Ocean General Circulation Model Research Articles

Distinct Controls on the Strength of the Abyssal Overturning Circulation: Channel versus Basin Dynamics

AbstractAlthough the reconfiguration of the abyssal overturning circulation has been argued to be a salient feature of Earth’s past climate changes, our understanding of the physical mechanisms controlling its strength remains limited. In particular, existing scaling theories disagree on the relative importance of the dynamics in the Southern Ocean versus the dynamics in the basins to the north. In this study, we systematically investigate these theories and compare them with a set of numerical simulations generated from an ocean general circulation model with idealized geometry, designed to capture only the basic ingredients considered by the theories. It is shown that the disagreement between existing theories can be partially explained by the fact that the overturning strengths measured in the channel and in the basin scale distinctly with the external parameters, including surface buoyancy loss, diapycnal diffusivity, wind stress, and eddy diffusivity. The overturning in the reentrant channel, which represents the Southern Ocean, is found to be sensitive to all these parameters, in addition to a strong dependence on bottom topography. By contrast, the basin overturning varies with the integrated surface buoyancy loss rate and diapycnal diffusivity but is mostly unaffected by winds and channel topography. The simulated parameter dependence of the basin overturning can be described by a scaling theory that is based only on basin dynamics.

Energy Sources Generation and Energy Cascades along the Kuroshio East of Taiwan Island and the East China Sea

There are multi-spatial-scale ocean dynamic processes in the western boundary current region, so the budget of energy source and sink in the Kuroshio Current area can describe the oceanic energy cycle and transformation more accurately. The slope of the one-dimensional spectral energy density varies between −5/3 and −3 in the wavenumber range of 0.02–0.1 cpkm, indicating an inverse energy cascade in the Kuroshio of Taiwan Island and the East China Sea. According to the steady-state energy evolution, an energy source must be present. The locations of energy sources were identified using the spectral energy transfer calculated by 24 years of Ocean General Circulation Model for the Earth Simulator (OFES) data. At the sea surface, the kinetic energy (KE) sources are mainly within 23.2°–25.6° Nand 28°–29° N at less than 0.02 cpkm and within 23.2°–25° N and 26°–30° N at 0.02–0.1 cpkm. The available potential energy (APE) sources are mainly within 22°–28° N and 28.6°–30° N at less than 0.02 cpkm and within 22.6°–24.6° N, 25.4°–28° N and 29.2°–30° N at 0.02–0.1 cpkm. Beneath the sea surface, the energy sources are mainly above 400 m depth. Wind stress and density differences are primarily responsible for the KE and APE sources, respectively. Once an energy source is formed, to maintain a steady state, energy cascades (mainly inverse cascades by calculating spectral energy flux) will be engendered. By calculating the energy flux at 600 m depth, KE changes from inflow (sink) to outflow (source), and the conversion depth of source and sink is 380 m. However, outflow of the APE behaves as the source.

A local sigma-point unscented Kalman filter for geophysical data assimilation

The prior covariance calculated in the Reduced-rank Sigma-Point Kalman filter (RSPKF) data assimilation method can be suboptimal as a result of finite number of sigma points, effects of sampling error, process error, and other factors. In this study, we analyse the performance of RSPKF by applying a localization scheme that combines local analysis and global generation of sigma points. The analysis at each model grid for the entire domain are updated using only the observations local to the analysis grid. The localization enables a high rank approximation of the background error covariance in a local subspace of greatly lower dimension than the global domain using a small number of sigma points. The global-analysis vector is constructed combining the local analyses at all model grid points. The global analysis-covariance matrix is generated in the ensemble subspace and the global sigma points are constructed following the RSPKF algorithm. Numerical experiments of our method utilized the Lorenz-96 model. The performance of the localization scheme is assessed in the presence of varying parameters such as the number of sigma points (k), inflation factor (ϕ), localization radius (d) and the number of model variables (Nx). When the localization is implemented, the number of sigma points required to achieve the minimum RMSE is significantly reduced compared to a case where no localization is used, for three different cases of model variables (Nx). We also show that the approximate number of sigma points used to obtain optimal estimate is independent of the state dimension Nx of the model. This further highlights the importance of localization in RSPKF, making it a potential candidate for data assimilation in oceanic or atmospheric General Circulation Models (GCMs).

Coupling framework (1.0) for the PISM (1.1.4) ice sheet model and the MOM5 (5.1.0) ocean model via the PICO ice shelf cavity model in an Antarctic domain

Abstract. The past and future evolution of the Antarctic Ice Sheet is largely controlled by interactions between the ocean and floating ice shelves. To investigate these interactions, coupled ocean and ice sheet model configurations are required. Previous modelling studies have mostly relied on high-resolution configurations, limiting these studies to individual glaciers or regions over short timescales of decades to a few centuries. We present a framework to couple the dynamic ice sheet model PISM (Parallel Ice Sheet Model) with the global ocean general circulation model MOM5 (Modular Ocean Model) via the ice shelf cavity model PICO (Potsdam Ice-shelf Cavity mOdel). As ice shelf cavities are not resolved by MOM5 but are parameterized with the PICO box model, the framework allows the ice sheet and ocean components to be run at resolutions of 16 km and 3∘ respectively. This approach makes the coupled configuration a useful tool for the analysis of interactions between the Antarctic Ice Sheet and the global ocean over time spans of the order of centuries to millennia. In this study, we describe the technical implementation of this coupling framework: sub-shelf melting in the ice sheet component is calculated by PICO from modelled ocean temperatures and salinities at the depth of the continental shelf, and, vice versa, the resulting mass and energy fluxes from melting at the ice–ocean interface are transferred to the ocean component. Mass and energy fluxes are shown to be conserved to machine precision across the considered component domains. The implementation is computationally efficient as it introduces only minimal overhead. Furthermore, the coupled model is evaluated in a 4000 year simulation under constant present-day climate forcing and is found to be stable with respect to the ocean and ice sheet spin-up states. The framework deals with heterogeneous spatial grid geometries, varying grid resolutions, and timescales between the ice and ocean component in a generic way; thus, it can be adopted to a wide range of model set-ups.

AMOC and Climate Responses to Dust Reduction and Greening of the Sahara during the Mid-Holocene

AbstractNorth Africa was green during the mid-Holocene [about 6000 years ago (6 ka)] and emitted much less dust to the atmosphere than in the present day. Here we use a fully coupled atmosphere–ocean general circulation model, CESM1.2.2, to test the impact of dust reduction and greening of the Sahara on the Atlantic meridional overturning circulation (AMOC) during this period. Results show that dust removal leads to a decrease of AMOC by 6.2% while greening of the Sahara with 100% shrub (100% grass) cover causes an enhancement of the AMOC by 6.1% (4.8%). The AMOC is increased by 5.3% (2.3%) when both the dust reduction and green Sahara with 100% shrub (100% grass) are considered. The AMOC changes are primarily due to the precipitation change over the west subtropical North Atlantic, from where the salinity anomaly is advected to the deep-water formation region. Global-mean surface temperature increases by 0.09° and 0.40°C (0.25°C) when global dust is removed and when North Africa and the Arabian region are covered by shrub (grass), respectively, showing a dominating effect of vegetation over dust. The comparison between modeled and reconstructed sea surface temperature is improved when the effect of vegetation is considered. The results may have implications for climate impact of future wetting over North Africa, either through global warming or through building of solar farms and wind farms.

Система оперативного моделирования Северного Ледовитого океана и прилегающих к нему акваторий на основе российской модели INMOM-Арктика

A regional σ-model INMOM-Arctic has been prepared on the basis of the Russian ocean general circulation model INMOM (Institute of Numerical Mathematics Ocean Model) to reproduce the current state and short-term forecast of the Arctic Ocean (AO) hydrothermodynamics. The model is implemented in a rotated spherical coordinate system with the poles located at 60°E and 120° W on the geographic equator, which makes it possible to use a quasi-uniform resolution of ~ 3,7 km in the Arctic Basin. Data on temperature, salinity, horizontal velocity components and sea level taken from the CMEMS ocean products are used at the AO open boundaries. To take into account the tidal effect in the INMOM-Arctic model at open boundaries, the time series of the tidal sea level is set based on the data of the TPXO 9 atlas (TOPEX/Poseidon Global Tidal Model) with a spatial resolution of 1/30°. To calculate the atmospheric impact, the researches use the atmospheric circulation data from the Era 5 global reanalysis with a spatial resolution of 0,25×0,25° and with a temporal resolution of 1 hour.

A new presentation of the Indian Ocean shallow overturning circulation from a vertical perspective

The calculation of the meridional overturning streamfunction in the southern Indian Ocean is biased by the Indonesian Throughflow. Therefore, this study applies the vertical overturning streamfunction to diagnose the shallow overturning circulation in the Indian Ocean. Using the Ocean General Circulation Model for the Earth simulator output, improvements with the vertical overturning streamfunction compared with the meridional overturning streamfunction are explored. The results show that the vertical overturning streamfunction smoothly connects the shallow overturning circulations of the northern Indian Ocean and the southern Indian Ocean with the whole cycle of the subtropical cell and the cross-equatorial cell. The vertical overturning streamfunction shows a much cleaner shallow overturning circulation, which is underestimated by the meridional overturning streamfunction. It shows that the shallow overturning circulation has a magnitude of ~13 Sv (1 Sv 106 m3 s−1), of which the subtropical cell accounts for ~8 Sv. In addition, the vertical overturning streamfunction captures a clockwise overturning cell in the upper 600 m layer between 30°S and 34°S. This cell has a magnitude of about −5 Sv and probably corresponds to the wind-forced subtropical gyre. Therefore, the vertical overturning streamfunction provides a new approach for estimating the shallow overturning circulation in the Indian Ocean.摘要由于印尼贯穿流干扰了南印度洋经向翻转环流的计算, 本文应用垂向翻转流函数来诊断印度洋浅层经向翻转环流. 基于海洋环流模拟产品 (OFES), 本文探讨了垂向流函数相比经向翻转流函数的改进. 研究结果表明垂向翻转流函数能平滑连接南北印度洋的翻转环流, 包括完整的副热带翻转环流和跨赤道翻转环流. 经向翻转流函数低估了印度洋浅层翻转环流; 而垂向翻转流函数表示的印度洋浅层翻转环流受印尼贯穿流影响更小, 其评估的印度洋浅层翻转环流强度约13 Sv, 其中副热带翻转环流强度约8 Sv (1 Sv 106 m3 s−1). 此外, 垂向翻转流函数在印度洋南开边界 (30°S~34°S) 上600 m层表现为顺时针翻转结构, 其强度约−5 Sv, 可能对应风驱的副热带流系. 因此, 垂向翻转流函数提供了一种新方法来评估印度洋浅层翻转环流.

Projected changes in the climate of Pakistan using IPCC AR5-based climate models

Using an ensemble of 22 climate models from the 5th Annual Report of Intergovernmental Panel on Climate Change (IPCC AR5), the projected robustness and variability of temperature and precipitation for the data-sparse region of Pakistan is studied both on seasonal and annual time scales for the 21st century. The winter season in Pakistan is displaying ensemble-based spatially robust and progressively more relative warming in temperature under representative concentration pathway (RCP) 8.5 scenario as compared to RCP 4.5 scenario, both in the middle (2035−2064) and end (2070−2099) of 21st century projection periods. On the other hand, the ensemble-based relative changes in precipitation during the aforementioned two projected periods are spatially less robust. Most of the atmosphere–ocean general circulation models (AOGCMs) project a relative increase of 5−10% in annual precipitation in all the regions of Pakistan. On a seasonal time scale, most AOGCMs project a relative precipitation decrease (increase) during winter (summer) in central and in southern (central and northern) Pakistan. All the AOGCMs under both RCPs project an increase in temperature in all Pakistan, northern Pakistan, and southern Pakistan on annual, winter, and summer time scales.

The role of Arctic gateways on sea ice and circulation in the Arctic and North Atlantic Oceans: a sensitivity study with an ocean-sea-ice model

The impact of changes in volume, heat and freshwater fluxes through Arctic gateways on sea ice, circulation and fresh water and heat contents of the Arctic and North Atlantic Oceans is not fully understood. To explore the role played by each gateway, we use a regional sea-ice ocean general circulation model with a fixed atmospheric forcing. We run sensitivity simulations with combinations of Bering Strait (BS) and Canadian Arctic Archipelago (CAA) open and closed inspired by paleogeography of the Arctic. We show that fluxes through BS influence the Arctic, Atlantic and Nordic Seas while the impact of the CAA is more dominant in the Nordic Seas. In the experiments with BS closed, there is a change in the surface circulation of the Arctic with a weakening of the Beaufort Gyre by about thirty percent. As a consequence, the Siberian river discharge is spread offshore to the west, rather than being directly advected away by the Transpolar Drift. This results in a decrease of salinity in the upper 50 m across much of the central Arctic and East Siberian and Chukchi Seas. We also find an increase in stratification between the surface and subsurface layers after closure of BS. Moreover, closure of the BS results in an upward shift of the relatively warm waters lying between 50 and 120 m, as well as a reorganization of heat storage and transport. Consequently, more heat is kept in the upper layers of the Arctic Ocean, thus increasing the heat content in the upper 50 m and leading to a thinner sea ice cover. The CAA closing has a large impact on sea ice, temperature and salinity in the subarctic North Atlantic with opposite responses in the Greenland-Iceland-Norwegian Seas and Baffin Bay. It is also found that CAA being open or closed strongly controls the sea ice export through the Fram Strait. In all our experiments, the changes in temperature and salinity of the Barents and Kara Seas, and in fluxes through Barents Sea Opening are relatively small, suggesting that they are likely controlled by the atmospheric processes. Our results demonstrate the need to take into consideration the fluxes through the Arctic gateways when addressing the ocean and climate changes during deglaciations as well as for predictions of future climate.

Near-field wind mixing and implications on parameterization from float observations

AbstractA part of near-inertial wind energies dissipates locally below the surface mixed layer. Here, their role in the climate system is studied by adopting near-inertial near-field wind-mixing parameterization to a coarse-forward ocean general circulation model. After confirming a problem of the parameterization in the equatorial region, we investigate effects of near-field wind mixing due to storm track activities in the North Pacific. We found that, in the center of the Pacific Decadal Oscillation (PDO) around 170°W in the mid latitude, near-field wind mixing transfers the PDO signal into deeper layers. Since the results suggest that near-field wind mixing is important in the climate system, we also compared the parameterization with velocity observations by a float in the North Pacific. The float observed abrupt and local propagation of near-inertial internal waves and shear instabilities in the main thermocline along the Kuroshio Extension for 460 km. Vertical diffusivities inferred from the parameterization do not reproduce the enhanced diffusivities in the deeper layer inferred from the float. Wave-ray tracing indicates that wave trapping near the Kuroshio front is responsible for the elevated diffusivities. Therefore, enhanced mixing due to trapping should be included in the parameterization.

The impact of climate change on the economic perspectives of crop farming in Pakistan: Using the ricardian model

Climate change may have detrimental effects on income from crop farming in Pakistan and according to the IPCC (2014), the impacts of climate change will be more vigorous in the coming years. Despite a few studies we still have limited knowledge on how climate change can impact the crop farming economy in Pakistan. Considering Khyber Pakhtunkhwa (KP) province of Pakistan as the study area, we attempted to fill this gap by analyzing the economic impacts of climate change on the net income from crop farming. Data from the 635 farm households and also climatic variables data were collected from five agro-ecological zones (AEZ) of the KP. The Ricardian method was employed for analysis purposes. The results revealed that the increase of annual average temperature and decreases in rainfall is strongly associated with net revenue deficits. The effects of increasing precipitation are observed to be useful but an increase in temperature is forecasted to be harmful to the net revenue. The effect of increased temperature in the summer season on NR ranged between US$-29 to US$35, and the effect in the winter season ranged between US$-14 to US$28 per hectare. The effect of decreased annual precipitation on NR in the summer season and winter season ranged between US$ 0 to US$-24 and US$<-20 per hectare, respectively. This effect is particularly severe in the Central Valley Plains, Southern Piedmont Plains, and Western Dry Mountains. This study further evaluates the effects of the climatic forecasts by three Atmospheric Oceanic General Circulation Models (AOGCM). Our results will facilitate the policymakers to implement suitable strategies in KP to curb the deleterious effects of climate change on overall crop production and farmer income.

The 20th century global warming signature on the ocean at global and basin scales as depicted from historical reanalyses

AbstractMore than 90% of excess energy trapped by greenhouse gases is accumulated in the oceans. Ocean heat content (OHC) and its changes are therefore fundamental metrics to monitor climate change. However, due to sparse observation sampling before the 1950s, accurate observation‐based estimations only exist for the second half of the 20th century. A 16‐member ensemble of historical ocean reanalyses is used for the first time to compile a unique estimate of 20th century oceanic warming rates. The reanalyses combine dynamical ocean general circulation models with historical observations and observation‐based atmospheric forcing. Ocean heat content tendencies (OHCT) from the multireanalysis ensemble (MRE) agree well with independent estimates of ocean heat content, and show a coherent evolution with records of Earth's energy imbalance from atmospheric reanalyses and atmospheric CO2concentration at a range of timescales. OHCT from reanalyses proves to be a more effective climate change proxy, that is, more closely related to independent climate indexes, than observed surface warming tendencies that contain high‐frequency variability not related to the climate change signature, or historical coupled model simulations, which neglect interannual fluctuations. The ensemble mean estimate of the century‐long ocean warming rate is 0.26 ± 0.08 W m−2. The warming rate of 0.84 ± 0.21 W m−2estimated from 1993 onwards is unprecedented. The global decadal warming rate is persistently positive from about 1925 onwards, except for two neutral periods. The Indian Ocean exhibits the highest relative contribution to centennial heat accumulation, while the Atlantic Ocean plays an increasingly prominent role, especially during the 1995–2004 decade. These findings are in agreement with previous studies.

Volume Transport Variability in the Western Equatorial Pacific and its Relations to Halmahera Throughflow

This study investigates the coherency of volume transport between Halmahera throughflow and current major system in the western equatorial Pacific Ocean (Mindanao Current – MC, New Guinea Coastal/Under Current – NGCC/NGCUC, and North Equatorial Counter Current – NECC). The validated daily ocean general circulation model datasets of INDESO (2010-2014) were used in this study. The results showed that the estimated average transport volume was 25.6 Sv flowing southward through MC, 34.5 Sv flowing eastward through NECC, 18.3 Sv flowing northwestward through NGCC/NGCUC, and 2.5 Sv flowing southward through the Halmahera Sea. The variability of volume transport was dominated by intraseasonal, semiannual, and annual time-scales. The increased transport of NECC corresponded to the intensification of MC and NGCC/NGCUC transports. NGCC/ NGCUC significantly controlled the South Pacific water inflow into the Halmahera Sea because of the positively high correlation between NGCC/NGCUC transport and Halmahera throughflow transport.

HIDRA 1.0: deep-learning-based ensemble sea level forecasting in the northern Adriatic

Abstract. Interactions between atmospheric forcing, topographic constraints to air and water flow, and resonant character of the basin make sea level modelling in the Adriatic a challenging problem. In this study we present an ensemble deep-neural-network-based sea level forecasting method HIDRA, which outperforms our set-up of the general ocean circulation model ensemble (NEMO v3.6) for all forecast lead times and at a minuscule fraction of the numerical cost (order of 2×10-6). HIDRA exhibits larger bias but lower RMSE than our set-up of NEMO over most of the residual sea level bins. It introduces a trainable atmospheric spatial encoder and employs fusion of atmospheric and sea level features into a self-contained network which enables discriminative feature learning. HIDRA architecture building blocks are experimentally analysed in detail and compared to alternative approaches. Results show the importance of sea level input for forecast lead times below 24 h and the importance of atmospheric input for longer lead times. The best performance is achieved by considering the input as the total sea level, split into disjoint sets of tidal and residual signals. This enables HIDRA to optimize the prediction fidelity with respect to atmospheric forcing while compensating for the errors in the tidal model. HIDRA is trained and analysed on a 10-year (2006–2016) time series of atmospheric surface fields from a single member of ECMWF atmospheric ensemble. In the testing phase, both HIDRA and NEMO ensemble systems are forced by the ECMWF atmospheric ensemble. Their performance is evaluated on a 1-year (2019) hourly time series from a tide gauge in Koper (Slovenia). Spectral and continuous wavelet analysis of the forecasts at the semi-diurnal frequency (12 h)−1 and at the ground-state basin seiche frequency (21.5 h)−1 is performed. The energy at the basin seiche in the HIDRA forecast is close to that observed, while our set-up of NEMO underestimates it. Analyses of the January 2015 and November 2019 storm surges indicate that HIDRA has learned to mimic the timing and amplitude of basin seiches.

Towards multiscale modeling of ocean surface turbulent mixing using coupled MPAS-Ocean v6.3 and PALM v5.0

Abstract. A multiscale modeling approach for studying the ocean surface turbulent mixing is explored by coupling an ocean general circulation model (GCM) MPAS-Ocean with the Parallelized Large Eddy Simulation Model (PALM). The coupling approach is similar to the superparameterization approach that has been used to represent the effects of deep convection in atmospheric GCMs. However, the focus of this multiscale modeling approach is on the small-scale turbulent mixing and their interactions with the larger-scale processes in the ocean, so that a more flexible coupling strategy is used. To reduce the computational cost, a customized version of PALM is ported on the general-purpose graphics processing unit (GPU) with OpenACC, achieving 10–16 times overall speedup as compared to running on a single CPU. Even with the GPU-acceleration technique, a superparameterization-like approach to represent the ocean surface turbulent mixing in GCMs using embedded high fidelity and three-dimensional large eddy simulations (LESs) over the global ocean is still computationally intensive and infeasible for long simulations. However, running PALM regionally on selected MPAS-Ocean grid cells is shown to be a promising approach moving forward. The flexible coupling between MPAS-Ocean and PALM allows further exploration of the interactions between the ocean surface turbulent mixing and larger-scale processes, as well as future development and improvement of ocean surface turbulent mixing parameterizations for GCMs.

A Barotropic Solver for High-Resolution Ocean General Circulation Models

High-resolution global ocean general circulation models (OGCMs) play a key role in accurate ocean forecasting. However, the models of the operational forecasting systems are still not in high resolution due to the subsequent high demand for large computation, as well as the low parallel efficiency barrier. Good scalability is an important index of parallel efficiency and is still a challenge for OGCMs. We found that the communication cost in a barotropic solver, namely, the preconditioned conjugate gradient (PCG) method, is the key bottleneck for scalability due to the high frequency of the global reductions. In this work, we developed a new algorithm—a communication-avoiding Krylov subspace method with a PCG (CA-PCG)—to improve scalability and then applied it to the Nucleus for European Modelling of the Ocean (NEMO) as an example. For PCG, inner product operations with global communication were needed in every iteration, while for CA-PCG, inner product operations were only needed every eight iterations. Therefore, the global communication cost decreased from more than 94.5% of the total execution time with PCG to less than 63.4% with CA-PCG. As a result, the execution time of the barotropic modes decreased from more than 17,000 s with PCG to less than 6000 s with CA-PCG, and the total execution time decreased from more than 18,000 s with PCG to less than 6200 s with CA-PCG. Besides, the ratio of the speedup can also be increased from 3.7 to 4.6. In summary, the high process count scalability when using CA-PCG was effectively improved from that using the PCG method, providing a highly effective solution for accurate ocean simulation.

Process-Based Modeling of the High Flow of a Semi-Mountain River under Current and Future Climatic Conditions: A Case Study of the Iya River (Eastern Siberia)

The purpose of the study was to analyze the formation conditions of catastrophic floods in the Iya River basin over the observation period, as well as a long-term forecast of the impacts of future climate change on the characteristics of the high flow in the 21st century. The semi-distributed process-based Ecological Model for Applied Geophysics (ECOMAG) was applied to the Iya River basin. Successful model testing results were obtained for daily discharge, annual peak discharge, and discharges exceeding the critical water level threshold over the multiyear period of 1970–2019. Modeling of the high flow of the Iya River was carried out according to a Kling–Gupta efficiency (KGE) of 0.91, a percent bias (PBIAS) of −1%, and a ratio of the root mean square error to the standard deviation of measured data (RSR) of 0.41. The preflood coefficient of water-saturated soil and the runoff coefficient of flood-forming precipitation in the Iya River basin were calculated in 1980, 1984, 2006, and 2019. Possible changes in the characteristics of high flow over summers in the 21st century were calculated using the atmosphere–ocean general circulation model (AOGCM) and the Hadley Centre Global Environment Model version 2-Earth System (HadGEM2-ES) as the boundary conditions in the runoff generation model. Anomalies in values were estimated for the middle and end of the current century relative to the observed runoff over the period 1990–2019. According to various Representative Concentration Pathways (RCP-scenarios) of the future climate in the Iya River basin, there will be less change in the annual peak discharge or precipitation and more change in the hazardous flow and its duration, exceeding the critical water level threshold, at which residential buildings are flooded.

Sea Surface Height Variability in the 30–120 km Wavelength Band From Altimetry Along‐Track Observations

AbstractReprocessed altimetry along‐track sea surface height (SSH) observations from multiple missions are analyzed for the SSH variability in the 30–120 km wavelength band. This band of variability is missing in the widely used global multi‐mission mapped SSH data set, known as the Archiving, Validation and Interpretation of Satellite Oceanographic Data SSH product. The SSH variability is derived from along‐track SSH variance spectra after subtracting noise floors, and its global spatial patterns are consistent among the missions and the output of a high‐resolution ocean general circulation model simulation. High SSH variability in the 30–120 km wavelength band is found in the intratropics of all ocean basins (25°S–25°N), the western boundary current outflow regions, and in five regions along the Antarctic Circumpolar Current: Agulhas Retroflection zone, Kerguelen Plateau, Macquarie Ridge, Pacific Antarctic Ridge, and Drake Passage. The average slope of the along‐track SSH spectra in the 30–120 km wavelength band in the intratropics falls between −2.14 and −1.56, consistent with the Garrett‐Munk spectral slope. The spatial patterns of the SSH variability could be used as a metric to evaluate high‐resolution numerical models and to shed light on what to expect from future high‐resolution altimetry missions.

A global eddy-resolving ocean forecast system in China – LICOM Forecast System (LFS)

ABSTRACT A global eddy-resolving forecast system LFS is developed based on a primitive ocean general circulation model. The system's configuration, forecast experiments, and a preliminary evaluation of the preoperational system are shown. In the preoperational stage, since the full data assimilation has not yet been set up for LFS, the initial state is obtained by nudging the ocean temperature and salinity from an ocean analysis dataset. Despite this, the LFS demonstrates a generally good performance in short-term oceanography forecasting, except for sea level anomaly (SLA). The median values of the 1-day forecast leading root mean square error (RMSE) for the sea surface temperature (SST), SLA, upper 2000 m temperature, and salinity are approximately 0.52°C, 0.10 m, 0.57°C and 0.13 psu, respectively. Although there are slight warm biases in the forecasted SST, the forecasts of temperature and salinity in the thermocline by the LFS are comparable with the results of operational oceanography systems under the framework of the Intercomparison and Validation Task Team. However, the forecast SLA has a relatively large RMSE related to the absence of direct observational constraints in the initial state. Further investigations are needed to improve the performance of LFS.

Simulation of global distribution of rare earth elements in the ocean using an ocean general circulation model

In this study, we report our ocean general circulation model simulations of the global distribution of rare earth elements (REEs) in the ocean. As previously reported (Oka et al. in Glob Biogeochem Cycles 23:1–16, 2009), the vertical profiles of REEs in the North Pacific Ocean are strongly controlled by the reversible scavenging process, and the systematic differences between REEs can be reproduced in the model by selecting an appropriate model parameter which controls affinity to particles. We here demonstrate that the external REE input from the coastal regions also plays a role in controlling the vertical profiles of dissolved REE and their inter-basin differences. The role of the external inputs is especially important for light REEs, such as neodymium (Nd). The linear increase in Nd concentration in the North Pacific Ocean cannot be sufficiently reproduced by the reversible scavenging alone; rather, a combination of the reversible scavenging and the external inputs is necessary. On the other hand, the distribution of heavy REEs, such as lutetium (Lu), can be broadly reproduced without the external inputs, suggesting that Lu has similarity with conservative nutrient-like tracer. When compared with REE observations compiled from both the recently obtained GEOTRACES dataset and pre-GEOTRACES reported data, our simulations successfully reproduced the overall features of these observations. Observational data suggested that the vertical profiles of REEs are not the same among the basins; our model simulations demonstrate that this feature can be clearly reproduced by considering both the reversible scavenging and the external REE inputs from the coastal regions.