The Role of Clouds and Surface Heat Fluxes in the Maintenance of the 2013–2016 Northeast Pacific Marine Heatwave

Abstract

Starting in late 2013, the Northeast (NE) Pacific Ocean experienced anomalously warm sea surface temperatures (SSTs) that persisted for over 2 years. This marine heatwave, known as “the Blob,” produced many devastating ecological impacts with socioeconomic implications for coastal communities. The warm waters observed during the NE Pacific 2013/2016 marine heatwave altered the surface energy balance and disrupted ocean–atmosphere interactions in the region. In principle, ocean–atmosphere interactions following the formation of the marine heatwave could have perpetuated warm SSTs through a positive SST‐cloud feedback. The actual situation was more complicated. While reanalysis data show a decrease in boundary layer cloud fraction and an increase in downward shortwave radiative flux at the surface coincident with warm SSTs, this was accompanied by an increase in longwave radiative fluxes at the surface, as well as an increase in sensible and latent heat fluxes out of the ocean mixed layer. The result is a small negative net heat flux anomaly (compared to the anomalies of the individual terms contributing to the net heat flux). This provides new information about the midlatitude ocean–atmosphere system while it was in a perturbed state. More specifically, a mixed layer heat budget reveals that anomalies in both the atmospheric and oceanic processes offset each other such that the anomalously warm SSTs persisted for multiple years. The results show how the atmosphere–ocean system in the NE Pacific is able to maintain itself in an anomalous state for an extended period of time.

Physical drivers of the summer 2019 North Pacific marine heatwave

“…Whereas, the peak latent heat flux anomalies precede the largest shortwave anomalies by several months, and may be thought of as a triggering mechanism for this event brought about by the weakened North Pacific High. These results echo an earlier study by Schmeisser et al 34 , which revealed that the mechanisms for the development of these mid-latitude marine heatwaves can differ from those for their maintenance. We encourage future studies on this topic once more data becomes available during the lifespan of Blob 2.0.…”

Two types of warm blobs in the Northeast Pacific and their potential effect on the El Niño

“…The magnitude of temperature tendency was smaller after the first peak due to the balance from negative effect of heat flux term. By contrast, vertical entrainment and diffusion were the leading factors to drive the single‐peak warm blob and also mostly cause its termination (Figure 10b), which was somewhat different compared to previous literature (Bond et al ., 2015; Liang et al ., 2017; Schmeisser et al ., 2019). Although the heat flux term was generally positive mainly contributed by the LH anomalies (not shown) before the peak, it was much smaller compared to the double‐peak events (Figure 10a), in accordance with the weaker SLP and easterly anomalies (Figure 5a–e).…”

“…Although our present study improves the understanding on the warm blobs in NE Pacific, several issues remain to be resolved. First, we should separate different contributions from SW, LW, SH, and LH to better understand some specific atmospheric processes such as the cloud physics (Schmeisser et al ., 2019) and improve the predictability of warm blobs, which is relatively limited at the current stage due to the stochastic processes in the atmosphere (Hu et al ., 2017). Second, as vertical processes largely contribute to the warm blobs, some dynamical processes, such as the reemergence mechanism (Namias and Born, 1970; Alexander and Deser, 1995), may exert impacts on the warm blob as suggested by Hu et al .…”

“…Remotely in the Southern Ocean, similar marine heatwaves have been identified and described as regions with large‐scale and persistent warm SST anomalies (Pearce and Feng, 2013). In particular, an extreme warm blob event experienced a multi‐year lifetime in NE Pacific, characterized by the record‐breaking high temperature and longest persistence in records from 2013 to early 2016 (Bond et al ., 2015; Di Lorenzo and Mantua, 2016; Peterson et al ., 2016; Gentemann et al ., 2017; Joh and Lorenzo, 2017; Schmeisser et al ., 2019). Most recently, another prominent warm anomaly developed to certain levels in the boreal summer of 2019 (Amaya et al ., 2020).…”

“…Researchers have been making efforts to explore the onset and maintenance mechanisms responsible for the anomalous warm blob (used hereafter) in NE Pacific, our focused region in this study (e.g., Bond et al ., 2015; Liang et al ., 2017). Many factors could potentially contribute to the persistence of warm blob: the atmospheric ridge (Bond et al ., 2015), the circulation anomalies such as North Pacific Oscillation (NPO; Rogers, 1981) (Di Lorenzo and Mantua, 2016), the coupling between the North Pacific Gyre Oscillation (NPGO; Di Lorenzo et al ., 2008) and Pacific Decadal Oscillation (PDO; Mantua et al ., 1997) (Di Lorenzo and Mantua, 2016; Joh and Lorenzo, 2017), ocean advection as well as surface heat flux (Bond et al ., 2015; Schmeisser et al ., 2019), ocean–atmosphere interaction (Tseng et al ., 2017), and salinity effects (Zhi et al ., 2019). However, most of above results are based on the single warm blob during 2013–2016 and concentrate on one specific aspect related to the warm blob.…”