1 Introduction

Arctic mixed‐phase clouds (AMPCs) are a key component of the Arctic climate system that affect the delicate energy balance over frozen surfaces (Morrison et al., 2012). One of the least understood processes regarding AMPCs is aerosol‐cloud interactions, specifically those pertaining to cloud ice formation by mineral or biological ice nucleating particles (INPs; Solomon et al., 2018). Biologically derived INPs (Bio‐INPs; INPs including microbes and their exudates) that form ice greater than −15 °C are typically thought to be of terrestrial origin—yet marine bio‐INPs have been shown to form ice at temperatures up to −3 °C (McCluskey et al., 2018a; DeMott et al., 2016; Irish et al., 2017; Irish et al., 2019; Schnell, 1975). AMPC temperatures are often greater than −10 °C on average, are most prevalent in late summer, and can exist down to the surface (Shupe, 2011; Shupe et al., 2006, 2011), while the cloud‐driven mixed layer can also extend down to the surface (Shupe et al., 2013). These statistics indicate that marine bio‐INPs could play a critical role in cloud formation, especially in the summer when emitted from open water (Wex et al., 2019). Yet field observations of such INPs are exceedingly rare and are mainly reported in midlatitude or Southern Ocean regions (Bigg, 1973; Schnell, 1977).

Measurements of INPs in both the ocean and atmosphere are essential to directly link the thriving marine sources of primary productivity (production of organic matter by phytoplankton) to the clouds above. Schnell (1977) measured INPs in sub‐Arctic seawater and air and found cases where atmospheric INPs were comparable or much lower in abundance than those found in the seawater, indicating the ocean was likely the source of the atmospheric INPs. The Arctic summer possesses quintessential conditions for proliferation of primary productivity when open water and daylight hours are at their maxima (Moore et al., 2018). In a recent study, Gabric et al. (2018) demonstrate a clear connection between sea ice extent, productivity, and marine biogenic aerosols. Additionally, recent modeling work has demonstrated that marine organic aerosols lead to increases in cloud ice in polar regions during the summer (Huang et al., 2018). However, to date no studies have reported INP enhancements from naturally occurring phytoplankton blooms, which can link such sources to the air above. We present results from a summertime expedition in the Bering and Chukchi Seas demonstrating how bio‐INPs likely from a phytoplankton bloom became airborne in the lower Arctic atmosphere.

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4 Conclusions and Broader Implications

Our results demonstrate that flow dynamics and mixing, in the presence of wind forcing, can strongly impact INP populations from the bottom of the shelf to the air above the surface (Figure 4). Nutrient‐rich AW flows into Bering Strait, enabling proliferation of a phytoplankton bloom in the surface waters. The photo‐inhibited bacteria residing in and near the sediment below the bloom are dormant and unproductive since vertical export is limited by the rapid lateral transport and food is not available to them. As the flow progresses northward and slows, the nutrients are drawn down, and the phytoplankton starts to decay and sink, providing a steady source of carbon to the seafloor. Bacterial respiration and growth ensue at the deposition zone north of Bering Strait. At this point, northerly wind forcing results in the transport of the active bacteria from the bottom deposition zone to the surface via two mechanisms: upwelling on the cyclonic side of the reversed circulation and turbulent wind mixing. The strong winds also likely induced surface bubble breaking and aerosolization of the bacterial material. Some of these materials were likely proficient INPs that may assist cloud glaciation processes at relatively warm temperatures should they become vertically mixed through the marine boundary layer to cloud levels. Shupe et al. (2006, 2013) report relatively low and warm summertime AMPCs and occasional vertical mixing in the Arctic marine boundary layer, which makes it plausible that the characteristic INPs studied here may reach levels in which they realistically assist cloud glaciation. However, we note that our findings are qualitative in nature and a more quantitative assessment of the relationships between the various measurements should be conducted in future studies.

Figure 4

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Conceptual model of processes associated with the bloom. Bottom axis represents relative distance and time. Numbers correspond to the steps in the process, namely, (1) nutrient input, (2) primary production, (3) slowing of the current, (4) deposition of nutrient exhaustion, (5) wind‐forced turnover, and (6) aerosolization. The blue with the minus sign indicates inactive microbes, while the green with the plus sign indicates actively producing microbes. Blue and white arrows indicate water and air movement, respectively. DBO3 = Distributed Biological Observatory transect 3.

More broadly, the processes that invigorate AMPC formation are poorly quantified, but results such as ours can help elucidate the sources, abundance, and efficiency of Arctic INPs in addition to the mechanisms that promote aerosolization from the marine environment. One thought‐provoking question that arises from this study is as follows: Are shelf deposition zones in the Arctic a key source of airborne INPs? We believe that this question warrants further study, especially considering the ongoing ecological shifts in this region, and potentially other regions, rich in benthic and pelagic activity. Additionally, with the projected increase in Arctic blooms under warmer climate scenarios, quantifying INP abundance and effectiveness from such sources with similar particulate dispersal processes as those reported here is crucial to estimating the Arctic INP and ultimately the Arctic aerosol budget. Another question that should be addressed in future observations is as follows: Will a warmer climate, and thus a more productive Arctic Ocean, serve as a dominant source of INPs that affect AMPC formation? Results from the current work provides motivation to augment the frequency and spatial coverage of targeted Arctic INP, ecological, and oceanographic measurements in the future.

Full paper here.