Impact of freshwater discharge from the Greenland ice sheet on North Atlantic climate variability

Using a coupled ocean–atmosphere general circulation model, we investigated the impact of Greenland ice sheet melting on North Atlantic climate variability. The positive-degree day (PDD) method was incorporated into the model to control continental ice melting (PDD run). Models with and without the PDD method produce a realistic pattern of North Atlantic sea surface temperature (SST) variability that fluctuates from decadal to multidecadal periods. However, the interdecadal variability in PDD run is significantly dominated in the longer time scale compared to that in the run without PDD method. The main oscillatory feature in these experiments likely resembles the density-driven oscillatory mode. A reduction in the ocean density over the subpolar Atlantic results in suppression of the Atlantic Meridional Overturning Circulation (AMOC), leading to a cold SST due to a weakening of northward heat transport. The decreased surface evaporation associated with the cold SST further reduces the ocean density and thus, simultaneously acts as a positive feedback mechanism. The southward meridional current associated with the suppressed AMOC causes a positive tendency in the ocean density through density advection, which accounts for the phase transition of this oscillatory mode. The Greenland ice melting process reduces the mean meridional current and meridional density gradient because of additional fresh water flux, which suppress the delayed negative feedback due to meridional density advection. As a result, the oscillation period becomes longer and the transition is more delayed.     

CO2 feedbacks and the 100 K year cycle in climate

Summary In an earlier paper (Lindzen, 1986), it was shown that allowing CO₂ to vary with snow/sea ice position could lead to a greatly enhanced response in glaciation to 100 K year orbital forcing—even when 20 K year forcing was much stronger. In that model, snow/sea ice position (SSIP) and glaciation were different: the former was the forcing for the latter. However, SSIP and glaciation were not decorrelated. Observations (Berner et al., 1979; Lorius et al., 1985; Neftel et al., 1982) suggest that CO₂ may be independently related to both SSIP and glaciation. In the present paper, we allow (in a highly simplified manner) such independent dependence, and show how it alters the earlier results. Briefly, the dependence of CO₂ on glaciation can contribute to and even cause a highly enhanced response to the 100 K year component of the forcing. However, the CO₂ dependence on SSIP is, on the whole, more effective in this regard. Thus, we expect time series of CO₂ to show variation on the faster time scales than does glaciation.With 5 Figures     

An investigation of the small ice cap instability in the Southern Hemisphere with a coupled atmosphere-sea ice-ocean-terrestrial ice model

 A simple climate model has been developed to investigate the existence of the small ice cap instability in the Southern Hemisphere. The model consists of four coupled components: an atmospheric energy balance model, a thermodynamic snow-sea ice model, an oceanic mixed layer model and a terrestrial ice model. Results from a series of experiments involving different degrees of coupling in the model show that the instability appears only in those cases when an explicit representation of the Antarctic ice sheet is not included in the model. In order to determine which physical processes in the ice sheet model lead to a stabilization of the system we have conducted several sensitivity experiments in each of which a given ice sheet process has been removed from the control formulation of the model. Results from these experiments suggest that the feedback between the elevation of the ice sheet and the snow accumulation-ice ablation balance is responsible for the disappearance of the small ice cap instability in our simulation. In the model, the mass balance of the ice sheet depends on the air temperature at sea level corrected for altitude and it is, therefore, a function of surface elevation. This altitude-mass balance feedback effectively decouples the location of the ice edge from any specific sea level isotherm, thus decreasing the model sensitivity to the albedo-temperature feedback, which is responsible for the appearance of the instability. It is also shown that the elevation-radiative cooling feedback tends to stabilize the ice sheet, although its effect does not seem to be strong enough to remove the instability. Another interesting result is that for those simulations which include the terrestrial ice model with elevation-dependent surface mass balance, hysteresis is exhibited, where for a given level of external forcing, two stable solutions with different, non-zero ice-sheet volume and area and different air and ocean temperature fields occur. However, no unstable transition between the two solutions is ever observed. Our results suggest that the small ice cap instability mechanism could be unsuitable for explaining the inception of glaciation in Antarctica. Received: 14 April 1997 / Accepted: 22 October 1997     

A Boolean delay equation model of an interdecadal Arctic climate cycle

A Boolean delay equation (BDE) model is presented for the interdecadal Arctic and Greenland Sea climate cycle recently proposed by Mysak, Manak and Marsden. It is shown that 15- to 20-year oscillations can occur in the model for a variety of time delays in the BDEs. However, both the period and structure of the oscillations are sensitive to the initial conditions. In an extended model, in which the convection in the Greenland Sea is dependent upon the ice conditions during each of several previous years as well as the current year, the solution structure is more realistic, with two jumps per period of oscillation.     

A strategy for frequency spectra of quaternary climate records

The paleoclimatic variability at frequencies ranging from 10^–4 cycle per year (cpy) to 10^–5 cpy is investigated using a set of four deep-sea cores from the Atlantic, Pacific and Indian Oceans. Dominant features are the presence of orbital frequencies corresponding to mean periods of 117.7, 43.6, 24.9 and 19.3 kyr. These are statistically significant according to such advanced spectral tools as Blackman-Tukey, maximum entropy and the highly efficient Thomson technique. However, the main purpose of this paper is methodological, describing the statistical analyses of time series with modern methods in order to stress their relative power, advantages and disadvantages. The more advanced statistical methods confirm the coincidence of the dominant periods in the deep sea cores and those in the astronomical elements, including combination tones. Three frequency bands of high paleoclimatic variability centred at 15.4, 13 and 10.8 kyr are indeed also detected. These two last periods are very close to those predicted by the climatic non-linear model of Ghil and Le Treut and found by Pestiaux et al. and Yiou et al.     

A comparison of the static and flow methods for the detection of ice nuclei

The use of the membrane-filter processing chamber to study ice nuclei concentrations has become wide-spread since its introduction by Bigg et al. in 1961. The technique is convenient because of the simplicity of its operation and because it could be run remote from the place of field study. It has however been found to suffer from a number of drawbacks, namely, the volume effect, the chamber height effect, the vapour depletion effect, etc.Comparison of the results obtained by running a traditional filter processor and a continuous flow chamber under identical temperature and humidity conditions for polluted Manchester air has shown that the latter technique detects more ice nuclei than the former one by a factor of about 14±4. These results confirm that the filter technique suffers from the vapour depletion effect. The present results are in agreement with Bigg et al., Mossop and Thorndike, and King.In the light of our findings the filter technique does not appear to be a standard method. Therefore the ice nuclei data obtained with the filter method should not be extended to clouds in order to study their microphysical properties.     

Investigating the variability of the Arctic sea ice thickness in response to a stochastic thermodynamic atmospheric forcing

The natural low frequency variability of the sea-ice thickness in the Arctic is investigated based on a 10 000 years simulation with a one-dimensional thermodynamic sea-ice model forced by random perturbations of the air surface temperature and solar radiation. The simulation results suggest that atmospheric random perturbations are integrated by the sea-ice. Moreover those perturbations occurring at the onset of ice melting force the largest ice thickness anomalies, which are successively amplified in summer by the albedo feedback and damped in winter by the feedback of the heat conduction through the ice. They also result in a global shift of the melting season which, in the mean annual cycle, leads to earlier melting as compared to the mean climatological cycle. The power spectrum of the ice anomalies suggests that the thickness of the perennial ice should vary preferentially on a time scale of approximately 20 years. The shape of the spectrum is consistent with that of a first order Markov process in which the characteristic time scale of the ice fluctuations would be the relaxation time scale associated with the linear feedback. The equivalent Markov model is constructed by linearizing the ice growth rate anomaly equations and allows us to derive an analytical expression of the feedback and of the forcing of the anomalies. The characteristic time scale depends explicitly on those model parameters involved in the atmosphere-ice interaction but also on the mean seasonal characteristics of the forcing and of the ice thickness. Received: 18 August 1999 / Accepted: 10 May 2000     

Sensitivity of the last glacial inception to initial and surface conditions

We investigate the sensitivity of simulations of the last glacial inception (LGI) with respect to initial (size of the Greenland ice sheet) and surface (state of ocean/vegetation) conditions and two different CO₂ reconstructions. Utilizing the CLIMBER-2 Earth system model, we obtain the following results: (a) ice-sheet expansion in North America at the end of the Eemian can be reduced or even completely suppressed when pre-industrial or Eemian ocean/vegetation is prescribed. (b) A warmer surrounding ocean and, in particular, a large Laurentide ice sheet reduce the size of the Greenland ice sheet before and during the LGI. (c) A changing ocean contributes much stronger to the expansion of the Laurentide ice sheet when we apply the CO₂ reconstruction according to Barnola et al. (Nature 329:408–414, 1987) instead of Petit et al. (Nature 399:429–436, 1999). (d) In the fully coupled model, the CO₂ reconstruction used has only a small impact on the simulated ice sheets but it does impact the course of the climatic variables. (e) For the Greenland ice sheet, two equilibrium states exist under the insolation and CO₂ forcing at 128,000 years before present (128 kyear BP); the one with an ice sheet reduced by about one quarter as compared to its simulated pre-industrial size and the other with nearly no inland ice in Greenland. (f) Even the extreme assumption of no ice sheet in Greenland at the beginning of our transient simulations does not alter the simulated expansion of northern hemispheric ice sheets at the LGI.     

On the role of high latitude ice,snow, and vegetation feedbacks in the climatic response to external forcing changes

A seasonal energy balance climate model containing a detailed treatment of surface and planetary albedo, and in which seasonally varying land snow and sea ice amounts are simulated in terms of a number of explicit physical processes, is used to investigate the role of high latitude ice, snow, and vegetation feedback processes. Feedback processes are quantified by computing changes in radiative forcing and feedback factors associated with individual processes. Global sea ice albedo feedback is 5–8 times stronger than global land snowcover albedo feedback for a 2% solar constant increase or decrease, with Southern Hemisphere cryosphere feedback being 2–5 times stronger than Northern Hemisphere cryosphere feedback.In the absence of changes in ice extent, changes in ice thickness in response to an increase in solar constant are associated with an increase in summer surface melting which is exactly balanced by increased basal winter freezing, and a reduction in the upward ocean-air flux in summer which is exactly balanced by an increased flux in winter, with no change in the annual mean ocean-air flux. Changes in the mean annual ocean-air heat flux require changes in mean annual ice extent, and are constrained to equal the change in meridional oceanic heat flux convergence in equilibrium. Feedback between ice extent and the meridional oceanic heat flux obtained by scaling the oceanic heat diffusion coefficient by the ice-free fraction regulates the feedback between ice extent and mean annual air-sea heat fluxes in polar regions, and has a modest effect on model-simulated high latitude temperature change.Accounting for the partial masking effect of vegetation on snow-covered land reduces the Northern Hemisphere mean temperature response to a 2% solar constant decrease or increase by 20% and 10%, respectively, even though the radiative forcing change caused by land snowcover changes is about 3 times larger in the absence of vegetational masking. Two parameterizations of the tundra fraction are tested: one based on mean annual land air temperature, and the other based on July land air temperature. The enhancement of the mean Northern Hemisphere temperature response to solar constant changes when the forest-tundra ecotone is allowed to shift with climate is only 1/3 to 1/2 that obtained by Otterman et al. (1984) when the mean annual parameterization is used here, and only 1/4 to 1/3 as large using the July parameterization.The parameterized temperature dependence of ice and snow albedo is found to enhance the global mean temperature response to a 2% solar constant increase by only 0.04 °C, in sharp contrast to the results of Washington and Meehl (1986) obtained with a mean annual model. However, there are significant differences in the method used here and in Washington and Meehl to estimate the importance of this feedback process. When their approach is used in a mean annual version of the present model, closer agreement to their results is obtained.     

ON THE ICE NUCLEATION BY CONTACT ON THE INNER AND OUTER LAYER OF A WATER DROP*

Nucleation of ice by contact takes place when an aerosol particle collides with a surface of supercooled water drop.Aerosol particle may either bounce off the water surface or be captured on it.McCully et al.(1956) and Rosinski et al.(1963) have shown that capture of hydrophilic particles was four times larger than of hydrophobic ones.Hydrophobic aerosol particles will nucleate ice preferentially during the brief time of contact when they bounce off the surface.     

Solar irradiance forcing of centennial climate variability during the Holocene

Centennial climate variability during the Holocene has been simulated in two 10,000 year experiments using the intermediate-complexity ECBilt model. ECBilt contains a dynamic atmosphere, a global 3-D ocean model and a thermodynamic sea-ice model. One experiment uses orbital forcing and solar irradiance forcing, which is based on the Stuiver et al. residual ¹⁴C record spliced into the Lean et al. reconstruction. The other experiment uses orbital forcing alone. A glacier model is coupled off-line to the climate model. A time scale analysis shows that the response in atmospheric parameters to the irradiance forcing can be characterised as the direct response of a system with a large thermal inertia. This is evident in parameters like surface air temperature, monsoon precipitation and glacier length, which show a stronger response for longer time scales. The oceanic response, on the other hand, is strongly modified by internal feedback processes. The solar irradiance forcing excites a (damped) mode of the thermohaline circulation (THC) in the North Atlantic Ocean, similar to the loop-oscillator modes associated with random-noise freshwater forcing. This results in a significant peak (at time scales 200–250 year) in the THC spectrum which is absent in the reference run. The THC response diminishes the sea surface temperature response at high latitudes, while it gives rise to a signal in the sea surface salinity. A comparison of the model results with observations shows a number of encouraging similarities.     

Centennial-scale climate change from decadally-paced explosive volcanism: a coupled sea ice-ocean mechanism

Northern Hemisphere summer cooling through the Holocene is largely driven by the steady decrease in summer insolation tied to the precession of the equinoxes. However, centennial-scale climate departures, such as the Little Ice Age, must be caused by other forcings, most likely explosive volcanism and changes in solar irradiance. Stratospheric volcanic aerosols have the stronger forcing, but their short residence time likely precludes a lasting climate impact from a single eruption. Decadally paced explosive volcanism may produce a greater climate impact because the long response time of ocean surface waters allows for a cumulative decrease in sea-surface temperatures that exceeds that of any single eruption. Here we use a global climate model to evaluate the potential long-term climate impacts from four decadally paced large tropical eruptions. Direct forcing results in a rapid expansion of Arctic Ocean sea ice that persists throughout the eruption period. The expanded sea ice increases the flux of sea ice exported to the northern North Atlantic long enough that it reduces the convective warming of surface waters in the subpolar North Atlantic. In two of our four simulations the cooler surface waters being advected into the Arctic Ocean reduced the rate of basal sea-ice melt in the Atlantic sector of the Arctic Ocean, allowing sea ice to remain in an expanded state for?>?100 model years after volcanic aerosols were removed from the stratosphere. In these simulations the coupled sea ice-ocean mechanism maintains the strong positive feedbacks of an expanded Arctic Ocean sea ice cover, allowing the initial cooling related to the direct effect of volcanic aerosols to be perpetuated, potentially resulting in a centennial-scale or longer change of state in Arctic climate. The fact that the sea ice-ocean mechanism was not established in two of our four simulations suggests that a long-term sea ice response to volcanic forcing is sensitive to the stability of the seawater column, wind, and ocean currents in the North Atlantic during the eruptions.     

Are polynyas self‐sustaining?

Abstract

In this paper, 441 Conductivity Temperature Depth (CTD) casts from the North Water (NOW) Polynya study were used to calculate geostrophic currents between the 10 and 200 dbar surface during April, May and June 1998. Results for April and May indicated a surface intensified southward flow of 10 to 15 cm s^–1 with a small return flow along the Greenland coast in agreement with inferred currents described by Melling et al. (2001) and surface ice drifts found by Wilson et al. (2001). Southward transports at this time were 0.4–0.55 Sv in April and May. In June, however, surface currents diminished markedly: southward transports declined to 0.1–0.35 Sv, coincident with a decrease in directly measured winds over the polynya and in the surface barometric pressure difference between Grise Fjord and the Carey Islands that was used as a surrogate for the local north wind speed. There was no evident decrease in air pressure difference between Resolute and Grise Fjord, indicative of the strength of the north wind over the eastern Arctic in general. The results are consistent with present thinking that the NOW Polynya is primarily a latent heat polynya, forced by dominant north winds. The idea, broached here, is that the polynya creates its own microclimate which sustains the polynya's ice‐free condition after its initial formation. The mechanism is identified by an anomalous low pressure region associated with surface buoyancy flux in the polynya and is pursued through the application of a simple geostrophic adjustment model that suggests two self‐sustaining mechanisms. Firstly, the frontal intrusion of the cold ambient terrestrial air mass drives a significant surface wind that transports frazil ice to the edge of the polynya before it can congeal. Secondly, rotation at these high latitudes restricts the penetration of the front into the polynya, essentially insulating the centre from freezing temperatures.     

On the Arctic Ocean ice thickness response to changes in the external forcing

Submarine and satellite observations show that the Arctic Ocean ice cover has undergone a large thickness reduction and a decrease in the areal extent during the last decades. Here the response of the Arctic Ocean ice cover to changes in the poleward atmospheric energy transport, F _wall, is investigated using coupled atmosphere-ice-ocean column models. Two models with highly different complexity are used in order to illustrate the importance of different internal processes and the results highlight the dramatic effects of the negative ice thickness—ice volume export feedback and the positive surface albedo feedback. The steady state ice thickness as a function of F _wall is determined for various model setups and defines what we call ice thickness response curves. When a variable surface albedo and snow precipitation is included, a complex response curve appears with two distinct regimes: a perennial ice cover regime with a fairly linear response and a less responsive seasonal ice cover regime. The two regimes are separated by a steep transition associated with surface albedo feedback. The associated hysteresis is however small, indicating that the Arctic climate system does not have an irreversible tipping point behaviour related to the surface albedo feedback. The results are discussed in the context of the recent reduction of the Arctic sea ice cover. A new mechanism related to regional and temporal variations of the ice divergence within the Arctic Ocean is presented as an explanation for the observed regional variation of the ice thickness reduction. Our results further suggest that the recent reduction in areal ice extent and loss of multiyear ice is related to the albedo dependent transition between seasonal and perennial ice i.e. large areas of the Arctic Ocean that has previously been dominated by multiyear ice might have been pushed below a critical mean ice thickness, corresponding to the above mentioned transition, and into a state dominated by seasonal ice.     

Planktonic dimethylsulfide and cloud albedo: An estimate of the feedback response

Partial control of climate by the biosphere may be possible through a chain of processes that ultimately links marine plankton production of dimethylsulfide (DMS) with changes in cloud albedo (Charlson et al., 1987). Changes in cloud optical properties can have profound impacts on atmospheric radiation transfer and, hence, the surface environment. In this study, we have developed a simple model that incorporates empirically based parameterizations to account for the biological control of cloud droplet concentration in a first attempt to estimate the strength of the DMS-cloud albedo feedback mechanism. We find that the feedback reduces the global climatic response to imposed perturbations in solar insolation by less than 7%. Likewise, it modifies the strength of other feedbacks affecting surface insolation over oceans by roughly the same amount. This suggests that the DMS-cloud albedo mechanism will be unable to substantially reduce climate sensitivity, although these results should be confirmed with less idealized models when more is known about the net production of DMS by the marine biosphere and its relation to aerosol/cloud microphysics and climate.     

Multiple sea-ice states and abrupt MOC transitions in a general circulation ocean model

Sea ice has been suggested, based on simple models, to play an important role in past glacial–interglacial oscillations via the so-called “sea-ice switch” mechanism. An important requirement for this mechanism is that multiple sea-ice extents exist under the same land ice configuration. This hypothesis of multiple sea-ice extents is tested with a state-of-the-art ocean general circulation model coupled to an atmospheric energy–moisture-balance model. The model includes a dynamic-thermodynamic sea-ice module, has a realistic ocean configuration and bathymetry, and is forced by annual mean forcing. Several runs with two different land ice distributions represent present-day and cold-climate conditions. In each case the ocean model is initiated with both ice-free and fully ice-covered states. We find that the present-day runs converge approximately to the same sea-ice state for the northern hemisphere while for the southern hemisphere a difference in sea-ice extent of about three degrees in latitude between the different runs is observed. The cold climate runs lead to meridional sea-ice extents that are different by up to four degrees in latitude in both hemispheres. While approaching the final states, the model exhibits abrupt transitions from extended sea-ice states and weak meridional overturning circulation, to less extended sea ice and stronger meridional overturning circulation, and vice versa. These transitions are linked to temperature changes in the North Atlantic high-latitude deep water. Such abrupt changes may be associated with Dansgaard–Oeschger events, as proposed by previous studies. Although multiple sea ice states have been observed, the difference between these states is not large enough to provide a strong support for the sea-ice-switch mechanism.     

包含详细水分循环和海冰物理过程的一维。气候模式与气候系统内反馈机制的研究

紧于水分循环和海冰物理过程是气候变化研究中两个比较薄弱的环节，而在以往的一些气候模式中往往简化甚至忽略了其中之一，给气候变化的研究带来了一定的不确定性。因此，我们设计了包含详细水分循环和海冰物理过程的一维气候模式，着重研究了存在，气候系统内部的反馈机制，得到以下几点结论：（１）无论在地表还是在大气中，水汽反馈和冰雪反照率反馈均为很强的正反馈，前者比后者要强一些，冰雪反照率反馈在极区比其它地区强一些。（２）降水过程无论在地表还是在大气中均表现为负反馈。（３）在大气中，蒸发过程表现为很强的正反馈；在地表，蒸发过程在中低纬度表现为很强的负反馈，而在高纬度却表现为正反馈。（４）大气中的潜热输送无论在大气中还是在地表均表现为正反馈，共正反馈效应通过放大水汽的温室效应体现出来。（５）大气中的感热输送无论在大气中还是在地表均表现为较弱的负反馈，其负反馈作用通过抑制冰雪反照率反馈而表现出来。（６）不同反馈之间的合成不是两者简单地线性相加，而是以一种北线性方式相互作用。     

The small ice cap instability in seasonal energy balance models

Results from a two-dimensional energy balance model with a realistic land-ocean distribution show that the small ice cap instability exists in the Southern Hemisphere, but not in the Northern Hemisphere. A series of experiments with a one-dimensional energy balance model with idealized geography are used to study the roles of the seasonal cycle and the land-ocean distribution. The results indicate that the seasonal cycle and land-ocean distribution can influence the strength of the albedo feedback, which is responsible for the small ice cap instability, through two factors: the temperature gradient and the amplitude of the seasonal cycle. The land-ocean distribution in the Southern Hemisphere favors the small ice cap instability, while the land-ocean distribution in the Northern Hemisphere does not. Because of the longitudinal variations of land-ocean distribution in the Northern Hemisphere, the behavior of ice lines in the Northern Hemisphere cannot be simulated and explained by the model with zonally symmetric land-ocean distribution. Model results suggest that the small ice cap instability may be a possible mechanism for the formation of the Antarctic icesheet. The model results cast doubt, however, on the role of the small ice cap instability in Northern Hemisphere glaciations. Offprint requests to: J Huang     

沙尘气溶胶作为冰核对阿克苏地区一次多单体型强对流风暴降水及其微物理过程影响的数值模拟研究

将DeMott冰核浓度参数化方案引入到WRF中尺度数值模式中,模拟了新疆阿克苏地区一次多单体型强对流风暴,并对背景大气条件和沙尘条件下气溶胶作为冰核,对云中微物理结构和降水变化的影响进行了敏感性试验和对比分析,结果显示:在背景大气条件和沙尘条件下增加冰核浓度对降水中心强度影响较小,并且总体上看降水分布变化不大,但是降水局部的变化量较明显;不同背景条件下IN（Ice Nuclei）浓度的增加使得冰晶和雪的质量混合比和数浓度均有较大幅度的增加,其中雪的主要源项为凝华增长过程,而霰增长主要来源于冰相粒子碰并过冷云滴,并且在背景大气和沙尘条件下增加IN都使得霰的数浓度增加,尺度减少。     

Thunderstorm charging: Laboratory experiments clarified

Further laboratory experiments of thunderstorm charging by ice crystal collisions with riming graupel pellets have led to results that may help account for a major difference between two independent studies reported earlier. The problem concerns values of the cloud liquid water content required to change the sign of graupel charging. The present work has noted and investigated differences in the techniques of cloud nucleation used in the two studies. In one set of measurements by Takahashi, ice crystals are nucleated continuously in a supercooled water droplet cloud, while in the other experiments by Jayaratne et al. and Saunders et al., the droplet cloud is nucleated only briefly. These two nucleation methods lead to differences in the mass of cloud particles collected on a sampling probe used to determine the liquid water content of the cloud. In the continuous nucleation case, the probe collects sufficient numbers of ice crystals to lead to a substantial overestimate of the cloud liquid water content.