Forecasting the El Niño type well before the spring predictability barrier

<p>The El Ni&#241;o Southern Oscillation (ENSO) is the most important driver of interannual global climate variability and can trigger extreme weather events and disasters in various parts of the globe. Depending on the region of maximal warming, El Ni&#241;o events can be partitioned into 2 types, Eastern Pacific (EP) and Central Pacific (CP) events. The type of an El Ni&#241;o has a major influence on its impact and can even lead to either dry or wet conditions in the same areas on the globe. Here we show that the zonal difference &#916;T_WP-CPbetween the sea surface temperature anomalies (SSTA) in the equatorial western Pacific and central Pacific is predictive of the <em>type </em>of an upcoming El Ni&#241;o. When at the end of a calendar year, &#916;T_WP-CPis positive, an El Ni&#241;o event developing in the following year will probably be an EP event, otherwise a CP event. Between 1950 and present, the index correctly indicates the type of 18 out of 21 El Ni&#241;o events (p = 9.1&#8901;10^-4).<br />For early actionable forecasts, the index has to be combined with a forecast for the actual onset of an El Ni&#241;o event. The previously introduced climate network-based forecasting approach provides such forecasts for the <em>onset</em> of El Ni&#241;o events also by the end of the calendar year before onset. Thus a combined approach can provide reliable forecasts for both the onset and the type of an event: at a lead time of about one year, 2/3 of the EP El Ni&#241;o forecasts and all CP El Ni&#241;o forecasts in the regarded period are correct. The combined model has considerably more predictive power than the current operational type forecasts with a mean lead time of about 1 month and should allow early mitigation measures.</p>