A Discussion of the possible ENSO evolution in 2003        Issued on 24 April 2003

 1. Introduction

        This is a discussion of our estimate of the ENSO situation in 2003.  What prompted us to issue such a discussion is because as we prepare the seasonal tropical cyclone activity forecasts for 2003, we find that the ENSO signal appears to indicate tropical cyclone activity similar to the situation associated with a cold event (see 2003 tropical cyclone activity forecast).

        A few years ago, our group has examined evolution of warm events in the past (Chan and Xu 2000) and proposed that the warm event could become a cold event in the spring [the Spring (SP) type] or in the summer [the Summer (SU) type], or it could simply fade away (the NON type).  If we compare the current situation with the results obtained by Chan and Xu (2000), it appears that the evolution up to March 2003 resembles the composites for either the SP or the SU type, but is quite unlike the NON type.  In section 2, comparisons are made between the 2002-2003 situation and the composites given in Chan and Xu (2000).  The latest forecasts from different models and centres are then discussed in section 3.  Based on the comparisons of the current observations and these different forecasts, a qualitative estimate of the possible ENSO condition in 2003 is made in section 4.

2. Comparisons between observations and previous composite results

a. Zonal wind anomalies along the equator

        The 1000-hPa zonal wind anomalies averaged between 5^oS and 5^oN (Fig. 1a) shows easterly anomalies in the equatorial Indian Ocean (EIO) from around September 2002 that propagate eastward so that by March 2003 (the latest data available), easterly anomalies are found east of ~120^oE.  This time-longitude cross-section is very much similar to the composite for the SU-type events (Fig. 1c) but different from the SP- or NON-type events (Figs. 1b and 1d respectively).

Fig. 1.	Longitude-time cross-section of the 1000-hPa monthly zonal wind anomalies in the equatorial area (5oS-5oN) for (a) 2002-2003, (b) SP-type composite, (c) SU-type composite and (d) NON-type composite.  (Figs. 1b-d extracted from Chan and Xu 2000).  The arrow indicates the direction of propagation of the easterly anomalies.

b. Sea-level pressure (SLP) anomalies

         The SLP anomalies over the Pacific averaged between November 2002 and February 2003 (Fig. 2a) show strong negative anomalies in the northeast Pacific and positive anomalies over eastern part of Australia and the southwest Pacific.  Anomalies over the western North Pacific are weakly positive.  This pattern suggests a strengthening of the subtropical high especially over the western Pacific, especially in the Southern Hemisphere.  Such a pattern appears to lie somewhere between those of the SP and SU types (Figs. 2b and 2c respectively) but unlike that of the NON-type (Fig. 2d). 

Fig. 2.	SLP anomalies for the period of November (-1) to February (0) for (a) 2002-2003, (b) SP-type composite, (c) SU-type composite and (d) NON-type composite.  The shaded areas in red and blue indicate SLPA > 0.5 hPa and < -0.5 hPa respectively.  Unit: 0.01 hPa.

        As mentioned in Chan and Xu (2000), a major difference between the SP and SU episodes lies in the SLP anomalies in the eastern Pacific.  A comparison is therefore made between the situation in 2002-2003 and those from the three types (Fig. 3).  It appears that the SLP variations with time are quite similar to those of the SP composite.

Fig. 3.	Latitude-time section of SLP anomalies over the eastern Pacific (180o - 120oW) for the (a) situation in 2002-2003, (b) SP-type composite, (c) SU-type composite and (d) NON-type composite.  The shaded areas indicate SLPA < 0. Unit: 0.01 hPa.

c. Summary

        From these comparisons, it appears that at least up to March 2003, the conditions in the Pacific resemble those of previous cases in which a La Nia event developed either in the spring or summer, but are less likely similar to those in which the warm event simply weakens without becoming a cold event (i.e. becoming neutral).  In other words, based on these “ analogs ”, it is possible that 2003 could be a cold event year.

3. Model forecasts

        A summary of the various model forecasts suggests that most of them predict a neutral condition in 5 to 8 months time (Table. 1). 

        Compared with the same forecasts made three months ago, it appears that the number of models that favours a continuation of a warm event becomes less and less.  Indeed, a plot of the actual forecast values of the various models shows a continuous “ downward ” adjustment of the predicted SST anomaly (Fig. 4).

Fig. 4.	Summary of the average forecasts from all (a) dynamical and (b) statistical models made from December 2002 to April 2003.

        Of particular note are the ECMWF ensemble forecasts of the NINO-area anomalies (the Nino plumes) (Fig. 5).  The individual plumes and the ensemble mean show that the forecasts call for a continuous decrease in SST anomalies in Nino3.4, reaching the threshold for a cold event by the summer.

Fig. 5.	(a) Nino plumes of the Nino3.4 anomalies from ECMWF made from 1 February, March and April 2003.  (b) Ensemble mean values of the forecasts from December 2002 to April 2003.

        To summarize, while most of the models predict neutral conditions for 2003, a tendency for moving towards a cold event appears to be increasing.  In fact, the latest ECMWF ensemble mean suggests the Nio3.4 SST anomaly to reach the threshold of a cold event by late summer.

 4. Summary and prediction

        An apparent discrepancy seems to exist between what may be expected when comparing the current situation with that of past ENSO episodes and the forecasts from various models.  However, what appears to be consistent is that 2003 would unlikely be a warm event year.  Furthermore, for most of the past cases in which a warm event becomes a cold event, the Nio3.4 SST anomalies fell below 0.5^oC no later than April.  The model forecasts for this year are all showing this trend (see Figs. 4 and 5).  Therefore, if history is any indication, it is likely that 2003 would be a La Nia year.

        Because of the relatively large deviation between the “ analog ” forecast and those of the models (dynamical and statistical), our estimate of the ENSO situation in 2003 at this time would call for neutral conditions with the possibility of a cold event developing later in the year.

References

Chan, J. C. L. and J. J. Xu, 2000: Physical mechanisms responsible for the transition from a warm to a cold state of the El Nino Southern Oscillation. J. Climate, 13, 2056-2071. Abstract