The 1997/98 ENSO event

Multivariate ENSO Index Index is based on 6 parameters relevant to phase of ENSO: sea level pressurezonal and meridional wind at surface sea surface temperatureair temperature cloudiness

A weakening and reversal of trade winds in Western and central Pacific led to anomalous warm SSTs east of date line in early 1997 Warm waters (> 29C migrated eastwards, collapse of cold tongue in east, which failed to develop.

El Nino developed so rapidly that each month between June-Dec 1997set a new monthly record high was set for SST in eastern equatorial Pacific.

Development of El Nino in equatorial Pacific was significantly modulated by higher frequency variability. Weakening and reversal of trades was punctuated by a number of westerly wind bursts of increasing intensity along the equator (linked to several MJOs) Ocean currents forced by these WWBs, advected warm water eastwards. Also, WWBs forced downwelling K-waves (thermocline depth decreased by 90m in eastern Pacific by end of 97) – opposite anomaly in western Pacific.

Rossby waves excited by the initial weakening of the trades propagated westwards towards. Net result was a flattening of the thermocline and disapperance of “normal” east-west gradient across the equator, resulting in a weakening of the trades etc etc etc (Bjerknes Feedback).

Note anomalous SSTs in early 1998, SST anomalies superimposed on seasonal warming. Cooling did not take place until winds returned close to normal (easterly) in the middle of Very abrupt change associated with upwelling due to “normal” easterly wind strength. Change to La Nina.

Note build up of heat content in Western Pacific at least a year before onset of the 97/98 El Nino – due to stronger than normal trade winds (linked to weak La Nina). Although these conditions helped to set the stage for the development of El Nino – the onset did not occur until the intensification of the MJO in late Observed heat content anomalies (from 0 to 400m) averaged between 2N and 2S from the TAO array

The delayed oscillator theory and the 1997/98 El Nino Evolution of equatorial Pacific on interannual timescales is governed by interplay between large-scale equatorial wave processes and atmosphere- ocean feedbacks. Strictly the DO requires no new forcing and downwelling Kelvin waves should result from a reflection of a downwelling Rossby wave at the Western boundary. In reality, the only downwelling Kelvin waves seen in 1997/98 were associated with a succession of MJOs. DO theory predicts demise of El Nino to be associated with reflected upwelling Rossby waves at the western boundary – which was seen to occur after the onset of the El Nino. In addition there was direct wind-forcing of upwelling Kelvin waves associated with appearance of easterly wind anomalies in the West Pacific in late 1997 early 1998; so the demise was likely associated with upwelling Kelvin waves due to two different processes.