Science1 publisherNot yet confirmed elsewhere2 min readPublished
Record-breaking El Nino is forecast to keep strengthening until its December peak
Forecasters expect this year's record-breaking El Nino to strengthen further before it peaks around December. The physics shows which way rainfall moves in Indonesia and coastal Peru, but it does not say how severe the drought or flooding will be.
The Scientist · Science desk
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What happened
- During El Nino the east-to-west trade winds weaken, leaving more warm water in the central and eastern Pacific while the western Pacific can turn relatively cooler.
- Sinking air over the western Pacific can leave Indonesia and nearby regions much drier than usual and raise the risk of forest and peatland fires in Indonesia.
- The zone of strongest evaporation and rainfall moves east with the warm water, bringing much wetter conditions to parts of the eastern Pacific, including coastal Peru.
- As the temperature gap between the western and eastern Pacific shrinks, the trade winds can weaken further and the whole Walker circulation weakens and shifts east.
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Why it matters
- decision Because a weakening of the trade winds feeds on itself, planners have reason to treat a strengthening event through to a December peak as their base case.
- constraint Water and crop planners still need regional forecasts to gauge how large a local drought or flood will be before committing resources, since the ocean record measures the event at its source.
- exposure Indonesian forest and peatland managers are the most directly exposed group in this account, because the drying it describes comes with a stated fire risk.
- exposure Teleconnections carry the shifted atmospheric heating thousands of kilometres away, so regions far from the Pacific cannot plan as if this were a Pacific-only event.
The forecast of further strengthening [5] rests on how the Pacific normally holds itself in place. Warm water in the west makes air rise. Cooler water in the east makes it sink, and that contrast keeps the trade winds blowing from east to west [9]. The winds keep warm surface water piled up around Indonesia and draw colder water up from depth in the east [6]. Scientists call this mutual reinforcement of ocean and atmosphere the Bjerknes feedback [9].
El Nino is the same loop running backwards. Weaker winds push less warm water west [10]. The temperature gap narrows, and a narrower gap weakens the winds again [11]. Given that physics, a forecast that this year's event will keep building toward its December peak is no surprise [5].
The pattern being disrupted is large. In a single month, parts of Indonesia can get more rain than Lima gets across an entire year [8]. Moist air rising over the warm west condenses into cloud and rain, releasing heat into the atmosphere [7]. High up, that air spreads out, sinks over the cooler east and flows back west at the surface as the trade winds, a loop called the Walker circulation [7].
The phys.org explainer does not identify which record this year's El Nino broke, or by how much [4]. Its sentences about impacts are hedged. Indonesia and the surrounding region "can become much drier than usual" [2], and El Nino "can bring much wetter conditions" to parts of the eastern Pacific, including coastal Peru [1].
Outside the Pacific, the explainer follows El Nino's reach through teleconnections. As the main zone of rising moist air moves east, the heat released by condensation moves with it. That shifted heating then changes weather thousands of kilometres away [3].
I think the planning case is right about direction. Forecasters expect a stronger event into December [5]. The feedback loop gives a physical reason to expect the trade winds to keep weakening while it builds [11]. A record set in the Pacific [4] measures the size of the disturbance at its source. How large a drought or flood it produces in any one river basin or farming district will have to come from regional forecasts.
What to watch
- Whether the event strengthens as forecasters expect and reaches its peak around December on schedule.
- Publication of the measure behind the claim that this El Nino has already surpassed previous records, and the margin by which it did.
- Regional seasonal outlooks that convert the eastward shift in atmospheric heating into rainfall forecasts for places reached by teleconnections.