Some bays of the subtropical west coast of South America, usually protected from the winds, have been observed as upwelling shadows (US), with anomalous high sea surface temperatures. Different arguments have tried to explain it. Using observational, satellite-derived, and reanalysis data, we studied the relationship between synoptic-scale atmospheric phenomena and the US in one of these areas, the Gulf of Arauco (GA), Chile (37°S), a highly valued one productive semi-enclosed bay. With about nine years (2011 – 2019) of a daily global 1 km sea surface temperature data set (G1SST) an upwelling shadow index permitted identify 324 US days. Then, 40 upwelling shadow events, periods with at least three consecutive US days, were found. The composite analysis proposes that the migratory anticyclones moving eastward at southern Chile induce coastal southerly wind intensification due to coastal low-level jet (CLLJ) formation, which triggers upwelling shadows through coastal upwelling outside of GA. The same anticyclone induces a warm low-pressure area increasing the sea surface temperature (SST) inside the Gulf. When the coastal low reaches its mature stage, wind magnitude is reduced, and weak northerly winds are observed at the coast. The weakening of the net convective fluxes, possibly due to this CLLJ relaxation, also increases the SST and exacerbates the upwelling shadow. Another process that could increase SST within the GA is the northerly meridional advection of warm water. However, the observed warming rates (>1◦C/day) suggest that it could only partially explain the warming within the GA.