Salinity

 Copilot:

Climate researchers have warned us of the possibility of a shut-down of AMOC,the circulation of warm seawater up the Eastern Coast of North America, and it's turntoward Northern Europe. This is what keeps Europe warmer than North America atMontreal latitudes. it is weakening, but a total collapse would take one hundred years, sothis is not an immediate worry.

I ended up having a chat with Copilot about oceanic circulation - to me, very surprising.Below:

AMOC; explain to me the salinity cycle. Is salt constantly accumulating at the bottom of the sea?

Short answer:

No — salt does not accumulate at the bottom of the ocean.

Ocean salinity is governed by a continuous global cycle of inputs, mixing, and removal. The Atlantic Meridional Overturning Circulation (AMOC) is one of the major systems that redistributes salt, but it does not cause salt to sink and pile up on the seafloor. Salt stays dissolved in seawater and moves with currents.

Below is the clear, structured explanation you asked for.

🌊 What “salinity” actually is

Salinity is the concentration of dissolved ions in seawater — mainly sodium (Na⁺) and chloride (Cl⁻). These ions remain fully dissolved, even in deep water. They do not precipitate or settle like sand or silt.

Why?

Because the ocean is far from saturated with salt. It would need to be ~10× saltier before NaCl crystals could form.

🌍 The global salinity cycle (step‑by‑step)

1. Sources of salt

Rivers bring dissolved minerals from weathered rocks.

Hydrothermal vents add ions from deep crustal fluids.

Volcanic emissions contribute chloride and sulfur species.

Atmospheric deposition (sea spray returning to ocean).

2. Processes that remove salt

Salt is removed extremely slowly, but it is removed:

Evaporite formation — in restricted basins (e.g., Mediterranean in the past), seawater can evaporate enough to precipitate salt layers.

Biological uptake — tiny amounts incorporated into shells or tissues.

Sediment adsorption — some ions bind to clays.

Subduction — sediments containing salts get dragged into Earth’s mantle.

These processes keep ocean salinity roughly stable over millions of years.

3. Mixing and redistribution

Ocean circulation — including the AMOC — constantly mixes salt:

Warm, salty water moves northward at the surface.

Cold, dense water sinks and returns southward at depth.

(This is the conveyor‑belt mechanism described in Woods Hole’s AMOC overview .)

Salinity differences help drive density contrasts that power overturning.

But salt does not sink independently — only water masses with certain temperature/salinity combinations sink.

🧂 Why salt does not accumulate at the bottom

Reason 1 — Salt stays dissolved

The deep ocean is cold, but not salty enough for precipitation.

Even the densest deep water is only ~35–36 PSU (practical salinity units).

Reason 2 — Sinking water carries salt back upward eventually

Deep water formed in the North Atlantic flows south, then mixes and eventually upwells in the Indian and Pacific Oceans.

This overturning cycle takes ~1000 years, but it prevents any long‑term buildup.

Reason 3 — The ocean is well‑mixed over long timescales

Vertical mixing, turbulence, and upwelling constantly stir the water column.

🌡️ How AMOC fits into the salinity story

AMOC is driven partly by salinity‑controlled density:

Salty water is denser than fresh water.

In the North Atlantic, surface water becomes cold and salty enough to sink.

(This sinking of cold, salty water is a key AMOC mechanism .)

Changes in salinity can strengthen or weaken AMOC.

Recent research shows:

AMOC slowdown amplifies salinity variability in the North Atlantic .

Subpolar salinity is a useful indicator of AMOC mean state .

But again: AMOC redistributes salt — it does not deposit it on the seafloor.

🧠 A non‑obvious insight

The ocean’s salt budget is controlled not by “salt sinking,” but by freshwater fluxes:

Rain, river input, and ice melt dilute salinity.

Evaporation concentrates salinity.

These freshwater changes strongly influence AMOC stability and tipping behavior (as noted in AMOC stability analyses ).

In other words:

Freshwater changes matter more than salt movement itself.

✔️ Final takeaway

Salt does not accumulate at the bottom of the sea.

The ocean’s salinity is maintained by:

continuous input from rivers and vents,

slow removal through geological processes,

and global mixing driven by systems like the AMOC.

If you want, I can also explain:

Why the Atlantic is saltier than the Pacific

How salinity anomalies can trigger AMOC weakening

The role of evaporation in creating the Mediterranean’s extreme salinity

Which direction would you like to explore next?

Why is the Atlantic saltier than the Pacific?

The Atlantic is saltier than the Pacific because it loses more freshwater than it gains, while the Pacific gains more freshwater than it loses.

This imbalance — driven by evaporation, rainfall, river input, and ocean‑circulation geometry — creates a persistent salinity contrast between the two basins. Below is the full, structured explanation.

🌡️ 1. The Atlantic has higher evaporation

Warm, dry subtropical air masses over the Atlantic remove large amounts of freshwater through evaporation.

When water evaporates, salt stays behind, raising salinity.

This effect is especially strong in the North Atlantic subtropics, one of the saltiest open‑ocean regions on Earth.

🌧️ 2. The Pacific receives more rainfall

The Pacific basin is larger and has stronger rainfall belts (ITCZ + monsoon systems).

More precipitation means more freshwater added → diluted salinity.

This is one of the biggest reasons the Pacific averages ~34–35 PSU while the Atlantic averages ~35–37 PSU.

🏞️ 3. The Pacific receives more river runoff

Major rivers (Columbia, Yukon, Amur, Yangtze, Mekong, etc.) drain into the Pacific, adding large volumes of freshwater.

The Atlantic has fewer massive river systems by comparison.

More river input → lower salinity.

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❄️ 4. Arctic meltwater flows mainly into the Pacific

Freshwater from melting Arctic ice enters both basins, but the Pacific receives a larger share through the Bering Strait.

This further reduces Pacific salinity.

🌊 5. Ocean circulation patterns reinforce the difference

The Atlantic exports salty water and imports fresher water:

The AMOC draws warm, salty water northward in the Atlantic.

The Pacific does not have an equivalent overturning circulation.

The Atlantic also receives salty water from the Mediterranean outflow, which boosts its salinity. Meanwhile, the Pacific’s geometry and circulation promote freshwater accumulation, keeping it less salty.

📈 6. The contrast is actually increasing

Recent observational studies show the salinity gap between the Atlantic and Pacific is widening, driven by climate‑related changes in evaporation, rainfall, and ocean dynamics.

🧠 Non‑obvious insight

The Atlantic is saltier not because it has more salt added — but because it loses more freshwater.

Ocean salinity differences are overwhelmingly controlled by the freshwater cycle, not by salt inputs.

✔️ Final takeaway

The Atlantic is saltier than the Pacific because:

It evaporates more

It rains less

It receives less river runoff

It gets less Arctic meltwater

Its circulation traps and concentrates salt

The Mediterranean outflow adds extra salt

All these factors combine to make the Atlantic consistently — and increasingly — saltier than the Pacific.

If you want, I can also explain:

Why the Mediterranean is one of the saltiest seas

How salinity differences affect AMOC stability

Why the Atlantic and Pacific “don’t mix” sharply at Panama

Just tell me which direction you want to explore.

Is there ever calibration between the two?

Short answer: No — there is no “calibration” event where the Atlantic and Pacific equalize their salinity.

 But there are slow, indirect mechanisms that prevent the two oceans from drifting infinitely apart in salinity.

Here’s the clear breakdown.

🌊 1. The Atlantic and Pacific are not directly connected

Since the closure of the Central American Seaway ~3 million years ago, no deep or surface water flows directly between the two basins.

 The Panama Isthmus acts as a hard barrier.

So there is no direct mixing that could “calibrate” salinity between them.

🌍 2. They do communicate — but only through the global circulation loop

Even though they don’t touch, the oceans are connected through the global thermohaline circulation, the “conveyor belt” that takes ~1000 years to complete.

The sequence is:

Atlantic forms deep water (cold + salty).

That deep water flows south → around Antarctica → into the Indian and Pacific basins.

It slowly upwells in the Pacific.

Surface waters eventually return toward the Atlantic via the Indonesian Throughflow and the Agulhas leakage.

This is the only mechanism that allows the basins to influence each other.

But it is not a calibration — it is a slow redistribution.

🌧️ 3. Freshwater fluxes keep the basins different

Even with global circulation, the Atlantic and Pacific remain very different because their freshwater budgets are fundamentally different:

Atlantic: evaporation > precipitation → saltier

Pacific: precipitation + river input > evaporation → fresher

As long as this imbalance persists, the salinity contrast persists.

No circulation can “fix” that.

🧂 4. The conveyor belt exports Atlantic salt to the Pacific

This is the closest thing to “calibration.”

Because the Atlantic forms deep water, it exports salty water into the global abyss.

 That water eventually upwells in the Pacific, making the Pacific slightly saltier than it otherwise would be.

But the Pacific still receives so much freshwater that it stays less salty overall.

So the conveyor belt acts like a slow leak, not a calibration.

🧠 5. A non‑obvious insight

The Atlantic is saltier because it is the basin where deep water forms.

 Deep water formation requires high salinity, so the Atlantic is “locked” into being saltier.

The Pacific cannot form deep water because it is too fresh — and because it is too warm at the surface.

This asymmetry is self‑reinforcing.

✔️ Final takeaway

There is no calibration event between the Atlantic and Pacific.

 Instead:

They remain different because of freshwater fluxes.

They influence each other only through the slow global overturning circulation.

The Atlantic exports salt, but not enough to equalize the basins.

The salinity contrast is stable and persistent — and actually increasing with climate change.

If you want, I can also explain:

Why closing the Panama Seaway changed global climate

Whether AMOC slowdown will change Atlantic salinity

How salinity differences affect sea level between basins

Just tell me which direction you want to explore.

When did this closure occur, and how long did it take?

Louise, this is a great question — and the timing and duration of the Panama Seaway closure are actually more complex than most people realize. It wasn’t a single moment; it was a multi‑million‑year tectonic process with several stages.

Here’s the clear, structured breakdown.

🧱 1. When did the closure begin?

Geological evidence shows the seaway began shoaling (becoming shallower) around:

~15 million years ago (Ma)

Volcanic arc collisions and uplift in Central America started restricting deep-water flow.

The connection was still open, but weaker.

🏞️ 2. Major restriction phase

By:

~10–6 Ma

The seaway had become a narrow, shallow strait.

Deep-water exchange between the Atlantic and Pacific had largely stopped.

Surface water still flowed between the basins.

This is when salinity differences began to grow.

🧱 3. Final closure

Most geological and paleoceanographic evidence points to:

~3.0–2.8 Ma

This is when:

The last shallow marine connection disappeared.

Land animals could cross between North and South America.

Atlantic–Pacific water exchange ceased completely.

This is the date usually cited as the closure.

⏳ 4. How long did the closure take?

From first restriction (~15 Ma) to final closure (~3 Ma):

≈ 12 million years

But if you focus only on the final phase — when the seaway went from shallow to fully closed — that took:

≈ 3–4 million years

So the closure was:

Slow

Stepwise

Geologically messy

Driven by uplift, volcanism, sedimentation, and plate movement

🌍 5. Why the duration matters

The long, gradual closure allowed:

The Atlantic to slowly become saltier

AMOC to strengthen over time

Global climate to shift toward the ice‑age cycles

Species migrations (Great American Biotic Interchange)

If the closure had been sudden, climate impacts would have been far more abrupt.

✔️ Final takeaway

Started: ~15 million years ago

Mostly restricted: ~10–6 million years ago

Fully closed: ~3 million years ago

Total duration: ~12 million years (or ~3–4 million for the final phase)

If you want, I can also explain:

How scientists know when the seaway closed (sediment cores, isotopes, fossils)

How the closure helped trigger Northern Hemisphere glaciation

Whether reopening the seaway (hypothetically) would collapse AMOC

Just tell me which direction you want to explore next.

How did the closure help trigger Northern Hemisphere glaciation?

Louise, this is one of those beautiful Earth‑system stories where tectonics, ocean physics, and climate feedbacks all lock together.

The closure of the Panama Seaway didn’t directly create ice — but it re‑engineered the global ocean in a way that made Northern Hemisphere glaciation possible for the first time in millions of years.

Here’s the clear, step‑by‑step chain of causality.

🌎 1. Closing the seaway made the Atlantic saltier

Once the Atlantic could no longer exchange water with the fresher Pacific:

Evaporation dominated in the Atlantic

Salt accumulated

Surface waters became denser

This was the key precondition for the next step.

🌊 2. Denser Atlantic water strengthened the AMOC

Higher salinity → higher density → more vigorous sinking in the North Atlantic.

A stronger AMOC meant:

More warm water transported northward

More evaporation over the Atlantic

Even saltier water returning south at depth

A self‑reinforcing loop

This intensified the northward heat pump.

❄️ 3. More heat transported north paradoxically created more snow

This is the counterintuitive part.

Warm water flowing northward:

Evaporates more moisture

Sends that moisture into the atmosphere

Delivers it to high latitudes

Where it falls as snow, not rain

Because high latitudes were already cooling due to long‑term orbital cycles, the extra moisture increased snowfall, not melting.

More snowfall → more ice accumulation.

🧊 4. Ice sheets grow when snowfall exceeds summer melt

The strengthened AMOC created:

Wetter, snowier Arctic and sub‑Arctic winters

Summers that were not warm enough to melt all the new snow

Gradual buildup of ice sheets in Canada, Greenland, and Scandinavia

This is the moment Earth entered the Pleistocene glacial–interglacial cycles.

🌍 5. Ice sheets amplified cooling through feedbacks

Once ice sheets formed, they:

Reflected more sunlight (albedo feedback)

Cooled the atmosphere

Altered jet streams

Locked Earth into a new climate mode

The Northern Hemisphere became glaciation‑capable for the first time.

🧠 A non‑obvious insight

The Panama closure didn’t “cause” ice ages by cooling the planet.

It caused ice ages by moistening the Arctic.

The Arctic became a place where:

Warm Atlantic water → more evaporation

More evaporation → more snowfall

More snowfall → more ice

More ice → more cooling

This moisture‑delivery mechanism is the hidden lever.

✔️ Final takeaway

The closure of the Panama Seaway helped trigger Northern Hemisphere glaciation by:

Making the Atlantic saltier

Strengthening AMOC

Pumping more moisture northward

Increasing snowfall

Allowing ice sheets to grow

Locking Earth into the Pleistocene ice‑age cycles

It was a tectonic event that reshaped global climate for millions of years.

If you want, I can also explain:

Why the Pacific couldn’t form deep water even before the closure

Whether reopening the seaway (hypothetically) would collapse AMOC

How this event changed CO₂ and global vegetation

Just tell me which direction you want to explore.

What could happen if we reopened the seaway?

Louise, reopening the Panama Seaway would be one of the most climate‑disruptive geological events imaginable. It would fundamentally rewire the global ocean, collapse the modern Atlantic–Pacific salinity contrast, and almost certainly shut down the AMOC. The consequences would ripple through climate, ecosystems, sea level, and even human habitability.

Here’s the clear, structured breakdown of what would happen.

🌊 1. Atlantic–Pacific mixing would resume

Right now, the Atlantic is salty and dense; the Pacific is fresher and lighter.

Reopening the seaway would allow:

Fresh Pacific water to flow into the Atlantic

Salty Atlantic water to leak into the Pacific

This would rapidly reduce the Atlantic’s salinity.

The Atlantic’s “salt reservoir” would drain into the Pacific like a breached dam.

🧊 2. AMOC would weaken — or collapse entirely

AMOC depends on salty, dense water sinking in the North Atlantic.

If the Atlantic suddenly became fresher:

Surface water would lose density

Deep‑water formation would slow

The overturning circulation would weaken

A full collapse becomes likely

This is the same mechanism scientists worry about with Greenland meltwater — but reopening Panama would be far more dramatic.

A collapsed AMOC means:

Northern Europe cools sharply

North Atlantic storms intensify

Tropical rain belts shift

Sea level rises along the U.S. East Coast

This is not subtle.

🌧️ 3. Global rainfall patterns would reorganize

The closure of the seaway millions of years ago reshaped rainfall.

Reopening it would reverse many of those changes:

The Atlantic would become wetter

The Pacific would become saltier and drier

Monsoon systems would shift

The ITCZ (tropical rain belt) would migrate westward

Amazon rainfall could decline

Central America’s climate would transform

Rainfall redistribution is one of the biggest impacts.

🌡️ 4. Northern Hemisphere would warm, not cool

This is the counterintuitive part.

The original closure helped trigger glaciation by strengthening AMOC and increasing snowfall.

Reopening the seaway would:

Weaken AMOC

Reduce northward moisture transport

Reduce snowfall over high latitudes

Make it harder for ice sheets to grow

The Arctic would warm faster.

Greenland’s ice loss would accelerate.

🌍 5. Sea level would change differently in each basin

Because salinity affects density:

Fresher Atlantic water → expands → sea level rises

Saltier Pacific water → contracts → sea level falls slightly

Plus, a weakened AMOC raises sea level along:

U.S. East Coast

Gulf of Mexico

Caribbean

This is a known AMOC‑collapse signature.

🐠 6. Marine ecosystems would be disrupted

Mixing two basins with different chemistry would:

Change nutrient distributions

Alter oxygen levels

Reshape fisheries

Disrupt coral systems

Possibly trigger extinctions in sensitive species

The Atlantic and Pacific have evolved separately for millions of years.

Reconnecting them would be ecologically violent.

🧠 7. A non‑obvious insight

Reopening the seaway would not just change ocean currents — it would erase the modern climate mode that has existed for 3 million years.

The entire Pleistocene glacial–interglacial rhythm is tied to the Atlantic’s ability to form deep water.

Undoing the closure would undo the climate system we live in.

✔️ Final takeaway

Reopening the Panama Seaway would:

Freshen the Atlantic

Salt up the Pacific

Collapse or severely weaken AMOC

Shift global rainfall

Warm the Arctic

Raise sea level along Atlantic coasts

Disrupt ecosystems

End the climate regime that produced ice ages

It would be one of the most dramatic climate changes Earth could experience without external forcing.

If you want, I can also explore:

Whether AMOC collapse would make Québec colder or warmer

How fast the salinity equalization would occur

What the world looked like before the seaway closed

Whether any modern engineering project could ever do this (spoiler: no)

Just tell me where you want to go next.

Would it make Quebec colder or warmer?

It’s a sharp, personal question—“what does all this mean where I live?”

Short version:

If we reopened the Panama Seaway and that led to a strong weakening or collapse of AMOC, Québec would likely get colder in winter than in a world with warming but no AMOC collapse—but not back to preindustrial cold, and summers could stay warm or even hot.

Let’s unpack that just enough to make it feel real.

Big picture: AMOC collapse = North Atlantic cooling

AMOC today: carries warm, salty water north, helping keep Eastern Canada and Western Europe milder than their latitude suggests.

- If it collapses: climate models show sharp cooling over the far North Atlantic and modest cooling over much of the rest of the Northern Hemisphere, including Canada.

So for Québec:

Winters:

Likely colder than in a “normal” global‑warming scenario (no AMOC collapse).

More frequent cold spells, stronger temperature gradients over the North Atlantic.

- Summers:

Still influenced by global greenhouse warming.

Could remain warm or hot; the AMOC signal is strongest in winter and over the ocean.

Net effect: compared to a world with 2–3 °C global warming and no AMOC collapse, Québec would probably feel less warmed overall, especially in winter, but not “saved” from climate change—just shifted into a different, more chaotic pattern.

A subtle but important nuance

Models show that:

Global mean temperature drops a bit with AMOC collapse (cooling offsets part of warming).

- But regional impacts are uneven—Scandinavia, Russia, Canada, and North Atlantic coasts get hit with the biggest cooling.

So for you in Québec, the story is:

Relative to a warmed world without AMOC collapse, reopening the seaway (and collapsing AMOC) would likely make winters colder, summers still warm, and the overall climate more unstable, not simply “better” or “worse.”

If you want, we can zoom in next on:

How this would change snowpack and freezing rain in southern Québec

What it might mean for St‑Lawrence sea ice and local weather pattern

                                                            

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Is there a stict separation in sealife between the Atlantic and Pacific oceans (including sharks)?

No, there's not a strict separation, but there is a very significant biological divide between the Atlantic and Pacific that developed after the rise of the Isthmus of Panama. The formation of the isthmus isolated Caribbean and Pacific marine populations, leading to extensive speciation and diversification on each side.

A useful way to think about it is:

🐟 Coastal and shallow-water species: often strongly separated

Many species that once formed a single population were split into Atlantic and Pacific descendants when the seaway closed. Scientists call these "geminate species" pairs. The separation cut off gene flow and allowed the populations to evolve independently.

For example:

Numerous reef fish

Snapping shrimp

Sea urchins

Molluscs and corals

have Atlantic and Pacific relatives that look similar but are now distinct species.

🦈 Sharks: less separated than reef species

Sharks are interesting because many species are highly mobile.

Some sharks occur in both oceans, including species such as:

Tiger sharks

Blue sharks

Whale sharks

Shortfin makos

These species can cross entire ocean basins, so the Atlantic-Pacific distinction is much weaker for them than for reef fish.

However, there are also shark species that are largely restricted to one side or have developed separate populations over time.

🌍 The biggest barrier isn't the Panama Canal

People often imagine the Panama Canal reconnects the oceans biologically. In reality, it does very little compared with the ancient seaway because:

It is freshwater in large sections.

It uses locks rather than a continuous sea-level channel.

It is heavily managed.

So it is not re-establishing free marine interchange on anything like the pre-closure scale.

🐋 What about whales, tuna, and other ocean wanderers?

Large pelagic animals often occur in both oceans because they can move around:

via the Arctic (for some species),

around South America,

or through evolutionary distributions established before the closure.

For them, the Atlantic-Pacific divide is much less important than for organisms tied to coastlines, reefs, estuaries, or specific currents.

🧠 A fascinating example

If you collected a reef fish from the Caribbean coast of Panama and a superficially similar fish from the Pacific coast of Panama, they are often not the same species, despite being only ~80 km apart across the isthmus. Millions of years of isolation have made the two sides biologically distinct.

So the best answer is:

There is no strict Atlantic-Pacific separation of marine life, especially for highly migratory animals like many sharks, whales, and tuna. But for coastal and reef organisms, the Isthmus of Panama created one of the strongest biogeographic barriers on Earth, leading to millions of years of independent evolution on the two sides.

One particularly interesting follow-up is that the Atlantic and Pacific sides of Panama today have dramatically different ecosystems despite being geographically close. The Caribbean side has extensive coral reefs, while the Pacific side is more influenced by nutrient-rich upwelling and tends to support very different marine communities.

Can a human swimmer feel the difference in salinity between both ocean beaches?

Yes, but probably not in the way most people imagine.

A typical swimmer cannot usually jump into the Atlantic one day and the Pacific the next and immediately say, "This one is 1 PSU saltier." The average salinity difference between the open Atlantic and open Pacific is only on the order of 1 to 2 parts per thousand, which is quite small.

However, swimmers can notice effects associated with salinity:

🌊 Buoyancy

Saltier water is denser, so you float slightly higher.

For example:

In the Dead Sea (extremely salty), the difference is obvious.

Between the Atlantic and Pacific, the difference is much more subtle.

A strong swimmer might feel marginally more buoyant in the saltier Atlantic, but most people would not reliably distinguish the oceans in a blind test.

👁️ Eyes and taste

A more noticeable difference is often:

how much the water stings your eyes,

how salty it tastes if some gets in your mouth.

But local conditions often overwhelm the Atlantic-Pacific average difference.

For example:

Near a large river mouth, seawater can be dramatically fresher.

In a dry tropical region, seawater can be much saltier.

The Caribbean is often saltier than many Pacific coastal areas.

A swimmer comparing beaches in Panama might notice a bigger difference than someone comparing beaches in Nova Scotia and British Columbia.

🏊 What if you stood on both sides of Panama?

This is where it gets interesting.

The Caribbean side tends to be:

warmer,

clearer,

saltier.

The nearby Pacific side tends to be:

less salty,

more nutrient-rich,

often murkier because of upwelling and larger tidal ranges.

Many people would perceive the water as different, but they would probably be responding to a combination of:

salinity,

temperature,

clarity,

waves,

and local ecology,

rather than salinity alone.

🦈 A shark's perspective

Many marine animals can detect salinity changes far better than humans can. Sharks, fish, and even some invertebrates use salinity as an environmental cue. For them, the Atlantic-Pacific difference is biologically meaningful even when it's barely perceptible to us.

A fun Québec comparison

If you've ever floated in the St. Lawrence Estuary and then in the Gulf of St. Lawrence, you've experienced a much larger salinity change than the average Atlantic-versus-Pacific difference. Most people notice that immediately because the estuary contains so much river water from the St. Lawrence.

So the answer is:

A human swimmer might notice a slight difference in buoyancy or taste, but most people could not reliably identify "Atlantic" versus "Pacific" from salinity alone. Local conditions usually dominate what the swimmer actually feels.