Dr Joanne Williams
As part of the Sea Level group at NOC, I'm currently working on global tide and surge reanalysis, seeking to improve the understanding of coastal flood-risk around the world. I'm working with Kevin Horsburgh on the Atlantos project.
Storm surge refers to the excess sea-level that is caused by weather systems. In the UK, where we have very large tidal range, one of the effects of a storm can be to change the timing of high tide as well as the maximum water level. In analysing past events from tide-gauge records we have to be very careful how we separate these effects. Rather than just take the difference between continuously measured water level and predicted tide (the non-tidal residual) we sometimes use "skew-surge", the difference between peak observation and prediction, even if these are at a different time.
I've also worked with Chris Hughes and Mark Tamisiea, studying bottom pressure records and their use in measuring sea-level, mass changes in the ocean, and ocean circulation. Our spectral maps of bottom pressure and sea-level from ocean models are now available.
The work on weighing the ocean has highlighted to us again that bottom pressure recorders are subject to non-linear drift, that can be at least as great as annual cycles or long-term trends. We have found that this drift can be better characterised by proper accounting of known annual cycles, and are working on techniques to improve this. You can get our code from the PSMSL website. We are also interested in alternative techniques to measure bottom pressure, such as combining thermosteric moorings and satellite altimetry.
I'm also interested in using global ocean model data, from for example the NEMO or OCCAM models, to analyse bottom pressure and sea-level where no records exist. This has led to some interesting findings about the comparison of tide-gauge records (necessarily at the shore) and altimetry (usually some distance away). We have found that whether it is valid to use these inter-changeably is dependant on latitude and the time-scales of interest. More details can be found here.
I have worked at the National Oceanography Centre (formerly the Proudman Oceanographic Laboratory) since 2008, mostly part-time. My PhD, in Thin-film Rimming Flows was from the University of Nottingham, with David Riley, Stephen Hibberd and Henry Power. Prior to that I was employed at Malvern in research on aircraft communication systems, and have a maths MSc from Bristol and BA from Oxford.