Nature of global large‐scale sea level variability in relation to atmospheric forcing: A modeling study

The relation between large-scale sea level variability and ocean circulation is studied using a numerical model. A global primitive equation model of the ocean is forced by daily winds and climatological heat fluxes corresponding to the period from January 1992 to January 1994. The physical nature of sea level's temporal variability from periods of days to a year is examined on the basis of spectral analyses of model results and comparisons with satellite altimetry and tide gauge measurements. The study elucidates and diagnoses the inhomogeneous physics of sea level change in space and frequency domain. At midlatitudes, large-scale sea level variability is primarily due to steric changes associated with the seasonal heating and cooling cycle of the surface layer. In comparison, changes in the tropics and high latitudes are mainly wind driven. Wind-driven variability exhibits a strong latitudinal dependence in itself. Wind-driven changes are largely baroclinic in the tropics but barotropic at higher latitudes. Baroclinic changes are dominated by the annual harmonic of the first baroclinic mode and is largest off the equator; variabilities associated with equatorial waves are smaller in comparison. Wind-driven barotropic changes exhibit a notable enhancement over several abyssal plains in the Southern Ocean, which is likely due to resonant planetary wave modes in basins semienclosed by discontinuities in potential vorticity. Otherwise, barotropic sea level changes are typically dominated by high frequencies with as much as half the total variance in periods shorter than 20 days, reflecting the frequency spectra of wind stress curl. Implications of the findings with regards to analyzing observations and data assimilation are discussed.

[1]  J. O'Brien,et al.  Interannual variability of the equatorial Pacific in the 1960's , 1981 .

[2]  Brian D. Beckley,et al.  Global mesoscale variability from collinear tracks of SEASAT altimeter data , 1983 .

[3]  J. Dukowicz,et al.  Implicit free‐surface method for the Bryan‐Cox‐Semtner ocean model , 1994 .

[4]  P. Malanotte‐Rizzoli,et al.  An approximate Kaiman filter for ocean data assimilation: An example with an idealized Gulf Stream model , 1995 .

[5]  A. Chave,et al.  Evidence for local and nonlocal barotropic responses to atmospheric forcing during BEMPEX , 1990 .

[6]  M. Ghil,et al.  Data assimilation in meteorology and oceanography , 1991 .

[7]  Richard Smith,et al.  Global Ocean Circulation from Satellite Altimetry and High-Resolution Computer Simulation , 1996 .

[8]  Jin Wu.  Wind‐stress coefficients over sea surface from breeze to hurricane , 1982 .

[9]  Interactions between the atmosphere, oceans and crust: Possible oceanic signals in Earth rotation , 1993 .

[10]  C. Wunsch,et al.  How well does a 1/4° global circulation model simulate large-scale oceanic observations? , 1996 .

[11]  K. Wyrtki.  Fluctuations of the Dynamic Topography in the Pacific Ocean , 1975 .

[12]  Carl Wunsch,et al.  The Vertical Partition of Oceanic Horizontal Kinetic Energy , 1997 .

[13]  C. Hughes,et al.  Bottom pressure correlations in the south Atlantic , 1996 .

[14]  Ralph J. Slutz,et al.  A Comprehensive Ocean-Atmosphere Data Set , 1987 .

[15]  Andreas Oschlies,et al.  Assimilation of Geosat altimeter data into an eddy-resolving primitive equation model of the North Atlantic Ocean , 1996 .

[16]  S. Manabe,et al.  A Global Ocean-Atmosphere Climate Model. Part II. The Oceanic Circulation , 1975 .

[17]  C. Wunsch.  Bermuda sea level in relation to tides, weather, and baroclinic fluctuations , 1972 .

[18]  Sol Hellerman,et al.  Normal Monthly Wind Stress Over the World Ocean with Error Estimates , 1983 .

[19]  A. E. Gill,et al.  The theory of the seasonal variability in the ocean , 1973 .

[20]  K. Wyrtki.  Sea Level and the Seasonal Fluctuations of the Equatorial Currents in the Western Pacific Ocean , 1974 .

[21]  Y. Han.  A numerical world ocean general circulation model. Part II. A baroclinic experiment , 1984 .

[22]  Anthony Rosati,et al.  The sea surface pressure formulation of rigid lid models. Implications for altimetric data assimilation studies , 1995 .

[23]  R. Greatbatch.  A note on the representation of steric sea level in models that conserve volume rather than mass , 1994 .

[24]  S. Levitus.  Climatological Atlas of the World Ocean , 1982 .

[25]  Yi Chao,et al.  A Comparison Between the TOPEX/POSEIDON Data and a Global Ocean General Circulation , 1995 .

[26]  R. Davidson,et al.  A note on the barotropic response of sea level to time‐dependent wind forcing , 1995 .

[27]  Detlef Stammer,et al.  Steric and wind-induced changes in TOPEX/POSEIDON large-scale sea surface topography observations , 1997 .

[28]  Mark A. Cane,et al.  Modeling Sea Level During El Niño , 1984 .

[29]  Sarah T. Gille,et al.  Gulf Stream surface transport and statistics at 69°W from the Geosat altimeter , 1990 .

[30]  J. Willebrand,et al.  The Oceanic Response to Large-Scale Atmospheric Disturbances , 1980 .

[31]  A. Semtner.  Modeling Ocean Circulation , 1995, Science.

[32]  Carl Wunsch,et al.  The global frequency-wavenumber spectrum of oceanic variability estimated from TOPEX/POSEIDON altimetric measurements , 1995 .

[33]  R. Leben,et al.  Tracking Loop Current eddies with satellite altimetry , 1993 .

[34]  R. Pacanowski,et al.  Parameterization of Vertical Mixing in Numerical Models of Tropical Oceans , 1981 .