El Niño in Peru: understanding the phenomenon and its effects

What is called El Niño

A name from the sailors of northern Peru

The name comes from Peru’s north coast. In 1892, in the bulletin of the Geographical Society of Lima, Navy captain Camilo Carrillo reported that the sailors of Paita knew of a corriente del Niño, a warm current so called “no doubt because it becomes more visible and palpable after the Christmas feast”, the feast of the Christ Child. The year before, Luis Carranza had described in the same bulletin an exceptional warm current and the great floods of 1891 on the north coast (IGP, 2017).

In the 20th century, scientists linked this warming to vast swings in atmospheric pressure over the tropical Pacific, then understood that it was a single phenomenon, made up of exchanges between the ocean and the atmosphere. Today it is called ENSO: El Niño-Southern Oscillation. Internationally, the name El Niño ended up referring to the warming of the central Pacific, and no longer only to the sea off Peru. Hence a source of confusion that accounts for much of this page.

The Pacific in normal times

Along the equator, the trade winds, steady winds, blow from east to west, from South America towards Asia. They push the warm surface water westwards: according to NOAA, the sea surface is about half a metre higher in Indonesia than in Ecuador, and the water there is about 8°C warmer than in the eastern Pacific (NOAA PMEL).

Below the surface, the thermocline separates warm water from the cold deep layers. It is deep in the west and rises near South America. Along the coasts of Peru and Ecuador, followed northwards by the cold Humboldt Current, cold, nutrient-rich water rises from the depths: this is upwelling. It feeds plankton, then the whole food chain: according to IMARPE, the Peruvian Marine Research Institute, the Peruvian sea is the most important upwelling area in the world, and the Peruvian anchovy (anchoveta) is its key species.

In the atmosphere, warm, moist air rises over the western Pacific, where it forms clouds and rain, flows east at altitude, then sinks, dry and stable, over the eastern Pacific: this is the Walker circulation (NOAA Climate.gov). Together with the cold sea, this sinking air helps make the Peruvian coast a desert.

During El Niño

During an El Niño episode, the trade winds weaken over the central and western Pacific, and warm water spreads east. The thermocline flattens: it sinks near South America and rises in the west. Upwelling continues, but it draws on warmer, less nutrient-rich water: plankton production falls, with knock-on effects all the way to the fisheries. Rain follows the warm water eastwards, with floods in Peru and droughts in Indonesia and Australia (NOAA PMEL).

The ocean and the atmosphere feed off each other: weaker trade winds let the sea warm up, and a warmer sea weakens the trade winds further. That is why an episode, once under way, can last for months.

El Niño, La Niña and neutral years

ENSO has three states, according to the World Meteorological Organization (WMO). El Niño is its warm phase: unusually warm surface waters in the central and eastern tropical Pacific. La Niña is its cold phase: stronger trade winds than usual and colder waters in those same regions. In between, neutral conditions are simply those described above. La Niña sometimes follows El Niño, but not always (NOAA PMEL).

There is no regular calendar. According to NOAA, El Niño returns on average every two to seven years, and an episode usually lasts nine to twelve months; the WMO says an El Niño episode can last up to eighteen months, and a La Niña episode up to three years. The official chronology of ENFEN, the Peruvian commission in charge of studying El Niño, lists 25 coastal El Niños between 1950 and 2024, lasting from three to seventeen months (ENFEN, 2024). An episode can begin at almost any time of year, but its strongest effects on rainfall on the north coast come in the southern summer, from February to April, when the sea is warmest on average.

The Pacific El Niño and coastal El Niño

This is the most useful distinction for understanding what you read in the press or in bulletins.

Two regions of the ocean

To track the phenomenon, weather and ocean services measure sea temperature in defined regions of the ocean. Two matter for Peru:

• the Niño 3.4 region, in the central equatorial Pacific, between 5° north and 5° south, and between 170° and 120° west. This is the one followed by the US agency NOAA, the WMO and most countries, because its temperature is the most closely linked to variations in the atmosphere on a global scale;
• the Niño 1+2 region, off Ecuador and northern Peru, between the equator and 10° south, and between 90° and 80° west. This is the one that says most about how warm the sea is along the Peruvian coast.

Two linked phenomena, but not the same

The two regions often warm together, but not always. To avoid misunderstandings, ENFEN introduced two separate notions in 2012: coastal El Niño (El Niño costero), when the sea warms off the Peruvian coast, and central Pacific El Niño, when it is the middle of the ocean that warms; with their cold counterparts, coastal La Niña and central Pacific La Niña.

Recent episodes show how the combinations vary (IGP, 2017):

• in 1997-1998, both regions warmed strongly: it was both a coastal El Niño and a central Pacific El Niño;
• in 2015-2016, the central Pacific saw one of the warmest episodes ever measured, but much less rain fell on the north coast than in 1997-1998: according to the IGP, the remote effects of the central warming, which tend to reduce rainfall in Peru, outweighed the warming of the coast;
• in 1925 and in 2017, the sea warmed abruptly off Peru while the central Pacific was cold or neutral: two coastal El Niños, with very heavy rain on the coast.

Warming in the central Pacific also matters for Peru. According to the IGP, a central Pacific El Niño can cause droughts in the Andes: this is one of its “teleconnections”, the remote effects carried through the atmosphere.

The indices and the words in the bulletins

Each region has its index, a three-month average of the difference between sea temperature and its normal value.

The RONI, NOAA’s relative index, subtracts from the Niño 3.4 temperature anomaly the average anomaly of all tropical seas (NOAA CPC). NOAA explains the choice as follows: the temperature of the tropical oceans is changing, so a thirty-year reference average lags behind; and El Niño’s effects depend on the difference between the east-central Pacific and the rest of the tropics. The thresholds, however, stay the same. The ICEN, for its part, has been recalculated by ENFEN with rolling reference periods, updated every five years, because of changes in the global climate.

Strength labels do not carry over from one system to the other. For ENFEN, a coastal El Niño is weak, moderate, strong or extraordinary according to the peak reached by the ICEN (above +0.5, +1.3, +2.1 and +3.5°C); NOAA speaks of weak, moderate, strong or very strong El Niño according to its own thresholds. A “strong coastal El Niño” and a NOAA “strong El Niño” therefore do not describe the same thing.

Finally, ENFEN publishes in each of its official statements the status of its alert system: “not active”, “watch” when an episode becomes more than 50% likely in the coming months, or “alert” when it is imminent, under way or expected to continue (ENFEN, 2026). The commission brings together IMARPE, which chairs it, the SENAMHI weather service, the Navy (DHN), the Geophysical Institute of Peru (IGP), the National Water Authority (ANA), INDECI and CENEPRED, the disaster risk prevention agency.

How the effects vary across Peru

Rainy season, anomaly, local event

Three things are often mixed up. The usual rainy season comes back every year: in the Andes, from December to March, with its ordinary landslides (see when to go). A climate anomaly such as El Niño changes, for several months, the odds of the season being wetter or drier, warmer or cooler, depending on the region. A local event, finally, is a storm, a flood or a huaico, on a given day, in a given place. El Niño changes the odds; it does not tell you what the weather will be on a particular day in a particular place.

Region by region

According to ENFEN, El Niño can trigger heavy rain in arid areas such as the Peruvian coast or, on the contrary, cause a deficit in the highlands and the Amazon rainforest during the southern summer. The effects below are possible, not automatic: they depend on the strength of the episode, on which region of the ocean is warming and on the season.

Sources: ENFEN (statement 17-2026 and technical note 01-2024), IGP (2017), the Andina news agency’s summary for past episodes.

Temperatures. A warm episode heats the sea and, with it, the air along the whole coast: days and nights are warmer and more humid than usual.

Huaicos and catchments

A huaico is a flow of mud and stones that rushes down a ravine (a quebrada) after heavy rain, tearing up sediment as it goes. In Arequipa, at the foot of the volcanoes, the IGP prefers to speak of a lahar, because the flow carries volcanic deposits, which are denser; in February 2026, lahars coming down from Misti and Chachani hit about ten districts of the city (IGP).

What counts is the catchment: all the slopes whose waters drain towards the same river. Heavy rain falling at 3,000 metres can make a river burst its banks or set off a huaico tens of kilometres further down, under a dry sky. That is what happens in the valleys east of Lima, such as the Rímac valley at Chosica and Chaclacayo or the Huaycoloro quebrada, hit notably in 1998. The sky above the city centre says little about the danger in a valley or beside a riverbed, even a dry one.

Huaicos are not specific to El Niño: the rainy season brings them every year in the Andes. A marked episode can make them more frequent in regions where they are usually rare, such as the coast.

Why no two episodes are alike

The big episodes have left their mark on people’s memories, but they are not the rule: of the 25 coastal El Niños recorded by ENFEN between 1950 and 2024, eleven were weak, seven moderate, five strong and only two extraordinary, in 1982-1983 and 1997-1998.

NOAA RONI values consulted on 12 October 2026 (they may be revised); coastal periods and strengths: ENFEN chronology (technical note 01-2024); effects: IGP, IMARPE, PRODUCE and the Andina news agency’s summary.

The case of 2017 says it all. Rated only “moderate” by the coastal index, with no El Niño in the central Pacific, it was nevertheless, through its rain and floods, the third most intense episode in Peru in at least a hundred years according to SENAMHI, which compares it with the coastal El Niño of 1925. According to the authorities at the time, quoted by the Andina news agency, the floods killed 162 people and affected nearly one and a half million. Conversely, the 2015-2016 episode, very strong in the central Pacific, had much smaller effects on rainfall on the north coast than that of 1997-1998.

The strength of an ocean index and the scale of the consequences are two different things. NOAA says so itself: the strength of an episode does not necessarily match the strength of its effects; it makes certain effects more likely, without ever guaranteeing them (NOAA CPC). In Peru, these consequences also depend on the season in which the coastal sea warms most, and on how slopes and quebrada beds have been built on, which the IGP cites among the causes of the damage from the Arequipa lahars in 2026.

The effects on daily life

Fishing and the anchovy

The Peruvian anchovy lives in cold water, between 14 and 18°C. When the sea warms, its shoals pull back towards the coast and, in intense episodes, move south. During the extraordinary episodes of 1982-1983 and 1997-1998, the anchovy gathered very close to the shore and towards the centre and south of the country; in 2017, the coastal episode, short and limited to the north, left the anchovy more widely spread, and fishing was not greatly affected (IMARPE, 2019).

In June 2023, on IMARPE’s advice, the Ministry of Production (PRODUCE) did not open the first anchovy fishing season in the north and centre of the country: exploratory fishing was finding 86.3% juveniles. Fishing at that point, the minister said, would have depleted the resource (El Peruano).

Yet the effects vary between activities. When anchovy grows scarce, warm-water species come closer: bonito, yellowfin tuna, skipjack, mahi-mahi (“perico”). During the 2026 episode, ENFEN announced their arrival near the coast, and PRODUCE planned measures to let fishers take advantage of these “resources of opportunity”.

Ecosystems and farming

At sea, the fall in plankton ripples through the whole food chain (NOAA PMEL). On land, the effects are mixed: according to the Andina news agency’s summary, the rain turns the dry forests of the north coast green, brings up temporary pastures and recharges groundwater; the heat favours rice on the coast but can reduce other crops, such as potatoes, and floods destroy canals and water intakes.

Water, roads and services

Depending on the region, El Niño can bring too much water or too little, sometimes both in the same country at the same time: in October 2026, the Ministry of Health put its facilities on alert for both risks at once, heavy rain and water shortages. In the big episodes, roads and bridges paid a heavy price: in 1997-1998, damage to transport was the second-largest item of losses after the productive sectors, according to the Andina news agency’s summary.

Health

Heat and standing water favour the Aedes aegypti mosquito, which carries dengue, and past major episodes were followed by a rise in several infectious diseases. The Ministry of Health issues alerts when the situation calls for it. For a traveller, protection does not change: repellent and covering clothes, by day as well as in the evening, especially on the north coast and in the Amazon (see mosquitoes).

What it changes for a trip

There is no general rule: neither “cancel everything” nor “nothing changes”. What counts is your itinerary, the season and what the bulletins say when you set off.

Should you change your itinerary?

Generally no, but some stages call for more attention during a marked episode, especially in the southern summer:

• the north coast, from Tumbes to Trujillo, and its beaches, where rain, floods and cut roads are most likely;
• the roads climbing from the coast into the Andes and the valleys prone to huaicos, such as the Carretera Central east of Lima;
• long road journeys from one region to another, more exposed to closures than a domestic flight.

On the other hand, El Niño tends rather to reduce rainfall in the southern Andes; in Cusco and on the way to Machu Picchu, it is mainly the ordinary constraints of the rainy season that count.

What to check before a journey

• SENAMHI weather warnings for the regions you cross, from yellow to red according to the level of danger (SENAMHI);
• road conditions: SUTRAN’s road alert map (alertasviales.sutran.gob.pe) and Provías Nacional’s National Road Network maps;
• INDECI’s emergency reports for the region;
• the view of the transport company (bus company, train, airline), the accommodation or the agency, who know the state of the road the day before;
• your own country’s travel advice, for Switzerland the FDFA’s.

Lima, the north coast, the Andes: three situations

In Lima, daily life goes on. Expect more heat and humidity, and sometimes a shower. Stay away from riverbeds and the bottom of quebradas after heavy rain upstream, and check before taking the Carretera Central into the Andes.

On the north coast, a stay is still possible, but it calls for more flexibility during a marked episode: ask your accommodation how access and supplies work in heavy rain, and choose bookings you can change. The north coast is also the region where the sea warms most.

In the Andes, mountain roads and trails can be cut by landslides in the rainy season, with or without El Niño. A reputable agency adapts a trek or an excursion to the state of the ground.

Keeping a reasonable margin

• A spare day before an international flight or an important connection.
• Changeable bookings for the most exposed stages.
• A fallback activity for each outdoor day, as suggested on the when to go page.
• Insurance that covers trip interruption, to be checked before you leave.

El Niño and climate change

Two phenomena overlap without being the same. El Niño is part of the climate’s natural variability: it has been described on the Peruvian coast since at least the 19th century, long before the current warming. Background warming is the gradual rise in average temperatures caused by human activity.

What is observed. According to the IPCC, human influence has not changed the major modes of tropical variability, ENSO among them, beyond their natural variations (high confidence). The WMO likewise says there is no evidence that climate change increases the frequency or intensity of El Niño episodes, but that it can amplify their effects, because the atmosphere holds more energy and moisture. It also points out that 2024 was the warmest year on record, owing to the combined effect of the 2023-2024 episode and background warming.

What is projected. For the IPCC, it is virtually certain that ENSO will remain the main mode of year-to-year variability in a warmer world. Models do not agree on any change in the amplitude of El Niño-related sea temperature variations (medium confidence). On the other hand, it is very likely that ENSO-related rainfall variability will increase markedly in the second half of the century under medium and high emissions scenarios (IPCC, Technical Summary).

What remains uncertain. The projections mainly concern the central Pacific El Niño. Coastal El Niño, more local and faster, has been much less studied, which makes it harder to forecast, the IGP stressed in 2017. Background warming also poses a measurement problem: NOAA switched to a relative index because the average temperature of the tropical oceans is changing, and ENFEN revised the way the ICEN is calculated, citing changes in the global climate.

For Peru, the practical consequence is simple to state: El Niño episodes now occur on a warmer ocean and atmosphere than in the past, which can amplify some of their effects, such as heat, without any single episode being attributable to warming alone.

Forecasts, bulletins and current information

What a seasonal forecast can tell you, and what it cannot

A seasonal forecast gives probabilities for the coming months and for large regions: a wetter or warmer season than normal on the north coast, for example. It does not say whether it will rain in Piura on 15 February, or whether a road will be cut on a given day. ENFEN points out that its alert statuses rest on probabilistic forecasts, with a degree of uncertainty, and that they are updated as observations come in.

Three kinds of information not to confuse

An observed state, a probability of change and a local warning therefore have neither the same scale nor the same use.