Horizontal variability in chlorophyll fluorescence and potential temperature

Abstract Chlorophyll fluorescence and temperature were measured with a horizontal resolution of 4 m on two isopycnals and two isobars along a 1000 km meridional transect in the eastern North Pacific. Probability density functions (PDFs) of the magnitude of fluctuations in temperature and fluorescence are compared at each of a large range of length scales. While they are nearly identical at larger scales, the shapes of the PDFs of fluorescence and temperature differ significantly at small scales (∼1 km and less). This difference may indicate that at these scales, temperature and fluorescence distributions are controlled by distinct mechanisms. By comparing the phase of wavelet transforms of each of these two tracers, the tendency for temperature and fluorescence gradients to line up is investigated over a large range of spatial scales. At horizontal length scales of order 10 km and larger, the wavelet phase difference between temperature and fluorescence tends to be close to 0° or 180°—that is, the gradients tend to align, either in phase or 180° out of phase. At smaller scales, the distribution of phase difference on isopycnals is uniform—there is no tendency for gradients to coincide. 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. These analyses suggest the horizontal distributions of temperature and fluorescence at large scales were both governed by advective processes. At intermediate scales, of order 1–10 km, a renovating wave model suggests that gradient alignment could be destroyed by turbulent diffusion or by rapid local phytoplankton growth.