GFDL - Geophysical Fluid Dynamics Laboratory

Ocean color and climate

The centers of the subtropical oceanic gyres are relative blue and clear, with some bands of sunlight penetrating over 100m. The edges of the gyres are much more turbid, with penetration length scales only 5-10m. We have been studying the impact of this additional shortwave absorption in a range of models. Naively one would expect a turbid ocean to be warmer at the surface and cooler at depth. Circulation and mixing can alter this picture substantially. Click here to hear and Earth and Sky radio show on this work

Results so far:

1. Sweeney, C.O., A. Gnanadesikan, S.M. Griffies, M.J. Harrison, A. Rosati and B.L. Samuels, 2005: Impacts of shortwave penetration depth on the large-scale circulation and heat transport, J. of Physical Oceanography,35, 1103-1119.PDF

This paper demonstrated that differences in how the penetration of solar radiation into the water column is represented in ocean models can have a surprising impact on the cold tongue, with deeper penetration of radiation producing warming near the surface. The mechanism is similar to the “Equatorial Thermostat” proposed by Amy Clement.

2. Anderson, W.G., A. Gnanadesikan, R.W. Hallberg, J. Dunne and B.L. Samuels, 2007:  Impact of ocean color on the maintenance of the Pacific Cold Tongue, Geophysical Res. Lett. , 34, GB4006, doi:10.1029/2006GB002907. PDF

This paper demonstrates that in our coupled climate model with an density-coordinate ocean, changes in ocean color away from the equator can have profound impacts on the structure of the cold tongue and amplitude of El Nino. In a fully coupled model, the equatorial thermostat mechanism found in (1) does not appear to hold.

3. Gnanadesikan, A. and W.G. Anderson, 2009: Ocean water clarity and the ocean general circulation in a coupled climate model, J. of Physical Oceanography, 39, 314-332. PDF

Here we examine the oceanic response to increasing solar penetration in different regions. We show that the response is very dependent on where the additional heating is added and that the resulting shifts in winds and ocean currents can be quite large.

4. Anderson, W.G., A. Gnanadesikan and A.T. Wittenberg, 2009: Regional impacts of ocean color on tropical Pacific variability, Ocean Science 5, 313-327. Link

We demonstrate that the changes in the mean state shown in (3) can have major impacts on the amplitude of El Nino. The results are highly dependent on the regions involved and on the coupled response of the atmosphere. Ocean-only simulations suggest that increasing penetration of solar radiation would always damp El Nino, whereas we find that increasing the penetration of solar radiation along the gyre margins actually makes the Pacific more variable.

5. Gnanadesikan, A., K. Emanuel, G.A. Vecchi, W.G. Anderson and R. Hallberg, 2010: How ocean color can steer Pacific tropical cyclones,  Geophys. Res. Lett, 37, L18802, 2010.Link

We show that the presence of ocean color in the subtropical gyres actually attracts hurricanes and typhoons to these regions, away from the equatorial ocean. Click here to learn more.

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