Changes In Meridional Heat Transport Research Articles

Meridional Heat Transport in the DeepMIP Eocene Ensemble: Non‐CO2 and CO2 Effects

AbstractThe total meridional heat transport (MHT) is relatively stable across different climates. Nevertheless, the strength of individual processes contributing to the total transport are not stable. Here we investigate the MHT and its main components especially in the atmosphere, in five coupled climate model simulations from the Deep‐Time Model Intercomparison Project (DeepMIP). These simulations target the early Eocene climatic optimum, a geological time period with high CO2 concentrations, analog to the upper range of end‐of‐century CO2 projections. Preindustrial and early Eocene simulations, at a range of CO2 levels are used to quantify the MHT changes in response to both CO2 and non‐CO2 related forcings. We found that atmospheric poleward heat transport increases with CO2, while oceanic poleward heat transport decreases. The non‐CO2 boundary conditions cause more MHT toward the South Pole, mainly through an increase in the southward oceanic heat transport. The changes in paleogeography increase the heat transport via transient eddies at the northern mid‐latitudes in the Eocene. The Eocene Hadley cells do not transport more heat poleward, but due to the warmer atmosphere, especially the northern cell, circulate more heat in the tropics, than today. The monsoon systems' poleward latent heat transport increases with rising CO2 concentrations, but this change is counterweighted by the globally smaller Eocene monsoon area. Our results show that the changes in the monsoon systems' latent heat transport is a robust feature of CO2 warming, which is in line with the currently observed precipitation increase of present day monsoon systems.

Seasonal temperatures in West Antarctica during the Holocene

The recovery of long-term climate proxy records with seasonal resolution is rare because of natural smoothing processes, discontinuities and limitations in measurement resolution. Yet insolation forcing, a primary driver of multimillennial-scale climate change, acts through seasonal variations with direct impacts on seasonal climate1. Whether the sensitivity of seasonal climate to insolation matches theoretical predictions has not been assessed over long timescales. Here, we analyse a continuous record of water-isotope ratios from the West Antarctic Ice Sheet Divide ice core to reveal summer and winter temperature changes through the last 11,000 years. Summer temperatures in West Antarctica increased through the early-to-mid-Holocene, reached a peak 4,100 years ago and then decreased to the present. Climate model simulations show that these variations primarily reflect changes in maximum summer insolation, confirming the general connection between seasonal insolation and warming and demonstrating the importance of insolation intensity rather than seasonally integrated insolation or season duration2,3. Winter temperatures varied less overall, consistent with predictions from insolation forcing, but also fluctuated in the early Holocene, probably owing to changes in meridional heat transport. The magnitudes of summer and winter temperature changes constrain the lowering of the West Antarctic Ice Sheet surface since the early Holocene to less than 162 m and probably less than 58 m, consistent with geological constraints elsewhere in West Antarctica4–7.

Synergy of In Situ and Satellite Ocean Observations in Determining Meridional Heat Transport in the Atlantic Ocean

AbstractThe meridional overturning circulation (MOC) is a major driver in global redistribution of heat, which modulates global climate and weather. Altimeter sea surface height measurements and in situ hydrographic data are combined in this work to derive monthly synthetic temperature and salinity (T/S) profiles along zonal transbasin sections in both the North (26.5°N) and South (20°S, 25°S, 30°S, and 35°S) Atlantic Ocean since 1993, which are then used to estimate the MOC and meridional heat transport (MHT). Consistent with previous studies, the results indicate that the MHT is highly correlated with the MOC at all five latitudes. At 26.5°N the mean MHT from synthetic profiles (1.09 ± 0.21 PW) is slightly lower than that from the RAPID‐MOCHA‐WBTS array (1.20 ± 0.28 PW), but their interannual variabilities show good agreements. Both the geostrophic and Ekman components contribute significantly to the MHT variability, with the geostrophic component dominating during 1993–2004 and the Ekman component dominating during 2005–2014. In the South Atlantic, the MHT seasonal cycle strengthens southward from 20°S to 34.5°S. On interannual time scale, the MHT at 20°S and 25°S experiences larger variations than that at 30°S and 34.5°S, in part due to the fact that the geostrophic and Ekman components work together to strengthen the MHT changes, while they tend to work against each other at 30°S and 34.5°S. Results shown here suggest that integrating data from different observing platforms provides better means to estimate the MOC and MHT in near real time.

Large‐scale ocean circulation‐cloud interactions reduce the pace of transient climate change

AbstractChanges to the large‐scale oceanic circulation are thought to slow the pace of transient climate change due, in part, to their influence on radiative feedbacks. Here we evaluate the interactions between CO2‐forced perturbations to the large‐scale ocean circulation and the radiative cloud feedback in a climate model. Both the change of the ocean circulation and the radiative cloud feedback strongly influence the magnitude and spatial pattern of surface and ocean warming. Changes in the ocean circulation reduce the amount of transient global warming caused by the radiative cloud feedback by helping to maintain low cloud coverage in the face of global warming. The radiative cloud feedback is key in affecting atmospheric meridional heat transport changes and is the dominant radiative feedback mechanism that responds to ocean circulation change. Uncertainty in the simulated ocean circulation changes due to CO2 forcing may contribute a large share of the spread in the radiative cloud feedback among climate models.

Inter-hemispheric asymmetry in the sea-ice response to volcanic forcing simulated by MPI-ESM (COSMOS-Mill)

Abstract. The decadal evolution of Arctic and Antarctic sea ice following strong volcanic eruptions is investigated in four climate simulation ensembles performed with the COSMOS-Mill version of the Max Planck Institute Earth System Model. The ensembles differ in the magnitude of the imposed volcanic perturbations, with sizes representative of historical tropical eruptions (1991 Pinatubo and 1815 Tambora) and of tropical and extra-tropical "supervolcano" eruptions. A post-eruption Arctic sea-ice expansion is robustly detected in all ensembles, while Antarctic sea ice responds only to supervolcano eruptions, undergoing an initial short-lived expansion and a subsequent prolonged contraction phase. Strong volcanic forcing therefore emerges as a potential source of inter-hemispheric interannual-to-decadal climate variability, although the inter-hemispheric signature is weak in the case of eruptions comparable to historical eruptions. The post-eruption inter-hemispheric decadal asymmetry in sea ice is interpreted as a consequence mainly of the different exposure of Arctic and Antarctic regional climates to induced meridional heat transport changes and of dominating local feedbacks that set in within the Antarctic region. Supervolcano experiments help to clarify differences in simulated hemispheric internal dynamics related to imposed negative net radiative imbalances, including the relative importance of the thermal and dynamical components of the sea-ice response. Supervolcano experiments could therefore serve the assessment of climate models' behavior under strong external forcing conditions and, consequently, favor advancements in our understanding of simulated sea-ice dynamics.

Consistent Differences in Climate Feedbacks between Atmosphere–Ocean GCMs and Atmospheric GCMs with Slab-Ocean Models*

Abstract Climate sensitivity is generally studied using two types of models. Atmosphere–ocean general circulation models (AOGCMs) include interactive ocean dynamics and detailed heat uptake. Atmospheric GCMs (AGCMs) with slab ocean models (SOMs) cannot fully simulate the ocean’s response to and influence on climate. However, AGCMs are computationally cheaper and thus are often used to quantify and understand climate feedbacks and sensitivity. Here, physical climate feedbacks are compared between AOGCMs and SOM-AGCMs from the Coupled Model Intercomparison Project phase 3 (CMIP3) using the radiative kernel technique. Both the global-average (positive) water vapor and (negative) lapse-rate feedbacks are consistently stronger in AOGCMs. Water vapor feedback differences result from an essentially constant relative humidity and peak in the tropics, where temperature changes are larger for AOGCMs. Differences in lapse-rate feedbacks extend to midlatitudes and correspond to a larger ratio of tropical- to global-average temperature changes. Global-average surface albedo feedbacks are similar between models types because of a near cancellation of Arctic and Antarctic differences. In AOGCMs, the northern high latitudes warm faster than the southern latitudes, resulting in interhemispheric differences in albedo, water vapor, and lapse-rate feedbacks lacking in the SOM-AGCMs. Meridional heat transport changes also depend on the model type, although there is a large intermodel spread. However, there are no consistent global or zonal differences in cloud feedbacks. Effects of the forcing scenario [Special Report on Emissions Scenarios A1B (SRESa1b) or the 1% CO2 increase per year to doubling (1%to2x) experiments] on feedbacks are model dependent and generally of lesser importance than the model type. Care should be taken when using SOM-AGCMs to understand AOGCM feedback behavior.

Decadal‐scale changes in meridional heat transport across 24°N in the Pacific Ocean

We investigated the long‐term variability of the meridional heat transport across 24°N in the Pacific Ocean in two ways: (1) by adding the surface net heat flux to the temporal change rate of the ocean heat content (OHC) north of 24°N and (2) by integrating the product of meridional velocity and potential temperature over the 24°N section in numerical models. Decadal‐scale changes with amplitudes of approximately 0.1–0.2 PW were detected consistently in the 5‐year running means of the meridional heat transport in the 1980s and 1990s from these different analyses. The heat transport increased during the 1980s, which is closely linked with the change in the OHC. While both the Ekman and Kuroshio transports were responsible for the change in the 1980s, the contribution of the former was dominant. On the other hand, the variation in the Kuroshio temperature transport dominated meridional heat transport in the 1990s. The air‐sea net heat fluxes of the atmospheric reanalysis data sets integrated north of 24°N increased in the 1990s, which indicates that the increase of the lateral heat transport was distributed mainly to the atmosphere, particularly in the Kuroshio and Kuroshio‐Oyashio Extension (KOE) regions. However, there are also large discrepancies in the surface heat flux among the data sets. To reveal the impact of the decadal‐scale changes in meridional heat transport on the atmosphere, it is crucial to obtain more accurate values for the air‐sea heat exchange and OHC by using a sustainable observing system and numerical models.

Fish tooth δ18O revising Late Cretaceous meridional upper ocean water temperature gradients

The oxygen isotope composition of fossil fish teeth, a paleo–upper ocean temperature proxy exceptionally resistant to diagenetic alteration, provides new insight on the evolution of the low- to middle-latitude thermal gradient between the middle Cretaceous climatic optimum and the cooler latest Cretaceous period. The new middle Cretaceous low to middle latitude thermal gradient agrees with that previously inferred from planktonic foraminifera δ 18 O recovered from Deep Sea Drilling Project and Ocean Drilling Program drilling sites, although the isotopic temperatures derived from δ 18 O of fish teeth are uniformly higher by ∼3–4 °C. In contrast, our new latest Cretaceous thermal gradient is markedly steeper than those previously published for this period. Fish tooth δ 18 O data demonstrate that low- to middle-latitude thermal gradients of the middle Cretaceous climatic optimum and of the cooler latest Cretaceous are similar to the modern one, despite a cooling of 7 °C between the two periods. Our new results imply that no drastic changes in meridional heat transport are required to explain the Late Cretaceous climate. Based on climate models, such a cooling without any change in the low to mid- decrease dle latitude thermal gradient supports an atmospheric CO 2 as the primary driver of the climatic evolution recorded during the Late Cretaceous.

An Estimation of the Climatic Effects of Stratospheric Ozone Losses during the 1980s

In order to study the potential climatic effects of the ozone hole more directly and to assess the validity of previous lower resolution model results, the latest high spatial resolution version of the Atmospheric and Environmental Research, Inc., seasonal radiative dynamical climate model is used to simulate the climatic effects of ozone changes relative to the other greenhouse gases. The steady-state climatic effect of a sustained decrease in lower stratospheric ozone, similar in magnitude to the observed 1979-90 decrease, is estimated by comparing three steady-state climate simulations: 1) 1979 greenhouse gas concentrations and 1979 ozone, II) 1990 greenhouse gas concentrations with 1979 ozone, and III) 1990 greenhouse gas concentrations with 1990 ozone. The simulated increase in surface air temperature resulting from nonozone greenhouse gases is 0.272 K. When changes in lower stratospheric ozone are included, the greenhouse warming is 0.165 K, which is approximately 39% lower than when ozone is fixed at the 1979 concentrations. Ozone perturbations at high latitudes result in a cooling of the surface-troposphere system that is greater (by a factor of 2.8) than that estimated from the change in radiative forcing resulting from ozone depiction and the model's 2 x CO, climate sensitivity. The results suggest that changes in meridional heat transport from low to high latitudes combined with the decrease in the infrared opacity of the lower stratosphere are very important in determining the steady-state response to high latitude ozone losses. The 39% compensation in greenhouse warming resulting from lower stratospheric ozone losses is also larger than the 28% compensation simulated previously by the lower resolution model. The higher resolution model is able to resolve the high latitude features of the assumed ozone perturbation, which are important in determining the overall climate sensitivity to these perturbations.

Synchronous, high‐frequency oscillations in tropical sea surface temperatures and North Atlantic Deep Water production during the Last Glacial Cycle

Stable isotopic measurements of G. sacculifer and C. wuellerstorfi in a core from the western equatorial Atlantic imply that there are parallel, suborbital oscillations in surface water hydrography and deep water circulation occurring during oxygen isotope stages 2 and 3. Low values of G. sacculifer δ18O accompany high values of C. wuellerstorfi δ13C, linking warmer sea surface temperatures (SSTs) in the tropics with increased production of lower North Atlantic Deep Water (NADW). The amplitude of the δ18O oscillations is 0.6‰ (or 2°–3°C), which is superimposed on a glacial/interglacial amplitude of about 2.1‰. Using the G. sacculifer δ18O data, we calculate that surface waters were colder during stage 2 than calculated by CLIMAP [1976, 1981]. The longer‐period (>2 kyr) oscillations in air temperature recorded in the Greenland and Antarctic ice cores appear to correlate with oscillations in sea surface temperature in the equatorial Atlantic. The magnitude of these oscillations in tropical SST is too large to have resulted from changes in meridional heat transport caused by the global conveyor alone. The apparent synchroneity of equatorial SST and polar air temperature changes, as well as the amplitude of the SST changes at the equator, are consistent with the climate effects expected from changes in the atmosphere's greenhouse gas content (H2Ovapor, CO2, and CH4).

Ocean Response to Surface Heat Anomalies

An ocean general circulation model (OGCM) is used to study the response of ocean heat and mass transport to positive and negative heat flux anomalies at the ocean surface. As expected, tropical and low-latitude mixed layers respond rapidly (e-folding time about 50-70 years) to external forcing, while the response of the high-latitude mixed layer, especially the Southern Ocean and northern North Atlantic, is very slow (e-folding time greater than 300 yr). The overall response is faster for negative than positive heat flux anomaly at the surface. The meridional heat transport changes by 15% in the first 50 yr in the southern high latitudes. Surprisingly, for the next 400-500 yr the change is very small. The analysis shows that the meridional mass transport intensifies in response to a negative surface heat flux anomaly but weakens in response to a positive heat flux anomaly. For example, at model year 100 the North Atlantic Deep Water (NADW) is reduced from about 18 Sv to about 10 Sv for the positive heat flux experiment but increased to about 26 Sv for the negative heat flux experiment.

An Ice-Free Cretaceous? Results from Climate Model Simulations

We have quantitatively investigated the mechanisms that could explain the warm, equable climate that is believed to have been typical of the mid-Cretaceous (100 million years ago). By performing simulations with a climate model based on zonal energy balance, we demonstrate that past changes in geography were important in bringing about climatic change. However, the meridional distribution of Cretaceous temperatures cannot be successfully simulated unless additional physical "feedback mechanisms" are included in the model. These mechanisms may involve cloud and meridional heat transport changes. We also conclude that paleoclimatologists should reexamine carefully both existing data and their interpretations with regard to reconstruction of Cretaceous tropical and polar surface temperatures.