Ocean Boundary Conditions Research Articles

The TOPAZ monitoring and prediction system for the Atlantic and Arctic Oceans

Ocean data assimilation systems allow the combination of remote sensing and in-situ ocean observations with primitive equation ocean general circulation models (OGCM). They thus provide initial conditions for short-term ocean currents forecasts (ten days to one month) and boundary conditions for nested high-resolution models of coastal seas and can be operated in hindcast to reproduce past events over long periods. At a time when offshore activities are moving toward deeper waters and ice-covered seas, accurate monitoring and forecasting of the environment (particularly ocean currents and seaice) cannot be neglected. This paper presents the TOPAZ system, being the Arctic component of the MERSEA integrated system and one of the contributors to the GODAE international initiative. The system is based on the latest scientific developments in terms of ocean modelling with the Hybrid Coordinate Ocean Model (HYCOM) and data assimilation with the Ensemble Kalman Filter (EnKF). Validation results of the system and applications in nested regional models are presented.

Satellite‐derived aerosol optical depth over dark water from MISR and MODIS: Comparisons with AERONET and implications for climatological studies

Although the current Multiangle Imaging Spectroradiometer (MISR) and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite passive remote sensing midvisible aerosol optical thickness (AOT) products are accurate overall to about 0.05 or 20%, they differ systematically on a global, monthly average basis, by about 0.03 to 0.05. Some key climate change and other applications require accuracies of 0.03 or better. The instruments are sufficiently stable and well characterized, and have adequate signal‐to‐noise, to realize such precision. However, assumptions made in the current standard aerosol retrieval algorithms produce AOT biases that must be addressed first. We identify the causes of AOT discrepancies over dark water under typical, relatively low AOT conditions and quantify their magnitudes on the basis of detailed analysis. Examples were selected to highlight key issues for which there are coincident MISR, MODIS, and Aerosol Robotic Network (AERONET) observations. Instrument calibration and sampling differences, assumptions made in the MISR and MODIS standard algorithms about ocean surface boundary conditions, missing particle property or mixture options, and the way reflectances used in the retrievals are selected each contribute significantly to the observed differences under some circumstances. Cloud screening is also identified as a factor, though not fully examined here, as are the relatively rare high‐AOT cases over ocean. Specific algorithm upgrades and further studies indicated by these findings are discussed, along with recommendations for effectively using the currently available products for regional and global applications.

The Dynamics of Idealized Convection Schemes and Their Effect on the Zonally Averaged Tropical Circulation

In this paper, the effect of a simple convection scheme on the zonally averaged tropical general circulation is examined within an idealized moist GCM to obtain broad classifications of the influence of convection on the Tropics. This is accomplished with a simplified convection scheme in the style of Betts and Miller. The scheme is utilized in a moist GCM with simplified physical parameterizations (gray radiation, with zonally symmetric, slab mixed layer ocean boundary conditions). Comparisons are made with simulations without a convection scheme [i.e., with large-scale condensation (LSC) only], with the moist convective adjustment (MCA) parameterization, and with various formulations and parameter sets with a simplified Betts–Miller (SBM) scheme. With the control run using the SBM scheme, the Tropics become quieter and less dependent on horizontal resolution as compared with the LSC or MCA simulations. The Hadley circulation mass transport is significantly reduced with the SBM scheme, as is the ITCZ precipitation. An important factor determining this behavior is the parameterization of shallow convection: without shallow convection, the convection scheme is largely ineffective at preventing convection from occurring at the grid scale. The sensitivities to convection scheme parameters are also examined. The simulations are remarkably insensitive to the convective relaxation time, and only mildly sensitive to the relative humidity of the reference profile, provided significant large-scale condensation is not allowed to occur. The changes in the zonally averaged tropical circulation that occur in all the simulations are understood based on the convective criteria of the schemes and the gross moist stability of the atmosphere.

Convectively Coupled Kelvin Waves in an Idealized Moist General Circulation Model

The dynamics of convectively coupled Kelvin waves and their dependence on convection scheme parameters are studied within a simplified moist general circulation model. The model consists of the primitive equations on the sphere over zonally symmetric aquaplanet, slab mixed layer ocean boundary conditions, and idealized physical parameterizations including gray radiative transfer and a simplified Betts–Miller convection scheme. This framework allows the authors to study the dependence of Kelvin waves on quantities such as the gross moist stability in a clean manner. A control simulation with the model produces convectively coupled Kelvin waves that are remarkably persistent and dominate the variability within the Tropics. These waves propagate with an equivalent depth of ≈40 m. Linear regression analysis with respect to a Kelvin-filtered time series shows that the waves are driven by evaporation–wind feedback and have structures broadly consistent with theoretical predictions for Kelvin waves. Next, the determination of the speed and structure of the Kelvin waves is studied by examining the response of the waves to changes in convection scheme parameters. When the convective relaxation time is lengthened, the waves are damped and eventually are completely eliminated. The propagation speed additionally increases with longer relaxation time. Then changes to a convection scheme parameter that essentially controls the fraction of convective versus large-scale precipitation are examined. When some large-scale precipitation occurs, the waves increase in strength, propagate more slowly, and move to larger scales. However, when mostly large-scale precipitation occurs, the Kelvin wave disappears, and the Tropics are dominated by tropical storm–like variability. The decrease in speed is related here to the gross moist stability of the atmosphere, which is reduced with increased large-scale precipitation.

Near-surface wind speed retrieval from space-based, multi-angle imaging of ocean sun glint patterns

The relationship between the MISR multi-angle observations of sun glint pattern width over ocean and the near-surface wind speed is explored. With this relationship, we develop an algorithm that can constrain the standard, wind-driven glint/white cap model that defines the ocean–surface boundary condition in MISR aerosol retrievals. The key geometric factor determining wind retrieval quality is the angular distance between the reflection vector and the closest MISR multi-angle observation — the minimum Sun Glint Angle (SGA). MISR-retrieved winds are within 2.4 m/s of buoy-measured values for minimum SGA below about 15° for the 38 cases studied, and usually better than 1.1 m/s, except one for which the wind speed was extremely low. For minimum SGA above about 15°, MISR angular glint pattern reflectance sampling is not adequate to perform wind retrievals. An ambiguity in the retrieval that can occur for minimum SGA greater than 6° or 7° is resolved by the minimum observed absolute reflectance. We also confirm the high accuracy of the Cox–Munk ocean surface reflectance model, used to analyze the glint pattern dependence on wind speed. The wind-retrieval method could be used in an operational multi-angle aerosol retrieval algorithm to dynamically constrain the ocean surface boundary condition when glint pattern angular sampling is adequate.

Sensitivity of global tropospheric ozone and fine particulate matter concentrations to climate change

An integrated global model of climate, tropospheric gas phase chemistry, and aerosols has been used to investigate the sensitivity of global ozone and fine particulate matter concentrations to climate change. Two simulations corresponding to present (1990s) and future (2050s) climates have been performed and compared. A future climate has been imposed using ocean boundary conditions corresponding to the Intergovernmental Panel on Climate Change SRES A2 scenario for the 2050s decade, resulting in an increase in the global annual average values of the surface air temperature by 1.7°C, the lower tropospheric specific humidity by 0.9 g H2O/kg air, and the precipitation by 0.15 mm d−1. Present‐day anthropogenic emissions have been used in both simulations while climate‐sensitive natural emissions were allowed to vary with the simulated climate. The tropospheric ozone burden in the future climate run decreased by 5%, and its lifetime decreased from 27.8 to 25.3 days. The tropospheric ozone change is driven primarily by increased ozone loss rates through ozone photolysis in the presence of water vapor, which on a global scale, more than compensate for the increased ozone chemical production associated with increased temperatures. At the model surface layer, over remote regions, ozone mixing ratios decreased by 1–3 ppbv, while polluted regions showed a relatively smaller decrease of 0–1 ppbv and increased by 1–5 ppbv in some cases. The global burdens and lifetimes of fine particulate matter species in the future climate run decreased by 2 to 18% because of increased wet deposition loss rates associated with increased precipitation. At the model surface layer, there are regions of decreases and increases in the concentrations of fine particulate matter species. The increased surface layer concentrations of some fine particulate matter species is primarily driven by lower regional‐scale precipitation and increased secondary production, where applicable. The robustness of the predicted regional‐scale changes for fine particulate matter species is strongly dependent upon the predicted regional‐scale precipitation changes.

The effect of tidal inlets on open coast storm surge hydrographs

Bridge scour modeling requires storm surge hydrographs as open ocean boundary conditions for coastal waters surrounding tidal inlets. These open coast storm surge hydrographs are used to accurately determine both horizontal and vertical circulation patterns, and thus scour, within the inlet and bay for an extreme event. At present, very little information is available on the effect that tidal inlets have on these open coast storm surge hydrographs. Furthermore, current modeling practice enforces a single design hydrograph along the open coast boundary for bridge scour models. This study expands on these concepts and provides a more fundamental understanding on both of these modeling areas. A numerical parameter study is undertaken to elucidate the influence of tidal inlets on open coast storm surge hydrographs. Four different inlet–bay configurations are developed based on a statistical analysis of existing tidal inlets along the Florida coast. The length and depth of the inlet are held constant in each configuration, but the widths are modified to include the following four inlet profiles: 1) average Florida inlet width; 2) 100 m inlet width; 3) 500 m inlet width; and 4) 1000 m inlet width. Results from these domains are compared to a control case that does not include any inlet–bay system in the computational domain. The Advanced Circulation, Two-Dimensional Depth-Integrated (ADCIRC-2DDI) numerical code is used to obtain water surface elevations for all studies performed herein. The code is driven by astronomic tides at the open ocean boundary, and wind velocities and atmospheric pressure profiles over the surface of the computational domains. Model results clearly indicate that the four inlet–bay configurations do not have a significant impact on the open coast storm surge hydrographs. Furthermore, a spatial variance amongst the storm surge hydrographs is recognized for open coast boundary locations extending seaward from the mouth of the inlet. In addition, a storm surge study of Hurricane Ivan in the vicinity of Escambia Bay along the Panhandle of Florida is performed to assess the findings of the numerical parameter study in a real-life scenario. The main conclusions from the numerical parameter study are verified in the Hurricane Ivan study: 1) the Pensacola Pass–Escambia Bay system has a minimal effect on the open coast storm surge hydrographs; and 2) the open coast storm surge hydrographs exhibit spatial dependence along typical open coast boundary locations. The results and conclusions presented herein have implications toward future bridge scour modeling efforts.

Intercomparison of Arctic Regional Climate Models: Modeling Clouds and Radiation for SHEBA in May 1998

Abstract To improve simulations of the Arctic climate and to quantify climate model errors, four regional climate models [the Arctic Regional Climate System Model (ARCSYM), the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS), the High-Resolution Limited-Area Model (HIRHAM), and the Rossby Center Atmospheric Model (RCA)] have simulated the annual Surface Heat Budget of the Arctic Ocean (SHEBA) under the Arctic Regional Climate Model Intercomparison Project (ARCMIP). The same lateral boundary and ocean surface boundary conditions (i.e., ice concentration and surface temperature) drive all of the models. This study evaluated modeled surface heat fluxes and cloud fields during May 1998, a month that included the onset of the surface icemelt. In general, observations agreed with simulated surface pressure and near-surface air properties. Simulation errors due to surface fluxes and cloud effects biased the net simulated surface heat flux, which in turn affected the timing of the simulated icemelt. Modeled cloud geometry and precipitation suggest that the RCA model produced the most accurate cloud scheme, followed by the HIRHAM model. Evaluation of a relationship between cloud water paths and radiation showed that a radiative transfer scheme in ARCSYM was closely matched with the observation when liquid clouds were dominant. Clouds and radiation are of course closely linked, and an additional comparison of the radiative transfer codes for ARCSYM and COAMPS was performed for clear-sky conditions, thereby excluding cloud effects. Overall, the schemes for radiative transfer in ARCSYM and for cloud microphysics in RCA potentially have some advantages for modeling the springtime Arctic.

High‐latitude forcing of interior ocean δ13C

Transit time distribution probability density functions (TTDs) are used to investigate the possible role of changing boundary conditions in driving the δ13C signal in the interior of a steady state ocean. We use idealized examples to investigate the general question of how a conservative tracer propagates from the surface ocean to interior ocean and to illustrate how a given tracer boundary signal will be “filtered” with increasing distance from its source region. We show that tracers in the deep southeast Atlantic Ocean will respond much more strongly to changes in the surface Southern Ocean than to changes in the high‐latitude North Atlantic, while the opposite is true for waters at intermediate depths. The impact of a change in the Southern Ocean surface δ13C on a profile from the western South Atlantic is estimated using model‐derived transit time distributions, and it is shown that significant deep ocean δ13C variations can be expected on glacial‐interglacial timescales, even under a steady state circulation regime. Records of δ13C from the high‐latitude North Atlantic and Southern Ocean are used as a proxy for glacial‐interglacial changes in the surface ocean boundary condition in regions of deepwater formation. By convolving these high‐latitude boundary conditions with model‐derived TTDs, we are able to explain a significant part of the observed variability in benthic δ13C records spanning the last glacial cycle(s) from locations as diverse as the equatorial Atlantic Ocean, the Cape Basin, and the equatorial Pacific. This suggests that changing boundary conditions may be driving a significant fraction of benthic δ13C variability previously attributed to changes in ocean circulation. Furthermore, we show that our results predict a slightly higher δ13C than observed in high‐productivity regions, consistent with the concept of a productivity‐induced low‐δ13C overprint.

Transit-Time Distributions in a Global Ocean Model

Abstract Results from a simulation of the ocean “transit-time distribution” (“TTD”) for global and regional ocean surface boundary conditions are presented based on a 5000-yr integration using the Parallel Ocean Program ocean general circulation model. The TTD describes the probability that water at a given interior point in the ocean was at some point on the ocean surface a given amount of time ago. It is shown that the spatial distribution of ocean TTDs can be understood in terms of conventional wisdom regarding time scales and pathways of the ventilated thermocline and the thermohaline circulation–driven deep-ocean circulation. The true mean age from the model (the first moment of the TTD) is demonstrated to be very large everywhere, because of very long-tailed distributions. Regional TTD distributions are presented for distinct surface boundary subregions, and it is shown how these can help in the interpretation of the global TTD. The spatial structure of each regional TTD is shown to become essentially the same at relatively long times. The form of the TTD at a given point in the ocean can be very simple, but some regions do exhibit more complicated multimodal distributions. The degree to which a simple functional approximation to the TTD is able to predict the spatial and temporal evolution of selected idealized tracers (for which interior sources and sinks are known or zero) with knowledge of only the tracer surface boundary condition is explored.

Reproducibility of Seasonal Land Surface Climate

Abstract In this study, the sensitivity of the continental seasonal climate to initial conditions is estimated from an ensemble of decadal simulations of an atmospheric general circulation model with the same specifications of radiative forcings and monthly ocean boundary conditions, but with different initial states of atmosphere and land. As measures of the “reproducibility” of continental climate for different initial conditions, spatiotemporal correlations are computed across paired realizations of 11 model land surface variables in which the seasonal cycle is either included or excluded—the former case being pertinent to climate simulation and the latter to seasonal prediction. It is found that the land surface variables that include the seasonal cycle are impacted only marginally by changes in initial conditions; moreover, their seasonal climatologies exhibit high spatial reproducibility. In contrast, the reproducibility of a seasonal land surface anomaly is generally low, although it is substantially higher in the Tropics; its spatial reproducibility also markedly fluctuates in tandem with warm and cold phases of the El Niño–Southern Oscillation. However, the overall degree of reproducibility depends on the particular land surface anomaly considered. It is also shown that the predictability of a land surface anomaly implied by its reproducibility statistics is consistent with what is inferred from more conventional predictability metrics. Implications of these results for climate model intercomparison projects and for operational forecasts of seasonal continental climate also are elaborated.

Global Microwave Satellite Observations of Sea Surface Temperature for Numerical Weather Prediction and Climate Research

Obtaining global sea surface temperature (SST) fields for the ocean boundary condition in numerical weather prediction (NWP) models and for climate research has long been problematic. Historically, such fields have been constructed by a blending of in situ observations from ships and buoys and satellite infrared observations from the Advanced Very High Resolution Radiometer (AVHRR) that has been operational on NOAA satellites since November 1981. The resolution of these global SST fields is limited by the sparse coverage of in situ observations in many areas of the World Ocean and cloud contamination of AVHRR observations, which can exceed 75% over the subpolar oceans. As clouds and aerosols are essentially transparent to microwave radiation, satellite microwave observations can greatly improve the sampling and resolution of global SST fields. The Advanced Microwave Scanning Radiometer on the NASA Earth Observing System (EOS) Aqua satellite (AMSR-E) is providing the first highly accurate and global satellite microwave observations of SST. The potential for AMSR-E observations to improve the sampling, resolution, and accuracy of SST fields for NWP and climate research is demonstrated from example SST fields and from an investigation of the sensitivity of NWP models to specification of the SST boundary condition.

Estimation of ammonia exchange at the land–ocean boundary condition of Sundarban mangrove, northeast coast of Bay of Bengal, India

Abstract The Sundarban mangrove forest (4264 km 2 ) is about 3% of the total area of the world mangrove is interdependent on the coastal aquatic subecosystem (1781 km 2 ). Ammonia concentration and flux measurements in the mangrove ecosystem and measurements in such tropical land–ocean boundary conditions are few and far between. A micrometeorological method was used to measure ammonia exchange over Sundarban mangrove forest and aquatic subecosystem. Results showed both emission and gaseous dry deposition in the forest area at a rate 1.79×10 3 and 2.26×10 3 kg N km –2 yr −1 , respectively. Flux of ammonia in the aquatic subecosystem was found always from air to water with a rate of 775.7 kg N km –2 yr −1 . The annual particulate dry and wet atmospherically deposited ammonia flux were calculated to be 69.0 and 21.5 kg N km –2 yr −1 , respectively. Exchange of gaseous ammonia in the mangrove forest showed net deposition with a rate of 4.90×10 2 kg N km –2 yr −1 . Annual imported flux from the proximate terrestrial source to this virgin forest was found to be 4.26×10 6 kg N yr −1 out of which 32.9% was deposited to the coastal water. Ammonia compensation point for mangrove vegetation was found to occur between 51 and 421 nmol m −3 and its variation was related to wind velocity, temperature, and humidity.

Long-Range Predictability in the Tropics. Part I: Monthly Averages

Abstract The sensitivity to initial and boundary conditions of monthly mean tropical long-range forecasts (1–14 weeks) during Northern Hemisphere winter is studied with a numerical model. Five predictability experiments with different combinations of initial conditions and prescribed ocean boundary conditions are conducted in order to investigate the temporal and spatial characteristics of the perfect model forecast skill. It is shown that initial conditions dominate a tropical forecast during the first 3 weeks and that they influence a forecast for at least 8 weeks. The initial condition effect is strongest over the Eastern Hemisphere and during years when the El Niño–Southern Oscillation (ENSO) phenomenon is weak. The relatively long sensitivity to initial conditions is related to a complex combination of dynamic and thermodynamic effects, and to positive internal feedbacks of large-scale convective anomalies. At lead times of more than 3 weeks, boundary forcing is the main contributor to tropical predictability. This effect is particularly strong over the Western Hemisphere and during ENSO. Using persisted instead of observed sea surface temperatures leads to useful forecast results only over the Western Hemisphere and during ENSO.

The Impact of SST Specification on ECMWF Surface Wind Stress Fields in the Eastern Tropical Pacific

Abstract The impact of SST specification on low-level winds in the operational ECMWF numerical weather prediction model is investigated in the eastern tropical Pacific from comparisons of ECMWF wind stress fields with QuikSCAT satellite scatterometer observations of wind stress during the August–December cold seasons of 2000 and 2001. These two time periods bracket the 9 May 2001 change from the Reynolds SST analyses to the Real-Time Global SST (RTG_SST) analyses as the ocean boundary condition in the ECMWF model. The ocean–atmosphere interaction in the eastern tropical Pacific that is clearly evident in QuikSCAT wind stress divergence and curl fields is also evident in the ECMWF winds, but is more than twice as strong in the 2001 cold season as in the 2000 cold season, due primarily to the improved spatial and temporal resolution of the RTG_SST analyses compared with the Reynolds SST analyses. While a significant improvement compared with 2000, the response of the 2001 ECMWF wind stress field to SST is only about half as strong as the coupling inferred from QuikSCAT data and satellite observations of SST from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). It is concluded that the underrepresentation of the ocean–atmosphere coupling is attributable partly to underrepresentation of SST gradients in the RTG_SST fields and partly to inadequacies of the ECWMF model. The latter may be due to errors in the parameterization of boundary layer processes or to insufficient horizontal or vertical resolution in the model.

Toward a unified theory of tidally-averaged estuarine salinity structure

Equations are developed for the tidally-averaged, width-averaged estuarine salinity and circulation in a rectangular estuary. Width and depth may vary along the length of the channel, as may coefficients of vertical turbulent mixing and along channel diffusion. The system is reduced to a single first-order, nonlinear, ordinary differential equation governing the section-averaged salinity. A technique for specifying the ocean boundary condition is given, and solutions are found by numerical integration. Under different assumptions for the diffusion it is possible to reproduce the few existing analytical solutions, in particular the Hansen and Rattray (1965) Central Regime solution, and Chatwin's (1976) solution.The mathematical framework allows easy comparison of the results of different channel geometries and mixing coefficients. Of particular interest is the along-channel distribution of the diffusive fraction of up-estuary salt flux. It is shown that the Hansen and Rattray solution is always diffusion-dominated near the mouth. A theory is presented for estimating the diffusion coefficient within a tidal excursion of the mouth. It is shown that the resulting rapid along-channel increase of diffusion may explain some observed patterns of salinity structure: a decrease in both stratification and along-channel salinity gradient near the mouth. The theory is applied to the Delaware Estuary and Northern San Francisco Bay, and shows reasonable agreement with observed sensitivities of salt intrusion distance to river flow.

Atmospheric Response to Modified CLIMAP Ocean Boundary Conditions during the Last Glacial Maximum*

Global climate simulations are conducted to examine the sensitivity of the Last Glacial Maximum (LGM) climate to prescribed sea surface temperatures (SSTs) that are modified from the Climate: Long-range Investigation, Mapping, and Prediction (CLIMAP) study. Based on the consensus from various LGM proxy data, the SSTs are cooled by 48C uniformly in the Tropics (308N‐308S) relative to CLIMAP, and the high-latitude sea ice extent is reduced. Compared to results from a simulation with CLIMAP SSTs, the modified LGM SSTs cause significant opposing changes in the hemispheric and regional-scale atmospheric circulation, which are most pronounced in the winter hemisphere. For instance, there is significant weakening of the midlatitude circulation and reduction of 500-hPa eddy kinetic energy and midlatitude precipitation resulting from the decreased meridional temperature gradient in the modified SST simulation. In contrast, reduced sea ice extent during the boreal winter causes increased regional baroclinicity and intensified atmospheric circulation in the western North Pacific and the North Atlantic. Cooled tropical SSTs also increase the land‐ocean temperature contrast, which strengthens the Asian summer monsoon circulation. Both LGM simulations produce enhanced low-level convergence and increased precipitation along the South Pacific convergence zone (SPCZ) relative to present day, despite the cooler LGM climate. The SPCZ orientation and intensity are closely linked to the distribution of South Pacific SSTs. Comparison of surface temperature estimates from land- and ocean-based proxy data with model output suggests that uniform cooling of the tropical SSTs and modification of the high-latitude sea ice extent may be sufficient to accurately simulate the first-order characteristics of the LGM climate.

Ice–ocean boundary conditions for coupled models

Coupled general circulation models are becoming more sophisticated, particularly with respect to the sea ice component and the increasing use of free surface formulations in the ocean. It is therefore timely to revisit the boundary conditions at the sea ice–ocean interface to ensure that (a) mass and energy are conserved, (b) the physics represented is as realistic as possible, and (c) numerical instabilities are avoided. We present here an overview of recent practice from the GISS, NCAR CCSM2.0 and MPI Hamburg coupled models. A new formulation of the basal sea ice fluxes, discussions of lateral melt and snow–ice formation, coupling strategies for the sea ice dynamics component, and interactions with dynamic free surfaces are presented.

Assimilation of SST Data into a Real-Time Coastal Ocean Forecast System for the U.S. East Coast*

Abstract The real-time, three-dimensional, limited-area Coastal Ocean Forecast System (COFS) has been developed for the northwestern Atlantic Ocean and implemented at the National Centers for Environmental Prediction. COFS generates a daily nowcast and 1-day forecast of water level, temperature, salinity, and currents. Surface forcing is provided by 3-h forecasts from the National Weather Service's Eta Model, a mesoscale atmospheric prediction model. Lateral oceanic boundary conditions are based on climatic data. COFS assimilates in situ sea surface temperature (SST) observations and multichannel satellite SST retrievals for the past 48 h. SST predictions from the assimilating and nonassimilating versions of COFS were compared with independent observations and a 14-km-resolution multichannel SST analysis. The assimilation of SST data reduced the magnitude and the geographic extent of COFS's characteristic positive temperature bias north of the Gulf Stream. The root-mean-square SST differences between the ...

Role of the western Pacific Ocean boundary conditions during 1980–1998in the El Niño-Southern Oscillation events simulated by a coupled ocean-atmospheremodel

[1] In situ and satellite sea level data sets over 1980–1998 are used to estimate interannual variations of the geostrophic zonal transport from the western Pacific Ocean into the Celebes Sea. Then the transport component due to the Pacific wind-driven and fully reflected equatorial waves is removed. Finally, the residual variations, named Indonesian throughflow correction (ITFC), are used to correct for the model closed western boundary. ITFC inflows/outflows are leading the warm/cold events by a few months. They are then prescribed at the model western boundary over 1980–1998 to compare with closed boundary simulations. An ITFC inflow anomaly makes the eastern Pacific slightly warmer and trade winds in the central Pacific slightly weaker. Indeed, these quantities are simulated by both experiments in very good agreement with observations. More importantly, prescribing the ITFC greatly improves two characteristics of the model that play a key role in the coupled simulations. The first is the RMS variability of sea level versus longitude; the ITFC shifts to the west the position of the minimum. The second is the basin average sea level; the ITFC largely amplifies its fluctuations, stimulating the charge and discharge of the system. Consequently, the ITFC can have a very large impact on El Nino-Southern Oscillation forecasts. For example, the 1982/1983 and 1997/1998 1-year lead forecasts which fail to predict the warm events with closed boundaries are successful with the ITFC. The impact of the ITFC on coupled simulations is not linearly dependent on the ITFC value itself: In addition to the fluctuating conditions at the western boundary, results depend on how close to instability the system is at each time step.