Jieshun Zhu

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Papers by Jieshun Zhu

Climate drift of AMOC, North Atlantic salinity and arctic sea ice in CFSv2 decadal predictions

by Jieshun Zhu and S. Zhang

Climate Dynamics, 2014

Sea Surface Temperature Predictions in NCEP CFSv2 Using a Simple Ocean Initialization Scheme

Monthly Weather Review, 2015

In contrast to operational climate predictions based on sophisticated ocean data assimilation sch... more In contrast to operational climate predictions based on sophisticated ocean data assimilation schemes at the National Centers for Environmental Predictions (NCEP), this study applied a simple ocean initialization scheme to the NCEP latest seasonal prediction model, the Climate Forecast System, version 2 (CFSv2). In the scheme, sea surface temperature (SST) was the only observed information applied to derive ocean initial states. The physical basis for the method is that, through air-sea coupling, SST is capable of reproducing some observed features of ocean evolutions by forcing the atmospheric winds. SST predictions based on the scheme are compared against hindcasts from the National (lately North American) Multimodel Ensemble (NMME) project.

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Variability of Summer Rainfall in Northeast China and Its Connection with Spring Rainfall Variability in the Huang-Huai Region and Indian Ocean SST

by Jieshun Zhu, Bhaskar Jha, and Zeng-Zhen Hu

Journal of Climate, 2014

In this work, the variability of summer [June-August (JJA)] rainfall in northeast China is examin... more In this work, the variability of summer [June-August (JJA)] rainfall in northeast China is examined and its predictors are identified based on observational analyses and atmospheric modeling experiments. At interannual time scales, the summer rainfall anomaly in northeast China is significantly correlated with the rainfall anomaly over the Huang-Huai region (328-388N, 1058-1208E) in late spring (April-May). Compared with climatology, an earlier (later) rainy season in the Huang-Huai region favors a wet (dry) summer in northeast China. Also, this connection has strengthened since the late 1970s. In addition to the impact of the sea surface temperature anomaly (SSTA) in the tropical Indian Ocean, the local soil moisture anomalies caused by the rainfall anomaly in the Huang-Huai region in late spring generate summer general circulation anomalies, which contribute to the rainfall anomaly in northeast China. As a result, when compared with the SSTA, the rainfall anomaly in the Huang-Huai region in late spring can be used as another and even better predictor for the summer rainfall anomaly in northeast China.

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Model study of the Labrador Sea variability at interannual and decadal time scales

Eddy permitting model study of the Labrador Sea interannual and interdecadal variability from 194... more Eddy permitting model study of the Labrador Sea interannual and interdecadal variability from 1948 to 2005 is present. The NEMO coupled sea-ice ocean model is used in this study, which is implemented for the region of the North Atlantic with 1/4 degree horizontal resolution and 46 vertical levels. The surface atmospheric forcing is calculated with NCEP atmospheric reanalysis. Variability of the surface momentum, heat and water fluxes for the period of simulations is presented. It's relation to the dominant atmospheric pattern in the North Atlantic is discussed. Water mass characteristics and water transport in the Labrador Sea and their interannual and interdecadal variability are studied. Energetic analysis of mean and eddy flows is used to explain the mechanism of formation of long-term variability in the model solution.

Dependence of computational stability of nonlinear advection equation on the Initial Value

On the role of oceanic entrainment temperature (Te) in decadal changes of El Niño/Southern Oscillation

An improved Hybrid Coupled Model: ENSO Simulations

Improving ENSO Simulation by Parameterizing the Subsurface Entrainment Temperature

Variability and predictability of Northeast China climate during 1948–2012

by Jieshun Zhu, Bhaskar Jha, and Zeng-Zhen Hu

A Model Study of the Relationship between Sea-Ice Variability and Surface and Intermediate Water Mass Properties in the Labrador Sea

Atmosphere-Ocean, 2014

Sea ice plays an important role in the variability of the Labrador Sea especially in its most wes... more Sea ice plays an important role in the variability of the Labrador Sea especially in its most western part adjacent to an important region of deep convection. Winter-to-winter re-emergence and propagation of both sea-ice concentration (SIC) and sea surface temperature anomalies have been observed following years of high SIC in this region. They have potentially important links to water mass properties and freshwater and heat transports in the subpolar North Atlantic. This article builds on the results of two precursor papers and presents results from a coupled sea-ice-ocean model study of the interannual variability of sea ice in the Labrador Sea. The relationships between SIC and water column properties in the subpolar North Atlantic are assessed. Winters with high SIC and strong surface cooling are found to be conducive to intensified convection. Surface and mid-depth temperature and salinity anomalies are observed in the Labrador Sea and the northwestern North Atlantic during winters with anomalous Labrador Sea SIC. These anomalies are found to propagate along the major circulation patterns in the subpolar North Atlantic and to persist for up to three years. RÉSUMÉ [Traduit par la rédaction] La glace de mer joue un rôle important dans la variabilité de la mer du Labrador, surtout dans sa partie la plus à l'ouest adjacente à une grande région de convection profonde. On a observé que la réapparition et la propagation, d'un hiver à l'autre, d'anomalies de concentration de glace de mer (CGM) et de température de la surface de la mer suivent des années de forte CGM dans cette région. Ces anomalies pourraient être étroitement liées aux propriétés des masses d'eau et au transport d'eau douce et de chaleur dans l'Atlantique Nord subpolaire. Cet article fait suite aux résultats de deux articles précédents et présente les résultats d'une étude de la variabilité interannuelle de la glace de mer dans la mer du Labrador faite à l'aide d'un modèle couplé glace de merocéan. Nous examinons la relations entre la CGM et les propriétés de la colonne d'eau dans l'Atlantique Nord subpolaire. Nous constatons que les hivers de CGM élevée et de fort refroidissement en surface favorisent une convection intensifiée. Nous observons des anomalies de température et de salinité à la surface et à mi-profondeur dans la mer du Labrador et dans le nord-ouest de l'Atlantique Nord durant les hivers où se produisent des anomalies de CGM dans la mer du Labrador. Nous trouvons que ces anomalies se propagent le long des principales configurations de circulation dans l'Atlantique Nord subpolaire et perdurent jusqu'à trois ans.

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North Atlantic atmospheric and ocean inter-annual variability over the past fifty years – Dominant patterns and decadal shifts

by Jieshun Zhu and T. Hauser

Progress in Oceanography, 2014

Please cite this article as: Hauser, T., Demirov, E., Zhu, J., Yashayaev, I., North Atlantic atmo... more Please cite this article as: Hauser, T., Demirov, E., Zhu, J., Yashayaev, I., North Atlantic atmospheric and ocean inter-annual variability over the past fifty years -dominant patterns and decadal shifts, Progress in Oceanography (2014), doi: http://dx.Abstract The atmosphere and ocean of the North Atlantic have undergone significant changes in the past century. To understand these changes, their mechanisms, and their regional implications requires a quantitative understanding of processes in the coupled ocean and atmosphere system. Central to this understanding is the role played by the dominant patterns of ocean and atmospheric variability which define coherent variations in physical characteristics over large areas.

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Model simulations of mesoscale eddies and deep convection in the Labrador Sea

by Ying Zhang and Jieshun Zhu

Advances in Atmospheric Sciences, 2014

Deep convection in the Labrador Sea is confined within a small region in the southwest part of th... more Deep convection in the Labrador Sea is confined within a small region in the southwest part of the basin. The strength of deep convection in this region is related to the local atmospheric and ocean characteristics, which favor processes of deep convection preconditioning and intense air-sea exchange during the winter season. In this study, we explored the effect of eddy-induced flux transport on the stratification of the Labrador Sea and the properties of deep convection. Simulations from an eddy-resolving ocean model are presented for the Labrador Sea. The general circulation was well simulated by the model, including the seasonal cycle of the deep Labrador Current. The simulated distribution of the surface eddy kinetic energy was also close to that derived from Topex-Poseidon satellite altimeter data, but with smaller magnitude. The energy transfer diagnostics indicated that Irminger rings are generated by both baroclinic and barotropic processes; however, when they propagate into the interior basin, the barotropic process also disperses them by converting the eddy energy to the mean flow. In contrast to eddy-permitting simulations, deep convection in the Labrador Sea was better represented in the eddyresolving model regarding their lateral position. Further analysis indicated that the improvement might be due to the lateral eddy flux associated with the resolved Irminger rings in the eddy-resolving model, which contributes to a realistic position of the isopycnal dome in the Labrador Sea and correspondingly a realistic site of deep convection. Citation: Zhu, J. S., E. Demirov, Y. Zhang, and A. Polomska-Harlick, 2014: Model simulations of mesoscale eddies and deep convection in the Labrador Sea.

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Improved reliability of ENSO hindcasts with multi-ocean analyses ensemble initialization

by Jieshun Zhu and Peitao Peng

Climate Dynamics, 2013

Currently, ensemble seasonal forecasts using a single model with multiple perturbed initial condi... more Currently, ensemble seasonal forecasts using a single model with multiple perturbed initial conditions generally suffer from an ''overconfidence'' problem, i.e., the ensemble evolves such that the spread among members is small, compared to the magnitude of the mean error. This has motivated the use of a multi-model ensemble (MME), a technique that aims at sampling the structural uncertainty in the forecasting system. Here we investigate how the structural uncertainty in the ocean initial conditions impacts the reliability in seasonal forecasts, by using a new ensemble generation method to be referred to as the multiple-ocean analysis ensemble (MAE) initialization. In the MAE method, multiple ocean analyses are used to build an ensemble of ocean initial states, thus sampling structural uncertainties in oceanic initial conditions (OIC) originating from errors in the ocean model, the forcing flux, and the measurements, especially in areas and times of insufficient observations, as well as from the dependence on data assimilation methods. The merit of MAE initialization is demonstrated by the improved El Niño and the Southern Oscillation (ENSO) forecasting reliability. In particular, compared with the atmospheric perturbation or lagged ensemble approaches, the MAE initialization more effectively enhances ensemble dispersion in ENSO forecasting. A quantitative probabilistic measure of reliability also indicates that the MAE method performs better in forecasting all three (warm, neutral and cold) categories of ENSO events. In addition to improving seasonal forecasts, the MAE strategy may be used to identify the characteristics of the current structural uncertainty and as guidance for improving the observational network and assimilation strategy. Moreover, although the MAE method is not expected to totally correct the overconfidence of seasonal forecasts, our results demonstrate that OIC uncertainty is one of the major sources of forecast overconfidence, and suggest that the MAE is an essential component of an MME system.

Evaluation of the CFSv2 CMIP5 decadal predictions

Climate Dynamics, 2014

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Salinity anomaly as a trigger for ENSO events

by Jieshun Zhu, Magdalena Balmaseda, and Rong-hua Zhang

Scientific reports, 2014

According to the classical theories of ENSO, subsurface anomalies in ocean thermal structure are ... more According to the classical theories of ENSO, subsurface anomalies in ocean thermal structure are precursors for ENSO events and their initial specification is essential for skillful ENSO forecast. Although ocean salinity in the tropical Pacific (particularly in the western Pacific warm pool) can vary in response to El Niño events, its effect on ENSO evolution and forecasts of ENSO has been less explored. Here we present evidence that, in addition to the passive response, salinity variability may also play an active role in ENSO evolution, and thus important in forecasting El Niño events. By comparing two forecast experiments in which the interannually variability of salinity in the ocean initial states is either included or excluded, the salinity variability is shown to be essential to correctly forecast the 2007/08 La Niña starting from April 2007. With realistic salinity initial states, the tendency to decay of the subsurface cold condition during the spring and early summer 2007 ...

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A nonlinear statistical ensemble model for short-range rainfall prediction

Theoretical and Applied Climatology, 2014

ABSTRACT Following the practice of the numerical weather ensemble prediction, a nonlinear statist... more ABSTRACT Following the practice of the numerical weather ensemble prediction, a nonlinear statistical ensemble prediction model has been developed based on a neural network technique with a Particle Swarm Optimization (PSO) algorithm. The model is validated by short-range climate forecasts of monthly mean rainfall at 37 stations in Guangxi, China during the first rainy season (April, May, and June). Independent prediction results show that the Particle Swarm Optimization Neural Network ensemble prediction model is clearly better than the traditional linear statistical method, such as the multiple regression method and the stepwise regression method. It is also suggested that by applying multiple ensemble members with each member objectively determined by the PSO algorithm, the generalization capacity of the ensemble prediction model is enhanced, demonstrating a vast range of possibilities for operational short-range climate prediction.

Eddy-permitting simulations of the sub-polar North Atlantic: impact of the model bias on water mass properties and circulation

Ocean Dynamics, 2010

Some previous studies demonstrated that model bias has a strong impact on the quality of long-ter... more Some previous studies demonstrated that model bias has a strong impact on the quality of long-term prognostic model simulations of the sub-polar North Atlantic Ocean. Relatively strong bias of water mass characteristics is observed in both eddy-permitting and eddy-resolving simulations, suggesting that an increase of model resolution does not reduce significantly the model bias. This study is an attempt to quantify the impact of model bias on the simulated water mass and circulation characteristics in an eddy-permitting model of the sub-polar ocean. This is done through comparison of eddy-permitting prognostic model simulations with the results from two other runs in which the bias is constrained by using spectral nudging. In the first run, the temperature and salinity are nudged towards climatology in the whole column. In the second run, the spectral nudging is applied in the surface 30 m layer and at depths below 560 m only. The biases of the model characteristics of the unconstrained run are similar to those reported in previous eddy-permitting and eddy-resolving studies. The salinity in the surface and intermediate waters of the Labrador Sea waters increases with respect to the climatology, which reduces the stability of the water column. The deep convection in the uncon-strained run is artificially intensified and the transport in the sub-polar gyre stronger than in the observations. In particular, the transport of relatively salty and warm Irminger waters into the Labrador Sea is unrealistically high. While the water mass temperature and salinity in the run with spectral nudging in the whole column are closest to the observations, the depth of the winter convection is underestimated in the model. The water mass characteristics and water transport in the run with spectral nudging in the surface and deep layers only are close to observations and at the same time represent well the deep convection in terms of its intensity and position. The source of the bias in the prognostic model run is discussed.

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On the mechanism of interannual variability of the Irminger Water in the Labrador Sea

Journal of Geophysical Research, 2011

1] The mechanism of variability of the North Atlantic subpolar gyre (SPG) and its relation to the... more 1] The mechanism of variability of the North Atlantic subpolar gyre (SPG) and its relation to the North Atlantic Oscillation (NAO) is investigated using an ocean general circulation model. In this study we conducted three model experiments. The first two were forced with idealized positive (NAO + ) and negative (NAO − ) NAO-like forcing, including variations at decadal time scales. The third experiment was forced with the National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) reanalysis. The decadal variability of the volume transport, the sea surface temperature in the North Atlantic Current (SSTA1), and the Irminger Water temperature (IWT) in the NAO − experiment have 2-3 times smaller magnitude than in the NAO + experiment. The decadal variations in the strength of circulation in the NAO + experiment covaries negatively with SSTA1 and IWT anomalies. A similar covariability of these parameters is not found in the NAO − simulations. The results from the model experiment forced with the NCEP/NCAR reanalysis from 1958 to 2005 show a shift in the subpolar ocean response to the atmospheric variability in the early 1980s. The amplitude of quasi-decadal variability of the IWT and SSTA1 and their correlation are high after 1980 (r = 0.79) and weaker in the period between 1958 and 1980 (r = 0.1). The IWT after 1980 is well correlated (r = 0.67) to the subpolar gyre transport index (SPGI, defined as the minimum value of the annual mean anomaly of the model barotropic stream function). This correlation is weaker and negative (r = −0.09) in the period from 1958 to 1980. We explain this shift in the covariability of the SSTA1, IWT, and SPGI with the antisymmetric response of the SPG to atmospheric variations at decadal time scale under positive and negative NAO index. The NAO sign changed in the early 1980s from predominantly negative to positive phase. In the 1980s and 1990s the model SPGI variability follows closely the decadal variations of the NAO index with a delay of about 3 years. Similar covariability between the SPGI and the NAO and related negative covariance of SSTA1 and IWT with the SPGI are not observed in the model simulations of the period from 1958 to 1980.

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Prediction skill of North Pacific variability in NCEP Climate Forecast System Version 2: Impact of ENSO and beyond

by Jieshun Zhu and Yuanhong Guan

Journal of Climate, 2014

This work examines the impact of El Niño-Southern Oscillation (ENSO) on the prediction skill of N... more This work examines the impact of El Niño-Southern Oscillation (ENSO) on the prediction skill of North Pacific variability (NPV) in retrospective predictions of the NCEP Climate Forecast System, version 2. It is noted that the phase relationship between ENSO and NPV at initial conditions (ICs) affects the prediction skill of NPV. For average lead times of 0-6 months, the prediction skills of sea surface temperature anomalies (SSTAs) in NPV (defined as the NPV index) increase from 0.42 to 0.63 from the cases of an out-of-phase relation between the Niño-3.4 and NPV indices in ICs to the cases of an in-phase relation. It is suggested that when ENSO and NPV are in phase in ICs, ENSO plays a constructive role in the NPV development and enhances its signals. Nevertheless, when ENSO and NPV are out of phase, some pronounced positive NPV events are still predictable. In these cases, the North Pacific is dominated by strong positive SSTAs, which may overcome the opposing influence from the tropical Pacific and display predictability.

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The Role of Ocean Dynamics in the Interaction between the Atlantic Meridional and Equatorial Modes

Journal of Climate, 2012

This study examines a mechanism of the interaction between the tropical Atlantic meridional and e... more This study examines a mechanism of the interaction between the tropical Atlantic meridional and equatorial modes. To derive robust heat content (HC) variability, the ensemble-mean HC anomalies (HCA) of six state-of-the-art global ocean reanalyses for 1979-2007 are analyzed. Compared with previous studies, characteristic oceanic processes are distinguished through their dominant time scales. Using the ensemble empirical mode decomposition (EEMD) method, the HC fields are first decomposed into components with different time scales. The authors' analysis shows that these components are associated with distinctive ocean dynamics. The high-frequency (first three) components can be characterized as the equatorial modes, whereas the low-frequency (the fifth and sixth) components are featured as the meridional modes. In between, the fourth component on the time scale of 3-4 yr demonstrates ''mixed'' characteristics of the meridional and equatorial modes because of an active transition from the predominant meridional to zonal structures on this time scale. Physically, this transition process is initiated by the discharge of the off-equatorial HCA, which is first accumulated as a part of the meridional mode, into the equatorial waveguide, which is triggered by the breakdown of the equilibrium between the cross-equatorial HC contrast and the overlying wind forcing, and results in a major heat transport through the equatorial waveguide into the southeastern tropical Atlantic. It is also shown that remote forcing from El Niñ o-Southern Oscillation (ENSO) exerts important influence on the transition from the equatorial to meridional mode and may partly dictate its time scale of 3-4 yr. Therefore, the authors' results demonstrate another mechanism of the equatorial Atlantic response to the ENSO forcing.

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