Professor Chris W. Hughes

Who am I?

I am science leader of POL's Sea Level group. The group includes people with expertise in tide gauges, geophysics and geodesy, coastal flooding and modelling, and various aspects of ocean dynamics and sea level change, and incorporates the Permanent Service for Mean Sea Level and the the National Tidal and Sea Level Facility.

My particular expertise is in deep-ocean dynamics, 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, 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.

I am employed at POL by the Natural Environment Research Council, and am a Visiting Professor in the University of Liverpool's Department of Earth and Ocean Sciences.

What is Geodesy?

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 the 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. Compared to this smooth 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.

Journal publications

Submitted

Hughes, C. W. and S. D. P. Williams, 2010: The color of sea level: The importance of spatial variations in spectral shape for assessing the significance of trends. J. Geophys. Res. (submitted Jan 2010).

In press

Hibbert, A., H. Leach, P. Woodworth, C. W. Hughes and V. Roussenov, 2010: Quasi-biennial modulation of the Southern Ocean coherent mode. Q. J. Roy. Met. Soc. (in press).

2010

Hughes, C. W., A. F. Thompson and C. Wilson, 2010: Identification of jets and mixing barriers from sea level and vorticity measurements using simple statistics. Ocean Modelling (32), 44-57.

2009

Bingham, R. J. and C. W. Hughes, 2009: The geostrophic dynamics of meridional transport variability in the sub-polar North Atlantic. J. Geophys. Res. (114), C12029

Milne, G., R. Gehrels, M. Tamisiea and C. W. Hughes, 2009: Identifying the causes of sea-level change Nature Geoscience (2), 471-478.

Bingham, R. J. and C. W. Hughes, 2009: The signature of the Atlantic meridional overturning circulation in sea level along the east coast of North America. Geophys. Res. Lett., 36, L02603.

2008

Bingham, R. J., K. Haines and C. W. Hughes, 2008: Calculating the ocean's mean dynamic topography from a mean sea surface and a geoid. J. Atmos. Oceanic Tech., (25) 1808-1822.

Bingham, R. J. and C. W. Hughes, 2008: Determining North Atlantic meridional transport variability from pressure on the western boundary: A model investigation. Journal of Geophysical Research (114) C09008.

Roussenov, V., R. G. Williams, C. W. Hughes and R. Bingham, 2008: Boundary wave communication of bottom pressure and overturning changes for the North Atlantic. Journal of Geophysical Research (114), C08042.

Hughes, C. W., 2008: A form of potential vorticity equation for depth-integrated flow with a free surface. J. Phys. Oceanogr. (38) 1131-1136.

Hughes, C. W. and C. Wilson, 2008: Wind work on the geostrophic ocean circulation: An observational study of the effect of small scales in the wind stress. J. Geophys. Res. (113), C02016.

Bingham, R. J. and C. W. Hughes, 2008: The relationship between sea-level and bottom pressure variability in an eddy-permitting ocean model. Geophys. Res. Lett., (35), L03602.

Hughes, C. W., and R. J. Bingham, 2008: An oceanographer's guide to GOCE and the geoid. Ocean Science, (4), 15-29, 2008. .

2007

Bingham, R. , C. W. Hughes, V. Roussenov and R. G. Williams, 2007: Meridional coherence of the North Atlantic meridional overturning circulation. Geophys. Res. Lett. (34) L23606.

Heywood, K. J., J. L. Collins, C. W. Hughes and I. Vassie, 2007: On the detectability of internal tides in Drake Passage. Deep-Sea Res. I (54), 1972-1984.

Williams, R. G., C. Wilson, and C. W. Hughes, 2007: Ocean and atmosphere storm tracks: the role of eddy vorticity forcing. J. Phys. Oceanogr. 37(9), 2267-2289.

Hughes, C. W., V. N. Stepanov, L. -L. Fu, B. Barnier and G. W. Hargreaves, 2007: Three forms of variability in Argentine Basin ocean bottom pressure. J. Geophys. Res., 112, C01011.

2006

Stepanov, V. N. and C. W. Hughes, 2006: Propagation of signals in basin-scale ocean bottom pressure from a barotropic model. J. Geophys. Res. (Oceans)., 111, C12002.

Rietbroek, R., P. LeGrand, B. Wouters, and C. W. Hughes, 2006: Comparison of in situ bottom pressure data with GRACE gravimetry, in the Crozet-Kerguelen region Geophys. Res. Letters, 33, L21601.

Woodworth, P. L., C.W. Hughes, D.L. Blackman, V.N. Stepanov, S.J. Holgate, P.R. Foden, S. Mack, G.W. Hargreaves, M.P. Meredith and G. Milinevsky, 2006:Antarctic Peninsula sea levels: A real-time system for monitoring Drake Passage transport. Antarctic Science, 18(3), 429-436.

Jackson, L., C. W. Hughes, and R. G. Williams, 2006: The role of bottom pressure torques and friction in basin and channel flows. J. Phys. Oceanogr., 36(9), 1786--1805.

Bingham, R. J. and C. W. Hughes, 2006: Observing seasonal bottom pressure variability in the North Pacific with GRACE. Geophys. Res. Letters, 33, L08607.

Hughes, C. W., and M. P. Meredith, 2006: Coherent sea level fluctuations along the global continental slope. Phil. Trans. Roy. Soc. Lond. A, 364, 885-901.

2005

Hughes, C. W., 2005: Nonlinear vorticity balance of the Antarctic Circumpolar Current. J. Geophys. Res 110, C11008.

Hughes, C. W. and P. LeGrand, 2005: Future benefits of time-varying gravity missions to ocean circulation studies. Earth, Moon, and Planets 94, 73-81.

Meredith, M. P. and C. W. Hughes, 2005: On the sampling timescale required to reliably monitor interannual variability in the Antarctic circumpolar transport. Geophys. Res. Lett., 32, L03609.

2004

Meredith, M. P., P. L. Woodworth, C. W. Hughes, and V. N. Stepanov, 2004: Changes in ocean transport through Drake Passage during the 1980s and 1990s, forced by changes in the Southern Annular Mode. Geophys. Res. Lett., 31, L09609.

Hughes, C. W., and V. N. Stepanov, 2004: Ocean dynamics associated with rapid J2 fluctuations: Importance of circumpolar modes and identification of a coherent Arctic mode. J. Geophys. Res. 109 C06002.

Meredith, M. P., and C. W. Hughes, 2004: On the wind forcing of bottom pressure variability at Amsterdam and Kerguelan Islands, Southern Indian Ocean. J. Geophys. Res. 109 C03012.

Stepanov, V. N., and C. W. Hughes, 2004: Parameterization of ocean self-attraction and loading in numerical models of the ocean circulation. J. Geophys. Res. 109, C03037.

2003

Hughes, C. W., and V. Stepanov, 2003: Feasibility and contribution to ocean circulation studies of bottom pressure determination. Space Science Reviews 108(1), 217-224.

Jakobsen, P. K., M. H. Ribergaard, D. Quadfasel, T. Schmith, and C. W. Hughes, 2003: Near-surface circulation in the northern North Atlantic as inferred from Lagrangian drifters: Variability from the mesoscale to interannual. J. Geophys. Res. 108(C8), 3251.

Hughes, C. W., P. L. Woodworth, M. P. Meredith, V. Stepanov, T. Whitworth, and A. Pyne, 2003: Coherence of Antarctic sea levels, Southern hemisphere Annular Mode, and flow through Drake Passage. Geophys. Res. Lett. 30(9), 1464.

Meredith, M. P., C. W. Hughes, and P. R. Foden, 2003: Downslope convection north of Elephant Island, Antarctica: Influence on deep waters and dependence on ENSO. Geophys. Res. Lett. 30(9), 1462.

Johannessen J. A., G. Balmino, C. Le Provost, R. Rummel, R. Sabadini,H. Sunkel, C. C. Tscherning, P. Visser, P. Woodworth, C. W. Hughes, P. Legrand, N. Sneeuw, F. Perosanz, M. Aguirre-Fernandez, H. Rebhan, M. R. Drinkwater, 2003: The European Gravity Field and Steady-State Ocean Circulation Explorer satellite mission: Its impact on geophysics. Surveys In Geophysics 24 (4): 339-386.

2002

Hughes, C.W., 2002: Sverdrup-like theories of the Antarctic Circumpolar Current. J. Mar. Res., 60(1) 1-17 - More.

Killworth, P.D., and C.W. Hughes, 2002: The Antarctic Circumpolar Current as a free equivalent-barotropic jet. J. Mar. Res., 60(1) 19-45 - More.

Hughes, C.W., 2002: Torques exerted by a shallow fluid on a non-spherical, rotating planet. Tellus 54A, 56-62.

Hughes, C.W., 2002: Zonal jets in and near the Coral Sea, seen by satellite altimetry. Geophys Res. Letters, 29(9) 44-1 - 44-4.

Owen, G.W., I.D. Abrahams, A.J. Willmott, and C.W. Hughes, 2002: On the scattering of baroclinic Rossby waves by a ridge in a continuously stratified ocean. J. Fluid Mech. 465 131-155.

2001

Hughes, C.W., and E. R. Ash, 2001: Eddy forcing of the mean flow in the Southern Ocean. J. Geophys. Res. (Oceans), 106(C2), 2713-2722 - More .

Gille, S.T., and C.W. Hughes, 2001: Aliasing of high-frequency variability by altimeter observations: Evaluation from bottom pressure recorders. Geophys. Res. Letters, 28, 1755-1758 - More .

Hughes, C.W., and B. A. de Cuevas, 2001: Why western boundary currents in realistic oceans are inviscid: a link between form stress and bottom pressure torques. J. Phys. Oceanogr., 31(10) 2871-2885.

2000

Hughes, C.W., 2000: A theoretical reason to expect inviscid western boundary currents in realistic oceans. Ocean Modelling 2(1-2) 73-83

1999

Hughes, C.W., M.P. Meredith and K. Heywood, 1999: Wind-Driven Transport Fluctuations through Drake Passage: A Southern Mode. J. Phys. Oceanogr., 29, 1971 - 1992.

Hughes, C.W., & S. Carnochan, 1998: Rossby wave propagation in the Southern Ocean from satellite altimetry. Advances in Space Research 22(11) 1557-1560.

1998

Hughes, C.W., M.S. Jones and S. Carnochan, 1998: Use of Transient Features to Identify Eastward Currents in the Southern Ocean. J. Geophys. Res. (Oceans) 103 (C2) 2929-2944 - More .

1997

Hughes, C.W., 1997: Comments on "On the Obscurantist Physics of 'Form Drag' in Theorizing about the Circumpolar Current". J. Phys Oceanogr. 27(1), 209-210.

1996

Hughes, C.W. 1996: The Antarctic Circumpolar Current as a Waveguide for Rossby Waves. J. Phys. Oceanogr., 26(7), 1375-1387.

Hughes, C.W. 1996: The Use of Topographic Wave Modes to Solve for the Barotropic Mode of a Rigid Lid Ocean Model. J. Atmosph. Oceanic Tech., 13(3) 751-761.

Woodworth, P.L., J.M. Vassie, C.W. Hughes, and M.P. Meredith, 1996: A Test of TOPEX/POSEIDON's Ability to Monitor Flows Through Drake Passage. J. Geophys. Res. (Oceans) 101(C5), 11,935-11,947 - More .

Hughes, C.W. & M.J. Smithson, 1996: Bottom pressure correlations in the south Atlantic. Geophys. Res. Letters, 23(17), 2243-2246 - More .

1995

Hughes, C.W. 1995: Rossby Waves in the Southern Ocean: A Comparison of TOPEX/POSEIDON Altimetry with Model Predictions. J. Geophys. Res. 100(C8), 15933-15950 - More .

Hughes, C.W. & P.D. Killworth, 1995: Effects of bottom topography in the large-scale circulation of the Southern Ocean. J. Phys. Oceanogr., 25(11), 2485--2497.