Journal of Geophysical Research-Oceans最新文献

The bathymetry of the northern Gulf of Mexico is strongly influenced by diapirism of subsurface salt. A competition between salt dynamics and the depositional mechanics of sediment laden density flows over geological timescales controls the scale of seafloor depressions, which are the dominant bathymetric features of the margin. Salt domes create topographic highs, and salt removal to the domes creates topographic lows, with sediment deposition driving the gravitational dynamics. The strength of bathymetric self-organization into depressions is inferred through analysis of a vast bathymetric data set made public by the U.S. Bureau of Ocean and Energy Management. Depression geometric scales follow Pareto distributions, and their tail indexes aid inference of the strength of bathymetric self-organization, with lower tail indexes linked to greater self-organization. A comparison is made of margin subregions defined by pseudo-flow drainage density maps, which inversely relates to the pre-deformation thickness of subsurface salt. Tail indexes of distributions decrease with the thickness of the underlying salt. This is linked to the merger of depressions, which is enhanced when depressions can grow wider and deeper, as occurs over thick salt fields, and the development of salt structure. The manner of self-organization results in most of the margin's ponded sediment accommodation residing in relatively few depressions that have reliefs exceeding 100 m. This relief is sufficient to induce sedimentation from even the thickest turbidity currents, which can further drive gravitational dynamics. The bathymetric complexity of depressions is also greatest over regions with the thickest salt, further supporting enhanced self-organization.

Operational wave forecasts for the Great Lakes originate from the NOAA Great Lakes Waves Unstructured version 2 system. The model uses a simple ice blocking (IC0) parameterization for ice-wave damping, so ice-covered portions of the lakes are treated as land in the modeling system. Although simple and effective, the simple block can impede forecasting by eliminating wave forecast guidance from areas with thin or partial ice cover. We evaluate 12 ice-wave damping parameterizations within WAVEWATCH III (WW3, version 6.07.1) for Lake Erie, by comparing model results against wave observations made at several locations using moored acoustic wave and current profilers during the winters of 2010–2011 and 2012–2013. The comparisons show that the IC4M4 module performs the best among 12 ice modules with a root mean square error (RMSE) of 0.32–0.39 m and a root bias of −0.06 to −0.11 m, outperforming the existing IC0 parameterization (RMSE: 0.46–0.59 m; bias: −0.23 to −0.34 m) during the 2010–2011 analysis year. WW3 ice modules are mostly derived from measurements and studies of the Arctic and Antarctic Ocean. The dominant wave frequency is about 0.05∼0.10 Hz in the Arctic Ocean compared to 0.15∼0.2 Hz in the lake. Thus formulas built on frequency based on the studies from deep oceans may not be suitable for the shallow lakes because they cause too much damping. Although the IC4M4 ice module is from the study of the Antarctic Ocean , the wave attenuation formula based on incoming wave height is also suitable for Lake Erie.

A thermohaline staircase detection algorithm, applied to mooring and ice-tethered profiler data, systematically assessed the variability of fine-scale, diffusive-convective staircase abundance in the Arctic Ocean thermoclines in 2004–2023. Over that period, staircase occurrence statistically decreased in both the Amerasian and Eurasian basins, with thinner, shallower staircase layers preferentially decreasing over the Eurasian Basin's slope. In stark contrast to the Amerasian Basin, seasonality of detected staircase occurrence was pronounced in the Eurasian Basin and appeared to be increasing. Interannual and long-term variability of detectable staircase abundance and background thermocline density stratification were correlated, negatively so in the Amerasian Basin and positively in the Eurasian Basin, indicating reversed sensitivities of staircase constructive and destructive processes to stratification. Seasonal and long-term staircase variabilities in both basins were consistent with known environmental contrasts and tendencies, including upper freshening of the stronger, thicker Amerasian Basin halocline, the shift toward deeper winter ventilation of the weaker Eurasian Basin halocline, and more near-surface velocity shear over the Eurasian Basin's slope. There is no reason to believe that climate change will stop anytime soon, and we have good cause to believe that the observed tendencies in staircase structure will persist.

Wind stress deviates both in magnitude and direction under light wind conditions due to the modulation effect of swell waves. Eddy covariance flux measurements were conducted from an offshore wind tower in the northern South China Sea to investigate the influence of swell-induced perturbations on the evolution of wind velocity fluctuations and wind stress reductions. Direct evidence of the wind field being affected by dominant swell waves was observed at an observational height of 20 m, indicating that swell-induced perturbations can penetrate the wave boundary layer to at least this height. Time series analysis implies that a stable atmosphere helps preserve the observed prominent peak in wind velocity spectra, extending it to a higher range under large wave age. Therefore, swell-induced perturbations are more easily observed at night. Additionally, our results show that swell wave modulation on wind stress is not merely restricted to moments with pronounced peaks in velocity spectra, indicating that wind stress reduction is not an exception. The most significant reduction occurs at wind speeds around 2–4 m/s, leading to negative wind stress, which implies momentum transfer from the ocean to the atmosphere. Our conclusion illustrates that the deviation of wind stress magnitude increases under stable atmospheric conditions; however, with a simple correction model, the modified wind stress is comparable to the observed values.

The Congolese upwelling system (CoUS), located along the West African coast north of the Congo River, is one of the most productive and least studied systems in the Gulf of Guinea. The minimum sea surface temperature in the CoUS occurs in austral winter, when the winds are weak and not particularly favorable to coastal upwelling. Here, for the first time, we use a high-resolution regional ocean model to identify the key atmospheric and oceanic processes that control the seasonal evolution of the mixed layer temperature in a 1°-wide coastal band from 6°S to 4°S. The model is in good agreement with observations on seasonal timescales, and in particular, it realistically reproduces the signature of the surface upwelling during the austral winter, the shallow mixed layer due to salinity stratification, and the signature of coastal wave propagation. The analysis of the mixed layer heat budget for the year 2016 reveals a competition between warming by air-sea fluxes, dominated by the incoming shortwave radiation throughout the year, and cooling by vertical mixing at the base of the mixed layer, as other tendency terms remain weak. The seasonal cooling is induced by vertical mixing, where local wind-driven dynamics play a secondary role compared to subsurface processes. A subsurface analysis shows that remotely forced coastal-trapped waves raise the thermocline from April to August, which strengthens the vertical temperature gradient at the base of the mixed layer and leads to the mixing-induced seasonal cooling in the Congolese upwelling system.

Properties of the Fraser River plume in the Strait of Georgia, BC, are significantly influenced by the tide. However, the dynamics and magnitude of this tidal influence on the plume area are not known. Here, we use 17 years of daily satellite observations of suspended particulate matter to understand the tidal variability of the plume area. A consistent inverse relationship between the Fraser River plume area and the tidal elevation with a phase lag of 1–2 hr is revealed from two independent analyses: one by correcting for temporal aliasing and extracting tidal signal from the whole image set and the other using only same-day image pairs. The plume area increases/decreases by about 20% after ebb/flood tides, and a lower river discharge typically leads to a more dramatic tidal variation in the plume area. A simple analytical model based on the volume conservation and salinity balance equations is developed to analyze the mechanism of the tidal variability in the plume size. The observed tidal patterns of the plume area are largely reproduced using tidally modulated plume salinity (observed from instrumented ferries) and river discharge (from numerical model outputs). Tidal flux in both river discharge and entrainment rate is found to be important in explaining plume area variability.

In this study, we examine intensive observational measurements from a 12-glider array in the South China Sea, and reveal that tropical cyclone “Haitang” created the conditions for the development of several types of forced submesoscale instabilities within a mesoscale anticyclonic eddy. The anticyclonic eddy shed from the Kuroshio loop current in the Luzon Strait and propagated toward the South China Sea. Fine-scale temperature and salinity observation from gliders captured the complex mesoscale frontal structure induced by mesoscale strain around anticyclonic eddy (AE). Various favorable conditions for submesoscale instabilities show significantly different spatial distributions as well as temporal evolution characteristics in the AE. Analyses indicate that the occurrence probability of forced symmetric instability (SI) and gravitational instability (GI) during the tropical cyclone (TC) period (∼5 days) is found to be 2 times higher than that during the non-TC period (∼25 days). Heat loss creates conditions for GI in the upper part of the negative potential vorticity (PV) layer, and GIs tend to be distributed inside the eddy. Strong wind stress induced by the TC promotes the injection of negative PV through cross-front Ekman buoyancy flux, leading to the occurrence of SI at the edge of the eddy. During the TC, stable wind fields are more favorable for the development of submesoscale instability compared to rotating wind fields. The effect of strong winds breaks the normal diurnal cycle of SI, creating conditions for active submesoscale instabilities at midday. These findings help us to understand submesoscale air-sea interaction processes.

The advent of under-ice profiling float and biologging techniques has enabled year-round observation of the Southern Ocean and its Antarctic margin. These under-ice data are often overlooked in widely used oceanographic datasets, despite their importance in understanding seasonality and its role in sea ice changes, water mass formation, and glacial melt. We develop a monthly climatology of the Southern Ocean (south of 40°S and above 2,000 m) using Data Interpolating Variational Analysis, which excels in multi-dimensional interpolation and consistent handling of topography and horizontal advection. The climatology successfully captures thermohaline variability under sea ice, previously hard to obtain, and outperforms other observation-based products and state estimate simulations in data fidelity, with smaller root-mean-square errors and biases. To demonstrate its multi-purpose capability, we present a qualitative description of the seasonal variation, including (a) the surface mixed layer, (b) the water mass volume census, (c) the Antarctic Slope Front, and (d) shelf bottom waters. The circumpolar variation in the extent of dense shelf water—including its presence outside the four major formation sites—and the annual volume overturning that reaches deep waters are revealed for the first time. The present work offers a new monthly climatology of the Southern Ocean and the Antarctic margin, which will be instrumental in investigating the seasonality and improving ocean models, thereby making valuable winter observations more accessible. We further highlight the quantitative significance of under-ice data in reproducing ocean conditions, advocating for their increased use to achieve a better Southern Ocean observing system.

The Southern Ocean spring phytoplankton bloom impacts regional food webs and the marine carbon cycle, but we do not fully understand which drivers—environmental, ecological, or biological—control the timing of the onset of the spring bloom. Nutrients, particularly iron, are likely replete in the austral winter, but the importance of underwater light availability and grazing pressure are topics of ongoing discussion. Furthermore, in the extreme polar winter, phytoplankton physiology may impart additional constraints on the bloom onset. We analyzed biogeochemical (BGC) Argo profiles from the Pacific sector of the Southern Ocean, and a one-dimensional water column turbulence model forced by reanalysis data. Though the surface mixed layer defines where density is homogenous, the presence of enhanced turbulence and the active mixing of constituents, such as chlorophyll fluorescence, is better estimated by the depth of active mixing that we estimate from the turbulence model. We identified two regimes: one north of the subantarctic front where bloom onsets occur around July, before the seasonal maximum in mixing depth and when light availability remained near its annual minimum value. It is likely that changes in the phytoplankton loss rate control the bloom onset in this region. South of the subantarctic front, bloom onsets occur closer to austral spring following enhanced light availability, suggesting that bloom onset is primarily controlled by phytoplankton growth rather than loss terms. Our analysis shows that new insights can be gained into spring bloom phenology from the combination of float and model data.

The interannual relationships between the cross-equatorial cell (CEC) in the Indian Ocean (IO) and the El Niño-Southern Oscillation (ENSO) are examined through observational data and numerical simulations. The findings indicate a notable intraseasonal variation in the boreal summer IO CEC responses to preceding ENSO events, showing a weakening in early summer (May–June) and a strengthening in late summer (August–September) subsequent to an El Niño events. These contrasting responses are primarily driven by opposite meridional Ekman transport anomalies, characterized by anomalous northward Ekman transport in early summer and southward transport in late summer. Further analysis reveals that ENSO-induced surface zonal wind anomalies predominantly influence these anomalous meridional Ekman transport in the upper layer, accounting for over 80% of the variation. In early summer, an antisymmetric wind pattern over the IO, induced by El Niño in the decaying spring, along with the westward extension of the anomalous western North Pacific anticyclone (WNPAC), generates anomalous easterlies over the North IO (NIO), leading to weakened meridional Ekman transport and a diminished CEC. Simultaneously, those anomalous easterlies and the associated weakened CEC in early summer trigger a wind-CEC-SST (WCS) negative feedback mechanism. The resulting anomalous northward transport and reduced upwelling lead to sea surface temperature (SST) warming in the northern NIO, creating a northward surface ocean temperature gradient in the July–August period. This gradient, along with the eastward retreat of the WNPAC, causes anomalous NIO westerlies in late summer, enhancing meridional Ekman transport and ultimately strengthening the CEC.