Wave- and current-supported turbidity currents (WCSTCs) are one of the sediment delivery mechanisms from the inner shelf to the shelf break. Therefore, they play a significant role in the global cycles of geo-chemically important particulate matter. Recent observations suggest that WCSTCs can transform into self-driven turbidity currents close to the continental margin. However, little is known regarding the critical conditions that grow self-driven turbidity currents on WCSTCs. This is in part due to the knowledge gaps in the dynamics of WCSTCs regarding the role of density stratification. Especially the effect of sediment entrainment, and the parameters thereof, on density stratification and the amount of sediment suspension, has been overlooked. To this end, this study revisits the existing theoretical framework for a simplified WCSTC, in which waves are absent, i.e., alongshelf current-supported turbidity current (ACSTC). A depth-integrated advection model is developed for suspended sediment concentration. The analyses of the model, which are verified by turbulence-resolving simulations, indicate that the amount of suspended sediment load is regulated by the equilibrium among density stratification, positive feedback between entrainment and cross-shelf gravity force, and settling flux dissociated with density stratification. It is also found that critical density stratification is not a necessary condition for equilibrium. A quantitative relation is developed for the critical conditions for self-driven turbidity currents, which is a function of bed shear stress, entrainment parameters, bed slope, and sediment settling velocity. In addition, the suspended sediment load is analytically estimated from the model developed.

A document by Qian Liu. Click on the document to view its contents.

Nickel (Ni) is a micronutrient that plays a role in nitrogen uptake and fixation in the modern ocean may have impacted rates of methanogenesis on geological timescales. Here we present the results of a diagnostic model of global ocean Ni fluxes which addresses key questions about the biogeochemical processes which cycle Ni in the modern oceans. Our approach starts with extrapolating the sparse available observations of Ni data from the GEOTRACES project into a global gridded climatology of ocean Ni concentrations. Three different machine learning techniques were tested, each relying on marine tracers with better observational coverage such as macronutrient concentrations and physical parameters. The ocean transport of this global Ni concentration field is then estimated using the OCIM2 ocean circulation inverse model, revealing regions of net convergence or divergence. These diagnostics are not based on any assumption about Ni biogeochemical cycling, but their spatial patterns can be interpreted as reflecting biogeochemical processes. We find that the spatial pattern of Ni uptake in the surface ocean is similar to phosphate (P) uptake, but not silicate (Si) uptake, suggesting that Ni is not incorporated into diatom frustules. We find that Ni:P ratios at uptake do not decrease with Ni concentrations approaching 2 nM, which challenges the hypothesis of a ~2 nM pool of non-bioavailable Ni in the surface ocean. Finally, the net regeneration of Ni occurs deeper in the ocean than P remineralization, which could be explained by reversible scavenging or the presence of a refractory Ni phase.

Optical properties of seawater can provide valuable insight into distributions of dissolved organic carbon (DOC) and particulate organic carbon (POC), provided that their interrelationships are well understood. We examined relationships between DOC and POC, and absorption, backscatter, and fluorescence in a river-fed lagoon system in the coastal Alaskan Arctic during late summer of 2018 and 2019. Over both years analytically measured DOC levels were inversely correlated with salinity (r2 = 0.97) and DOC was positively correlated with dissolved organic material fluorescence (fDOM; r2 = 0.67). However, DOC showed strong correlation with the absorption coefficient at 440 nm (ag(440)) only in 2018 (r2 = 0.95 versus r2 = 0.00056 in 2019). Vertical structure of fDOM in our study area corresponded with density profiles more strongly in 2018 than in 2019, but higher levels of fDOM, ag(440), and backscatter near the bottom in 2019 suggest prior wind-driven mixing or bottom resuspension events. In 2018 and 2019, the spectral slope of the absorption coefficient between 412 and 550 nm was strongly correlated with DOC concentration (r2 = 0.70), and spectral backscattering coefficients were well correlated with POC concentration (r2 = 0.90, 0.71, and 0.59 for 470, 532, and 660 nm respectively). These interannual patterns in the distribution of DOC and POC and their respective relationships with optical proxies likely reflect regional climatological factors such as precipitation over the adjacent watersheds, wind patterns, and residual sea ice in late summer.

The wintertime Kuroshio sea surface temperature (SST) front have the significant climate effects on southern China. The study demonstrates a close relationship between heavy precipitation over Southern China and Kuroshio SST front in winter. More than half winter heavy rainfall events in Southern China are proved to be resulted from strong High-frequency Variability events of the sea surface Wind Coupled with Precipitation (HV-WCP) over Kuroshio SST front. One day before strong HV-WCP events, the initial precipitation appears over Middle-lower Yangtze River due to the significantly enhanced frontal intensity. Then the precipitation generates low level cyclone and southeasterly wind anomalies, after it moving into Kuroshio front area because of the winter monsoon. The significant marine atmospheric boundary layer (MABL) height gradient over Kuroshio leads to plentiful moisture transporting from MABL into free atmosphere and enhances the local precipitation again. This process further causes the large-scale stratus rainband extending to Southern China and enhancing the heavy rainfall locally. Especially in 2008 winter, several processes of a strong HV-WCP event followed by continuous weak ones are conducive to the low-temperature-precipitation disaster in Southern China

The global area and distribution of shallow water ecosystems (SWEs), and their projected responses to climate change, are fundamental for evaluating future changes in their ecosystem functions, including biodiversity and climate change mitigation and adaptation. Whereas previous studies have focused on a few SWEs, we modelled the global distribution of all major SWEs (seagrass meadows, macroalgal beds, tidal marshes, mangroves, and coral habitats) from current conditions (1986−2005) to 2100 under the RCP2.6 and RCP8.5 emission scenarios. Our projections show that global coral habitat shrank by as much as 75% by 2100 with warmer ocean temperatures, but macroalgal beds, tidal marshes, and mangroves remained about the same because photosynthetic active radiation (PAR) depth did not vary greatly (macroalgal beds) and the shrinkage caused by sea-level rise was offset by other areas of expansion (tidal marshes and mangroves). Seagrass meadows were projected to increase by up to 11 % by 2100 because of the increased PAR depth. If the landward shift of tidal marshes and mangroves relative to sea-level rise was restricted by assuming coastal development and land use, the SWEs shrank by 91.9% (tidal marshes) and 74.3% (mangroves) by 2100. Countermeasures may be necessary for coastal defense in the future; these include considering the best mix of SWEs and coastal hard infrastructure because the significant shrinkage in coral habitat could decrease wave energy. However, if appropriate coastal management is achieved, the other four SWEs, which relatively have high CO2 absorption rates, can help mitigate the climate change influences.

We present a 700 km airborne electromagnetic survey of late-spring fast ice and sub-ice platelet layer (SIPL) thickness distributions, from McMurdo Sound to Cape Adare, providing a first-time inventory of thickness close to its annual maximum. The overall modal consolidated ice (including snow) thickness was 1.9 m, less than its mean of 2.6±1.0 m. Our survey was partitioned into level and rough ice, and SIPL thickness was estimated under level ice. Although results show a prevalence of level ice, with a mode of 2.0 m and mean of 2.0±0.6 m, rough ice covered 41% of the transect by length, 50% by volume, with a mode of 3.3 m and mean of 3.2±1.2 m. The thickest 10% of rough ice was almost 6 m on average, and a 2 km segment in Moubray Bay had a thickness greater than 8 m, demonstrating the overwhelming influence of deformation against coastal features. The fast ice was thus significantly thicker than adjacent pack ice. The presence of a significant SIPL was observed in Silverfish Bay, offshore Hells Gate Ice Shelf, New Harbour, and Granite Harbour where the SIPL transect volume was a significant fraction (0.30) of the consolidated ice volume. The thickest 10% of SIPLs had an average thickness of nearly 3 m, and near Hells Gate Ice Shelf the SIPL was almost 10 m thick, implying vigorous heat loss to the ocean (~ 90Wm-2). We conclude that polynya-induced deformation and interaction with continental ice influence fast ice thickness in the western Ross Sea.

Realistically approximating the basal melting of ice shelves is critical for reliable climate model projections and the process representations in ice-ocean interaction. In this regard, extensive research attributes the massive thinning of vulnerable ice shelves to basal melting enhancement driven by ocean water warming, focusing mainly on oceanic warm water intrusion into the sub-shelf basins. However, climate models mainly underestimated the impacts of probable small-scale processes at the ice-ocean interface on basal melting by using smooth ice base topographies. This paper provides new insights into how small-scale features on the ice-ocean interface contribute to basal melting enhancement and spatial distribution. We developed a time-dependent, two-dimensional ice-shelf plume model as an optimal tool that allows a high-resolution representation of basal topography and with the unique ability to provide valuable information from the mixed boundary layer between ocean and ice shelves. In an exemplary case study for the floating ice tongue of the 79◦ North Glacier, systematic sensitive analyses were performed with the developed model. Our results show that the sub-km-scale basal channels with realistic dimensions increase the mean basal melt rate and generate extreme and sizeable lateral variability of melting at the grounding line. This mechanism is not reproducible with the tuning of drag coefficient. Besides, it reveals that the subglacial discharge in the channels has contradicting effects of reducing the melt rate by refreshing the sea water and increasing the freezing point while increasing the melt rate due to high water speed. However, the latter was dominant in our experiments.

Export of sinking particles from the surface ocean is critical for carbon sequestration and to provide energy to the deep biosphere. The magnitude and spatial patterns of this export have been estimated in the past by \emph{in situ} particle flux observations, satellite-based algorithms, and ocean biogeochemical models; however, these estimates remain uncertain. Here, we use a recent machine learning reconstruction of global ocean particle size distributions from Underwater Vision Profiler 5 (UVP5) measurements to estimate carbon fluxes by sinking particles (35 $\mu$m - 5 mm equivalent spherical diameter) from the surface ocean. We combine global maps of particle size distribution properties with empirical relationships constrained against \emph{in situ} flux observations to calculate particulate carbon export from the euphotic zone and wintertime mixed layer depths. The new flux reconstructions suggest a less variable seasonal cycle in the tropical ocean, and a more persistent export in the Southern Ocean than previously recognized. Smaller particles (less than 420 $\mu$m) contribute most of the flux globally, while larger particles become more important at high latitudes and in tropical upwelling regions. Export from the wintertime mixed layer globally exceeds that from the euphotic zone, suggesting shallow particle recycling and net heterotrophy in the deep euphotic zone. These estimates open the way to fully three-dimensional global reconstructions of particle fluxes in the ocean, supported by the growing database of \emph{in situ} optical observations.

Ice sheets such as Pine Island and Thwaites Glaciers which terminate at their ice shelves in the eastern Amundsen Sea, West Antarctica, are losing mass faster than most others about the continent. The mass loss is due to basal melting, this affected by a deep current thought to be guided by bottom bathymetry that transports warm Circumpolar Deep Water (CDW) from the continental shelf break towards the ice shelves. This current and associated heat transport are controlled by the near-surface winds that vary on a range of timescales due to both anthropogenic and natural effects. In this study we use idealised models to reproduce essential features of the Amundsen Sea circulation and heat transport. The aim is to elucidate the role of bathymetric features in shaping the circulation and in enabling heat transport from the deep ocean onto the continental shelf. Bathymetric variations along the continental slope enhance on-shelf heat transport by inducing breaks in the Antarctic Slope Front that separates off-shelf CDW from the colder, fresher shelf waters. The idealised model results imply that a ridge that blocks deep westward inflow from the Bellingshausen Sea leads to the existence of a deep cyclonic circulation on the shelf. Part of this circulation is an eastward undercurrent that flows along the continental shelf break. The broader cyclonic circulation transports heat that has been recently fluxed onto the shelf towards the south. These fundamental investigations will help refine the aims of future fieldwork and modelling.

We present the first investigation of subsidence due to sediment compaction and consolidation in two laboratory-scale river delta experiments. Spatial and temporal trends in subsidence rates in the experimental setting may elucidate behavior which cannot be directly observed at sufficiently long timescales, except for in reduced scale models such as the ones studied. We compare subsidence between a control experiment using steady boundary conditions, and an otherwise identical experiment which has been treated with a proxy for highly compressible marsh deposits. Both experiments have non-negligible compactional subsidence rates across the delta-top, comparable in magnitude to our boundary condition relative sea level rise of 250 μm/h. Subsidence in the control experiment (on average 54 μm/h) is concentrated in the lowest elevation (<10mm above sea level) areas near the coast and is likely due to creep induced by a rising water table near the shoreface. The treatment experiment exhibits larger (on average 126 μm/h) and more spatially variable subsidence rates controlled mostly by compaction of recent marsh deposits within one channel depth (_10 mm) of the sediment surface. These rates compare favorably with _eld and modeling based subsidence measurements both in relative magnitude and location. We find that subsidence “hot spots” may be relatively ephemeral on longer timescales, but average subsidence across the entire delta can be variable even at our shortest measurement window. This suggests that subsidence rates in a given decade or century may exceed thresholds for marsh platform drowning, even if the long term trend does not.

We use in situ measurements of suspended mud to assess the flocculation state of the lowermost freshwater reaches of the Mississippi River. The goal of the study was to assess the flocculation state of the mud in the absence of seawater, the spatial distribution of floc sizes within the river, and to look for seasonal differences between summer and winter. The data was also used to examine whether measured floc sizes could explain observed vertical distributions of suspended sediment concentration through a Rouse profile analysis. The surveys were conducted at the same location during summer and winter at similar discharges and suspended sediment concentrations, and in situ measures of the size distribution of the mud over the longitudinal, transverse, and vertical directions within the river were obtained using a specially developed underwater imaging system. These novel observations show that mud in the Mississippi is flocculated with median floc sizes ranging from 50 to 200 microns depending on location and season. On average flocs were found to be 40 microns larger during summer than in winter and to slightly increase in size moving downriver from the Bonnet Carré Spillway to Venice, LA. Floc size statistics varied little over the depth or laterally across the river at a given station. Bulk settling velocities calculated from size measurements matched values obtained from a Rouse profile analysis at stations with sandy beds, but underestimated settling velocities using the same equation parameters for measurements made during winter over muddy beds.

El Niño-Southern Oscillation (ENSO) is the most prominent interannual climate variability in the tropics and exhibits diverse features in spatiotemporal patterns. This paper develops a simple multiscale intermediate coupled stochastic model to capture the ENSO diversity and complexity. The model starts with a deterministic and linear coupled interannual atmosphere, ocean, and sea surface temperature (SST) system. It can generate two dominant linear solutions representing the eastern Pacific (EP) and the central Pacific (CP) El Niños, respectively. In addition to adopting a stochastic model for characterizing the intraseasonal wind bursts, another simple stochastic process is developed to describe the decadal variation of the background Walker circulation. The latter links the two dominant modes in a simple nonlinear fashion and advances the modulation of the strength and occurrence frequency of the EP and the CP events. Finally, cubic nonlinear damping is adopted to parameterize the relationship between subsurface temperatures and thermocline depth. The model succeeds in reproducing the spatiotemporal dynamical evolution of different types of ENSO events. It also accurately recovers the strongly non-Gaussian probability density function, the seasonal phase locking, the power spectrum, and the temporal autocorrelation function of the SST anomalies in all the three Niño regions (3, 3.4 and 4) across the equatorial Pacific. Furthermore, both the composites of the SST anomalies for various ENSO events and the strength-location bivariate distribution of equatorial Pacific SST maxima for the El Niño events from the model simulation highly resemble those from the observations.

Detailed pathway of wave energy exchange between the Pacific and Indian Oceans through the Indonesian archipelago and associated energy dissipation are investigated by using a reduced gravity model with realistic coastline. The wave energy flux analysis that can be applicable for all latitudes in a linear shallow water system is adopted. The energy fluxes diagnosed from the model outputs for the incoming Rossby waves from the Pacific clearly indicate two major energy pathways to the Indian Ocean; one turning southward in the Halmahera Sea and reaches the Indian Ocean via the Banda Sea and the Timor Passage, the other passing through the Makassar and Lombok Straits. The former route, however, is shifted to the western side of the island chain within the Banda Sea due to energy trapping around the island chain. It is also found that strong energy dissipation occurs along the northern coast of New Guinea when the period of the incoming Rossby wave is shorter than 1.5 year. In the case of the Kelvin waves from the Indian Ocean, it is found that the major energy pathway is through the Lombok and Makassar Straits to the Pacific Ocean. However, there appears another pathway along the eastern side of the Sulawesi Island in the Banda Sea to exit through the Molucca Sea only when the wave period is shorter than about one month. This secondary pathway makes it easier for the wave energy from the Indian Ocean to reach the western Pacific Ocean for the short period waves.

The debate over the historical and future evolution of the Atlantic Meridional Overturning Circulation (AMOC) has united scientists around a single topic, but this community has yet to unite around a single definition of the AMOC. In an effort to focus the debate around dynamics rather than semantics, we recommend that the community universally adopt a definition of the AMOC in density coordinates. We present evidence that the traditional depth space definition is insufficient at capturing elements of this circulation, especially at high latitudes where the northward and southward limbs of the AMOC are separated horizontally rather than vertically. Instead, the AMOC in density coordinates more realistically captures the water mass transformation process at high latitudes, shifts the maximum AMOC from the subtropical to the subpolar North Atlantic where the majority of the deep waters are formed, and depicts the peak in meridional heat transport associated with the subtropical gyre.

A current along a sloping bottom gives rise to upwelling, or downwelling Ekman transport within the stratified bottom boundary layer (BBL), also known as the bottom Ekman layer. In 1D models of slope currents, geostrophic vertical shear resulting from horizontal buoyancy gradients within the BBL is predicted to eventually bring the bottom stress to zero, leading to a shutdown, or \lq arrest \rq \, , of the BBL. Using 3D ROMS simulations, we explore how the dynamics of buoyancy adjustment in a current-ridge encounter problem differs from 1D and 2D temporal spin up problems. We show that in a downwelling BBL, the destruction of the ageostrophic BBL shear, and hence the bottom stress, is accomplished primarily by nonlinear straining effects during the initial topographic counter. As the current advects along the ridge slopes, the BBL deepens and evolves toward thermal wind balance. The onset of negative potential vorticity (NPV) modes of instability and their subsequent dissipation partially offsets the reduction of the BBL dissipation during the ridge-current interaction. On the upwelling side, although the bottom stress weakens substantially during the encounter, the BBL experiences a horizontal inflectional point instability and separates from the slopes before sustained along-slope stress reduction can occurred. In all our solutions, both the upwelling and downwelling BBLs are in a partially arrested state when the current separates from the ridge slope, characterized by a reduced, but non-zero bottom stress on the slopes.

We examine how zooplankton influence phytoplankton bloom phenology from the top-down, then use inverse modelling to infer the distribution and drivers of mean community zooplankton grazing dynamics based on the skill with which different simulated grazing formulations are able to recreate the observed seasonal cycle in phytoplankton biomass. We find that oligotrophic (eutrophic) biomes require more (less) efficient grazing dynamics, characteristic of micro- (meso-) zooplankton, leading to a strong relationship between the observed mean annual phytoplankton concentration in a region and the optimal grazing parameterization required to simulate it's observed phenology. Across the globe, we found that a type III functional response consistently exhibits more skill than a type II response, suggesting the mean dynamics of a coarse model grid-cell should offer stability and prey refuge at low biomass concentrations. These new observationally-based global distributions will be invaluable to help constrain, validate and develop next generation of biogeochemical models.

The Dotson Ice Shelf has resisted acceleration and ice-front retreat despite high basal-melt rates and rapid disaggregation of the neighboring Crosson Ice Shelf. Because of this lack of acceleration, previous studies have assumed that Dotson is stable. Here we show clear evidence of Dotson's destabilization as it decelerates, contrary to the common assumption that ice-flow deceleration is synonymous with stability. Ungrounding of a series of pinning points initiated acceleration in the Upper Dotson in the early 2000s, which subsequently slowed ice flow in the Lower Dotson. Discharge from the tributary Kohler Glacier into Crosson increased, but non-proportionally. Using ICESat and ICESat-2 altimetry data we show that ungrounding of the remaining pinning points is linked to a tripling in basal melt rates between 2006-2016 and 2016-2020. Basal melt rates on Crosson doubled over the same period. The higher basal melt at Lower Dotson is consistent with the cyclonic ocean circulation in the Dotson cavity, which tends to lift isopycnals and allow warmer deep water to interact with the ice. Given current surface-lowering rates, we estimate that several remaining pinning points in the Upper Dotson will unground within one to three decades. The grounding line of Kohler Glacier will retreat past a bathymetric saddle by the late 2030s and merge into the Smith West Glacier catchment, raising concern that reconfiguration of regional ice-flow dynamics and new pathways for the intrusion of warm modified Circumpolar Deep Water could further accelerate grounding-line retreat in the Dotson-Crosson Ice Shelf System.

The tsunami-generated magnetic field is a magnetic field that show up with the moving of tsunami. In the previous studies, researchers claimed that the tsunami-generated magnetic field arrives earlier than the tsunami sea level change based on analytical solutions and numerical simulations. In this paper, we used the world's first simultaneous data of sea level change and magnetic field in the 2009 Samoa and 2010 Chile tsunamis to study the relation between these two physical quantities. We found that the vertical component of tsunami magnetic field arrives earlier than the sea level change. Moreover, the horizontal component of tsunami magnetic field arrives even earlier than the vertical component. The tsunami magnetic field was also revealed that it can be used to estimate the tsunami wave height very accurately. We investigated the observed tsunami magnetic field by our 3-D time-domain simulation. However, the currently existing tsunami source models were unable to reproduce the observation in our research area. We confirmed that a better source model can improve the simulation. It follows that our high precision tsunami wave height data converted from the magnetic field should be used to construct a better tsunami source model.

Models and observations suggest that particle flux attenuation is lower across the mesopelagic zone of anoxic environments compared to oxic environments. Flux attenuation is controlled by microbial metabolism as well as aggregation and disaggregation by zooplankton, all of which also shape the relative abundance of differently sized particles. Observing and modeling particle spectra can provide information about the contributions of these processes. We measured particle size spectrum profiles at one station in the oligotrophic Eastern Tropical North Pacific Oxygen Deficient Zone (ETNP ODZ) using an underwater vision profiler (UVP), a high-resolution camera that counts and sizes particles. Measurements were taken at different times of day, over the course of a week. Comparing these data to particle flux measurements from sediment traps collected over the same time-period allowed us to constrain the particle size to flux relationship, and to generate highly resolved depth and time estimates of particle flux rates. We found that particle flux attenuated very little throughout the anoxic water column, and at some time-points appeared to increase. Comparing our observations to model predictions suggested that particles of all sizes remineralize more slowly in the ODZ than in oxic waters, and that large particles disaggregate into smaller particles, primarily between the base of the photic zone and 500 m. Acoustic measurements of multiple size classes of organisms suggested that many organisms migrated, during the day, to the region with high particle disaggregation. Our data suggest that diel-migrating organisms both actively transport biomass and disaggregate particles in the ODZ core.

Model simulations of past climates are increasingly found to compare well with proxy data at a global scale, but regional discrepancies remain. A persistent issue in modeling past greenhouse climates has been the temperature difference between equatorial and (sub-)polar regions, which is typically much larger in simulations than proxy data suggest. Particularly in the Eocene, multiple temperature proxies suggest extreme warmth in the southwest Pacific Ocean, where model simulations consistently suggest temperate conditions. Here we present new global ocean model simulations at 0.1° horizontal resolution for the middle-late Eocene. The eddies in the high-resolution model affect poleward heat transport and local time-mean flow in critical regions compared to the non-eddying flow in the standard low-resolution simulations. As a result, the high-resolution simulations produce higher surface temperatures near Antarctica and lower surface temperatures near the equator compared to the low-resolution simulations, leading to better correspondence with proxy reconstructions. Crucially, the high-resolution simulations are also much more consistent with biogeographic patterns in endemic-Antarctic and low-latitude-derived plankton, and thus resolve the long-standing discrepancy of warm subpolar ocean temperatures and isolating polar gyre circulation. The results imply that strongly eddying model simulations are required to reconcile discrepancies between regional proxy data and models, and demonstrate the importance of accurate regional paleobathymetry for proxy-model comparisons.

As a consequence of a diminishing sea ice cover in the Arctic, activity is on the rise. The position of the sea ice edge, which is generally taken to define the extent of the ice cover, changes in response to dynamic and thermodynamic processes. Forecasts for sea ice expansion due to an advancing ice edge will provide information that can be of significance for operations in polar regions. However, the value of this information depends on the quality of the forecasts. Here, we present methods for examining the quality of forecasted sea ice expansion and the geographic location where the largest expansion are expected from the forecast results. The algorithm is simple to implement, and an examination of two years of model results and accompanying observations demonstrates the usefulness of the analysis.

El Niño-Southern Oscillation (ENSO) shows a large diversity of events, whose modulation by climate variability and change, and their representation in climate models, limit our ability to predict their impact on ecosystems and human livelihood. Here, we introduce a new framework to analyze probabilistic changes in event-location and -intensity, which overcomes existing limitations in studying ENSO diversity. We find robust decadal variations in event intensities and locations in century-long observational datasets, which are associated with perturbations in equatorial wind-stress and thermocline depth, as well as extra-tropical anomalies in the North and South Pacific. A large fraction of CMIP5 and CMIP6 models appear capable of simulating such decadal variability in ENSO diversity, and the associated large-scale patterns. Projections of ENSO diversity in future climate change scenarios strongly depend on the magnitude of decadal variations, and the ability of climate models to reproduce them realistically over the 21st century.

Tropical instability waves (TIWs) in the equatorial eastern Pacific (EEP) exhibit 25–40-day westward-propagating fluctuations with seasonal and inter-annual variations, which are stronger during July–December and La Niña periods. They likely transfer their energy northward by forming barotropic Rossby waves (BTRWs). Long-term near-bottom current measurements at 10.5°N and 131.3°W during 2004–2013 revealed a spectral peak at 25–40 days, where significant coherences were found with satellite-measured sea surface height in a wide region of EEP with maxima approximately 5°N. Simulated deep currents from a data-assimilated ocean model concur with the observed near-bottom currents, and both currents vary seasonally and interannually, consistent with the typical characteristics of TIW. Further analyses using 25–40-day bandpass-filtered barotropic velocity data from the model revealed that they reasonably satisfied the theoretical dispersion relation of TIW-induced BTRW (BTRWTIW). We reconfirmed BTRWTIW propagating northward above 10°N in the northeastern Pacific by in-situ observations.