Temperature Trends and Seasonality in Neritic and Transitional Waters of the Southern Bay of Biscay from 1998 to 2023: Relationships with Local Factors and Climate-Ocean Teleconnections

Temporal and spatial variations in water temperature were analysed from 23 time series across two contrasting Basque coast estuaries over a 26-year period (1998-2023). We examined long-term trends, seasonality, and intra- and inter-estuary variability, as well as links to local hydrometeorological factors and North Atlantic climate-ocean teleconnections. Time-series decomposition, clustering, and regression and correlation analyses were used. Water temperature interannual and seasonal patterns differed mainly between outer neritic waters and shallow transitional waters. Most water masses exhibited an overall warming trend, with temperature increases until 2003-2006, followed by declines until 2013-2015, and a sharp rise in 2020-2023, largely driven by spring temperatures. Warming was strongest (0.24-0.25 °C decade-1) in middle-estuary waters, while no trend or even cooling occurred in inner above-halocline waters of the stratified system. Spring, particularly May, experienced the highest warming rates, reaching 0.49 °C and 0.80 °C decade-1, respectively, in transitional waters of the mixed estuary. Seasonal temperature minima and maxima occurred earlier in surface transitional waters than in neritic waters and deep transitional waters of the stratified system. Over time, annual maxima advanced, minima were delayed, and spring temperatures rose, this extending the warm period. Air temperature, especially over-sea, was the primary driver of water temperature trends. River flow modulated the trends to annual and seasonal scales, inducing cooling, mainly in spring. AMO index was the teleconnection best related to water temperature, while links with the NAO index and the EA pattern were more restricted to specific seasons.