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The Agulhas Current (AC) is a critical component of global ocean circulation. However, due to a lack of multidecadal observations, it is not clear how the AC has changed in response to anthropogenic forcing. A recent observational study suggests a broadening and slight weakening of the AC in the past few decades, while others suggest a strengthening of the AC during the historical period. In this paper, we find substantial internal variability of the AC on decadal to multidecadal time scales in high-resolution models. We show that the AC consistently exhibits two modes of decadal and multidecadal (i.e., low frequency) variability in a series of high-resolution climate models: a uniform mode that is largely associated with changes in the AC strength and a dipole mode that is mainly related to width changes of the AC. We demonstrate that the uniform mode is mainly forced externally by the decadal variations of the wind field and presents a decline under global warming, suggesting a weakening of the AC in response to anthropogenic forcing. The dipole mode, on the other hand, is mainly due to internal dynamics and does not show a trend during the historical period. Using a quasigeostrophic model that captures the dipole mode, we attribute the dipole mode to low-frequency potential vorticity changes in the western boundary, driven by a divergence of relative potential vorticity due to eddy activity. Thus, our results present further context for the interpretation of the AC responses in a changing climate based on a short observational record. [ABSTRACT FROM AUTHOR]

This paper investigates the structure and dynamics of the Equatorial Undercurrent (EUC) of the Indian Ocean by analyzing in situ observations and reanalysis data and performing ocean model experiments using an ocean general circulation model and a linear continuously stratified ocean model. The results show that the EUC regularly occurs in each boreal winter and spring, particularly during February and April, consistent with existing studies. The EUC generally has a core depth near the 20°C isotherm and can be present across the equatorial basin. The EUC reappears during summer-fall of most years, with core depth located at different longitudes and depths. In the western basin, the EUC results primarily from equatorial Kelvin and Rossby waves directly forced by equatorial easterly winds. In the central and eastern basin, however, reflected Rossby waves from the eastern boundary play a crucial role. While the first two baroclinic modes make the largest contribution, intermediate modes 3-8 are also important. The summer-fall EUC tends to occur in the western basin but exhibits obvious interannual variability in the eastern basin. During positive Indian Ocean dipole (IOD) years, the eastern basin EUC results largely from Rossby waves reflected from the eastern boundary, with directly forced Kelvin and Rossby waves also having significant contributions. However, the eastern basin EUC disappears during negative IOD and normal years because westerly wind anomalies force a westward pressure gradient force and thus westward subsurface current, which cancels the eastward subsurface flow induced by eastern boundary-reflected Rossby waves. Interannual variability of zonal equatorial wind that drives the EUC variability is dominated by the zonal sea surface temperature (SST) gradients associated with IOD and is much less influenced by equatorial wind associated with Indian monsoon rainfall strength. [ABSTRACT FROM AUTHOR]

The effect of climate warming in response to rising atmospheric CO2 on the ventilation of the ocean remains uncertain. Here we make theoretical advances in elucidating the relationship between ideal age and transit time distribution (TTD) in a time-dependent flow. Subsequently, we develop an offline tracer-transport model to characterize the ventilation patterns and time scales in the time-evolving circulation for the 1850–2300 period as simulated with the Community Earth System Model version 1 (CESMv1) under a business-as-usual warming scenario. We found that by 2300 2.1% less water originates from the high-latitude deep water formation regions (both hemispheres) compared to 1850. In compensation, there is an increase in the water originating from the subantarctic. We also found that slowing meridional overturning circulation causes a gradual increase in mean age during the 1850–2300 period, with a globally averaged mean-age increase of ∼110 years in 2300. Where and when the water will be re-exposed to the atmosphere depends on the post-2300 circulation. For example, if we assume that the circulation persists in its year-2300 state (scenario 1), the mean interior-to-surface transit time in year 1850 is ∼1140 years. In contrast, if we assume that the circulation abruptly recovers to its year-1850 state (scenario 2), the mean interior-to-surface transit time in 1850 is only ∼740 years. By 2300, these differences become even larger; in scenario 1, the mean interior-to-surface transit time increases by ∼200 years, whereas scenario 2 decreases by ∼80 years. The dependence of interior-to-surface transit time on the future ocean circulation produces an additional unavoidable uncertainty in the long-term durability of marine carbon dioxide removal strategies. Significance Statement: The ocean's circulation, when altered by climate warming, can affect its capacity to absorb heat and CO2, which are crucial for the global climate. In our study, we investigated how global warming, caused by rising CO2 levels, might impact the ocean circulation—the way water moves from deep ocean to the surface and vice versa. We discovered that by 2300, if we continue on our current warming trajectory, the origins of water within the ocean will shift, with less coming from deep, cold zones near the poles and more from subantarctic regions. As a result, deep water will take longer time before it resurfaces than shallow water. How quickly this water travels from deep regions to the surface could change, depending on the state of future ocean circulation. If the circulation remains as predicted in 2300, this journey will take longer. Conversely, if it reverts to the pattern in 1850, the process will be quicker. This variability introduces added uncertainty to strategies aimed at mitigating climate change by storing CO2 in the ocean. Our work highlights the intricate ways in which climate change can influence our oceans, potentially affecting our plans to mitigate global warming. [ABSTRACT FROM AUTHOR]

A rapid warming and freshening of the Southern Ocean have been observed over the past several decades and are attributed to anthropogenic climate change. In this study, ocean model perturbation experiments are conducted to separate roles of individual surface forcing in the Southern Ocean temperature and salinity changes. Model-based findings are compared with results from a theoretical framework including three idealized processes defined on the θ–S diagram. Under the future scenario of CO2 doubling, the heat flux forcing dominates the large-scale warming, deepening of isopycnals, and spiciness changes along isopycnals, which can be captured by an idealized pure warming process to represent the subduction of surface heat uptake. The poleward-intensifying westerly winds account for 24% of the enhanced warming between 35° and 50°S and would have comparable contribution as the heat flux forcing after removing the global ocean warming effect. In contrast, the widespread freshening in the Southern Ocean driven by increased surface freshwater input is largely compensated by the wind-driven saltening. The response to freshwater forcing could not be approximated as a similar pure freshening process as the induced cooling and freshening have comparable effects on density. The wind-driven changes are primarily through the local heave of isopycnals, thus resembling an idealized pure heave process, but contain considerable spiciness signals especially in the midlatitude Southern Ocean, resulting from anomalous northward transport and subduction of heat and salt that are largely density-compensating. These distinct signatures of individual surface forcing help us to better understand observed and projected changes in the Southern Ocean. Significance Statement: Considerable changes including a rapid warming and freshening have been observed in the Southern Ocean as it absorbs most of the extra heat from the anthropogenic climate change, receives increased surface freshwater input, and experiences a poleward shift and intensification of the westerly winds. The purpose of this study is to distinguish different contributions from surface heat flux, freshwater flux, and wind forcing to the Southern Ocean temperature and salinity changes, based on ocean model experiments and three idealized processes from a theoretical framework. Our study reveals distinct signatures of individual surface forcing that help us to understand linkages between changes seen at the surface and in the interior Southern Ocean. [ABSTRACT FROM AUTHOR]

The mean circulation and volume budgets in the upper 1200 m of the Maluku Sea are studied using multiyear current meter measurements of four moorings in the Maluku Channel and of one synchronous mooring in the Lifamatola Passage. The measurements show that the mean current in the depth range of 60–450 m is northward toward the Pacific Ocean with a mean transport of 2.07–2.60 Sv (1 Sv ≡ 106 m3 s−1). In the depth range of 450–1200 m, a mean western boundary current (WBC) flows southward through the western Maluku Sea and connects with the southward flow in the Lifamatola Passage. The mean currents in the central-eastern Maluku Channel are found to flow northward at this depth range, suggesting an anticlockwise western intensified gyre circulation in the middle layer of the Maluku Sea. Budget analyses suggest that the mean transport of the intermediate WBC is 1.83–2.25 Sv, which is balanced by three transports: 1) 0.62–0.93 Sv southward transport into the Seram–Banda Seas through the Lifamatola Passage, 2) 0.97–1.01 Sv returning to the western Pacific Ocean through the central-eastern Maluku Channel, and 3) a residual transport surplus, suggested to upwell to the upper layer joining the northward transport into the Pacific Ocean. The dynamics of the intermediate gyre circulation are explained by the potential vorticity (PV) integral constraint of a semienclosed basin. Significance Statement: The Indonesian Throughflow plays an important role in the global ocean circulation and climate variations. Existing studies of the Indonesian Throughflow have focused on the upper thermocline currents. Here we identify, using mooring observations, an intermediate western boundary current with the core at 800–1000-m depth in the Maluku Sea, transporting intermediate waters from the Pacific into the Seram–Banda Seas through the Lifamatola Passage. Potential vorticity balance suggests an anticlockwise gyre circulation in the intermediate Maluku Sea, which is evidenced by the mooring and model data. Transport estimates suggest northward countercurrent in the upper Maluku Sea toward the Pacific, supplied by the Lifamatola Passage transport and upwelling from the intermediate layer in the Maluku Sea. Our results suggest the importance of the intermediate Indonesian Throughflow in global ocean circulation and overturn. More extensive investigations of the Indo-Pacific intermediate ocean circulation should be conducted to improve our understanding of global ocean overturn and heat and CO2 storages. [ABSTRACT FROM AUTHOR]

South China Sea (SCS) abyssal circulation largely contributes to water renewal, energy budget, and sedimentary processes in the deep ocean. The three-dimensional abyssal circulation west of the Luzon Strait (LS) in the northern SCS was investigated using an array comprising 27 current- and pressure-recording inverted echo sounders. Over 400 days of measurements from June 2018 to July 2019 showed a narrow and strong (∼70 km, ∼2.3 cm s−1 at 2500 dbar) northward current near the steep eastern boundary, while a wide and weak (∼180 km, ∼1.5 cm s−1 at 2500 dbar) southwestward current lies along the subdued western boundary. The circulation showed conspicuous cyclonic patterns with a volume transport of ∼1.21 ± 0.93 Sv (1 Sv ≡ 106 m3 s−1) and ∼1.59 ± 0.95 Sv below 2500 dbar along the eastern and western boundaries, respectively. The current near the LS was strong in late autumn and early winter but weak in late winter and spring, following the seasonal variation of LS deep-water overflow. However, the southwestward current in the interior SCS was stronger in summer and early autumn but weaker in late winter and early spring. The different seasonal patterns identified near the LS and the interior SCS are attributed to the propagation of seasonal variation. The weak current along the western boundary in August 2018 and February 2019 was dominated by LS deep-water overflow with a time lag of ∼7.5 months. Although eddies in the upper ocean may also contribute to such variation through pressure work, the effect is minor. Significance Statement: Cyclonic circulation in the deep South China Sea (SCS) largely contributes to water renewal, energy budget, and sedimentary processes and influences the transport of dissolved elements, minerals, and pollutants. As an important part of the SCS throughflow, an in-depth analysis of the SCS abyssal circulation may also contribute to understanding Indonesian Throughflow and global climate change. The three-dimensional abyssal circulation west of the Luzon Strait was investigated using large-scale data from June 2018 to July 2019, which provided unprecedented coverage of abyssal circulation in the northeast SCS. The study provides important observational evidence for the existence of SCS abyssal cyclonic circulation. Detailed spatiotemporal structure of abyssal circulation and its variations are presented, and related dynamic processes are discussed. [ABSTRACT FROM AUTHOR]

Spatial and temporal variations of nutrient-rich upwelled water across the major eastern boundary upwelling systems are primarily controlled by the surface wind with different, and sometimes contrasting, impacts on coastal upwelling systems driven by alongshore wind and offshore upwelling systems driven by the local wind stress curl. Here, concurrently measured wind fields and satellite-derived chlorophyll-a concentration, along with a state-of-the-art ocean model simulation, spanning 2008–18 are used to investigate the connection between coastal and offshore physical drivers of the Benguela upwelling system (BUS). Our results indicate that the spatial structure of long-term mean upwelling derived from Ekman theory and the numerical model is fairly consistent across the entire BUS and is closely followed by the chlorophyll-a pattern. The variability of the upwelling from the Ekman theory is proportionally diminished with offshore distance, whereas different and sometimes opposite structures are revealed in the model-derived upwelling. Our result suggests the presence of submesoscale activity (i.e., filaments and eddies) across the entire BUS with a large modulating effect on the wind-stress-curl-driven upwelling off Lüderitz and Walvis Bay. In Kunene and Cape Frio upwelling cells, located in the northern sector of the BUS, the coastal upwelling and open-ocean upwelling frequently alternate each other, whereas they are modulated by the annual cycle and are mostly in phase off Walvis Bay. Such a phase relationship appears to be strongly seasonally dependent off Lüderitz and across the southern BUS. Thus, our findings suggest that this relationship is far more complex than is currently thought and seems to be sensitive to climate changes, with short- and far-reaching consequences for this vulnerable marine ecosystem. [ABSTRACT FROM AUTHOR]

Antarctic glacial meltwater is thought to play an important role in determining large-scale Southern Ocean climate trends, yet recent modeling efforts have proceeded without a good understanding of how its vertical distribution in the water column is set. To rectify this, here we conduct new large-eddy simulations of the ascent of a buoyant meltwater plume after its escape from beneath an Antarctic ice shelf. We find that the meltwater's settling depth is primarily a function of the buoyancy forcing per unit width of the source and the ambient stratification, consistent with the classical theory of turbulent buoyant plumes and in contrast to previous work that suggested an important role for centrifugal instability. Our results further highlight the significant role played by localized variability in stratification; this helps explain observed interannual variability in the vertical meltwater distribution near Pine Island Glacier. Because of the vast heterogeneity in mass loss rates and ambient conditions at different Antarctic ice shelves, a dynamic parameterization of meltwater settling depth may be crucial for accurately simulating high-latitude climate in a warming world; we discuss how this may be developed following this work, and where the remaining challenges lie. [ABSTRACT FROM AUTHOR]

The Subantarctic Mode Water (SAMW) plays an essential role in the global heat, freshwater, carbon, and nutrient budgets. In this study, decadal changes in the SAMW properties in the southern Indian Ocean (SIO) and associated thermodynamic and dynamic processes are investigated during the Argo era. Both temperature and salinity of the SAMW in the SIO show increasing trends during 2004–18. A two-layer structure of the SAMW trend, with more warm and salty light SAMW but less cool and fresh dense SAMW, is identified. The heaving and spiciness processes are important but have opposite contributions to the temperature and salinity trends of the SAMW. A significant deepening of isopycnals (heaving), peaking at σθ = 26.7–26.8 kg m−3 in the middle layer of the SAMW, expands the warm and salty light SAMW and compresses the cool and fresh dense SAMW corresponding to the change in subduction rate during 2004–18. The change in the SAMW subduction rate is dominated by the change in the mixed layer depth, controlled by the changes in wind stress curl and surface buoyancy fluxes. An increase in the mixed layer temperature due to weakening northward Ekman transport of cool water leads to a lighter surface density in the SAMW formation region. Consequently, density outcropping lines in the SAMW formation region shift southward and favor the intrusion and entrainment of the cooler and fresher Antarctic surface water from the south, contributing to the cooling/freshening trend of isopycnals (spiciness). Subsequently, the cooler and fresher SAMW spiciness anomalies spread in the SIO via the subtropical gyre. Significance Statement: Subantarctic Mode Water is a distinct water mass with vertically uniform properties in the Southern Hemisphere's subtropical gyres. Climate change is imprinted in the SAMW through the ventilation at the base of the winter mixed layer. The ocean modulation associated with wind-forced large-scale waves and circulation also plays an essential role in heat, salinity, and water mass redistribution. A net increase in volume-weighted potential temperature and salinity of the SAMW is found during the Argo era since 2004, resulting from a combination of climate change and ocean modulation through opposite heaving and spiciness processes. This study improves our understanding of the dynamics and thermodynamics involved in the SAMW formation during rapid climate change. [ABSTRACT FROM AUTHOR]

It is generally accepted that the El Niño–Southern Oscillation (ENSO) dominates interannual climate variability. Yet, its genesis and maintenance mechanisms are still under intense debate, with no scientific consensus. Some authors argued that the westerly winds originating over the equatorial Indian Ocean are significantly enhanced and extend eastward in the western and central equatorial Pacific during El Niño events, thus advecting the warm pool eastward along the equator and causing SST anomalies. However, this assertion is unlikely to be quantitatively supported by observational data. Here we present detailed observational data and modeling evidence to demonstrate that the westerly winds had little change in intensity in the western equatorial Pacific, with a wider zonal extent only during most El Niño events, and with a slight increase even in the most pronounced 1997 El Niño. Instead, an eastward equatorial current near the equator has been observed and is considered to play a significant role in shifting the eastern edge of the warm pool eastward, elevating SSTs in the central and eastern equatorial Pacific and giving rise to El Niño, with the interactions between the eastward warm pool and the upwelling in the eastern cold tongue ascertaining the amplitudes of SST anomalies. [ABSTRACT FROM AUTHOR]

A key consequence in climate change is the warming of deep waters, away from the faster warming rates of near-surface subtropical and tropical waters. Since surface and near-surface oceanic temperatures have been measured far more frequently in time and space than deep waters (>2000 m), deep measurements become quite valuable. Semi-enclosed basins, such as the Gulf of Mexico, are of particular interest as the waters below sills that connect with the neighboring oceans have residence times much longer than upper layers. Within the western Gulf of Mexico, near-bottom measurements at ~3500-m depths at four sites show a stable linear warming trend of ~16 ± 2 m°C decade−1 for the period 2007–18, and CTD data from eight oceanographic cruises occurring from 2003 to 2019 show a trend of ~18 ± ~2 m°C decade−1 from the bottom to ~2000 m below the surface. The bottom geothermal heat flux is a contributing factor to be considered in the warming and renewal of such waters, but it has not changed over millennia and is therefore unlikely to be the cause of the observed trend. The densest waters that spill into the Gulf of Mexico, over the Yucatan Channel sill, must mix substantially during their descent and in the near-bottom interior, losing their extreme values. A simple box model connects the observed warming, well within the Gulf interior, with that expected in the densest waters that spill from the North Atlantic into the Cayman Basin through Windward Passage and suggests that the source waters at the entrance to the Caribbean have been warming for at least 100 years. [ABSTRACT FROM AUTHOR]

Regional connectivity is important to the global climate salinity response, particularly because salinity anomalies do not have a damping feedback with atmospheric freshwater fluxes and may therefore be advected over long distances by ocean circulation, resulting in nonlocal influences. Climate model intercomparison experiments such as CMIP5 exhibit large uncertainty in some aspects of the salinity response, hypothesized here to be a result of ocean dynamics. We use two types of Lagrangian particle tracking experiments to investigate pathways of exchange for salinity anomalies. The first uses forward trajectories to estimate average transport time scales between water cycle regimes. The second uses reverse trajectories and a freshwater accumulation method to quantitatively identify remote influences in the salinity response. Additionally, we compare velocity fields with both resolved and parameterized eddies to understand the impact of eddy stirring on intergyre exchange. These experiments show that surface anomalies are readily exchanged within the ocean gyres by the mean circulation, but intergyre exchange is slower and largely eddy driven. These dynamics are used to analyze the North Atlantic salinity response to climate warming and water cycle intensification, where the system is broadly forced with fresh surface anomalies in the subpolar gyre and salty surface anomalies in the subtropical gyres. Under these competing forcings, strong intergyre eddy fluxes carry anomalously salty subtropical water into the subpolar gyre which balances out much of the local freshwater input. [ABSTRACT FROM AUTHOR]

An intrahalocline eddy was observed on the Chukchi slope in September of 2015 using both towed CTD and microstructure temperature and shear sections. The core of the eddy was 6°C, significantly warmer than the surrounding −1°C water and far exceeding typical temperatures of warm-core Arctic eddies. Microstructure sections indicated that outside of the eddy the rate of dissipation of turbulent kinetic energy ε was quite low . However, at the edges of the eddy core, ε was elevated to . Three different processes were associated with elevated ε. Double-diffusive steps were found at the eddy's top edge and were associated with an upward heat flux of 5 W m−2. At the bottom edge of the eddy, shear-driven mixing played a modest role, generating a heat flux of approximately 0.5 W m−2 downward. Along the sides of the eddy, density-compensated thermohaline intrusions transported heat laterally out of the eddy, with a horizontal heat flux of 2000 W m−2. Integrating these fluxes over an idealized approximation of the eddy's shape, we estimate that the net heat transport due to thermohaline intrusions along the eddy flanks was 2 GW, while the double-diffusive flux above the eddy was 0.4 GW. Shear-driven mixing at the bottom of the eddy accounted for only 0.04 GW. If these processes continued indefinitely at the same rate, the estimated life-span would be 1–2 years. Such eddies may be an important mechanism for the transport of Pacific-origin heat, freshwater, and nutrients into the Canada Basin. [ABSTRACT FROM AUTHOR]

The salt transport by the wind-driven gyres and the meridional overturning circulation (MOC) is studied in an idealized-geometry primitive equation ocean model. Two narrow continents, running along meridians, divide the model domain into two basins of different widths connected by a re-entrant channel south of 52.5°S. One of the continents, representing the Americas, is longer than the other, representing Europe/Africa. Two different configurations of the model are used: the "standard" one, in which the short continent is west of the wide basin, and the "exchanged" one, in which the short continent is west of the narrow basin. In both cases, deep water is formed in the basin to the west of the short continent. Most residual transport of the MOC's upper branch enters this basin by flowing along open streamlines that pass westward south of the short continent before proceeding northward. The meridional salt transport in the upper ocean of the sinking basin is decomposed into two portions: transport along open streamlines and transport by closed streamlines (gyres). In the Northern Hemisphere of the basin in which deep water is formed, the total northward salt transport per unit width along open streamlines is larger in the standard configuration than in the exchanged configuration. This larger salt transport is caused by two factors: a larger northward advection of salt by the interbasin transport and a larger cross-streamline salt transport out of the subpolar gyre. It is concluded that increasing interbasin flow south of Africa would likely bring more salt into the Atlantic Ocean. [ABSTRACT FROM AUTHOR]

A subpolar marginal sea, like the Nordic seas, is a transition zone between the temperature-stratified subtropics (the alpha ocean) and the salinity-stratified polar regions (the beta ocean). An inflow of Atlantic Water circulates these seas as a boundary current that is cooled and freshened downstream, eventually to outflow as Deep and Polar Water. Stratification in the boundary region is dominated by a thermocline over the continental slope and a halocline over the continental shelves, separating Atlantic Water from Deep and Polar Water, respectively. A conceptual model is introduced for the circulation and water mass transformation in a subpolar marginal sea to explore the potential interaction between the alpha and beta oceans. Freshwater input into the shelf regions has a slight strengthening effect on the Atlantic inflow, but more prominently impacts the water mass composition of the outflow. This impact of freshwater, characterized by enhancing Polar Water outflow and suppressing Deep Water outflow, is strongly determined by the source location of freshwater. Concretely, perturbations in upstream freshwater sources, like the Baltic freshwater outflow into the Nordic seas, have an order of magnitude larger potential to impact water mass transports than perturbations in downstream sources like the Arctic freshwater outflow. These boundary current dynamics are directly related to the qualitative stratification in transition zones and illustrate the interaction between the alpha and beta oceans. [ABSTRACT FROM AUTHOR]

Volume, heat, and salt transports through the Soya Strait are estimated based on measurements from high-frequency ocean radars during 2003-15 and all available hydrographic data. The baroclinic velocity structure derived from the climatological geopotential anomaly is combined with the sea surface gradient obtained from radar-derived surface velocities to estimate the absolute velocity structure. The annual-mean volume, heat, and salt transports are 0.91 Sv (1 Sv ≡ 106 m3 s−1), 25.5 TW, and 31.15 × 106 kg s−1, respectively. The volume transport exhibits strong seasonal variations, with a maximum of 1.41 Sv in August and a minimum of 0.23 Sv in January. The seasonal amplitude and phase roughly correspond to those of the Tsushima-Korea Strait. Time series of the monthly transport is presented for the 12 yr, assuming that the baroclinic components are the monthly climatological values. In cold seasons (November to April), the monthly volume transport is strongly correlated with the sea level difference between the Japan and Okhotsk Seas, and an empirical formula to estimate the transport from the sea level difference is introduced. It is likely that the sea level setup by the wind stress along the east coast of Sakhalin determines the sea level difference, which explains the seasonal and interannual wintertime variations of transport through the strait. The annual flux of water through the Soya Strait with a density greater than 26.8 σ θ, a potential source of Okhotsk Sea Intermediate Waters, is estimated to be 0.18 Sv. [ABSTRACT FROM AUTHOR]

This study first detects the decadal variability in the depths of the East/Japan Sea (EJS) Intermediate Water (ESIW) using in situ observations and relates it to strong El Niño-Southern Oscillations (ENSOs). Using multitaper cross-spectrum and cyclostationary empirical orthogonal function analysis, this study found significant coherences at the 99% confidence level and opposite phases between Niño-3 and the ESIW isopycnal depths in the Ulleung Basin (UB) at a period of 14.1 yr during 1968-2002. This suggests a teleconnection between strong ENSOs and the ESIW, the cause of which is explored. When a strong El Niño (EN) develops, the Pacific-North American pattern is intensified by the EN-related Rossby wave interfering constructively with the climatological stationary wave. The amplified wave propagates upward into the stratosphere and breaks, weakening the polar vortex. The EN-related geopotential height increases over the pole with poleward converging air and decreases over the Yakutsk Basin (YB), indicating a negative northern annular mode with the south-to-north gradient balancing the easterly anomaly that responds to vortex weakening. The converged air at the pole warms adiabatically and raises the height as it sinks. This height distribution, including the east-to-west gradient balancing the southward flow, induces the polar vortex split into two with the colder one in the YB where the EJS is closer than from the pole. The EN-related northwesterly wind directing toward the EJS is also strong. Thus, the coldest air with negative wind stress curls reaches the EJS quickly and forms more ESIW, which converges into the UB, causing the observed decadal isopycnal fluctuations. [ABSTRACT FROM AUTHOR]

A weakly damped mode of variability, corresponding to the oceanic signature of the Atlantic multidecadal oscillation (AMO) was found through the linear stability analysis of a realistic ocean general circulation model. A simple two-level model was proposed to rationalize both its period and damping rate. This model is extended here to three levels to investigate how the mode can draw energy from the mean flow, as found in various ocean and coupled models. A linear stability analysis in this three-level model shows that the positive growth rate of the oscillatory mode depends on the zonally averaged isopycnal slope. This mode corresponds to a westward propagation of density anomalies in the pycnocline, typical of large-scale baroclinic Rossby waves. The most unstable mode corresponds to the largest scale one (at least for low isopycnal slope). The mode can be described in four phases composing a full oscillation cycle: 1) basin-scale warming of the North Atlantic (AMO positive phase), 2) decrease in upper-ocean poleward transport [hence a reduction of the Atlantic meridional overturning circulation (AMOC)], 3) basin-scale cooling (negative AMO), and 4) AMOC intensification. A criterion is developed to test, in oceanic datasets or numerical models, whether this multidecadal oscillation is an unstable oceanic internal mode of variability or if it is stable and externally forced. Consistent with the classical theory of baroclinic instability, this criterion depends on the vertical structure of the mode. If the upper pycnocline signature is in advance of the deeper pycnocline signature with respect to the westward propagation, the mode is unstable and could be described as an oceanic internal mode of variability. [ABSTRACT FROM AUTHOR]

The seasonal variation of the South Equatorial Current (SEC) bifurcation off the Australian coast in the South Pacific (SP) is investigated with observations and a nonlinear, reduced-gravity, primitive equation model of the upper ocean. The mean SEC bifurcation latitude (SBL) integrated over the upper thermocline is around 17.5°S, almost 2° south of the position predicted by Sverdrup theory. For its seasonal variation, the SBL reaches its southernmost position in June/July and its northernmost position in November/December. The south-north migration of 2.7° is twice as large as its counterpart in the North Pacific. It is found that the large seasonal amplitude of the SBL results from the combined effect of Low-Lat-SP and Non-Low-Lat-SP processes. The Low-Lat-SP process (referred to as the Rossby wave dynamics forced by the wind stress curl over the low-latitude SP) accounts for almost ⅔ of the SBL seasonal variability, and the Non-Low-Lat-SP processes account for ⅓. Both of these processes are responsible for its south-north migration but in different ways. The Low-Lat-SP wind forcing determines the offshore upper-layer thickness (ULT) via Rossby wave propagation, while the Non-Low-Lat-SP wind forcing determines the alongshore ULT via coastal Kelvin wave propagation. A simple bifurcation model is proposed under the framework of linear Rossby wave dynamics. It is found that the seasonal bifurcation latitude is predominantly determined by the spatial pattern of the wind and baroclinic Rossby wave propagation. This model explains the roles of local/remote wind forcing and baroclinic adjustment in the south-north migration and peak seasons of the bifurcation latitude. [ABSTRACT FROM AUTHOR]

The seasonal response of sea surface height anomaly (SSHA) to wind stress curl (WSC) in the northern South China Sea (NSCS) and the Kuroshio intrusion through the Luzon Strait is analyzed using observations and models. The dominant response to WSC is through simple Ekman pumping, while effects of β appear as the weaker second empirical orthogonal function mode. The Luzon Strait intrusion is shown to be largely deterministic using a model forced by realistic wind in the North Pacific Ocean, and it contributes significantly to the SSH variability in the NSCS. The WSC accounts for 62%, while intrusion 38% of the total forcing, but the latter alters the forced Rossby wave response. Without the intrusion, westward propagation is too fast, resulting in incorrect balance and erroneous annual SSH variability in the NSCS. [ABSTRACT FROM AUTHOR]

The interannual variability of the upper-ocean circulation forced by seasonally varying monsoonal wind is investigated in a two-layer quasigeostrophic (QG) model, with the aim to understand the low-frequency variability of the South China Sea (SCS) circulation. It is demonstrated that the seasonally varying monsoonal wind can force the upper-ocean circulation with significant internal variability, which is mainly associated with the intrinsic nonlinear dynamics of the summer double-gyre system. This arises from the fact that the intrinsic variability, characterized by the Rossby wave adjustment in the winter single-gyre system, is much weaker than that in the summer double-gyre system driven by the intergyre eddy potential vorticity flux through barotropic instability. [ABSTRACT FROM AUTHOR]

Moored observations are used to investigate the seasonal change in vertical structure of the cross-shelf circulation at a midshelf location in the northern California Current System. A streamwise-normal coordinate system is employed to eliminate meander- and eddy-induced biases in the cross-shelf flow that are unaccounted for with an alternative, commonly applied approach. The resulting flow structure develops an organized pattern midway through the upwelling season. In particular, under upwelling-favorable conditions an onshore return layer occurs just beneath the offshore surface flow, and a third offshore-directed layer exists at depth that does not appear to satisfy Ekman dynamics (to within 9 m of the bottom). Both subsurface layers strengthen in time over the upwelling season. Mechanisms to explain the mean structure are evaluated, and it is suggested that the timing of the development and strengthening of both the interior onshore return flow and the offshore near-bottom layer are consistent with the seasonally changing direction and magnitude of the large-scale alongshelf sea level gradient. The change to a poleward sea level gradient initiates a seasonal relaxation of upwelled isopycnals that likely leads to the near-bottom flow. Late-season enhancement of the interior onshore return flow is related to the alongshelf surface wind stress but appears to form as a consequence of offshore transport in the near-bottom layer and the need to satisfy coastal mass balance. [ABSTRACT FROM AUTHOR]

Antarctic Circumpolar Current (ACC) fronts, defined as water mass boundaries, have been known to respond to large-scale atmospheric variabilities, especially the Southern Hemisphere annular mode (SAM) and El Niño-Southern Oscillation (ENSO). Distinct patterns of localized variability in meridional front displacements during 1992-2011 are derived from the analysis of satellite sea surface height data. Major basin-scale differences are found between the southeast Pacific (150°-90°W) and the southeast Indian (75°-150°E) sectors of the ACC. Frontal positions in the southeast Pacific show large year-to-year meridional fluctuations, attributed mostly to ENSO and in part SAM, and no apparent seasonal cycles or long-term trends. In contrast, summer (winter) frontal locations in the southeast Indian extend farther to the south (north) of their long-term mean distribution. A southward drift of ACC fronts is indicated over the Indian sector during the past two decades. This long-term shift is not directly related to the atmospheric variabilities, but this is most likely in response to changes in large-scale ocean circulation, in particular to the poleward expansion of the Indian subtropical gyre. The existence of these localized, contrasting variability patterns suggests that a circumpolar-averaging analysis could possibly smooth out a local climate signal, with an emphasis on a basin-scale investigation for climate studies in the Southern Ocean. [ABSTRACT FROM AUTHOR]