Disentangling Physical Forcings Influencing Exchange Flow in a Multi-basin Fjord System: Chiloé Inner Sea, Patagonia

Abstract. Fjords mediate land-sea exchange of water masses, nutrients, and dissolved oxygen in high latitude coastal areas. The renewal of fjord waters is sensitive to shifts in runoff, winds, and tides that could be exacerbated due to climate change. In this study we quantify the relative contribution of rivers, wind, tides, and boundary temperature and salinity to estuarine exchange flow in the multi–basin Chiloé Inner Sea (CIS). We use a suite of controlled 3D simulations that toggle each driver on/off and analyze the outputs with the Total Exchange Flow (TEF) framework. TEF separates exchange inflow and outflow via salinity classes, allowing salt import to be a proxy for renewal potential. Idealized experiments isolating exchange flow drivers show that internal baroclinicity from rivers sets the baseline two–layer exchange flow and explains∼45–55 % of the variability of the exchange inflow (Q_in), the dynamic stratification (∆S), and the salt import (Q_in∆S) along the CIS. Wind modulates the baseline exchange flow by (∼20–50 %), enhancing Q_in when wind stress is down–estuary while strong wind of either sign erodes ∆S and thus salt import. Tides contribute less on average than wind to the exchange inflow (∼ 10–20 %) but their influence changes spatially. In interior basins the influence of tides is minimal, while at sills and channel constrictions spring tides tend to increase Q_in and often Q_in∆S without a proportional collapse of ∆S. Offshore temperature and salinity variability, also called boundary baroclinicity, is of secondary influence, yet is consequential near the CIS mouth in winter, and can at times weaken or reverse the internally driven exchange. Renewal of deep fjord waters requires both ample exchange inflow and dynamic stratification. Our results indicate that in the CIS, the likelihood of salt import is greatest under moderate down–estuary winds coincident with high runoff and spring tides, whereas persistent up–estuary winds or strong wind events suppress import despite large Q_in. The exchange inflow along the CIS was found to have dominant synoptic (3–12 d; wind/storms), monthly (25–35) and intraseasonal (40–70 d) periodicities that are driven primarily by atmospheric phenomena in the Southern Hemisphere. These results highlight new mechanisms and periodicities important for deep-water renewal events to take place, and therefore can help anticipate when renewal will be suppressed and low dissolved oxygen could become a risk in the fjords and channels of the CIS.