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(a) Quasi‐geostrophic vertical velocity (m/day) at 50 m depth from Conductivity‐Temperature‐Depth Survey 2, which took place between 29 May 2014 12:30 P.M. and 30 May 2014 10:04 P.M. The yellow line indicates the track of GS2 (deep glider; date is indicated in Figure 1c). Gray contours correspond to dynamic height (cm) interpolated from the Conductivity‐Temperature‐Depths. (b) Mesoscale vertical transport of chlorophyll‐a estimated as the product of the mesoscale vertical velocity wQG and chlorophyll‐a, expressed in milligram per square meter per day, for GS2, where wQG is estimated from the QG omega equation. Red/blue colors denote upward/downward fluxes of phytoplankton (chlorophyll) in the frontal region due to the secondary circulation. Gray contours denote Chlorophyll (mg/m³).
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Oceanic fronts are dynamically active regions of the global ocean that support upwelling and downwelling with significant implications for phytoplankton production and export. However (on time scales ≳ the inertial time scale), the vertical velocity is 10³–10⁴ times weaker than the horizontal velocity and is difficult to observe directly. Using int...
Citations
... The timing of the onset of MHW-1 in May provides optimal light conditions which, given sufficient supply of nutrients, may contribute to the initiation of the phytoplankton bloom. The series of eddies which ensue, support both geostrophic eddy stirring and transient submesoscale dynamics along the edges of the eddies which may in turn provide both an upward and downward transport path for nutrient fluxes and carbon export (Callbeck et al., 2017;Ruiz et al., 2019). ...
Between May and August 2018, two separate marine heatwaves (MHWs) occurred in the Arkona Sea in the western Baltic Sea. These heatwaves bookended an extended period of phytoplankton growth in the region. Data from the Ocean and Land Colour Instrument (OLCI) on board the European Sentinel-3 satellite revealed an eddy-like structure containing high chlorophyll a (Chl-a) concentrations (ca. 25 mg.m⁻³) persisting for several days at the end of May in the Arkona Sea. Combining ocean colour observations, a coupled bio-optical ocean model and a particle tracking model, we examined the three dimensional relationship between these co-occurring MHW and phytoplankton bloom events. We find that the onset of the MHW in May provided the optimal conditions for phytoplankton growth, i.e. sufficient light and nutrients. Wind-driven surface eddy circulation, geostrophic eddy stirring and transient submesoscale dynamics along the edges of the eddy provided a transport path for nutrient fluxes and carbon export, and helped to sustain the phytoplankton bloom. The bloom may have indirectly had an enhancing effect on the MHW, through the impact of water constituent-induced heating rates on air-sea energy fluxes. The subsurface signature of the MHW plays a critical role in de-coupling surface and subsurface dynamics and terminating the phytoplankton bloom. Subsurface temperature anomalies of up to 8°C between 15 and 20 m depth are found to persist up to 15 days after the surface signature of the MHW has disappeared. The study reveals how surface and subsurface dynamics of MHWs and phytoplankton blooms are connected under different environmental conditions. It extends our knowledge on surface layer processes obtained from satellite data.
... Resolving oceanic vertical velocities, w, at the submesoscale, defined here as O(1-10) km spatial scales and O(1) Rossby number, is important for the vertical transport of heat, carbon, and nutrients between the surface and deep ocean (e.g., Ruiz et al., 2019;Su et al., 2018;Uchida et al., 2019). Directly measuring w is difficult as w is several orders of magnitude smaller than the horizontal velocity, and w is noisy since it includes the wave field and turbulent fluctuations. ...
... Directly measuring w is difficult as w is several orders of magnitude smaller than the horizontal velocity, and w is noisy since it includes the wave field and turbulent fluctuations. However, recent studies suggest it may be possible to infer w at the submesoscale from surface signatures since strong up-and down-welling are known to be associated with surface fronts, convergence, and cyclonic vorticity (D'Asaro et al., 2018;Freilich & Mahadevan, 2021;Ruiz et al., 2019;Tarry et al., 2021). We also expect that surface lateral velocity data will continue to improve in resolution and accuracy, leading to better estimates of divergence and vorticity at smaller scales. ...
... The inputs to the ML models include the surface density ρ, and the horizontal surface velocities u → h = (u,v) in the x and y directions, respectively. We also calculate the surface divergence δ = u x + v y , vorticity ζ = v x u y , and crossshore density gradient ρ y to use as inputs, because w is known to be related to those quantities (D'Asaro et al., 2018;Ruiz et al., 2019;Tarry et al., 2021). The dominant density gradient is in the cross-shore direction. ...
Plain Language Summary
Vertical velocities, w, are associated with ocean currents that move toward or away from the ocean surface and are important for connecting the surface and deep ocean. It is extremely difficult to measure w directly, but measurements of other variables that are related to w, such as horizontal currents that move along the surface in the north‐south or east‐west directions, can be exploited to predict w. Here, we investigate the feasibility of inferring w from other more easily measurable data. We compare three machine learning methods to see which is best at finding relationships between more easily measurable variables (the input data) and w at different depths. We test how using different input variables, adding noise to the input data, or changing the spatial resolution of the input data, impact the w predictions. Our results show that machine learning models are successful at reconstructing the 3D w field using high‐resolution (∼1 km) surface data, and in particular, surface horizontal velocities are the most important to include. This study shows that data‐driven methods are promising for relating remotely sensed surface measurements of the ocean to vertical velocities below the surface, which can help provide us with a better understanding of the 3D ocean.
... As the theory predicted, w is downward (upward) on the heavy (light) side of the front, with a vertical extent up to 150 m depth (Fig. 4a). It reaches its maximum at the location of the maximum temperature gradient at ~ 50 m depth, and the magnitudes of maximum upward and downward velocities are almost same, about 170 m d −1 , which is larger than most of the vertical velocity observed in the vicinity of submesoscale fronts 54,55 . The intense vertical motions enhance the exchange of heat, salt and material between the upper layer and ocean interior. ...
Submesoscale fronts, with horizontal scale of 0.1–10 km, are key components of climate system by driving intense vertical transports of heat, salt and nutrients in the ocean. However, our knowledge on how large the vertical transport driven by one single submesoscale front can reach remains limited due to the lack of comprehensive field observations. Here, based on high-resolution in situ observations in the Kuroshio-Oyashio Extension region, we detect an exceptionally sharp submesoscale front. The oceanic temperature (salinity) changes sharply from 14 °C (34.55 psu) to 2 °C (32.7 psu) within 2 km across the front from south to north. Analysis reveals intense vertical velocities near the front reaching 170 m day⁻¹, along with upward heat transport up to 1.4 × 10⁻² °C m s⁻¹ and salinity transport reaching 4 × 10⁻⁴ psu m s⁻¹. The observed heat transport is much larger than the values reported in previous observations and is three times as that derived from current eddy-rich climate models, whereas the salinity transport enhances the nutrients concentration with prominent implications for marine ecosystem and fishery production. These observations highlight the vertical transport of submesoscale fronts and call for a proper representation of submesoscale processes in the next generation of climate models.
... Neglecting ageostrophic and nonhydrostatic effects could also lead to misestimation of vertical velocities (Mahadevan & Tandon, 2006). These effects of submesoscale along-isopycnal motions on vertical excursion of tracers have also been demonstrated in recent observational and modeling studies (Cao & Jing, 2022;Qu et al., 2022;Ruiz et al., 2019). ...
Mesoscale and submesoscale processes have crucial impacts on ocean biogeochemistry, importantly enhancing the primary production in nutrient‐deficient ocean regions. Yet, the intricate biophysical interplay still holds mysteries. Using targeted high‐resolution in situ observations in the South China Sea, we reveal that isopycnal submesoscale stirring serves as the primary driver of vertical nutrient transport to sustain the dome‐shaped subsurface chlorophyll maximum (SCM) within a long‐lived cyclonic mesoscale eddy. Density surface doming at the eddy core increased light exposure for phytoplankton production, while along‐isopycnal submesoscale stirring disrupted the mesoscale coherence and drove significant vertical exchange of tracers. These physical processes play a crucial role in maintaining the elevated phytoplankton biomass in the eddy core. Our findings shed light on the universal mechanism of how mesoscale and submesoscale coupling enhances primary production in ocean cyclonic eddies, highlighting the pivotal role of submesoscale stirring in structuring marine ecosystems.
... Gliders are autonomous underwater vehicles that allow sustained collection at high spatial resolution (1 km) and low costs compared to conventional oceanographic methods. Many studies confirmed the feasibility of using coastal and deep gliders to monitor the spatial and low-frequency variability of the coastal ocean (Alvarez et al., 2007;Heslop et al., 2012;Ruiz et al., 2019;Zarokanellos et al., 2022). In this work we used the observations from two gliders in the Balearic Sea as a part of the Calypso 2022 experiment. ...
Combining remote-sensing data with in-situ observations to achieve a comprehensive 3D reconstruction of the ocean state presents significant challenges for traditional interpolation techniques. To address this, we developed the CLuster Optimal Interpolation Neural Network (CLOINet), which combines the robust mathematical framework of the Optimal Interpolation (OI) scheme with a self-supervised clustering approach. CLOINet efficiently segments remote sensing images into clusters to reveal non-local correlations, thereby enhancing fine-scale oceanic reconstructions. We trained our network using outputs from an Ocean General Circulation Model (OGCM), which also facilitated various testing scenarios. Our Observing System Simulation Experiments aimed to reconstruct deep salinity fields using Sea Surface Temperature (SST) or Sea Surface Height (SSH), alongside sparse in-situ salinity observations. The results showcased a significant reduction in reconstruction error up to 40% and the ability to resolve scales 50% smaller compared to baseline OI techniques. Remarkably, even though CLOINet was trained exclusively on simulated data, it accurately reconstructed an unseen SST field using only glider temperature observations and satellite chlorophyll concentration data. This demonstrates how deep learning networks like CLOINet can potentially lead the integration of modeling and observational efforts in developing an ocean digital twin.
... This spatial structure is emblematic of the coupling between surface-enhanced submesoscale processes and deeper mesoscale dynamics. This coupling can locally intensify fronts and generate more rapid subduction, as established by our modeling efforts herein (Extended Data Text 9) and previously 18,26 . Together, the modeling studies demonstrate downward instantaneous vertical velocities of up to 100m day -1 , even below the surface, generating sustained time-integrated vertical transport in intrusions in excess of~10m day - 1 18,26,27 . ...
... We used a submesoscale resolving process study ocean model initialized with the observed hydrographic conditions and POC distribution (Extended Data Text 9). Certain aspects of the large-scale Mediterranean circulation are particular, resulting in quasi-stationary fronts that have previously been thought to be critical for generating intrusions 16,18,22 . The process study model we use here generalizes the frontal export process beyond the quasi-steady topographically-influenced conditions in the Alborán Sea and instead quantifies the influence of globally-relevant submesoscale and mesoscale baroclinic instability. ...
Subtropical oceans contribute significantly to global primary production, but the fate of the picophytoplankton that dominate in these low nutrient regions is poorly understood. Working in the subtropical Mediterranean, we demonstrate that subduction of water at ocean fronts generates 3D intrusions with uncharacteristically high carbon, chlorophyll, and oxygen that extend below the sunlit photic-zone into the dark ocean. These contain "fresh" picophytoplankton assemblages that resemble the photic-zone regions where the water originated. Intrusions propagate depth-dependent seasonal variations in microbial assemblages into the ocean interior. Strikingly, the intrusions included dominant biomass contributions from non-photosynthetic bacteria and enrichment of enigmatic heterotrophic bacterial lineages. Thus, the intrusions not only deliver material that differs in composition and nutritional character from sinking detrital particles, but also drive shifts in bacterial community composition, organic matter processing, and interactions between surface and deep communities. Modeling efforts paired with global observations demonstrate that subduction can flux similar magnitudes of particulate organic carbon as sinking export, but is not accounted for in current export estimates and carbon cycle models. Intrusions formed by subduction are a particularly important mechanism for enhancing connectivity between surface and upper mesopelagic ecosystems in stratified subtropical ocean environments that are expanding due to the warming climate.
... It is important to consider the influence of submesoscale processes in the KE system, which are most pronounced during winter and spring [40]. Submesoscale features exhibit higher vertical velocities compared to eddies [41], making them important contributors to phytoplankton production by transporting nutrients and phytoplankton into the sunlit ...
... It is important to consider the influence of submesoscale processes in the KE system, which are most pronounced during winter and spring [40]. Submesoscale features exhibit higher vertical velocities compared to eddies [41], making them important contributors to phytoplankton production by transporting nutrients and phytoplankton into the sunlit ocean [42][43][44]. This transport mechanism plays a significant role in promoting phytoplankton production. ...
The Kuroshio Extension (KE) System exhibits highly energetic mesoscale phenomena, but the impact of mesoscale eddies on marine ecosystems and biogeochemical cycling is not well understood. This study utilizes remote sensing and Argo floats to investigate how eddies modify surface and subsurface chlorophyll-a (Chl-a) concentrations. On average, cyclones (anticyclones) induce positive (negative) surface Chl-a anomalies, particularly in winter. This occurs because cyclones (anticyclones) lift (deepen) isopycnals and nitrate into (out of) the euphotic zone, stimulating (depressing) the growth of phytoplankton. Consequently, cyclones (anticyclones) result in greater (smaller) subsurface Chl-a maximum (SCM), depth-integrated Chl-a, and depth-integrated nitrate. The positive (negative) surface Chl-a anomalies induced by cyclones (anticyclones) are mainly located near (north of) the main axis of the KE. The second and third mode represent monopole Chl-a patterns within eddy centers corresponding to either positive or negative anomalies, depending on the sign of the principal component. Chl-a concentrations in cyclones (anticyclones) above the SCM layer are higher (lower) than the edge values, while those below are lower (higher), regardless of winter variations. The vertical distributions and displacements of Chl-a and SCM depth are associated with eddy pumping. In terms of frequency, negative (positive) Chl-a anomalies account for approximately 26% (18%) of the total cyclones (anticyclones) across all four seasons. The opposite phase suggests that nutrient supply resulting from stratification differences under convective mixing may contribute to negative (positive) Chl-a anomalies in cyclone (anticyclone) cores. Additionally, the opposite phase can also be attributed to eddy stirring, trapping high and low Chl-a, and/or eddy Ekman pumping. Based on OFES outputs, the seasonal variation of nitrate from winter to summer primarily depends on the effect of vertical mixing, indicating that convective mixing processes contribute to an increase (decrease) in nutrients during winter (summer) over the KE.
... Much of the recent work on the biogeochemical impacts of submesoscales has relied on local field surveys (Little et al. 2018, Marrec et al. 2018, de Verneil et al. 2019, Ruiz et al. 2019, Tzortzis et al. 2021. Such field campaigns are extremely difficult to implement due to the inherent difficulty of sampling submesoscales, which are constantly and rapidly changing over a few days. ...
Fine-scale currents, O(1–100 km, days–months), are actively involved in the transport and transformation of biogeochemical tracers in the ocean. However, their overall impact on large-scale biogeochemical cycling on the timescale of years remains poorly understood due to the multiscale nature of the problem. Here, we summarize these impacts and critically review current estimates. We examine how eddy fluxes and upscale connections enter into the large-scale balance of biogeochemical tracers. We show that the overall contribution of eddy fluxes to primary production and carbon export may not be as large as it is for oxygen ventilation. We highlight the importance of fine scales to low-frequency natural variability through upscale connections and show that they may also buffer the negative effects of climate change on the functioning of biogeochemical cycles. Significant interdisciplinary efforts are needed to properly account for the cross-scale effects of fine scales on biogeochemical cycles in climate projections.
Expected final online publication date for the Annual Review of Marine Science, Volume 16 is January 2024. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
... Additionally, it is important to consider the influence of submesoscale processes in the KE system, which are most pronounced during winter and spring [38]. Submesoscale features exhibit higher vertical velocities compared to eddies [39], making them important contributors to phytoplankton production by transporting nutrients and phytoplankton into the sunlit ocean [40][41][42]. This transport mechanism plays a significant role in promoting phytoplankton production. ...
The Kuroshio Extension(KE) System exhibits highly energetic mesoscale phenomena, but the impact of mesoscale eddies on marine ecosystems and biogeochemical cycling is not well understood. This study utilizes remote sensing and Argo floats to investigate how eddies modify surface and subsurface chlorophyll (Chl-a) concentrations. On average, cyclones (anticyclones) induce positive (negative) surface Chl-a anomalies, particularly in winter. This occurs because cyclones (anticyclones) lift (deepen) isopycnals and nitrate into (out of) the euphotic zone, stimulating (depressing) the growth of phytoplankton. Consequently, cyclones (anticyclones) result in greater (smaller) subsurface Chl-a maximum (SCM), depth-integrated Chl-a, and depth-integrated nitrate. The positive (negative) surface Chl-a anomalies induced by cyclones (anticyclones) are mainly located near (north of) the main axis of the KE. The second and third mode represent monopole Chl-a patterns within eddy centers corresponding to either positive or negative anomalies, depending on the sign of the principal component. Chl-a concentrations in cyclones (anticyclones) above the SCM layer are higher (lower) than the edge values, while those below are lower (higher), regardless of winter variations. The vertical distributions and displacements of Chl-a and SCM depth are associated with eddy pumping. In terms of frequency, negative (positive) Chl-a anomalies account for approximately 26% (18%) of the total cyclones (anticyclones) across all four seasons. The opposite phase suggests that nutrient supply resulting from stratification differences under convective mixing may contribute to negative (positive) Chl-a anomalies in cyclone (anticyclone) cores. Additionally, the opposite phase can also be attributed to eddy stirring, trapping high and low Chl-a, and/or eddy Ekman pumping. Based on OFES outputs, the seasonal variation of nitrate from winter to summer primarily depends on the effect of vertical mixing, indicating that convective mixing processes contribute to an increase (decrease) in nutrients during winter (summer) over the KE.
... Despite numerous local observations and a strong theoretical basis for the physical processes affecting phytoplankton growth over fronts (e.g., recent studies by Marrec et al., 2018;Little et al., 2018;de Verneil et al., 2019;Ruiz et al., 2019;Uchida et al., 2020;Kessouri et al., 2020;Tzortzis et al., 2021), their integrated contribution at the scale of regional biomes is still largely unknown. Ephemeral fronts move and dissipate continuously on timescales of days to weeks and are thus particularly difficult to sample. ...
... Moreover, our assessment of the effect of fronts on phytoplankton, based on surface Chl a, is probably a lower estimate given that the episodic nutrient injections due to submesoscale vertical velocities at fronts can get consumed before reaching the surface (Johnson et al., 2010), leading to phytoplankton enhancements that often do not reach the surface and are more intense at the subsurface (Mouriño et al., 2004;Ruiz et al., 2019). In addition, there may be photo-inhibition which prevents phytoplankton from being near the surface in oligotrophic regions, even though they may be impacted by frontal motions. ...
Fronts affect phytoplankton growth and phenology by locally reducing stratification and increasing nutrient supplies. Biomass peaks at fronts have been observed in situ and linked to local nutrient upwelling and/or lateral transport, while reduced stratification over fronts has been shown to induce earlier blooms in numerical models. Satellite imagery offers the opportunity to quantify these induced changes in phytoplankton over a large number of fronts and at synoptic scales. Here we used 20 years of sea surface temperature (SST) and chlorophyll a (Chl a) satellite data in a large region surrounding the Gulf Stream to quantify the impact of fronts on surface Chl a (used as a proxy for phytoplankton) in three contrasting bioregions, from oligotrophic to blooming ones, and throughout the year. We computed an heterogeneity index (HI) from SST to detect fronts and used it to sort fronts into weak and strong ones based on HI thresholds. We observed that the location of strong fronts corresponded to the persistent western boundary current fronts and weak fronts to more ephemeral submesoscale fronts. We compared Chl a distributions over strong fronts, over weak fronts, and outside of fronts in the three bioregions. We assessed three metrics: the Chl a excess over fronts at the local scale of fronts, the surplus in Chl a induced at the bioregional scale, and the lag in spring bloom onset over fronts. We found that weak fronts are associated with a local Chl a excess weaker than strong fronts, but because they are also more frequent, they contribute equally to the regional Chl a surplus. We also found that the local excess of Chl a was 2 to 3 times larger in the bioregion with a spring bloom than in the oligotrophic bioregion, which can be partly explained by the transport of nutrients by the Gulf Stream. We found strong seasonal variations in the amplitude of the Chl a excess over fronts, and we show periods of Chl a deficit over fronts north of 45∘ N that we attribute to subduction. Finally we provide observational evidence that blooms start earlier over fronts by 1 to 2 weeks. Our results suggest that the spectacular impact of fronts at the local scale of fronts (up to +60 %) is more limited when considered at the regional scale of bioregions (less than +5 %) but may nevertheless have implications for the region's overall ecosystem.
