Journal of Geophysical Research-Oceans最新文献

Estimation of sea surface upward longwave radiation (SULR) are generally dependent on sea surface temperature (SST), in accordance with the Stefan-Boltzmann law. The consensus is that the inaccuracy in estimating SST contributes to bias in the estimation. This study proposes the incorporation of the spray layer in estimating upward longwave radiation, as when the sea surface is covered by the spray layer, it becomes the primary source of longwave radiation emitted to the atmosphere. Analysis of ERA5 data suggests that the bias in sea SULR is associated with the temperature of the sea spray layer and the whitecap coverage. Incorporating the spray layer in the estimation of sea SULR results in a decrease of the overestimation in ERA5 by over 15%. The revised scheme, implemented in CESM2.1.3, led to an average percentage bias improvement around 16%. The percentage bias improvement in localized areas of the southeastern Pacific and the southeastern Indian Oceans reaches up to 40%. The regions where bias has been effectively reduced are mainly found within latitudes of 30°, and the negative bias in the Southern Ocean is increased. This study offers a novel insight into the errors in estimating sea SULR.

The transformation of internal waves on continental shelves is important to mass transfer, nutrient replenishment, and heat transfer. Yet, the transfer of energy from larger to smaller scale or between nonlinear internal waves (NLIW) themselves remains poorly understood. We present 1 month of through water column observations of temperature and currents on the southeast continental shelf of the Bay of Biscay, a region where internal wave dynamics have never been described. Over the shallower part of the shelf, a relatively strong baroclinic dynamic exists, with the mode-1 internal tide and NLIW generating currents more than three times the barotropic tide. The nature of these features varied greatly over the subtidal timescales, which we correlate to wind-driven currents and the associated modulation of background stratification. In addition to the well-documented processes of internal tide steepening and NLIW polarity reversal, we present novel observations of colocated elevation and depression (termed opposite polarity) NLIW. While this colocation has been previously studied theoretically, it has not been described with in situ observations to date. In agreement with theory, we observed these waves when the wind-driven dynamics resulted in double pycnocline stratification. We found that the collocated waves of depression and elevation propagate independently on the upper and lower pycnocline, respectively. We use direct estimates of wave speed to infer the potential for interaction between waves of opposite polarity and discuss the potential relevance for other regions worldwide where double pycnocline background conditions are observed.

Internal solitary waves (ISWs) typically exhibit a strong tendency to preserve their waveforms and amplitudes upon collision, but this preservation breaks down when two ISWs obliquely interact. Although oblique wave-wave interactions have been frequently observed in the oceans worldwide and studied extensively through theoretical research and satellite imagery, the fundamental underwater dynamics and spatiotemporal evolution of these interactions remain poorly understood. This study presents a detailed investigation of oblique ISW interactions in the southern Andaman Sea, integrating long-term in situ mooring data with high-resolution 3D numerical simulations. The observations show that when two ISW packets interact obliquely, they merge into a new, chaotic waveform with significant disruption to the horizontal velocity field. The resulting merged ISW is asymmetric, narrow, and steep, exhibiting a particular velocity structure with enhanced current shears. It is worth noting that the amplitude (energy) of the merged ISW is approximately 25% (79%) larger than the combined amplitude of the two incident ISWs, highlighting the strong nonlinearity that has been long proposed by previous theoretical studies. Theoretical analyses suggest that the observed interaction is strong and non-phase-conserving, and the resulting merged ISW is a part of an evolving Mach stem. Additionally, model simulations indicate that ISWs originating from three distinct source regions interact in multiple ways, each producing unique variations in crest patterns and wave intensity, which is far more complex than earlier theoretical predictions. This study underscores the critical role of oblique wave-wave interactions in altering the 3D characteristics of ISWs, including both their underwater dynamics and along-crest variability.

The Bay of Bengal is a dynamic region that experiences intense freshwater runoff, extreme meteorological events, and seasonally reversing surface currents. The region is particularly susceptible to anthropogenic climate change, driven in part by large air-sea fluxes, persistent freshwater stratification, and low overturning rates. Predicting how this system is likely to change in the future is paramount for planning effective adaption and mitigation strategies. Using a relocatable, coupled physics-ecosystem regional coastal ocean model (NEMO-ERSEM), we investigate potential future changes in surface circulation and coastal nitrate pathways around the coast of the Bay of Bengal from 1980 to 2060, using a “business-as-usual” climate change scenario. We find that future surface currents are reduced in the northern Bay of Bengal(summer) and strengthened in the southern Bay of Bengal (fall). Coastal nitrate transports mirror this asymmetric change and decrease by as much as 14% in the northern Bay of Bengal, perpetuating a positive feedback loop whereby the northern Bay of Bengal becomes progressively fresher and more nutrient-rich, strengthening surface stratification and increasing the risk of toxic algal blooms and eutrophication events. Conversely, in the southern Bay of Bengal, coastal nitrate transports increase by 52% that promotes localized diatom blooms despite reduced regional river runoff. This work highlights the need for more rigorous scenario testing in the region and presents new challenges for mitigating the impact of anthropogenic climate change across South Asia.

This study aimed to examine the variations in vorticity in the northern South China Sea (NSCS) resulting from the cut-off of the Kuroshio main path east of Taiwan Island. Absolute dynamic topography and satellite altimeter eddy tracking data from 1993 to 2021 were employed to analyze the momentum ratio of the Kuroshio and mesoscale eddies concerning Kuroshio cut-off events through their interaction. The results revealed 12 events where mesoscale cyclonic eddies east of Taiwan Island cut off the Kuroshio main path, resulting in intensified loop currents in the Luzon Strait and increased negative vorticity in the NSCS. Six of these events occurred between 1998 and 2013, coinciding with the negative Pacific Decadal Oscillation (PDO) regime. During this period, negative vorticity triggered by Kuroshio cut-off events rose by 79% in the NSCS, compared to 34% during the positive PDO regime. The Kuroshio interacted with mesoscale cyclonic eddies east of Taiwan for 28 days during the negative PDO regime and 41 days during the positive PDO regime. The Kuroshio velocity strengthened east of Luzon Island but weakened east of Taiwan Island during the negative PDO regime, elucidating the observed disparity. The average duration between the occurrence of mesoscale cyclonic eddy impinging on the Kuroshio east of Taiwan Island and the peak negative vorticity observed in the NSCS was approximately 28 days. This was comparable to data obtained using field measurements.

Transformation of light to dense waters by atmospheric cooling is key to the Atlantic Meridional Overturning Circulation in the Subpolar Gyre. Convection in the center of the Irminger Gyre contributes to the formation of the densest waters east of Greenland. We present a 19-year (2002–2020) weekly time series of hydrography and convection in the central Irminger Sea based on (bi-)daily mooring profiles supplemented with Argo profiles. A 70-year annual time series of shipboard hydrography shows that this mooring period is representative of longer-term variability. The depth of convection varies strongly from winter to winter (288–1,500 dbar), with a mean March mixed layer depth (MLD) of 470 dbar and a mean maximum density reached of 27.70 ± 0.05 kg m⁻³. The densification of the water column by local convection directly impacts the sea surface height in the center of the Irminger Gyre and thus large-scale circulation patterns. Both the observations and a Price-Weller-Pinkel mixed layer model analysis show that the main cause of interannual variability in MLD is the strength of the winter atmospheric surface forcing. Its role is three times as important as that of the strength of the maximum stratification in the preceding summer. Strong stratification as a result of a fresh surface anomaly similar to the one observed in 2010 can weaken convection by approximately 170 m on average, but changes in surface forcing will need to be taken into account as well when considering the evolution of Irminger Sea convection under climate change.

Based on the novel energetic analysis tools, namely multiscale window transform and multiscale energy and vorticity analysis, this study investigates the seasonal variability of eddy kinetic energy (EKE) in the greater Agulhas Current system (GACS), which is divided into three subdomains based on the horizontal structures of background flows: the Mozambique Channel (MZC) subdomain, the northern Agulhas Current (NAC) subdomain, and the Agulhas retroflection (ARF) subdomain. Results show that the seasonality of EKE is spatially inhomogeneous. In the MZC subdomain, the EKE is strongest in spring and weakest in autumn, whereas in the NAC and ARF subdomains, the EKE level is highest in summer and lowest in winter. In all the three subdomains, the seasonal cycle of the barotropic instability of the mean flow corresponds well with that of the EKE. In contrast, the nonlocal transportation that mainly works on the redistribution of EKE is out of phase with the seasonality of EKE. Regarding the local wind forcing and baroclinic instability, they both have weak impacts on the EKE evolution, contributing power only about 10% of the barotropic instability. Moreover, neither of their seasonal cycles is consistent with the seasonality of EKE. Therefore, it is the barotropic instability of the mean flow that controls the seasonal variability of the EKE in the GACS.

The distribution of helium isotopes in the upper kilometer of the water column along the GP15 section in the central Pacific reflects the large-scale patterns of upwelling hydrothermal ³He in the tropics and sub-polar gyre, tracing two important pathways whereby bottom water exits from the deep Pacific. Heavy noble gas saturation anomalies, particularly in the upper two hundred meters of the water column, are more strongly increased by seasonal radiative heating, while lighter noble gas saturation anomalies are increased more by air injection processes. A similar, seasonally persistent radiative heating feature was observed in the Equatorial Undercurrent, and appears to be replicated in climate system model simulations. The origin of this feature, however, remains a mystery. A heuristic component model explains the noble gas saturation anomaly distributions, separating the influences of air injection, barometric pressure and radiative heating/cooling. Results show cohesive spatial patterns consistent with where water masses originate, their circulation, and gas exchange dynamics in relation to their formation regions. Using this model, we diagnose the distribution of “non-atmospheric” ⁴He in shallow waters, which parallels the helium isotope anomaly and silica distributions.

The Kuroshio intrusion from the Luzon Strait significantly affects ecosystems in the South China Sea (SCS), especially during the Northeast Monsoon, a time when field observations are notably sparse and where vertical mixing induced by strong winds can obscure the effects of the Kuroshio intrusion. In this study, we address these gaps by reanalyzing data from 20 cruises (5,067 samples) in the SCS between 2004 and 2015. We also carried out two dedicated field cruises during the Northeast and the Southwest Monsoon in 2018. Field observations from both cruises revealed a consistent unimodal relationship between total chlorophyll a (Chla) concentrations in the upper 50 m of the water column and the index of the Kuroshio intrusion. Specifically, a strong Kuroshio intrusion during the Northeast Monsoon significantly enhanced Chla concentrations in the northern SCS. This enhanced Chla concentration during the Northeast Monsoon was primarily driven by increases of Synechococcus and nanophytoplankton that contrasted with the dominance of Prochlorococcus during the Southeast Monsoon. Long-term remote sensing data corroborated these findings and demonstrated a consistent pattern wherein intrusion by the Kuroshio led to elevated Chla concentrations, particularly during the Northeast Monsoon. There was a significant positive correlation between the intensity of the Kuroshio intrusion and the magnitude of the Chla increase. Furthermore, these findings suggested a concerning possibility: weakening of the Kuroshio intrusion intensity over time might diminish future biogeochemical effects on SCS ecosystems. Continued monitoring and research will be crucial to understanding and responding to these changes.

Recent work has revealed the presence of an offshore near-surface plume of dissolved trace elements in the South Atlantic Ocean (SAO). Dissolved Fe (dFe) supply from the Congo plume is equivalent to ∼40% of the annual atmospheric dFe supply to the SAO. However this plume is not captured by biogeochemical models, raising questions about its exact sources. To help understand the potential source mechanisms, we use particle tracking experiments to investigate elemental distributions. Results suggest that elevated concentrations of some elements in the Congo plume are primarily sourced from river discharge and wet atmospheric deposition with minimal influence from shelf sediments. River discharge is the main source in shelf regions and some off-shelf regions, whereas atmospheric deposition dominates the area to the southwest of the Congo River outflow. A quantitative analysis along 3$��°�$S specifically for dFe suggests a decrease in the contribution of river discharge from 90% to 30% moving off-shelf, with a corresponding increase in the contribution of atmospheric deposition. Within the shelf zone, atmospheric deposition accounts for roughly 20%–40% and could be a major source of dFe around the river mouth. Integration of data from cruise GA08 reinforces the finding that wet deposition augments the concentrations of dFe, manganese (dMn), and cobalt (dCo) at distances over 1,000 km from the river mouth. Given present-day patterns of nitrate, Fe, and Co limitation for primary producers in the SAO, changing rainfall patterns may have long-term implications for both regional elemental budgets and ecologically dependent processes sensitive to trace element ratios.