The chatter amongst the El Niño experts has ticked up significantly this week, with excitement building about whether this fall will witness the arrival of The Big One.
Why all the fuss? And what are the chances of a major El Niño event next winter?
While considerable uncertainty surrounds the conception of an El Niño event, most scientists agree that it helps to have the following features in place:
1) An ocean that is primed for an event (enough time elapsed since the last major event). The last Big One was the 1997-1998 El Niño, and before that was the 1982-1983 El Niño, and before that the 1972-1973 El Niño. By this simple metric, we are due for a Big One. Given that the last moderate El Niño event was 2009-2010, we are at least due for One. Some El Niño scientists describe the pacemaker for the El Niño-Southern Oscillation events as a "recharge oscillator", whereby heat accumulates in the western Pacific ocean until it is discharged to the atmosphere by a cascade of positive feedbacks triggered by . . . (see #2 below).
2) A strong burst of westerly winds along the equator in the west Pacific. Usually, trade winds blow steadily from the east to the west ('easterly' winds) across the tropical Pacific, and they maintain the temperature structure of the equatorial Pacific: cool waters in the East, and warm waters in the West. Essentially, the trade winds keep the 'thermocline' (the boundary between warm surface waters and the cool deeper waters) close to the surface in the eastern Pacific. When a burst of westerly winds occurs in the western Pacific, it triggers a 'downwelling wave' that propagates eastward along the equatorial thermocline. When it reaches the eastern Pacific, it pushes the thermocline to deeper depths, leading to warmer sea-surface temperatures in this region roughly 2 months after the westerly wind burst. [For more information on ENSO physics, see this recent and exhaustive review by some ENSO heavy-hitters, in press in "Coral Reefs of the Eastern Pacific" (Springer).]
At a certain point (and this is where the mystery comes in), the warming in the central to eastern Pacific is strong enough to undermine the strength of the trade winds themselves. The thermocline further deepens in the east, and surface ocean temperatures warm even more. At that point, the system is moving inevitably towards an El Niño event that will reach its maturity sometime between November and February - a well-choreographed and now-familiar dance between the tropical Pacific atmosphere and ocean.
So what's got all the experts staring at the tropical Pacific data this week?
Well, this, as measured by an array of moored ocean buoys spanning the tropical Pacific, the Tropical Atmosphere Ocean array:
This time-longitude plot tracks the history of westerly wind events, and where they have occurred along the equator. You can see that we are in the midst of a very strong westerly wind burst that began roughly 2 weeks ago in the western equatorial Pacific.
To put the current wind event in context, here is a time-longitude plot of westerly winds over the last 20 years, again from the TAO array:
It will be several months before we know whether The Big One is on its way, but a moderate-sized El Niño event seems almost inevitable. Indeed, a full ensemble of statistical and dynamical forecasts of ENSO can be seen below, courtesy of the IRI, with most models projecting El Niño conditions by JJA (otherwise known as boreal summer).
If this El Niño is The Big One, then this is the chance of a lifetime for scientists to get out in the field and collect data that can illuminate the physical lifecycle of such an extraordinary event and its global impacts on weather patterns, ecosystems, and human systems. This may be as much advance warning as we will ever have - 6 precious months.
Unfortunately, funding agencies are ill-equipped to meet the needs of such "rapid response" science. The federal research vessels are booked out years in advance. Even if someone could convince NSF to mount a multi-disciplinary armada to the tropical Pacific, there is always the risk that it might not develop into The Big One. Nonetheless, whisper campaigns are mushrooming up as we speak, aiming to make an urgent and compelling case that the benefits far outweigh the risks of pursuing such science. Where traditional funding avenues fail, foundation support and/or access to ships of opportunity (yachts or for-hire research vessels) may allow some of the most pressing and low-hanging science to go forward.
Such a coordinated observational campaign is all the more pressing because the TAO array - the only source of direct observations of the tropical Pacific atmospheric winds and subsurface ocean temperature so critical to El Niño's evolution - is losing buoys at an alarming rate because the ship that serviced them has been sidelined by NOAA's funding gaps. Data return has dropped to 36% (M. McPhaden, pers. comm.) - the plots you see above are heavily infilled using the few buoys that remain. Just today, the TAO homepage posted a disclaimer warning of poor data quality. This depressing moment in the history of US ocean science is discussed in a recent Nature News piece.
So, will the tropical Pacific ocean and atmosphere conceive The Big One this spring? I don't know, but it certainly has gone way past flirtation.