Along-Isopycnal Variability in Temperature and Chlorophyll Fluorescence in the North Pacific

Planktonic chlorophyll in the ocean may be regarded as a reactive tracer. Two mechanisms which could lead to small- and meso-scale structure in the horizontal chlorophyll field are: 1) cholophyll sources or sinks, e.g. spatially varying growth/death rates of phytoplankton; and 2) advection--stirring of large-scale chlorophyll gradients by ocean currents. In the latter case, chlorophyll behaves as a conservative tracer, and would be expected to display a distribution statistically similar to that of other passive conservative tracers. Further, simple stirring models demonstrate that the locations of enhanced gradients in all tracers would be expected to coincide where the strain has been greatest. However, the directions of enhanced gradients formed in this way may be either parallel or antiparallel, depending on initial conditions and on the direction of the strain. Chlorophyll fluorescence and temperature were measured with a horizontal resolution of 4 meters on the 1025.5 kg/m³ isopycnal along a 1000 km meridional transect in the eastern North Pacific. Probability density functions (PDFs) of fluctuation magnitudes of the concentrations of these two tracers are compared at each of a large range of lengthscales. At the smallest scales, the PDFs of fluorscence and potential temperature differ significantly. They are nearly identical at larger scales. By comparing the phase of wavelet transforms of each of the tracers, the tendancy for temperature and fluorscence gradients to line up is investigated over a large range of spatial scales. At horizontal lengthscales of order 10 km and larger, the wavelet phase difference between temperature and fluorscence tends to be close to 0 or 180 degrees--that is, the gradients tend to align, either in phase or 180 degrees out of phase. At smaller scales, the distribution of phase difference is uniform--there is no tendancy for gradients to coincide. These analyses suggest that at small scales, the horizontal temperature and fluorescence distributions observed were generated by different dynamics, but at larger scales both were governed by advective processes.