Maintenance of near-surface stratification in Doubtful Sound, a New Zealand fjord

The surface waters of Doubtful Sound, a glacially carved fjord in south-wes tern New Zealand, feature a quasi-permanent low-salinity-layer (LSL). The L SL is maintained gear round by the extreme precipitation in the catchment ( similar to7 m yr(-1)) and discharge from a hydroelectric power station (sim ilar to 450 m(3) s(-1)). The robust, stable LSL has been shown to play a ma jor role in controlling intertidal and subtidal community structure. By con trast, little is known about the dynamics of the LSL itself. The work prese nted here elucidates the response of the LSL to perturbations in the wind s tress and rainfall. Frequency-domain analysis of salinity data collected from an array of moore d instruments revealed that the LSL responded to perturbations in both the winds and rainfall. However, the specific roles of the wind stress and rain could not be adequately resolved in these analyses. By contrast, simulatio ns of the response of the LSL using a three-dimensional primitive equation model revealed that strong up-fjord directed wind events set up a storm sur ge at the head of the fjord. This surge significantly deepens the LSL at th e head of the fjord and retards or reverses estuarine circulation. The subs equent relaxation of the surge after the abatement of the wind stress resul ted in a redistribution of buoyancy throughout the fjord over a two-day per iod. It is shown that the development and relaxation of the storm surge is a major process controlling the maintenance of the near-surface stratificat ion. (C) 2000 Academic Press.