Natural forcing of climate during the last millennium: fingerprint of solar variability

The variability of the climate during the last millennium is partly forced by changes in total solar irradiance (TSI). Nevertheless, the amplitude of these TSI changes is very small so that recent reconstruction data suggest that low frequency variations in the North Atlantic Oscillation (NAO) and in the thermohaline circulation may have amplified, in the North Atlantic sector and mostly in winter, the radiative changes due to TSI variations. In this study we use a state-of-the-art climate model to simulate the last millennium. We find that modelled variations of surface temperature in the Northern Hemisphere are coherent with existing reconstructions. Moreover, in the model, the low frequency variability of this mean hemispheric temperature is found to be correlated at 0.74 with the solar forcing for the period 1001?C1860. Then, we focus on the regional climatic fingerprint of solar forcing in winter and find a significant relationship between the low frequency TSI forcing and the NAO with a time lag of more than 40?years for the response of the NAO. Such a lag is larger than the around 20-year lag suggested in other studies. We argue that this lag is due, in the model, to a northward shift of the tropical atmospheric convection in the Pacific Ocean, which is maximum more than four decades after the solar forcing increase. This shift then forces a positive NAO through an atmospheric wave connection related to the jet-stream wave guide. The shift of the tropical convection is due to the persistence of anomalous warm SST forcing the anomalous precipitation, associated with the advection of warm SST by the North Pacific subtropical gyre in a few decades. Finally, we analyse the response of the Atlantic meridional overturning circulation to solar forcing and find that the former is weakened when the latter increases. Changes in wind stress, notably due to the NAO, modify the barotropic streamfunction in the Atlantic 50?years after solar variations. This implies a wind-driven modification of the oceanic circulation in the Atlantic sector in response to changes in solar forcing, in addition to the variations of the thermohaline circulation.     

The medieval solar activity maximum

Paleoclimatic studies of the Medieval Solar Maximum (c. A.D. 1100–1250, corresponding with the span of the Medieval Warm Epoch) may prove useful because it provides a better analog to the present solar forcing than the intervening era. The Medieval Solar Activity Maximum caused the cosmogenic isotope production minimum during the 12th and 13th Centuries A.D. reflected by ¹⁴C and¹⁰Be records stored in natural archives. These records suggest solar activity has returned to Medieval Solar Maximum highs after a prolonged period of reduced solar activity. Climate forcing by increased solar activity may explain some of this century's temperature rise without assuming unacceptably high climate sensitivity. By analogy with the Medieval Solar Activity Maximum, the contemporary solar activity maximum may be projected to last for 150 years. The maximum temperature increase forced by increased solar activity stays well below the predicted doubled atmospheric CO₂ greenhouse forcing.     

Carbon-14 production compared to oxygen isotope records from Camp Century,Greenland and Devon Island,Canada

Carbon-14 production rate variations that are not explainable by geomagnetic changes are thought to be in antiphase with solar activity and as such should be in antiphase with paleotemperature records or proxy temperature histories such as those obtainable from oxygen isotope analyses of ice cores. Oxygen isotope records from Camp Century, Greenland and Devon Island Ice Cap are in phase with each other over thousands of years and in antiphase to the ¹⁴C production rate residuals.     

Bi-decadal variability excited in the coupled ocean–atmosphere system by strong tropical volcanic eruptions

Decadal and bi-decadal climate responses to tropical strong volcanic eruptions (SVEs) are inspected in an ensemble simulation covering the last millennium based on the Max Planck Institute—Earth system model. An unprecedentedly large collection of pre-industrial SVEs (up to 45) producing a peak annual-average top-of-atmosphere radiative perturbation larger than ?1.5 Wm^?2 is investigated by composite analysis. Post-eruption oceanic and atmospheric anomalies coherently describe a fluctuation in the coupled ocean–atmosphere system with an average length of 20–25 years. The study provides a new physically consistent theoretical framework to interpret decadal Northern Hemisphere (NH) regional winter climates variability during the last millennium. The fluctuation particularly involves interactions between the Atlantic meridional overturning circulation and the North Atlantic gyre circulation closely linked to the state of the winter North Atlantic Oscillation. It is characterized by major distinctive details. Among them, the most prominent are: (a) a strong signal amplification in the Arctic region which allows for a sustained strengthened teleconnection between the North Pacific and the North Atlantic during the first post-eruption decade and which entails important implications from oceanic heat transport and from post-eruption sea ice dynamics, and (b) an anomalous surface winter warming emerging over the Scandinavian/Western Russian region around 10–12 years after a major eruption. The simulated long-term climate response to SVEs depends, to some extent, on background conditions. Consequently, ensemble simulations spanning different phases of background multidecadal and longer climate variability are necessary to constrain the range of possible post-eruption decadal evolution of NH regional winter climates.     

A 1,000-year ice core record of interannual to multidecadal variations in atmospheric circulation over the North Atlantic

Interannual to multidecadal modes in ocean/atmosphere dynamics in the North Atlantic region have been identified using sea salt aerosol proxy records from northern Greenland ice cores over the last 1,000 years. Sea salt concentrations show a consistent relationship with anomalies in the meridional pressure gradient over the North Atlantic region over all considered time scales. These pressure anomalies are connected to shifts in storm tracks, leading to lower pressure and higher storm activity, hence, higher sea salt export over the Greenland ice sheet. Two modes of long-term variability with a period of 10.4 years and 62 years could be identified. The latter is connected to long-term changes in sea surface temperature (SST) as documented by a high correlation of North Atlantic SST with our sea salt record over the last 150 years. Long-term reconstruction of these modes shows that the 10.4-year cycle has been a phenomenon persistent over the last millennium while the 62-year cycle has been mainly active after 1700. Accordingly, the longer-term persistence of this multidecadal variability in sea salt points also to significant variations in SST over the last 300 years.     

耦合模式ICM模拟的近千年气候特征

利用中国科学院大气物理研究所季风系统研究中心发展的气候模式(Integrated Climate Model,ICM)开展了近千年气候模拟试验,考察了模式对过去千年温度和大气涛动变化的模拟,并分析了全球季风百年到千年尺度的变化。结果表明:模式对百年尺度气候变率有较好的模拟能力,900~1200年北半球平均表面温度偏高,1500~1800年温度偏低,模拟的北半球、南半球平均表面温度都呈现出了19世纪至2000年的快速增暖。模式对大气涛动百年尺度变化的模拟与重建资料存在较大的不同。全球季风在850~1050年、1150~1200年和1300~1420加强,在1210~1300年和1600~1850年减弱。1875~2000年全球季风指数呈直线上升趋势。中世纪气候异常期(MWP)季风强度在全球大部分季风区域增加,小冰期(LIA)则相反。20世纪暖期(PWP)全球季风强度显著增加,其中赤道西太平洋增加超过1 mm/d。     

A dissection of the surface temperature biases in the Community Earth System Model

Based upon the climate feedback-responses analysis method, a quantitative attribution analysis is conducted for the annual-mean surface temperature biases in the Community Earth System Model version 1 (CESM1). Surface temperature biases are decomposed into partial temperature biases associated with model biases in albedo, water vapor, cloud, sensible/latent heat flux, surface dynamics, and atmospheric dynamics. A globally-averaged cold bias of ?1.22 K in CESM1 is largely attributable to albedo bias that accounts for approximately ?0.80 K. Over land, albedo bias contributes ?1.20 K to the averaged cold bias of ?1.45 K. The cold bias over ocean, on the other hand, results from multiple factors including albedo, cloud, oceanic dynamics, and atmospheric dynamics. Bias in the model representation of oceanic dynamics is the primary cause of cold (warm) biases in the Northern (Southern) Hemisphere oceans while surface latent heat flux over oceans always acts to compensate for the overall temperature biases. Albedo bias resulted from the model’s simulation of snow cover and sea ice is the main contributor to temperature biases over high-latitude lands and the Arctic and Antarctic region. Longwave effect of water vapor is responsible for an overall warm (cold) bias in the subtropics (tropics) due to an overestimate (underestimate) of specific humidity in the region. Cloud forcing of temperature biases exhibits large regional variations and the model bias in the simulated ocean mixed layer depth is a key contributor to the partial sea surface temperature biases associated with oceanic dynamics. On a global scale, biases in the model representation of radiative processes account more for surface temperature biases compared to non-radiative, dynamical processes.     

Ice and climate modeling: An editorial essay

The growth and decay of ice sheets are driven by forces affecting the seasonal cycles of snowfall and snowmelt. The external forces are likely to be variations in the earth's orbit which cause differences in the solar radiation received. Radiational control of snowmelt is modulated by the seasonal cycles of snow albedo and cloud cover. The effects of orbital changes can be magnified by feedbacks involving atmospheric CO₂ content, ocean temperatures and desert areas. Climate modeling of the causes of the Pleistocene ice ages involves modeling the interactions of all components of the climate system; snow, sea ice, glacier ice, the ocean, the atmosphere, and the solid earth. Such modeling is also necessary for interpreting oxygen isotope records from ice and ocean as paleoclimatic evidence.     

Role of the ocean in controlling atmospheric CO2 concentration in the course of global glaciations

Responses of ocean circulation and ocean carbon cycle in the course of a global glaciation from the present Earth conditions are investigated by using a coupled climate-biogeochemical model. We investigate steady states of the climate system under colder conditions induced by a reduction of solar constant from the present condition. A globally ice-covered solution is obtained under the solar constant of 92.2% of the present value. We found that because almost all of sea water reaches the frozen point, the ocean stratification is maintained not by temperature but by salinity just before the global glaciation (at the solar constant of 92.3%). It is demonstrated that the ocean circulation is driven not by the surface cooling but by the surface freshwater forcing associated with formation and melting of sea ice. As a result, the deep ocean is ventilated exclusively by deep water formation in southern high latitudes where sea ice production takes place much more massively than northern high latitudes. We also found that atmospheric CO₂ concentration decreases through the ocean carbon cycle. This reduction is explained primarily by an increase of solubility of CO₂ due to a decrease of sea surface temperature, whereas the export production weakens by 30% just before the global glaciation. In order to investigate the conditions for the atmospheric CO₂ reduction to cause global glaciations, we also conduct a series of simulations in which the total amount of carbon in the atmosphere?Cocean system is reduced from the present condition. Under the present solar constant, the results show that the global glaciation takes place when the total carbon decreases to be 70% of the present-day value. Just before the glaciation, weathering rate becomes very small (almost 10% of the present value) and the organic carbon burial declines due to weakened biological productivity. Therefore, outgoing carbon flux from the atmosphere?Cocean system significantly decreases. This suggests the atmosphere?Cocean system has strong negative feedback loops against decline of the total carbon content. The results obtained here imply that some processes outside the atmosphere?Cocean feedback loops may be required to cause global glaciations.     

A comparison between medieval and current climate warming using the Przewalskii’s juniper tree-ring data

Comparison with the climate of the past centuries has demonstrated until recently the unprecedented warming at the scale of the last millennium at least. This is embodied in the latest report of the Intergovernmental Panel on Climate Change. However, recently the studies have appeared putting this statement into question. A new 1000-year long reconstruction based on the tree-ring variations of the long-living Chinese junipers (Sabina Przewalskii Kom.) growing in the northeastern part of the Tibetan Plateau reveals that the climate during and immediately after the medieval maximum of solar activity was warmer that the present-day one, all subsequent cooling coincided with the periods of low solar activity, and the warming in the 1970s–1990s followed a new maximum of the solar activity which peak fell on the 1960s.     

Selected topics in arctic atmosphere and climate

This paper summarizes the main elements of four IPY projects that examine the Arctic Atmosphere. All four projects focus on present conditions with a view to anticipating possible climate change. All four investigate the Arctic atmosphere, ocean, ice, and land interfacial surfaces. One project uses computer models to simulate the dynamics of the Arctic atmosphere, storms, and their interactions with the ocean and ice interface. Another project uses statistical methods to infer transports of pollutants as simulated in large-scale global atmospheric and oceanic models verifying results with available observations. A third project focuses on measurements of pollutants at the ice-ocean?Catmosphere interface, with reference to model estimates. The fourth project is concerned with multiple, high accuracy measurements at Eureka in the Canadian Archipelago. While these projects are distinctly different, led by different teams and interdisciplinary collaborators, with different technical approaches and methodologies, and differing objectives, they all strive to understand the processes of the Arctic atmosphere and climate, and to lay the basis for projections of future changes. Key findings include: ? Decreased sea ice leads to more intense storms, higher winds, reduced surface albedo, increased surface air temperature, and enhanced vertical mixing in the upper ocean. ? Arctic warming may affect toxic chemicals by remobilizing persistent organic pollutants and augmenting mercury deposition/retention in the environment. ? Changes in sea ice can dramatically change processes in and at the ice surface related to ozone, mercury and bromine oxide and related chemical/physical properties. ? Structure and properties of the Arctic atmospheric??troposphere to stratosphere??and tracking of transport of pollution and smoke plumes from mid-latitudes to the poles.     

The initiation of ice sheet growth,Milankovitch solar radiation variations,and the 100 ky ice age cycle

Much work has gone into deciphering the causes of the large scale glacial/interglacial variations in the climate system over the last 900 000 years. While variations on the 41 thousand year (ky) and 23 ky time scales seem to be linearly linked to the variations in the distribution of solar radiation at the top of the atmosphere, Milankovitch solar radiation variations, the causes of the dominant 100 ky cycle in the geologic record are still unknown. One of the aspects of this cycle that is not well understood is how large scale ice sheet growth is initiated. Here we describe the mechanisms by which large scale ice sheet growth may have been initiated by the changes in the seasonal and latitudinal distribution of solar radiation over the past 160 ky. This is done through the use of a coupled energy balance climate-thermodynamic sea ice model that includes a hydrologic cycle which computes precipitation, and a land surface energy balance which determines the net accumulation of snow and ice. Results indicate that the initiation of ice sheet growth is possible during times of extremely low summer solstice solar radiation as a result of a large decrease in ablation during the critical melt season.     

A model of late Pleistocene ice sheet growth with realistic geography and simplified cryodynamics and geodynamics

A global two-dimensional one-level seasonal energy-balance model is asynchronously coupled to vertically integrated ice-flow models (which depend both on latitude and longitude) to study the response of the atmosphere-ocean-cryosphere-lithosphere system to solar forcing for the last ice age cycle of the late Pleistocene. The model simulates the position of the North American and European ice sheet complexes at the last glacial maximum satisfactorily. Both the geographic distributions of the ice volumes delivered by the model and their masses are a reasonable approximation to those inferred on the basis of relative sea level data (Tushingham and Peltier 1990). The sensitivity of the coupled model over the last glacial-interglacial cycle to solar forcing is nevertheless low, which suggests that further physical mechanisms will have to be added to the model (such as explicit basal sliding and ice shelves which would respond to sea-level variations and therefore permit marine incursions), if it is to adequately simulate the terminations that control the 10⁵ year ice age cycle. One should also incorporate long-term variations of the greenhouse gases (Manabe et al. 1985b).This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dümenil     

Changes in ice-season characteristics of a European Arctic lake from 1964 to 2008

The long-term ice record (from 1964 to 2008) of an Arctic lake in northern Europe (Lake Kilpisj?rvi) reveals the response of lake ice to climate change at local and regional scales. Average freeze-up and ice breakup occurred on 9 November and 19 June, respectively. The freeze-up has been significantly delayed at a rate of 2.3 d per decade from 1964 onward (P?<?0.05). No significant change has taken place in ice breakup. Annual average ice thickness has become smaller since the mid-1980s (P?<?0.05). Air temperature during the early ice season significantly affected the ice thickness. The freeze-up date exhibits the highest correlation with the 2-month average daily minimum air temperature centered at the end of October, while the ice breakup date exhibits the highest correlation with the 2-month average daily maximal air temperature centered in mid May. A 1°C increase in the surface air temperature corresponds to a freeze-up later by 3.4?days and an ice breakup earlier by 3.6?days. Snow cover is a critical factor in lake-ice climatology. For cumulative November to March precipitation of less than 0.13?m, the insulating effect of the snow dominated, while higher rates of precipitation favored thicker ice due to the formation of snow ice. Variations in ice records of Lake Kilpisj?rvi can serve as an indicator of climate variations across the northern Europe. The North Atlantic Oscillation (NAO) does not significantly affect the ice season there, although both the local air temperatures and winter precipitation contain a strong NAO signal.     

Warm winds from the Pacific caused extensive Arctic sea-ice melt in summer 2007

During summer 2007 the Arctic sea-ice shrank to the lowest extent ever observed. The role of the atmospheric energy transport in this extreme melt event is explored using the state-of-the-art ERA-Interim reanalysis data. We find that in summer 2007 there was an anomalous atmospheric flow of warm and humid air into the region that suffered severe melt. This anomaly was larger than during any other year in the data (1989?C2008). Convergence of the atmospheric energy transport over this area led to positive anomalies of the downward longwave radiation and turbulent fluxes. In the region that experienced unusual ice melt, the net anomaly of the surface fluxes provided enough extra energy to melt roughly one meter of ice during the melting season. When the ocean successively became ice-free, the surface-albedo decreased causing additional absorption of shortwave radiation, despite the fact that the downwelling solar radiation was smaller than average. We argue that the positive anomalies of net downward longwave radiation and turbulent fluxes played a key role in initiating the 2007 extreme ice melt, whereas the shortwave-radiation changes acted as an amplifying feedback mechanism in response to the melt.     

Urban air pollution: process identification, impact analysis and evaluation of forecast potential

The daily variations in the atmospheric pollutants like suspended particulate matter (SPM), respirable suspended particulate matter (RSPM), sulphur dioxide (SO₂) and nitrogen dioxide (NO₂) depend on both local meteorological processes and various natural as well as anthropogenic sources and sinks. It was shown in an earlier work (Goswami and Baruah in Mon Wea Rev 136(9):3597?C3607, 2008) that the daily variations in SPM over a location could be simulated quite well by considering daily meteorological fields from NCEP Reanalysis in combination with a model for natural and anthropogenic sources over Delhi. In the present work, we extend the scope of the model to include three other pollutants: RSPM, SO₂ and NO₂. While the basic conservation equations and the meteorological fields are common to all the three (and SPM) pollutants, the sources and sinks for each is modeled in a species-specific manner to obtain maximum skill. As we do not consider active chemistry, the present model provides the minimal dynamics of pollution over an urban location in terms of annual load; the model error is about 10% on the average, with no significant bias for any of the species.     

Solar irradiance during the last 1200 years based on cosmogenic nuclides

Based on a quantitative study of the common fluctuations of ¹⁴C and ¹⁰Be production rates, we have derived a time series of the solar magnetic variability over the last 1200 years. This record is converted into irradiance variations by linear scaling based on previous studies of sun‐like stars and of the sun's behavior over the last few centuries. The new solar irradiance record exhibits low values during the well‐known solar minima centered at about 1900, 1810 (Dalton) and 1690 ad (Maunder). Further back in time, a rather long period between 1450 and 1750 ad is characterized by low irradiance values. A shorter period is centered at about 1200 ad , with irradiance slightly higher or similar to present day values. It is tempting to correlate these periods with the so‐called "little ice age" and "medieval warm period", respectively. An accurate quantification of the climatic impact of this new irradiance record requires the use of coupled atmosphere–ocean general circulation models (GCMs). Nevertheless, our record is already compatible with a global cooling of about 0.5‐1°C during the "little ice age", and with a general cooling trend during the past millenium followed by global warming during the 20th century (Mann et al., 1999).     

大气随机动力学与可预报性

偶然性与必然性过程及其相互转化是世界事物变化复杂性的根源。根据布朗运动统计理论,提出了分子热运动是不稳定流体中湍流形成之源,由此形成不同宏观尺度的随机运动是大气运动固有的属性。观测事实表明,太阳辐射作为决定大气运动与变化的主要因子,它的变化具有随机性,是大气的随机强迫因子,它对气候变化具有决定性影响。地-气相互作用是一个时变的非线性相互反馈的耦合过程,形成了下边界对大气复杂的随机强迫作用,其界面交换耦合随机动力学模式尚待建立。由于大气过程固有的随机性以及随机的外强迫耦合作用,大气确定性预报的时效是有界的,它决定于预报对象的不确定性及其空间尺度与时间尺度,以及预报时效内的大气不确定性。由此,客观存在着大气过程的报不准关系。     

An assessment of the surface climate in the NCEP climate forecast system reanalysis

This paper analyzes surface climate variability in the climate forecast system reanalysis (CFSR) recently completed at the National Centers for Environmental Prediction (NCEP). The CFSR represents a new generation of reanalysis effort with first guess from a coupled atmosphere?Cocean?Csea ice?Cland forecast system. This study focuses on the analysis of climate variability for a set of surface variables including precipitation, surface air 2-m temperature (T2m), and surface heat fluxes. None of these quantities are assimilated directly and thus an assessment of their variability provides an independent measure of the accuracy. The CFSR is compared with observational estimates and three previous reanalyses (the NCEP/NCAR reanalysis or R1, the NCEP/DOE reanalysis or R2, and the ERA40 produced by the European Centre for Medium-Range Weather Forecasts). The CFSR has improved time-mean precipitation distribution over various regions compared to the three previous reanalyses, leading to a better representation of freshwater flux (evaporation minus precipitation). For interannual variability, the CFSR shows improved precipitation correlation with observations over the Indian Ocean, Maritime Continent, and western Pacific. The T2m of the CFSR is superior to R1 and R2 with more realistic interannual variability and long-term trend. On the other hand, the CFSR overestimates downward solar radiation flux over the tropical Western Hemisphere warm pool, consistent with a negative cloudiness bias and a positive sea surface temperature bias. Meanwhile, the evaporative latent heat flux in CFSR appears to be larger than other observational estimates over most of the globe. A few deficiencies in the long-term variations are identified in the CFSR. Firstly, dramatic changes are found around 1998?C2001 in the global average of a number of variables, possibly related to the changes in the assimilated satellite observations. Secondly, the use of multiple streams for the CFSR induces spurious jumps in soil moisture between adjacent streams. Thirdly, there is an inconsistency in long-term sea ice extent variations over the Arctic regions between the CFSR and other observations with the CFSR showing smaller sea ice extent before 1997 and larger extent starting in 1997. These deficiencies may have impacts on the application of the CFSR for climate diagnoses and predictions. Relationships between surface heat fluxes and SST tendency and between SST and precipitation are analyzed and compared with observational estimates and other reanalyses. Global mean fields of surface heat and water fluxes together with radiation fluxes at the top of the atmosphere are documented and presented over the entire globe, and for the ocean and land separately.     

Exciting natural modes of variability by solar and volcanic forcing: idealized and realistic experiments

Using multi-millenium simulations performed with the three-dimensional climate model ECBILT-CLIO, we analyze how variations in the external forcing can excite low-frequency modes of climate variability. We find that prescribing an idealized, abrupt decrease in solar irradiance can trigger a large perturbation of the oceanic thermohaline circulation (THC) associated with a cooling of more than 5 °C in the North Atlantic over decades to centuries. Using more realistic scenarios that include the variations of solar irradiance and the influence of volcanic eruptions, such large perturbations of the THC are not triggered. Nevertheless, modifications of the forcing can strongly modify the probability of very cold years in the North Atlantic. During those cold years, sea-ice covers a large part of the Nordic Seas and the inflow of warm Atlantic waters at high latitudes is strongly reduced. Those processes induce a temporarily, strong local amplification of the forcing and generate modifications of the atmospheric conditions. Simulations of the last millenium climate using realistic forcing reveal that the probability to have such very cold years in the model is higher during the period AD 1300–1850 than during the first centuries of the second millenium or during the twentieth century. This might explain the higher variability observed during this period in some climate records in the Nordic Seas.