On the “spring predictability barrier” for strong El Nio events as derived from an intermediate coupled model ensemble prediction system

Using predictions for the sea surface temperature anomaly(SSTA) generated by an intermediate coupled model(ICM)ensemble prediction system(EPS), we first explore the "spring predictability barrier"(SPB) problem for the 2015/16 strong El Nio event from the perspective of error growth. By analyzing the growth tendency of the prediction errors for ensemble forecast members, we conclude that the prediction errors for the 2015/16 El Nio event tended to show a distinct season-dependent evolution, with prominent growth in spring and/or the beginning of the summer. This finding indicates that the predictions for the 2015/16 El Nio occurred a significant SPB phenomenon. We show that the SPB occurred in the 2015/16 El Nio predictions did not arise because of the uncertainties in the initial conditions but because of model errors. As such, the mean of ensemble forecast members filtered the effect of model errors and weakened the effect of the SPB, ultimately reducing the prediction errors for the 2015/16 El Nio event. By investigating the model errors represented by the tendency errors for the SSTA component,we demonstrate the prominent features of the tendency errors that often cause an SPB for the 2015/16 El Nio event and explain why the 2015/16 El Nio was under-predicted by the ICM EPS. Moreover, we reveal the typical feature of the tendency errors that cause not only a significant SPB but also an aggressively large prediction error. The feature is that the tendency errors present a zonal dipolar pattern with the west poles of positive anomalies in the equatorial western Pacific and the east poles of negative anomalies in the equatorial eastern Pacific. This tendency error bears great similarities with that of the most sensitive nonlinear forcing singular vector(NFSV)-tendency errors reported by Duan et al. and demonstrates the existence of an NFSV tendency error in realistic predictions. For other strong El Nio events, such as those that occurred in 1982/83 and 1997/98, we obtain the tendency errors of the NFSV structure, which cause a significant SPB and yield a much larger prediction error. These results suggest that the forecast skill of the ICM EPS for strong El Nio events could be greatly enhanced by using the NFSV-like tendency error to correct the model.

Processes involved in the second-year warming of the 2015 El Nio 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 Nio 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 Nio 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 Nio event are consistent with current El Nio and Southern Oscillation theories.

Research on the Response of the Upper Layer Heat Structure in the Western Pacific Warm Pool to the Mean Madden-Julian Oscillation

By using the long-term observed hydro-meteorological data (1985-2002) from the Tropical Atmosphere Ocean System (TAO) during the international Tropical Ocean and Global Atmosphere (TOGA) experiment, the key parameters of the Sea Surface Temperature (SST), thermocline depth, surface sensible heat flux and latent heat flux, and the pseudo wind stress in the Westen Equatorial Ocean are calculated in this paper. On the basis of the calculation, the response of upper layer heat structure in the Westen Pacific Warm Pool to the mean Madden-Julian Oscillation (MJO) and its relation to the El Nio events are analyzed. The results show that within the MJO frequency band (42-108 d), the distributions of sea surface wind stress and upper ocean temperature have several spatial-temporal variation structures. Among these structures, the type-I surface pseudo wind stress field plays the role of inhibiting the eastward transport of ocean heat capacity, while the type-II strengthens the heat capacity spreading eastward. Therefore the type-II surface pseudo wind stress field is the characteristic wind field that provokes El Nio events. During calm periods (July-September) of the wind stress variations, the sensible and latent heat capacity fluxes change considerably, mostly in the region between 137°-140°E, while to the east of 150°E, the heat capacity flux changes less.\ In the mean MJO state, the type-I surface pseudo wind stress field structure dominates in the Western Pacific. This is why El Nio events can not occur every year. However, when the type-II and type-III surface pseudo wind stress field structures are dominant, an El Nio event is likely to occur. In this case, if the heat capacity of the Western Pacific Warm Pool is transported eastward and combined with the Equatorial Pacific heat capacity spreading eastward, El Nio events will soon occur.