PERIODIC FRONTOGENESIS IN A REGION OF FRESH-WATER INFLUENCE

Observations are presented from a series of three conductivity-tempera ture-depth (CTD) surveys of the salinity and temperature structure of Liverpool Bay, a region that is strongly influenced by the input of fr esh water from the rivers of northwest England. The surveys demonstrat e the development, seaward movement, and eventual decay of a haline fr ont. The frontogenesis is driven by the relaxation of a freshwater-ind uced horizontal density gradient following the decrease in tidal range at neap tides. It results in the area of Liverpool Bay being stratifi ed for a period of 8 d before the increase in tidal mixing as the spri ng tide approaches returns the region to its initial vertically mixed state. In Liver-pool Bay this process usually repeats on the spring-ne ap cycle, though strong wind-mixing may prevent the frontogenesis and subsequent stratification. Analysis with a 1-dimensional numerical mod el suggests that relaxation of an initially nonlinear horizontal densi ty field, creating the front, is triggered by the stability produced b y tidal straining of the water column during the ebb half-cycle. The r eduction in tidal mixing energy approaching neap tide does not lead to frontogenesis without this initial stability. Such a regular stratifi cation signal will have a marked effect on the local environment. The periodic frontogenesis will act as a tidal pump, moving buoyant substa nces in the water column offshore, while the onshore residual currents lower in the water column will move deeper dissolved substances insho re. The cycling of stability on the spring-neap time scale is consider ably faster than the seasonal cycle of thermal stratification in the s helf seas, but is similar in creating the conditions required for phyt oplankton blooms. Conditions favorable for enhanced primary production may therefore occur frequently in such regions of freshwater influenc e.