Shallow Mixed Layer Depth Research Articles

Ocean Alkalinity Enhancement in Deep Water Formation Regions Under Low and High Emission Pathways

AbstractOcean Alkalinity Enhancement (OAE) is an ocean‐based Carbon Dioxide Removal (CDR) method to mitigate climate change. Studies to characterize regional differences in OAE efficiencies and biogeochemical effects are still sparse. As subduction regions play a pivotal role for anthropogenic carbon uptake and centennial storage, we here evaluate OAE efficiencies in the subduction regions of the Southern Ocean, the Northwest Atlantic, and the Norwegian‐Barents Sea region. Using the ocean biogeochemistry model FESOM2.1‐REcoM3, we simulate continuous OAE globally and in the subduction regions under high (SSP3‐7.0) and low (SSP1‐2.6) emission scenarios. The OAE efficiency calculated by two different metrics is higher (by 8%–30%) for SSP3‐7.0 than for SSP1‐2.6 due to a lower buffer factor in a high‐ world. All subduction regions show a CDR potential (0.23–0.31; PgC uptake per Pg alkaline material) consistent with global OAE for both emission scenarios. Calculating the efficiency as the ratio of excess dissolved inorganic carbon (DIC) to excess alkalinity shows that the Southern Ocean and the Northwest Atlantic are as efficient as the global ocean (0.79–0.85), while the Norwegian‐Barents Sea region has a lower efficiency (0.65–0.75). The subduction regions store a fraction of excess carbon below 1 km that is 1.9 times higher than the global ocean. The excess surface alkalinity and thus uptake and storage follow the mixed‐layer depth seasonality, with the majority of the excess flux occurring in summer at shallow mixed layer depths. This study therefore highlights that subduction regions can be efficient for OAE if optimal deployment strategies are developed.

Observational evidence of overlooked downwelling induced by tropical cyclones in the open ocean

Tropical cyclones (TCs) cause severe natural hazards and drive intense upper ocean cooling through a series of oceanic and atmospheric physical processes, including vertical mixing and upwelling. Among these processes, TC-induced warming of near-surface waters in the open ocean has rarely been noted. This study provides a detailed analysis of upper ocean responses to 30 TC events observed by two buoys in the western North Pacific between 2016 and 2021. Supplemented with numerical experiments, we suggest that downwelling frequently occurs at the periphery of upwelling regions (around the radius of the 34 knot wind speed) following the passage of a TC. Downwelling is identified via pronounced warm anomalies under a shallow mixed layer depth, and its dynamics are attributed to negative wind stress curl and current-induced convergence. These findings highlight the important role played by TC-induced downwelling and offer insights for reconsidering the influence of TCs on biogeochemical processes.

Winter storms drive offshore transport and modulate phytoplankton blooms in Northern Taiwan, China

The East China Shelf Sea (ECSS) is subject to high-frequency storms during winter and spring, with these storm processes serving as a significant driving factor for initiating the outward diffusion of materials from the inner shelf. Inner shelf waters tend to be rich in nutrients, thus their horizontal diffusion is crucial to the marine primary productivity of the outer shelf. However, our understanding of the processes that govern the offshore water mass transport under winter storm oscillations and the mechanisms driving their impact on phytoplankton is currently limited. In this study, we used satellite and reanalysis data from 2003 to 2020 to investigate the modulation mechanism of winter offshore currents (WOC) on phytoplankton bloom (PB) stimulated by storms in Northern Taiwan (NTW). The results revealed that at synoptic scales, winter storms result in higher sea surface height (SSH) and potential energy on the inner side of the front. However, during periods of weakened storms (for example, during north wind relaxation or a shift to southerly winds), a strong WOC is produced in the top 30–50 m of the ocean due to continuous SSH adjustments caused by changed pressure gradients and induced by topography. PB occurs in the offshore waters of the front, driven by a continuous nutrient supply and an increase in photosynthetically available radiation (PAR). On an interannual scale, offshore transport is controlled by the storm's duration and the extent of its weakening. As the climate warms and winter winds in NTW have consistently weakened over the past 20 years, lower turbulent kinetic energy (TKE) and a shallower mixed layer depth (MLD) will promote phytoplankton growth. The results suggest that water level fluctuations caused by high-frequency changes in winter storms are strongly linked to the offshore sediment transport from near-shore waters, impacting the material transport and the health of the ecosystem on the continental shelf.

The abnormal track of super typhoon Hinnamnor (2022) and its interaction with the upper ocean

The upper ocean response and feedback to super typhoon Hinnamnor (2022) were systematically investigated considering multi-satellite, Array for Real-time Geostrophic Oceanography (Argo) float and reanalysis data. Hinnamnor experienced three main stages: a rapid intensification (RI) stage with a high moving speed along a westward track, a rapid weakening (RW) stage with a low moving speed along a northward sudden-turning track and a re-intensification (Re-I) stage with a moderate speed along a northward track. At the westward straight-line stage, despite the thin warm water layer (D26 < 40 m) and shallow mixed layer depth (MLD) (<20 m), its fast movement (∼8.1 m/s) tended to generate weaker sea surface temperature (SST) cooling (approximately 1 °C) and shorter cooling exposure. Consequently, the negative feedback decreased (negative feedback factor FSST:approximately−0.47to−0.20), causing Hinnamnor's rapid intensification (25 knots/6 h). Although the MLD was deeper (approximately 30 m) and the warm water layer was thicker (approximately 60 m) at the sudden-turning stage, the low translation speed (<2 m/s) tended to generate stronger SST cooling (approximately 5.6 °C) and longer cooling exposure, both of which intensified the negative feedback (FSST:approximately−0.8to−0.4), causing Hinnamnor's rapid weakening (−22 knots/6 h). At the northward stage, Hinnamnor re-intensified under a weak warm eddy with a moderate moving speed (3.6–5.4 m/s). An intriguing leftward bias in SST cooling occurred due to the preexisting cold eddy (CE) on the left side of the track. Analysis of the Argo floats in the RI and RW regions showed that the enhanced upwelling and vertical mixing yielded more significant SST cooling in the CE region at the RW stage due to the CE and low translation speed (<2 m/s). The differences between the RI and RW regions were remarkable when using the Hybrid Coordinate Ocean Model (HYCOM) global ocean model output.

Spatial and seasonal variations of near-inertial kinetic energy in the upper Ross Sea and the controlling factors

The spatial distribution and seasonal variation of near-inertial kinetic energy (NIKE) in the upper Ross Sea (RS) are examined using the 1/4° NEMO3.6-LIM3 sea ice physical–biological coupled model. The annual-mean surface and mixed layer-integrated NIKE have large values at the shelf break around Iselin Bank and the northeast area outside the continental shelf. The spatial distribution of the surface and mixed layer-integrated NIKE in the austral summer RS is mainly controlled by the near-inertial wind stress magnitude (NIWSM), which acts as the energy source. The northeast high NIKE agrees with the large NIWSM there. Semidiurnal tides only contribute to the NIKE peak around the Iselin Bank and shelf break. The change of mixed layer depth (MLD) can induce spatial discrepancies between surface and mixed layer-integrated NIKE. The shallower MLD in the western RS limits the near-inertial energy in a thin layer, which has induced the large surface NIKE there. The NIKE in the upper RS has a pronounced seasonal variation with the largest surface and mixed layer-integrated NIKE both observed during austral summer. The seasonal variation of NIWSM and sea ice concentration play an important role in inducing the seasonal variation of NIKE. While the NIWSM acts as the main energy source of NIKE, the presence of sea ice can prevent the wind from directly acting on the surface ocean, inducing the low NIKE during austral winter.

Effects of Vertical Water Column Temperature on Distribution of Juvenile Tuna Species in the South China Sea

In this study, we conducted two surveys in the central and southern parts of the South China Sea, in autumn 2012 and spring 2013. Six juvenile tuna species were caught in each survey. Gradient forest analysis (GFA) and a generalized additive model (GAM) were used to analyze the relationship between the catch per unit effort (CPUE) for the juvenile tuna species and six sea temperature indices for the South China Sea. In the GFA, the temperature difference between the sea surface and 50 m depth (D50) showed the highest importance to CPUE than other indices, which indicates that D50 was the best predictor of the abundance of juvenile tuna species. The GAM analysis showed that lower deep-water temperature, a shallow mixed layer depth, and a higher difference in temperature between the surface and deeper water were associated with increased CPUE. The results indicate that a relatively rapid decrease in vertical water temperature is favorable for the aggregation of juvenile tuna. These results contribute to understanding of the distribution mechanism of juvenile tuna species in the South China Sea and provide a scientific basis for the rational development and utilization of tuna resources.

Long‐term patterns in ecosystem phenology near Palmer Station, Antarctica, from the perspective of the Adélie penguin

AbstractClimate change is leading to phenological shifts across a wide range of species globally. Polar oceans are hotspots of rapid climate change where sea ice dynamics structure ecosystems and organismal life cycles are attuned to ice seasonality. To anticipate climate change impacts on populations and ecosystem services, it is critical to understand ecosystem phenology to determine species activity patterns, optimal environmental windows for processes like reproduction, and the ramifications of ecological mismatches. Since 1991, the Palmer Antarctica Long‐Term Ecological Research (LTER) program has monitored seasonal dynamics near Palmer Station. Here, we review the species that occupy this region as year‐round residents, seasonal breeders, or periodic visitors. We show that sea ice retreat and increasing photoperiod in the spring trigger a sequence of events from mid‐November to mid‐February, including Adélie penguin clutch initiation, snow melt, calm conditions (low winds and warm air/sea temperature), phytoplankton blooms, shallow mixed layer depths, particulate organic carbon flux, peak humpback whale abundances, nutrient drawdown, and bacterial accumulation. Subsequently, from May to June, snow accumulates, zooplankton indicator species appear, and sea ice advances. The standard deviation in the timing of most events ranged from ~20 to 45 days, which was striking compared with Adélie penguin clutch initiation that varied &lt;1 week. In general, during late sea ice retreat years, events happened later (~5 to &gt;30 days) than mean dates and the variability in timing was low (&lt;20%) compared with early ice retreat years. Statistical models showed the timing of some events were informative predictors (but not sole drivers) of other events. From an Adélie penguin perspective, earlier sea ice retreat and shifts in the timing of suitable conditions or prey characteristics could lead to mismatches, or asynchronies, that ultimately influence chick survival via their mass at fledging. However, more work is needed to understand how phenological shifts affect chick thermoregulatory costs and the abundance, availability, and energy content of key prey species, which support chick growth and survival. While we did not detect many long‐term phenological trends, we expect that when sea ice trends become significant within our LTER time series, phenological trends and negative effects from ecological mismatches will follow.

The ocean response to El Nino Southern Oscillation (ENSO) in the Bali Sea: a model study

The Bali Sea (BS) is connected to the Java Sea and Makassar Strait in the north and is one of the main outflow straits of the Indonesian Throughflow (ITF) in Lombok Strait (LS). Local ocean dynamics and their variability may be controlled significantly by the ITF. This study aims to investigate the interannual variation of circulation, mixed layer depth, and isotherm of 20°C, by performing a simulation of fine- a resolution 1/84° ocean circulation model of CROCO between 2010-2020 with a focus on El Nino 2015 (EN2015) and La Nina 2020 (LN2020) events. The model validation reproduces well the satellite-derived SST and SSH dataset with a correlation coefficient of >0.9. The model demonstrated an anti-clockwise circulation of the ITF in the BS, entering via the northern and exiting into the LS via the southern area. This circulation changes drastically during the ENSO event, where the ITF transport inflow is weaker and shallower during EN2015, compared to a much stronger and deeper ITF inflow during LN2020. Accordingly, shallower mixed layer depth and depth of isotherm of 20°C are found during EN2015, and vice versa. Coherence of these parameters with NINO3.4 is very significant (>-0.89) with a phase lag of 1.7 months.

Simulated Impact of Ocean Alkalinity Enhancement on Atmospheric CO2 Removal in the Bering Sea

AbstractOcean alkalinity enhancement (OAE) has the potential to mitigate ocean acidification (OA) and induce atmospheric carbon dioxide (CO2) removal (CDR). We evaluate the CDR and OA mitigation impacts of a sustained point‐source OAE of 1.67 × 1010 mol total alkalinity (TA) yr−1 (equivalent to 667,950 metric tons NaOH yr−1) in Unimak Pass, Alaska. We find the alkalinity elevation initially mitigates OA by decreasing pCO2 and increasing aragonite saturation state and pH. Then, enhanced air‐to‐sea CO2 exchange follows with an approximate e‐folding time scale of 5 weeks. Meaningful modeled OA mitigation with reductions of &gt;10 μatm pCO2 (or just under 0.02 pH units) extends 100–100,000 km2 around the TA addition site. The CDR efficiency (i.e., the experimental seawater dissolved inorganic carbon (DIC) increase divided by the maximum DIC increase expected from the added TA) after the first 3 years is 0.96 ± 0.01, reflecting essentially complete air‐sea CO2 adjustment to the additional TA. This high efficiency is potentially a unique feature of the Bering Sea related to the shallow depths and mixed layer depths. The ratio of DIC increase to the TA added is also high (≥0.85) due to the high dissolved carbon content of seawater in the Bering Sea. The air‐sea gas exchange adjustment requires 3.6 months to become (&gt;95%) complete, so the signal in dissolved carbon concentrations will likely be undetectable amid natural variability after dilution by ocean mixing. We therefore argue that modeling, on a range of scales, will need to play a major role in assessing the impacts of OAE interventions.

The generation mechanism of cold eddies and the related heat flux exchanges in the upper ocean during two sequential tropical cyclones

The impacts of two sequential tropical cyclones (TCs), Kyarr and Maha, [from October 24 to November 06, 2019, over the Arabian Sea (AS)] on upper ocean environments were investigated using multiple satellite observations, Argo float profiles and numerical model outputs. To obtain a realistic TC strength, the Weather Research and Forecasting (WRF) model was used to reproduce Kyarr and Maha. During Kyarr and Maha, three distinct cold patches were observed at the sea surface with a maximum sea surface cooling of approximately 5°C. The comparison between WRF model simulation results and ERA5 wind field showed that the WRF model simulation indicated high simulation accuracy with respect to the SST decrease in the AS under the influence of Kyarr and Maha’s wind stress curls. Meanwhile, concentration of chlorophyll a (chl-a) and positive relative vorticity of sea surface also appeared in the three cold patch areas. Through the use of eddy detection algorithms, three mesoscale cold cyclonic eddies were identified along the track of TC Kyarr, and the locations of these cold eddies were highly correlated with three obvious negative sea surface height anomalies (SSHAs). The radii of the three cold eddies were 69 km, 50 km, and 41 km. With a focus on the thermodynamic responses of the three cold eddy fields to Kyarr and Maha, the central regions of the three cold eddies were explored. The central regions of the three cold eddies exhibited relatively shallow mixed-layer depths (MLDs) and low mixed-layer temperatures (MLTs). The depth integrated heat (DIH) content was also calculated to explore the heat flux exchanges occurring in different layers in the upper 200 m of the centre of each eddy. The results showed that DIH in each eddy centre varied by one order of magnitude, accounting for between 127.3 MJ m-2 and 1220.0 MJ m-2 of heat loss. This study suggests that the effect of long forcing time on intense positive wind stress curls can produce upwelling caused by Ekman response, which is the main influencing factor of the three cold eddies generation mechanism. At the same time, the positive relative vorticity injected into the sea surface also has some contribution. TC-induced vertical mixing and upwelling (strengthened by unstable structures inside the cold eddies) cause substantial redistribution of the DIH, and related heat flux exchanges at different layers occur in the eddy fields.

Summer net community production in the northern Chukchi Sea: Comparison between 2017 and 2020

The Arctic Ocean environment is drastically changing because of global warming. Although warming-induced processes, such as the decrease in sea-ice extent and freshening of the surface layer, have the potential to alter primary production, the changes that will likely occur in their production and their mechanisms are still poorly understood. To assess the potential changes in net community production, which is a measure of biological carbon pump, in response to climate change, we observed the O2/Ar at the surface of the northern Chukchi Sea in the summers of 2017 and 2020. The net community production (NCP) estimates that we derived from O2/Ar measurements were largely in the range of 1 – 11 mmol O2 m-2 d-1 in the northern Chukchi and Beaufort Seas, close to the lower bounds of the values in the global oceans. The average NCP of 1.5 ± 1.7 mmol O2 m-2 d-1 in 2020 was substantially lower than 7.1 ± 7.4 mmol O2 m-2 d-1 in 2017, with the most pronounced decrease occurring in the ice-free region of the northern Chukchi Sea; the NCP of the ice-free region in 2020 was only 12% of that in 2017. The decrease in NCP in 2020 was accompanied by a lower salinity of &amp;gt; 2, which resulted in shallower mixed layer depths and stronger stratification. We speculated that the anomalously low pressure near the east Russian coast and the lack of strong winds contributed to the strong stratification in 2020. With a continuing decrease in the extent of sea ice, the northern Chukchi Sea will likely experience earlier phytoplankton blooms and nitrate exhaustion. Unless winds blow strong enough to break the stratification, the biological pump in late summer is likely to remain weak.

Extreme harmful algal blooms, climate change, and potential risk of eutrophication in Patagonian fjords: Insights from an exceptional Heterosigma akashiwo fish-killing event

The Patagonian fjords have experienced intense harmful algal blooms (HABs) in the last decade, affecting important aquaculture areas in southern Chile. Climatic anomalies have recently triggered ‘super blooms’ of opportunistic toxic microalgal genera, especially due to persistent thermal stratification which likely provides an optimal niche for HABs development in fjord systems. In March-April 2021, an intense and widespread bloom of the raphidophyte Heterosigma akashiwo caused high salmon mortalities (>6,000 t) in the Comau fjord, Los Lagos Region. A climate variability analysis showed the effects of the positive phase of the Southern Annular Mode (SAM > 1.2 hPa) overcame those of La Niña (Niño3.4 = -0.9 °C) leading to an intense drought on the northern part of Patagonia with record low rainfall (the 2nd driest summer in the last 70 years) and increased water temperature. A regional satellite analysis revealed an extreme and persistent shallow Mixed Layer Depth (MLD) during summer periods since 2019 within the inland seas. In situ vertical fine-resolution measurements during the bloom event showed high cell abundances in the first 3 m of the water column (max. ∼ 70,000 cells mL−1), associated with warmer water temperature (∼15.5 – 17.5 °C), low salinity (∼25–30 psu), moderate to high dissolved oxygen (5 – 8.5 mg/L) and extremely high fluorescence signals in dense superficial cell aggregations (max. 74.9 µg/L). A 18S rRNA metabarcoding analysis formally confirmed the presence of H. akashiwo and its almost monospecific bloom development at the water surface. HPLC pigment analysis showed the carotenoid fucoxanthin in high proportion (48.8 %) compared to other photosynthetic pigments, becoming a potential pigment biomarker for early satellite H. akashiwo detection. Cell growth and cytotoxic in vitro experiments revealed high phenotypic plasticity of Chilean H. akashiwo against sudden changes in salinity. An RTgill-W1 gill cell assay revealed high cytotoxic activity (viability down to ∼ 50 – 30 % of controls) only at high cell abundances (>40,000H. akashiwo cells mL−1), which was in accordance with histological examination of moribund salmon that showed gill damage and circulatory disorders mainly due to long-term exposure to hypoxic conditions and not to potent cytotoxic effects. The Party-MOSA particles dispersion model revealed a high retention of water masses within the Comau fjord during the H. akashiwo outbreak, a scenario that may have boosted fish kills due to enhanced cells patchiness, ichthyotoxins persistence and hypoxic conditions. A historical dissolved inorganic nutrient data analysis showed that inner Patagonian fjords maintain low N and P concentrations including those environments considered of high eutrophication risk. Low N:P (<16:1) ratios measured at Comau fjord during the 2021 suggests that toxic flagellates growth could be favored over diatoms; however, low N:Si (<1:1 – N deficiency) evidences a clear need for better understanding of the role of mixotrophy in the persistence of the 2021H. akashiwo bloom for several weeks. These results highlight the fact that HABs responses against climate drivers and potential eutrophication are not universal and need to be assessed yearly and locally, particularly because extreme droughts and intensive aquaculture in northern Patagonia are expected to continue throughout the 21st century.

Drivers of upper ocean heat content extremes around New Zealand revealed by Adjoint Sensitivity Analysis

Marine heatwaves can have devastating ecological and economic impacts and understanding what drives their onset is crucial to achieving improved prediction. A key knowledge gap exists around the subsurface structure and temporal evolution of MHW events in continental shelf regions, where impacts are most significant. Here, we use a realistic, high-resolution ocean model to identify marine heatwaves using upper ocean heat content (UOHC) as a diagnostic metric. We show that, embedded in the inter-annual variability of UOHC across the Tasman Sea, regional UOHC around New Zealand varies at short temporal and spatial scales associated with local circulation which drives the onset of extreme events with median duration of 5–20 days. Then, using a novel application of Adjoint Sensitivity Analysis, we diagnose the regional drivers of extreme UOHC events and their 3-dimensional structure. We compute the sensitivity of UOHC to changes in the ocean state and atmospheric forcing over the onset of MHW events using ensembles of between 34 and 64 MHW events across 4 contrasting regions over a 25-year period. The results reveal that changes in regional UOHC on short (5-day) timescales are largely driven by local ocean circulation rather than surface heat fluxes. Where the circulation is dominated by boundary currents, advection of temperature in the mixed layer dominates the onset of extreme UOHC events. Higher magnitude MHW events are typically associated with shallower mixed layer and thermocline depths, with higher sensitivity to temperature changes in the upper 50–80 m. On the west coast, where boundary currents are weak, UOHC extremes are sensitive to density changes in the upper 1,000 m and likely caused by downwelling winds. Our results highlight the importance of understanding the different temporal and spatial scales of UOHC variability. Understanding the local circulation associated with heat content extremes is an important step toward accurate MHW predictability in economically significant shelf seas.

Unravelling the roles of Indian Ocean Dipole and El-Niño on winter primary productivity over the Arabian Sea

The Northern Arabian Sea (NAS) is a highly productive basin in the Indian Ocean. The marked seasonal variation of surface winds has the major control on the oceanic primary productivity in the NAS. A regional physical and biogeochemical coupled model was used to examine the control of marine physical processes on the primary productivity in the NAS. It is found that the pure positive Indian Ocean Dipole (PPIOD) has positive anomalies, whereas a co-occurrence of El-Niño and positive IOD (CEPIOD) events leads to negative anomalies in winter-time Chlorophyll-a (Chl-a) concentration during 2000–2018. The CEPIOD events are characterized by weaker winter convective mixing than PPIOD events. The evolution of this discrepancy in the convective mixing process is sufficiently explained by the presence of weaker northeasterly (dry and cold) winds (<2.4 m/s) and a lower net heat flux loss (<- 47 W/m2) during CEPIOD as compared to the PPIOD events. Higher convective mixing results in intense convective cooling, a deeper mixed layer depth (MLD), increased nutrients’ injection through entrainment, and a stronger winter bloom, whereas weak convective mixing causes warming and a shallow MLD, resulting in weak winter bloom. The higher-mixing and surface-cooling condition was observed more often in PPIOD than in CEPIOD years. These conditions promote vertical convective mixing that maintains nutrients’ supply in the near-surface layers and supports primary productivity in PPIOD years.

Long-term variability of satellite derived total alkalinity in the southwest Bay of Bengal

The seasonal and inter-annual variability of Total Alkalinity (TA) concentration was studied in the Bay of Bengal from 2003 to 2019 by using MODIS-Aqua derived sea surface temperature (SST) and sea surface salinity (SSS) products. The satellite derived TA showed a positive relationship with in-situ TA with (R2 = 0.67, RMSE = ±27.53 μMol/kg, SEE = ±32.16 and uncertainty error = 2287μMol/kg). The seasonal SST, SSS and TA portray the clear seasonal pattern between the seasons without any rapid change increase or decrease in trend observed over the years. In contrast to other seasons, the spring inter-monsoon was observed to have a warm surface water temperature with high salinity and TA. Strong wind and excessive cloud cover during the summer monsoon result in the reduction of ocean surface heat, which favours sea surface cooling and shallow mixed layer depth, resulting in low SST, SSS, and TA compared to the spring inter-monsoon. During fall inter-monsoon, the reversal of East India coastal current directs warm water from north to south and the weak wind that prevails in this region enhances stratification. During winter, low-saline water compensates the static stability loss by thermal inversion from the sea surface resulting in surface cooling with coldest SST, low SSS and TA during this period.

Variations in Phytoplankton Primary Production Driven by the Pacific Decadal Oscillation in the East/Japan Sea

AbstractThe Pacific Decadal Oscillation (PDO) regime is a major factor not only for the physical properties of the ocean but also for fishery and water resources. However, only a few studies have examined the impact of the PDO on the marine ecosystem in the East/Japan Sea. Therefore, in this study, the relationship between PDO and primary production (PP), and subsequent effects on the marine ecosystem were investigated in the East/Japan Sea using satellite data sets. PDO index showed a negative relationship with sea surface temperature (SST) and the contribution of the small phytoplankton to the total PP during the study period, whereas the mixed layer depth (MLD) and the PP showed a positive relationship with PDO index. The shallower MLD during the negative PDO phase indicates that vertical mixing may be weakened due to the stronger stratification caused by the higher SST than observed during the positive PDO phase. Consequently, we hypothesized that weakened vertical mixing may reduce nutrient supply to the euphotic layer, providing small‐sized phytoplankton favored environmental conditions during the negative PDO. It is noteworthy that PDO‐induced shoaling of the MLD was mainly observed in winter, which may influence the annual PP of the following year. This study shows that the annual PP in the East/Japan Sea can be largely affected through interactions between SST, MLD and subsequent changes in nutrient regime according to the PDO regime, which subsequently affects potential fishery resources in the East/Japan Sea.

Citizen scientists’ dive computers resolve seasonal and interannual temperature variations in the Red Sea

Dive computers have the potential to provide depth resolved temperature data that is often lacking especially in close to shore, but spatiotemporal assessment of the robustness of this citizen science approach has not been done. In this study, we provide this assessment for the Red Sea, one of the most dived areas in the world. A comparison was conducted between 17 years of minimum water temperatures collected from SCUBA dive computers in the northern Red Sea (23–30° N, 32–39.4° E), satellite-derived sea surface temperatures from the Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) optimal interpolation product, and depth-banded monthly mean in-situ temperature from the TEMPERSEA dataset, which incorporates data originating from several in-situ recording platforms (including Argo floats, ships and gliders). We show that dive computer temperature data clearly resolve seasonal patterns, which are in good agreement in both phase and amplitude with OSTIA and TEMPERSEA. On average, dive computer temperatures had an overall negative bias of (–0.5 ± 1.1) °C compared with OSTIA and (–0.2 ± 1.4) °C compared with TEMPERSEA. As may be expected, increased depth-related biases were found to be associated with stratified periods and shallower mixed layer depths, i.e., stronger vertical temperature gradients. A south-north temperature gradient consistent with values reported in the literature was also identifiable. Bias remains consistent even when subsampling just 1% of the total 9310 dive computer datapoints. We conclude that dive computers offer potential as an alternative source of depth-resolved temperatures to complement existing in situ and satellite SST data sources.

Sea Surface Salinity Changes in Response to El Niño–like SST Warming and Relevant Ocean Dynamics in the Tropical Pacific under the CMIP6 Abrupt-4XCO2 Scenario

Abstract Based on the abrupt-4XCO2 scenario in phase 6 of the Coupled Model Intercomparison Project (CMIP6), this study investigates the response of the rainfall changes to El Niño–like SST warming and the role of ocean dynamical processes in the salinity changes in the tropical Pacific. The results show that the Walker circulation weakening and eastward shift, related to El Niño–like SST warming, dominates the zonal precipitation change. Precipitation decreases (increases) in the Maritime Continent (the equatorial Pacific), partly offsetting the effect of specific humidity. At the same time, the El Niño–like warming triggers convergence of meridional winds, which leads to a precipitation increase in the equatorial Pacific and a decrease in the intertropical convergence zone and the South Pacific convergence zone, following the “warmer-get-wetter” mechanism. Unlike the spatial pattern of precipitation changes, the sea surface salinity changes become fresher in the tropical western Pacific, related to the precipitation and the mean horizontal advection. The precipitation increase leads to negative salinity anomalies in the equatorial central Pacific. The westward climatological zonal currents transport the negative salinity anomalies westward. The meridional currents advect the salinity anomalies to both sides of the equator, partly offsetting the contribution of the freshwater flux on the salinity change. In addition, shallower mixed layer depth and weakening upwelling bring less high-salinity water to the surface and impact salinity redistribution through the vertical process in the equatorial regions.

A lens-shaped, cold-core anticyclonic surface eddy in the northern South China Sea

Typically, compared with the normal ocean, an anticyclonic eddy has higher sea surface temperature (SST), greater surface mixed-layer depth (MLD), and a bowl-shaped structure under the action of geostrophy. This study is the first to report an abnormal anticyclonic eddy characterized by a lens-shaped structure, cold core, and shallower MLD, which were observed in situ in the northern South China Sea (SCS) in September 2021. The SST at core of the anticyclonic eddy was 0.4°C lower than that in its peripheral region. The MLD at the center of the eddy was about half of that outside the eddy. Below the surface mixed layer, a lens-shaped structure containing relatively well-mixed water was observed between the two high-gradient layers. Within this lens-shaped structure, the isothermal layers were stretched, but accompanied by water mixing that was about one third of that at the upper and lower bounds of the structure. This eddy originated from a Kuroshio Current intrusion in late October 2020 and died in November 2021, such that its lifespan exceeded 1 year. The shedding of the Kuroshio Loop into the SCS under strong air–sea interactions under continuous sea surface cooling in winter was considered a key mechanism for the generation of the cold core of the eddy. The lens-shaped structure formed below the surface mixed layer and was maintained through geostrophic balance by the subsurface maximum speeds of the Kuroshio Current intrusion (50–100 m), thereby forming a shallower MLD in the eddy center. The subsurface speed maximum within the eddy was also observed by a shipborne acoustic Doppler current profiler at 10 months after its formation, confirming the hypothesized mechanism. This type of abnormal anticyclonic may be a common phenomenon in the northern SCS and has potential implications for the local biogeochemistry, local heat budget, and regional oceanic models.

Role of Mixed Layer Depth in the Location and Development of the Northeast Pacific Warm Blobs

AbstractWarm blobs are persistent warm anomalies in the Northeast Pacific (NEP) upper ocean. Here, we assess the role of mixed layer depth (MLD) in their location and development based on ocean‐atmosphere reanalysis data. We find that warm blobs occur more frequently over 165°–130°W and 35°–50°N with shallow MLD. They are largely confined to the mixed layer, although substantial portions exist beneath it in summer when the MLD shoals. Based on a mixed‐layer heat budget analysis, we reveal that anomalous MLD and heat flux contribute dominantly to the surface heat flux term over the study area from May to July and from September to March, respectively, and have a large effect on warm blob development. Therefore, the MLD has important implications for the location and seasonal evolution of warm blobs and temperature diagnosis over the NEP.