Climatology of the Sea Temperature from Long-Term In Situ Observations: Northern Chilean Patagonia

Abstract. Ocean climatologies are essential for understanding the mean state and variability of marine systems, providing reference conditions for detecting anomalies, assessing ecosystem functioning and identifying long-term trends. In this study, we present the first high-resolution, four-dimensional temperature climatology for Northern Chilean Patagonia, a region characterised by the presence of a complex network of fjords and channels. The climatology was generated from approximately three million in situ observations collected over the past 75 years from multiple datasets. Using the Data Interpolating Variational Analysis (DIVAnd) tool, temperature fields were reconstructed with a horizontal resolution of about 900 m and over 32 vertical levels, producing monthly fields from the surface to 400 m depth. The resulting climatology accurately reproduces the main spatial and seasonal temperature patterns reported in previous studies. Surface temperatures show strong summer gradients between the open Pacific and Northern Patagonia, while enclosed regions such as Reloncaví Sound, Comau and Puyuhuapi Fjords exhibit pronounced seasonal amplitude compared with the more stable Corcovado Gulf. Below surface, temperature gradients become smoother and the thermocline deepens in summer, typically ranging between 10 and 50 m. At 100 m depth, temperatures are relatively constant throughout the year, but spatial variations are still observed, with Ancud Gulf and Reloncaví Sound being slightly warmer. This climatology provides a refined and physically consistent representation of the oceanographic structure of Northern Patagonia, offering an essential baseline for future modelling, monitoring and climate-change assessments in a region where satellite observations are limited by persistent cloud cover and complex topography. All climatologies generated in this study are freely available online.