Sensitivity of the Indian Ocean circulation to phytoplankton forcing using an ocean model

Abstract Most ocean general circulation models (OGCMs) do not take into account the effect of space- and time-varying phytoplankton on solar radiation penetration, or do it in a simplistic way using a constant attenuation depth, even though one-dimensional experiments have shown potential significant effect of phytoplankton on mixed-layer dynamics. Since some ocean basins are biologically active, it is necessary for an OGCM to take water turbidity into account, even if it is not coupled with a biological model. Sensitivity experiments carried out with the Massachusetts Institute of Technology (MIT) OGCM with spatially and temporally-varying pigment concentration from Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data during 1998–2003 reveal the effect of ocean turbidity on tropical Indian Ocean circulation. Variations of light-absorbing phytoplankton pigments change the vertical distribution of solar heating in the mixed layer, thereby affecting upper-ocean circulation. A simulation was performed from 1948 to 2003 with a constant minimum pigment concentration of 0.02 mg m− 3 while another simulation was performed from September 1997 to December 2003 with variable pigment concentration, and the differences between these two simulations allow us to quantify the effects of phytoplankton on solar radiation penetration in the ocean model. Model results from a period of 6 years (1998–2003) show large seasonal variability in the strength of the meridional overturning circulation (MOC), meridional heat transports (MHT), and equatorial under current (EUC). The MOC mass transport changes by 2 to 5 Sv (1 Sv = 106 m3 s− 1) between boreal winter (January) and boreal summer (July), with a corresponding change in the MHT of ∼ 0.05 PW (1 PW = 1015 W) in boreal winter, which is close to the expected change associated with a significant climate change [Shell, K., Frouin, R., Nakamoto, S., & Somerville, R.C.J. (2003): Atmospheric response to solar radiation absorbed by phytoplankton. Journal of Geophysical Research, 108(D15), 4445. doi:10.1029/2003JD003440.]. In addition, changes in phytoplankton pigments concentration are associated with a reduction in the EUC by ∼ 3 cm s− 1. We discuss the possible physical mechanisms behind this variability, and the necessity of including phytoplankton forcing in the OGCM.

[1]  W. Collins,et al.  The NCEP–NCAR 50-Year Reanalysis: Monthly Means CD-ROM and Documentation , 2001 .

[2]  Antonio J. Busalacchi,et al.  Effects of Penetrative Radiation on the Upper Tropical Ocean Circulation , 2002 .

[3]  Peter A. Rochford,et al.  The Impact of Water Turbidity on Interannual Sea Surface Temperature Simulations in a Layered Global Ocean Model , 2004 .

[4]  W. Large,et al.  Oceanic vertical mixing: a review and a model with a nonlocal boundary layer parameterization , 1994 .

[5]  E. Schneider,et al.  Sensitivity of the Simulated Annual Cycle of Sea Surface Temperature in the Equatorial Pacific to Sunlight Penetration , 1998 .

[6]  Thomas M. Smith,et al.  An Improved In Situ and Satellite SST Analysis for Climate , 2002 .

[7]  J. Yoder,et al.  Seasonal and ENSO variability in global ocean phytoplankton chlorophyll derived from 4 years of SeaWiFS measurements , 2003 .

[8]  Antonio J. Busalacchi,et al.  Oceanic processes associated with anomalous events in the Indian Ocean with relevance to 1997–1998 , 2000 .

[9]  R. Reynolds,et al.  The NCEP/NCAR 40-Year Reanalysis Project , 1996, Renewable Energy.

[10]  R. Reed.  Heat fluxes over the eastern tropical Pacific and aspects of the 1972 El Niño , 1983 .

[11]  J. Marotzke,et al.  The dynamics of ocean heat transport variability , 2001 .

[12]  C. Paulson,et al.  Irradiance Measurements in the Upper Ocean , 1977 .

[13]  K. Wyrtki,et al.  An Equatorial Jet in the Indian Ocean , 1973, Science.

[14]  Robert Frouin,et al.  Sensitivity of equatorial Pacific Ocean circulation to solar radiation absorbed by phytoplankton , 2005, SPIE Optics + Photonics.

[15]  P. K. Kundu,et al.  A numerical investigation of dynamics, thermodynamics and mixed-layer processes in the Indian Ocean , 1993 .

[16]  B. Subrahmanyam,et al.  Numerical simulation of seasonal and interannual Indian Ocean upper layer circulation using Miami Isopycnic Coordinate Ocean Model , 2003 .

[17]  Trevor Platt,et al.  Biological control of surface temperature in the Arabian Sea , 1991, Nature.

[18]  J. Fischer,et al.  Equatorial currents and transports in the upper central Indian Ocean: Annual cycle and interannual variability , 1999 .

[19]  D. Antoine,et al.  Heating rate within the upper ocean in relation to its bio-optical state , 1994 .

[20]  Peter J. Webster,et al.  Coupled ocean–atmosphere dynamics in the Indian Ocean during 1997–98 , 1999, Nature.

[21]  L. Perelman,et al.  Hydrostatic, quasi‐hydrostatic, and nonhydrostatic ocean modeling , 1997 .

[22]  Robert Frouin,et al.  Atmospheric response to solar radiation absorbed by phytoplankton , 2003 .

[23]  C. Wunsch.  Response of an Equatorial Ocean to a Periodic Monsoon , 1977 .

[24]  C. Mobley.  Light and Water: Radiative Transfer in Natural Waters , 1994 .

[25]  M. Redi.  Oceanic Isopycnal Mixing by Coordinate Rotation , 1982 .

[26]  L. Perelman,et al.  A finite-volume, incompressible Navier Stokes model for studies of the ocean on parallel computers , 1997 .

[27]  J. Knauss,et al.  Equatorial Undercurrent of the Indian Ocean , 1964, Science.

[28]  F. Schott,et al.  Heat fluxes of the Indian Ocean from a global eddy-resolving model , 1997 .

[29]  Tong Lee,et al.  Seasonal Cycles of Meridional Overturning and Heat Transport of the Indian Ocean , 1998 .

[30]  R. Pacanowski,et al.  Seasonal Heat Transport in a Primitive Equations Model of the Tropical Indian Ocean , 1996 .

[31]  Robert Frouin,et al.  Response of the equatorial Pacific to chlorophyll pigment in a mixed layer isopycnal ocean general circulation model , 2001 .

[32]  Kenneth L. Denman,et al.  A Time-Dependent Model of the Upper Ocean , 1973 .

[33]  P. Gent,et al.  Isopycnal mixing in ocean circulation models , 1990 .

[34]  F. Schott,et al.  The monsoon circulation of the Indian Ocean , 2001 .

[35]  Robert Frouin,et al.  Chlorophyll modulation of sea surface temperature in the Arabian Sea in a mixed‐layer isopycnal general circulation model , 2000 .

[36]  B. Goswami,et al.  A dipole mode in the tropical Indian Ocean , 1999, Nature.