Using monthly reanalysis data of the National Center for Environmental Research/National Center for Atmospheric Research(NCEP/NCAR) and Objectively Analyzed Air-Sea Heat Flux(OAFlux) gathered during the winter,singular vector decomposition(SVD) analysis was conducted to reveal the coupled mode between the Kuroshio marine heating anomaly and the geopotential height at 500 hPa(Z500) over the North Pacific.The first SVD mode showed that when the northern Kuroshio marine heating anomaly was positive,the Z500 in the central and western sections of the North Pacific was anomalously low.By composing the meteorological field anomalies in the positive(or negative) years,it has been revealed that while the Aleutian Low deepens(or shallows),the northwesterly wind overlying the Kuroshio strengthens(or weakens) and induces the near-surface air to be cool(or warm).Furthermore,this increases(or decreases) the upward heat flux anomaly and cools(or warms) the sea surface temperature(SST) accordingly.In the vicinity of Kuroshio and its downstream region,the vertical structure of the air temperature along the latitude is baroclinic;however,the geopotential height is equivalently barotropic,which presents a cool trough(or warm ridge) spatial structure.The divergent wind and vertical velocities are introduced to show the anomalous zonal circulation cell.These are characterized by the rising(or descending) air in the central North Pacific,which flows westward and eastward toward the upper troposphere,descends(or rises) in the Kuroshio and in the western section of North America,and then strengthens(or weakens) the mid-latitude zonal cell(MZC).
Within a theoretical ENSO model, the authors investigated whether or not the errors superimposed on model parameters could cause a significant "spring predictability barrier" (SPB) for El Nio events. First, sensitivity experiments were respectively performed to the air-sea coupling parameter, α and the thermocline effect coefficient μ. The results showed that the uncertainties superimposed on each of the two parameters did not exhibit an obvious season-dependent evolution; furthermore, the uncertainties caused a very small prediction error and consequently failed to yield a significant SPB. Subsequently, the conditional nonlinear optimal perturbation (CNOP) approach was used to study the effect of the optimal mode (CNOP-P) of the uncertainties of the two parameters on the SPB and to demonstrate that the CNOP-P errors neither presented a unified season-dependent evolution for different El Nio events nor caused a large prediction error, and therefore did not cause a significant SPB. The parameter errors played only a trivial role in yielding a significant SPB. To further validate this conclusion, the authors investigated the effect of the optimal combined mode (i.e. CNOP error) of initial and model errors on SPB. The results illustrated that the CNOP errors tended to have a significant season-dependent evolution, with the largest error growth rate in the spring, and yielded a large prediction error, inducing a significant SPB. The inference, therefore, is that initial errors, rather than model parameter errors, may be the dominant source of uncertainties that cause a significant SPB for El Nio events. These results indicate that the ability to forecast ENSO could be greatly increased by improving the initialization of the forecast model.
Using the sea surface temperature (SST) predicted for the equatorial Pacific Ocean by the Flexible Global Ocean-Atmosphere-Land System Model-gamil (FGOALS-g),an analysis of the prediction errors was performed for the seasonally dependent predictability of SST anomalies both for neutral years and for the growth/decay phase of El Ni o/La Ni a events.The study results indicated that for the SST predictions relating to the growth phase and the decay phase of El Ni o events,the prediction errors have a seasonally dependent evolution.The largest increase in errors occurred in the spring season,which indicates that a prominent spring predictability barrier (SPB) occurs during an El Ni o-Southern Oscillation (ENSO) warming episode.Furthermore,the SPB associated with the growth-phase prediction is more prominent than that associated with the decay-phase prediction.However,for the neutral years and for the growth and decay phases of La Ni a events,the SPB phenomenon was less prominent.These results indicate that the SPB phenomenon depends extensively on the ENSO events themselves.In particular,the SPB depends on the phases of the ENSO events.These results may provide useful knowledge for improving ENSO forecasting.
