The truth about 26C and the relative roles of the ocean and atmosphere at the hurricane air–sea interface

Several studies dating back to the 1940's have documented the observation that tropical cyclones usually do not reach or maintain hurricane intensity once underlying sea surface temperatures (SST) fall below 26C. For decades, the widely held belief as to why this occurs is that the available ocean thermal energy necessary to maintain the intensity of the hurricane is simply insufficient when SSTs fall below this threshold. As such, many recent studies have attempted to link the ocean heat content (OHC) diagnostic parameter, which is undefined at SSTs below 26C, to subsequent changes in storm intensity. Results from this multi-hurricane study suggest that the criticality of the 26C threshold as it relates to storm maintenance is instead more closely linked to atmospheric thermodynamic conditions typically observed within the hurricane inner core environment. Findings also suggest that the primary thermodynamic term controlling the magnitude of surface moisture flux within the high wind inner core decreases in magnitude as storms move northward. A reduction in the near-surface atmospheric moisture gradient (qSST – qa10m or “Δq”) is found to be primarily driven by cooler ocean conditions typically observed at higher latitudes. Results also indicate that the explained variance associated with Δq under high wind conditions for deep tropic (<26N) and subtropic (26-29N) hurricanes was largely driven by atmospheric forcing (qa10m). At higher latitudes (>29-32.5N), Δq variability was equally influenced by inner core qa10m and qSST.