Journal of Geophysical Research-Oceans最新文献

Understanding the dynamics of thermohaline intrusions is crucial for predicting changes in water masses and their impact on marine ecosystems, especially in highly stratified semi-enclosed seas and other coastal environments. We use high-resolution (up to 1–2 m) Argo profiling float data collected over 15 years in the Black Sea, an excellent test area for studying thermohaline intrusions. Our analysis challenges the conventional view of stagnant intermediate and deep waters, revealing active mixing processes that reshape the thermohaline structure. We identified two main mechanisms driving these intrusions, related to dense water inflows from the Marmara Sea and boundary mixing enhanced by frontal instabilities. Argo data also allowed us to identify areas with favorable conditions for double-diffusive processes. The variability of intrusions is due to changes in the thermohaline state of the upper ocean as well as to quasi-periodic changes in the inflow caused by local conditions. Trends in the intensity and frequency of intrusions indicate shifts in water mass properties that are likely to be associated with climate variability and extreme weather events. Such trends can affect nutrient cycling, oxygen distribution and the overall stability of the water column, thereby affecting biogeochemical cycles and the resilience of marine ecosystems. Similar ventilation mechanisms may operate in other highly stratified marine systems, such as the Baltic Sea and the Arctic Ocean, so our findings may have wider implications for understanding climate-induced changes in water masses at regional and global scales.

Changes in ocean salinity are essential for the stratification of the upper ocean and the regional marine ecosystem. In this study, 10 sets of large ensemble experiments and multi-model ensembles from the Coupled Model Intercomparison Project Phase 6 (CMIP6) are used to investigate the effect of anthropogenic forcing on upper ocean salinity in the South China Sea (SCS). In most models, surface salinity increases during the historical period due to external forcing. Using the salinity budget, we find that a decrease in freshwater flux, particularly precipitation, is responsible for the increase in salinity, although horizontal advection also contributes to the change. Single forcing experiments reveal that the change in salinity in the SCS is mainly attributed to anthropogenic forcing, while the influence of natural forcing is relatively small. Anthropogenic aerosols (AAs) can decrease the dynamic and thermal components of precipitation, resulting in a considerable increase in salinity. In contrast, anthropogenic greenhouse gases (GHGs) have less effect on long-term trend in SCS salinity because the GHG forcing leads to an increased thermal response of precipitation but a decreased dynamic response. Additionally, we use the Community Earth System Model version 1 (CESM1) to evaluate the role of different aerosol emission sources in modulating the salinity change in the SCS. The experimental results show that aerosol emissions from Asia dominate the salinity change in the SCS by changing the local Hadley circulation. In contrast, the contribution of aerosol emissions from North America and Europe (NAEU) is much smaller.

Sea level variations in the coastal zone can differ significantly from those in the open ocean and can be highly spatiotemporally coherent in the alongshore direction. Yet, where and how coastal sea levels exhibit variations that emerge as persistent and recurrent patterns along the world's coastlines remain poorly understood. Here, we use a Bayesian mixture model to identify large-scale patterns of coherent modes of monthly coastal sea level variations from coastal altimetry and tide gauge data. We determine nine clusters of coherent coastal sea level variability that explain a majority of the monthly variance measured by tide gauges (1993–2020). The analysis of along track altimetry data enables us to detect several additional clusters in ungauged regions, such as the Indian Ocean or around the South Atlantic basin, which have so far been poorly described. Although some clusters (e.g., at the eastern boundary of the Pacific, the western tropical Pacific, and the marginal and semi-enclosed seas) are highly correlated with climate modes, other clusters share very little variability with the considered climate modes at the monthly timescale. Knowledge of these coherent regions thus motivates and enables further investigations on the impacts of local and remote forcing on coastal sea level variability, and the extent to which coastal sea level variability is decoupled from the adjacent deep ocean.

The Deep Cross-Equatorial Cell (DCEC) is the primary branch of Indian Ocean Meridional Overturning Circulation (MOC) in the tropical Indian Ocean, essential for energy redistribution, water exchange, and diapycnal mixing. However, the mechanisms behind its interannual variability remain limited. This study utilized two reanalysis data sets and a series of ocean model experiments with a Hybrid Coordinate Ocean Model and a Linear Ocean Model to investigate the underlying mechanisms. Model experiments highlight the critical role of direct local wind forcing and eastern boundary waves induced by remote equatorial wind forcing in influencing the DCEC variability. Specifically, through the first mode of baroclinic dynamics, direct wind forcing initiates reverse meridional flow at the DCEC core (around 8°S) in both surface and deep ocean layers, leading to interannual variations of the DCEC. During transitions of climate modes like ENSO and Indian Ocean Dipole from positive to negative phases, both positive and negative DCEC anomalies intensify. In addition to direct local wind forcing, the delayed-time Rossby waves reflected from the eastern boundary excited by the equatorial easterly wind in the previous year make substantial contributions (37.8%). The interplay of faster baroclinic Rossby waves at lower latitudes and slower baroclinic Rossby waves at higher latitudes alters the basin-wide pressure gradient, ultimately amplifying interannual DCEC anomalies in the subsequent year.

The Indian summer monsoon, which brings heavy precipitation to the densely populated Indian subcontinent, plays an important role in the development of a coastal upwelling circulation that brings colder, nutrient-rich water to the surface. Although the western shores of the Arabian Sea (AS) and Bay of Bengal (BoB) both experience upwelling-favorable winds during June-August, only the AS coastline exhibits significant surface cooling. In contrast, the BoB remains warm and its upwelling is characterized by a transient, weak sea surface temperature (SST) response confined to the east coast of India. A weaker mean alongshore wind stress and coastal circulation do not sufficiently explain the lack of SST response in the BoB. Here, we examine other reasons for the differing behavior of these two coastal margins. Firstly, we show that while winds are persistently upwelling-favorable in the western AS, intraseasonal wind variability in the BoB induces intermittent upwelling. Secondly, the vertical density stratification is controlled by salinity in the BoB, and upwelled waters are saltier, but only marginally cooler than surface waters. By contrast, the density in the AS is temperature-controlled, and upwelled waters are substantially colder than the surface. Additionally, satellite-based SST in the BoB does not adequately resolve the upwelling signal. Using a numerical model, we find that salinity stratification has a greater influence on the mean SST, while wind frequency alters near-shore SST and its temporal variability. This work has implications for the sensitivity of upwelling regions and their response to wind stress and stratification in a warming climate.

Marine heatwaves (MHWs) are characterized as extreme ocean warming events, which have multifaceted impacts on marine ecosystems. The warming of the ocean associated with MHWs can extend into the deep ocean. In the study, five main types of MHWs in the South China Sea (SCS) are identified with various vertical structures, including shallow, surface-extension-reversed, surface-extension-intensified, deep, and surface-extension-intensified-reversed. Different types of MHWs have different spread patterns and depths. Shallow and surface-extension-reversed MHWs are common in coastal areas, and surface-extension-intensified MHW events often occur in deep areas. The vertical structures of MHWs are affected by ocean dynamical processes, particularly vertical processes and horizontal heat advection. These MHWs can alter ocean chlorophyll concentration to varying degrees and locations, contingent on their vertical thermal profiles. Furthermore, an analysis of the impacts of a long-lived MHW event in the SCS in 2020 revealed that the MHWs could alter chlorophyll concentration. These results help to better understand the physical drivers of localized MHWs and their potential impacts in the context of climate change.

Oxygen deficient zones (ODZs) contain elevated concentrations of dissolved, reduced iron (Fe(II)), presumably sourced from shelf sediments. As that Fe(II) is transported offshore, it is oxidized and scavenged to the continental slope. The redox reactions involved and their influence on transport are poorly constrained. Here, the in situ oxidation of Fe(II) by nitrate and or nitrite and incorporation into iron oxyhydroxides was studied in a free floating sediment trap array in the Eastern Tropical North Pacific ODZ. Particle-dependent Fe(II) half-lives ranged from 43 to 132 days, with the slowest rates at each station within the core of the ODZ. The very slowest rates were at an offshore station with the lowest Fe(II) concentrations. We conclude that iron oxidation in this region is likely a microbially driven process. An inverse model described the characteristic distribution of Fe(II) within ODZs by coupling a benthic source with our oxidation rate data. While oxidation was assumed to be first order with respect to Fe(II), apparent second order kinetics yielded the best fit, presumably because microbial Fe(II) oxidizer abundance is proportional to Fe(II) concentration. The fit was also improved by incorporating an Fe(II) source within the ODZ from remineralization of sinking particles. While this source was at odds with thermodynamics in a nitrate-dominated regime, we showed that Fe(II) production occurs in anaerobic, nitrate-replete mesocosm, provided that large particles are present. Such particles may harbor nitrate-depleted microenvironments that create conditions thermodynamically favorable for iron reduction. These experiments provided a justification for incorporating a remineralization term into the model.

The Deep Western Boundary Current (DWBC) is a major conduit for the equatorward export of dense waters formed in the subpolar North Atlantic and Nordic Seas that constitute the lower limb of the Atlantic Meridional Overturning Circulation. Here, we investigate the extent to which there is coherent propagation of property anomalies along the DWBC from the Labrador Sea exit to 26.5°N. Past studies have focused on relationships between DWBC anomalies at selected sites. Here we use a hydrographic data set (EN4) that covers the time period of 1970–2020 to examine coherence continuously along the boundary current. Our findings reveal sharp differences between the upper and deep Labrador Sea Water (uLSW, dLSW). Specifically, dLSW property anomalies are highly correlated at all points downstream to the Labrador Sea exit. Furthermore, the lags that yield maximum correlations uniformly increase with distance along the boundary. uLSW, however, shows a sharp decline in coherence along the boundary such that the anomalies downstream are poorly correlated with those at the Labrador Sea exit and the lag times are not monotonic. Most of the decline in uLSW coherence occurs from the Labrador Sea exit to Flemish Cap, where local variability at uLSW densities is large. Our study sheds light on the competition between advected property variability and local property variability that impacts the identification of anomalies downstream. The uLSW and dLSW differences expressed along the DWBC are also evident offshore, consistent with past Lagrangian studies.

Using a recently developed 1/12th degree regional ocean model, we establish a link between U.S. East Coast sea level variability and offshore upper ocean heat content change. This link manifests as a cross-shore mass redistribution driven by an offshore thermosteric sea level response to subsurface warming or cooling. Approximately 50% of simulated monthly to interannual coastal sea level variance south of Cape Hatteras can be statistically accounted for by this mechanism, realized as a function of regional ocean hypsometry, gyre scale warming, and the depth dependence of density change. This response to offshore warming explains the nonstationarity of U.S. East Coast sea level covariance, a specifically observed and modeled behavior after $��\sim �$ 2010. Since approximately 2010, elevated rates of sea level rise south of Cape Hatteras can be partly explained as the result of shoreward mass redistribution due to offshore subsurface warming within the North Atlantic subtropical gyre. These results reveal a mechanism that connects local coastal sea level to a broader region and identifies the influence of regional heat content changes on coastal sea level. This analysis presents a framework for identifying new regions that may be susceptible to enhanced sea level rise due to ocean warming and helps bridge the gap between quantifying large scale change and anticipating local coastal impacts that can make flooding and storm surge more acutely damaging.

We present an assessment of the water mass dynamics in a reanalysis of the Mediterranean Sea with a focus on the Algero-Provencal basin. We use a θ-S-based algorithm to compute the fractions of the main western Mediterranean water masses: Atlantic and modified Atlantic Waters (AW and mAW), Western and Eastern Intermediate Waters (WIW and EIW), Tyrrhenian and Western Mediterranean Deep Water (TDW and WMDW). The reanalysis retains the known mean characteristics of the water masses, their seasonal to interannual variabilities, and main circulation patterns when compared with the literature. The imprint of winter mixing is particularly obvious with coherent variations of water mass volumes, mainly the yearly creation of WIW from mAW on northernmost shelves and of WMDW from all surface and intermediate layers during years of deep water formation (DWF). The results also highlight some unrealistic events of variability of the WMDW volume that are likely due to the data assimilation process. Recomputation of water mass volumes and transports without these altered years allowed to highlight the possible disruption of large-scale barotropic cyclonic circulation in the East Algerian basin in response to major DWF events over the Gulf of Lion and the induced surface consequence on Algerian Eddies' trajectories.