Prof Chris W Hughes
Role/area of expertise
Location & Contact Details
Prof Chris W Hughes
Group: Marine Physics and Ocean Climate.
Subgroup: Sea Level and Ocean Climate.
Based in Liverpool, I am Professor of Sea Level Science at the University of Liverpool (university profile page)in a joint appointment with the National Oceanography Centre, where I am a part of the Sea Level group, which covers diverse aspects of sea level science from tide gauge technology to geodesy and geophysics to coastal flooding. The group includes the global tide gauge data service: the Permanent Service for Mean Sea Level, and the National Tidal and Sea Level Facility.
My particular expertise is in the relationship between ocean dynamics and sea level, particularly the interaction of ocean flows with topography, and with eddies. I study these using theory, numerical model diagnostics, and observations, particularly satellite altimetry and tide gauge (sea level) measurements, satellite gravity, and ocean bottom pressure measurements. Sea level and bottom pressure are both intimately linked to the earth's gravity field and rotation. Ocean dynamics are also strongly controlled by the earth's rotation, so much so that I tend to consider ocean dynamics as rotational dynamics (of a fluid), rather than fluid dynamics (rotating). As a result I take a strongly geodetic approach to oceanography, and call myself a geodetic oceanographer.
Geodesy is the study of the earth's shape, its gravity field and its rotation (orientation in space). All of these things are inter-related, and they are much more complicated than you would first think. Nonetheless, geodesists are now able to combine systems such as GPS and other satellite positioning systems, VLBI (very accurate tracking of the position in the sky of extremely distant radio sources), and gravity measurements, together with satellites measuring the position of the sea surface, to measure global-scale position and level changes to an accuracy which ranges from millimetres to a few tens of centimetres depending on precisely which quantity you are interested in.
The most straightforward link with oceanography is via sea level. If the ocean were quiescent - no waves, no wind or atmospheric pressure variation, no heating or cooling - it would settle down to a constant level. What that means is that the shape of the sea surface would be determined by a balance between gravitational attraction, pressure, and centrifugal forces. If you froze this ocean, and placed a marble anywhere on its surface, the marble would not roll anywhere. In short, the surface, known as the geoid, is an equipotential of gravity (gravity = gravitation + centrifugal force). But that surface is not a simple shape. As a first approximation it is an ellipsoid, with an equatorial radius of about 6,378 km and a polar radius about 21 km shorter. On top of that smooth ellipsoidal shape, the geoid has undulations of up to 100 m, with bumps down to scales of only a few km reflecting the mass distribution (mountains, for small scales, and deeper features for large scales) of the earth.
Oceanographers are interested in the fact that the sea surface isn't level, because that tells us about currents in the ocean. But the surface only departs from a level surface by less than 2 metres, so most of the shape of the ocean is simply the geoid. In fact, we care about sea level changes as small as fractions of a centimetre, so in order to interpret sea level measurements, we would like to know the geoid to that accuracy. That is what satellite missions such as GRACE and GOCE are edging towards. For more technical details on geoids and gravity, see An Oceanographer's Guide to GOCE and the Geoid.
Other links between oceanography and geodesy concern ocean bottom pressure and angular momentum. Bottom pressure is a measure of the weight of a column of water plus air, and the mass of that column results in a gravitational force which can be measured. In this way, the GRACE satellites use gravity measurements to infer bottom pressure changes. Bottom pressure is especially interesting because it changes much less than sea level, and is particularly sensitive to integrals of ocean flows (either the depth integral of northward current, or the zonal integral of northward current at constant depth). This makes bottom pressure very valuable as a means of monitoring ocean flows. It also represents the force exerted by the ocean on the solid earth, exchanging angular momentum with the earth (and influencing its rotation), as well as causing deformation of the solid earth.
Some Geodetic Oddities
- The moon is drifting away from the earth at a rate of about 4 cm per year. This is a result of tidal friction, mostly in the oceans, which is causing the earth's rotation to slow down, increasing the length of day by about 2 milliseconds per century (as the earth slows, the moon must drift away in order to conserve angular momentum). This makes it possible to measure how much energy is dissipated by tides.
- It is impossible to dig a vertical-sided hole through the earth.
- Tidal gravity forces and the varying distribution of weight of the oceans due to tides make the solid earth flex. As a result the land moves up and down, typically by a few tens of cm, twice a day.
Membership of Committees and Boards
Member of the IAPSO Commission on Mean Sea-Level and Tides, since 2007
Member of GGOS Working Group on Contributions to Earth Systems Monitoring, since 2011
Member of NERC Space Geodesy Facilities Steering Committee since 2011
Latest Publications for Prof Chris W Hughes
Higginson, S.; Thompson, K.R.; Woodworth, P.L.; Hughes, C.W.. 2015 The tilt of mean sea level along the east coast of North America. Geophysical Research Letters, 42 (5). 1471-1479. 10.1002/2015GL063186
Wilson, Chris; Hughes, Christopher W.; Blundell, Jeffrey R.. 2015 Forced and intrinsic variability in the response to increased wind stress of an idealized Southern Ocean. Journal of Geophysical Research: Oceans, 120 (1). 113-130. 10.1002/2014JC010315
Woodworth, Philip L.; Gravelle, Médéric; Marcos, Marta; Wöppelmann, Guy; Hughes, Chris W.. 2015 The status of measurement of the Mediterranean mean dynamic topography by geodetic techniques. Journal of Geodesy, 89 (8). 811-827. 10.1007/s00190-015-0817-1
Hughes, Chris W.; Bingham, Rory J.; Roussenov, Vassil; Williams, Joanne; Woodworth, Philip L.. 2015 The effect of Mediterranean exchange flow on European time mean sea level. Geophysical Research Letters, 42 (2). 466-474. 10.1002/2014GL062654
Williams, Joanne; Hughes, Chris W.; Tamisiea, Mark. 2015 Detecting trends in bottom pressure measured using a tall mooring and altimetry. Journal of Geophysical Research: Oceans, 120 (7). 5216-5232. 10.1002/2015JC010955
Rye, Craig D.; Naveira Garabato, Alberto C.; Holland, Paul R.; Meredith, Michael P.; Nurser, A.J. George; Hughes, Chris W.; Coward, Andrew C.; Webb, David J.. 2014 Rapid sea-level rise along the Antarctic margins in response to increased glacial discharge. Nature Geoscience, 7 (10). 732-735. 10.1038/ngeo2230
Woodworth, Philip L.; Morales Maqueda, Miguel Á.; Roussenov, Vassil M.; Williams, Richard G.; Hughes, Chris W.. 2014 Mean sea-level variability along the northeast American Atlantic coast and the roles of the wind and the overturning circulation. Journal of Geophysical Research: Oceans, 119 (12). 8916-8935. 10.1002/2014JC010520
Tamisiea, Mark E.; Hughes, Chris W.; Williams, Simon D.P.; Bingley, Richard M.. 2014 Sea level: measuring the bounding surfaces of the ocean. Philosophical Transactions of the Royal Society of London, A, 372 (2025). 20130336. 10.1098/rsta.2013.0336
Hughes, C.W.; Williams, Joanne; Coward, A.C.; de Cuevas, B.A.. 2014 Antarctic circumpolar transport and the southern mode: a model investigation of interannual to decadal timescales. Ocean Science, 10 (2). 215-225. 10.5194/os-10-215-2014
Elipot, Shane; Frajka-Williams, Eleanor; Hughes, Chris W.; Willis, Joshua. 2014 The observed North Atlantic Meridional Overturning Circulation, its Meridional Coherence and Ocean Bottom Pressure. Journal of Physical Oceanography, 44 (2). 517-537. 10.1175/JPO-D-13-026.1
Williams, J.; Hughes, C.W.; Tamisiea, M.E.; Williams, S.D.P.. 2014 Weighing the ocean with bottom-pressure sensors: robustness of the ocean mass annual cycle estimate. Ocean Science, 10 (4). 701-718. 10.5194/os-10-701-2014
Elipot, Shane; Hughes, Chris; Olhede, Sofia; Toole, John. 2013 Coherence of western boundary pressure at the RAPID WAVE array: boundary wave adjustments or deep western boundary current advection? Journal of Physical Oceanography, 43 (4). 744-765. 10.1175/JPO-D-12-067.1
Hughes, Chris W.; Elipot, Shane; Morales Maqueda, Miguel Angel; Loder, John W.. 2013 Test of a method for monitoring the geostrophic meridional overturning circulation using only boundary measurements. Journal of Atmospheric and Oceanic Technology, 30. 789-809. 10.1175/JTECH-D-12-00149.1
Panet, I.; Flury, J.; Biancale, R.; Gruber, T.; Johannessen, J.; van den Broeke, M.; van Dam, T.; Gegout, P.; Hughes, C.; Ramillien, G.; Sasgen, I.; Seoane, L.; Thomas, M.. 2013 Earth system mass transport mission (e.motion): A concept for future Earth gravity field measurements from space. Surveys in Geophysics , 34 (2). 141-163. 10.1007/s10712-012-9209-8
Williams, Joanne; Hughes, Christopher W.. 2013 The coherence of small island sea-level with the wider ocean: a model study. Ocean Science, 9. 111-119. 10.5194/os-9-111-2013
Williams, Joanne; Hughes, Chris W.. 2012 Coherence of off-shore steric height and island sea level. In: European Geosciences Union General Assembly 2012, Vienna, 22-27 Apr 2012.
Hughes, Christopher W.; Tamisiea, Mark E.; Bingham, Rory J.; Williams, Joanne. 2012 Weighing the ocean: Using a single mooring to measure changes in the mass of the ocean. Geophysical Research Letters, 39 (17). L17602. 10.1029/2012GL052935
Woodworth, Philip; Hughes, Christopher; Bingham, Rory; Gruber, Thomas. 2012 Towards Worldwide Height System Unification using Ocean Information. Journal of Geodetic Science, 2 (4). 302-318. 10.2478/v10156-012-0004-8
Hughes, Chris W.; Tamisiea, Mark E.; Bingham, Rory J.; Williams, Joanne. 2012 Weighing the ocean: How a single mooring in the mid-Pacific can monitor changes in ocean mass. In: European Geosciences Union General Assembly 2012, Vienna, 22-27 Apr 2012. (Unpublished)