IUGG 2003 Abstract

JSP09 Physical Aspects of Air-Sea Interaction (IAPSO, IAMAS)  Friday, July 11 PM Location: Site B, Room 20 Presiding Chairs:L. Shay, P. Black

TIME [ 1415 ] [ JSP09/11P/B20-002 ]

SENSITIVITY OF THE EQUATORIAL PACIFIC OCEAN CIRCULATION TO CHLOROPHYLL MODULATION OF PENETRATIVE SOLAR IRRADIANCE IN AN OGCM

Kyozo UEYOSHI(SCRIPPS INSTITUTION OF OCEANOGRAPHY, UNIV. OF CALIFORNIA, SAN DIEGO)

Detlef STAMMER(SCRIPPS INSTITUTION OF OCEANOGRAPHY, UNIV. OF CALIFORNIA, SAN DIEGO) Shoichiro NAKAMOTO ( Advanced Earth Science and Technology Organization, Bussan Bldg. Annex, 1-1-15 Nishi-shinbashi, Minato-ku, Tokyo 105-0003, Japan ) Bulusu SUBRAHAMANYAM ( Center for Ocean Atmospheric Prediction Studies, Florida State University, Tallahassee, FL 32306-2840, USA ) Prasanna KUMAR ( Physical Oceanography Division, National Institute of Oceanography, Dona Paula, Goa 403 004, India ) Kei MUNEYAMA ( Japan Marine Science and Technology Center, 2-15 Natsushima-cho, Yokosuka, 237, Japan )

The sensitivity of the dynamics and thermodynamics to details in the vertical profile of short-wave radiative heating rate is investigated in the MIT z-coordinate ocean general circulation model with initial focus to the tropical Pacific. In this study we examine the hypothesis that surface chlorophyll pigment induces geostrophic current at subsurface level in the equatorial Pacific ocean, proposed by Nakamoto et al.(2001).For this purpose the biological vertical heating rate parameterization of Morel and Anotine(1994) is used to simulate the effects of space-time varying phytoplankton content in the upper ocean in conjunction with monthlymean SeaWifs chlorophyll pigment concentrations. The results from this simulation are compared to those from a control run in which a clear water body is mimicked by assigning a minimum constant pigment concentration value in the same parameterization scheme. The difference between these two simulations is regarded as the effects of phytoplankton on the penetration of solar radiation in the model ocean. A year-round, higher concentration of chlorophyll pigment in theequatorial Pacific leads to a decrease of the mixed layerdepth due to heat accumulation in the mixed layer accompaniedby equatorward shoaling of mixed layer base. In the central equatorial Pacific between 5N and 5S,anomalous geostrophic currents are generated which balance with the slope of the mixed layer shoaling toward the equator.Anomalous eastward currents are seen along the isopycnal surface of~26 sigma t, extending from the ~200 m depth in the western equatorial Pacific (160E) to the surface level in the eastern equatorial Pacific at 90W. The divergence of anomalous surface currents in the eastern equatorial Pacificis accompanied by meridional convergence in the level below,consistent with the enhanced water mass in the equatorial undercurrents inSeaWifs chlorophyll pigment ocean. Anomalous vertical velocity induced by modulated penetration of solarradiation by phytoplankton leads to upwelling of cold water along the equatorextending from the eastern equatorial Pacific towards the dateline.This upwelling is associated with surface divergence and subsurface convergence due to heat localization and subsequent geostrophic balance in the equatorial Pacific. Our results with SeaWifs chlorophyll pigment data generally support the hypothesis of Nakamoto et al. (2001) that phytoplankton in the equatorial Pacific induces subsurface currents that merges into the equatorialundercurrents, resulting in biomass-induced upwelling in the eastern equatorial Pacific. The results from several different absorption models for the shortwave solar radiation are currently being examined and will also be presented.