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Zheng F.,CAS Institute of Atmospheric Physics | Wang H.,National Meteorological Center | Wan L.,Key Laboratory of Research on Marine Hazards Forecasting
Acta Oceanologica Sinica

The 2002/03 El Niño event, a new type of El Niño with maximum warm anomaly occurring in the central equatorial Pacific, is known as central-Pacific (CP) El Niño. In this study, on the basis of an El Niño prediction system, roles of the initial ocean surface and subsurface states on predicting the 2002/03 CP El Niño event are investigated to determine conditions favorable for predicting El Niño growth and are isolated in three sets of hindcast experiments. The hindcast is initialized through assimilation of only the sea surface temperature (SST) observations to optimize the initial surface condition (Assim_SST), only the sea level (SL) data to update the initial subsurface state (Assim_SL), or both the SST and SL data (Assim_SST+SL). Results highlight that the hindcasts with three different initial states all can successfully predict the 2002/03 El Niño event one year in advance and that the Assim_SST+SL hindcast performs best. A comparison between the various sets of hindcast results further demonstrates that successful prediction is significantly affected by both of the initial surface and subsurface conditions, but in different developing phases of the 2002/03 El Niño event. The accurate initial surface state can easier trigger the prediction of the 2002/03 El Niño, whereas a more reasonable initial subsurface state can contribute to improving the prediction in the growth of the warm event. © 2015 The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg Source

Zu Z.,Key Laboratory of Research on Marine Hazards Forecasting | Mu M.,CAS Qingdao Institute of Oceanology | Mu M.,Laboratory for Ocean and Climate Dynamics | Dijkstra H.A.,University Utrecht
Journal of Physical Oceanography

Within a three-dimensional ocean circulation model, the nonlinear optimal initial perturbations (NOIP) of sea surface salinity (SSS) and sea surface temperature (SST) to excite variability in the Atlantic meridional overturning circulation (AMOC) were obtained under prescribed heat and freshwater flux boundary conditions, using the conditional nonlinear optimal perturbation (CNOP) method. After 10 years, the optimal SSS and SST perturbations lead to reductions of the AMOC by 3.6 and 2.5 Sv (1 Sv = 106 m3 s-1), respectively, followed by multidecadal oscillations with a period of about 50 years. During the first 30 years, nonlinear processes have an important influence on the AMOC strength: convection strengthens the AMOC during years 0-2, zonal density advection promotes the slowdown of the AMOC during years 7-20, and meridional density advection inhibits the slowdown of meridional velocities in the upper ocean during years 5-18. The linear optimal initial perturbation (LOIP) was also computed using the first singular vector (FSV) method. For SSS perturbations with an amplitude of 0.5 psu, the LOIP will cause an underestimation of the amplitude of the multidecadal AMOC variability by about 1 Sv, compared to that induced by the NOIP. This underestimation will become more significant as the amplitudes of SSS perturbations increase. © 2016 American Meteorological Society. Source

Jiang L.,University of Oxford | Jiang L.,Key Laboratory of Research on Marine Hazards Forecasting | Borthwick A.G.L.,University of Oxford | Kramer T.,Budapest University of Technology and Economics | Jozsa J.,Budapest University of Technology and Economics
International Journal of Computational Fluid Dynamics

A horizontally variable density flow model is used to simulate hydraulic bore interactions with idealised urban obstacles. The 2D non-linear shallow water equations are solved using a second-order Monotonic Upstreamcentered Schemes for Conservation Laws-Hancock Godunov-type HLLC approximate Riemann scheme. Validation test results are reported for wave propagation over a hump, a constant-density circular dam break and two 1D dam breaks involving different spatial distributions of solute concentration. Detailed parameter studies are then considered for hydraulic bore interactions with single and multiple-square obstacles under subcritical, critical and supercritical flow conditions. In all cases, reflected and diffracted wave patterns are generated immediately after the bore impacts the obstacle(s). Later, the incident bore reconstitutes itself downstream of the obstacle(s). Variable density flows are also considered, with the upstream volumetric concentrations set to values corresponding to water-sediment mixture densities of 1165 and 1495 kg/m3. It is found that the upstream Froude number, gap spacing between obstacles and upstream to downstream density difference influence the strength of the bore-structure interaction, run-up at the front face of the obstacle(s), and subsequent wave-wave interactions. © 2011 Taylor & Francis. Source

Zheng F.,CAS Institute of Atmospheric Physics | Li J.,Beijing Normal University | Li J.,Joint Center for Global Change Studies | Wang L.,Key Laboratory of Research on Marine Hazards Forecasting | And 3 more authors.
Journal of Climate

New evidence suggests that interannual variability in zonal-mean meridional circulation and precipitation can be partially attributed to the Southern Hemisphere annular mode (SAM), the dominant mode of climate variability in the Southern Hemisphere (SH) extratropics. A cross-seasonal correlation exists between the December-February (DJF) SAM and March-May (MAM) zonal-mean meridional circulation and precipitation. This correlation is not confined to the SH: it also extends to the Northern Hemisphere (NH) subtropics. When the preceding DJF SAM is positive, counterclockwise, and clockwise meridional cells, accompanied by less and more precipitation, occur alternately between the SH middle latitudes and NH subtropics in MAM. In particular, less precipitation occurs in the SH middle latitudes, the SH tropics, and the NH subtropics, but more precipitation occurs in the SH subtropics and the NH tropics. A framework is built to explain the cross-seasonal impact of SAM-related SST anomalies. Evidence indicates that the DJF SAM tends to lead to dipolelike SST anomalies in the SH extratropics, which are referred to in this study as the SH ocean dipole (SOD). The DJF SOD can persist until the following MAM when it begins to modulate MAM meridional circulation and large-scale precipitation. Atmospheric general circulation model simulations further verify that MAM meridional circulation between the SH middle latitudes and the northern subtropics responds to the MAM SOD. © 2015 American Meteorological Society. Source

Wang L.,Key Laboratory of Research on Marine Hazards Forecasting | Wang L.,Peking University | Wang Z.,Key Laboratory of Research on Marine Hazards Forecasting | Li J.,Beijing Normal University | Zheng F.,CAS Institute of Atmospheric Physics
Atmospheric Science Letters

Seesaw pattern of the perturbation potential energy in the extratropics and the Arctic (defined as PEAS) is observed. The PEAS is closely related to the Arctic Oscillation, kinetic energy and zonal wind in the middle- and high-latitude. The PEAS may influence atmospheric circulation via northern migration of anomalous Ferrel cell, accompanying by suppressed subtropical jet stream and enhanced polar jet stream. Anomalous warm conditions associated with the PEAS are observed in high-latitude of Eurasia, whereas cold anomalies are presented in Greenland, northeast Africa and southwest China, which is mainly attributed to the horizontal temperature advection. © 2015 Royal Meteorological Society. Source

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