Intermediate Ocean Model Research Articles

A Hybrid Coupled Ocean-Atmosphere Model and Its Simulation of ENSO and Atmospheric Responses

A new hybrid coupled model (HCM) is presented in this study, which consists of an intermediate tropical Pacific Ocean model and a global atmospheric general circulation model. The ocean component is the intermediate ocean model (IOM) of the intermediate coupled model (ICM) used at the Institute of Oceanology, Chinese Academy of Sciences (IOCAS). The atmospheric component is ECHAM5, the fifth version of the Max Planck Institute for Meteorology atmospheric general circulation model. The HCM integrates its atmospheric and oceanic components by using an anomaly coupling strategy. A 100-year simulation has been made with the HCM and its simulation skills are evaluated, including the interannual variability of SST over the tropical Pacific and the ENSO-related responses of the global atmosphere. The model shows irregular occurrence of ENSO events with a spectral range between two and five years. The amplitude and lifetime of ENSO events and the annual phase-locking of SST anomalies are also reproduced realistically. Despite the slightly stronger variance of SST anomalies over the central Pacific than observed in the HCM, the patterns of atmospheric anomalies related to ENSO, such as sea level pressure, temperature and precipitation, are in broad agreement with observations. Therefore, this model can not only simulate the ENSO variability, but also reproduce the global atmospheric variability associated with ENSO, thereby providing a useful modeling tool for ENSO studies. Further model applications of ENSO modulations by ocean-atmosphere processes, and of ENSO-related climate prediction, are also discussed.

Processes involved in the second-year warming of the 2015 El Niño event as derived from an intermediate ocean model

The tropical Pacific experienced a sustained warm sea surface condition that started in 2014 and a very strong El Nino event in 2015. One striking feature of this event was the horseshoe-like pattern of positive subsurface thermal anomalies that was sustained in the western-central equatorial Pacific throughout 2014–2015. Observational data and an intermediate ocean model are used to describe the sea surface temperature (SST) evolution during 2014–2015. Emphasis is placed on the processes involved in the 2015 El Nino event and their relationships with SST anomalies, including remote effects associated with the propagation and reflection of oceanic equatorial waves (as indicated in sea level (SL) signals) at the boundaries and local effects of the positive subsurface thermal anomalies. It is demonstrated that the positive subsurface thermal anomaly pattern that was sustained throughout 2014–2015 played an important role in maintaining warm SST anomalies in the equatorial Pacific. Further analyses of the SST budget revealed the dominant processes contributing to SST anomalies during 2014–2015. These analyses provide an improved understanding of the extent to which processes associated with the 2015 El Nino event are consistent with current El Nino and Southern Oscillation theories.

Maximum likelihood parameter estimation for a high-dimensional system by particle-based filters

Particle-based filters are applied to estimate poorly-known parameters in an oceanic thermohaline circulation model. The model is a stochastic partial differential equation that conceptually represents the Atlantic meridional overturning circulation. It exhibits rapid transitions between strong (temperature-dominated) and weak (salinity-dominated) circulations, and its stability depends on a stochastic term in the equation.Accurate estimation of the variance of the random term is critical in characterizing the circulation system, and the Maximum Likelihood Estimation is utilized via particle-based filters to estimate the parameter. The performance of different types of particle-based filters such as the Sequential Importance Resampling filter, Ensemble Kalman filter, and Maximum entropy filter are compared by the results of parameter estimation of the intermediate ocean model. The results show that all the three filters correctly estimate the stochastic parameter and, more importantly, suggest that particle-based filters are a promising tool for parameter estimation for high-dimensional systems.

Dynamic mechanism of interannual zonal displacements of the eastern edge of the western Pacific warm pool

The eastern edge of the western Pacific warm pool (WPWP) in the upper layer (shallower than 50m) exhibits significant zonal displacements on interannual scale. Employing an intermediate ocean model, the dynamic mechanism for the interannual zonal displacement of the WPWP eastern edge in the upper layer is investigated by diagnosing the dynamic impacts of zonal current anomalies induced by wind, waves (Kelvin and Rossby waves), and their boundary reflections. The interannual zonal displacements of the WPWP eastern edge in the upper layer and the zonal current anomaly in the equatorial Pacific west of 110°W for more than 30 years can be well simulated. The modeling results show that zonal current anomalies in the central and eastern equatorial Pacific are the dominant dynamic mechanism for the zonal displacements of the eastern edge of the upper WPWP warm water. Composite analyses suggest that the zonal current anomalies induced by waves dominate the zonal displacement of the WPWP eastern edge, whereas the role played by zonal wind-driven current anomalies is very small. A sensitivity test proves that the zonal current anomalies associated with reflected waves on the western and eastern Pacific boundaries can act as a restoring force that results in the interannual reciprocating zonal motion of the WPWP eastern edge.

The Roles of Atmospheric Stochastic Forcing (SF) and Oceanic Entrainment Temperature (Te) in Decadal Modulation of ENSO

AbstractThe El Niño–Southern Oscillation (ENSO) has been observed to exhibit decadal changes in its properties; the cause and implication of such changes are strongly debated. Here the authors examine the influences of two particular attributors of the ocean–atmospheric system. The roles of stochastic forcing (SF) in the atmosphere and decadal changes in the temperature of subsurface water entrained into the mixed layer (Te) in modulating ENSO are compared to one another using coupled ocean–atmosphere models of the tropical Pacific climate system. Two types of coupled models are used. One is an intermediate coupled model (ICM) and another is a hybrid coupled model (HCM), both of which consist of the same intermediate ocean model (IOM) with an empirical parameterization for Te, constructed via singular value decomposition (SVD) analysis of the IOM simulated historical data. The differences in the ICM and HCM are in the atmospheric component: the one in the ICM is an empirical feedback model for wind stress (τ), and that in the HCM is an atmospheric general circulation model (AGCM; ECHAM4.5). The deterministic component of atmospheric τ variability, representing its signal response (τSig) to an external SST forcing, is constructed statistically by an SVD analysis from a 24-member ensemble mean of the ECHAM4.5 AGCM simulations forced by observed SST; the SF component (τSF) is explicitly estimated from the ECHAM4.5 AGCM ensemble and HCM simulations. Different SF representations are specified in the atmosphere: the SF effect can be either absent or present explicitly in the ICM, or implicitly in the HCM where the ECHAM4.5 AGCM is used as a source for SF. Decadal changes in the ocean thermal structure observed in the late 1970s are incorporated into the coupled systems through the Te parameterizations for the two subperiods before (1963–79) and after (1980–96) the climate shift (T63–79e and T80–96e), respectively.The ICM and HCM simulations well reproduce interannual variability associated with El Niño in the tropical Pacific. Model sensitivity experiments are performed using these two types of coupled models with different realizations of SF in the atmosphere and specifications of decadal Te changes in the ocean. It is demonstrated that the properties of ENSO are modulated differently by these two factors. The decadal Te changes in the ocean can be responsible for a systematic shift in the phase propagation of ENSO, while the SF in the atmosphere can contribute to the amplitude and period modulation in a random way. The relevance to the observed decadal ENSO variability in the late 1970s is discussed.

Response of tropical Pacific interannual variability to decadal entrainment temperature change in a hybrid coupled model

n n The role of decadal changes in ocean thermal structure, as observed in the tropical Pacific in the late 1970s, in modulating El Nino/Southern Oscillation (ENSO) is examined using a hybrid coupled model (HCM), consisting of an atmospheric general circulation model (AGCM; ECHAM4.5) and an intermediate ocean model (IOM) with an empirical parameterization for the temperature of subsurface water entrained into the mixed layer (T-e), which is constructed via a singular value decomposition (SVD) analysis of historical data. A standard HCM run produces irregularity of ENSO. Two perturbation runs are conduced by introducing the decadal changes in T-e estimated from two subperiods before and after the climate shift (T-e(63-79) and T-e(80-96)), respectively. Together with previous studies using an intermediate coupled model (ICM) consisting of the same IOM, different modulating effects of T-e in the ocean and stochastic forcing in the atmosphere are demonstrated on the properties of ENSO. The former is responsible for changes in the phase propagation of ENSO, while the latter can contribute to the amplitude and period modulation.

An empirical parameterization for the salinity of subsurface water entrained into the ocean mixed layer (Se) in the tropical Pacific

An empirical parameterization for Se is proposed and tested in an intermediate ocean model (IOM) of the Tropical Pacific Ocean. An inverse modeling approach is first adopted to estimate Se from a sea surface salinity (SSS) anomaly (SSSA) model using observed in‐situ SSS measurements, simulated upper ocean currents, and freshwater flux (evaporation minus precipitation, E‐P) data. A relationship between Se and sea level (SL) anomalies is then obtained by utilizing an empirical orthogonal function (EOF) technique. This empirical scheme is able to estimate Se anomalies reasonably well in the equatorial Pacific Ocean and can be used to parameterize Se fields in terms of SL anomalies for use in SSSA calculations. An optimized Se parameterization naturally leads to a balanced depiction of the subsurface effect on SSS variability in association with entrainment and vertical mixing. As a result, SSSA simulations can be potentially improved in the Tropical Pacific.

An Empirical Parameterization of Subsurface Entrainment Temperature for Improved SST Anomaly Simulations in an Intermediate Ocean Model

AbstractAn empirical model for the temperature of subsurface water entrained into the ocean mixed layer (Te) is presented and evaluated to improve sea surface temperature anomaly (SSTA) simulations in an intermediate ocean model (IOM) of the tropical Pacific. An inverse modeling approach is adopted to estimate Te from an SSTA equation using observed SST and simulated upper-ocean currents. A relationship between Te and sea surface height (SSH) anomalies is then obtained by utilizing a singular value decomposition (SVD) of their covariance. This empirical scheme is able to better parameterize Te anomalies than other local schemes and quite realistically depicts interannual variability of Te, including a nonlocal phase lag relation of Te variations relative to SSH anomalies over the central equatorial Pacific. An improved Te parameterization naturally leads to better depiction of the subsurface effect on SST variability by the mean upwelling of subsurface temperature anomalies. As a result, SSTA simulations are significantly improved in the equatorial Pacific; a comparison with other schemes indicates that systematic errors of the simulated SSTAs are significantly small—apparently due to the optimized empirical Teparameterization. Cross validation and comparisons with other model simulations are made to illustrate the robustness and effectiveness of the scheme. In particular it is demonstrated that the empirical Te model constructed from one historical period can be successfully used to improve SSTA simulations in another.

Impact of Atmospheric Intraseasonal Variability in the Indian Ocean: Low-Frequency Rectification in Equatorial Surface Current and Transport

An ocean general circulation model (OGCM) is used to investigate the low-frequency (period longer than 90 days) rectification of atmospheric intraseasonal variability (10–90-day periods) in zonal surface current and transport of the equatorial Indian Ocean. A hierarchy of OGCM solutions is found in an actual tropical Indian Ocean basin for the period of 1988–2001. To help to identify and isolate nonlinear processes, a linear continuously stratified model and a 4-layer intermediate ocean model are also used. Results from the OGCM solutions suggest that intraseasonal atmospheric forcing acts to weaken the equatorial seasonal surface currents. Amplitudes of the spring and autumn eastward surface jets, the Wyrtki jets (WJ), and the westward surface current during January–March are reduced by as much as 15–25 cm s−1 by intraseasonal atmospheric forcing, and strengths of the rectification exhibit a significant interannual variability. Important processes that cause the low-frequency rectification are asymmetric response of mixed layer depth to easterly and westerly winds, entrainment, and upwelling of momentum. During spring and autumn, the westerly (easterly) phase of an intraseasonal event enhances (weakens or even reverses) the seasonal westerly winds, increases (decreases) equatorial convergence and entrainment, and thus deepens (thins) the mixed layer. A net, westward current is generated over an event mean because easterly wind acts on a thinner surface mixed layer whereas westerly wind acts on a thicker one. In contrast, during January–March when the seasonal winds are equatorial easterlies, surface currents are westward and equatorial undercurrents (EUC) develop. The rectified surface currents are eastward, which reduces the westward surface flow. This eastward rectification results largely from the vertical advection and entrainment of the EUC. The seasonal-to-interannual variability of the rectified surface flow is determined primarily from the seasonal cycle and interannual variability of the background state. Seasonal-to-interannual variability of the intraseasonal wind forcing also contributes. The rectified low-frequency zonal volume (heat) transports integrated over the entire water column along the Indian Ocean equator are persistently eastward with an amplitude of 0– 15 × 106 m3 s−1 (0–1.2 pW). This is because westerly winds generate equatorial downwelling, advecting the surface eastward momentum downward and giving an eastward subsurface current. Easterly winds cause equatorial upwelling and produce an eastward pressure gradient force that drives an eastward subsurface current. This eastward subsurface current is advected upward by upwelling. The mean effect over an intraseasonal event at the equator is to increase the eastward transport in the water column. In the layers above the thermocline, the rectified zonal volume (heat) transports are in the same direction as the rectified surface currents. Results from this paper may have important implications for understanding climate variability because modification of WJ strength and transports can affect the SST and heat storage in the equatorial Indian Ocean warm pool.

A new approach to improved SST anomaly simulations using altimeter data: Parameterizing entrainment temperature from sea level

Observed Topex/Poseidon/Jason‐1 (T/P/J) altimeter sea level (SL) data are used to improve an empirical parameterization of the temperature of subsurface water entrained into the mixed layer (Te). An inverse modeling method is first adopted to estimate Te from a SST anomaly (SSTA) equation using observed SST and simulated upper ocean currents from an intermediate ocean model (IOM). An empirical relationship between anomalies of Te (estimated) and SL (observed) from T/P/J altimeter is then constructed by utilizing a singular value decomposition (SVD) of their covariance. As compared with SSTA simulations using modeled SL data, the use of T/P/J SL data allows observed information about dynamic ocean adjustments to be built directly into the empirical Te model being used to parameterize the thermocline effect on SSTAs. The improved Te model leads to better SSTA simulations in the tropical Pacific. Cross validation is made to examine the sensitivity of the SSTA simulations to the period chosen for training the Te model. The proposed approach provides a new way to more effectively use T/P/J altimeter data in climate studies.

Interhemispheric Coherence of Tropical Climate Variability: Effect of the Climatological ITCZ

The intertropical convergence zone (ITCZ) is the rising branch of the global Hadley circulation and often considered as the climatic axis of symmetry. A full-physics atmospheric general circulation model is coupled with an intermediate ocean model to investigate the effect of ITCZ's meridional configuration on the space-time structure of climate variability. In the control experiment where the model settles into a north-south symmetric climatology, strong interhemispheric interaction takes place and sea surface temperature (SST) anomalies are organized into an anti-symmetric dipole pattern with the equator as the nodal line. The trade winds intensify (weaken) over the anomalously cold (warm) side of the equator, indicative of a positive feedback between surface wind, evaporation and SST (WES). When the mean ITCZ is displaced into the Northern Hemisphere by perturbing the shape of continents, SST variability is significantly reduced at low-frequencies, especially with periods greater than 5 years, as a result of reduced interhemispheric interaction. The nodal line now coincides with the northward-displaced ITCZ, and the SST correlation across this nodal line is greatly reduced to a statistically insignificant level. Calculations with a simple baroclinic model of the atmosphere indicate that the departure of the climatic axis of symmetry from the geographic equator weakens the WES feedback and hence interhemispheric interaction. Implications for tropical Atlantic variability are discussed. In particular, our result of ITCZ’s modulation of interhemispheric interaction is consistent with the seasonality of cross-equatorial SST gradient variability: it peaks in boreal spring when the mean ITCZ is nearly symmetric about the equator and is significantly reduced in other seasons when the Atlantic ITCZ is displaced into the Northern Hemisphere.

El Niño and La Niña sea surface temperature anomalies: Asymmetry characteristics associated with their wind stress anomalies

The asymmetric nature of El Niño and La Niña sea surface temperature (SST) anomalies is investigated by the use of National Centers for Environmental Prediction reanalysis data and various ocean and atmosphere models. It is demonstrated that the relatively weak SST anomalies during La Niña compared with those of El Niño are related to the westward shift of wind stress anomalies by 10°–15°. The asymmetric characteristics of atmospheric responses are confirmed by the general circulation model experiments with the two different SST anomalies, which have equal amplitude but are of opposite sign from each other. The experiments with an intermediate ocean model and a hybrid coupled model clearly show that the SST anomalies over the equatorial Pacific become weaker as the zonal wind stress shifts to the west. Not only the amplitude but also the oscillation timescale of the SST anomaly is shown to be sensitive to the location of wind stress anomalies. The duration of La Niña, which is rather shorter than that of El Niño, is also related to the longitudinal displacement of the wind stress anomaly.

Simulation of the Intraseasonal Oscillation in the ECHAM-4 Model: The Impact of Coupling with an Ocean Model*

Abstract Three 15-yr integrations were made with the ECHAM-4 atmospheric GCM (AGCM); in the first integration, the model lower boundary conditions were the observed monthly mean sea surface temperatures, and, in the second, the AGCM was coupled to the University of Hawaii 2.5-layer intermediate ocean model. In the third simulation, the SST climatology generated in the coupled run was used to create monthly mean SSTs, which were then used to drive the AGCM in an uncoupled configuration similar to the first run. The simulation of the intraseasonal oscillation (ISO) in these three runs was compared with data from the NCEP reanalysis and outgoing longwave radiation from NOAA polar-orbiting satellites, with particular emphasis on the boreal summer ISO. The overall effect of coupling the AGCM to the ocean model is to improve the intraseasonal variability of the model. Upon coupling, the simulated boreal winter ISO becomes more spatially coherent and has a more realistic phase speed. In the May–June Asian monsoo...

Formation and Variability of a Northerly ITCZ in a Hybrid Coupled AGCM: Continental Forcing and Oceanic–Atmospheric Feedback*

Abstract Despite the equatorial symmetry of the annual-mean insolation, the intertropical convergence zone (ITCZ) and the collocated band of high sea surface temperature (SST) assume perennial northern latitudes over the eastern Pacific and Atlantic. An atmospheric general circulation model is coupled with an intermediate ocean model to study continental forcing and oceanic–atmospheric interaction that act to break the equatorial symmetry. The model reaches a statistically symmetric mean state under perfectly symmetric conditions with the continental coasts running along meridians. When a bulge of landmass is added to the eastern continent north of the equator, it initiates a coupled ocean–atmosphere wave front that propagates westward across the ocean basin, cooling the ocean surface and suppressing deep convection on and south of the equator. As a result, the ITCZ shifts into the Northern Hemisphere. In contrast to this basinwide response, little latitudinal asymmetry develops in the coupled model when ...

An El‐Nino Prediction System using an intermediate ocean and a statistical atmosphere

An El‐Nino prediction model is developed based on an intermediate ocean model similar to the Cane and Zebiak (CZ) and a statistical atmosphere model. The present ocean model differs from CZ in the parameterization of sub‐surface temperature and the basic state. The predictability skill of the present model is better than that of CZ. The better performance is particularly distinctive for early stage of the prediction everywhere in the domain and in the central Pacific for all period of prediction. It is suggested that the better performance for the early stage is due to the use of SST anomalies in the initialization, and the better performance in the central Pacific results from a better representation of subsurface temperature in the present model.

On the seasonal sea surface temperature variations in the tropical Pacific

In this paper, we investigated physical processes that control the seasonal variations of sea surface temperature in the tropical Pacific, using an intermediate ocean model. It is found that the westward propagation of sea surface temperature along the equator is attributed to dynamic response of the ocean to the wind (that consists of 3-dimensional temperature advection), whereas the northward propagation of sea surface temperature in the eastern Pacific results from the thermodynamic response of the ocean to the surface heat flux, primarily shortwave radiation that includes the effect of low-level stratus clouds. The remote response of the eastern Pacific sea surface temperature to seasonally varying wind in the western Pacific is of secondary importance, compared to the local wind forcing. The results suggest that the mechanism that controls the seasonal cycle of sea surface temperature is different from that associated with El Nino-Southern Oscillation.