El Niño & La Niña: How Ocean Cycles Shape the Weather

Weather is a complicated, interconnected system that goes way beyond the local prediction. The distinction between the transient weather conditions and the more stable context of climate is inherent: whereas weather can change within hours, climate changes within decades. One of the most impressive examples of this difference is the El Nino-Southern Oscillation, or ENSO a natural and periodic process within the Pacific Ocean, that alternates between warm (El Nino) and cold (La Nina) every few years. These changes in surface temperature of the seas do not remain loyal to the tropics, but they restructure global atmospheric circulation. As the Pacific warms or cools considerably, the effects are felt like the rock tossed in a stream. Jet streams scrimper, monsoons change their tunes and rain, and drought reallocate themselves between continents. It is this international teleconnection that has made ENSO one of the most impactful climate phenomena on the planet that silently drives all processes like rainfall in South America, heatwaves across the Asian region, and storm seasons in the Atlantic. 

What ENSO Is (The Basics) 

ENSO refers to the coupled ocean-atmosphere loop in the tropical Pacific which switches between a warm (El Nino) and a cool (La Nina) periodicity period of approximately 2-7 years.  

Normal (Neutral) Pacific: Normal (neutral) year This occurs during a normal year when the strong easterly trade winds blow in a westward direction along the equator accumulating warm surface water in the western Pacific around Indonesia. This generates a warm pool of water in the west and upwells water that is cold and upwells South America. According to the description at NOAA, these trades keep the waters warmer on the west and chilled and upwelled water on the east. The resulting Walker circulation brings rising air and heavy rain above the hot west pacific and sinking air on the east. 

El Nino: During El Nino years, there is an abnormal high pressure in the western Pacific and trade winds become weak or even anti clockwise. The warm water which is traditionally contained in the west moves in an eastern direction. The thermocline south of America becomes much deeper and the upwelling is weakened thus the eastern pacific becomes warmer. The rain belt is pushed eastwards: usually wet areas of the western pacific become dry but the central/eastern pacific receives a lot of rain.  

In summary,  

El Nino = weak trades + warm eastern Pacific + eastward shifting rain. 

La Nina: La Nina is basically the counterpart of that which is cold. Even more powerful trade winds push warm water further into the west and increase upwelling off South America. This increases the slope of the thermocline and coldness in the eastern pacific. The precipitation is very wet with the rain moving far west, hence leaving Indonesia/Australia wet and central/eastern Pacific dry.  

In short,  

La Nina = high trades and cool eastern pacific and rain pushed westward. 

ENSO was initially discovered in the 1960s when Jacob Bjerknes discovered the close ocean-atmosphere interaction that causes such swings. These cycles of warm/cool occur not regularly. 

How ENSO Moves the Atmosphere 

ENSO alters atmospheric circulation by modifying heat transport, winds and storm-tracks. The heat patterns in the ocean boosts the tropical convection, as the deep cumulonimbus storms take the surface heat to the upper atmospheres; during El Nino the active convection in the warm eastern pacific creates the large scale Rossby waves that travel north-south and change distant weather patterns, spreading the effects of El Nino to other parts of the world. ENSO also breaks the equatorial Walker circulation: El Nino weakens or inverts easterly trade winds and flattens the east west overturning cell owing to the increased pressure in the western Pacific, whereas La Nina reinforces both the trades and the Walker circulation, which shows the close interaction between ocean warming and atmospheric winds that NOAA pointed to. These shifts extend to the midlatitudes in the form of jet stream shifts: El Nino usually intensifies and bends the Pacific jet eastwards pushing storms west and south of the United States of America to give rise to wetter-than-average winters there, whereas the La Nina usually gives rise to a split jet or a displaced jet that leaves the U.S. South drier. Together, the wind variations of ENSO cause changes in the path of world storms, and El Nino is more likely to cause colder and wetted winters in the southern U.S and in portions of the Americas, where La Nina tends to cause the opposite effect. 

What ENSO Changes in the Sky 

El Nino-Southern Oceans (ENSO) significantly changes the weather patterns of the world as it rearranges rainfall, temperature and storms. Changes in oil temperature in the Pacific are changing the precipitation patterns on large scales, causing floods in parts of the world and droughts in others: El Nino causes the hot water to move eastwards, preventing rainfall in Indonesia, Australia and large parts of Southeast Asia and causing heavy rains in the central and eastern Pacific, including Peru, Ecuador and much of the southwestern United States; La Nina causes the opposite of this, with monsoons intensifying and flood threat increasing in Southeast Asia and Australia, and drying the eastern Pacific and southwestern United States, as seen in the climat ENSO also causes extreme temperatures--El Nino years are usually marked by global warming as ocean heat is emitted into the atmosphere, which has caused some of the hottest years on record, but La Nina tends to cool the climate, although the effect is lessening as climate warming rises steadily in general, especially in 2025, with record warmth recorded even with a La Nina taking place. Such redistributions of heat increase local extremes, and El Nino is associated with extreme heat waves and La Nina with cold waves in certain regions of the midlatitudes. ENSO also affects tropical and midlatitude storms due to changes in vertical wind shear and jet streams: El Nino increases shear in the Atlantic and Indian Oceans, where cyclone formation becomes more difficult, whereas the central and eastern pacific typhoons are often more active and La Nina more often redistributes the tracks of hurricanes and winter storms across regions. 

Regional Weather Shifts 

South Asia (Bangladesh/India): El Nino tends to ruin the summer monsoon of South Asia. NASA reports that during El Nino, the Indian monsoon is weaker than usual, and this implies that the countries such as Bangladesh experience below-average monsoon rains. Instead, La Nina enhances the monsoon. 

Southeast Asia and Australia: El Nino is notorious in causing drought in Indonesia, Malaysia and Australia - warm pacific waters remaining east cause these western basins to be dry. La Nina does the reverse: the areas experience heavy rains and floods. Again, the El Nino that caused the drought in 1997-98 had a sequence of brown (dry) anomalies running down Indonesia/Australia. 

Africa: East Africa will see increased rainfall during El Nino, but southern Africa will be wetter during La Nina years. According to NASA, La Nina causes unusual heavy rain in southeastern Africa meaning that El Nino causes drier spells in southeastern Africa. All in all, ENSO may push the Horn of Africa towards floods (El Nino) or deep drought (La Nina), which has happened in the recent years. 

Australia: Australia is a sensitive ENSO country. El Nino is associated with warm, dry weather - widely known to cause lethal fire seasons and droughts. La Nina usually causes excessive rainfall on an above-average level. Global maps made by NASA indicate that Australia experiences intense droughts during intense El Ninos. 

Americas: El Nino winters typically cause an outburst of a stormy Pacific jet in the southern U.S. and Mexico (wet) but leave the Pacific Northwest and Canada drier. La Nina moves the jet northwards and tends to produce drier southern U.S. and wetter Pacific Northwest. El Nino usually leads to floods in Peru/Ecuador (and some of the U.S. Southeast) but dries up in the Amazon, whereas La Nina does the reverse (more in the Amazon, drier in Peru). 

Monsoons, Floods & Droughts 

ENSO has massive impacts on seasonal rains extreme. Monsoons in El Nino years of South Asia are usually late or weak. As an illustration, on the developing El Niino conditions, the July 2009 monsoon was “off to a bad start over India and Bangladesh. Satellite imagery indicated that India/Bangladesh got almost forty percent less than usual rainfall during the week. Overall, ENSO can be translated as floods or drought, depending on the area: according to NASA, El Nino may cause flood-causing rain or drought that kills crops in Asia. Indeed, the 2009 El Niño caused a monsoon drought in South Asia. In comparison, the years of La Nina are characterized by very wet monsoons and floods. In this way, the phases of ENSO bars the flood risk as well as the drought risk, El Nino and La Nina increase drought risk and flood risk, respectively, in monsoon Asia and Australia. 

Heat, Humidity & Human Stress 

Extremes caused by ENSO may be dangerous under the conditions of the combination of heat and humidity. Hazards of heatwave are specifically associated with ENSO phenomena. Powerful El Ninos is accompanied by major heatwaves: recent record global highs were experienced during El Niño years, and even in the year of La Nina WMO recorded previously unseen warmness in early 2025. High humidity tends to add to the effects of ENSO-enhanced heat in areas such as South Asia. Late monsoons or starving rains may spell dry dusty air that overheats in the day, but traps heat in the night. This is intensified in urban centres by the effect of heat-island and the combined heat/humidity levels are more deadly. In a word, ENSO increases heat stress: NOAA even issues warnings of ENSO-associated heatwaves, and WMO has reported months where La Niña still produced record heat. 

Cyclones & Severe Storms 

The tropical cyclone seasons are highly influenced by ENSO phase. Vertical wind shear is the most important element. The warming of Pacific with El Nino is more likely to enhance shear in the tropical Atlantic and Indian basin that suppresses hurricanes and Bay-of-Bengal cyclones. On the other hand, La Nina lowers shear, and this enables more storm to develop. Indicatively, recent reports indicate that La Nina years (and negative IOD) are positive in high post-monsoon cyclones in Bay of Bengal and El Nino years' experience a substantial decline in the Bay cyclogenesis. El Nino has stronger upper winds that tend to compress the formation of hurricanes in the Atlantic, whereas the calmer pattern of the Atlantic seasons is nourished by La Nina. The opposite is true of Pacific basins: El Niño usually implies additional Pacific typhoons. ENSO also affects storm tracks: in addition, El Nino can cause midlatitude storms to be diverted into some areas (e.g. southern U.S.) and La Nina to divert them in others. To conclude, certain years are stormier or less stormy based on ENSO: La Nina normally results in more frequent and severe cyclones in the Bay of Bengal and Atlantic, whereas El Nino often leads to the reverse. 

The Supporting Cast: Ocean–Atmosphere Side Players  

ENSO is certainly the lead actor in our world weather play but it does not act alone. Consider the Pacific as a big stage, but there are other side players in the Indian and the Atlantic oceans who can either exaggerate the performance of ENSO or entirely ruin its script.  

The Indian Ocean Dipole (IOD): The Pacific’s Influential Neighbor  

The Indian Ocean Dipole is to the Pacific king, ENSO, what power player neighbor is. It is an equivalent of a teeter-totter of sea surface temperatures, except that it is between Africa and Indonesia.  

• The Synergizer: To the extent that a “Positive IOD” (warm water near Africa) combines with El Nino, it is bad news to Australia and Southeast Asia and just doubles down on droughts and bushfires.  

• The Spoiler: On the other hand, a Negative IOD may actually quash an El Nino buzz with rainfall to dry locales that should have gotten dry. In some cases such as in Bangladesh, the IOD will determine whether the monsoon would bring a life-giver or a flood-maker.  

The Madden-Julian Oscillation (MJO): The Global Pulse  

The MJO is a travelling thunderstorm party in case ENSO is a season mood. It is a huge wave of rain and wind, which encompasses the earth in 30-60 days. It is so fast that it can temporarily overpower El Nino by accelerating the westerly bursts of wind required to move warm water southwards towards America. It is the immediate fire that tends to ignite the ENSO fire of long-term duration.  

Decadal Cycles: The Long Game  

Lastly, there is the Pacific Decadal Oscillation (PDO). This is ENSO at slow motion loop, moving after every 20-30 years. During the warm phase of the PDO it behaves as a tailwind to El Nino, increasing the frequency and the severity of its occurrence. This is the time we are in wherein such background cycles are becoming less foreseeable and where the side players are more crucial in our predictions than ever before.  

The Ripple Effect: Making Weather a Systems Stress Test.  

When ENSO changes the thermostat on the globe, the effects do not end at the rain gauges or heat readings. They rippling through farms, cities, hospitals, and homes, they bled, and even now, when the skies are clear, they remain. Consider ENSO as more than a pattern of climate, and the final test to our contemporary society.  

The Dominos of Food and Finance.  

Food security is always the first domino to fall. ENSO basically reinvents the regulations of farmers: it falls too late, too intensively, or even evaporates. El Nino can lead to a plummeting of drought-ridden rice and wheat production in South and Southeast Asia and the sudden, violent downpours of La Nina can lead to the flooding of fields as the crops are on the verge of harvest.  

The disturbance reaches under the water, as well. El Nino disables the nutrient elevator (upwelling) that supports the most productive fisheries in the world in the ocean. Caused by fish population collapse or migration, means of survival of coastal populations are destroyed in one night. A land and sea food supply crunch causes the world to run out of food and food prices peak, making an impending Pacific weather change a local cost-of-living crisis that only strikes low-income families the most and the hardest.  

A Crisis for Body and Breath  

The health effects are consequent, which usually comes noiselessly but inexorably. Warm conditions generate high temperatures that result in increased incidences of heat exhaustion and cardiovascular stress in workers who are outside or in highly populated urban heat islands. At the same time, altered rainfall patterns provide a Goldilocks zone of disease; floods initiate water-borne epidemics such as cholera, whereas water shortage due to drought undermines hygiene and makes malnutrition the killer.  

Even the air that we breathe is not spared. When droughts caused by ENSO reduce forests to tinderboxes, mega-fires in areas such as the Amazon or Indonesia cover continents with toxic haze. These occasions are not only known to cause asthma, but they also confine urban pollution at ground level making ordinary smog a breathing crisis.  

The Big Picture  

When collectively viewed, the effects of an ENSO are a sobering sight: ENSO seeks the weak points in our infrastructure and rips them open. It evaluates the ability of our health systems to manage a surge, whether our supply chains can manage a shock or not and whether our cities can shield the weak. With global warming, these tests are increasingly common, meaning that, in a globalized world, there can be no distant weather anymore.   

Same pattern, Higher Stakes ENSO in a Warming World.  

ENSO represented is not a new phenomenon. Pacific has been oscillating between the El Nino and La Nina for thousands of years, which is natural rhythm which the world is accustomed to. New, however, is the background on which this cycle now functions. The rise in climate change has quietly increased baseline of the whole planet, making the ENSO not a well-known seasonal change but a high-stake personal experience of global risk.  

Consider it as follows: the pacific engine is the same given that we have replaced the fuel with something much more combustible.  

A Larger Fuel Tank  

The oceans form the start of the story. With the rise in global temperatures, oceans in the world are becoming a giant sponge by absorbing over 90 percent of the additional heat held by greenhouse gases. When the El Nino occurs nowadays it is not merely warm, but it is overlaid on top of already record-breaking waters.  

This forms a huge source of energy. Basic physics has it that as the air warms more water is contained in it, about 7 per cent more water exists in the air per 1degC of warming and this means that more water is available to fall out of the sky. This is the reason recent ENSO activities have been associated with supercharged rainfall as well as abnormally intense storms. The trend has not been reduced, but the fuel tank is significantly bigger, and the ensuing floods are more disastrous, as well as the storms themselves are increasingly more regular.  

The "Higher Baseline" for Heat  

There is no greater indication of the warming signal than in our temperature charts. The heatwaves do not begin with a normal starting point, but rather with a point where there is already the existence of fever. Some regions such as South Asia, Australia and some parts of the Americas already anticipate hotter weather in the years of El Nino. When you add long-term global warming to the top of that, those occurrences spearhead temperatures into dangerous, record-shattering realms at higher rates and longer durations.  

It is not merely a matter of the increased heat on a thermometer, but it is a mass health emergency. An increase in baselines implies that nights remain hotter, and human bodies cannot recuperate after a scorching day. In thick, wet urban areas, such as Dhaka to Bangkok, this compounded heat becomes stress to the extreme of human survivability, and it puts a strain on the power grids and health systems that never envision this new reality.   

The "Swing" and the Uncertainty  

Scientists are also closely monitoring the "variability" of the cycle. There is growing evidence that we are entering an era of more frequent "extreme" El Niño and La Niña events. We are also seeing signs of faster transitions between the two, a phenomenon known as "weather whiplash", where a region might swing from a bone-dry, fire-prone El Niño straight into a record-flooding La Niña without a moment to recover.  

While there is still scientific debate about exactly how ENSO’s frequency will change, one thing is clear: uncertainty does not mean safety. It means our planning must be more flexible and our infrastructure more resilient.  

The Multiplier of Risk  

The key takeaway is that ENSO is no longer acting on a stable planet. The familiar teeter-totter remains, but its impacts are being amplified by warmer oceans, hotter air, and more vulnerable societies. In this warming world, ENSO does more than just shape the weather, it acts as a risk multiplier, finding the weak points in our global systems and pushing them to the breaking point  

Can We Predict It? Tracking a Moving Target  

Among the chaotic mess of global climate patterns, ENSO is something of a success story. While we struggle to predict if it will rain next Tuesday, we can often see an El Niño or La Niña coming months in advance. This makes it one of the most predictable large-scale climate phenomena on Earth, but in the world of meteorology, “predictable” doesn’t mean “precise,” and that distinction is where the real challenge lies.  

The High-Tech Toolbox  

Modern ENSO forecasts typically provide a lead time of about 6 to 9 months. This foresight isn't a guess; it’s built on a massive, high-tech network of "eyes" and "ears" across the equator. A fleet of TAO/TRITON buoys across the Pacific, measuring water temperature from the surface down to 500 meters. Often, a plume of warm water (a Kelvin Wave) moves toward South America deep underwater long before the surface warms. When satellites and ocean sensors see these "dominoes" lining up, they know the system has momentum.  

Probabilities, Not Promises  

Despite the tech, ENSO forecasts are probabilistic, not deterministic. You will rarely hear a scientist say, “An El Niño will happen.” Instead, they report a “70% chance of El Niño conditions by winter.” This reflects the inherent "noise" of the system, a sudden, random burst of wind near Indonesia can either supercharge a developing El Niño or snuff it out entirely.  

There is also a seasonal hurdle known as the “Spring Predictability Barrier.” During the Northern Hemisphere spring (March–May), the signals are often too quiet to read reliably. A forecast made in February is frequently no better than a coin toss; however, once we cross into June, the accuracy of our models skyrockets as the ocean and atmosphere "lock in" their dance for the year.  

The Local Puzzle  

Even more challenging is predicting the "flavor" of the event. Knowing the phase is only the first step. Whether a specific region, like the coastal belt of Bangladesh or the Australian outback, gets hit with a flood or a drought depends on how ENSO interacts with local geography and other players like the Indian Ocean Dipole. A strong El Niño is a huge red flag, but it isn't a guarantee of a specific local outcome.  

This uncertainty can be frustrating for policymakers, but imperfect forecasts are still incredibly powerful. They shift the global mindset from reaction (sending aid after a disaster) to anticipation (preparing for the risk). ENSO prediction may not provide a perfect script, but it gives us a crucial window of time to prepare for the performance.  

Turning Odds into Action: Early Warning & Preparedness  

A forecast only matters if someone can use it. ENSO predictions become truly powerful when they move beyond technical scientific reports and translate into early warning and early action. This is where preparedness saves lives, not by reacting to the headlines of a disaster, but by preventing them from being written in the first place.  

Agriculture: Reducing the Element of Surprise  

In the world of farming, climate-informed planning can be the difference between a lost harvest and a resilient one. When farmers know months ahead that an El Niño may weaken monsoon rains, they can shift planting dates, choose drought-tolerant seeds, or adjust irrigation schedules. Conversely, during La Niña years, advance warning allows communities to prepare for excess rainfall, protecting seeds, improving drainage, and reinforcing storage facilities. The goal isn’t perfect prediction; it’s the strategic reduction of surprise.  

Health: Priming the System  

Health systems also gain a massive advantage from early warnings. Heat alerts triggered by ENSO-informed forecasts allow hospitals and clinics to prepare for spikes in heat-related illnesses before the wards are overwhelmed. Public health messaging can be deployed early to encourage hydration, limit outdoor labor during peak heat, and protect vulnerable populations like the elderly. In flood-prone regions, these windows of time support disease surveillance and the pre-positioning of clean water supplies, stopping outbreaks of water-borne killers like cholera before they begin.  

The Urban Shield  

For the world’s rapidly growing cities, readiness depends entirely on timing. Clearing drainage systems of debris, lowering reservoir levels to create capacity for heavy rain, and protecting power infrastructure all require significant lead time. ENSO forecasts provide that window. In a dense urban environment, a single extreme rainfall event can overwhelm systems; having six months to prepare can save billions in infrastructure damage and countless lives.  

Financing the Future  

Increasingly, these forecasts are being linked to innovative disaster risk financing. Rather than waiting for a crisis to appear on the news to raise funds, some governments and humanitarian agencies now release money automatically when certain climate thresholds are crossed. This shift from reaction to anticipation is transformative. It allows for "forecast-based action", paying to evacuate livestock or reinforce homes before the storm arrives.  

The lesson is simple: early warning only works when it leads to early action. Preparedness turns abstract climate knowledge into tangible protection. In an era of extremes, it is our most effective tool for ensuring that a shift in Pacific winds doesn't become a tragedy on land.  

Weather, Rights & Governance: The Equity of the Air  

ENSO doesn’t hit everyone the same way. While the climate pattern itself is a global phenomenon, its impacts expose a deeply unequal reality: who gets hurt, who gets warned, and who gets protected. In this sense, ENSO is far more than a meteorological event, it is a story of governance, power, and human rights.  

When floods, heatwaves, or droughts strike, the most vulnerable populations suffer first and worst. Low-income communities often occupy high-risk geography, living in flood-prone zones or working outdoors in life-threatening heat. Whether it is a subsistence farmer in South Asia or a family in an informal urban settlement, these groups have fewer buffers, less savings, weaker infrastructure, and limited access to healthcare. ENSO doesn't necessarily create these inequalities, but it acts as a magnifying glass, finding the cracks in society and prying them wide open.  

Access to early warning is perhaps the sharpest dividing line. In many nations, a seasonal forecast translates into a sophisticated web of text alerts, public advisories, and coordinated local response plans. In others, that same life-saving information remains trapped in a technical report, never reaching the village elders, local officials, or small-scale fishers who need it most. A forecast that isn't accessible, understandable, and trusted isn't a tool, it's just data. Closing the "last mile" gap is the difference between a community that thrives and one that is devastated.  

This is where governance moves from technical to political. Anticipatory action, the decision to act before a disaster strikes, is a choice. Governments must decide whether to invest in resilience today or pay for the catastrophe tomorrow. While waiting for a disaster to occur and then providing aid is often more "visible" for political optics, it is far more costly in terms of lives and livelihoods.  

As climate risks intensify, a powerful new idea is gaining ground: climate information as a human right. If we can see a disaster coming months in advance, failing to prepare is no longer an unavoidable tragedy, it is a policy failure.  

In a warming world, good governance isn’t just about being a good first responder; it’s about being a better planner. It means treating early warning not as a luxury for the wealthy, but as a fundamental protection that every citizen deserves. As the Pacific continues to pulse, our ability to govern that pulse will determine the survival of millions.