Amplification of marine heatwaves in shallow coastal bays

Much of the work on marine heat waves focuses on oceanic environments, but many important coastal ecosystems, such as seagrass, are located in coastal bays with shallow depths and limited exchange with the ocean through tidal inlets. The shallowness of the bays permits heating of the water column during clear, hot summer days that can elevate water temperatures to values significantly higher than offshore sea-surface temperatures. In this study, 35-year records of daily average water temperature from a shallow bay on the Atlantic coast of the lower Delmarva Peninsula, Virginia, and from a nearby ocean site are compared, with a focus on summertime marine heat waves. Surface water temperature at the two sites is well correlated (0.93) and average summer temperature at both sites has increased significantly over the last 35 yrs. Marine heat waves were identified using the criteria proposed by Hobday et al. Mean and median summer water temperatures averaged 3°C warmer in the bays than the ocean. As a result, a majority of the summer days in the bays would quality as experiencing marine heat wave conditions if thresholds from the ocean were used to identify heat waves in the bays. When separate thresholds are defined for the bays and ocean, 15 marine heat waves were identified as occurring simultaneously at the bay and ocean site over the 35 years of record. An additional 15 marine heat waves were identified for the ocean site. Most of these were also reflected by temperature increases in the bays, but the temperatures were not high enough for long enough to meet the marine heat wave criteria. Four heat waves were identified at the bay site but not at the ocean site. In both locations, the numbers of heat waves have increased over time.