Publication

The Atlantic meridional overturning circulation is an important component of the climate system because of its role in the heat transport. Its strength is sensitive to the surface density but mechanisms of the effect of Southern Ocean freshwater anomalies are relatively unknown. Here, we investigate the impact of Antarctic ice sheet meltwater on the Atlantic meridional overturning circulation using an earth system model of intermediate complexity. The meltwater over the Pacific sector of the Southern Ocean is transported to the east and, after passing the Drake Passage, travels northward reaching the North Atlantic. Furthermore, Southern Ocean cooling induces a northward shift of the Intertropical Convergence Zone, leading to more precipitation in the tropical Atlantic. Consequently, the reduced salinity weakens the Atlantic meridional overturning circulation. Additional experiments, in which the duration period of freshwater hosing was varied while keeping its total amount constant, indicate that the amplitude and the duration of the meltwater play crucial roles in determining the degree of reduction in Atlantic meridional overturning circulation.

The k-means clustering of reanalysis datasets is used to classify the intrinsic atmospheric circulation in Asia based on the daily geopotential height of 500 hPa during the boreal summer (June, July and August) for 1958–2020. Among the total clusters of atmospheric circulation patterns in Asia, two distinct clusters of atmospheric circulation are characterized by a significant increasing trend in occurrence. They correspond to different zonal wave numbers (1–2 vs. 3–4) of atmospheric circulation in the mid-to-high latitudes of the Northern Hemisphere, and their associated sea surface temperature structures are not the same in the tropical Pacific and the North Atlantic. Further analysis indicates that two clusters of atmospheric circulation alter the amount of summer monsoon precipitation in East Asia, and an increasing amount of precipitation in southern China in recent decades could be explained by the combined effect of an increasing trend in the frequency of occurrence of two clusters of atmospheric circulation. Finally, a long-term simulation of the Community Earth System Model version 2 suggests that anthropogenic forcing may be responsible for the increasing trend of occurrence of the two atmospheric circulations in Asia.

We investigate the influence of tides on the exchange of water between the Arabian Gulf and the Sea of Oman through the Strait of Hormuz using a high-resolution numerical model. Two numerical simulations are contrasted, one with and one without tidal forcing. We find that tides suppress exchange through the Strait, by ∼20% in the annual mean, being largest in the summer (∼30%) and diminishing in the winter (∼13%). Tides enhance the parameterised shear-driven vertical mixing inside the Gulf and Strait, mixing warm, relatively fresh surface waters downward thus reducing the density of bottom waters flowing outwards. This reduces the lateral difference of density between Gulf and Sea of Oman and hence the exchange through the Strait. Maximum reductions occur in summer when both the vertical stratification and mixing is the largest.

The tropical climate variabilities, such as Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO), are accompanied by changes in the tropical deep convection which can influence the atmospheric circulation in the Southern Hemisphere (SH). To investigate each role of IOD and ENSO in the September-November (SON) circulation, we examine teleconnection patterns associated with IOD and ENSO events using the ERA5 monthly averaged data from 1979 to 2020. Our approach is to calculate the power spectral density (PSD) of the sea level pressure (SLP) and meridional wind and geopotential height at 300 hPa that are decomposed by zonal wave numbers (ZWNs), and to compute their correlations with IOD and ENSO at each latitudinal band. The main results are that IOD (ENSO) is negatively (positively) correlated with PSDs of ZWN2 and ZWN3 (ZWN1) at 300 hPa in the SH middle latitudes. Considering the Rossby wave train, IOD (ENSO) considerably affects the variability of the ZWN3 (ZWN1) pattern, which influences the meridional exchange of momentum. Additionally, the relationship between IOD and ZWN3 has become tighter in recent years, which is not seen in that with ENSO. The IOD and ENSO events also modify the SLP patterns and meridional surface winds, modulating the sea ice extent in the Southern Ocean. During the highly positive 2019 IOD event, the variability of the middle latitudes atmospheric circulation was considerably larger than climatology, suggesting a higher chance of more extreme weather patterns associated with more frequent intense IOD events in the warming climate.

Mesoscale eddies are prevalent throughout the global ocean and have significant implications on the exchange of heat, salt, volume, and biogeochemical properties. These small-scale features can potentially influence regional and global climate systems. However, the effects of climate change on ocean eddies remain uncertain due to limited long-term observational data. To address this knowledge gap, our study focuses on examining the impact of greenhouse warming on surface mesoscale eddy characteristics, utilizing a high-resolution climate simulation project. Our model experiments provided valuable insights into the potential effects of greenhouse warming on mesoscale eddies, suggesting that mesoscale eddies will likely become more frequent under greenhouse warming conditions and exhibit larger amplitudes and radii, especially in regions characterized by strong ocean currents such as the Antarctic Circumpolar Current and western boundary currents. However, a distinctive pattern emerged in the Gulf Stream, with increases in eddy occurrence and radius and significant decreases in eddy amplitude. This phenomenon can be attributed to the relationship between eddy lifespans and their properties. Specifically, in the Kuroshio Current, the amplitude of eddies increased due to the increased occurrence of long-lived eddies. In contrast, in the Gulf Stream, the amplitude of eddies decreased significantly due to the decreased occurrence of long-lived eddies. This distinction arises from the fact that long-lived eddies can accumulate more energy than shorter-lived eddies throughout their lifetime. These findings provide valuable insights into the complex dynamics of mesoscale eddies in a warming world.

Recent changes in the Arctic sea-ice are strongly influenced by the recent increase in heat transport from vigorous Atlantic inflows, so-called Atlantification. This Atlantification can induce physical and ecological changes near the Atlantic gateway. Here, we used the observational data sets and 26 Earth system models to estimate Atlantic water intrusion, and firstly suggest the impact of Atlantification on marine productivity in the Barents Sea in a warming climate, especially on boreal spring. In a warming climate, the heat transport across the Barents Sea Opening (BSO) is projected to be enhanced (45.5 ± 34.9 TW) by the end of the 21st century compared to the present climate. This poleward intrusion of the Atlantic water is likely to increase productivity with the largest increase in spring (70%). In a warming climate, the productivity is enhanced by Atlantification-induced changes in physical states—ocean temperature, circulations, stratification, and sea-ice. Based on inter-model analyses, we estimated that the Atlantification can explain approximately 26% of the productivity changes in the Barents Sea. Thus, Atlantification is critical for future changes in biological productivity and physical states over the Arctic Ocean.

Marine heatwaves (MHWs), referring to anomalously high sea surface temperatures, have drawn significant attention from marine scientists due to their broad impacts on the surface marine ecosystem, fisheries, weather patterns, and various human activities. In this study, we examined the impact of the distribution of Changjiang diluted water (CDW), a significant factor causing oceanic property changes in the East China Sea (ECS) during the summer, on MHWs. The surface salinity distribution in the ECS indicates that from June to August, the eastern extension of the CDW influences areas as far as Jeju Island and the Korea Strait. In September, however, the CDW tends to reside in the Changjiang estuary. Through the Empirical Orthogonal Function analysis of the cumulative intensity of MHWs during the summer, we extracted the loading vector of the first mode and its principal component time series to conduct a correlation analysis with the distribution of the CDW. The results revealed a strong negative spatial correlation between areas of the CDW and regions with high cumulative intensity of MHWs, indicating that the reinforcement of stratification due to low-salinity water can increase the intensity and duration of MHWs. This study suggests that the CDW may still influence the spatial distribution of MHWs in the region, highlighting the importance of oceanic environmental factors in the occurrence of MHWs in the waters surrounding the Korean Peninsula.

Accurate representation of the Atlantic Meridional Overturning Circulation (AMOC) in global climate models is crucial for reliable future climate predictions and projections. In this study, we used 42 coupled atmosphere–ocean global climate models to analyze low-frequency variability of the AMOC driven by the North Atlantic Oscillation (NAO). Our results showed that the influence of the simulated NAO on the AMOC differs significantly between the models. We showed that the large intermodel diversity originates from the diverse oceanic mean state, especially over the subpolar North Atlantic (SPNA), where deep water formation of the AMOC occurs. For some models, the climatological sea ice extent covers a wide area of the SPNA and restrains efficient air–sea interactions, making the AMOC less sensitive to the NAO. In the models without the sea-ice-covered SPNA, the upper-ocean mean stratification critically affects the relationship between the NAO and AMOC by regulating the AMOC sensitivity to surface buoyancy forcing. Our results pinpoint the oceanic mean state as an aspect of climate model simulations that must be improved for an accurate understanding of the AMOC.

Langmuir circulation (LC) plays an important role in deepening the mixed layer, especially when the ocean is weakly stratified. LC parameterization has been known to improve the accuracy of the mixed-layer depth, sea surface temperature, and ocean ventilation in the Southern Ocean. However, changes in ocean dynamics and biogeochemical processes owing to LC mixing have rarely been investigated. In this study, we implemented LC parameterization to a physical–biogeochemical coupled model and examined the changes in water properties and circulation and their effects on biogeochemical processes in the Southern Ocean. The LC effect enhances the turbulent mixing length scale and turbulent kinetic energy, especially in the subantarctic region, resulting in the deepening of the mixed layer. Increased vertical mixing causes deeper warm and saline water to reach the surface layer and weakens the surface meridional velocity by transferring momentum deeper. The weakening of the meridional velocity decreases equatorward freshwater transport, causing the surface water to become saltier and denser north of 50°S adjacent to the formation site of Subantarctic Mode Water, which enhances ocean ventilation eventually. Additionally, the weak equatorward velocity drives the retreat of sea-ice south of 60°S, leading to cooler and fresher water in the southern region of 60°S. Changes in the sea-ice distribution, mixed-layer depth, and dissolved inorganic carbon distribution alter the air-sea CO2 exchange, suggesting that the LC parameterization in the model can ameliorate 10% of the air-sea CO2 exchange. The LC effects on biogeochemical tracers, such as iron (Fe) and dissolved inorganic carbon, are mainly determined by changes in ocean circulation, and the modest improvement in primary productivity is mainly attributed to the changes in the Fe distribution in the high-nutrient, low-chlorophyll region, where Fe availability limits primary production. Although the direct LC effects occur near the surface, the altered meridional circulation spreads the effects to a deeper layer and improves the overall representation of physical, biogeochemical tracers and air-sea CO2 exchange in the Southern Ocean, suggesting that a simple LC parameterization can improve the ocean model.

The ocean mixed layer model (OMLM) is improved using the large eddy simulation (LES) and the inverse estimation method. A comparison of OMLM (Noh model) and LES results reveals that underestimation of the turbulent kinetic energy (TKE) flux in the OMLM causes a negative bias of the mixed layer depth (MLD) during convection, when the wind stress is weak or the latitude is high. It is further found that the entrainment layer thickness is underestimated. The effects of alternative approaches of parameterizations in the OMLM, such as nonlocal mixing, length scales, Prandtl number, and TKE flux, are examined with an aim to reduce the bias. Simultaneous optimizations of empirical constants in the various versions of Noh model with different parameterization options are then carried out via an iterative Green’s function approach with LES data as constraining data. An improved OMLM is obtained, which reflects various new features, including the enhanced TKE flux, and the new model is found to improve the performance in all cases, namely, wind-mixing, surface heating, and surface cooling cases. The effect of the OMLM grid resolution on the optimal empirical constants is also investigated.

Marine extreme temperature events (METs), including marine heatwaves (MHWs) and cold spells, have recently gained much attention owing to their vital influence on the marine ecosystem and social economy. Since METs can alter the upper ocean stratification and wintertime convective mixing in the northwestern North Pacific subtropical gyre (NPSG), their activities may modulate phytoplankton blooms by regulating entrainment of the subtropical mode water (STMW) with high NO. Furthermore, because STMW formed in the previous winter reemerges east of its formation site in the following winter, the METs activities imprinted in STMW affect phytoplankton blooms remote from its formation site. Here, we examined the relationship between the MET activities, STMW volume, and phytoplankton blooms using satellite observations and a data-assimilative coupled physical-biogeochemical model dataset. MET activities appearing in the STMW formation region during winter regulate the formation of STMW and the supply of NO from the subsurface, with the latter controlling the spring/autumn blooms in that region under NO-limited conditions. Subsequently, this water mass is transported eastward in the subsurface within the northern flank of the NPSG before reemerging east of the STMW formation site the following spring. This process results in a negative lag-correlation between MET activities and surface chlorophyll in the reemergence region; for example, MHWs in winter at the STMW formation site tend to lower the surface chlorophyll concentration one year later in the reemergence region. Our study suggests that the oceanic processes allow one year of predictability of the marine ecosystems by monitoring METs in the STMW formation site.

The predictability of the sea surface temperature (SST) in seasonal forecast systems is crucial for accurate seasonal predictions. In this study, we evaluated the prediction of SST in the Global Seasonal forecast system version 5 (GloSea5) hindcast, particularly focusing on the western North Pacific (WNP), where the SST can modify atmospheric convection and the East Asian weather. GloSea5 has a cold SST bias in the WNP that grows over at least 7 months. The bias originates from the surface net heat flux. At the beginning of model integration, the ocean receives excessive heat from the atmosphere because of the predominant positive bias in the downward shortwave radiation (SW), which rapidly decreased within a few days as cloud cover builds. Then, the negative bias in the latent heat (LH) flux increases over time and induces a negative bias in the surface net heat flux. Although the magnitude of the negative bias in LH flux gradually decreases, it remains the most significant contributor to the negative bias in the net heat flux bias for more than 250 days. Uncoupled ocean model experiments showed that the ocean model is unlikely to be the primary source of the SST bias.

The Southern Ocean, an important region for the uptake of anthropogenic carbon dioxide (CO2), features strong surface currents due to substantial mesoscale meanders and eddies. These features interact with the wind and modify the momentum transfer from the atmosphere to the ocean. Although such interactions are known to reduce momentum transfer, their impact on air-sea carbon exchange remains unclear. Using a 1/20° physical-biogeochemical coupled ocean model, we examined the impact of the current-wind interaction on the surface carbon concentration and the air-sea carbon exchange in the Southern Ocean. The current-wind interaction decreased winter partial pressure of CO2 (pCO2) at the ocean surface mainly south of the northern subantarctic front. It also reduced pCO2 in summer, indicating enhanced uptake, but not to the same extent as the winter loss. Consequently, the net outgassing of CO2 was found to be reduced by approximately 17% when including current-wind interaction. These changes stem from the combined effect of vertical mixing and Ekman divergence. A budget analysis of dissolved inorganic carbon (DIC) revealed that a weakening of vertical mixing by current-wind interaction reduces the carbon supply from below, and particularly so in winter. The weaker wind stress additionally lowers the subsurface DIC concentration in summer, which can affect the vertical diffusive flux of carbon in winter. Our study suggests that ignoring current-wind interactions in the Southern Ocean can overestimate winter CO2 outgassing.

We diagnose the ocean’s residual overturning circulation of the Arabian Gulf in a high-resolution model and interpret it in terms of water-mass transformation processes mediated by air–sea buoyancy fluxes and interior mixing. We attempt to rationalize the complex three-dimensional flow in terms of the superposition of a zonal (roughly along axis) and meridional (transverse) overturning pattern. Rates of overturning and the seasonal cycle of air–sea fluxes sustaining them are quantified and ranked in order of importance. Air–sea fluxes dominate the budget so that, at zero order, the magnitude and sense of the overturning circulation can be inferred from air–sea fluxes, with interior mixing playing a lesser role. We find that wintertime latent heat fluxes dominate the water-mass transformation rate in the interior waters of the Gulf leading to a diapycnal volume flux directed toward higher densities. In the zonal overturning cell, fluid is drawn in from the Sea of Oman through the Strait of Hormuz, transformed, and exits the Strait near the southern and bottom boundaries. Along the southern margin of the Gulf, evaporation plays an important role in the meridional overturning pattern inducing sinking there.