Two‐dimensional observations of temperature microstructure in a coastal region

Abstract

Temperature microstructure is mapped in two dimensions for a section 6.7 km long and 150 m deep in water which is 200 m deep. The section is located on the continental shelf southeast of Point Conception on the southern California coast. Regions of mixing activity are composed of numerous individual events, some of which are observed to collapse onto isopycnal surfaces when plotted in a distance‐density coordinate system. Mixing regions with the temperature variance dissipation rate χ exceeding 10−6°C2 s−1 are observed to have isopycnal extents of up to 3 km. Comparison of the distributions of N and χ reveals no consistent relationship between mixing activity and the ambient stratification. Terms in the two‐dimensional budget equation for temperature variance (T′)2 are compared with χ for a volume bounded by isopycnal surfaces which is about 17 m thick and which spans the data section. The budget for (T′)2 is consistent with a balance between diapycnal production and dissipation of (T′)2 yielding an effective diapycnal diffusivity of heat of 2.0 × 10−4 m2 s−1. Water within the averaging volume is thermally stratified and the measured temperature variance dissipation rate may be used to estimate the rate at which kinetic energy is irreversibly lost to mixing of local density gradients. This rate is found to be about 3.3×10−8 W kg−1 within the averaging volume and is about a factor of 3 smaller than the kinetic energy dissipation rate ε typically found in surface mixed layers.