Temporal variations of the seasonal sea level harmonics throughout the 20th and early 21st century along the United States Gulf coast are investigated. A significant amplification of the annual sea level cycle from the 1990s onward is found, with both lower winter and higher summer sea levels in the eastern Gulf. Ancillary data are used to build a set of multiple regression models to explore the mechanisms driving the decadal variability and recent increase in the annual cycle. The results suggest that changes in the air surface temperature toward warmer summers and colder winters and changes in mean sea level pressure explain most of the amplitude increase. The changes in the seasonal sea level cycle are shown to have almost doubled the risk of hurricane induced flooding associated with sea level rise since the 1990s for the eastern and north-eastern Gulf of Mexico coastlines.