Journal of Geophysical Research-Oceans最新文献

Based on the coupled conditional nonlinear optimal perturbation (C-CNOP) method, this study explores the season-dependent predictability barrier (PB) affecting the forecasts of two types of El Niño (central Pacific, CP; eastern Pacific, EP) events by using CMIP6 models. It is found that CP (EP) El Niño forecasts often occurs summer (spring) PB, and only powerful season-dependent PB can lead to large prediction errors. Further investigating the initial causes of the largest prediction errors and strongest PB, we find that the spatial pattern of initial errors consistently exhibits the sea temperature anomaly dipole of east positive–west negative in the equatorial Pacific, and errors over upper layers of North (South) Pacific are similar to the negative Victoria mode (South Pacific Meridional Mode). Physically, the mode evolution of initial errors in the equatorial Pacific, North and South Pacific are all positive feedback processes, which together ultimately lead to large cold biases over the central-eastern (CP) or eastern (EP) equatorial Pacific in December. Analysis shows that the initial error mode of North Pacific mainly affects the cold bias of the central Pacific, whereas the mode of South Pacific mostly controls the bias in the eastern Pacific. These initial error modes found in this study can have more serious impacts on forecasts of two types of El Niño events than that in previous studies. The results of this study offer valuable scientific guidance for the adaptive observation of ENSO, which will likely be able to maximize the prediction skills for two types of El Niño events.

The northern East China Sea is one of the Large Marine Ecosystems, which experiences the fastest ocean warming across the globe. With the anticipation of stimulated Tsushima Warm Current (TWC) transport and rapid ocean warming, the importance of picoplankton will increase in this ecosystem. Thus, the spatiotemporal distribution of picoplankton was investigated in the northern East China Sea, where variability of TWC can modulate temperature and stratification. High contributions of warm and less saline Tsushima Surface Warm Water (TSWW) increased water temperature and stratification. Picoplankton were responsible for 75% of the relative contribution of chl a. PicoC:PicoChla ratios increased with distance from the coasts and fluctuated with depth, indicating that an elevated contribution of Tsushima Warm Water diminished the photosynthetic activity of picoplankton. Synechococcus I population decreased with salinity. High contribution of TSWW increased Prochlorococcus and Synechococcus II population, related with stratification and rising temperature driven by TSWW, respectively. Picoplankton-derived carbon contents were mostly attributed either to picoeukaryotes or to Synechococcus depending on the TSWW contribution. This suggests that the rising TSWW contribution can intensify the role of Synechococcus in picoplankton-derived organic matter in the northern East China Sea.

Land-fast sea-ice (fast ice) in McMurdo Sound grows through heat loss to the atmosphere and through heat loss to the ocean due to the presence of supercooled water. In this paper, we present a fast-ice thickness data set covering 1986–2022, providing a baseline of interannual variability in fast-ice thickness. Fast ice thicknesses are related to atmospheric and oceanic drivers on monthly and seasonal timescales to provide one of the longest timeseries of drivers of interannual fast-ice thickness variability from high-quality, in situ observations. We select a 14 km by 20 km area of level fast-ice over which atmospheric and oceanic drivers have negligible spatial variation, allowing us to resolve temporal variability in drivers and thickness. A statistical significance testing approach is adopted which only considers drivers that have a plausible physical mechanism to influence fast-ice thickness. We demonstrate that the fast-ice cover in McMurdo Sound is thicker in years when surface air temperature is colder, average (southerly) wind speed is higher, and there are fewer southerly storms. Nonetheless, we show that monthly averaged drivers have limitations and often do not produce strong correlations with thickness or fast-ice persistence. Consequently, most of the variability in fast-ice thickness cannot be explained by a single driver. No long-term trend in fast-ice thickness was found in eastern McMurdo Sound, thickness being influenced by a combination of drivers. Future event-based analyses, relating storms to fast-ice persistence, are needed. The present study provides a baseline against which these extreme events and long-term trends can be assessed.

A recent study using the first 21 months of the OSNAP time series revealed that the export of dense waters in the eastern subpolar North Atlantic―as part of the Atlantic Meridional Overturning Circulation (MOC)―can be almost wholly attributed to surface-forced water mass transformation (SFWMT) in the Irminger and Iceland basins, thus suggesting a minor role for other means of transformation, such as diapycnal mixing. To understand whether this result is valid over a period that exceeds the current observational record, we use four different ocean reanalysis products to investigate the relationship between surface buoyancy forcing and dense water production in this region. We also reexplore this relationship with the now available 6-year OSNAP time series. Our analysis finds that although surface transformation in the eastern subpolar gyre dominates the production of deep waters, mixing processes downstream of the Greenland Scotland Ridge are also responsible for the production of waters carried within the AMOC's lower limb both in the observations and reanalyses. Further analysis of the reanalyses shows that SFWMT partly explains MOC interannual variability, the remaining portion can be attributed to basin storage and mixing. Compared to the observations, the reanalyses exhibit stronger MOC variance but comparable SFWMT variance on interannual timescales.

River plumes are crucial in transporting terrestrial materials from rivers to oceans. Knowledge gaps, however, still exist in understanding the transport pathway and the ultimate fate of riverine water in coastal oceans. This study conducted a 50-year climatological numerical simulation to investigate the long-term transport processes of Changjiang River water in the East Asian shelf seas. The Changjiang River water exhibits distinct seasonal patterns near the estuary mouth and in the coastal area south of the estuary, and it tends to be retained within the shelf seas, which influences its far-field transport. The Changjiang River water takes less than 1 year to reach the eastern shelf edge of the East China Sea and over 12 years to enter the Bohai Sea. The Kuroshio current impedes the cross-shelf transport of Changjiang water, with water in the Kuroshio region over 6 years old. The Taiwan Warm Current not only acts as a barrier that regulates the pathways of Changjiang River water but also serves as an important conduit for water exiting the East China Sea. The Changjiang River water leaves the estuary through four branches, forming eight major transport pathways in the Yellow and East China Seas. Approximately 85% of Changjiang River water flows through the Tsushima/Korea Straits, about 14% exits from the shelf edge of the East China Sea, and less than 1% passes through the Taiwan Strait. The results underscore the importance of water renewal and shelf circulation in the long-term transport of river water within coastal oceans.

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.