Physical drivers of the summer 2019 North Pacific marine heatwave

Abstract

Summer 2019 observations show a rapid resurgence of the Blob-like warm sea surface temperature (SST) anomalies that produced devastating marine impacts in the Northeast Pacific during winter 2013/2014. Unlike the original Blob, Blob 2.0 peaked in the summer, a season when little is known about the physical drivers of such events. We show that Blob 2.0 primarily results from a prolonged weakening of the North Pacific High-Pressure System. This reduces surface winds and decreases evaporative cooling and wind-driven upper ocean mixing. Warmer ocean conditions then reduce low-cloud fraction, reinforcing the marine heatwave through a positive low-cloud feedback. Using an atmospheric model forced with observed SSTs, we also find that remote SST forcing from the central equatorial and, surprisingly, the subtropical North Pacific Ocean contribute to the weakened North Pacific High. Our multi-faceted analysis sheds light on the physical drivers governing the intensity and longevity of summertime North Pacific marine heatwaves.

Responses of ichthyoplankton assemblages to the recent marine heatwave and previous climate fluctuations in several Northeast Pacific marine ecosystems

“…Layered on top of long‐term warming trends (Frölicher et al., 2018), current climate projections suggest that MHWs will become more intense under global warming (Oliver et al., 2018). Indeed, in 2019, ocean temperatures in the Northeast Pacific were again much warmer than average during the summer and fall (Amaya et al., 2020). Preliminary larval observations from the Gulf of Alaska and British Columbia for 2019 appear consistent with many of the patterns shown in our analyses for 2014–2016 as abundances of many common species were again low.…”

Subsurface Evolution and Persistence of Marine Heatwaves in the Northeast Pacific

“…A similar situation played out during the summer of 2019 when a resurgence of Blob‐like surface conditions intensified in the NE Pacific. Weakened surface wind speeds, driven by atmospheric teleconnections associated with SST anomalies in the tropical Pacific, resulted in reduced evaporative heat loss from the ocean to atmosphere and limited wind‐driven mixing, resulting in a MHW off the U.S. West Coast (Amaya et al, 2020). Increased shortwave radiation and a positive SST‐cloud feedback helped to maintain the MHW over an exceptionally shallow summertime mixed layer (Amaya et al, 2020).…”

“…Weakened surface wind speeds, driven by atmospheric teleconnections associated with SST anomalies in the tropical Pacific, resulted in reduced evaporative heat loss from the ocean to atmosphere and limited wind‐driven mixing, resulting in a MHW off the U.S. West Coast (Amaya et al, 2020). Increased shortwave radiation and a positive SST‐cloud feedback helped to maintain the MHW over an exceptionally shallow summertime mixed layer (Amaya et al, 2020). Here, we show evidence for the role of salinity anomalies in increasing upper ocean stability and describe the propagation and persistence of the 2019–2020 NE Pacific MHW in the subsurface.…”

The Continuum of Northeast Pacific Marine Heatwaves and Their Relationship to the Tropical Pacific

“…Xu, T., Newman, M., Capotondi, A., & Di Lorenzo, E. (2021) Despite the record-breaking nature of the 2013-2015 event, there have been other intense Northeast Pacific warm events before and since (e.g., D. Amaya et al, 2016;D. J. Amaya et al, 2020;Oliver et al, 2018).…”

“…Despite the record‐breaking nature of the 2013–2015 event, there have been other intense Northeast Pacific warm events before and since (e.g., D. Amaya et al., 2016; D. J. Amaya et al., 2020; Oliver et al., 2018). In fact, prolonged oceanic warm events occur over a range of intensities and durations (Scannell et al., 2016).…”