Hezi Gildor

Sea ice as the glacial cycles’ Climate switch: role of seasonal and orbital forcing

A box model of the coupled ocean, atmosphere, sea ice, and land ice climate system is used to study glacial-interglacial oscillations under seasonally and orbitally varying solar forcing. The dominant 100 kyr oscillation in land ice volume has the familiar sawtooth shape of climate proxy records, and to zeroth order, it does not depend on the seasonal and Milankovitch forcing. The sea ice controls, via its albedo and insulating effects, the atmospheric moisture fluxes and precipitation that enable the land ice sheet growth. This control and the rapid growth and melting of the sea ice allow the sea ice to rapidly switch the climate system from a growing ice sheet phase to a retreating ice sheet phase and to shape the oscillations sawtooth structure. A specific physical mechanism is proposed by which the insolation changes act as a pacemaker, setting the phase of the oscillation by directly controlling summer melting of ice sheets. This mechanism is shown to induce deglaciations during periods of lower summer insolation. Superimposed on the 100 kyr are the linear Milankovitch-forced frequencies of 19, 23, and 41 kyr. The transition from 41 kyr glacial cycles to 100 kyr cycles one million years ago may be explained as being due to the activation of the sea ice switch at that time. This would be the case if sea ice extent was more limited during the warmer climate of the early Pleistocene.

A sea ice climate switch mechanism for the 100-kyr glacial cycles

A box model of the ocean-atmosphere-sea ice-land ice climate system is used to study a novel mechanism for self-sustained oscillations of the climate system on a time scale of 100,000 years, without external forcing. The oscillation in land ice volume has the familiar sawtooth shape of climate proxy records. The most novel aspect of the climate oscillations analyzed here is the crucial role played by the sea ice. The sea ice acts as a “switch” of the climate system, switching it from a growing land glaciers mode to a retreating land glaciers mode and shaping the oscillations sawtooth structure. A simple explanation of the 100-kyr timescale is formulated on the basis of the mechanism seen in the model. Finally, rapid sea ice changes such as those seen in our model, and their drastic effects on the climate system, may provide an explanation to some of the rapid climate changes observed to be a part of the variability at all timescales in the paleorecord.

Coherent Resonant Millennial-Scale Climate Oscillations Triggered by Massive Meltwater Pulses

The role of mean and stochastic freshwater forcing on the generation of millennial-scale climate variability in the North Atlantic is studied using a low-order coupled atmosphere‐ocean‐sea ice model. It is shown that millennial-scale oscillations can be excited stochastically, when the North Atlantic Ocean is fresh enough. This finding is used in order to interpret the aftermath of massive iceberg surges (Heinrich events) in the glacial North Atlantic, which are characterized by an excitation of Dansgaard‐Oeschger events. Based on model results, it is hypothesized that Heinrich events trigger Dansgaard‐Oeschger cycles and that furthermore the occurrence of Heinrich events is dependent on the accumulated climatic effect of a series of Dansgaard‐Oeschger events. This scenario leads to a coupled ocean‐ice sheet oscillation that shares many similarities with the Bond cycle. Further sensitivity experiments reveal that the timescale of the oscillations can be decomposed into stochastic, linear, and nonlinear deterministic components. A schematic bifurcation diagram is used to compare theoretical results with paleoclimatic data.

A Simple Time-Dependent Model of SST Hot Spots

Abstract The authors introduce a simple model for the time-dependent evolution of tropical “hot spots,” or localized regions where the sea surface temperature (SST) becomes unusually high for a limited period of time. The model consists of a simple zero-dimensional atmospheric model coupled to an ocean mixed layer. For plausible parameter values, steady solutions of this model can become unstable to time-dependent oscillations, which are studied both by linear stability analysis and explicit time-dependent nonlinear simulation. For reasonable parameter values, the oscillations have periods ranging from intraseasonal to subannual. For parameter values only slightly beyond the threshold for instability, the oscillations become strongly nonlinear, and have a recharge–discharge character. The basic mechanism for the instability and oscillations comes from cloud-radiative and wind-evaporation feedbacks, which play the same role in the dynamics and are lumped together into a single parameterization. This is pos...

Sea-ice switches and abrupt climate change

We propose that past abrupt climate changes were probably a result of rapid and extensive variations in sea–ice cover. We explain why this seems a perhaps more likely explanation than a purely thermohaline circulation mechanism. We emphasize that because of the significant influence of sea ice on the climate system, it seems that high priority should be given to developing ways for reconstructing high–resolution (in space and time) sea–ice extent for past climate–change events. If proxy data can confirm that sea ice was indeed the major player in past abrupt climate–change events, it seems less likely that such dramatic abrupt changes will occur due to global warming, when extensive sea–ice cover will not be present.

A coral reef refuge in the Red Sea.

The stability and persistence of coral reefs in the decades to come is uncertain due to global warming and repeated bleaching events that will lead to reduced resilience of these ecological and socio-economically important ecosystems. Identifying key refugia is potentially important for future conservation actions. We suggest that the Gulf of Aqaba (GoA) (Red Sea) may serve as a reef refugium due to a unique suite of environmental conditions. Our hypothesis is based on experimental detection of an exceptionally high bleaching threshold of northern Red Sea corals and on the potential dispersal of coral planulae larvae through a selective thermal barrier estimated using an ocean model. We propose that millennia of natural selection in the form of a thermal barrier at the southernmost end of the Red Sea have selected coral genotypes that are less susceptible to thermal stress in the northern Red Sea, delaying bleaching events in the GoA by at least a century.

Physical Mechanisms Behind Biogeochemical Glacial-Interglacial CO2 Variations

The atmospheric concentration ofCO2 has un- dergonesignicantandfairlyregularchangesonatimescale of 100 kyr during the at least last four glacial-interglacial cycles. Here we present a novel coupled physical-biogeo- chemical mechanism for these variations. Previous studies had to arbitrarily specify the behavior of the physical cli- matesysteminordertoinvokeabiogeochemical mechanism for the glacial CO2 changes, be it an arbitrarily specied changeintheverticaloceanmixing(Toggweiler,1999), orar- bitrarily specied sea ice cover changes (Stephens and Keel- ing, 2000). Instead, we present here a new, self-consistent, qualitative physical mechanism for both the vertical mixing and sea ice cover changes. In this mechanism, the cooling of North Atlantic Deep Water due to northern hemisphere glaciationistransportedsouthwardbythethermohalinecir- culation,andcoolsthedeepwaterupwellingintheSouthern Ocean. This, in turn, aects the Southern Ocean stratica- tion, reducesthe rate of vertical mixing of thesurface water with the deep water and increases the sea ice cover. We also explain the continuous time evolution between glacial and interglacial states rather than treat them as two steady states, and are able to model explicitly for the rst time the amplication of the glacial-interglacial variability of the physical climate system by the ocean biogeochemistry.

Progress in Paleoclimate Modeling

This paper briefly surveys areas of paleoclimate modeling notable for recent progress. New ideas, including hypotheses giving a pivotal role to sea ice, have revitalized the low-order models used to simulate the time evolution of glacial cycles through the Pleistocene, a prohibitive length of time for comprehensive general circulation models (GCMs). In a recent breakthrough, however, GCMs have succeeded in simulating the onset of glaciations. This occurs at times (most recently, 115 kyr B.P.) when high northern latitudes are cold enough to maintain a snow cover and tropical latitudes are warm, enhancing the moisture source. More generally, the improvement in models has allowed simulations of key periods such as the Last Glacial Maximum and the mid-Holocene that compare more favorably and in more detail with paleoproxy data. These models now simulate ENSO cycles, and some of them have been shown to reproduce the reduction of ENSO activity observed in the early to middle Holocene. Modeling studies have demonstrated that the reduction is a response to the altered orbital configuration at that time. An urgent challenge for paleoclimate modeling is to explain and to simulate the abrupt changes observed during glacial epochs (i.e., Dansgaard–Oescher cycles, Heinrich events, and the Younger Dryas). Efforts have begun to simulate the last millennium. Over this time the forcing due to orbital variations is less important than the radiance changes due to volcanic eruptions and variations in solar output. Simulations of these natural variations test the models relied on for future climate change projections. They provide better estimates of the internal and naturally forced variability at centennial time scales, elucidating how unusual the recent global temperature trends are.

Red Sea during the Last Glacial Maximum: Implications for sea level reconstruction

The Red Sea is connected to the Indian Ocean via a narrow and shallow strait and exhibits a high sensitivity to atmospheric changes and a reduced sea level. We used an ocean general circulation model to investigate the hydrography and circulation in the Red Sea in response to reduced sea level and modified atmospheric conditions occurring during the Last Glacial Maximum (LGM). The model salinity shows high sensitivity to sea level reduction together with a mild atmospheric impact. Sea level reduction affects the stratification and alters the circulation pattern at the Strait of Bab el Mandab, which experiences a transition from a submaximal flow to a maximal flow. The best correlation to reconstructed conditions during LGM exists when the water depth of the Hanish Sill (the shallowest part in the Strait of Bab el Mandab) is 33 ± 10.75 m, which would be affected by a sea level lowering of approximately 105 m. Our results support the reconstructed maximum salinity of around 57 practical salinity units because of a simple model (that takes into account mixing processes along the strait) and comparison of the surface salinity gradient to reconstructions based on isotopic records from sedimentary cores. The salinity and δ18O are sensitive to the mixing process at the strait, and the sensitivity increases as the sea level is further reduced. A local relative sea level reduction of approximately 105 m is also in close agreement with the inference of the LGM low stand of the sea at the location of the sill based on the ICE-5G (VM2) model.

Evidence for Submesoscale Barriers to Horizontal Mixing in the Ocean from Current Measurements and Aerial Photographs

Abstract Ocean submesoscale (∼2–20 km) mixing processes play a major role in ocean dynamics, in physical–biological interactions (e.g., in the dispersion of larvae), and in the dispersion of pollutants. In this paper, horizontal mixing on a scale of a few kilometers is investigated, from observations of surface currents, using highly resolved (300 m) high-frequency radar. These results show the complexity of ocean mixing on scales of a few kilometers and the existence of temporary barriers to mixing that can affect the dispersion of biological materials and pollutants. These barriers are narrow [O(100 m)] and can survive for a few days. The existence of these barriers is supported in simultaneous aerial photographs. The barriers observed here may require a new approach to the way horizontal mixing is parameterized in ocean and climate models.