Subtidal dynamics in a deep fjord of southern Chile

Abstract

This study presents results from an unprecedented program of measurements performed in Reloncavi Fjord (RF), Chile. This fjord is located around 41°35′S, 72°30′W, making it one of the most equatorward fjords in the world. The main objectives of this study were to describe and quantify the mean circulation inside the fjord and to evaluate its along- and cross-fjord subtidal momentum balance. The depth of the fjord is about 450m near the mouth and lacks a well defined sill. There, the mean flow showed a three layered structure. Surface (<5m depth) and deep (>100m depth) outflow layers delimited an inward-flowing layer. The along-fjord wind stress (τy) shifted markedly during the beginning of the austral spring (September 2008). In winter, the mean τy was predominantly out-fjord, similar to the mean along-fjord currents (v) observed in the upper layer. In contrast, the spring τy was mainly toward the fjord head, i.e., opposite to the surface current. During periods of large up-fjord winds (τy>0.04Nms−2) surface currents were reversed. Near the surface (depths <5m), v and τy were well correlated in spring. The horizontal baroclinic pressure gradient force was typically of order 10−5ms−2 at the surface, both along- and cross-fjord. The estimated magnitudes of the other terms in the along-fjord momentum equation indicated that advective terms were the most relevant. But near Puelo, where the fjord abruptly bends and the main river flow enters the fjord, both the advective and frictional forces were relevant, particularly in February 2009. In the cross-fjord component of the momentum equation, the Coriolis term 〈fv〉 was the most relevant in winter. At Puelo the advective term 〈u(∂u/∂x)〉 was the most important in summer. The importance of the Coriolis force in the cross-fjord momentum balance suggested that v may approximately be in geostrophic balance. The correlation between the mean along-fjord geostrophic velocities and the mean observed currents had an R2 >0.79 for all measurements and reached R2 ∼0.85 in winter.