The Great Decoupling 1: How Ocean Stratification is Boosting Global Warming

As ocean layers decouple, a record-breaking 2025 heat pulse prepares to collide with a 2026 El Niño, supercharging global weather and accelerating non-linear warming.

The End of the Linear Slope

For decades, climate change was discussed as a steady, predictable climb in global average temperatures. Most climate scientists, including the IPCC, reported on linear rates of warming since the 1980s. Recent observations however show temperatures no longer following a steady linear path, but accelerating.¹ Satellite observations of the Earth’s Energy Imbalance show continued increases in the amount of energy being absorbed by the planet.² Since this energy flow rate controls the warming, its increase drives the acceleration which is now being seen in every measure of the system including air and sea surface temperatures, ocean heat content at every depth and the frequency of weather extremes.

There is however another worrying trend emerging from the data, that of non-linear step changes or jumps in global temperature. The first of these was seen in 2015-2016, the next is unfolding today following the 2023-2024 jump and another one, is potentially approaching in 2027.

The jumps we are seeing are linked to El Niño events in the Pacific Ocean. The El Niño Southern Oscillation (ENSO) is the strongest natural phenomena affecting global climate. The warmer El Niño events drive short term rises in global mean temperature, while the cooler La Niña events drive cooler periods, or at least they used to…

In 2016, following the warming spike from a very strong El Niño event, temperatures didn’t return to the baseline trend during the subsequent La Niña.³ This was the first signs of a system that is no longer buffering heat, but amplifying it with each El Niño spike, forcing the steam engine of Earth into a higher gear.⁴⁵⁶ The same thing happened following the 2023 El Niño. As expected, 2024 was the hottest year on record, but 2025 did not drop to the prior trend line, despite a relatively weak La Niña phase that has run to this day. The three year temperature average for 2023-2025 was over 1.5ºC⁷ and 2025 itself was more than 0.25ºC warmer than the 5 years leading up to the 2023 El Niño.

NOAA is now predicting a 60% chance of a new El Niño event in late 2026.⁸ If one does develop, we are not just looking at a warm 24 months; we are looking at the release of another “loaded” planetary battery as the heat accumulating in the Pacific is released to the atmosphere, driving another jump in global temperatures that is unlikely to be recovered from. A new record of 1.7ºC could be reached in 2027⁹ and once again, maybe here to stay.

To understand why this could be happening we need to look at the structural changes underway in the world’s oceans.

The “Invisible Ceiling” and the Stratification Trap

Despite humanity’s interest in surface air temperature, the oceans are the real story of global warming. About 90% of the trapped energy and therefore the warming, is in the oceans of the world. Water has a very high specific heat capacity, meaning it takes approximately 3,400 times more energy to heat the same volume of water by 1ºC than air. The oceans therefore act as a huge heat sink or buffer, regulating the climate system. They absorb additional energy in their interior and release it slowly over time as the energy imbalance fluctuates. As the chart below shows however, the highest proportion of the accumulating heat is being absorbed by the upper layers of the ocean (the paler blues).¹⁰ Note also the thin purple line at the bottom, this is the atmospheric take up of heat (just 2%) where we observe close to 1.5ºC of warming. That just shows how much energy we are dealing with here.

Total Earth system energy gains. Note the massive heat uptake in the oceans, concentrating in the surface layers, but also the thin purple line which shows the atmospheric energy take-up, just 2% of the Earth’s energy accumulation. From von Shuckmann et al.

Up until recently, the oceans have calmly absorbed most of our human induced warming, but things are starting to change. The very structure of the oceans is under threat.

The core of the crisis lies in something called Ocean Stratification.¹¹ Rather than composing of a uniform water mass, the ocean depths are arranged in layers connected by global currents that move and mix waters at different depths. The layers are separated weakly by density differences. Lighter water tends to sit above denser waters. Salinity and temperature control the water’s density with warm and fresh water being less dense than cold and saline waters.

The invisible ceiling created by Ocean Stratification which prevents mixing and traps heat in the surface layers where it can interact with the atmosphere.

Mixing between layers occurs in a variety of ways. Overturning currents including the AMOC are well known examples, but the majority of the mixing and therefore heat transfer happens through mode water (shallower) and intermediate water (deeper) mass formation. Water masses of homogeneous density are formed during winter when the surface oceans cool down and the denser water starts to sink. This process happens in highly concentrated areas where water masses of different densities meet in the oceans. In these areas horizontal density gradients are bent upward to the surface. It’s like a slide where denser water can slide down and spread out. Water mass formation is strongly influenced by surface winds and density stratification.¹²

New observations indicate that this process could be jeopardised.¹³ Multiple studies indicate an emerging decline in winter mode water formation in both the North Pacific and North Atlantic.¹⁴¹⁵¹⁶¹⁷ If mode water mass formation rates do decline or even break down in some areas, it would speed up heat accumulation within the top 300m, amplifying surface warming. This is one example of how changes in vertical mixing and upper ocean heat content can drive the non-linear spread of Marine Heat Waves over the extratropical oceans.

If mode and intermediate water mass formation rates decline or even break down in some areas, it would speed up heat accumulation within the top 300m and thereby contribute to the non-linear spread of Marine Heat Waves.¹⁸

Winds are another important temperature control. Besides stirring and mixing the upper oceans across density gradients they drive evaporation rates which cool the surface oceans down (just like blowing a hot mug of coffee). During calm periods when the winds stop blowing, evaporation shuts down.¹⁹²⁰ The barrier is a micrometer layer at the very surface of the water where the oceans and the atmosphere merge into one. If this thin layer saturates with water vapour, it prevents further evaporation. Winds disrupt and break this barrier, carrying the water vapour away, allowing further evaporation and heat transfer. It’s amazing how the conditions inside the space of mere micrometers control the heat content of something as vast as the oceans, and how non-linear and complex the mechanisms of the planet truly are. If summer winds are weak over the oceans they warm faster and upper ocean stratification increases.

Normally, the ocean is a dynamic, mixing body of water, where water masses exchange across weaker layers of different density. Over longer periods of time any slight warming of the climate would not affect these processes much as the stratified layers would maintain their balance. However as warming accelerates, the upper ocean is becoming significantly warmer and less dense than the water below as heat penetration takes time. This is now happening too quickly for the normal heat exchange mechanisms and currents to balance out. Warm water is observed to be accumulating increasingly in the surface layers²¹²²²³²⁴ as mixing with the waters below starts to be suppressed.²⁵ This further strengthens the stratification and so on, in a feedback loop.

Ocean stratification is measured and expressed in the rather confusing units of 10^-7 s^-2 which quantifies the strength of stratification over the layered water column. It represents the frequency at which a displaced parcel of water will oscillate vertically within a stable density gradient using the Brunt–Väisälä method, and indicates oceanic stratification strength. Higher values mean stronger stratification (more stable layering), while lower values indicate weaker stratification. The chart below shows the global ocean stratification increasing over time. Note the jump following the 2016 El Niño and then again, in 2023 an even larger jump.

Average Ocean Stratification. Note the sharp increases around 2015/16 and 2023. Data source http://www.ocean.iap.ac.cn/ftp/images_files/Stratification_global_time_series.txt

This increase in stratification, especially strong in the upper layers, create a set of physical barriers, or a “lid”, that starts to suppress vertical mixing. This leads to dangerous non-linear feedback loops:

• Heat Trapping: Stratification is increasing fastest in the upper layers of the oceans which shifts the ratio of ocean heat uptake to shallower depth’s over large areas. This signal is now starting to emerge as accelerating heat uptake in the upper layer, as shown in the graph below. This creates a heat depot that the atmosphere can put a lid on (with weak winds)²⁶²⁷ only to tap into more efficiently (with strong winds).²⁸²⁹ One feeds the other, feeding back again. In both cases, the coupling between the upper ocean and atmosphere intensifies. First the upper ocean warms faster, then releases more heat. A recharge discharge behaviour that operates from the regional³⁰³¹ to the planetary level.³²³³

• The Cloud Gap: As the surface warms, Sea Surface Temperature (SST) and atmospheric circulation patterns change leading to contracted tropical convection,³⁴³⁵³⁶ supporting poleward displacements of narrowing storm tracks, and shifting SST patterns,³⁷ regulating cloud cover over large regions. The net effect is a decline in cloud cover over the oceans. This allows more solar radiation to reach the water, heating it up even faster, again in a feedback loop.³⁸

• Wind-stratification feedback: As the lid strengthens, the winds need more energy to physically mix the upper ocean through wave dynamics. As the oceans stratify, basin wide warming signals appear together with weakening surface winds.³⁹⁴⁰ It could be part of the reason that data from 2008-2017 shows a decline in surface winds over 2/3 of the oceans, besides large scale circulation changes.⁴¹ It could be another vicious cycle of a self-reinforcing feedback loop driving further upper ocean stratification, surface warming, and ocean heat uptake concentrating at shallower depths.

It’s not yet clear to what extent large areas of exceptional warm oceans weaken wind speeds. In general, surface winds control the temperature of the surface oceans through evaporation rate and mixing ratios. When SSTs suddenly spiked in 2023, winds must have been weak over large ocean areas to allow such enormous SST anomalies to persist for such extended periods of time. To what extent had it been a forced reaction between the upper oceans and the atmosphere? The scientific discussion has really only just started.

Looking at the Ocean Heat Content (OHC) observations at different depths in the graphic below, it is clear that the shallower waters in blue are heating far faster than the deeper waters in green and grey.

Ocean Heat Content at three depths relative to the 2006-2015 mean baseline. OHC for depths below 2000m has only been available since 1992. Data: http://www.ocean.iap.ac.cn/pages/dataService/dataService.html?navAnchor=dataService

In 2023 and 2024 most of the heat uptake took place in the upper 300m of the oceans⁴²⁴³ while ocean stratification showed a massive peak. Then in 2025 another jump in ocean heat uptake by 23 zettajoules of energy (For context, 23 ZJ is roughly 200 times the total annual electricity generation of the entire human race). This is roughly an 8 fold increase of the annual mean values from 1958-1985 of ~2.9ZJ.⁴⁴ All this heat entered the oceans from the top. The danger is that stratification will now breach thresholds from where a growing share of accelerating ocean heat uptake will be stored at shallower depths. Such a development would feed back on itself. Sea surface temperatures remaining at record levels indicates that this worst of all feedback’s could be starting to kick in as the meta control of SSTs is ocean heat content over the first 100 - 300m.⁴⁵⁴⁶

Marine Heat Waves: A symptom and a cause

The term Marine Heat Waves (MHW) has only been defined in the last 10 years. Since then their prevalence has increased rapidly as the graphs below show.⁴⁷ On the left is the average MHW days each year from 1983 to 2024. Note the exponential increase in the persistence of MHWs. On the right is the total MHW coverage for the year by severity. In 2024, 91% of the global ocean surface experienced at least one MHW. On this graph, note the increase in severity as well as the coverage increase.

Marine Heat Wave occurrence and coverage. Graph from Blunden et al.

From an ocean perspective, MHWs are often a symptom of strengthening stratification, and declining upper mixed layer depth.⁴⁸⁴⁹⁵⁰⁵¹ The upper well mixed layer becomes thinner and heats up faster. This then traps heat accumulation closer to the surface which becomes a heat depot.⁵² As clouds decline and winds weaken, the upper thinner water layer heats up even faster. Thereby, the density of the upper layer is further reduced. Stratification increases further preventing mixing with layers below, trapping the heat in and exposing the already warm water to more solar radiation.⁵³ Passing eddies can then transfer the signal to the subsurface – warm core eddies pumping water downward in their centre.⁵⁴⁵⁵

One such example from 2023 was the MHW in the North Atlantic which persisted for over a year. A first of its kind for intensity, scale, and persistence.⁵⁶ During its existence, cloud cover was reduced, winds declined and stratification increased. These factors acted as feedbacks strengthening and perpetuating the MHW.

The North Atlantic sea surface temperature anomaly from the 2023 MHW. https://earth.nullschool.net/#2023/06/14/0000Z/ocean/surface/currents/anim=off/overlay=sea_surface_temp_anomaly/orthographic=-32.41,31.34,470

Of more concern however today, is the North Pacific. It started with a “Blob”, which had been a MHW off the west coast of the US that persisted from 2014-2016. In the following years MHWs intensified and spread fast. Just ten years later, in 2025, the MHW stretched across the whole North Pacific during the summer months. This has been accompanied by a huge build up of heat content in the upper layers of the ocean during the last decade. The North Pacific has now started to release more heat into the atmosphere during the winter. Not only that, a fast warming North Pacific has a huge impact on large scale circulation which can for example, favour more El Niño events.⁵⁷

It’s the rapidly accumulating heat in the mid-latitude regions that could have a significant impact on the warming rate by releasing, during the winter, rising amounts of latent heat driven by an upper ocean - winter storm feedback. Again one feeds the other all the way to the planetary level if stronger ENSO amplitudes couple strongly with other climate modes all the way to the polar regions.⁵⁸

Atmospheric Explosion: The Valencia Warning

The energy trapped by stratification eventually has to go somewhere. As ever more heat is accumulating within the top 300m, more of it escapes into the atmosphere as water vapour, turning the air into a high-octane fuel for storms.

The 2024 Valencia flooding provided a grim “proof of concept.” With Mediterranean sea surface temperatures at record highs, the storm didn’t just follow standard physics; it exhibited Super-Clausius-Clapeyron scaling. Instead of the expected 7% increase in rain per degree of warming, rainfall intensity jumped by 20%. The result was a year’s worth of rain (771.8mm or 32 inches) in just 16 hours. The flooding claimed 230 lives and caused huge economic losses.

This intensification was driven by enhanced atmospheric moisture from warmer sea surface temperatures, leading to increased convective available potential energy, stronger updrafts, and microphysical changes including elevated graupel (soft hail) concentrations.⁵⁹

Schematic showing the influence of climate change driven warming on the various drivers of the Valencia tragedy. Graphic from Calvo-Sancho et al.

2026/2027: The next jump in global temperatures

As we look toward a potential 2026 El Niño, the “pre-conditioning” is unprecedented.

Despite 2025 being a neutral to weak La Niña year, ocean heat content set new records by a substantial margin. Such a jump should have only been possible during an extreme La Niña since colder equatorial surface water accelerates ocean heat uptake while the system loses less energy to space. In the past ocean heat uptake to greater depths slowed down surface warming. This process appears to be stalling. The ocean surface layers now absorb so much energy that is increasingly being trapped within the upper 300m, accelerating surface warming and through its connection with the atmosphere, air warming.

When the El Niño begins to return, part of the stored energy will start to be released into the atmosphere. Potentially in late 2026, with peak values over the winter months. We are likely to see global temperatures jump to +1.6°C or even +1.7°C during 2027.

A deadly principle seems to be emerging. With each tenth of a degree of warming, extreme events increase disproportionally. Persistence of each jump in temperature with no La Niña cooling phases create a very worrying prognosis. We have not experienced these temperatures before and can only guess at the weather extremes, fire season, floods, droughts and crop disturbances that will now emerge. Whatever does emerge is then unlikely to retreat, its just the next step in the escalation of climate damage.

If an El Niño fails to develop in 2026 then unfortunately that won’t let us off the hook. It would just mean that the oceans will increasingly accumulate even more heat (aided by the neutral to La Nina conditions) to produce an even larger temperature jump in a following year when a strong El Niño does eventually emerge. The longer it takes, the stronger the force and worse the impacts. Until then stratification and MHWs will likely further intensify.

It happened during 2014-2016, then in 2023/24, so it will happen again...

A warning seems warranted. We are in the process of triggering an ocean-atmosphere feedback that may well never have happened before in Earth’s history. This is because it depends on the warming rate rather than the pure temperature. Previous spikes in emissions and warming, such as the PETM,⁶⁰ have occurred over millennia rather than a century allowing systems some time to gain balance. This is not the case today.

Conclusion: The End of the Buffer

We can no longer afford to prepare for “averages.” The 2024 Valencia floods and the North Pacific’s “Super Heatwave” are not outliers; they are the early tremors of a system reaching a thermal breaking point. Our global infrastructure—from drainage systems to food supply chains—was built for a linear world that no longer exists.

As the ocean surface continues to decouple from the deep, we are losing our greatest climate buffer, disrupting the “safe” storage of almost unimaginable amounts of energy. The upcoming 2026/2027 El Niño will not simply be a repeat of the past; it will be a global ventilation event, dumping years of “trapped” energy into a fragile atmosphere. We must monitor the ocean’s “pulse” with the same urgency we monitor the air. The lid is popping and the climate no longer resets, it only moves forward.

Ocean stratification is emerging as a vitally important aspect of the changing climate with major global implications. It is driven by a complex network of feedbacks but has the potential to disrupt the planet’s energy flows in hugely significant ways. Over half of the energy accumulating as a result of our emissions is being stored in the top 700m of the oceans compared to just 2% in the atmosphere.

In a follow-up article we examine the implications of stratification on ocean health, acidification, oxygenation, carbon sequestration and biodiversity, all of which also have climate change implications.

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