The warming (solar insolation) and freshening (sea ice melting and riverine water inputs) of the Arctic Ocean during summer increase stratification and suppress the upward mixing of nutrients into the euphotic zone (Codispoti et al., 2013). However, sea ice is now thinner and less compact (Kwok, 2018; Perovich et al., 2020); thus, the Arctic Ocean is more responsive to wind stress (Kwok et al., 2013), which enhances the nutrient supply (Bluhm et al., 2015). Shelf-break upwelling is becoming more prominent in the Arctic as the sea ice edge retreats poleward with ongoing climate change, exposing the shelf break to more direct wind forcing (Arrigo et al., 2014; Carmack & Chapman, 2003; Tremblay et al., 2011). Recently, Lewis et al. (2020) reported that annual net primary production (NPP) increased by 57% over the Arctic Ocean between 1998 and 2018. They also found that increased chlorophyll-a (Chl-a) was responsible for the sustained increase in annual NPP between 2009 and 2018, particularly along the interior shelf break. These results suggest that additional nutrients were supplied from increased vertical mixing near the shelf break into the nutrient-depleted upper euphotic zone (Arrigo & van Dijken, 2015; Lewis et al., 2020) and that the changes in ocean circulation in response to recent sea ice loss and increased wind mixing could significantly influence biological production (Ardyna & Arrigo, 2020). The Arctic Ocean is experiencing radical modifications in its hydrographic properties and in its overall circulation (Ardyna & Arrigo, 2020). For example, Polyakov et al. (2017) reported that the recent increase in Abstract Atlantic-origin cold saline water has previously not been considered an important contributor to the nutrient supply in the Pacific Arctic due to the effective insulation by the overlying Pacific-origin waters that separate the surface mixed layer from the deeper Atlantic Water. Based on hydrographic observations in the northwestern Chukchi Sea from 2015 to 2017, we demonstrate that the intrusion of Atlantic-origin cold saline water into the halocline boundary between Pacific and Atlanticorigin waters in 2017 lifted Pacific-origin nutrients up to the surface layer. We find that the cyclonic atmospheric circulation in 2017 was considerably strengthened, leading to lateral intrusions of two bodies of cold halocline water from the Eurasian marginal seas into the northwestern Chukchi Sea. Our results reveal that the intrusions of cold halocline waters caused unprecedented shoaling of the nutricline and anomalously high surface phytoplankton blooms in typically highly oligotrophic surface waters in the region during summer. Plain Language Summary Nutrient depletion, especially nitrogen, in Arctic surface waters during the summer is common due to biological uptake and intense stratification caused by sea ice melting and riverine water inputs, which restricts the upward mixing of nutrients into the euphotic zone. Although Atlantic-origin cold saline water has previously not been considered an important ...

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In recent decades, Antarctic ice sheets have rapidly retreated, thus contributing to rising sea levels. An estimated 2720 billion tonnes of ice was lost from Antarctica between 1992 and 2017, corresponding to a global sea-level rise of about 7.6 mm (Shepherd et al., 2018). In particular, grounded ice reduction in West Antarctica accounted for ∼86% of the total Antarctic ice loss. The rapid ice reduction in West Antarctica caused by the increase in glacial flow is believed to be driven by the thinning of the buttressing ice shelves, in turn associated with increasing ocean melt. Notably, the fastest rate of decline in ice volume was observed in the Amundsen Sea sector during the late 2000s (Turner et al., 2017), with some potential anthropogenic origins (Holland et al., 2019).The Dotson Ice Shelf (DIS) is about 70 km long and 50 km wide, and is situated between the Martin Peninsula (MP) and the Bear Peninsula (BP) on the Marie Byrd Land coast, in the Amundsen Sea embayment, West Antarctica (Figure 1). It buttresses the flow of the Kohler and Smith glaciers. A rapid thinning of the DIS has been

In contrast with the midlatitudes and tropics, the stratification of the Arctic Ocean is essentially salinity-driven (Carmack, 2007;Timmermans & Jayne, 2016). Low-salinity waters of the upper water column are cold and play a major role in isolating sea ice at the surface from the heat carried by the underlying Atlantic Waters (AW, defined as S A > 34.9 g kg −1 , Θ > 0°C; Rudels et al., 1996). The low-salinity waters (with a wide range of salinities 0 < S A < 34.9 g kg −1 ), which make up the halocline and the surface mixed layer, are commonly called freshwaters (e.g.,

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