Diurnal variability on the continental shelf of Southern Brazil

摘要

This descriptive paper presents experimental data on diurnal cycling in the upper ocean on the shelf of Southern Brazil. Data were collected by three Brazilian research vessels at 10 individual sites, between 24 degrees S and 33 degrees S, During the measurements, the ships were stationed or drifted. At every location, series of temperature, salinity and density profiles were taken in the upper 50 m, accompanied by shipboard meteorology observations. At some locations, measurements of surface and underwater solar irradiance were also performed. The duration of time series varied from 8-72 h, The sampling intervals ranged between 0.5 and 3 h but normally were of one hour. Total duration of the series was 186 h, and the total number of individual hydrographic profiles in the dataset amounts to 159. The diurnal cycle of temperature was determined by fitting a sum of diurnal and semi-diurnal Fourier harmonics to the data. The diurnal range of SST varied from 0.1 degrees C to over 1.2 degrees C, controlled mostly by cloudiness and wind speed, The average range was about 0.5 degrees C for summer and 0.4 degrees C for winter observations, In most cases the observed diurnal cycle of temperature was confined to the uppermost 10 to 15 m. The phase lag between solar irradiance and sea temperature was normally about 4 h at the surface and increased downwards, typically by about one hour per every meter of depth. The sensible and latent fluxes exhibited regular diurnal variability, The divergence of the turbulent heat flux at a given depth in the bulk of the water column showed pronounced diurnal cycle, sometimes changing from - 150 to + 50 W m(-3) in the course of 8 to 12 h. A strong diurnal cycle of stratification was observed in the northern part of the region, with the average night-time values of buoyancy frequency smaller than the day-time ones approximately by one order of magnitude, while south of 31 degrees S stratification was mostly salinity controlled and represented small diurnal variation. In 3 of the observations, the night-time surface cooling led to convective instabilities and related overturns soon after midnight, resulting in rapid restratification of the water column and sign reversal of turbulent heat Bur. The vertical size of the overturns was about 10 m in two of the three cases and over 20 m in the third case, while the temporal scale was 1-3 h. (C) 1999 Published by Elsevier Science Ltd. All rights reserved. [References: 25]