CPClimate of the PastCPClim. Past1814-9332Copernicus PublicationsGöttingen, Germany10.5194/cp-12-543-2016The effect of low ancient greenhouse climate temperature gradients on the ocean's overturning circulationSijpWillem P.w.sijp@unsw.edu.auEnglandMatthew H.ARC Centre of Excellence for Climate System Science, University of New South Wales, Sydney, NSW 2052, AustraliaWillem P. Sijp (w.sijp@unsw.edu.au)29February201612254355210September20159October20159February2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://cp.copernicus.org/articles/12/543/2016/cp-12-543-2016.htmlThe full text article is available as a PDF file from https://cp.copernicus.org/articles/12/543/2016/cp-12-543-2016.pdf
We examine whether the reduced meridional temperature gradients of past
greenhouse climates might have reduced oceanic overturning, leading to a more
quiescent subsurface ocean. A substantial reduction of the pole-to-Equator
temperature difference is achieved in a coupled climate model via an altered
radiative balance in the atmosphere. Contrary to expectations, we find that
the meridional overturning circulation and deep ocean kinetic energy remain
relatively unaffected. Reducing the wind strength also has remarkably little
effect on the overturning. Instead, overturning strength depends on deep
ocean density gradients, which remain relatively unaffected by the surface
changes, despite an overall decrease in ocean density. Ocean poleward heat
transport is significantly reduced only in the Northern Hemisphere, as now
the circulation operates across a reduced temperature gradient, suggesting a
sensitivity of Northern Hemisphere heat transport in greenhouse climates to
the overturning circulation. These results indicate that climate models of
the greenhouse climate during the Cretaceous and early Paleogene may yield a
reasonable overturning circulation, despite failing to fully reproduce the
extremely reduced temperature gradients of those time periods.
Introduction
The polar warmth of the greenhouse climates in the Earth's past represents
a fundamentally different climate state to that of today, with a strongly
reduced temperature difference between the Equator and the poles
e.g.. show that oceanic poleward
heat transport reduces the meridional temperature gradient by causing an
enhanced greenhouse effect in extratropical latitudes. The ocean's meridional
overturning circulation (MOC) redistributes heat across the globe,
constituting a prime driver for oceanic poleward heat transport. The goal of
this study is to examine the MOC in a past greenhouse climate characterised
by weaker meridional temperature gradients, with mainly palaeoclimate but also
future climate perspectives.
Greenhouse conditions prevailed during the Cretaceous (146–66 Ma),
with significantly warmer temperatures than present ,
especially at high latitudes . find
that the earliest Cretaceous climate was stable and warm, with
a significantly lower meridional temperature gradient than today. Global
warmth increased during Cenomanian (94–101 Ma) to Turonian
(90–94 Ma) times and reaching its maximum during the Late Turonian
. Oxygen isotope data point to high tropical
temperatures of 33–34 ∘C
and southern subpolar Atlantic Ocean temperatures of
30–32 ∘C , suggesting extreme polar warmth.
Deep and bottom ocean temperatures exceeded 10 ∘C
at this time, indicating high polar winter temperatures
for this period as deepwater formation takes place during winter.
Greenhouse conditions extend beyond the Mesozoic, persisting over many
millions of years and ending with the initiation of the Antarctic ice cap at
the Eocene/Oligocene transition (EOT, 33 Ma)
e.g.. The early Eocene (56–48 Ma) saw
particularly reduced meridional temperature gradients
. High-latitude winter warmth
is reflected also in Eocene deep ocean temperatures, thought to have been
around 10 ∘C higher than today
. The low meridional temperature
gradients of the past greenhouse climates pose a significant problem to our
general understanding of how the climate works, as climate models fail to
simulate past polar amplification of the greenhouse effect, and the
responsible climate feedbacks remain unclear .
The phenomenon of polar amplification of global climatic change is present in
model projections of future climate e.g.. One
immediately obvious mechanism is the surface albedo feedback, where warming
leads to snow and ice melt and thus greater solar energy absorption. However,
global climate model (GCM) studies also find feedbacks involving
increased longwave forcing and latent heat transport
important in amplifying polar climate
change in the present-day. During the past greenhouse climates, feedbacks
related to the cryosphere were absent, and these atmospheric factors must be
even more important in causing polar warmth.
One promising recent line of research into explaining past polar and winter
warmth comes from , who propose that the Eocene high latitudes
were kept warm by deep atmospheric convection. Unlike today, in the absence
of the insulating sea-ice, more moisture and heat could enter the atmosphere.
They proposed this destabilised the winter air column, initiating atmospheric
convection and creating deep optically thick convective clouds, in sharp
contrast to today's stratified subpolar atmosphere. At high latitude, such
cloud cover would have a net warming effect, creating a positive feedback on
surface ocean temperature. This mechanism is most pronounced in winter,
suggesting a clue to the mild winters of greenhouse climates.
propose a mechanism whereby enhanced low-latitude ocean heat
transport can warm the mid- to high latitudes, without cooling the Tropics
see also. Here, extratropical
ocean warming arising from enhanced heat transport triggers a convective
adjustment of the troposphere. Enhanced greenhouse trapping associated with
convective moistening of the upper troposphere in the midlatitude storm
tracks leads to warming that extends to the poles by atmospheric processes.
The enhanced ocean heat transport causing these effects could for instance
arise from the altered geography at the time of the greenhouse climates
e.g. via a tropical circumpolar ocean;.
The very low meridional temperature gradients of the past greenhouse climates
pose a significant challenge to numerical climate models
, particularly
for the early Eocene. The modelling challenge here is that increased
greenhouse gases may yield balmy simulated polar regions, but they also
overheat the Tropics. , finding extratropical
cloud-related effects in their Eocene simulation, suggest a possible end to
the low-gradient problem, provided that very high CO2 concentrations
(4400 ppm) are prescribed. find reasonable agreement
between models and sea surface temperature (SST) data for the Eocene at high CO2
(2500–6500 ppm), with the important exception of the Southwest
Pacific and the Arctic. show that the convective cloud
feedback proposed by is active in an atmospheric GCM in
modern configuration with an atmospheric CO2 concentration of
2240 ppm and in a coupled GCM in Eocene configuration at 560 ppm.
In broad agreement with the work of and ,
recent advances in modelling the low-gradient greenhouse climates involve an
enhanced greenhouse effect in the extratropical regions. Indeed, radiative
feedbacks play an important role in amplifying the temperature effects of
opening the Drake Passage in the model study of . This
indicates that, while theories involving such feedbacks may vary in detail,
their broad predictions about the atmosphere's radiative balance should be
given serious consideration as a possible explanation of past greenhouse
warmth. This motivates research on the effects of the extremely low
temperature gradients ensuing from enhanced high-latitude greenhouse warmth
on the world ocean, which is the central focus of this paper.
Classical geostrophic scaling
suggests a linear relationship between the meridional density gradient and
the ocean's meridional overturning strength. Furthermore, a relationship of
this kind is also maintained in subsequent friction-based scaling arguments
and formulas , and has been
found in ocean models e.g..
There are two proposed driving mechanisms of the present Atlantic Meridional
Overturning Circulation , involving a balance between
deep sinking and low-latitude diapycnal upwelling driven by downward
turbulent diffusion of heat, or wind-driven upwelling in the Southern Ocean
associated with the absence of a Drake Passage gap . The
choice of numerical model parameters could influence the relative importance
of each mechanism, and potentially the overturning response to temperature
gradients. However, show that meridional density
differences set the overturning strength in both scenarios. Density gradients
remain relatively constant in our experiments, suggesting robustness of our
results. The absence of a deep Drake Passage gap in our experiments suggests
the importance of diapycnal upwelling as a deep water removal mechanism.
At higher temperatures, temperature is the dominant factor in setting density
gradients. A reduced meridional temperature gradient could therefore be
expected to yield a reduced density gradient, and therefore, based on the
scaling arguments in the literature cited above, the more “sluggish” ocean
overturning that is sometimes informally invoked for the past greenhouse
climates. Here, by reduced overturning we mean an overall average reduction
over longer timescales in a relatively steady climate and in the absence of
sharp perturbations on decadal to centennial timescales.
The possible effect of low temperature gradients on the ocean's overturning
strength is therefore important to study. Furthermore, modelling low-gradient
greenhouse climates allows insight into how the overturning might behave in
future climate scenarios. In this paper we simulate a Cretaceous climate
under an enhanced extratropical greenhouse effect in an Earth system model.
We choose a Cretaceous model geography as it represents a period deep within
the greenhouse climate timeframe, yet similar to the Eocene geography in that
the continents are recognisable and both characterised by closed or narrow
Southern Ocean gateways. As such, our results are not confined to the
Turonian, but serve to illustrate a wider timeframe of greenhouse climates.
We find that the meridional overturning circulation remains surprisingly
robust with respect to these changes in climatic ocean forcing, including a reduction of the oceanic wind stress.
Model and experimental design
We use a modified version of the intermediate complexity coupled model
described in detail in , the so-called UVic model. This model
allows suits a process-based approach, allowing simulations over thousands of model
years. Furthermore, by controlling the atmospheric responses, it allows us to
tease out the oceanic response to an altered atmospheric radiative balance
associated with an enhanced extratropical greenhouse effect.
The model consists of an ocean general circulation model GFDL MOM
Version 2.2 coupled to a simplified one-layer
energy–moisture balance model for the atmosphere and a dynamic–thermodynamic
sea-ice model based on that of . Air–sea heat and
freshwater fluxes evolve freely in the model, while a non-interactive wind
field is employed. The turbulent kinetic energy scheme of
based on represents vertical mixing due to wind and vertical
velocity shear, as well as buoyancy. More information about the technical
modifications to the model can be found in . The model is
the Cretaceous UVic model used in , where geography
represents a time slice around 90 Ma of the Cretaceous Turonian
period (90–94 Ma), as shown in Fig. . Wind stress is taken
from a GENESIS model simulation described in , with
identical continental configuration and elevated CO2. The present
model differs from that in in terms of the cloud albedo.
This prescribed field is described below, and has been made symmetrical
around the Equator to remove the present-day bias that exists in the standard
UVic model, and has been smoothed.
Average sea surface temperature (SST, ∘C) for the simulation
with altered radiative balance, LOWGRAD, in (a) January, February,
March, (b) June, July, August and (c) SST difference
LOWGRAD – CNTRL. Model geography is that of the Turonian period
(90–94 Ma) of the Cretaceous.
The standard simulation, named CNTRL, is similar to the Cretaceous UVic model
described in . To examine the effect of a strongly reduced
meridional temperature gradient arising from a polar enhancement of the
greenhouse effect, we have run a modified version of the control case to
equilibrium, LOWGRAD, where the outgoing longwave radiation has been scaled
so as to yield an enhanced meridional profile, as shown in Fig. b.
This effectively models an enhanced greenhouse effect at high latitudes,
where outgoing longwave radiation from the top of the atmosphere is reduced,
thus appearing colder from space. Vice versa, the increased outgoing longwave
radiation at low latitudes implies a reduced greenhouse effect there. This is
to achieve a low meridional temperature gradient, to examine its effect on
the global ocean. The third simulation, LOWGRADWIND, is identical to LOWGRAD,
but with oceanic wind stress reduced by a factor of half. Atmospheric albedo is
constant across the experiments. Table lists the three
simulations. All simulations have been run in excess of 6000 model years to
reach a stable climate equilibrium.
Description of numerical equilibrium simulations.
NameExplanationCNTRLThe control simulationLOWGRADEnhanced polar greenhouse effect, as shown in Fig. aLOWGRADWINDAs LOWGRAD, but with ocean wind stress reduced by halfResults
Figure shows boreal winter (Fig. a) and summer
(Fig. b) SSTs in our modified simulation
LOWGRAD. Arctic ocean temperatures remain above 12 ∘C in winter and
above 16 ∘C in summer. Summer SST of the order of 20–24 ∘C
are attained at the Antarctic margin, with a localised drop to 18 ∘C
in the proto-Weddell Sea. Winter temperatures are generally around
16–18 ∘C, and lowest temperatures are attained near the Antarctic
margin in the proto-Weddell sea, just east of the southern tip of South
America. Here, winter temperatures drop to close to 14 ∘C. In
contrast, we will see that deep ocean temperatures are around 17 ∘C,
and therefore deep sinking must take place elsewhere. Indeed, analysis of
ocean convection shows that the deep sinking site is located at the Antarctic
margin in the Pacific sector of the Southern Ocean (Fig. ). The
deep ocean temperature is consistent with the winter surface temperature of
this region (Fig. b).
The MOC in the CNTRL simulation is very similar to that discussed in
, with a strong deep global overturning cell originating
at the Antarctic surface of 39.0 Sv (Sverdrup, one Sv is
106 m3 s-1) and weak shallow overturning in the Northern
Hemisphere of 10.1 Sv, as shown in Fig. a. This
overturning remains remarkably similar in LOWGRAD (Fig. b), with
a small increase in southern sinking strength of 43.0 Sv, and
a reduction in the shallow northern cell, despite the significant reduction
in meridional density gradient (Fig. f). Again, the southern sinking
cell in LOWGRADWIND is stronger than in CNTRL, 41.7 Sv, although now
the northern cell is eliminated (Fig. c). This suggests that the
shallow northern overturning cell in CNTRL is driven by both the wind and
meridional density gradients.
Figure d shows that the meridional SST gradient is significantly
reduced in the model upon the enhancement of the greenhouse effect at high
latitudes in LOWGRAD compared to the control case. Annually averaged SST of
around 20 ∘C is now attained around Antarctica, and the coldest
annually and zonally averaged Arctic temperatures are around 15 ∘C.
Tropical SST remains around 32–33 ∘C, due to the prescribed
increase in outgoing longwave radiation there, in accordance with our aim of
achieving a lower temperature gradient and not overheating the Tropics. The
reduced wind stress simulation LOWGRADWIND yields a very similar temperature
profile to LOWGRAD. In contrast to SST, sea surface salinity (SSS) remains
very similar for CNTRL and LOWGRAD, whereas reducing the winds in LOWGRADWIND
yields overall fresher values, particularly in the Northern Hemisphere
(Fig. e). As can be expected from the reduced SST gradient and the
relatively unchanged SSS gradients, sea surface density is significantly
reduced in LOWGRAD, and LOWGRADWIND, compared to CNTRL. With SSS gradients
relatively unchanged, this is due to the reduced SST gradient in the LOWGRAD
and LOWGRADWIND experiments.
Maximum convection depth in km in (a) CNTRL and
(b) LOWGRAD.
Global meridional overturning circulation streamfunction for
(a) the control case CNTRL, with colour plot representing the
streamfunction (b) the low temperature gradient case, LOWGRAD, with
colour plot representing LOWGRAD–CNTRL streamfunction difference and
(c) the low temperature and weak wind case, LOWGRADWIND, with colour
plot representing LOWGRADWIND–CNTRL streamfunction difference. Values are
given in Sv (1 Sv =106 m3 s-1). The strength of
the Southern Hemisphere overturning cell is given in parentheses in the captions.
Global zonal mean of (a) atmospheric albedo (fraction)
(b) outgoing longwave radiation (W m-2), (c) oceanic
poleward heat transport (petawatts), (d) sea surface
temperature (SST, ∘C), (e) sea surface salinity (SSS,
kg m-3) and (f) ocean potential density
(kg m-3) for the control case CNTRL (blue), the low temperature
gradient case LOWGRAD (green), and the low temperature and weak wind case
LOWGRADWIND (red). Bottom row as (f) at depths (g) 500 m
and (h) 1000 m.
The reduced meridional temperature gradient in LOWGRAD leads to a significant
reduction in oceanic poleward heat transport (PHT, Fig. c) compared
to CNTRL in both hemispheres. This is to be expected, as now the gyre
circulation and the overturning circulation work across a lower temperature
gradient. This is because the heat transport should be proportional to the
gyre strength and the temperature difference between the cool and warm
branches of these circulation patterns. Reduction in wind stress in
LOWGRADWIND leads to a further significant reduction in oceanic poleward heat
transport (compared to LOWGRAD) only in the Northern Hemisphere, where values
now become very small compared to the Southern Hemisphere. This is because
here the gyre strength is significantly reduced, further reducing heat
transport. Furthermore, the collapse of the weak shallow Northern Hemisphere
overturning cell that is driven by density gradients and winds (as discussed
above, Fig. ) also contributes to reduced heat transport. The
Southern Hemisphere is significantly less sensitive to the effects of
reducing the meridional temperature gradient and the wind stress because heat
transport there is dominated by the large Southern Hemisphere sinking cell,
and this cell is insensitive to the changes in wind and temperature. This
suggests that poleward heat transport in past greenhouse climates was
dominated by the ocean's overturning circulation.
Vertical profiles of global horizontal mean in ocean of
(a) temperature (∘C), (b) salinity (kg m-3) and
(c) ocean potential density (kg m-3) for the control case
CNTRL (blue), the low temperature gradient case LOWGRAD (green), and the low
temperature and weak wind case LOWGRADWIND (red).
Figure a shows that the vertical ocean potential temperature
gradient is significantly smaller in the model under the enhanced greenhouse
effect at high latitudes in LOWGRAD compared to the control case CNTRL. This
illustrates how the meridional surface temperature gradient maps into the
deep ocean. Deep ocean temperatures are around 9 ∘C in CNTRL,
whereas they are around 17 ∘C in LOWGRAD, corresponding to the very
mild winter temperatures at the simulated deep sinking regions at the Pacific
sector of the Antarctic coast. The warm deep ocean temperatures in the
LOWGRAD simulation are in good agreement with the early Turonian deep water
temperature estimates of 19 ∘C of .
Reducing the wind stress, as in LOWGRADWIND, does not lead to a significant
change in temperature profile. The vertical salinity gradients are somewhat
reduced in LOWGRAD compared to CNTRL (Fig. b), and more strongly
reduced in LOWGRADWIND. This indicates that the overall fresher SSS values in
LOWGRADWIND (Fig. e) arise from a general relocation of salt from
the shallow ocean to the deep ocean there. The vertical density gradient is
also reduced in LOWGRAD with respect to CNTRL (Fig. c), with
a vertical density difference between the top and bottom layer of
4.4 kg m-3 in CNTRL, and 3.7 kg m-3 in LOWGRAD. This
indicates an overall less stratified ocean when meridional temperature
gradients are reduced.
The overturning circulation pattern can also be recognised in the global
vertical profile of the meridional velocity south of 30∘ N, shown in
Fig. a. The velocity profile is very similar for CNTRL and LOWGRAD,
yet differs near the surface in LOWGRADWIND where the average velocity is
somewhat more southward. Examination of the overturning streamfunction
anomaly in Fig. c (colour background) indicates that this is related
to the reduction in the shallow northern cell. Meridional velocities cancel
around 2000 m depth in all three simulations. This depth lies in
between an upper southward moving branch and a lower northward moving branch,
in agreement with the meridional overturning streamfunction (Fig. ).
This depth range coincides with a local minimum in kinetic energy. Kinetic energy does not assume values close to zero at
this minimum, indicating that the cancellation of meridional velocities in
the horizontal average in part masks a horizontal gyre circulation.
Vertical depth profiles of horizontal mean (south of 30∘ N)
of (a) meridional velocity u (10-4 m s-1),
(b) latitudinal (y) derivative of density ∂ρ∂y
(kg m-4) and (c) the buoyancy
frequency N=-gρ0∂ρ∂z (in
units s-1) for the control case CNTRL (blue), the low temperature
gradient case LOWGRAD (green), and the low temperature and weak wind case
LOWGRADWIND (red). Bottom labels indicate units.
Comparison with data inferred from proxies
Subpolar Atlantic summer temperatures around 60∘ S are between
20 and 24∘ C in LOWGRAD (Fig. a). This is in
agreement with similar estimates by based on oxygen
isotope data for the Falkland Plateau, around 60∘ S palaeolatitude,
for the mid-Turonian, and lower than the 30–32 ∘C they reported for
a warm excursion during the late Turonian. The good deep ocean temperature
agreement with proxies (see below) indicates that our modification of the
atmospheric radiative balance yields a good simulation of polar surface
temperatures at the southern source regions. Equatorial temperatures are
around 32–33 ∘C for LOWGRAD and LOWGRADWIND in the zonal mean
(Fig. d). A localised warm pool in the Tethys, where temperatures
reach 36 ∘C (Fig. 1b), constitutes a significant departure from
the zonal mean. This is in agreement with the high tropical temperatures of
33–34 ∘C found by , and .
We intend to illustrate a wider timeframe of greenhouse climates, and will
therefore briefly discuss the Eocene. Southwest Pacific SSTs increased to
∼ 32 ∘C during the early Eocene (∼ 53 Ma ago), and decreased
to ∼ 21 ∘C by the late Eocene ∼ 36 Ma
ago. This later cooler Eocene temperature is close to our
Cretaceous simulation LOWGRAD in the Southwest Pacific. Tropical conditions
(around 25 to 30 ∘C SSTs) may have prevailed at the Canterbury Basin
(55∘ S palaeolatitude) from the late–early to early–middle Eocene
(50.7–46.5 Ma) according to , although uncertainties related
to TEX86 suggest peak temperatures at site 1172 may have been 28 ∘C
. present palaeotemperature
records that indicate that the Seymour Island middle and late Eocene SSTs
ranged between 10 to 17 ∘C. This is significantly cooler than the
temperatures found elsewhere for similar sub-polar latitudes. These findings
point to a significantly reduced meridional temperature gradient during the
Cretaceous and Eocene.
We find southern sinking in our simulations. Although it is difficult to
infer meridional overturning circulation polarity for the Turonian,
argue that more homogeneous deep ocean conditions before
the late Eocene arose from deep sinking in both hemispheres, and that a
period of enhanced heterogeneity during the late Cretaceous may have arisen
from changes in the ability of winds to make water in the deep ocean
circulate via a Tethyan circum-equatorial current that might have functioned
in analogous fashion to the Antarctic Circumpolar Current . For the Eocene,
, , , and
find proxy evidence for deep ocean ventilation from the south. A further
discussion of the warm polar conditions during the greenhouse climates,
including terrestrial proxies, can be found in .
Discussion and conclusion
showed that the deep ocean meridional density gradients at
the western boundaries determine the meridional flow in the deep western
boundary current responsible for the main meridional volume transport of each
ocean basin. The depth of the density gradient is taken inside the depth
range of the deep branch of the meridional overturning cell.
build on this finding, showing that the volume transport in the Atlantic
meridional overturning cell that crosses the Equator depends on the deep
buoyancy distribution, and provide a theoretical underpinning based on
a level of no motion between the upper and lower overturning branches.
Namely, meridional flow in the deep western boundary current of the North
Atlantic Deep Water outflow depends only on meridional density gradients
below the intermediate depth interface between the North Atlantic Deep Water
and the Antarctic Intermediate Water masses, and not above, due to vanishing
horizontal pressure gradients near the interface.
The analogue to the quiescent zone inside the overturning cell in our
Cretaceous model is the interface between the upper and lower branches of the
large southern sinking cell, at around 2000 m depth. Although
meridional density gradients (Fig. b) differ between CNTRL and
LOWGRAD at the surface (see also Fig. f), they are remarkably
similar below 500 m depth. According to and
, it is the deep density structure that sets the strength of
the deep overturning branch, and therefore the southern sinking cell. As
a result, the overturning circulation (Fig. ) and the oceanic
meridional velocity (Fig. a) remain remarkably similar when the
meridional SST gradient is strongly reduced due to an extratropical
enhancement of the greenhouse effect. In contrast, the upper branch of the
MOC is not primarily driven by the density gradients, as surface steric
height gradients become important there. Therefore, we do not link changes in
the upper branch of the MOC to changes in density gradients. Of course in the
hypothetical case where the SST gradient is entirely absent and salinity
effects are weak, overturning is expected to be absent. Nonetheless, the SST
gradients appropriate for the past greenhouse climates are not sufficiently
low to yield reduced overturning.
In today's ocean, the sub-thermocline ocean is filled with water sourced from
three climatically distinct regions: namely, the North Atlantic Deep Water,
the Antarctic Intermediate Water and the Antarctic Bottom Water formation
regions. This yields three corresponding vertically stacked water masses that
can be found across the globe, providing the deep ocean density differences
that drive flow there. Indeed, showed that it is the density
difference between Antarctic Intermediate Water and the underlying North
Atlantic Deep Water in the tropical Atlantic that drives the conveyor belt
circulation. In sharp contrast, the deep ocean is filled from only one source
in the Southern Ocean in our Cretaceous experiments. This is similar to the
simulation of where the Drake Passage is closed in the
present-day geography, and a large Southern Hemisphere cell fills the deep
ocean, leading to a sharp vertical density gradient in the thermocline, and
a more homogeneous deep ocean. In our experiments here, we also find that the
single confined surface origin of deep water leads to a relatively homogeneous
body of water masses below the thermocline. This is apparent from the
vertical density profile (Fig. c), and the vertical profile of the
buoyancy frequency N=-gρ0∂ρ∂z
shown in Fig. c. Only diapycnal diffusivity leads to small
density gradients within this water mass, driving the ocean currents. As
a result, the deep ocean density gradients remain remarkably similar across
the experiments, as shown by the vertical profiles of the meridional density
gradient (Fig. b) and the buoyancy frequency (Fig. c),
and zonal means of density (Fig. g and h). This is because the
deep water source region warms, and this leads to a uniform warming of the
deep ocean as there are no other source regions of note, and density
gradients remain relatively unchanged as they depend on isopycnal diffusivity.
note the contradiction of weak meridional temperature
gradients demanding increased PHT, while simultaneously implying weak
PHT. They show this to be a mere paradox, as extratropical radiative
balance responses to heat transport from low latitudes may warm the high
latitudes without cooling the Tropics. In our experiments, we do not examine
the mechanisms behind the altered radiative balance, and do not distinguish
between the mechanisms of or , or the
warming-enhancing cloud mechanism at play in the simulations of
. Instead, we have prescribed an altered radiative balance,
of a similar nature to that implied via these mechanisms, by enhancing the
extratropical greenhouse effect, and reducing it in the Tropics. As a result,
heat transport is reduced while the meridional temperature gradient is also
smaller, illustrating the paradox of .
In summary, a substantial reduction of the pole-to-Equator temperature
difference leaves the meridional overturning circulation relatively
unaffected in our Cretaceous model. Reducing the wind strength also has
remarkably little effect on the overturning. This is because overturning
strength depends on deep ocean density gradients, which remain relatively
unaffected by the applied radiative changes, despite an overall decrease in
ocean density. Ocean poleward heat transport is reduced, as now the
circulation operates across a reduced temperature gradient. These results
indicate that models of the greenhouse climate during the Cretaceous and
early Paleogene may yield a reasonable overturning circulation, despite
failing to fully reproduce the extremely reduced temperature gradients of
those time periods.
Acknowledgements
We thank the University of Victoria staff for support in usage of their
coupled climate model. We thank Sascha Floegel for the pleasant collaboration
leading to the Cretaceous model. This research was undertaken with the
assistance of resources provided at the Katana computation cluster at the
UNSW Faculty of Science. This project was supported by the Australian
Research Council, including support from an ARC Laureate Fellowship
(FL100 100 214) and the ARC Centre of Excellence for Climate System Science
(CE110 001 028).
Edited by: A. Haywood
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