The Summer Heat Balance of the Oregon Inner Shelf Over Two Decades: Mean and Interannual Variability

Summer temperature and velocity measurements from 14 years in 15 m of water over the inner shelf off Oregon were used to investigate interannual temperature variability and the capacity of the across-shelf heat flux to buffer net surface warming. There was no observable trend in summer mean temperatures, and the standard deviation of interannual variability (0.5 degrees C) was less than the standard deviation in daily temperatures each summer (1.6 degrees C, on average). Yet net surface heat flux provided a nearly constant source of heat each year, with a standard deviation less than 15% of the interannual mean. The summer mean across-shelf upwelling circulation advected warmer water offshore near the surface, cooling the inner shelf and buffering the surface warming. In most years (11 out of 14), this two-dimensional heat budget roughly closed with a residual less than 20% of the leading term. Even in years when the heat budget did not balance, the observed temperature change was negligible, indicating that an additional source of cooling was needed to close the budget. A comparison of the residual to the interannual variability in fields such as along-shelf wind stress, stratification, and along-shelf currents found no significant correlation, and further investigation into the intraseasonal dynamics is recommended to explain the results. An improved understanding of the processes that contribute to warming or cooling of the coastal ocean has the potential to improve predictions of the impact of year-to-year changes in local winds and circulation, such as from marine heat waves or climate change, on coastal temperatures. Plain Language Summary Long-term observations over 14 summer seasons at a site off the Oregon coast were used to understand the year-to-year changes in the ocean temperatures between 1 km offshore in 15 m of water and the shoreline. The summer mean temperature did not change at this site, and year-to-year changes in the summer mean temperature were less than the variability in the temperature within a single year. Warming at the surface, such as from solar heating, was also relatively consistent from year to year. A simple budget was presented to estimate the relative contributions of different processes that can warm or cool the water. The summer mean circulation pattern in the across-shelf direction, which transports warmer water offshore near the surface, cooled the inner shelf and buffered or balanced the surface warming in most years. It was not clear what other processes contribute to warming or cooling of the coastal ocean, but additional work is needed to understand the potential impact of year-to-year changes in the local winds and circulation, such as from marine heat waves or climate change, on coastal temperatures.