This paper
examines and attempts to predict shifts in ocean stratification. Oceans are
stratified, which means there are vertical differences in conditions and
properties with water depth. Heat, carbon, oxygen, salinity, density, and
nutrient variation with depth are characteristics of ocean stratification. It
is mainly variations in temperature, salinity, and density that control
stratification. The paper gives an explanation of ocean stratification:
“Ocean stratification describes the layering of seawater, as dictated by temperature, salinity and thereby density (Fig. 1). Warmer, fresher (less dense) water sits atop cooler, saltier (denser) water. As stratification strength changes with depth, several distinct layers form1 (Fig. 1): a mixed layer, with vertically homogeneous density (very weak stratification) that directly experiences the effects of air–sea exchanges; a ‘barrier layer’, separating a shallow salinity-dominated mixed layer from a deeper isotherm layer; and thermocline, halocline and pycnocline that separate the upper mixed layer and deeper ocean, marked by pronounced vertical gradients in temperature, salinity and density, respectively (Fig. 1). This stratification establishes stable conditions, limiting convection and acting as a barrier to vertical mixing2, in turn regulating exchange of heat, momentum, carbon, oxygen and nutrients.”
The authors note that
stratification can be defined in different ways and there are mathematical
formulas to calculate that do so, such as the Brunt–Väisälä frequency (N2). In
this method, density is the main variable and is influenced by temperature and
salinity. They note that stronger surface warming over recent years, along with
salinity changes due to changes in the global hydrologic cycle and ocean
dynamics, have led to a robust increase in ocean stratification. This increase
in stratification has led to decreases in mixing between layers and less
vertical heat exchange. They say increased stratification has also led to
decreased oxygen exchange and an increase in deoxygenation, and an increase in
CO2 and nitrous oxide, both greenhouse gases, being released to the air. There
are regional and seasonal changes in ocean stratification that also must be
considered. They explain how ocean stratification is stronger in the upper
ocean than in the deep ocean. They see the changes overall as a potential climate
feedback mechanism.
Observed increases in
stratification at 0-200m depths are most pronounced, especially in the summer
season. Stratification has also increased at 0-2000m depths, but seasonal
changes are much smaller than at 0-200m depths. The graphs below show changes
in salinity, temperature, density, and N2 with thermocline, pycnoclines,
including seasonal pycnoclines, in different oceans, mostly for 0-500m but with
one graph for 0-2000m showing the permanent pycnocline.
The authors note that
recognized oceanic oscillations influence what they call “remarkable
interannual and decadal variations in global upper-ocean stratification.”
“Observations also reveal remarkable
interannual and decadal variations in global upper-ocean stratification (Fig.
3). This interannual variability is dominant in the tropics and shows positive
correlations with ENSO (Fig. 3a, inset); ENSO strongly alters global ocean
temperature, salinity, mixed layer, boundary layer and thermocline66–68.
However, stratification variability in the North Indian Ocean is negatively
linked to the Indian Ocean Dipole69,70. Globally, decadal variations are
correlated with Atlantic Multidecadal Variability and Pacific Decadal
Variability (PDV), largely through changes in sea surface properties3,5,71
(Supplementary Fig. 4), but the North Atlantic Oscillation is important in
driving decadal stratification variability in the North Atlantic Ocean3.”
Figure 3, below, shows predicted increases in ocean stratification for different ocean depths under three scenarios: a big increase, a modest increase, or staying roughly the same.
The scenarios modeled are:
“Shared Socioeconomic Pathways, including SSP1-2.6 (a
low-emission scenario), SSP2-4.5 (a moderate-emission scenario) and SSP5-8.5 (a
high-emission scenario)18,91.”
I would argue that the higher
two warming scenarios are not reasonable expectations. Warming of 5-8.5 deg C
by 2100 is not a likelihood, and the higher end of the 2-4.5 °C scenario is
also not likely. The lower end of the 2-4.5 °C scenario and the higher end of
the 1-2.6 °C scenarios are the most plausible. Thus, the most likely scenario
is between the blue and orange lines, with the red line and range being a more
extreme and highly unlikely scenario.
The authors also note
differences between Northern and Southern Hemisphere Ocean stratification. They
also reiterate that stratification changes in summer are generally consistently
stronger and more statistically significant:
“There are also strong contrasts in stratification
signals between the northern and southern high latitudes. Overall, pronounced
stratification increases are evident in the Arctic Ocean (north of 70° N) in
all levels (Fig. 4a,f,k): 7.6 ± 7.5 × 10−6 s−2 (0.7 ± 0.7% dec−1)
and 1.3 ± 1.2 × 10−6 s−2 (0.7 ± 0.7% dec−1)
for 0–200 m and 0–2,000 m,
respectively. Here, the increases are dominated by salinity changes (Fig.
4c,h,m and Supplementary Fig. 6), specifically freshening associated with
sea-ice and land-ice melt81–84. In the Southern Ocean (south of 50° S), by
contrast, changes are far more subdued, with pockets of statistically
significant decreases in stratification since 1960 (Fig. 4a,f,k). These
stratification decreases largely reflect temperature effects (Fig. 4b,g,l),
notably surface cooling and subsurface warming12,85,86, that reduce
stratification.”
The table below and the next
two graphs show observed and projected changes in ocean stratification.
The potential impacts of
observed and projected changes in ocean stratification are explored in the
statement and graphic below.
“Observed and projected ocean stratification changes
have substantial Earth system consequences. Amongst other facets, these include
impacts on physical ocean attributes (ocean circulation, tides and mixing,
marine heatwaves), biogeochemical ocean attributes (greenhouse gas fluxes,
biogeochemical changes) and attributes that are more climatic (Earth surface
warming, climate modes, tropical cyclones and tipping points), all of which are
now discussed (Fig. 6).”
The authors note that
stronger stratification is likely associated with more surface warming, reduced
vertical mixing, and a lower efficiency of ocean heat uptake. They also note
that these changes are regional and that the reverse may happen in subtropical
ocean regions. The speed of ocean responses to surface heat is a factor for
different regions. Deep ocean warming is less likely to occur, especially in
the low emissions scenarios that are most likely. They also note that ocean
stratification affects and is affected by marine heatwaves. Ocean circulation
patterns are modulated by stratification in several regions. Fast surface
warming in the 200-400m depth range can alter important ocean currents. Below,
they note that the often-stated concern that the Atlantic Meridional
Overturning Circulation (AMOC) could be weakened by ocean stratification and
how that may happen:
“Yet many other circulation systems weaken because of
stratification changes. For instance, strong freshening in the subpolar North
Atlantic in a warmer climate enhances vertical stratification, reducing the
formation of North Atlantic Deep Water and thus slowing the AMOC132,138,139.
Indeed, with rising atmospheric CO2 concentrations, AMOC periodicity and
amplitude tend to decline, largely related to a more stratified subpolar North
Atlantic that changes the characteristics of westward-propagated oceanic baroclinic
Rossby waves140,141. The weakened AMOC decelerates the Gulf Stream138 and
remotely reduces the Indonesian Throughflow transport through interbasin
Kelvin-wave propagation along the coastal-equatorial waveguide114,132. Although
stratification in the North Atlantic is pivotal, palaeoclimate evidence
indicates that AMOC stability during the last deglaciation is mostly determined
by salinity stratification at ~34° S (refs.
142,143) (Fig. 7c).”
The graphs below show the
authors’ attempts to quantify the impacts of stratification on the global
climate system.
They note that more
warming-induced stratification in the Southern Ocean would reduce Antarctic
Deep Water formation and reduce ocean CO2 uptake. They also think it
decelerates deeper ocean currents, even as it accelerates upper ocean currents.
They consider the potential
effects of increased stratification on oceanic methane releases to the
atmosphere :
“… increased stratification inhibits the penetration of
dissolved gases into the near-surface layer and hampers CH4 fluxes to the
atmosphere. Ocean temperature changes at the bottom affect the stability of
methane clathrates, and possibly encourage methane release from marine
sediments177,180. Also, ocean warming and upper-ocean stratification change
affect CH4through phytoplankton growth, zooplankton egestion and other
processes.”
They also consider the
effects of increased stratification on ocean oxygenation, noting that it is
considered to be a key driver of ocean deoxygenation, which affects marine
life.
Below, they note possible
effects on tropical cyclone intensity, some of which have already been
observed:
“Stratification can either amplify or subdue tropical
cyclone intensification68. For example, rising SST {sea surface temperature}
and ocean heat content associated with enhanced stratification will provide
more energy to the cyclones186,188. Increased stratification also inhibits
diapycnal mixing and reduces cyclone-induced surface cooling (cold wake)189.
Upper-ocean freshening caused by rainfall can also further intensify tropical
cyclones by increasing upper-ocean salinity stratification, which acts to suppress
cyclone-induced surface cooling190,191. This effect is even pronounced when
there is a freshwater-induced (rainfall or river systems) barrier layer190, by
increasing ocean stability and suppressing storm-induced vertical mixing and
cold wake. Thus, in a warming world with enhanced stratification, these effects
would cause an increase in cyclone intensity192, as already apparent in
observations192–194, albeit with uncertainty.”
However, they note as well
that increased stratification can also reduce tropical cyclone intensity under
some circumstances such as increased vertical mixing, which can lead to ocean
cooling.
They consider that increased
stratification can influence Pacific decadal-scale variability (PDV) as well as
more frequent cyclic changes such as the El Niño-Southern Oscillation (ENSO).
They also discuss the potential collapse of the AMOC under high emissions
scenarios, though these scenarios are quite unlikely, as I have noted.
The summary notes:
“Ocean stratification is an important oceanic process
with substantial climatic implications. Robust stratification increases have
been observed since the 1960s, with rates of 1.1 ± 0.2% dec−1,
0.8 ± 0.1% dec−1
and 1.8 ± 0.3% dec−1
for 0–200 m, 0–2,000 m and
seasonal pycnocline stratification, respectively (Table 1 and Fig. 3);
temperature contributes most strongly to these changes, but salinity can be
important regionally.”
The authors also note that
quantification of stratification changes needs to be improved and better
understood, especially along critical layers like the seasonal and permanent
pycnoclines, acknowledging that there are significant uncertainties. They
acknowledge data limitations and limitations in the current observation system
for quantifying stratification. They acknowledge a need for better modeling as
well as a better understanding of paleo-observations and proxies that can shed
light on past changes in ocean stratification.
“Although it is well accepted that ocean stratification
has increased, fundamental questions about the causes and impacts of these
changes remain. For instance, attribution of stratification changes is lacking,
necessitating investigations into the key mechanisms (winds, buoyancy or ocean
dynamics) and drivers (greenhouse gases, aerosols or climate variability) at
global and regional scales.”
“Ocean stratification is established as a crucial driver
of deoxygenation, but the magnitude of the contribution is little known182.
Likewise, influences on primary production, ocean biomass and the carbon cycle
have not been well quantified, nor have the compound effects of stratification
increases together with other ocean changes (such as warming, acidification and
deoxygenation). Finally, isolating the impacts of stratification from other
factors and phenomena is challenging — stratification is not an independent
variable. For example, the positive and negative feedbacks of stratification
change on tropical cyclones are mixed with temperature and salinity effects,
meaning that the direct net impact cannot be quantified. New analysis
approaches should be developed to clarify these effects, including model
experiments and theoretical analyses linked to observations.”
Thus, they note the complexity of the global systems they are modeling and the inherent uncertainties with attempts to quantify the sensitivities of global systems to changes in variables. I also note that one of the paper’s authors is the somewhat controversial scientist, Michael Mann. It makes me wonder if he was the one wanting the inclusion of the highly unlikely high emissions (5-8.5 °C) scenario. Most researchers think that global warming will peak somewhere between 2 and 2.5 °C, so why include scenarios that double or triple that? Perhaps because they look like hockey sticks? Mann has been challenged for his famous hockey stick graph that is likely weighted to show higher warming, and perhaps a similar thing has been done here. I have heard Mann speak and was surprised by his overt political attitude. Although I’m not challenging his science, I do think his strong politicization leads me to question his motives. I will also say that this kind of global system modeling is quite complex and often difficult to understand, even for low-mid level scientists like myself.
References
Ocean
stratification in a warming climate. Lijing Cheng, Guancheng Li, Kevin E.
Trenberth, Shang-Min Long, Yuanlong Li, Michael E. Mann, John Abraham, Yan Du,
Karina von Schuckman, Xuhua Cheng Maofeng Liu, Qihua Peng, Xun Gong , Zhanhong
Ma, & Huifeng Yuan. nature reviews earth & environment. September 2025.
Ocean
stratification in a warming climate
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