SEA RESEARCH

Postal address:

P.O. Box 59,

NL-1790 AB Den Burg (Texel)

The Netherlands

Visiting address:

Landsdiep 4

NL-1797 SZ ’t Horntje (Texel)

The Netherlands

Phone:

(+31) (0)222-369300

Fax:

(+31) (0)222-319674

E-mail: gerk ”at” nioz.nl

Phone: (+31) (0)222-369426

Research Topics:

The focus of my research activities has been on internal waves in the ocean.

They provide much of the ocean’s inner unrest, are ubiquitous, and can have vertical amplitudes of 100m or more (observable as periodic vertical displacements of the isotherms). In recent years, the scope of their interest has widened to large-scale ocean dynamics, even though they themselves are of relatively small scales (up to tens of km). This is because they provide a source for deep-ocean mixing, which is essential for maintaining the large-scale circulation (meridional overturning).

Although the internal-wave field fills a wide spectrum ranging from minutes to a day, two types of internal wave stand out (and form, in ways not yet fully understood, much of the source for the rest of the spectrum): near-inertial waves and internal tides. The former are primarily generated by atmospheric disturbances, the latter by tidal flow over sloping bottom. The main type of internal tide has a period of about 12h, while near-inertial waves usually have a slightly longer period (depending on latitude).

The propagation of near-inertial waves involves a number of subtleties which we have only recently begun to understand. Together with Victor Shrira (Keele University, UK), I’ve looked into the role of a component of the Coriolis force that is usually ignored (in the so-called “traditional approximation”). It turns out to play a crucial role in that it creates a sub-inertial short-wave limit; moreover, the sub-inertial waves can exist only in regions of weak stratification, singling out the abyssal ocean as their habitat (see our papers jgr ’05, jfm ’05, on my webpage “publications”). These two factors suggest a direct link to deep-ocean mixing. For a comprehensive overview on the “traditional approximation” in geophysical and astrophysical fluid dynamics, see rg ’08.

There are two simple (but mutually exclusive) ways of looking at the propagation of internal waves: one is the horizontal propagation as interfacial waves, the other the oblique propagation as a “beam” in a fluid of constant stratification. In the real ocean, with continuous but non-constant stratification, neither picture applies fully, but one recognizes elements of both, depending on (primarily) the strength of the thermocline. I examined this problem in jmr ’01, mainly by analytical means, thus explaining the mechanism responsible for the so-called “local generation” of internal solitons -- a phenomenon that has been observed in the Bay of Biscay.

In recent years I’ve also developed a numerical internal-tide generation model that is linear and hydrostatic but allows for an arbitrary vertical stratification in density. It can be used to study, for example, the influence of the thermocline on the generation and propagation of internal tides. A recent finding was that the permanent pycnocline (at a depth of about 900m) is an important factor in the energetics of internal-tide generation (dsr II ’04); here the region of study was the Bay of Biscay (see also dsr I ’04). The model has also been applied to Faeroe-Shetland Channel (jgr ’02), Mozambique Channel (jgr ’04), the Indonesian Straits (grl ’07), and to Great Meteor Seamount (os ’07). In this last paper, we showed that there is an indeterminacy in internal-tide energy flux profiles over sloping bottoms, a point that had passed unnoticed in the literature.

In the coming years I’ll also be working on the numerical modelling of the hydrodynamics in the Dutch Wadden Sea.