ENSO suppression due to weakening of the North Atlantic thermohaline circulation

Changes of the North Atlantic thermohaline circulation (THC) excite wave patterns that readjust the thermocline globally. This paper examines the impact of a freshwater-induced THC shutdown on the depth of the Pacific thermocline and its subsequent modification of the El Niño–Southern Oscillation (ENSO) variability using an intermediate-complexity global coupled atmosphere–ocean–sea ice model and an intermediate ENSO model, respectively. It is shown by performing a numerical eigenanalysis and transient simulations that a THC shutdown in the North Atlantic goes along with reduced ENSO variability because of a deepening of the zonal mean tropical Pacific thermocline. A transient simulation also exhibits abrupt changes of ENSO behavior, depending on the rate of THC change. The global oceanic wave adjustment mechanism is shown to play a key role also on multidecadal time scales. Simulated multidecadal global sea surface temperature (SST) patterns show a large degree of similarity with previous climate reconstructions, suggesting that the observed pan-oceanic variability on these time scales is brought about by oceanic waves and by atmospheric teleconnections

The sensitivity of Northern Hemisphere ice sheets to atmospheric forcing during the last glacial cycle using PMIP3 models

The evolution of Northern Hemisphere ice sheets through the last glacial cycle is simulated with the glacial index method by using the climate forcing from one General Circulation Model, COSMOS. By comparing the simulated results to geological reconstructions, we first show that the modelled climate is capable of capturing the main features of the ice-sheet evolution. However, large deviations exist, likely due to the absence of nonlinear interactions between ice sheet and other climate components. The model uncertainties of the climate forcing are examined using the output from nine climate models from the Paleoclimate Modelling Intercomparison Project Phase III. The results show a large variability in simulated ice sheets between the different models. We find that the ice-sheet extent pattern resembles summer surface air temperature pattern at the Last Glacial Maximum, confirming the dominant role of surface ablation process for high-latitude Northern Hemisphere ice sheets. This study shows the importance of the upper boundary condition for ice-sheet modelling, and implies that careful constraints on climate output is essential for simulating realistic glacial Northern Hemisphere ice sheets

Estimating Greenland surface melt is hampered by melt induced dampening of temperature variability

he positive degree-day (PDD) model provides a particularly simple approach to estimate surface melt from land ice based solely on air temperature. Here, we use a climate and snow pack simulation of the Greenland ice sheet (Modèle Atmosphérique Régional, MAR) as a reference, to analyze this scheme in three realizations that incorporate the sub-monthly temperature variability differently: (i) by local values, (ii) by local values that systematically overestimate the dampened variability associated with intense melting or (iii) by one constant value. Local calibrations reveal that incorporating local temperature variability, particularly resolving the dampened variability of melt areas, renders model parameters more temperature-dependent. This indicates that the negative feedback between surface melt and temperature variability introduces a non-linearity into the temperature – melt relation. To assess the skill of the individual realizations, we hindcast melt rates from MAR temperatures for each realization. For this purpose, we globally calibrate Greenland-wide, constant parameters. Realization (i) exhibits shortcomings in the spatial representation of surface melt unless temperature-dependent instead of constant parameters are calibrated. The other realizations perform comparatively well with constant parametrizations. The skill of the PDD model primarily depends, however, on the consistent calibration rather than on the specific representation of variability

The greening of Arabia: multiple opportunities for human occupation of the Arabian peninsula during the Late Pleistocene inferred from an ensemble of climate model simulations

Climate models are potentially useful tools for addressing human dispersals and demographic change. The Arabian Peninsula is becoming increasingly significant in the story of human dispersals out of Africa during the Late Pleistocene. Although characterised largely by arid environments today, emerging climate records indicate that the peninsula was wetter many times in the past, suggesting that the region may have been inhabited considerably more than hitherto thought. Explaining the origins and spatial distribution of increased rainfall is challenging because palaeoenvironmental research in the region is in an early developmental stage. We address environmental oscillations by assembling and analysing an ensemble of five global climate models (CCSM3, COSMOS, HadCM3, KCM, and NorESM). We focus on precipitation, as the variable is key for the development of lakes, rivers and savannas. The climate models generated here were compared with published palaeoenvironmental data such as palaeolakes, speleothems and alluvial fan records as a means of validation. All five models showed, to varying degrees, that the Arabia Peninsula was significantly wetter than today during the Last Interglacial (130 ka and 126/125 ka timeslices), and that the main source of increased rainfall was from the North African summer monsoon rather than the Indian Ocean monsoon or from Mediterranean climate patterns. Where available, 104 ka (MIS 5c), 56 ka (early MIS 3) and 21 ka (LGM) timeslices showed rainfall was present but not as extensive as during the Last Interglacial. The results favour the hypothesis that humans potentially moved out of Africa and into Arabia on multiple occasions during pluvial phases of the Late Pleistocene

Air-sea interactions during glacial Heinrich events

'Heinrich events' - massive iceberg discharges from Northern Hemisphere ice sheets during the last ice age - coincided with cold periods that were followed by abrupt warmings in the Northern Hemisphere. Climate reconstructions suggest that the associated freshwater pulses caused a temporary collapse of the Atlantic Meridional Overturning Circulation (AMOC) by stabilizing the stratification in the regions of deep water formation. In the present work a coupled atmosphere-ocean-sea ice model is employed under glacial boundary conditions to assess climate feedbacks after a simulated Heinrich event that lead to a fast recovery of the AMOC. Two main mechanisms have been identified. Initially, mixing and thermal processes weaken the stratification in the northern North Atlantic. Additionally, 300-400 years after the main collapse of the AMOC, the stratification is further destabilized by mean horizontal advection of anomalous saline waters within the subpolar gyre. In consequence the large-scale meridional overturning is reinitiated. The positive salinity anomaly originates from the tropical Atlantic and relies on air-sea coupling. Reduced poleward heat transport in the North Atlantic leads to a cooling north of the thermal equator. Due to advection of cold air and intensification of the northeasterly trade winds the Intertropical Convergence Zone is shifted southward and north equatorial precipitation is reduced. A dilution of the arising positive salinity anomaly is prevented because cross-equatorial oceanic surface flow is halted during the shut-down of the AMOC. Experiments with suppressed tropical air-sea coupling reveal that the recovery time of the AMOC is almost twice as long as in the coupled case. The impact of a shut-down of the AMOC on the Indian and Pacific Oceans can be decomposed into atmospheric and oceanic contributions. Temperature anomalies in the northern hemisphere are largely controlled by atmospheric teleconnections, whereas southern hemispheric ones mainly rely on ocean dynamical changes. Vertical diffusion is considered a key factor controlling the stability of the AMOC. This however may not be so after a shut-down. Whereas model simulations without air-sea coupling in the tropical Atlantic still show a strong sensitivity to vertical diffusion, this behaviour cannot be found in fully coupled simulations. Thus, after a Heinrich event the formation of a tropical salinity anomaly due to air-sea fluxes appears to be a more efficient negative feedback for the resumption of the AMOC than density homogenisation due to vertical diffusion

Ocean Current Changes

In this chapter the role of the ocean on climate and climate change is discussed in terms of the properties of oceans and in terms of the tools available to oceanographers. The details of the Atlantic Meridional Overturning Circulation (AMOC) are described with special reference to motivation, driving mechanisms, heat transport and the ocean's uptake of carbon and the ventilation of the deep ocean. The past changes of the AMOC and the Atlantic climate are also discussed. The chapter ends with a discussion of three questions: • Why should the AMOC change as a result of climate change? • Can we detect changes in the AMOC? • Is the AMOC already changing as a result of climate change

Pliocene aridification of Australia caused by tectonically induced weakening of the Indonesian throughflow

Tectonic changes of the Early to Mid-Pliocene largely modified the Indonesian Passages by constricting and uplifting the passages between today's New Guinea and Sulawesi. The associated changes in strength and water mass properties of the Indonesian throughflow (ITF) might have influenced the amount of heat transported from the Pacific to the Indian Ocean and thus contributed to Pliocene climate change of the Indo-Pacific. We study the climate response to changes in the geometry of the Indonesian Passages in an atmosphere–ocean general circulation model (AOGCM). We compare climate simulations with present-day topography and with a topography resembling the Early Pliocene situation in the Indo-Pacific, i.e. passages East of Sulawesi deepened and widened to the South. We find that transport through the Indonesian Archipelago is weakened in the constricted passage by 1.7 Sv and in the unchanged Makassar Strait West of Sulawesi by 3.5 Sv, while transport weighted temperature of the outflow into the Indian Ocean increases by 1 °C. Consistent with recent proxy evidence the reduction in ITF transport causes a decrease in subsurface temperatures in the Indian Ocean while surface waters of the equatorial Pacific exhibit an increase by up to 0.9 °C centred in the warm pool. As a local response to the sea surface temperature anomalies, we observe an anomalous precipitation dipole across the Indonesian passages with increased rainfall over the Pacific warm pool and decreased precipitation in the eastern Indian Ocean. The Australian continent experiences a pronounced aridification with mean annual precipitation rates dropping by 30% over most parts of the continent. Using an uncoupled vegetation model, we demonstrate that the simulated climate change might partly explain the observed Late Pliocene desertification of Australia