ENSO

                                                                           

An East to West wind forms over the Pacific, moving warm water at the Surface toward 

Asia. 

Trade winds weaken; warm water runs the other way. Upswelling stops!

Source for these illustrations: Russian-language Wikipedia

Trade winds return; upswelling is stroner than ever. Rainfall has shifted toward Indonesia!!

Measurements between Darwin Australia, and Tahiti keep track of relative pressure.

(Actually Tahiti minus Darwin). Both stations are just South of the equator, and - conveniently -

also on land!

Peru is South of the Equator. Shouldn't the situation run in reverse North of the equator?

Short answer: No — ENSO does not reverse North of the equator.  

And the reason why is one of the most elegant pieces of atmospheric physics on Earth.

Let me walk you through it clearly, Louise.

🌍 Why ENSO does NOT flip north of the equator

Even though Peru is south of the equator, ENSO is fundamentally an equatorial phenomenon, not a hemispheric one.

The equator behaves differently from the rest of the planet, and that changes everything.

Here’s the key:

⭐ At the equator, the Coriolis effect is essentially zero.

This means:

Winds don’t bend right (Northern Hemisphere)

Winds don’t bend left (Southern Hemisphere)

Instead, they blow straight east–west

And ENSO is entirely about east–west winds, not north–south winds.

So ENSO does not care whether Peru is north or south of the equator — it cares that Peru sits on the eastern side of the equatorial Pacific basin, where upwelling happens.

🌊 Why the pattern doesn’t reverse north of the equator

ENSO depends on three equatorial-only features:

1. Equatorial upwelling

Cold water rises along the equator because:

Winds blow straight west

The ocean surface is pulled away from the coast

Deep water replaces it

This upwelling happens on both sides of the equator, but the strongest region is near Peru/Ecuador, because the coastline is aligned with the prevailing winds.

If Peru were north of the equator, the physics would be the same.

2. Equatorial Kelvin waves

These waves:

Move only along the equator

Carry warm water eastward during El Niño

Cannot exist off the equator

This is why ENSO is centered on the equator, not on hemispheres.

3. The Pacific basin geometry

The Pacific is shaped so that:

Warm water piles up in the west (Indonesia)

Cold water upwells in the east (Peru/Ecuador)

This geometry does not flip north of the equator.

If Peru were magically moved to 5°N, ENSO would still behave the same.

🧭 So why does Peru matter even though it’s south of the equator?

Because Peru sits at the eastern boundary of the Pacific basin, where:

Trade winds push water away from the coast

Upwelling is strongest

ENSO’s temperature anomalies are most visible

Its latitude is irrelevant — its position in the basin is what matters.

What is the role of the thermocline off Peru in all this?

The thermocline off Peru is one of the most important physical players in ENSO. It is the “hinge” of the entire system — the part that moves the most, changes the fastest, and determines whether the Pacific is in El Niño, La Niña, or neutral conditions. Let me give you the clearest, step‑by‑step explanation of its role. 🌊 Concise takeaway The thermocline near Peru is normally very shallow, allowing cold, nutrient‑rich water to upwell. During El Niño, the thermocline deepens dramatically, shutting off upwelling and warming the eastern Pacific. During La Niña, it becomes even shallower, supercharging upwelling and cooling the region. The rise and fall of the thermocline at Peru is the oceanic signature of ENSO. 🧱 First: What is the thermocline? The thermocline is the boundary between: Warm surface water Cold deep water It is not a fixed depth — it moves up and down depending on winds and currents. Think of it as a flexible lid separating warm and cold layers. 🇵🇪 Why Peru’s thermocline is special The coast of Peru sits at the eastern edge of the Pacific basin, where: Trade winds push surface water away from the coast Cold deep water rises to replace it (upwelling) The thermocline is naturally very close to the surface This makes Peru the most sensitive location to changes in trade‑wind strength. 🔁 ENSO Mechanics: What the thermocline does ⭐ 1. Normal conditions Strong trade winds blow westward. Warm water piles up in Indonesia. The thermocline near Peru is shallow (close to the surface). Cold water easily upwells → cool SSTs near Peru. This is the “loaded spring” state. 🔥 2. El Niño: Thermocline deepens When trade winds weaken: Warm water sloshes eastward. The thermocline near Peru drops deeper — sometimes by 50–150 meters. Cold water can no longer reach the surface. Upwelling becomes warm instead of cold. SSTs rise dramatically. This is why El Niño begins in the eastern Pacific. The deep thermocline is the signature of El Niño. ❄️ 3. La Niña: Thermocline becomes even shallower When trade winds strengthen: Warm water is pushed west even harder. The thermocline near Peru rises closer to the surface. Upwelling intensifies. SSTs drop below normal. This is the mirror image of El Niño. 🧠 A non‑obvious insight ENSO is essentially the vertical movement of the thermocline across the Pacific. In the west (Indonesia), the thermocline is normally deep. In the east (Peru), it is normally shallow. ENSO events flatten or exaggerate this tilt: El Niño → thermocline becomes flat La Niña → thermocline becomes extra tilted This tilt controls where warm water sits and where cold water upwells. 🎯 Why Peru matters so much Peru is the best place on Earth to detect ENSO because: The thermocline is naturally shallow. Small changes in wind produce huge changes in upwelling. SST anomalies appear here first. The entire global climate responds to these anomalies. Peru is the “thermometer” of ENSO. 🌎 If you want, I can also explain: How Kelvin waves physically push the thermocline downward Why the thermocline tilt is the true ENSO indicator How Peru’s fisheries collapse during El Niño Why the Atlantic does not have a strong ENSO equivalent Just tell me where you want to go next.

What is sequence between neutral, El Nino and La Nina?

Short answer: ENSO does not follow a fixed, repeating sequence like Neutral → El Niño → La Niña → Neutral. Instead, the system cycles irregularly, but there is a typical pattern that emerges from the physics of the Pacific Ocean–atmosphere coupling. Below is the clearest, evidence‑based sequence, grounded in current scientific literature. 🌊 The Typical ENSO Sequence (Neutral → El Niño → La Niña → Neutral) ⭐ 1. ENSO‑Neutral (the baseline state) Trade winds blow steadily east → west. Warm water piles up in the western Pacific. Cold water upwells near South America. Sea‑surface temperatures in Niño 3.4 stay within ±0.5 °C of normal. Atmospheric pressure is lower near Darwin and higher near Tahiti (normal Walker Circulation). Neutral conditions can last years. They are the “reset” state of the system. ⭐ 2. El Niño (warm phase) Triggered when trade winds weaken: Warm water sloshes eastward. Upwelling near Peru collapses. Eastern/central Pacific warms by ≥ +0.5 °C for at least five overlapping 3‑month periods. Rainfall shifts eastward toward the central Pacific. El Niño typically lasts 9–18 months. ⭐ 3. La Niña (cold phase) Often—but not always—follows El Niño: Trade winds strengthen. Upwelling intensifies near Peru. Eastern/central Pacific cools by ≤ –0.5 °C for at least five overlapping 3‑month periods. Rainfall shifts westward toward Indonesia. La Niña can last 1–3 years, often longer than El Niño. ⭐ 4. Return to ENSO‑Neutral After La Niña, the system usually relaxes back to neutral: Trade winds return to normal strength. Thermocline tilt returns to normal. SST anomalies fade back toward zero. Neutral periods between events can last months or multiple years. 🔁 Is the sequence always Neutral → El Niño → La Niña? No. ENSO is irregular, not clock‑like. Observed patterns include: El Niño → Neutral → El Niño (rare but possible) La Niña → La Niña → La Niña (multi‑year La Niña, e.g., 2020–2023) Neutral → La Niña without an El Niño first El Niño → La Niña → Neutral → La Niña The system behaves like a sloshing basin driven by wind changes, not a strict cycle. 🧠 Why the sequence tends to go El Niño → La Niña This is the most important insight: El Niño spreads warm water eastward. When it ends, the warm water drains westward again. This “rebound” strengthens trade winds. Stronger trade winds favor La Niña. This is why La Niña often follows El Niño, but not always. 🎯 If you want, I can also explain: Why El Niño peaks in December Why La Niña lasts longer How Kelvin and Rossby waves drive the transitions How the thermocline tilt determines which phase comes next Just tell me what direction you want to explore

For the pressure reading, is it Darwin minus Tahiti, or vice versa?

The Southern Oscillation Index (SOI) uses the pressure difference between Tahiti and Darwin, and the convention is very clear in the scientific literature: SOI is based on (Tahiti MSLP – Darwin MSLP) So the subtraction is: ⭐ Pdiff = Tahiti pressure − Darwin pressure This is the value that gets standardized to produce the SOI. 🌡️ What the sign means Because the formula is Tahiti − Darwin, the interpretation is: Negative SOI (El Niño) Tahiti pressure drops Darwin pressure rises So Tahiti − Darwin becomes negative Trade winds weaken → warm water moves east → El Niño Positive SOI (La Niña) Tahiti pressure rises Darwin pressure drops So Tahiti − Darwin becomes positive Trade winds strengthen → cold upwelling intensifies → La Niña 🧠 Why this direction makes sense physically Tahiti represents the central/eastern Pacific pressure regime. Darwin represents the western Pacific warm pool pressure regime. When the Walker Circulation weakens (El Niño), Tahiti pressure falls relative to Darwin. When the Walker Circulation strengthens (La Niña), Tahiti pressure rises relative to Darwin. So the subtraction Tahiti − Darwin directly tracks the strength of the trade winds. 🎯 Summary It is Tahiti minus Darwin, not the other way around. Negative SOI → El Niño Positive SOI → La Niña If you want, I can also show you how this pressure difference physically relates to the thermocline tilt and the onset of El Niño/La Niña.