Guido Vettoretti

Investigating the Causes of the Response of the Thermohaline Circulation to Past and Future Climate Changes

The Atlantic thermohaline circulation (THC) is an important part of the earth’s climate system. Previous research has shown large uncertainties in simulating future changes in this critical system. The simulated THC response to idealized freshwater perturbations and the associated climate changes have been intercompared as an activity of World Climate Research Program (WCRP) Coupled Model Intercomparison Project/Paleo-Modeling Intercomparison Project (CMIP/PMIP) committees. This intercomparison among models ranging from the earth system models of intermediate complexity (EMICs) to the fully coupled atmosphere–ocean general circulation models (AOGCMs) seeks to document and improve understanding of the causes of the wide variations in the modeled THC response. The robustness of particular simulation features has been evaluated across the model results. In response to 0.1-S v( 1 Sv 10 6 m 3 s 1 ) freshwater input in the northern North Atlantic, the multimodel ensemble mean THC weakens by 30% after 100 yr. All models simulate some weakening of the THC, but no model simulates a complete shutdown of the THC. The multimodel ensemble indicates that the surface air temperature could present a complex anomaly pattern with cooling south of Greenland and warming over the Barents and Nordic Seas. The Atlantic ITCZ tends to shift southward. In response to 1.0-Sv freshwater input, the THC switches off rapidly in all model simulations. A large cooling occurs over the North Atlantic. The annual mean Atlantic ITCZ moves into the Southern Hemisphere. Models disagree in terms of the reversibility of the THC after its shutdown. In general, the EMICs and AOGCMs obtain similar THC responses and climate changes with more pronounced and sharper patterns in the AOGCMs.

Monsoon changes for 6000 years ago: Results of 18 simulations from the Paleoclimate Modeling Intercomparison Project (PMIP)

Amplification of the northern hemisphere seasonal cycle of insolation during the mid-Holocene causes a northward shift of the main regions of monsoon precipitation over Africa and India in all 18 simulations conducted for the Paleoclimate Modeling Intercomparison Project (PMIP). Differences among simulations are related to differences in model formulation. Despite qualitative agreement with paleoecological estimates of biome shifts, the magnitude of the monsoon increases over northern Africa are underestimated by all the models.

Mid-Holocene NAO: A PMIP2 model intercomparison

[1] The mid-Holocene (6000 years before present) North Atlantic Oscillation (NAO) from nine models in the Paleoclimate Modeling Intercomparison Project Phase 2 is studied, primarily through principal component analysis of winter time North Atlantic sea level pressure (SLP). Modeled mid-Holocene NAO and mean SLP show small changes compared to pre-industrial control runs, with a shift in mean state towards a more positive NAO regime for three of the models. Modeled NAO variability shows little change, with a small increase for some models in the fraction of time spent in the NAO-negative phase during the mid-Holocene. Proxy based reconstructions of the NAO indicate a more positive NAO regime compared to present day during the mid- Holocene. We hypothesise that there was a small NAO+ like shift in mean state during the mid-Holocene.

Dansgaard‐Oeschger oscillations predicted in a comprehensive model of glacial climate: A “kicked” salt oscillator in the Atlantic

During the period from 60,000 to 35,000 years ago, Summit-Greenland ice core records of the oxygen isotopic ratio 18O/16O exhibit intense millennium time scale oscillations. These Dansgaard-Oeschger oscillations have been interpreted to represent the variations in North Atlantic air temperature caused by correlative changes in the strength of North Atlantic Deep Water production. We apply a comprehensive model of glacial climate to unambiguously identify the mechanism responsible for this phenomenon. This is shown to involve a salt oscillation of relaxation oscillator form. This nonlinear oscillation does not require the existence of feedback due to freshwater release from grounded ice on the continents during the warm phase of the cycle.

Simulations of Mid-Holocene Climate Using an Atmospheric General Circulation Model

The authors describe a first paleoclimatological application of the Canadian Centre for Climate Modelling and Analysis atmospheric general circulation model (AGCM) to simulate the climate state 6000 calendar years before present (6 kyr BP). Climate reconstructions for this period are performed with both fixed SSTs and with the AGCM coupled to mixed layer ocean and thermodynamic sea‐ice modules. The most important difference between this epoch and the present involves the increased surface heating and cooling of the continental land masses in the Northern Hemisphere during summer and winter, respectively, which are a consequence of the modified orbital configuration. A comparison of a fixed SST experiment with a calculated SST experiment, incorporating a thermodynamic representation of oceanic response, is performed to assess the impact on the mid-Holocene climate. The results are also contrasted with those obtained on the basis of proxy climate reconstructions during this mid-Holocene optimum period. Of interest in this calculated SST experiment is the impact on the seasonal cycle of sea‐ice distribution due to the increased insolation at high latitudes during Northern Hemisphere summer. Also important is the fact that the mixed layer ocean in the simulation is found to further enhance the monsoon circulation beyond the enhancement found to occur due to the influence of modified orbital forcing alone. This increased response is found to be a consequence of the sensitivity of tropical SST to the amplification of the seasonal cycle due to the change in insolation forcing that was characteristic of the midHolocene period.

Post-Eemian glacial inception. Part I: The impact of summer seasonal temperature bias

Abstract Post-Eemian glacial inception [the transition between marine oxygen isotopic stage (MOIS) 5e and MOIS 5d] began approximately 117 000 years before present (117 kyr BP) and led to significant Northern Hemisphere glaciation within the ensuing 5000 yr. Previous sensitivity studies with atmospheric general circulation models (AGCMs) have had difficulty producing glacial nucleation in high northern latitude regions of the globe. A base simulation of this process has been conducted using the Canadian Centre for Climate Modelling and Analysis (CCCma) GCMII with mixed layer slab ocean model constrained so as to ensure that the model reproduces the set of Atmospheric Model Intercomparison Project 2 (AMIP2) modern sea surface temperatures (SSTs) under conditions of modern radiative forcing. This simulation demonstrates that entry into glacial conditions at 116 kyr BP requires only the introduction of post-Eemian orbital insolation and standard preindustrial CO2 concentrations. Two additional sensitivity ex...

Post-Eemian Glacial Inception. Part II: Elements of a Cryospheric Moisture Pump

Abstract This paper extends the analyses of the glacial inception process described in a previous paper (“Part I: The Impact of Summer Seasonal Temperature Bias”). The analyses described therein were based upon the use of the Canadian Centre for Climate Modelling and Analysis (CCCma) GCMII. Three simulations of the modern climate system were described that were, respectively, warm biased, unbiased, and cold biased with respect to the set of Atmospheric Model Intercomparison Project 2 SSTs and land surface temperatures in summer. These three control models were perturbed by the modification of the orbital insolation regime appropriate to the time 116 000 years before present (116 kyr BP) during which the most recent period of continental glaciation began. Two of the three simulations do deliver perennial snow cover in polar latitudes. Analyses of the land surface energy balance, hydrological cycle, and energetics of the atmosphere in the Northern Hemisphere polar region at 116 kyr BP discussed in greater d...

Interhemispheric air temperature phase relationships in the nonlinear Dansgaard‐Oeschger oscillation

It has previously been suggested that the Southern Ocean might act as a thermal reservoir in mediating the relationship between the northern and southern hemisphere air temperature signals during a Dansgaard-Oeschger cycle. On the basis of a climate model-based analysis, we demonstrate on the contrary that it is equatorial and subtropical North Atlantic thermocline waters that act so as to integrate and damp the northern hemisphere signal to fix the amplitude and phase of the southern hemisphere counterpart to the more intense North Atlantic component of the oscillation. The dynamics of the Southern Ocean component of the Atlantic meridional overturning circulation are critical in explaining the evolution of the interhemispheric temperature phase relationships. European Project for Ice Coring in Antarctica Dronning Maud Land-inferred air temperature variations are in phase with sea surface temperatures north of the Antarctic sea ice edge, suggesting that it is via a fast atmospheric pathway that this temperature signal is transmitted into the Antarctic ice core record.

The impact of insolation, greenhouse gas forcing and ocean circulation changes on glacial inception

In this study we employ the NCAR CCSM3 coupled model to investigate the onset of high northern latitude perennial snow cover. Two periods of Earth’s insolation history, that of the pre-industrial period and that of 116 ka before present (BP), are used as benchmarks in an investigation of the influences of interglacial greenhouse gas (GHG) concentration and insolation upon the occurrence of permanent summer snow cover. An additional two experiments at 10 ka and 51 ka into the future (AP) using a typical interglacial GHG level are used to investigate the length of the current interglacial. Results from this set of multicentury sensitivity experiments demonstrate the relative importance of forcings due to insolation and atmospheric greenhouse gases at the millennial scale, and of Atlantic ocean overturning strength (AMOC) at the century scale. We find that while areas of perennial snow cover are sensitive to GHG concentrations, they are much more sensitive to the contemporaneous insolation regime. The goodness of fit of the climatology of the control model to the modern observed climatology is found to influence the modeling results. While there is a strong correlation between AMOC decadal variability and high latitude surface temperature in our control climates, we find little change in AMOC strength during our simulations of 116 ka BP climate nor do we find significant correlation between high latitude snow accumulation and the AMOC. Both the 10 ka AP and 51 ka AP future simulations produce inception events which are much stronger than that of the equivalent pre-industrial simulation. The simulation of inception at 10 ka into the future suggests a maximum duration of the current interglacial of approximately 20 ka in the absence of modern anthropogenic forcing.

Congo Basin precipitation: Assessing seasonality, regional interactions, and sources of moisture

Precipitation in the Congo Basin was examined using a version of the National Center for Atmospheric Research Community Earth System Model (CESM) with water tagging capability. Using regionally defined water tracers, or tags, the moisture contribution from different source regions to Congo Basin precipitation was investigated. We found that the Indian Ocean and evaporation from the Congo Basin were the dominant moisture sources and that the Atlantic Ocean was a comparatively small source of moisture. In both rainy seasons the southwestern Indian Ocean contributed about 21% of the moisture, while the recycling ratio for moisture from the Congo Basin was about 25%. Near the surface, a great deal of moisture is transported from the Atlantic into the Congo Basin, but much of this moisture is recirculated back over the Atlantic in the lower troposphere. Although the southwestern Indian Ocean is a major source of Indian Ocean moisture, it is not associated with the bulk of the variability in precipitation over the Congo Basin. In wet years, more of the precipitation in the Congo Basin is derived from Indian Ocean moisture, but the spatial distribution of the dominant sources is shifted, reflecting changes in the mid-tropospheric circulation over the Indian Ocean. During wet years there is increased transport of moisture from the equatorial and eastern Indian Ocean. Our results suggests that reliably capturing the linkages between the large-scale circulation patterns over the Indian Ocean and the local circulation over the Congo Basin is critical for future projections of Congo Basin precipitation.