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From 11 April to 11 June 2018 a new type of ocean observing platform, the Saildrone surface vehicle, collected data on a round-trip, 60-day cruise from San Francisco Bay, down the U.S. and Mexican coast to Guadalupe Island. The cruise track was selected to optimize the science team’s validation and science objectives. The validation objectives include establishing the accuracy of these new measurements. The scientific objectives include validation of satellite-derived fluxes, sea surface temperatures, and wind vectors and studies of upwelling dynamics, river plumes, air–sea interactions including frontal regions, and diurnal warming regions. On this deployment, the Saildrone carried 16 atmospheric and oceanographic sensors. Future planned cruises (with open data policies) are focused on improving our understanding of air–sea fluxes in the Arctic Ocean and around North Brazil Current rings.

Given the importance of coastal upwelling systems to ocean productivity, fisheries, and biogeochemical cycles, their response to climate change is of great interest. However, there is no consensus on future productivity changes in these systems, which may be controlled by multiple drivers including wind‐driven and geostrophic transport, stratification, and source water properties. Here we use an ensemble of regional ocean projections and recently developed upwelling indices for the California Current System to disentangle these sometimes‐competing influences. Some changes are consistent among models (e.g., decreased mixed layer depth), while for others there is a lack of agreement even on the direction of future change (e.g., nitrate concentration in upwelled waters). Despite models' diverging projections of productivity changes, they agree that those changes are predominantly driven by subsurface nitrate concentrations, not by upwelling strength. Our results highlight the need for more attention to processes governing subsurface nutrient changes, not just upwelling strength. Plain Language Summary The California Current System is one of the world's eastern boundary upwelling systems—some of the most productive regions in the global ocean. These regions support a wide range of human activities, such as fisheries and tourism, motivating extensive research on how they might evolve under future climate change. A number of hypotheses have been offered to describe future physical and chemical change in these systems, and in terms of their impacts on primary production (which forms the base of the marine food web), these mechanisms may reinforce or oppose each other. Enhanced nutrient concentrations in upwelling source waters would support higher productivity, increased stratification would limit nutrient supply and productivity, and increased upwelling could enhance productivity to a point but limit productivity if it is too strong. There is no consensus on which mechanism(s) will predominantly drive future productivity changes. Here we provide a detailed analysis of projected physical and biogeochemical changes and how they relate to productivity changes. Even though different models project different futures, we find that in all of them the primary control on productivity is the nitrate concentration of subsurface waters, not the strength of upwelling, which has received more attention to date. Key Points Future changes in the California Current System are evaluated using an ensemble of downscaled ocean projections We evaluate changes in Ekman and geostrophic transports, water column structure, and subsurface nitrate concentrations Across models, phytoplankton biomass changes are more closely tied to subsurface nitrate concentration than upwelling strength

The Bølling‐Allerød deglacial event is marked by high diatom productivity and opal deposition throughout the subarctic Pacific. This opal could either constitute a strengthened biological pump and thus carbon sequestration, or a weakened biological pump and release of marine‐sequestered CO2 to the atmosphere. We quantify silicic acid supply at IODP Site U1340 in the Bering Sea using biogenic opal and δ30Si of Coscinodiscus, a diatom genus. These records, along with diatom environmental indicators, suggest the Bølling‐Allerød had high silicic acid availability related to a shift from stratification to seasonal upwelling dynamics. We thus propose the primary cause of the high productivity event was increased macronutrient supply from vertical exchange that injected old, nutrient‐rich, CO2‐rich waters into the surface. Enhanced CO2 release from the subarctic Pacific may help explain critical intervals of CO2 rise that occur at the onsets of the Bølling‐Allerød and PreBoreal. Plain Language Summary The subarctic Pacific experienced a period of remarkably high primary productivity from 14.7 to 12.9 thousand years ago. The growth and burial of diatoms, single‐celled algae with opal cell walls, deposited an opal‐rich layer in marine sediments across the entire region, including our site in the southern Bering Sea. To determine the cause of this productivity, we analyzed diatom opal and the diatom species present. The silicon isotopic composition of the opal suggests surface nutrients were more abundant during the high‐productivity event. This points to an increased connection between surface waters, where algae grow, and deep waters, which are rich in nutrients. The species present suggest deep mixing occurred each winter followed by a large spring bloom, indicating a different mode of oceanic circulation than the modern ocean. Summer production may have been limited by the rate of iron delivery, similar to the modern ocean. The combined effect of increased mixing and iron limitation is that, despite high productivity which sequestered some carbon, these events represent times when the subarctic Pacific was a net source of carbon to the atmosphere. The carbon released from the subarctic Pacific occurred during critical intervals of global change which ended the last glacial age. Key Points Diatom silicon isotopes suggest increased silicic acid availability during Bølling‐Allerød productivity in subarctic Pacific Diatom indicators suggest increased upwelling and iron limitation during high productivity events in the Subarctic Pacific Vertical mixing during the Bølling‐Allerød made the subarctic Pacific a source of atmospheric CO2 which contributed to global deglaciation

The north and central coast of Chile is influenced by El Niño-Southern Oscillation (ENSO) through oceanic and atmospheric teleconnections. However, it also experiences episodic oceanic warmings off central Chile (30°S) lasting a few months that are not necessarily associated with ENSO. These episodes, called “Chile Niño” events, besides their ecological and socio-economical impacts, have also the potential to influence tropical Pacific variability. Here, we investigate how realistically the models in the Coupled Model Intercomparison Project (CMIP, Phases 5 and 6) simulate Chile Niño/Niña (CN) events, and quantify their changes under anthropogenic forcing. Despite limitations of the global models in simulating realistically coastal upwelling dynamics, we show that they simulate reasonably well the observed spatial pattern, amplitude and seasonal evolution of CN events. They however fail to properly represent the positive skewness from observations. The analysis of a sub-group of models (36) that simulate ENSO realistically reveals that CN events increase in amplitude and variance in the future climate with no changes in their frequency of occurence. This is interpreted as resulting from compensating effects amongst changes in remote drivers and local feedbacks. In particular, ENSO variance increases while that of the South Pacific Oscillation decreases. Conversely, we found that while the Wind-Evaporation-SST feedback tends to increase and the coupling between mixed-layer depth and SST weakens, favoring the development of CN events, the thermocline and wind-SST feedbacks decrease. However, only the change in the thermocline feedback is correlated to changes in CN variance amongst the models, suggesting a dominant role of local oceanic stratification changes in constraining the sensitivity of CN to global warming.

Satellite images show that the Pearl River plume is entrained into the upwelling front in the northeastern South China Sea. To understand the processes and extend to other coastal zones, an idealized numerical model is used to investigate the upwelling dynamics in response to the arrival of the river plume. Upon forcing by an upwelling-favorable wind, the model reproduces the upwelling frontal jet with a stratified water column, which takes the river plume far away from the mouth of the estuary. The river plume introduces additional upwelling and downwelling at its inshore and offshore sides (defined as plume-related secondary upwelling circulation), respectively. For the initially unstratified water column, the plume-related secondary upwelling circulation is stronger and extends to deeper water than for the stratified condition. The surface boundary layer thins and the offshore current intensifies in the river plume. The variations in wind-driven current over the deep-water shelf in different stratified conditions are modulated by the vertical profiles of the eddy viscosity, which are shown by a one-dimensional numerical model. Offshore transport is reinforced when the head of the river plume arrives. Thereafter, it is changed by the cross-shore baroclinic geostrophic component of velocity, due to alongshore density variation by the river plume. The horizontal gradient of stress on the two sides of the river plume is responsible for the plume-related secondary upwelling circulation owing to different stress decay scales inside and outside the river plume.

Polar mesospheric clouds (PMCs), composed of ice particles, play a crucial role in mesospheric H2O redistribution, yet their formation mechanism remains incompletely understood. Using the Aeronomy of Ice in the Mesosphere (AIM) satellite observations, we reveal a previously unreported hemispheric asymmetry: southern hemisphere PMCs show a significant latitudinal decrease in column ice particle concentration, while their northern hemisphere counterparts exhibit zero trend. Our further analysis demonstrates that the column-averaged ice particle concentration (Nc) and radius (rc) are primarily governed by PMC height (h), rather than environmental temperature (Tenv). To explain these observations, we propose the charged meteoric smoke particle (MSP) nucleation (CMN) scheme, an altitude-dependent framework based on two key postulates: (1) charged-MSPs serve as ubiquitous ice nuclei throughout the PMC layer, and (2) ice particles grow predominantly in situ with negligible sedimentation. The CMN scheme naturally accounts for the observed vertical gradients in ice particle concentration (increasing with altitude due to charged-MSPs distribution) and size (decreasing with altitude due to H2O competition among ice particles). By eliminating sedimentation, the CMN scheme introduces a novel bottom-up H2O redistribution mechanism we term the cold-trap effect. This mechanism is driven by summer polar upwelling dynamics: upward H2O transport induces hydration, while simultaneous ice particle formation (facilitated by upwelling-induced cooling) blocks further H2O transport, ultimately causing dehydration above PMCs. While the traditional growth-sedimentation (GS) scheme and freeze-drying effect are well-validated, our CMN scheme and cold-trap effect provide an alternative paradigm particularly for understanding zonal and daily-scale PMC variability and associated H2O redistribution processes.

Upwelling is one of the major mechanisms responsible for the input of nutrients sustaining high levels of marine primary production. As a consequence of global change, variations in upwelling intensity may affect nutrient supply thus impacting marine food webs. In this study, we examine the effects of decadal variability of upwelling strength on nitrate supply and its influence on the nitrogen stable isotope composition at the base of the marine food web at the northern boundary of the Canary Current upwelling system (NW Spain) between 1989 and 2023. The study focused on the early upwelling season each year (March-June) to minimize the effects of nitrogen remineralization. Intertidal macroalgae (Phaeophyceae) and mussels ( Mytilus galloprovincialis ) were used as proxies for temporally integrated isotopic signals of nitrogen sources (δ 15 N). While no significant temporal trends for either upwelling strength, nitrate concentrations, or stable isotopes were found, three periods with characteristic upwelling and nutrient regimes were identified. A linear increase in δ 15 N, particularly in Fucus spp., associated with a decreasing contribution of upwelling-derived nitrogen suggest the influence of additional sources, likely of anthropogenic origin. Thus, no net change in productivity would be expected in this region despite quasi-decadal shifts in upwelling dynamics. Further insights on the origin and relevance of these sources can be gained through the investigation of river and runoff inputs and the use of more sensitive tracers, such as amino acid δ 15 N analysis in mussels.

Coastal upwelling dynamics are strongly affected by alongshore wind stress and nearshore wind stress curl. A coupled physical‐biogeochemical regional model and lagrangian diagnostics are used in the Peru current system to determine how the upwelling of nutrients and the primary productivity are impacted by the spatial structure of the nearshore wind stress. Three wind stress products derived from the ERS and QuikSCAT scatterometers and a smoothed QuikSCAT field, mainly differing in nearshore wind stress curl patterns, were used. Simulations are found to produce significantly different mean surface chlorophyll distributions and show that strong upwelling‐favorable nearshore wind stress curl may locally induce a wide coastal productive zone through upwelling of nutrient‐replete waters brought by a shoaling coastal undercurrent. Using wind stress products with realistic nearshore patterns is therefore crucial for the modeling of coupled physical‐biogeochemical coastal processes.

Coastal countercurrents (CCCs) are swift alongshore currents that flow opposite to the prevailing wind direction. In the world’s major upwelling regions, CCCs often develop after weakening or reversal of upwelling-favourable coastal winds. Using a process-oriented modelling approach, this paper explores the CCC development after a spatially confined upwelling-favourable wind event for a simplified coastal ocean with a straight coastline. Findings show that wind relaxation after a 5-day upwelling event can produce a swift CCC, ~ 5 km in width and ~ 0.3–0.4 m/s in speed, extending ~ 150 km along the coast. The development of such CCCs relies on two dynamical features. Firstly, spatially varying offshore Ekman transport near the upwind margin of wind field creates an alongshore pressure-gradient force that opposes the alongshore wind-stress force. After wind relaxation, the unbalanced alongshore pressure-gradient force provides the initial acceleration of the CCC. Secondly, the gravitational-rotational adjustment of the body of upwelled dense water induces a negative sea-level anomaly that sustains the alongshore pressure-gradient forcing of the CCC. Findings of this paper supplement previous theories on the formation of CCCs and enhance the understanding of the functioning of coastal upwelling systems.

The Arabian Sea (AS) was confirmed to be a net emitter of CO2 to the atmosphere during the international Joint Global Ocean Flux Study program of the 1990s, but since then few in situ data have been collected, leaving data-based methods to calculate air–sea exchange with fewer and potentially out-of-date data. Additionally, coarse-resolution models underestimate CO2 flux compared to other approaches. To address these shortcomings, we employ a high-resolution (1/24∘) regional model to quantify the seasonal cycle of air–sea CO2 exchange in the AS by focusing on two main contributing factors, pCO2 and winds. We compare the model to available in situ pCO2 data and find that uncertainties in dissolved inorganic carbon (DIC) and total alkalinity (TA) lead to the greatest discrepancies. Nevertheless, the model is more successful than neural network approaches in replicating the large variability in summertime pCO2 because it captures the AS's intense monsoon dynamics. In the seasonal pCO2 cycle, temperature plays the major role in determining surfacepCO2 except where DIC delivery is important in summer upwelling areas. Since seasonal temperature forcing is relatively uniform,pCO2 differences between the AS's subregions are mostly caused by geographic DIC gradients. We find that primary productivity during both summer and winter monsoon blooms, but also generally, is insufficient to offset the physical delivery of DIC to the surface, resulting in limited biological control of CO2 release. The most intense air–sea CO2 exchange occurs during the summer monsoon when outgassing rates reach ∼ 6 molCm-2yr-1 in the upwelling regions of Oman and Somalia, but the entire AS contributes CO2 to the atmosphere. Despite a regional spring maximum of pCO2 driven by surface heating, CO2 exchange rates peak in summer due to winds, which account for ∼ 90 % of the summer CO2 flux variability vs. 6 % for pCO2. In comparison with other estimates, we find that the AS emits ∼ 160 TgCyr-1, slightly higher than previously reported. Altogether, there is 2× variability in annual flux magnitude across methodologies considered. Future attempts to reduce the variability in estimates will likely require more in situ carbon data. Since summer monsoon winds are critical in determining flux both directly and indirectly through temperature, DIC, TA, mixing, and primary production effects on pCO2, studies looking to predict CO2 emissions in the AS with ongoing climate change will need to correctly resolve their timing, strength, and upwelling dynamics.