Robert S. Pickart

Under‐ice phytoplankton blooms in the Chukchi Sea are among the densest observed globally, and, together with sea ice microalgal and open water phytoplankton blooms, make the region one of the most productive in the Arctic Ocean. To better understand the factors influencing export of organic material captured by these blooms, free‐drifting and ice‐tethered particle interceptor trap arrays were deployed in the northeastern Chukchi Sea during the Tale of Three Systems cruise in June‐July 2023. Export fluxes of chlorophyll a (Chl a ), particulate organic carbon (POC), and particulate nitrogen (PN) were measured alongside taxonomic composition of collected material, hydrographic data, and sediment data. A dense under‐ice phytoplankton bloom was observed in the northern portion of the study area, and the ice edge was predominantly set by a persistent zonal water mass front throughout the sampling period. Outside the front, Chl a and POC fluxes were closely related to water column biomass, with the highest export fluxes observed beneath consolidated sea ice. In contrast, within the front, subduction driven by baroclinic instability dramatically enhanced export. South of the front, open water areas exhibited low biomass and rates of export, likely due to nutrient depletion by an earlier phytoplankton bloom. These findings demonstrate that the physical subduction of dense phytoplankton blooms can result in extremely high particle export fluxes on the shallow Chukchi Sea shelf.

In recent decades, evidence has emerged of phytoplankton growing beneath Arctic sea ice, in some cases forming massive blooms. Observed and predicted changes in the cryosphere highlight the need to better understand the development and progression of under‐ice blooms. During June and July 2023, as a part of the Tale of Three Systems (ToTS) field program, we characterized the environmental conditions that led to the initiation, peak, and decline of a substantial under‐ice phytoplankton bloom in the Chukchi Sea. By sampling two transects 7–8 times over a period of approximately 6 weeks, we are able to describe spatial and temporal patterns in hydrography, nutrient distributions, and phytoplankton biomass, allowing for a thorough analysis of a generally understudied phenomenon. Along both transects, phytoplankton biomass was five times higher under consolidated sea ice (maximum 661 mg Chl a m ⁻² ) than they were in open water (114 ± 135 mg Chl a m ⁻² ). Nitrate (NO ₃ ⁻ ) was the limiting nutrient and controlling factor of the magnitude of the bloom. The under‐ice environment also exhibited much greater surface water NO ₃ ⁻ depletion and dissolved oxygen (O ₂ ) supersaturation than in open water. The bloom was dominated by Thalassiosira spp., a large chain‐forming diatom, and much of the organic matter produced at the surface was exported below the mixed layer, suggesting tight benthic‐pelagic coupling. In the absence of widespread melt ponds, leads appeared to be the most important factor in alleviating light limitation of phytoplankton growth. An increasingly dynamic ice pack could therefore have important impacts on phytoplankton and the broader ecosystem.

Upwelling along southwest Greenland plays an important role in modulating shelf‐basin exchange and influencing deep convection in the Labrador Sea. Here we investigate its seasonal and interannual variability using the Overturning in the Subpolar North Atlantic Program West Greenland mooring observations and atmospheric reanalysis fields. Over the decade 2014–2024 we identified 142 upwelling events. During this time the number of events increased, primarily due to summer events, whereas the mean duration of events decreased for the first 7 years and then rebounded, largely reflecting winter variability. The latter is consistent with the winter North Atlantic Oscillation (NAO), whereas the relationship between the number of summer events and the summer NAO is counterintuitive. Two distinct atmospheric patterns were identified. Most events are associated with forward Greenland tip jets, whereby low‐pressure systems deepen in the vicinity of southeast Greenland resulting in enhanced northwesterly winds along southwest Greenland. In contrast, a large number of the summer events are shown to be related to an alternative atmospheric scenario characterized by a weak, distant low‐pressure system located south of Iceland coexisting with a high‐pressure system over the Labrador Sea. The tracks of these storms are typical of low‐NAO conditions, thus explaining the above‐noted counterintuitive relationship. Using a proxy we demonstrate that there was a regime shift in 1990/91 after which this type of summer upwelling became more common. Our results highlight a previously underappreciated role of high‐pressure systems in driving upwelling variability and provide new insights into the factors controlling upwelling along southwest Greenland.

Nutrient‐rich, Pacific‐origin winter water (PWW) is a primary source of nitrate for the western Arctic Ocean, spurring primary production at the base of the food web. This cold water exits the Chukchi Sea shelf via Herald Canyon and Barrow Canyon, which feed the eastward‐flowing Chukchi Shelfbreak Jet (CSJ) and the westward‐flowing Chukchi Slope Current, respectively. Using historical measurements of nitrate, hydrography, and velocity between 2002 and 2023, we characterize the spatial distribution of nitrate along the northern edge of the Chukchi shelf. Our analysis reveals that the nitrate concentration of PWW increases significantly to the west. Documenting the sources of nitrate in Herald Canyon and Barrow Canyon, and following its downstream evolution in the two currents, we demonstrate that the westward increase in nitrate is primarily associated with the CSJ. Our results shed light on the physical basis for seasonal phytoplankton growth and carbon sequestration in the region.

The Beaufort Shelf has historically been reported to exhibit limited polynya activity in winter. Yet, recent satellite observations show episodic recurrence of a large polynya west of Mackenzie Canyon, a site of significant shelf‐basin exchange. Here, we investigate satellite‐detected occurrences of this polynya over winters 2003–2025, including their relation to regional winds, ice drift, and ocean conditions. The polynya is observed to open when easterly winds drive rapid ice drift over the shelf, mechanically opening the ice near Qikiqtaruk (Herschel Island). Under strong and persistent forcing, open water extends northwestward, sometimes occupying large portions of the shelf. Its comparison to a 1‐D coastal polynya model suggests that this observed polynya growth could reflect contributions from ocean heating. Fluxes of interior ocean heat to the shelf are confirmed across two winters of mooring observations, which revealed coincident upwelling along the western flank of Mackenzie Canyon as polynyas formed. Warm upwelled waters were advected by a strong shelf current directed along the axis of polynya extension. Transported heat could suppress an estimated of daily ice growth over the shelf, comparable to that otherwise expected from the estimated surface heat losses. Recent years have featured several extreme polynyas, some exceeding 400 km in length. These events are rare and occur under exceptional wind forcing. However, increased ice drift speeds in the last decade coincide with more frequent and extensive openings, suggesting that large polynyas may be becoming a more prominent feature over the shelf as the mobility of the winter ice cover increases.

The West Greenland boundary current system plays a central role in modulating deep convection in the Labrador Sea. Here, we use 6 years of mooring data, 2014–2020, together with atmospheric and oceanic reanalysis fields, to investigate wind‐driven downwelling along the southwest Greenland shelf/slope. A total of 49 downwelling events were identified using timeseries of alongcoast wind and bottom density anomaly. On average, the events last 4.5 days and are characterized by increased southeasterly winds followed by an increase in alongstream velocity by ∼0.12 m/s and decrease in bottom density by ∼0.18 kg/m ³ . A cross‐stream Ekman cell develops, although the response is weaker offshore. The events are driven by low‐pressure systems originating from the southwest/south that progress into the Labrador Sea and generate strong southeasterly winds along southwest Greenland. More downwelling events occur in summer, but the ocean response is weaker due to less intense winds during this season. The largest number of events occurred in 2017–2018, coinciding with the period when the deepest convection occurred in the interior Labrador Sea over the past 30 years. The ocean reanalysis fields reveal significant positive anomalies of upper‐layer salinity and density in the interior Labrador Sea during the fall to early winter of 2017–2018. These anomalies likely reflect reduced seaward spreading of freshwater from the shelf due to the more frequent downwelling winds. Our results highlight the important role of wind‐driven downwelling along the west Greenland coast in preconditioning Labrador Sea deep convection, thereby influencing the large‐scale ocean circulation and climate system.

The shelfbreak jet of the Beaufort Sea is the main conduit of Pacific water progressing eastward from the Chukchi shelf, delivering heat, freshwater, and nutrients that impact the hydrographic state and ecosystem of the western Arctic Ocean. A mooring has been maintained in the center of the jet since 2002 (with a few gaps). Here we use this long timeseries to quantify the seasonal signal in water mass properties and transport of the jet and explore the interannual variability. The annual mean transport is 0.064 ± 0.018 Sv over the 20‐year period. It is largest during summer and decreases to near zero during the remainder of the year. Early in the record there was a single transport peak in July, but in recent years a second peak has occurred in October. Warm water first appears at the mooring site in July and lasts until December, after which winter water is the prevalent water mass advected by the jet. Using timeseries of ice thickness from the mooring, satellite ice concentration, and weather station wind data, we demonstrate that the transport of the shelfbreak jet is significantly reduced due to the combination of wind and ice cover. The reduction is greatest in the spring and fall when the pack‐ice is more mobile. The strength of the jet, and the fluxes of the individual water masses it advects, display pronounced interannual variability. The dominant year‐to‐year changes in transport are due to changes in the undisturbed transport; that is, in the absence of local winds.

Variability of the Atlantic Meridional Overturning Circulation (MOC) has drawn extensive attention due to its impact on the global redistribution of heat and freshwater. Here we present the latest time series (2014–2022) of the Overturning in the Subpolar North Atlantic Program and characterize MOC interannual variability. We find that any single boundary current captures ∼30% of subpolar MOC interannual variability. However, to fully resolve MOC variability, a wide swath across the eastern subpolar basin is needed; in the Labrador Sea both boundaries are needed. Through a volume budget analysis for the subpolar basins' lower limbs, we estimate the magnitude of unresolved processes (e.g., diapycnal mixing) required to close the mean budget (∼2 Sv). We find that in the eastern subpolar basin surface‐forced transformation variability is linked to lower limb volume variability, which translates to MOC changes within the same year. In contrast, this linkage is weak in the Labrador Sea.