The effects of reduced land fraction and solar forcing on the general circulation: results from the NCAR CCM

Land fraction and the solar energy at the top of the atmosphere (solar constant) may have been significantly lower early in Earth's history. It is likely that both of these factors played some important role in the climate of the early earth. The climate changes associated with a global ocean(i.e. no continents) and reduced solar constant are examined with a general circulation model and compared with the present-day climate simulation. The general circulation model used in the study is the NCAR CCM with a swamp ocean surface. First, all land points are removed in the model and then the solar constant is reduced by 10% for this global ocean case.Results indicate that a 4 K increase in air temperature occurs with global ocean simulation compared to the control. When solar constant is reduced by 10% under global ocean conditions a 23 K decrease in air temperature is noted. The global ocean warms much of the troposphere and stratosphere, while a reduction in the solar constant cools the troposphere and stratosphere. The largest cooling occurs near the surface with the lower solar constant.Global mean values of evaporation, water vapor amounts, absorbed solar radiation and the downward longwave radiation are increased under global ocean conditions, while all are reduced when the solar constant is lowered. The global ocean simulation produces sea ice only in the highest latitudes. A frozen planet does not occur when the solar constant is reduced—rather, the ice line settles near 30° of latitude. It is near this latitude that transient eddies transport large amounts of sensible heat across the ice line acting as a negative feedback under lower solar constant conditions keeping sea ice from migrating to even lower latitudes.Clouds, under lower solar forcing, also act as a negative feedback because they are reduced in higher latitudes with colder atmospheric temperatures allowing additional solar radiation to reach the surface. The overall effect of clouds in the global ocean is to act as a positive feedback because they are slightly reduced thereby allowing additional solar radiation to reach the surface and increase the warming caused by the removal of land. The relevance of the results to the “Faint-Young Sun Paradox” indicates that reduced land fraction and solar forcing affect dynamics, heat transport, and clouds. Therefore the associated feedbacks should be taken into account in order to understand their roles in resolving the “Faint-Young Sun Paradox”.     

Control of the Antarctic ice sheet by ocean–ice interaction

The Antarctic ice cap is the largest ice sheet of modern times. It is of considerable importance to predict the sea level variability due to the associated changes in ice volume. We present the results of a simple grounded ice sheet model, developed from Oerlemans [Oerlemans, J., 2002. Global dynamics of the Antarctic Ice Sheet, Climate Dynamics 19, 85–93.], in which the net oceanic evaporation influences the ice cap volume in two ways, through changes in: (i) the accumulation rate, and (ii) the mean sea level. The net evaporation changes are driven by the sea surface temperature (SST) anomaly time series of Howard [Howard, W.R., 1997. A warm future in the past, Nature, 388, 418–419.] for the subantarctic Southern Ocean over the period 220 kyr to the present. The effect of the waxing and waning of the northern hemisphere ice sheets is integrated into the model using an independent model, in which ice melting depends on the SST anomaly and ice calving depends on the sea level anomaly. A series of analytical expressions are derived for the related properties of the coupled ocean–ice system applicable over time scales of 100 kyr, which show, in particular, that the Antarctic ice cap volume changes are due mainly to the effects of the northern hemisphere ice sheets on sea level (which influences ice calving), rather than directly to changes in SST, and hence the ice cap volume is greatest during interglacial periods. This conclusion, which is independent of the specification of the ice melting regime for the northern hemisphere ice sheets, strongly suggests that the changes in accumulation flux estimated from the Vostok proxy temperature data and used in other studies of the Antarctic mass balance have been overestimated. A simple expression is also presented for the lag of ice cap volume to SST, and it is found that the predictions for the mean sea level variability are similar to observations for a melting flux of the northern hemisphere ice sheets about twice their accumulation flux due to the net oceanic evaporation, except during major deglaciations when these two fluxes appear to be of similar magnitude.     

Meltwater and the global ocean conveyor: northern versus southern connections

The sensitivity of the ocean circulation to changes in North Atlantic surface fluxes has become a major factor in explaining climate variability. The role of the Antarctic Bottom Water in modulating this variability has received much less attention, limiting the development of a complete understanding of decadal to millennial time-scale climate change. New analyses indicate that the southern deepwater source may change dramatically (e.g., experience a decrease of as much as two thirds during last 800 years). Such change can substantially alter the ocean circulation patterns of the last millennium. Additional analyses indicate that the Southern Hemisphere led the Northern Hemisphere changes in some of the glacial cycles of Pleistocene, implying a seesaw-type oscillation of the global ocean conveyor. The potential for melting of sea ice and ice sheets in the Antarctica associated with global warming can cause a further slowdown of the southern deepwater source. These results demand an assessment of the role of the Southern Ocean in driving changes of the global ocean circulation and climate. Systematic model simulation targeting the ocean circulation response to changes in surface salinity in the high latitudes of both Northern and Southern Hemispheres demonstrate that meltwater impacts in one hemisphere may lead to a strengthening of the thermohaline conveyor driven by the source in the opposite hemisphere. This, in turn, leads to significant changes in poleward heat transport. Further, meltwater events can lead to deep-sea warming and thermal expansion of abyssal water, that in turn cause a substantial sea-level change even without a major ice sheet melting.     

Discussion on sea level fluctuations along the Adriatic coasts coupling to climate indices forced by solar activity: Insights into the future of Venice

The North Atlantic Oscillation (NAO), which is a dominant circulation pattern in Northern Hemisphere winter, is known to affect sea-level variability in the Mediterranean Sea mainly through the hydrostatic response of water masses to pressure anomalies and changes in evaporation/precipitation budgets. In this study the influence of the NAO on sea levels along the Adriatic coasts is re-assessed in the attempt to uncover the potential causes of the observed high sensitivity of the northern basin to NAO fluctuations. The investigation is focused on the role of the NAO as forcing factor of the winds blowing in the area and of the freshwaters input from the Po River, both of which influence the hydrodynamics of the Northern Adriatic. In addition, some insights into the future of Venice are discussed on the basis of the hypothesis that NAO phases are modulated by the varying solar activity through the intensity of the Earth's geomagnetic activity.     

Role of Arctic sea ice in global atmospheric circulation: A review

Formed by the freezing of sea water, sea ice defines the character of the marine Arctic. The principal purpose of this review is to synthesize the published efforts that document the potential impact of Arctic sea ice on remote climates. The emphasis is on atmospheric processes and the resulting modifications in surface conditions such as air temperature, precipitation patterns, and storm track behavior at interannual timescales across the middle and low latitudes of the Northern hemisphere during cool months. Addressed also are the theoretical, methodological, and logistical challenges facing the current observational and modeling studies that aim to improve our awareness of the role that Arctic sea ice plays in the definition of global climate. Moving towards an improved understanding of the role that polar sea ice plays in shaping the global climate is a subject of timely importance as the Arctic environment is currently undergoing rapid change with little slowing down forecasted for the future.     

A comparison of simulated monsoon circulations and snow accumulation in Asia during the mid-Holocene and at the Last Glacial Maximum

Tropical climatology through the last glacial cycle is believed to have ranged from colder, windier conditions at the Last Glacial Maximum (LGM) to relatively warm, stable conditions during the Holocene. Changes in strength of the South Asian monsoon have previously been determined from a variety of proxy data and have been attributed primarily to changes in radiative forcing, although tropical sea surface temperature (SST) is known to play a fundamental role in regulating monsoon strength and is also believed to have changed throughout the late Quaternary.In this study, the monsoons simulated in a coupled atmosphere–ocean general circulation model (GCM) configured for the mid-Holocene (6000 years B.P.) and for the LGM (21,000 years B.P.) are compared. The colder and windier conditions simulated for the LGM produced a summer monsoon whose westerly winds are stronger and whose precipitation and snowfall into the eastern Himalaya are increased, with drier conditions over the rest of the Indian subcontinent and over most of southwest Asia.The mid-Holocene monsoon circulation is stronger than today, and annual mean snow accumulation is increased over the northwestern Himalaya. These changes in precipitation and snow accumulation are analyzed in terms of the altered atmospheric circulations, which are in turn driven by changes in radiative forcing, sea surface temperatures, and sea surface height. All of these factors are therefore demonstrated to be important in governing the spatial distribution of snow and ice deposition in the Himalaya during the late Quaternary, and are likely to have contributed to the observed asynchroneity of Himalayan glaciation and Northern Hemisphere ice sheet volume.     

Examining the sensitivity of Earth's climate to the removal of ozone, landmasses and enhanced ocean heat transport in the GENESIS global climate model

This paper examines the cloud radiative forcing and its impacts on the surface climate for global climate model simulations that use reduced ozone concentrations and land fractions as boundary conditions. In one simulation using present-day land continents, ozone concentrations are reduced to zero and compared to the present-day climate simulation. In the second set of simulations under global ocean conditions, the implied poleward transport of heat by the ocean is varied. The removal of ozone causes an increase in longwave cloud radiative forcing at the top of the atmosphere and the surface. The increase in longwave forcing melts sea-ice and snow at high latitudes leading 10–14°C warmer temperatures and globally a 2°C increase. The global ocean simulations lead to higher cloud fractions than present-day simulation. Without poleward transport of heat by the ocean, surface temperatures cool as a result of higher cloud fractions. Increasing the ocean heat transport by a factor of 3.33 brings about ice-free conditions. An 11°C difference in globally averaged surface air temperatures is found between the enhanced and zero poleward oceanic heat transport simulations. The longwave cloud radiative forcing from high cloud fractions enhance the surface warming in the polar regions during the winter season. Conversely, during the summer season, a high cloud fraction increases the shortwave cloud radiative forcing producing only moderately warm temperatures in the polar regions. High cloud fractions in polar regions during warm periods throughout geologic times may help to explain the reduced equator to pole temperature gradient.     

Detection of rapid climate change in Last Glacial fluvial successions in The Netherlands

Climate change during the Last Glacial is considered as a major forcing factor of fluvial system changes. A continuous succession of fluvial sediments, reflecting adaptations to climate change from the Weichselian Middle Pleniglacial (oxygen isotope stage 3) onwards, occurs in lowland river basins in the Netherlands.A comparison of the Pleniglacial and Late Glacial fluvial record in the Netherlands shows that climatic oscillations of similar magnitude did not produce changes in the fluvial sedimentary system of equal magnitude. The Late Glacial fluvial system proves to be highly sensitive to climate change. By contrast, many of the rapid climate changes that have occurred during oxygen isotope stage 3, according to the Greenland ice core record, are not detectable in the fluvial sediments. This can be explained by differences in the impact of the climate variations on drainage basin vegetation. During the Late Glacial, the tree line repeatedly shifted through the Netherlands, whereas the area remained within the tundra zone during the Middle Pleniglacial. Precipitation variations and permafrost aggradation and degradation have played a secondary role.The Weichselian fluvial succession in the Netherlands demonstrates that detection of a change in the fluvial sedimentary system and relating this change to climate change is subject to methodological limitations. The climatic significance of changes in the fluvial record should be carefully evaluated, as well as their chronology. The possibility that climate did not influence the fluvial system should always be considered as a null hypothesis in studies on fluvial successions.     

Enceladus' south polar sea

Recent observations of the south pole of Saturn's moon Enceladus by the Cassini spacecraft have revealed an active world, powered by internal heat. In this paper, we propose that localized subsurface melting on Enceladus has produced an internal south polar sea. Evidence for this localized sea comes from the shape of Enceladus, which does not match a differentiated body at its current orbital position. We show that melting induced by the observed heat flow at the south pole produces a large enough pit to match the shape of Enceladus with a differentiated rock and ice interior. Numerical modeling of melting and ice flow shows that the sea produced beneath the south pole is stable against inflow of ductile ice from its surroundings for the duration of the heating. The shape modification due to melting also produces a negative degree-two gravity anomaly, which can reorient the spin axis of Enceladus in order to place the sea at the pole.     

Testing the effects of ocean heat transport on climate

An atmospheric general circulation model is run with boundary conditions representing different amounts of equator-to-pole oceanic heat transport. Oceanic heat transport underneath sea ice is held fixed, minimizing positive feedbacks due to sea ice and thereby providing a lower bound on the effects of oceanic heat transport on climate. When oceanic heat transport is reduced, some compensating increases in atmospheric heat transport occur, but tropical surface temperatures increase and atmospheric circulation and precipitation patterns undergo significant changes. We conclude that the ability of the oceans to generate past and future climatic changes through transport of heat is substantial, even though it is limited by a tendency of the atmosphere to partly compensate for changes in oceanic heat transport.     

Simulations of the impact of orbital forcing and ocean on the Asian summer monsoon during the Holocene

The importance of orbital forcing and ocean impact on the Asian summer monsoon in the Holocene is investigated by comparing simulations with a fully coupled ocean–atmosphere general circulation model (FOAM) and with the atmospheric component of this model (FSSTAM) forced with prescribed modern sea-surface temperatures (SSTs). The results show: (1) the ocean amplifies the orbitally-induced increase in African monsoon precipitation, makes somewhat increase in southern India and damps the increase over the southeastern China. (2) The ocean could change the spatial distribution and local intensity of the orbitally-induced latitudinal atmospheric oscillation over the southeastern China and the subtropical western Pacific Ocean. (3) The orbital forcing mostly enhances the Asian summer precipitation in the FOAM and FSSTAM simulations. However, the ocean reduces the orbitally-induced summer precipitation and postpones the time of summer monsoon onset over the Asian monsoon region. (4) The orbital forcing considerably enhances the intensity of upper divergence, which is amplified by ocean further, over the eastern hemisphere. But the divergence is weaker in the FOAM simulations than in the FSSTAM simulations when the orbital forcing is fixed. (5) The orbital forcing can enhance the amplitude of precipitation variability over the subtropical Africa, the southeastern China and northwestern China, inversely, reduce it over central India and North China in the FOAM and FSSTAM simulations. The ocean obviously reduces the amplitude of precipitation variability over most of the Asian monsoon regions in the fixed orbital forcing simulations. (6) The areas characterized by increased summer precipitation in the long-term mean are mostly characterized by increased amplitude of short-term variability, whereas regions characterized by decreased precipitation are primarily characterized by decreased amplitude of short-term variability. However, the influences of orbital forcing or dynamical ocean on regional climate depend on the model.     

Coupled atmosphere-ocean models of Titan's past

We have developed a coupled atmosphere and ocean model of Titan's surface. The atmospheric model is a 1-D spectrally-resolved radiative-convective model. The ocean thermodynamics are based upon solution theory. The ocean, initially composed of CH4, becomes progressively enriched in ethane over time. The partial pressures of N2 and CH4 in the atmosphere are dependent on the ocean temperature and composition. We find that the resulting system is stable against a runaway greenhouse. Accounting for the decreased solar luminosity, we find that Titan's surface temperature was about 20 K colder 4 Gyr ago. Without an ocean, but only small CH4 lakes, the temperature change is 12 K. In both cases we find that the surface of Titan may have been ice covered about 3 Gyr ago. In the lakes case condensation of N2 provides the ice, whereas in the ocean case the ocean freezes. The dominant factor influencing the evolution of Titan's surface temperature is the change in the solar constant--amplified, if an ocean is present, by the temperature dependence of the solubility of N2. Accretional heating can dramatically alter the surface temperature; a surface thermal flux of 500 erg cm-2 sec-1, representative of small levels of accretional heating, results in a approximately 20 K change in surface temperatures.     

Liquid water on Mars

The factors which affect fusion and evaporation of ice under a variety of Martian surface conditions are examined. It is found that a frost or ice deposit will pass through a transient liquid phase in temperate latitudes during summer, if the ice is partly or wholly dust covered. The barrier to free gaseous diffusion which the surface material presents is, under favorable (and definable) conditions, more than adequate to force the water to remain in the liquid state until its evaporation is complete. Furthermore, for a realistic range of regolith particle sizes and porosities, and depths of burial of the ice, the lifetime of the ice can be considerably longer than the duration of a single diurnal warming cycle. Current knowledge of the seosonal and diurnal behavior of the atmospheric vapor is summarized and discussed as it relates to the availability of surface ice at temperate latitudes.     

A sensitivity study of the role of continental location and area on Paleozoic climate

The seasonal variation of the surface temperature is calculated for various idealized paleogeographic conditions with a 1.5-dimensional (1.5-D) coupled climate-sea ice model. The sensitivity of the annual and summer polar temperatures to the meridional oceanic heat transport and to the parameterizations adopted for the snow and sea ice albedos is examined in connection with the location and size of a polar global super-continent. It is shown that the high latitude summer temperatures remain below the freezing point in all numerical simulations with a polar super-continent, thus suggesting the potential role played by a large polar continental mass in the initiation of glaciations. These results are in agreement with a previous 1.5-D energy balance model (EBM) study but in conflict with two-dimensional (2-D) EBMs suggesting above-freezing high latitude summer temperatures in the case of a polar-centered super-continent. It is also found that the amount of seasonality is strongly dependent on the details of the surface albedo feedback parameterizations and could explain the various model diverging results.If a simplified temperature dependence of the silicate weathering rate controlling the long-term carbon cycle is included, the atmospheric CO₂ level is significantly increased in the case of a polar-centered continent but summer temperatures still remain below freezing.     

Land water storage contribution to sea level from GRACE geoid data over 2003–2006

Seasonal and inter-annual change in land water storage (expressed in terms of water volume change) over 27 large river basins worldwide are estimated from monthly GRACE geoids solutions computed at GFZ from February 2003 to February 2006. The largest annual water volume change is found in the Amazon basin, followed by the Parana, Ob, Orinoco, Tocantins, Niger, Congo, Ganges, Mekong, and Brahmaputra. In terms of trend over the 3-year period, positive and negative values are observed but in a number of cases computed trends are at the noise level. However significant negative trends are found in the Amazon, Ganges, Mississippi, Nile, Parana, and Zambezi basins, indicating water mass loss over that period. Positive trends (water mass gain) are marginally significant. We have computed the land water contribution to sea level change. On average over the 3-year time span, we find that the net effect is positive (net loss of water in terrestrial reservoirs), on the order of 0.19 +/− 0.06 mm/yr. If sustained over a longer time span than considered here, such a value may become comparable to the ice sheets contribution to sea level rise.     

Interannual variability of regional climate and its change due to the greenhouse effect

Interannual variability of regional climate was investigated on a seasonal basis. Observations and two global climate model (GCM) simulations were intercompared to identify model biases and climate change signals due to the enhanced greenhouse effect. Observed record length varies from 40 to 100 years, while the model output comes from two 100-year equilibrium climate simulations corresponding to atmospheric greenhouse gas concentrations at observed 1990 and projected 2050 levels. The GCM includes an atmosphere based on the NCAR CCM1 with the addition of the radiative effects of CH₄, N₂O and CFCs, a bulk layer land surface and a mixed-layer ocean with thermodynamic sea-ice and fixed meridional oceanic heat transport.Because comparisons of interannual variability are sensitive to the time period chosen, a climate ensemble technique has been developed. This technique provides comparisons between variance ratios of two time series for all possible contiguous sub-periods of a fixed length. The time autocorrelation is thus preserved within each sub-period. The optimal sub-period length was found to be 30 years, based on which robust statistics of the ensemble were obtained to identify substantial differences in interannual variability that are both physically important and statistically significant.Several aspects of observed interannual variability were reproduced by the GCM. These include: global surface air temperature; Arctic sea-ice extent; and regional variability of surface air temperature, sea level pressure and 500 mb height over about one quarter of the observed data domains. Substantial biases, however, exist over broad regions, where strong seasonality and systematic links between variables were identified. For instance, during summer substantially greater model variability was found for both surface air temperature and sea-level pressure over land areas between 20–50°N, while this tendency was confined to 20–30°N in other seasons. When greenhouse gas concentrations increase, atmospheric moisture variability is substantially larger over areas that experience the greatest surface warming. This corresponds to an intensified hydrologic cycle and, hence, regional increases in precipitation variability. Surface air temperature variability increases where hydrologic processes vary greatly or where mean soil moisture is much reduced. In contrast, temperature variability decreases substantially where sea-ice melts completely. These results indicate that regional changes in interannual variability due to the enhanced greenhouse effect are associated with mechanisms that depend on the variable and season.     

Thoughts on monitoring the effects of climate change on the surface elevation of the Greenland ice sheet

The effect of mass balance variations on changes in surface elevation of the Greenland ice sheet is examined in connection with the rise in sea level that will be caused by increased melting. Changes in surface elevation of several metres (of either sign) can occur in the ablation area of the ice sheet over a period of a few years as a result of random ablation forcing without being evidence of change in mean climate. Similar, but smaller, changes can occur in the accumulation area due to accumulation forcing. The ablation area of the ice sheet probably thickened from the mid-1970s to the mid-1980s as a result of lower ablation in that period but thinned again in the late 1980s as a result of higher ablation then. There is no evidence of any present trend of increased melting. Future climate warming will involve an accelerated thinning of the ablation area that could be detected in 1–2 decades against the background of natural fluctuations in surface elevation.     

Climate changes detected through the world's longest sea level series

The sea level series of Stockholm in the Baltic Sea, commencing already in 1774, is analysed in various ways together with contemporary climate data, in order to investigate long-term sea level changes and their relations to climate changes.First, a study of the eustatic rise of sea level, based on annual mean sea levels, is peformed, and compared with other sea level and climate studies. It is concluded that the general climatic rise of sea level has increased significantly (99.9%) from about 0.0 mm/year during the end of the Little Ice Age, to about 1.0 mm/year during the past century, characterized by melting of glaciers. Such sea level changes due to northern hemisphere climate variations since 800 A.D. have (hitherto) probably always kept within −1.5 and +1.5 mm/year, with an average fairly close to zero.Second, an investigation of the sea level variability, also based on annual mean sea levels, is performed together with temperature and wind variabilities. It is found that the interannual sea level variability of the Baltic Sea has decreased significantly (98%) from the end of the 1700s to the beginning of the 1900s; after that it has increased significantly (95%) again. Precisely the same is found to apply to winter climate or, more specifically, to the interannual winter temperature variability and the interannual winter wind variability. The common origin of all these long-term changes turn out to be two consecutive winter wind processes over the North and Baltic Seas, especially the Baltic entrance. From the end of the 1700s to the beginning of the 1900s, there has been a rapidly decreasing number of dominating winter winds from northeast, and after that there has been an increasing number of dominating winter winds from southwest. This may indicate corresponding long-term changes in the North Atlantic Oscillation.Third, using monthly mean sea levels together with corresponding wind data, seasonal variations are investigated. The seasonal sea level variation in the Baltic Sea has increased significantly (99%) since the early 1800s, together with a shift of the maximum from late summer to early winter. It is found that the main origin is a secular change of the winter wind conditions over the Baltic entrance, with increasing southwesterly winds in early winter. This might also be related to a long-term change in the North Atlantic Oscillation.     

大气、陆地水储量和海水质量分布变化与地球低阶引力场球谐系数的关系

大气、固体地球及海洋组成了一个复杂、变化的地球动力学系统，这一系统中的任一质量分布变化都将产生地球引力场变化。采用全球7000多个地面气象台站的月平均降水及温度资料、NCEP提供气压月均值、TOPEX／Poseidon卫星测高资料和WOA98海水温度及盐度模型计算了大气、陆地水储量和海水质量分布变化引起地球低阶引力场系数变化。比较综合大气、陆地水储量和海水质量分布变化对带谐项J2，J3，J4影响的计算结果和人卫激光卫星的测定结果，可以看出，大气、陆地水储量和海水质量分布变化是引起地球低阶引力场系数周年变化的重要激发源。     

The link between abrupt climate change and basin stratigraphy: a numerical approach

To use basin stratigraphy for studying past climate change, it is important to understand the influence of evolving boundary conditions (river discharge and sediment flux, initial bathymetry, sea level, subsidence) and the complex interplay of the redistribution processes (plumes, turbidity currents, debris flows). To provide understanding of this complexity, we have employed source to sink numerical models to evaluate which process dominates the observed variability in a sedimentary record of two coastal Pacific basins, Knight Inlet in British Columbia and the Eel Margin of northern California.During the last glacial period, the Eel River supplied comparatively more sediment with a less variable flux to the ocean, while today the river is dominated by episodic events. Model results show this change in the variability of sediment flux to be as important to the deposit character as is the change in the volume of sediment supply. Due to the complex interaction of flooding events and ocean storm events, the more episodic flood deposits of recent times are less well preserved than the flood deposits associated with an ice-age climate.In Knight Inlet, the evolving boundary conditions (rapidly prograding coastline, secondary transport by gravity flows from sediment failures) are a strong influence on the sedimentary record. The delta and gravity flow deposits punctuate the sedimentary record formed by hemipelagic sedimentation from river plumes. Missing time intervals due to sediment failures can take away the advantage of the otherwise amplified lithologic record of discharge events, given the enclosed nature of the fjord basin.