Scientific Home Page of Andreas Thurnherr (Contact Information)

Welcome to my homepage. I am a research scientist in the Division of Ocean and Climate Physics at the Lamont-Doherty Earth Observatory of Columbia University, where I study various aspects of the Ocean's circulation. (See my publications.) My preferred way of studying things is by observation. When I am not in the office or on a ship somewhere I am doing my darndest to enjoy life. Most of my activities (including this one) are perfectly legal.

Scientific Interests

Diapycnal Mixing and Upwelling in the Deep Ocean

In order to close the global overturning circulation, the production of dense water at high latitudes must be balanced elsewhere by buoyancy gain and upward vertical motion (upwelling). Observed mixing levels away from the near-surface layer and from topography are approximately one order of magnitude less than those required to close the global buoyancy budget. Therefore, most of the buoyancy gain must take place near the boundaries.

My interest lies primarily in the processes responsible for the enhanced mixing that has been observed near rough topography. In my interpretation the available observation point to the importance of processes associated with flows across sills, in particular near slow-spreading mid-ocean ridges.

St. Laurent, L. C. and A. M. Thurnherr, 2007: Intense mixing of lower thermocline water on the crest of the Mid-Atlantic Ridge. Nature, 448:680-683.

Kunze, E., E. Firing, J. M. Hummon, T. K. Chereskin and A. M. Thurnherr, 2006: Global abyssal mixin inferred from Lowered ADCP shear and CTD strain profiles. Journal of Physical Oceanography, 36(8):1553-1576.

Thurnherr, A. M. 2006: Diapycnal mixing associated with an overflow in a deep submarine canyon. Deep-Sea Research II, 53:194-206.

Thurnherr, A. M., L. C. St. Laurent, K. G. Speer, J. M. Toole and J. R. Ledwell 2005: Mixing Associated with Sills in a Canyon on the Mid-Ocean Ridge Flank. Journal of Physical Oceanography, 35(8): 1370-1381.

Thurnherr, A. M. and K. G. Speer, 2003: Boundary mixing and topographic blocking on the Mid-Atlantic Ridge in the South Atlantic. Journal of Physical Oceanography, 33(4): 848-862.

Thurnherr, A. M., K. J. Richards, C. R. German, G. F. Lane-Serff and K. G. Speer, 2002: Flow and mixing in the rift valley of the Mid-Atlantic Ridge. Journal of Physical Oceanography, 32(6): 1763-1778.

Thurnherr, A. M. (2000). Hydrography and Flow in the Rift Valley of the Mid-Atlantic Ridge. Ph. D. thesis, School of Ocean and Earth Science, Univ. of Southampton, England.

Determining the Large-Scale Ocean Circulation

Determining the quasi-steady or mean circulation over periods of a decade or more is one of the main goals of large-scale physical oceanography. One of the most widely used methods consists in determining the vertical shear of the geostrophic velocity field from hydrographic measurements, usually collected as (quasi-)synoptic data sets on time scales of weeks. Using such samples to draw inferences about a mean state requires an assumption of representativeness on the time and space scales of interest. In the presence of temporal variability a single synoptic sample cannot be expected to be truly representative. Formal methods, such as linear inversions, therefore require estimates of the temporal variability of the sampled fields, which are often assumed to be normally distributed about a mean state. The standard deviations are commonly assigned more-or-less arbitrarily, although output from numerical models is sometimes used. The available evidence for representativeness of synoptic hydrographic sections is ambiguous.

My interest lies in finding novel ways of analyzing oceanographic data in order to be able to draw conclusions about some sort of quasi-steady mean state.

See also:

Thurnherr, A. M. and K. G. Speer, 2004: Representativeness of meridional hydrographic sections in the western South Atlantic. Journal of Marine Research, 62: 37-65.

Thurnherr, A. M., 2003: Online hydrographic and tracer climatology of the South Atlantic.

Thurnherr, A. M. and K. G. Speer, 2003: Boundary mixing and topographic blocking on the Mid-Atlantic Ridge in the South Atlantic. Journal of Physical Oceanography, 33(4): 848-862.

Speer, K.G. and Maltrud, M.E. and Thurnherr, A.M., 2003: A Global View of Dispersion Above the Mid-Ocean Ridge, in Energy and Mass Transfer in Marine Hydrothermal Systems, edited by Halbach, P.E. €and Tunnicliffe, V. and Hein, J.R., Dahlem University Press, Dahlem Workshop Report, No. 89, Berlin, 287-302.

Circulation in the Vicinity of Hydrothermal Sites

Hydrothermal processes account for approximately one third of the global geothermal heat flux to the oceans and strongly affect their chemical composition. For some chemical species, hydrothermal input constitutes a source of similar magnitude to riverine input, while particle precipitation in hydrothermal plumes removes others. The most spectacular manifestations of hydrothermal circulation are the high-temperature (> 300 degrees centigrade) vent fields that give rise to particle-rich plumes rising hundreds of meters above the seafloor, where they spread laterally into the surrounding water column. These focused inputs of heat and chemicals support distinct and fascinating ecosystems.

My interest lies in studying the circulations (and, especially, the processes driving them) near hydrothermal hydrothermal vent fields. In this context I keep updating a website with float data from the Lau Basin. Additionally, I am currently involved in a large project to study larval dispersal on the East Pacific Rise near 10N.

See also:

Thurnherr, A. M., G. Reverdin, P. Bouruet-Aubertot, L.C. St. Laurent, A. Vangriesheim and V. Ballu, 2008: Hydrography and flow in the Lucky Strike segment of the Mid-Atlantic Ridge. Journal of Marine Research, 66: 347-372.

Speer, K.G. and Maltrud, M.E. and Thurnherr, A.M., 2003: A Global View of Dispersion Above the Mid-Ocean Ridge, in Energy and Mass Transfer in Marine Hydrothermal Systems, edited by Halbach, P.E. and Tunnicliffe, V. and Hein, J.R., Dahlem University Press, Dahlem Workshop Report, No. 89, Berlin, 287-302.

Hein, J. R., E. T. Baker, J.P. Cowen, C. R. German, E. Holzbecher, R. A. Koski, M. J. Mottl, N. V. Pimenov, S. D. Scott, and A. M. Thurnherr, 2003: Group Report: How Important Are the Material and Energy Fluxes from Hydrothermal Circulation to the Ocean?, in Energy and Mass Transfer in Marine Hydrothermal Systems, edited by Halbach, P.E. and Tunnicliffe, V. and Hein, J.R., Dahlem University Press, Dahlem Workshop Report, No. 89, Berlin, 337-355.

Thurnherr, A. M., K. J. Richards, C. R. German, G. F. Lane-Serff and K. G. Speer, 2002: Flow and mixing in the rift valley of the Mid-Atlantic Ridge. Journal of Physical Oceanography, 32(6): 1763-1778.

Mullineaux, L. S., K. G. Speer, A. M. Thurnherr, M. E. Maltrud and A. Vangriesheim, 2002: Implications of cross-axis flow for larval dispersal along mid-ocean ridges. Cahiers De Biologie Marine, 43(3-4): 281-284.

Thurnherr, A. M. and K. J. Richards, 2001: Hydrography and high-temperature heat flux of the Rainbow hydrothermal site (36 degrees 14 ' N, Mid-Atlantic Ridge). Journal of Geophysical Research-Oceans, 106(C5): 9411-9426.

Thurnherr, A. M. (2000). Hydrography and Flow in the Rift Valley of the Mid-Atlantic Ridge. Ph. D. thesis, School of Ocean and Earth Science, Univ. of Southampton, England.

Technical Interests

Lowered Acoustic Doppler Current Profilers

The use of Lowered Acoustic Doppler Current Profilers (LADCPs) to measure full-depth velocity profiles is becoming routine. Because of the scarcity of scattering particles at depth in many regions of the ocean the instruments are often operating near their noise levels, making data processing somewhat difficult. There are two methods/software packages that are commonly used to process LADCP data: an implementation of the shear method by Eric Firing (University of Hawaii) and a more recent inverse method developed by Martin Visbeck (LDEO). Besides implementing different methods the two software packages also use different statistical tests to filter bad data and therefore have different parameters that may have to be tuned to a particular instrument.

My interest lies primarily in the data processing of LADCP instruments. In the mid-term I would like to combine the advantages of the two data-processing methods.

See also:

Thurnherr A. M.: LADCP Processing How-To.

Thurnherr, A. M., M. Visbeck, and B. Huber, 2004: LADCP processing report and UNIX RDI communications software, Technical report.

Thurnherr, A. M., 2003: RDI Workhorse performance evaluation and shear-inversion software, Technical report.

Computer Animations

Before switching to oceanography, I was working in IT. One of my interest was (and is) scientific visualization and computer animations. In 1993 while working for the Swiss National Supercomputing Centre I created a short computer animation called Summer 93. [Use the VLC media player to view the animation on a Mac.] At different stages I have also created animations from current-meter, float, and from numerical-model data.