Graduate Thesis Or Dissertation

Analysis of Surface Heat Flux Anomalies to Understand Recent Northeast Pacific Marine Heatwave Events

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  • Several large-scale marine heatwave events occurred during the last 10 years in the North Pacific. A particular extreme marine heatwave in the North Pacific called the blob created an unprecedented high peak of sea surface temperature (SST) during 2013/14. MHW events had significant impacts on downstream weather and precipitation patterns and regional ecological dynamics. Some evidence suggests that the persistence of these warm ocean surface anomalies altered the Northern Hemisphere climate and weather variability, such as the precipitation distribution over North America. Also, warm surface water has less capacity for nutrients than cold upwelling water and created a chain reaction of ecosystem deterioration in the Gulf of Alaska. The conditions leading to the formation of these large-scale warm oceanic anomalies have not been well studied or understood. In this study, we tested the hypothesis that persistent atmospheric circulation anomalies were a key factor in generating and maintaining recent marine heatwaves in the North Pacific. To address this hypothesis, we analyzed ocean mixed-layer heat budgets from an ocean state estimate to isolate atmospheric processes contributing to the formation of the MHW during 2013-2014. We further used atmospheric reanalysis fields to resolve how atmospheric circulation affected surface turbulent (latent/sensible) and radiative (short/long wave) fluxes for two events during 2013/14 and 2019/20 in the Northeast Pacific to better understand the interactions between the atmospheric state and the upper ocean thermal structure. Our analysis shows that the net surface heat fluxes played a strong role in the formation of these two marine heat waves by inhibiting surface evaporative cooling and sensible heat loss. Furthermore, the heat fluxes anomalies are well correlated with the position of large-scale atmospheric ridging episodes in the North Pacific as represented by sea level pressure and surface wind anomalies. Ocean heat loss through the turbulent heat fluxes was reduced by more than 50 W/m2 preceding the formation of these marine heat waves. Analysis of the ocean mixed layer heat budget from the ocean state estimate indicates that the surface turbulent heat flux forcing was a key factor in generating these marine heat waves. We also found that the surface turbulent heat flux anomalies responsible for the MHWs were predominantly forced by anomalously warm and moist surface air anomalies driven by anomalous southerly winds. The wind anomalies were generated by persistent sea-level pressure anomalies during these events. Our conclusion is that these MHW events were primarily an ocean response to surface turbulent heat flux anomalies driven by anomalous atmospheric circulation patterns, which caused by the pressure ridge from the North Pacific High. Future studies should further investigate how the behavior and influence from the atmospheric forcing is related to low frequency climate scale oscillations such as the ENSO and PDO.
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