Variability Of Mixed Layer Depth Research Articles

Interannual Variability of Isopycnal Ocean Heat Content in the Subtropical Northeast Pacific

AbstractThe subsurface thermal anomaly originating from the mid‐latitude Pacific significantly influences climate variability in the tropical Pacific. Traditionally, the spiciness anomaly, defined on an isopycnal surface, is used to estimate this thermal anomaly. However, the thermal anomaly is confined to an isopycnal layer with a certain thickness, not spread on a specific surface. Its impact on the tropical Pacific depends on the amount of heat reaching the equator. Therefore, the thickness‐weighted isopycnal ocean heat content (IPOHC), defined as the ocean heat content in an isopycnal layer of a given potential density range, can more accurately estimate the subsurface thermal anomaly. This study focuses on the interannual variability of the IPOHC in the subtropical northeast Pacific, a region crucial for extratropical‐tropical oceanic exchange. The IPOHC in the 24.8–25.6 kg/m3 potential density layer exhibits a significant interannual variability with a 5–6‐year period. Subduction and salt fingering are identified as key processes driving this variability. Specifically, increased subduction volume and enhanced salt fingering both can induce positive IPOHC anomalies, while the opposite results in negative anomalies. Late winter variations in subduction volume and salt fingering strength, which originate from the winter mixed layer depth variability and halocline intensity variability, work together to cause the observed interannual IPOHC anomalies. We emphasize that the subduction‐induced IPOHC anomalies are determined by anomalous subduction volume rather than mixed layer temperature. Furthermore, salt fingering plays an essential role in driving substantial IPOHC anomalies in the deep, poorly‐ventilated halocline/thermocline.

Seasonal variability and long-term winter shoaling of the upper mixed layer in the southern Baltic Sea

The upper ocean mixed layer plays a crucial role in regulating the exchange between the ocean and the atmosphere. The Mixed Layer Depth (MLD) is a key parameter affecting the air-sea exchanges of momentum and heat and determining the upper ocean temperature. Numerous previous studies have investigated MLD variability in the global ocean or regional seas but no such studies were carried in the Baltic Sea. In this study, we present the first observational assessment of the MLD and its properties in the Southern Baltic Sea including quantification of its seasonal and long-term changes and identification of the multi-year winter shoaling. We calculated monthly maps of MLD in the southern Baltic Sea using a large number of historical CTD profiles collected in 1995–2021 from a combination of different data sets. To test the robustness of the results we compared the MLDs calculated using different threshold methods. Throughout the southern Baltic Sea, across its three basins, a distinct seasonality is evident in the MLD, with values varying from 12 m in July to 60 m in December–March. During winter the water column is well mixed down to the upper halocline depth and the MLD reaches about 45 m in the Bornholm Basin, 50 m in the Slupsk Furrow, and 60 m in the Gdansk Basin. The observed global warming and decadal changes in the salty inflows from the North Sea to the Baltic have had an impact on stratification by increasing water densities in the intermediate and deep layers. Consequently, density gradients have strengthened with depth while the upper ocean mixing has weakened during the winter season. The results reveal a significant winter shoaling of the mixed layer by 4 m per decade, driven by the increased stratification due to rising temperatures and salinity. These changes could have significant impacts on the dynamics and productivity of marine ecosystems.

Tropical Atlantic variability in EC-EARTH: impact of the radiative forcing

AbstractUnderstanding the impact of radiative forcing on climate variability and change in the Tropical Atlantic is crucial for different socio-economic sectors, given their substantial impacts in both local and remote regions. To properly evaluate the effect of a changing climate on the variability, the use of standard transient historical and scenario simulations requires very large ensembles. A computationally cheaper alternative implemented in this study consists of performing two 250-year-long atmosphere-ocean coupled simulations with EC-EARTH 3.3 (CMIP6 version) with fixed radiative forcing at the years 2000 and 2050, representative of present and future climate conditions, respectively. The changes in the leading modes of Tropical Atlantic variability (TAV), including the Atlantic Niño/Niña and the Subtropical North Atlantic pattern, have been assessed in three target seasons: spring (MAM), summer (JJ) and early winter (ND). While the change in sea surface temperature (SST) climatology shows homogeneous warming, the difference between future and present SST variability exhibits a distinct behaviour consistent along the seasonal cycle, with a decrease in the equatorial region and an increase at subtropical latitudes. This study explores the processes associated with the suppressed/enhanced TAV, with a particular focus on the less-explored early winter season. In agreement with previous studies, the Atlantic Meridional Overturning Circulation (AMOC) shows a weakening in strength, but the results also show an increase in variability. The AMOC-related deepening of the equatorial thermocline and the flattening linked to weakened trade winds are consistent with the suppressed SST variability of the Atlantic Niño/Niña. On the other hand, the enhanced SST variability at subtropical latitudes is mainly associated with an increase in turbulent heat flux variability, with a minor contribution of the mixed layer depth variability. Variability in turbulent heat flux is influenced primarily by latent heat flux, connected to changes in precipitation variability.

Trends in the ocean mixed‐layer depth in the western North Pacific tropical cyclone basin

AbstractThe upper ocean mixed‐layer depth (MLD) has been recognized as a crucial factor in understanding the impact of climate change on tropical cyclone (TC) intensity, while the specific MLD variations in the western North Pacific (WNP) basin have received comparatively little attention. Using two ocean reanalysis datasets and the output of 28 CMIP6 models, the climatological mean MLD and its trend in the TC active region are examined. Compared to the ORAS4 and EN4 datasets, the spatial pattern of the climatological mean MLD can be well simulated in the multi‐model ensemble (MME). The mean MLD decreases with increasing latitude and maintains the maximum in the equatorial region. The spatial patterns of MLD long‐term (1958–2017) and short‐term (1979–2017) trends are deepening in the TC active region, with the relatively large positive trends occurring to the southeastern WNP primarily between 5°N and 15°N. However, the simulated trend in the MME is dominant with shoaling MLD, and the shoaling of the MLD is also projected under the SSP5‐8.5 scenario. It is suggested that the bias on the simulated shoaling of the MLD in CMIP6 models may result from the overestimated enhancement of the upper‐ocean stratification. Thus, there is a discrepancy between the observed and simulated MLD trends in the WNP basin, which may have implications for TC intensity change under global warming scenarios.

Seasonal variability of Red Sea mixed layer depth: the influence of atmospheric buoyancy and momentum forcing

The seasonal and spatial evolution of the mixed layer (ML) in the Red Sea (RS) and the influence of atmospheric buoyancy and momentum forcing are analyzed for the 2001–2015 period using a high-resolution (1/100°, 50 vertical layers) ocean circulation model. The simulation reveals a strong spatiotemporal variability reflecting the complex patterns associated with the air–sea buoyancy flux and wind forcing, as well as the significant impact of the basin’s general and mesoscale circulation. During the spring and summer months, buoyancy forcing intensifies stratification, resulting in a generally shallow ML throughout the basin. Nevertheless, the results reveal local maxima associated with the influence of mesoscale circulation and regular wind induced mixing. Under the influence of surface buoyancy loss, the process of deepening of the ML commences in early September, reaching its maximum depth in January and February. The northern Gulf of Aqaba and the western parts of the northern RS, exhibit the deepest ML, with a gradual shoaling toward the south, primarily due to the surface advection of relatively fresh water that enters the basin from the Gulf of Aden. The mixed layer depth (MLD) variability is primarily driven by atmospheric buoyancy forcing, especially its heat flux component. Although evaporative fluxes dominate the annually averaged surface buoyancy forcing, they exhibit weak seasonal and spatial variability. Wind induced mixing exerts a significant impact on the MLD only locally, especially during summer. Of particular importance are strong winds channeled by topography, such as those in the vicinity of the Strait of Bab-Al-Mandeb and the straits connecting the two gulfs in the north, as well as lateral jets venting through mountain gaps, such as the Tokar Jet in the central RS. The analysis highlights the complex patterns of air-sea interactions, thermohaline circulation, and mesoscale activity, all of them strongly imprinted on the MLD distribution.

Composite vertical structures and spatiotemporal characteristics of abnormal eddies in the Japan/East Sea: a synergistic investigation using satellite altimetry and Argo profiles

Mesoscale eddies are omnipresent and play an important role in regulating Earth’s climate and ocean circulation in the global ocean. Here using the combination of satellite altimetry products and Argo float profile data, two types of abnormal eddies are investigated: WCEs(warm cyclonic eddies) and CAEs(cold anticyclonic eddies) with different cores than conventional eddies in the Japan/East Sea. By applying a classification method based on the calculation of the heat content anomalies in the upper ocean, it was found that 10% of the eddies that captured the Argo float profiles exhibited obvious abnormal features. Subsequently, their spatiotemporal distributions and characteristics were analyzed statistically. Three-dimensional structures of abnormal eddies were obtained via the composite analysis method, showing that the warm/cold and light/dense core of the composite WCE/CAE is confined to the upper 100 m of the ocean with a maximum temperature anomaly of approximately +1.0(-1.1)°C. The composite WCE had a double-core salinity structure with a salty core above 50 m and an inferior fresh core. Meanwhile composite CAE had a fresh single-core with a maximum magnitude of -0.05 psu. Abnormal eddies are pervasive in the Japan/East sea, a revaluation of the role of these eddies in ocean circulation and climate systems, such as heat and salt transport, air and sea interaction, and variability in mixed layer depth, is of great importance.

Mechanisms for Abrupt Summertime Circumpolar Surface Warming in the Southern Ocean

Abstract In recent years, the Southern Ocean has experienced unprecedented surface warming and sea ice loss—a stark reversal of the sea ice expansion and surface cooling that prevailed over the preceding decades. Here, we examine the mechanisms that led to the abrupt circumpolar surface warming events that occurred in late 2016 and 2019 and assess the role of internal climate variability. A mixed layer heat budget analysis reveals that these recent circumpolar surface warming events were triggered by a weakening of the circumpolar westerlies, which decreased northward Ekman transport and accelerated the seasonal shoaling of the mixed layer. We emphasize the underappreciated effect of the latter mechanism, which played a dominant role and amplified the warming effect of air–sea heat fluxes during months of peak solar insolation. An examination of the CESM1 large ensemble demonstrates that these recent circumpolar warming events are consistent with the internal variability associated with the Southern Annular Mode (SAM), whereby negative SAM in austral spring favors shallower mixed layers and anomalously high summertime SST. A key insight from this analysis is that the seasonal phasing of springtime mixed layer depth shoaling is an important contributor to summertime SST variability in the Southern Ocean. Thus, future Southern Ocean summertime SST extremes will depend on the coevolution of mixed layer depth and surface wind variability. Significance Statement This study examines how reductions in the strength of the circumpolar westerlies can produce abrupt and extreme surface warming across the Southern Ocean. A key insight is that the mixed layer temperature is most sensitive to surface wind perturbations in late austral spring, when the regional mixed layer depth and solar insolation approach their respective seasonal minimum and maximum. This heightened surface temperature response to surface wind variability was realized during the austral spring of 2016 and 2019, when a dramatic weakening of the circumpolar westerlies triggered unprecedented warming across the Southern Ocean. In both cases, the anomalously weak circumpolar winds reduced the northward Ekman transport of cool subpolar waters and caused the mixed layer to shoal more rapidly in the spring, with the latter mechanism being more dominant. Using results from an ensemble of coupled climate simulations, we demonstrate that the 2016 and 2019 Southern Ocean warming events are consistent with the internal variability associated with the Southern Annular Mode (SAM). These results suggest that future Southern Ocean surface warming extremes will depend on both the evolution of regional mixed layer depths and interannual wind variability.

Cessation of Labrador Sea Convection Triggered by Distinct Fresh and Warm (Sub)Mesoscale Flows

Abstract By ventilating the deep ocean, deep convection in the Labrador Sea plays a crucial role in the climate system. Unfortunately, the mechanisms leading to the cessation of convection and, hence, the mechanisms by which a changing climate might affect deep convection remain unclear. In winter 2020, three autonomous underwater gliders sampled the convective region and both its spatial and temporal boundaries. Both boundaries are characterized by higher subdaily mixed layer depth variability sampled by the gliders than the convective region. At the convection boundaries, buoyant intrusions—including eddies and filaments—instead of atmospheric warming primarily trigger restratification by bringing buoyancy with a comparable contribution from either fresh or warm intrusions. At the edges of these intrusions, submesoscale instabilities, such as symmetric instabilities and mixed layer baroclinic instabilities, seem to contribute to the decay of the intrusions. In winter, enhanced lateral buoyancy gradients are correlated with strong destabilizing surface heat fluxes and alongfront winds. Consequently, winter atmospheric conditions and buoyant intrusions participate in halting convection by triggering restratification while surface fluxes are still destratifying. This study reveals freshwater anomalies in a narrow area offshore of the Labrador Current and near the convective region; this area has received less attention than the more eddy-rich West Greenland Current, but is a potential source of freshwater in closer proximity to the region of deep convection. Freshwater fluxes from the Arctic and Greenland are expected to increase under a changing climate, and our findings suggest that they may play an active role in the restratification of deep convection.

The mixed-layer depth in the Ocean Model Intercomparison Project (OMIP): impact of resolving mesoscale eddies

Abstract. The ocean mixed layer is the interface between the ocean interior and the atmosphere or sea ice and plays a key role in climate variability. It is thus critical that numerical models used in climate studies are capable of a good representation of the mixed layer, especially its depth. Here we evaluate the mixed-layer depth (MLD) in six pairs of non-eddying (1∘ grid spacing) and eddy-rich (up to 1/16∘) models from the Ocean Model Intercomparison Project (OMIP), forced by a common atmospheric state. For model evaluation, we use an updated MLD dataset computed from observations using the OMIP protocol (a constant density threshold). In winter, low-resolution models exhibit large biases in the deep-water formation regions. These biases are reduced in eddy-rich models but not uniformly across models and regions. The improvement is most noticeable in the mode-water formation regions of the Northern Hemisphere. Results in the Southern Ocean are more contrasted, with biases of either sign remaining at high resolution. In eddy-rich models, mesoscale eddies control the spatial variability in MLD in winter. Contrary to a hypothesis that the deepening of the mixed layer in anticyclones would make the MLD larger globally, eddy-rich models tend to have a shallower mixed layer at most latitudes than coarser models do. In addition, our study highlights the sensitivity of the MLD computation to the choice of a reference level and the spatio-temporal sampling, which motivates new recommendations for MLD computation in future model intercomparison projects.

Near-inertial oscillations in the deep Gulf of Mexico

Near-inertial oscillations (NIOs) are important for maintaining turbulent mixing that affects ocean circulation, biogeochemistry, and climate. The spatial and temporal variability of NIOs in the deep part of the Gulf of Mexico (GoM) has rarely been reported. In this study, a collection of moored current observations was used to examine the spatiotemporal variability of NIOs in the GoM. In the upper layer (0–800 m), a strong seasonal variability of NIOs appears in the eastern GoM with larger amplitudes in winter than in summer and is attributed to the seasonal variability of surface winds and mixed layer depth. In the bottom layer (within 400 m above the bottom), strong NIOs are found in the middle and eastern GoM and show weaker seasonal variability. While winds still matter for the seasonal variability of NIOs in the bottom layer in the eastern GoM, low-frequency flows generally play a more important role in regulating NIOs through interactions with topography, particularly on the continental slope.

Inter‐Annual Variability in Phytoplankton and Nutrients in the Gulf of Elat/Aqaba

AbstractThe Gulf of Elat/Aqaba exhibits high inter‐annual variability in mixed layer depth. Observations from the northern Gulf show differences of hundreds of meters in winter mixing depth, which ranges between 300 m in years with shallow mixing and up to 700 m in years with deep mixing. Deep mixing events can occur in two consecutive years or after four consecutive years of shallow mixing. The mixing depth has an effect on the concentration of nutrients and chlorophyll (and other tracers) in the surface and deep water. Using a 3D coupled physical‐ecological model, we studied the effect of shallow versus deep mixing on the processes controlling the phytoplankton bloom and on nutrient accumulation in the deep water. We found that years with deep mixing are characterized by larger spatial variability in surface and integrated chlorophyll concentration during the mixing season. We found that horizontal advection is a dominant contributor for integrated phytoplankton concentration in years with deep mixing in the northern Gulf. Even when mixing was deep and nutrient limitation decreased, light limitation on phytoplankton integrated growth was enhanced in the north compared with the south. In addition, we showed that the nutrient accumulation in the deep water after a year with deep mixing in the northern Gulf was initially affected mostly by physical processes (such as advection and vertical mixing), and less from ecological regeneration, and switched to be dominated by ecological processes alone during the third year after mixing.

Origins of mesoscale mixed-layer depth variability in the Southern Ocean

Abstract. Mixed-layer depth (MLD) exhibits significant variability, which is important for atmosphere–ocean exchanges of heat and atmospheric gases. The origins of the mesoscale MLD variability in the Southern Ocean are studied here in an idealised regional ocean–atmosphere model (ROAM). The main conclusion from the analysis of the upper-ocean buoyancy budget is that, while the atmospheric forcing and oceanic vertical mixing, on average, induce the mesoscale variability of MLD, the three-dimensional oceanic advection of buoyancy counteracts and partially balances these atmosphere-induced vertical processes. The relative importance of advection changes with both season and average MLD. From January to May, when the mixed layer is shallow, the atmospheric forcing and oceanic mixing are the most important processes, with the advection playing a secondary role. From June to December, when the mixed layer is deep, both atmospheric forcing and oceanic advection are equally important in driving the MLD variability. Importantly, buoyancy advection by mesoscale ocean current anomalies can lead to both local shoaling and deepening of the mixed layer. The role of the atmospheric forcing is then directly addressed by two sensitivity experiments in which the mesoscale variability is removed from the atmosphere–ocean heat and momentum fluxes. The findings confirm that mesoscale atmospheric forcing predominantly controls MLD variability in summer and that intrinsic oceanic variability and surface forcing are equally important in winter. As a result, MLD variance increases when mesoscale anomalies in atmospheric fluxes are removed in winter, and oceanic advection becomes a dominant player in the buoyancy budget. This study highlights the importance of oceanic advection and intrinsic ocean dynamics in driving mesoscale MLD variability and underscores the importance of MLD in modulating the effects of advection on upper-ocean dynamics.

Seasonal variation of mixed layer depth and thermocline thickness from the ctd argo float data in the Southern Makassar Strait

The Makassar Strait (MS) is directly connected to the Sulawesi Sea in the north and the Java Sea and the Flores Sea in the south and is the main route of Indonesia Throughflow(ITF). Hence Stratification of water mass in the MS is influenced by the dynamics of these waters. This study aims to describe seasonal variations of the mixed layer depth and the thermocline thickness, identify ITF water masses, and the occurrence of coastal upwelling in the southern part of the MS. The results show that the North Pacific water origin of North Pacific Subtropical Water (NPSW) and North Pacific Intermediate Water (NPIW) are dominant, with a maximum salinity characteristic of 34.6-34.7 psu in the thermocline layer and a minimum salinity of 34.5 psu in the intermediate layer. However, near-surface fresh Java Seawater appears to be dominant during the northwest monsoon (NWM-DJF) period. Seasonal variation of the mixed layer depth is much deeper (93m) and vertical gradient of temperature and salinity are much stronger due to the presence of fresh Java Sea water advected by the eastward Java Monsoon Current. However, during the southeast monsoon (SEM-JJA) period the mixed layer depth is much shallower and the thermocline layer is much thicker (198 m), Associated with much stronger ITF and upwelled colder water from the upwelling area. The coastal upwelling event during the SEM period in the study are is characterized by an increase of isotherm of 25 °C, isohaline of 34.0 psu, and isopycnal of 22.0 kg/m3 from about 75 m depth to the sea surface layer.

Role of local and remote forcing on the decadal variability of Mixed Layer Depth in the Bay of Bengal

The Mixed Layer Depth (MLD) of an ocean acts as a storage of the incoming heat and releases via air-sea exchange processes. The present study tries to understand the decadal variability of MLD over the Bay of Bengal (BoB) and its association with local as well as external forcing. The time series and wavelet analysis confirm the significant decadal signals for the period from 1980 to 2015. The BoB MLD is found to have opposing trend prior to and post of 1998. In the decadal timescale, the BoB MLD is found to be comparably deep for regime-1 (1980–1998) and shallow for regime-2 (1999–2014) with stronger variation in the western bay. The local forcing like wind, wind-related processes, and Evaporation minus Precipitation (E-P) are found to have a prominent role to modulate the decadal variability of BoB MLD. The leading mode of the decadal Empirical Orthogonal Function (EOF-1) of MLD over the BoB shows a strong spatial pattern with the strongest in the western part of the Bay. The corresponding principal component shows a similar variability to Interdecadal Pacific Oscillation (IPO) signal. The enhancement of descending (ascending) branch of the Indian branch of Walker circulation (IWC) in the positive (negative) phase of IPO influences the changes over the equatorial Indian Ocean remotely modulate MLD of the coastal region of the BoB via the propagating coastal Kelvin waves. The decadal variability of MLD in the coastal region of the BoB is caused mainly by remote effect but the interior BoB is by the local factors.

Fragility of marine photosynthesis

Ecosystem fragility is an often used term in oceanography yet to this day it lacks a precise and widely accepted definition. Defining and subsequently quantifying fragility would be of great value, for such measures could be used to objectively ascertain the level of risk marine ecosystems face. Risk assessments could further be used to define the level of protection a given ocean region requires from economic activity, such as fisheries. With this aim we introduce to the oceanographic literature the concepts of marginal production and fragility, which we define for marine photosynthesis, the base of the oceanic food web. We demonstrate that marine photosynthesis is always fragile with respect to light, implying variability in surface irradiance acts unfavourably on biomass. We also demonstrate that marine photosynthesis can be both fragile and antifragile with respect to the mixed-layer depth, implying variability in mixed-layer depth can act both favourably and unfavourably on biomass. Quantification of marginal production and fragility is presented on data from two open ocean stations: Hawaii Ocean Time Series and Bermuda Atlantic Time-Series Study. Seasonal cycle of biomass is modelled and the effects of primary production fragility are analysed. A new tipping point for marine phytoplankton is identified in the form of a depth horizon. Using the new definitions presented here a rich archive of data can be used straightforwardly to quantify primary production fragility. The definitions can also be used to predict when primary production enters the fragile state during the seasonal cycle.

The Spatial and Temporal Impacts of Mixed Layer Depth (MLD) variations in West Sumatera Coastal Area

This paper investigates sea surface height to geoid (SSHG) and mixed layer depth (MLD) variations in the West Sumatera Coastal Area dominated Indian Ocean Dipole (IOD) evolutions using the data of MLD and (SSHG) from 2004 to 2017. The MLD variations in West Sumatera Coastal Area are explicitly investigated through spatial and temporal variations Empirical Orthogonal Functions (EOF). In the previous research, MLD variations in these extensive ocean-atmospheric circulations mainly focused on seasonal variations. The first results of EOF mode indicate 12% of the variability and show MLD variations in West Sumatera Coastal area during boreal fall (September – November). This result indicates that seasonal variation of MLD is slightly weak to the IOD event caused by ENSO signals from atmospheric teleconnection. On the other situation, the seasonal variations of SSHG is tightly related to the Indian Ocean Dipole (IOD) evolutions. The EOF results of SSHG shows 30% variabilities and is stronger than the MLD. The weak (dominant) pattern of these MLD and SSHG variations affects a shoaling layer over the west Sumatra coastal area associated with the negative Indian Ocean during 2010. We suggest that this MLD and SSHG product could be used as a reference for future studies and predict fish productivity.

Subantarctic Mode Water Variations in the Three Southern Hemisphere Ocean Basins During 2004–2019

AbstractSubantarctic mode water (SAMW) variations and their forcing mechanisms in the three Southern Hemisphere ocean basins are studied using Argo data and reanalysis products during the 2004–2019 period. In the Indian and Pacific Oceans, SAMW is characterized by potential vorticity (PV) minimums lower than 0.5 × 10−10 m−1 s−1. In the Atlantic Ocean, the PV minimum is generally lower than 1 × 10−10 m−1 s−1. The variations in SAMW thickness are significantly correlated with changes in the mixed layer depth (MLD) and show large differences among different ocean basins. The winter MLD in the Indian Ocean SAMW formation region is significantly positively correlated with the local wind stress curl (correlation coefficient = 0.61; 95% confidence level). In the Pacific and Atlantic SAMW formation regions, the variations in the winter MLD have significant negative correlations with the buoyancy flux; the correlation coefficients reach −0.57 and −0.67 (95% confidence level), respectively. These different mechanisms of the MLD variations in SAMW in the three ocean basins are related to the Southern Annular Mode (SAM) variation in winter. During positive (negative) SAM index periods, a strong positive (negative) wind stress curl anomaly occurs in association with a meridionally asymmetric westerly wind anomaly distribution. This phenomenon results in a corresponding MLD anomaly over the Indian Ocean SAMW formation region. In the Pacific and Atlantic SAMW formation regions, a negative (positive) westerly wind anomaly induces buoyancy gain (loss), leading to mixed layer shallowing (deepening) during positive (negative) SAM periods.

The role of winter net heat fluxes on the modulation of the upper mixed layer temperature and depth in the North Atlantic by the reanalysis data

The role of winter net heat fluxes (NHF) on the upper mixed layer (UML) temperature and depth variability over the North Atlantic is studied using ocean reanalysis data products. The monthly data from the ORA-S3 (1959–2011), GODAS (1980–2020), GECCO3 (1948–2018) datasets were used. The correlation coefficients were calculated between the anomalies of the NHF on the ocean surface and the UML temperature and depth with a shift of 1 month when the former were leading. In most of the North Atlantic, negative correlations between the UML temperature and the NHF preceding 1 month were found. Here, the UML temperature anomalies are probably caused by changes in heat exchange with the atmosphere. Areas of significant positive correlations across all data sets are concentrated in the area of the transition of the Gulf Stream to the North Atlantic Current. Almost the entire water area of the North Atlantic is occupied by positive correlations between the mixed layer depth in winter and the NHF on the ocean surface, with the latter leading for 1 month.

Reduction of Equatorial Obduction by Atmospheric Intraseasonal Oscillations in the Western and Central Pacific Ocean

AbstractObduction is the large‐scale, irreversible upward water transport from the permanent pycnocline to the surface mixed layer. Using Hybrid Coordinate Ocean Model (HYCOM), this study explores the impact of atmospheric intraseasonal oscillations (ISOs) on the Pacific equatorial obduction that plays a fundamental role in regulating the tropical Pacific climate. Parallel HYCOM experiments forced by atmospheric fields with and without ISO variability are compared to assess the effect of ISOs on the ocean. The results suggest that the ISOs can reduce the annual obduction rate of the western and central equatorial Pacific Ocean (WCEPO; 150°E−160°W, 5°S–5°N) by ∼12%, and their impact on the eastern Pacific is rather limited. The ISOs cause prominent intraseasonal variability of mixed layer depth (MLD) in the WCEPO primarily through surface wind forcing, which in turn affects the two key processes controlling the annual obduction rate. First, the intraseasonal MLD deepening events narrow down the total time windows for obduction by ∼30% and thereby give rise to the decrease in obduction rate; second, it leads to high‐frequency entrainment events and enhances the entrainment rate by ∼18%. The increased entrainment rate is overwhelmed by the reduced time windows for effective entrainment, and the net effect of ISOs is to reduce the obduction rate and modify its seasonal cycle. This work highlights the importance of atmospheric ISOs in the equatorial ocean dynamics, with implications for tropical surface temperature and climate variabilities and biogeochemical cycle.

Observational Study on the Variability of Mixed Layer Depth in the Bering Sea and the Chukchi Sea in the Summer of 2019

Based on the CTD data from 58 stations in the Bering Sea and the Chukchi Sea in the summer of 2019, the values of mixed layer depth (MLD) were obtained by using the density difference threshold method. It was concluded that the MLD can be estimated more accurately by using a criterion of 0.125 kg/m3 in this region. The average MLD in the Bering Sea basin was larger than that in the Bering Sea shelf, and both of them were smaller than that in the Bering Sea slope. The MLD increased northward in both the Chukchi Sea shelf and the Chukchi Sea slope. The farther northward, the greater the difference between the MLD calculated from temperature (MLDt) and the MLD calculated from density (MLDd). The water masses and their interaction played an important role in the variation of MLD in the northern Bering Sea shelf and Chukchi Sea. The MLD was large due to the vertically homogeneous Anadyr Water in the northwestern Bering Sea shelf. The horizontal advection of Bering Sea Anadyr Water and Alaska Coastal Water in the Bering Sea shelf led to shallower MLD in the central northern Bering Sea shelf. The westward advection of the Alaska Coastal Water caused shallow mixed layers (MLs) in some regions of the Chukchi Sea shelf in the summer of 2019. The observed large MLD at BL01 station near the Aleutian Island was caused by an anticyclonic eddy. The northward increase in the MLD in the Chukchi Sea was related to the low-salinity seawater from sea ice melting in summer. The spatial variation of MLD was also closely related to the surface momentum flux and the sea surface buoyancy flux. Stratification plays an even more important role in determining the variation of MLD. The ML in 2019 was shallower and warmer than those in previous years, especially in the Bering Sea shelf and Chukchi Sea where sea ice volume, thickness, and coverage were significantly larger than the Bering Sea basin, which was related to the small sea ice volume in winter and spring of 2019 compared to previous years.