A quasi-dipole structure in wind forcing over the California Current System, and the marine heat waves of 2014-2016
by Melanie Sewings, Associate Professor, College of Earth, Ocean, and Atmospheric Sciences, OSU
The northeast Pacific marine heat wave (MHW) of 2014–2016 was among the largest recorded MHWs. This MHW was caused by persistent mid-level atmospheric ridging and affected ecosystems from Alaska to Mexico. However, there was strong regional variability along the coast in the timing and strength of the MHW. By summer 2015, the MHW split in two parts, one in the Gulf of Alaska and one from central California to Baja. Similar regional variability within MHWs is common globally, but not well understood. Here, we show that the split MHW structure is explained by a regional, synoptic wind dipole mode we described in previous work. Satellite wind stress anomalies during the two weeks preceding the splitting of the MHW displayed an unusually persistent “southern wind relaxation” state. The unusual persistence of this state is consistent with the atmospheric ridging that caused the large-scale MHW. To determine whether changes in surface heat flux or wind-driven mixing were more likely responsible for the SST anomalies, we examined the heat budget for the surface mixed layer. The atmosphere-ocean heat flux anomalies off southern California were too small and/or the wrong sign to explain the SST anomalies during the splitting of the MHW. This suggests that changes in wind-driven vertical mixing and mixed-layer depth were more likely the cause of the splitting of the MHW. There is a systematic relation between the mid-level ridging that caused the MHW and the regional wind dipole with its associated SST anomaly dipole. This suggests that the spatial pattern of mid-latitude MHWs off western North America may be predictable, even if the timing is not. We suggest that future MHWs in the California Current System are likely to also develop a “split” structure.
Thursday, November 15, 2018 at 3:30pm to 4:30pm
Hatfield Marine Science Center, Guin Library Seminar Room
2030 SE Marine Science Dr, Newport, OR 97365
Free
Cinamon Moffett
5418670126
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