Global upper ocean heat content and climate variability

Observational data from the Global Temperature and Salinity Profile Program were used to calculate the upper ocean heat content (OHC) anomaly. The thickness of the upper layer is taken as 300 m for the Pacific/Atlantic Ocean and 150 m for the Indian Ocean since the Indian Ocean has shallower thermoclines. First, the optimal spectral decomposition scheme was used to build up monthly synoptic temperature and salinity dataset for January 1990 to December 2009 on 1° × 1° grids and the same 33 vertical levels as the World Ocean Atlas. Then, the monthly varying upper layer OHC field (H) was obtained. Second, a composite analysis was conducted to obtain the total-time mean OHC field ([`([`(H)])] \bar{\bar{H}} ) and the monthly mean OHC variability ( [(\textH)\tilde] \widetilde{\text{H}} ), which is found an order of magnitude smaller than [^(\textH)] \widehat{\text{H}} . Third, an empirical orthogonal function (EOF) method is conducted on the residue data ( [^(\textH)] \widehat{\text{H}} ), deviating from [(\textH)\tilde] \widetilde{\text{H}}  +  [(\textH)\tilde] \widetilde{\text{H}} , in order to obtain interannual variations of the OHC fields for the three oceans. In the Pacific Ocean, the first two EOF modes account for 51.46% and 13.71% of the variance, representing canonical El Nino/La Nina (EOF-1) and pseudo-El Nino/La Nina (i.e., El Nino Modoki; EOF-2) events. In the Indian Ocean, the first two EOF modes account for 24.27% and 20.94% of the variance, representing basin-scale cooling/warming (EOF-1) and Indian Ocean Dipole (EOF-2) events. In the Atlantic Ocean, the first EOF mode accounts for 49.26% of the variance, representing a basin-scale cooling/warming (EOF-1) event. The second EOF mode accounts for 8.83% of the variance. Different from the Pacific and Indian Oceans, there is no zonal dipole mode in the tropical Atlantic Ocean. Fourth, evident lag correlation coefficients are found between the first principal component of the Pacific Ocean and the Southern Oscillation Index with a maximum correlation coefficient (0.68) at 1-month lead of the EOF-1 and between the second principal component of the Indian Ocean and the Dipole Mode Index with maximum values (around 0.53) at 1–2-month advance of the EOF-2. It implies that OHC anomaly contains climate variability signals.     

Oscillating glacial northern and southern deep water formation from combined neodymium and carbon isotopes

While ocean circulation is driven by the formation of deep water in the North Atlantic and the Circum-Antarctic, the role of southern-sourced deep water formation in climate change is poorly understood. Here we address the balance of northern- and southern-sourced waters in the South Atlantic through the last glacial period using neodymium isotope ratios of authigenic ferromanganese oxides in thirteen deep sea cores from throughout the South Atlantic. The data indicate that northern-sourced water did not reach the Southern Ocean during the late glacial, and was replaced by southern-derived intermediate and deep waters. The high-resolution neodymium isotope record (~ 300 yr sample spacing) from two spliced deep Cape Basin sites indicates that over the last glacial period northern-sourced water mass export to the Southern Ocean was stronger during the major Greenland millennial warming intervals (and Southern Hemisphere cool periods), and particularly during the major interstadials 8, 12, and 14. Northern-sourced water mass export was weaker during Greenland stadials and reached minima during Heinrich Events. The benthic foraminiferal carbon isotopes in the same Cape Basin core reflect a partial control by Southern Hemisphere climate changes and indicate that deep water formation and ventilation occurred in the Southern Ocean during major Greenland cooling intervals (stadials). Together, neodymium isotopes and benthic carbon isotopes provide new information about water mass sourcing and circulation in deep Southern Ocean waters during rapid glacial climate changes. Combining carbon and neodymium isotopes can be used to monitor the relative proportion of northern- and southern-sourced waters in the Cape Basin to gain insight into the processes which control the carbon isotopic composition of deep waters. In this study we show that deep water formation and circulation was more important than biological productivity and nutrient regeneration changes for controlling the carbon isotope chemistry of Antarctic Bottom Water during millennial-scale glacial climate cycles. This observation also lends support to the hypothesis that ocean circulation is linked to interhemispheric climate changes on short timescales, and that ventilation in the glacial ocean rapidly switched between the northern and Southern Hemisphere on millennial timescales.     

全球海洋碳循环模式MOM4_L40对碳和营养物自然分布的模拟

本文介绍了国家气候中心发展的一个全球海洋碳循环环流模式,并分析评估了该模式的基本性能.该模式是在美国地球物理流体动力学实验室(GFDL,Geophysical Fluid Dynamics Laboratory)的全球海洋环流模式MOM4(Modular Ocean Model Version 4)基础上发展的一个垂直方向40层、包含生物地球化学过程的全球三维海洋碳循环环流模式,简称为MOM4_L40(Modular Ocean Model Version 4 With 40Levels).该模式在气候场强迫下长期积分1000年,结果分析表明,与观测相比,模式较好地模拟了海洋温度、盐度、总二氧化碳、总碱、总磷酸盐的表面和垂直分布特征.模拟的海洋总二氧化碳分布与观测基本相符,表层为低值区,其下为高值区,高值区域位于10°S—60°N之间,但2000m以上模拟值较观测偏小,2000m以下模拟值较观测偏大.总体来说,MOM4_L40模式是一个可信赖的海洋碳循环过程模拟研究工具.     

Regional and interannual variability of ecosystem dynamics in the Southern Ocean

To investigate regional and interannual variability of the ecosystem in the Southern Ocean, a coupled circumpolar ice–ocean–plankton model has been developed. The ice–ocean component (known as BRIOS-2) is based on a modified version of the s-coordinate primitive equation model (SPEM) coupled to a dynamic–thermodynamic sea-ice model. The biological model (BIMAP) comprises two biogeochemical cycles – silica and nitrogen – and a prognostic iron compartment to include possible effects of micronutrient limitation. Simulations with the coupled ice–ocean–plankton model indicate that the physical–biological interaction is not limited to the effect of a varying surface mixed-layer depth. In the Pacific sector, large anomalies in winter mixed-layer depth cause an increased iron supply and enhance primary production and plankton biomass in the following summer, whereas in the Atlantic sector variability in primary production is caused mainly by fluctuations of oceanic upwelling. Thus, the Antarctic Circumpolar Wave (ACW) induces regional oscillations of phytoplankton biomass in both sectors, but not a propagating signal. Furthermore, interannual variability in plankton biomass and primary production is strong in the Coastal and Continental Shelf Zone and the Seasonal Ice Zone around the Antarctic continent. Interannual variability induced by the ACW has large effects on the regional scale, but the associated variability in biogenic carbon fluxes is small compared to the long-term carbon sequestration of the Southern Ocean.     

On the influence of adequate Weddell Sea characteristics in a large-scale global ocean circulation model

Global ocean circulation models usually lack an adequate consideration of high-latitude processes due to a limited model domain or insufficient resolution. Without the processes in key areas of the global thermohaline circulation, the characteristics and flow of deep and bottom waters cannot be modeled realistically. In this study, a high-resolution (~20 km) ocean model focused on the Weddell Sea sector of the Southern Ocean is combined with a low-resolution (2^° × 2^°) global ocean model applying the state estimation technique. Temperature, salinity, and velocity data on two Weddell Sea sections from the regional model are used as constraints for the large-scale model in addition to satellite altimetry and sea-surface temperatures. The differences between the model with additional constraints and without document that the Weddell Sea circulation exerts significant influence on the course of the Antarctic Circumpolar Current with consequences for Southern Ocean water mass characteristics and the spreading of deep and bottom waters in the South Atlantic. Furthermore, a warming trend in the period 1993–2001 was found in the Weddell Sea and adjacent basins in agreement with float measurements in the upper Southern Ocean. Teleconnections to the North Atlantic are suggested but need further studies to demonstrate their statistical significance.     

Development and testing of a coupled ocean–atmosphere mesoscale ensemble prediction system

A coupled ocean–atmosphere mesoscale ensemble prediction system has been developed by the Naval Research Laboratory. This paper describes the components and implementation of the system and presents baseline results from coupled ensemble simulations for two tropical cyclones. The system is designed to take into account major sources of uncertainty in: (1) non-deterministic dynamics, (2) model error, and (3) initial states. The purpose of the system is to provide mesoscale ensemble forecasts for use in probabilistic products, such as reliability and frequency of occurrence, and in risk management applications. The system components include COAMPS® (Coupled Ocean/Atmosphere Mesoscale Prediction System) and NCOM (Navy Coastal Ocean Model) for atmosphere and ocean forecasting and NAVDAS (NRL Atmospheric Variational Data Assimilation System) and NCODA (Navy Coupled Ocean Data Assimilation) for atmosphere and ocean data assimilation. NAVDAS and NCODA are 3D-variational (3DVAR) analysis schemes. The ensembles are generated using separate applications of the Ensemble Transform (ET) technique in both the atmosphere (for moving or non-moving nests) and the ocean. The atmospheric ET is computed using wind, temperature, and moisture variables, while the oceanographic ET is derived from ocean current, temperature, and salinity variables. Estimates of analysis error covariance, which is used as a constraint in the ET, are provided by the ocean and atmosphere 3DVAR assimilation systems. The newly developed system has been successfully tested for a variety of configurations, including differing model resolution, number of members, forecast length, and moving and fixed nest options. Results from relatively coarse resolution (～27-km) ensemble simulations of Hurricanes Hanna and Ike demonstrate that the ensemble can provide valuable uncertainty information about the storm track and intensity, though the ensemble mean provides only a small amount of improved predictive skill compared to the deterministic control member.     

A climatic dataset of ocean vertical turbulent mixing coefficient based on real energy sources

Using data on wind stress, significant height of combined wind waves and swell, potential temperature, salinity and seawater velocity, as well as objectively-analyzed in situ temperature and salinity, we established a global ocean dataset of calculated wind- and tide-induced vertical turbulent mixing coefficients. We then examined energy conservation of ocean vertical mixing from the point of view of ocean wind energy inputs, gravitational potential energy change due to mixing (with and without artificially limiting themixing coefficient), and K-theory vertical turbulent parameterization schemes regardless of energy inputs. Our research showed that calculating the mixing coefficient with average data and artificial limiting the mixing coefficient can cause a remarkable lack of energy conservation, with energy losses of up to 90% and changes in the energy oscillation period. The data also show that wind can introduce a huge amount of energy into the upper layers of the Southern Ocean, and that tidesdo so in regions around underwater mountains. We argue that it is necessary to take wind and tidal energy inputs into account forlong-term ocean climate numerical simulations. We believe that using this ocean vertical turbulent mixing coefficient climatic dataset is a fast and efficient method to maintain the ocean energy balance in ocean modeling research.     

The roles of surface heat flux and ocean heat transport convergence in determining Atlantic Ocean temperature variability

The temperature variability of the Atlantic Ocean is investigated using an eddy-permitting (1/4°) global ocean model (ORCA-025) forced with historical surface meteorological fields from 1958 to 2001. The simulation of volume-averaged temperature and the vertical structure of the zonally averaged temperature trends are compared with those from observations. In regions with a high number of observations, in particular above a depth of 500 m and between 22° N and 65° N, the model simulation and the dataset are in good agreement. The relative contribution of variability in ocean heat transport (OHT) convergence and net surface heat flux to changes in ocean heat content is investigated with a focus on three regions: the subpolar and subtropical gyres and the tropics. The surface heat flux plays a relatively minor role in year-to-year changes in the subpolar and subtropical regions, but in the tropical North Atlantic, its role is of similar significance to the ocean heat transport convergence. The strongest signal during the study period is a cooling of the subpolar gyre between 1970 and 1990, which subsequently reversed as the mid-latitude OHT convergence transitioned from an anomalously weak to an anomalously strong state. We also explore whether model OHT anomalies can be linked to surface flux anomalies through a Hovmöller analysis of the Atlantic sector. At low latitudes, increased ocean heat gain coincides with anomalously strong northward transport, whereas at mid-high latitudes, reduced ocean heat loss is associated with anomalously weak heat transport.     

South Atlantic circulation in a world ocean model

The circulation in the South Atlantic Ocean has been simulated within a global ocean general circulation model. Preliminary analysis of the modelled ocean circulation in the region indicates a rather close agreement of the simulated upper ocean flows with conventional notions of the large-scale geostrophic currents in the region. The modelled South Atlantic Ocean witnesses the return flow and export of North Atlantic Deep Water (NADW) at its northern boundary, the inflow of a rather barotropic Antarctic Circumpolar Current (ACC) through the Drake Passage, and the inflow of warm saline Agulhas water around the Cape of Good Hope. The Agulhas leakage amounts to 8.7 Sv, within recent estimates of the mass transport shed westward at the Agulhas retroflection. Topographic steering of the ACC dominates the structure of flow in the circumpolar ocean. The Benguela Current is seen to be fed by a mixture of saline Indian Ocean water (originating from the Agulhas Current) and fresher Subantarctic surface water (originating in the ACC). The Benguela Current is seen to modify its flow and fate with depth; near the surface it flows north-westwards bifurcating most of its transport northward into the North Atlantic Ocean (for ultimate replacement of North Atlantic surface waters lost to the NADW conveyor). Deeper in the water column, more of the Benguela Current is destined to return with the Brazil Current, though northward flows are still generated where the Benguela Current extension encounters the coast of South America. At intermediate levels, these northward currents trace the flow of Antarctic Intermediate Water (AAIW) equatorward, though even more AAIW is seen to recirculate poleward in the subtropical gyre. In spite of the model’s rather coarse resolution, some subtle features of the Brazil-Malvinas Confluence are simulated rather well, including the latitude at which the two currents meet. Conceptual diagrams of the recirculation and interocean exchange of thermocline, intermediate and deep waters are constructed from an analysis of flows bound between isothermal and isobaric surfaces. This analysis shows how the return path of NADW is partitioned between a cold water route through the Drake Passage (6.5 Sv), a warm water route involving the Agulhas Current sheeding thermocline water westward (2.5 Sv), and a recirculation of intermediate water originating in the Indian Ocean (1.6 Sv).     

Wave effects in global ocean modeling: parametrizations vs. forcing from a wave model

One of the main challenges of the Copernicus Marine Service is the implementation of coupled ocean/waves systems that accurately estimate the momentum and energy fluxes provided by the atmosphere to the ocean. This study aims to investigate the impact of forcing the Nucleus for European Modelling of the Ocean (NEMO) ocean model with forecasts from the wave model of Météo-France (MFWAM) to improve classical air-sea flux parametrizations, these latter being mostly driven by the 10-m wind. Three wave-related processes, namely, wave-state-dependent stress, Stokes drift-related effects (Stokes-Coriolis force, Stokes drift advection on tracers and on mass), and wave-state-dependent surface turbulence, are examined at a global scale with a horizontal resolution of 0.25°. Three years of sensitivity simulations (2014–2016) show positive feedback on sea surface temperature (SST) and currents when the wave model is used. A significant reduction in SST bias is observed in the tropical Atlantic Ocean. This is mainly due to the more realistic momentum flux provided by the wave model. In mid-latitudes, the most interesting impact occurs during the summer stratification, when the wind is low and the wave model produces a reduction in the turbulence linked with wave breaking. Magnitudes of the large-scale currents in the equatorial region are also improved by 10% compared to observations. In general, it is shown that using the wave model reduces on average the momentum and energy fluxes to the ocean in tropical regions, but increases them in mid-latitudes. These differences are in the order of 10 to 20% compared with the classical parametrizations found in stand-alone ocean models.     

Long-term ocean simulations in FESOM: evaluation and application in studying the impact of Greenland Ice Sheet melting

The Finite Element Sea-ice Ocean Model (FESOM) is formulated on unstructured meshes and offers geometrical flexibility which is difficult to achieve on traditional structured grids. In this work, the performance of FESOM in the North Atlantic and Arctic Ocean on large time scales is evaluated in a hindcast experiment. A water-hosing experiment is also conducted to study the model sensitivity to increased freshwater input from Greenland Ice Sheet (GrIS) melting in a 0.1-Sv discharge rate scenario. The variability of the Atlantic Meridional Overturning Circulation (AMOC) in the hindcast experiment can be explained by the variability of the thermohaline forcing over deep convection sites. The model also reproduces realistic freshwater content variability and sea ice extent in the Arctic Ocean. The anomalous freshwater in the water-hosing experiment leads to significant changes in the ocean circulation and local dynamical sea level (DSL). The most pronounced DSL rise is in the northwest North Atlantic as shown in previous studies, and also in the Arctic Ocean. The released GrIS freshwater mainly remains in the North Atlantic, Arctic Ocean and the west South Atlantic after 120 model years. The pattern of ocean freshening is similar to that of the GrIS water distribution, but changes in ocean circulation also contribute to the ocean salinity change. The changes in Arctic and sub-Arctic sea level modify exchanges between the Arctic Ocean and subpolar seas, and hence the role of the Arctic Ocean in the global climate. Not only the strength of the AMOC, but also the strength of its decadal variability is notably reduced by the anomalous freshwater input. A comparison of FESOM with results from previous studies shows that FESOM can simulate past ocean state and the impact of increased GrIS melting well.

     

Analysis of the dynamic mechanisms of upwelling in deep ocean water caused by typhoons

Ocean Dynamics - A typhoon (hurricane) is a very strong local disturbance that can affect ocean water as deep as 1000 m. According to observations and numerical simulations, the decrease...     

Long-term ocean simulations in FESOM: evaluation and application in studying the impact of Greenland Ice Sheet melting

The Finite Element Sea-ice Ocean Model (FESOM) is formulated on unstructured meshes and offers geometrical flexibility which is difficult to achieve on traditional structured grids. In this work, the performance of FESOM in the North Atlantic and Arctic Ocean on large time scales is evaluated in a hindcast experiment. A water-hosing experiment is also conducted to study the model sensitivity to increased freshwater input from Greenland Ice Sheet (GrIS) melting in a 0.1-Sv discharge rate scenario. The variability of the Atlantic Meridional Overturning Circulation (AMOC) in the hindcast experiment can be explained by the variability of the thermohaline forcing over deep convection sites. The model also reproduces realistic freshwater content variability and sea ice extent in the Arctic Ocean. The anomalous freshwater in the water-hosing experiment leads to significant changes in the ocean circulation and local dynamical sea level (DSL). The most pronounced DSL rise is in the northwest North Atlantic as shown in previous studies, and also in the Arctic Ocean. The released GrIS freshwater mainly remains in the North Atlantic, Arctic Ocean and the west South Atlantic after 120 model years. The pattern of ocean freshening is similar to that of the GrIS water distribution, but changes in ocean circulation also contribute to the ocean salinity change. The changes in Arctic and sub-Arctic sea level modify exchanges between the Arctic Ocean and subpolar seas, and hence the role of the Arctic Ocean in the global climate. Not only the strength of the AMOC, but also the strength of its decadal variability is notably reduced by the anomalous freshwater input. A comparison of FESOM with results from previous studies shows that FESOM can simulate past ocean state and the impact of increased GrIS melting well.     

The Mutual Response of the Tropical Squall Line and the Ocean

—The Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) is used to investigate the mutual response of a tropical squall line and the ocean. Simulated squall line compares well with the observations, and consists of counterrotating vortices, and has a bow shape bulge toward the leading edge. In addition to these features, which are also shown in the previous numerical simulations, the unique results from the coupled simulation indicate that the air–sea interaction processes within the squall line are important. They affect both the atmosphere and the ocean locally. Simulated upper ocean displays significant response to the squall line with upwelling and baroclinicity. Depth of the ocean mixed layer in the coupled simulation becomes modified due to feedback processes. Ocean temperature acts as a destabilizing factor, and the salinity as a stabilizing factor. Surface turbulent fluxes from the coupled simulation are about 10% less than that of the uncoupled simulation. The SST in the coupled simulation decreases by about 0.21°C. Predicted squall line in the coupled simulation is weaker as compared to the uncoupled simulation. This is reflected in terms of differences in surface fluxes, cloud water, rain water and vertical velocities between the two simulations.     

Rapid intensification of Super Typhoon Haiyan: the important role of a warm-core ocean eddy

Super Typhoon Haiyan devastated portions of Southeast Asia, particularly the Philippines, on November 8, 2013. In this paper, observational data are used to analyze the intensification process of Super Typhoon Haiyan. Observational data showed that Typhoon Haiyan intensified and the maximum sustained winds increased to 59 ms^?1 after it encountered a double warm-core ocean eddy, while the central pressure of the typhoon dropped from 970 hPa to 920 hPa. Numerical simulations and observational data show that the presence of the warm-core eddy combined with SST increases due to climate change led to the rapid intensification of Super Typhoon Haiyan. Comparing these two factors, the warm-core ocean eddy, which brings significantly more heat into the upper ocean, plays the leading role in the intensification, with climate warming making a lesser contribution. Moreover, due to the increased thickness of the mixed layer associated with the warm-core ocean eddy, Super Typhoon Haiyan did not significantly decrease the sea surface temperature to the east of the Philippines, as is typical of typhoons, and the largest decrease was approximately 1 °C.     

Coccolith chemistry reveals secular variations in the global ocean carbon cycle?

The mismatch between the 100 and 400 k.y. components of Pleistocene climate and the relative power of those terms from the eccentricity of the Earth's orbit remains a challenge to the Milankovitch hypothesis. Coccolithophores have the potential to respond to parameters of orbital forcing other than insolation, and, as a critical component of the ocean carbon cycle, can act to modify the climate response. The first direct comparison of coccolith fraction Sr/Ca, alkenone abundance and automated coccolithophore counts, shows that CF Sr/Ca is largely driven by changing production of bloom species, with unusually high Sr/Ca ratios. The periods of high CF Sr/Ca and high bloom production mark periods of high global coccolithophore production, which correlate inversely with the low amplitude 100 and higher amplitude 400 k.y. eccentricity orbital frequency. ∼ 400 k.y. cycles of coccolithophore bloom production correspond to periods of enhanced carbonate accumulation in some parts of the ocean, deep ocean dissolution in others, positive shifts in global ocean δ¹³C, and acmes of Gephyrocapsa caribbeanica and Emiliania huxleyi. The link between production of coccolithophore blooms and eccentricity may be due to orbital control of silica leakage from the Southern Ocean, to the orbitally defined inverse correlation between insolation and growing season length and the asymptotic growth response to these parameters, or to changes in nutrient input from weathering. During the Pleistocene, the eccentricity induced coccolithophore acmes have no apparent influence on atmospheric carbon dioxide (pCO₂) due to the shift towards small bloom coccolithophores, or to coupling with increased diatom productivity, or the ballast effect of the calcium carbonate rain, such that Pleistocene climate has no significant variance at the largest amplitude eccentricity forcing of 400 k.y. Coccolithophores and their influence on the carbon cycle may act as a filter between the incident orbital forcing and resultant climate.     

Regional and global effects of southern ocean constraints in a global model

Global ocean circulation models do not usually take high-latitude processes into account in an adequate form due to a limited model domain or insufficient resolution. Without the processes in key areas contributing to the lower part of the global thermohaline circulation, the characteristics and flow of deep and bottom waters often remain unrealistic in these models. In this study, various sections of the Bremerhaven Regional Ice Ocean Simulation model results are combined with a global inverse model by using temperature, salinity, and velocity constraints for the Hamburg Large Scale Geostrophic ocean general circulation model. The differences between the global model with and without additional constraints from the regional model demonstrate that the Weddell Sea circulation exerts a significant influence on the course of the Antarctic Circumpolar Current with consequences for Southern Ocean water mass characteristics and the spreading of deep and bottom waters in the South Atlantic. The influence of the Ross Sea is found to be less important in terms of global influences. However, regional changes in the Pacific sector of the Southern Ocean are found to be of Ross Sea origin. The additional constraints change the hydrographic conditions of the global model in the vicinity of the Antarctic Circumpolar Current in such a way that transport values, e.g., in Drake Passage no longer need to be prescribed to obtain observed transports. These changes not only improve the path and transport of the Antarctic Circumpolar Current but affect the meso- and large-scale circulation. With a higher (lower) mean Drake Passage transport, the mean Weddell Gyre transport is lower (higher). Furthermore, an increase (decrease) in the Antarctic Circumpolar Current leads to a decrease (increase) of the circum-Australian flow, i.e., a decrease (increase) of the Indonesian Throughflow.     

Formulation of a new ocean salinity boundary condition and impact on the simulated climate of an oceanic general circulation model

The salinity boundary condition at the ocean surface plays an important role in the stability of long-term integrations of an oceanic general circulation model(OGCM) and in determining its equilibrium solutions.This study presents a new formulation of the salt flux calculation at the ocean surface based on physical processes of salt exchange at the air-sea interface.The formulation improves the commonly used virtual salt flux with constant reference salinity by allowing for spatial correlations between surface freshwater flux and sea-surface salinity while preserving the conservation of global salinity.The new boundary condition is implemented in the latest version of the National Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics/Institute of Atmospheric Physics Climate Ocean Model version 2(LICOM2.0).The impact of the new boundary condition on the equilibrium simulations of the model is presented.It is shown that the new formulation leads to a stronger Atlantic meridional overturning circulation(AMOC) that is closer to observational estimates.It also slightly improves poleward heat transport by the oceans in both the Atlantic and the global oceans.     

Study of seasonal variability and heat budget of the East Australian Current using two eddy-resolving ocean circulation models

Two mutually exclusive ocean models, Ocean general circulation model for the Earth Simulator (OFES) and the Bluelink ReANalysis (version 2.1; BRAN2.1), and the spin-up model (SPINUP4) of BRAN2.1 were used to investigate seasonal variability of the East Australian Current (EAC). These model outputs were tested against satellite and in situ data. The seasonally averaged sea surface temperature (SST) in the OFES and SPINUP4 shows a negative bias of 1 °C. However, the OFES, SPINUP4, and BRAN2.1 have a similar seasonal cycle in SST. The annual mean EAC transport computed at 28°S from the three models shows a good agreement with annual mean transport computed using the in situ data. However, they have considerable differences in terms of annual cycle. A better performance of the BRAN2.1 in simulating the temperature field is a result of data assimilation. The advection of heat across the open boundaries contributes ～50 % of the heat content change in the region. This study suggests that the advection by the EAC plays a significant role in heat content change of the region.     

On the interaction between a hurricane,the Gulf Stream and coastal sea level

Tropical storms and hurricanes in the western North Atlantic Ocean can impact the US East Coast in several ways. Direct effects include storm surges, winds, waves, and precipitation and indirect effects include changes in ocean dynamics that consequently impact the coast. Hurricane Matthew [October, 2016] was chosen as a case study to demonstrate the interaction between an offshore storm, the Gulf Stream (GS) and coastal sea level. A regional numerical ocean model was used, to conduct sensitivity experiments with different surface forcing, using wind and heat flux data from an operational hurricane-ocean coupled forecast system. An additional experiment used the observed Florida Current (FC) transport during the hurricane as an inflow boundary condition. The experiments show that the hurricane caused a disruption in the GS flow that resulted in large spatial variations in temperatures with cooling of up to ~?4 °C by surface heat loss, but the interaction of the winds with the GS flow also caused some local warming near fronts and eddies (relative to simulations without a hurricane). A considerable weakening of the FC transport (~?30%) has been observed during the hurricane (a reduction of ~?10 Sv in 3 days; 1Sv?=?10⁶ m³ s^?1), so the impact of the FC was explored by the model. Unlike the abrupt and large wind-driven storm surge (up to 2 m water level change within 12 h in the South Atlantic Bight), the impact of the weakening GS on sea level is smaller but lasted for several days after the hurricane dissipated, as seen in both the model and altimeter data. These results can explain observations that show minor tidal flooding along long stretches of coasts for several days following passages of hurricanes. Further analysis showed the short-term impact of the hurricane winds on kinetic energy versus the long-term impact of the hurricane-induced mixing on potential energy, whereas several days are needed to reestablish the stratification and rebuild the strength of the GS to its pre-hurricane conditions. Understanding the interaction between storms, the Gulf Stream and coastal sea level can help to improve prediction of sea level rise and coastal flooding.