Air Sea Analysis Of Marine Heatwaves In The Northeastern Pacific PDF Download

Marine heatwaves (MHWs) are oceanic heat extremes analogous to heat waves on land. These oceanic heat extremes threaten marine biology and ecosystem, and have shown a linkage with the coastal thermal environments. Under global warming, MHWs have found to become more frequent and last longer and have thus been a focus of climatic impact studies. How MHWs in the Northeastern Pacific impact downstream weather extremes in the US and how these MHWs are forced are the two questions addressed by this study. In this study, evidence is presented that MHWs impact not only marine ecosystems but also heat and precipitation extremes downwind of the MHWs. By analyzing 40 MHW events identified in the Northeast Pacific during 1982-2020, it is shown here that the climatic impacts of MHWs depend on their duration. Using 31 day as cut-off duration, MHWs are classified into short-duration (SD) and long-duration (LD) groups. These two groups of events are associated with different atmospheric circulation patterns which communicate distinct downstream hydroclimatic impacts. Specifically, the SD events are associated with eastward traveling waves while LD events are associated with stationary waves. As a result, the LD MHWs exert persistent and thus more significant impacts on hydroclimatic extremes in the United States (US). During LD MHWs, anomalously high temperatures (~3K) and reduced precipitation tend to occur in the western US, while reduced temperature and a more than 1.5mm day-1 increase in the daily precipitation tends to occur in the US Midwest. The anomalous temperature and precipitation in the western US during the LD MHWs provide favorable meteorological conditions for natural hazards such as the wildfires in California. Significantly different atmospheric circulation patterns during the SD and LD MHWs tend to be a result of different wave sources. Wave activity flux (WAF) analysis shows that the SD events are associated with local wave trains from the central subtropical Pacific while the LD events are associated with larger local WAFs in the central subtropical Pacific as well as wave propagation from Eurasia. With the pronounced downstream hydroclimatic impacts over the US, forcing mechanisms of the LD MHWs are examined. Mixed layer heat budget analysis shows that net surface heat fluxes across the air-sea interface play a more important role in modulating temperature anomalies than the horizontal heat advection by the ocean currents. Specifically, temperature change due to net surface heat flux contributes to the positive SSTAs during the development of MHWs and corresponds well with the total temperature change. In comparison, temperature change due to horizontal advection in the ocean is negligible for most cases and is featured by spatially incoherent patterns. Among the four heat flux terms that compose the net surface heat flux, latent heat flux contributes the most to the temperature change. However, the extent to which the latent heat flux contributes varies by case, ranging from 60% to 120%. This study reveals the significant hydroclimatic extremes in the US during the LD MHWs in the Northeast Pacific. Further examination into how the precipitation and temperature extremes could have evolved in the future can be performed using coupled model data. In addition, although net surface heat flux has been shown to be a key forcing mechanism of the LD MHWs, other factors such as vertical mixing and entrainment of water into the mixed layer should also be included in the heat budget analysis when high-resolution ocean modeling data is available in future studies.