Showing papers in "Climate Dynamics in 2014"

Global warming and 21st century drying

Abstract: Global warming is expected to increase the frequency and intensity of droughts in the twenty-first century, but the relative contributions from changes in moisture supply (precipitation) versus evaporative demand (potential evapotranspiration; PET) have not been comprehensively assessed. Using output from a suite of general circulation model (GCM) simulations from phase 5 of the Coupled Model Intercomparison Project, projected twenty-first century drying and wetting trends are investigated using two offline indices of surface moisture balance: the Palmer Drought Severity Index (PDSI) and the Standardized Precipitation Evapotranspiration Index (SPEI). PDSI and SPEI projections using precipitation and Penman-Monteith based PET changes from the GCMs generally agree, showing robust cross-model drying in western North America, Central America, the Mediterranean, southern Africa, and the Amazon and robust wetting occurring in the Northern Hemisphere high latitudes and east Africa (PDSI only). The SPEI is more sensitive to PET changes than the PDSI, especially in arid regions such as the Sahara and Middle East. Regional drying and wetting patterns largely mirror the spatially heterogeneous response of precipitation in the models, although drying in the PDSI and SPEI calculations extends beyond the regions of reduced precipitation. This expansion of drying areas is attributed to globally widespread increases in PET, caused by increases in surface net radiation and the vapor pressure deficit. Increased PET not only intensifies drying in areas where precipitation is already reduced, it also drives areas into drought that would otherwise experience little drying or even wetting from precipitation trends alone. This PET amplification effect is largest in the Northern Hemisphere mid-latitudes, and is especially pronounced in western North America, Europe, and southeast China. Compared to PDSI projections using precipitation changes only, the projections incorporating both precipitation and PET changes increase the percentage of global land area projected to experience at least moderate drying (PDSI standard deviation of ≤−1) by the end of the twenty-first century from 12 to 30 %. PET induced moderate drying is even more severe in the SPEI projections (SPEI standard deviation of ≤−1; 11 to 44 %), although this is likely less meaningful because much of the PET induced drying in the SPEI occurs in the aforementioned arid regions. Integrated accounting of both the supply and demand sides of the surface moisture balance is therefore critical for characterizing the full range of projected drought risks tied to increasing greenhouse gases and associated warming of the climate system.

ENSO representation in climate models: from CMIP3 to CMIP5

Abstract: We analyse the ability of CMIP3 and CMIP5 coupled ocean–atmosphere general circulation models (CGCMs) to simulate the tropical Pacific mean state and El Nino-Southern Oscillation (ENSO). The CMIP5 multi-model ensemble displays an encouraging 30 % reduction of the pervasive cold bias in the western Pacific, but no quantum leap in ENSO performance compared to CMIP3. CMIP3 and CMIP5 can thus be considered as one large ensemble (CMIP3 + CMIP5) for multi-model ENSO analysis. The too large diversity in CMIP3 ENSO amplitude is however reduced by a factor of two in CMIP5 and the ENSO life cycle (location of surface temperature anomalies, seasonal phase locking) is modestly improved. Other fundamental ENSO characteristics such as central Pacific precipitation anomalies however remain poorly represented. The sea surface temperature (SST)-latent heat flux feedback is slightly improved in the CMIP5 ensemble but the wind-SST feedback is still underestimated by 20–50 % and the shortwave-SST feedbacks remain underestimated by a factor of two. The improvement in ENSO amplitudes might therefore result from error compensations. The ability of CMIP models to simulate the SST-shortwave feedback, a major source of erroneous ENSO in CGCMs, is further detailed. In observations, this feedback is strongly nonlinear because the real atmosphere switches from subsident (positive feedback) to convective (negative feedback) regimes under the effect of seasonal and interannual variations. Only one-third of CMIP3 + CMIP5 models reproduce this regime shift, with the other models remaining locked in one of the two regimes. The modelled shortwave feedback nonlinearity increases with ENSO amplitude and the amplitude of this feedback in the spring strongly relates with the models ability to simulate ENSO phase locking. In a final stage, a subset of metrics is proposed in order to synthesize the ability of each CMIP3 and CMIP5 models to simulate ENSO main characteristics and key atmospheric feedbacks.

Future change of global monsoon in the CMIP5

Abstract: This study investigates future changes of Global Monsoon (GM) under anthropogenic global warming using 20 coupled models that participated in the phase five of Coupled Model Intercomparison Project (CMIP5) by comparing two runs: the historical run for 1850–2005 and the Representative Concentration Pathway (RCP) 4.5 run for 2006–2100. A metrics for evaluation of models’ performance on GM is designed to document performance for 1980–2005 and best four models are selected. The four best models’ multi-model ensemble (B4MME) projects the following changes in the twenty-first century under the RCP4.5 scenario. (1) Monsoon domain will not change appreciably but land monsoon domain over Asia tends to expand westward by 10.6 %. (2) The annual mean and range of GM precipitation and the percentage of local summer rainfall will all amplify at a significant level over most of the global region, both over land and over ocean. (3) There will be a more prominent northern-southern hemispheric asymmetry and eastern-western hemispheric asymmetry. (4) Northern Hemisphere (NH) monsoon onset will be advanced and withdrawal will be delayed. (5) Changes in monsoon precipitation exhibits huge differences between the NH and the Southern hemisphere (SH). The NH monsoon precipitation will increase significantly due to increase in temperature difference between the NH and SH, significant enhancement of the Hadley circulation, and atmospheric moistening, against stabilization of troposphere. There is a slight decrease of the Walker circulation but not significant against the inter-model spread. There are important differences between the CMIP 3 and CMIP5 results which are discussed in detail.

Regional and global projections of twenty-first century glacier mass changes in response to climate scenarios from global climate models

Abstract: A large component of present-day sea-level rise is due to the melt of glaciers other than the ice sheets. Recent projections of their contribution to global sea-level rise for the twenty-first century range between 70 and 180 mm, but bear significant uncertainty due to poor glacier inventory and lack of hypsometric data. Here, we aim to update the projections and improve quantification of their uncertainties by using a recently released global inventory containing outlines of almost every glacier in the world. We model volume change for each glacier in response to transient spatially-differentiated temperature and precipitation projections from 14 global climate models with two emission scenarios (RCP4.5 and RCP8.5) prepared for the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. The multi-model mean suggests sea-level rise of 155 ± 41 mm (RCP4.5) and 216 ± 44 mm (RCP8.5) over the period 2006–2100, reducing the current global glacier volume by 29 or 41 %. The largest contributors to projected global volume loss are the glaciers in the Canadian and Russian Arctic, Alaska, and glaciers peripheral to the Antarctic and Greenland ice sheets. Although small contributors to global volume loss, glaciers in Central Europe, low-latitude South America, Caucasus, North Asia, and Western Canada and US are projected to lose more than 80 % of their volume by 2100. However, large uncertainties in the projections remain due to the choice of global climate model and emission scenario. With a series of sensitivity tests we quantify additional uncertainties due to the calibration of our model with sparsely observed glacier mass changes. This gives an upper bound for the uncertainty range of ±84 mm sea-level rise by 2100 for each projection.

Recent intense hurricane response to global climate change

Abstract: An Anthropogenic Climate Change Index (ACCI) is developed and used to investigate the potential global warming contribution to current tropical cyclone activity. The ACCI is defined as the difference between the means of ensembles of climate simulations with and without anthropogenic gases and aerosols. This index indicates that the bulk of the current anthropogenic warming has occurred in the past four decades, which enables improved confidence in assessing hurricane changes as it removes many of the data issues from previous eras. We find no anthropogenic signal in annual global tropical cyclone or hurricane frequencies. But a strong signal is found in proportions of both weaker and stronger hurricanes: the proportion of Category 4 and 5 hurricanes has increased at a rate of ~25–30 % per °C of global warming after accounting for analysis and observing system changes. This has been balanced by a similar decrease in Category 1 and 2 hurricane proportions, leading to development of a distinctly bimodal intensity distribution, with the secondary maximum at Category 4 hurricanes. This global signal is reproduced in all ocean basins. The observed increase in Category 4–5 hurricanes may not continue at the same rate with future global warming. The analysis suggests that following an initial climate increase in intense hurricane proportions a saturation level will be reached beyond which any further global warming will have little effect.

Circulation effect: response of precipitation δ18O to the ENSO cycle in monsoon regions of China

Abstract: Inter-annual variation in the ratio of 18O to 16O of precipitation (δ18Op) in the monsoon regions of China (MRC, area approximately east of 100°E) has not yet been fully analyzed. Based on an analysis of the relationships between the time series of amount-weighted mean annual δ18O in precipitation (δ18Ow) and meteorological variables such as temperature, precipitation as well as atmospheric/oceanic circulation indices, it is recognized that the El Nino-Southern Oscillation (ENSO) cycle appears to be the dominant control on the inter-annual variation in δ18Op in the MRC. Further analysis shows that the trade wind plays a role in governing δ18Ow through affecting the intensity of the different summer monsoon circulations which are closely linked to the weakening (weaker than normal) and strengthening (stronger than normal) of the trade wind and gives the δ18Ow different values at or over inter-annual timescales. The southwest monsoon (SWM) drives long-distance transport of water vapor from Indian Ocean to the MRC, and along this pathway increasing rainout leads to more negative δ18Ow via Rayleigh distillation processes. In contrast, the southeast monsoon (SEM), which is consistent with the changes in the strength of the West Pacific subtropical high, drives short-distance water vapor transport from the West Pacific Ocean to the MRC and leads to less negative δ18Ow. Therefore, the δ18Ow value directly reflects the differences in influence between the SWM, which is strong when the SE trade wind is strong, and the SEM, which is strong when the SE trade wind is weak. In addition, the South China Sea Monsoon also transports local water vapor as well as plays a role in achieving the synchronization between the δ18Ow and ENSO. The author thus terms the δ18Op rhythm in the MRC the “circulation effect”. In turn, the δ18Op variation in the MRC has the potential to provide information on atmospheric circulation and the signal of δ18Op recorded in natural archives can then be used to deduce a long-term behavior of the tropical climate system.

Atmospheric impacts of Arctic sea-ice loss, 1979–2009: separating forced change from atmospheric internal variability

Abstract: The ongoing loss of Arctic sea-ice cover has implications for the wider climate system. The detection and importance of the atmospheric impacts of sea-ice loss depends, in part, on the relative magnitudes of the sea-ice forced change compared to natural atmospheric internal variability (AIV). This study analyses large ensembles of two independent atmospheric general circulation models in order to separate the forced response to historical Arctic sea-ice loss (1979–2009) from AIV, and to quantify signal-to-noise ratios. We also present results from a simulation with the sea-ice forcing roughly doubled in magnitude. In proximity to regions of sea-ice loss, we identify statistically significant near-surface atmospheric warming and precipitation increases, in autumn and winter in both models. In winter, both models exhibit a significant lowering of sea level pressure and geopotential height over the Arctic. All of these responses are broadly similar, but strengthened and/or more geographically extensive, when the sea-ice forcing is doubled in magnitude. Signal-to-noise ratios differ considerably between variables and locations. The temperature and precipitation responses are significantly easier to detect (higher signal-to-noise ratio) than the sea level pressure or geopotential height responses. Equally, the local response (i.e., in the vicinity of sea-ice loss) is easier to detect than the mid-latitude or upper-level responses. Based on our estimates of signal-to-noise, we conjecture that the local near-surface temperature and precipitation responses to past Arctic sea-ice loss exceed AIV and are detectable in observed records, but that the potential atmospheric circulation, upper-level and remote responses may be partially or wholly masked by AIV.

Temperature response to future urbanization and climate change

Abstract: This study examines the impact of future urban expansion on local near-surface temperature for Sydney (Australia) using a future climate scenario (A2). The Weather Research and Forecasting model was used to simulate the present (1990–2009) and future (2040–2059) climates of the region at 2-km spatial resolution. The standard land use of the model was replaced with a more accurate dataset that covers the Sydney area. The future simulation incorporates the projected changes in the urban area of Sydney to account for the expected urban expansion. A comparison between areas with projected land use changes and their surroundings was conducted to evaluate how urbanization and global warming will act together and to ascertain their combined effect on the local climate. The analysis of the temperature changes revealed that future urbanization will strongly affect minimum temperature, whereas little impact was detected for maximum temperature. The minimum temperature changes will be noticeable throughout the year. However, during winter and spring these differences will be particularly large and the increases could be double the increase due to global warming alone at 2050. Results indicated that the changes were mostly due to increased heat capacity of urban structures and reduced evaporation in the city environment.

The ocean's role in setting the mean position of the Inter-Tropical Convergence Zone

Abstract: Through study of observations and coupled climate simulations, it is argued that the mean position of the Inter-Tropical Convergence Zone (ITCZ) north of the equator is a consequence of a northwards heat transport across the equator by ocean circulation. Observations suggest that the hemispheric net radiative forcing of climate at the top of the atmosphere is almost perfectly symmetric about the equator, and so the total (atmosphere plus ocean) heat transport across the equator is small (order 0.2 PW northwards). Due to the Atlantic ocean’s meridional overturning circulation, however, the ocean carries significantly more heat northwards across the equator (order 0.4 PW) than does the coupled system. There are two primary consequences. First, atmospheric heat transport is southwards across the equator to compensate (0.2 PW southwards), resulting in the ITCZ being displaced north of the equator. Second, the atmosphere, and indeed the ocean, is slightly warmer (by perhaps 2 °C) in the northern hemisphere than in the southern hemisphere. This leads to the northern hemisphere emitting slightly more outgoing longwave radiation than the southern hemisphere by virtue of its relative warmth, supporting the small northward heat transport by the coupled system across the equator. To conclude, the coupled nature of the problem is illustrated through study of atmosphere–ocean–ice simulations in the idealized setting of an aquaplanet, resolving the key processes at work.

Influence of PDO on South Asian summer monsoon and monsoon–ENSO relation

Abstract: This study has investigated the possible relation between the Indian summer monsoon and the Pacific Decadal Oscillation (PDO) observed in the sea surface temperature (SST) of the North Pacific Ocean. Using long records of observations and coupled model (NCAR CCSM4) simulation, this study has found that the warm (cold) phase of the PDO is associated with deficit (excess) rainfall over India. The PDO extends its influence to the tropical Pacific and modifies the relation between the monsoon rainfall and El Nino-Southern Oscillation (ENSO). During the warm PDO period, the impact of El Nino (La Nina) on the monsoon rainfall is enhanced (reduced). A hypothesis put forward for the mechanism by which PDO affects the monsoon starts with the seasonal footprinting of SST from the North Pacific to the subtropical Pacific. This condition affects the trade winds, and either strengthens or weakens the Walker circulation over the Pacific and Indian Oceans depending on the phase of the PDO. The associated Hadley circulation in the monsoon region determines the impact of PDO on the monsoon rainfall. We suggest that knowing the phase of PDO may lead to better long-term prediction of the seasonal monsoon rainfall and the impact of ENSO on monsoon.

Climate model benchmarking with glacial and mid-Holocene climates

Abstract: Past climates provide a test of models’ ability to predict climate change. We present a comprehensive evaluation of state-of-the-art models against Last Glacial Maximum and mid-Holocene climates, using reconstructions of land and ocean climates and simulations from the Palaeoclimate Modelling and Coupled Modelling Intercomparison Projects. Newer models do not perform better than earlier versions despite higher resolution and complexity. Differences in climate sensitivity only weakly account for differences in model performance. In the glacial, models consistently underestimate land cooling (especially in winter) and overestimate ocean surface cooling (especially in the tropics). In the mid-Holocene, models generally underestimate the precipitation increase in the northern monsoon regions, and overestimate summer warming in central Eurasia. Models generally capture large-scale gradients of climate change but have more limited ability to reproduce spatial patterns. Despite these common biases, some models perform better than others.

Three Eurasian teleconnection patterns: spatial structures, temporal variability, and associated winter climate anomalies

Abstract: The Eurasian (EU) pattern is a distinct teleconnection pattern observed in boreal winter. Since the EU pattern was first identified, three types have been reported in the literature: the conventional EU pattern; the type 1 EU pattern, or Scandinavian (SCAND) pattern; and the type 2 EU pattern, or East Atlantic/West Russia (EATL/WRUS) pattern. Based on several reanalysis and observational datasets, the three EU patterns are extracted using the rotated empirical orthogonal function method. In order to provide a further distinction and understanding of the three EU patterns, a comprehensive side-by-side comparison is performed among them including their temporal variability, horizontal and vertical structure, related stationary Rossby wave activity, impact on climate, and possible driving factors associated with external forcing. The results reveal that all three EU patterns are characterised by a clear quasi-barotropic wave-train structure, but each has a distinct source and centre of action. Accordingly, their impacts on the precipitation and surface air temperature also differ from one other. Further evidence suggests that the conventional EU pattern is likely driven by anomalous sea surface temperatures (SST) over the North Atlantic, in which process the transient eddies are actively involved. The SCAND pattern is partly maintained by the vorticity source over Western Europe, which arises from the anomalous convergence/divergence over the Mediterranean and is efficiently driven by the tropical and southern Indian Ocean SST via divergent circulation. The EATL/WRUS pattern shows some linkage to the North American snow cover, and the involved process remains unclear and needs further investigation.

On the persistent spread in snow-albedo feedback

Abstract: Snow-albedo feedback (SAF) is examined in 25 climate change simulations participating in the Coupled Model Intercomparison Project version 5 (CMIP5). SAF behavior is compared to the feedback’s behavior in the previous (CMIP3) generation of global models. SAF strength exhibits a fivefold spread across CMIP5 models, ranging from 0.03 to 0.16 W m−2 K−1 (ensemble-mean = 0.08 W m−2 K−1). This accounts for much of the spread in 21st century warming of Northern Hemisphere land masses, and is very similar to the spread found in CMIP3 models. As with the CMIP3 models, there is a high degree of correspondence between the magnitudes of seasonal cycle and climate change versions of the feedback. Here we also show that their geographical footprint is similar. The ensemble-mean SAF strength is close to an observed estimate of the real climate’s seasonal cycle feedback strength. SAF strength is strongly correlated with the climatological surface albedo when the ground is covered by snow. The inter-model variation in this quantity is surprisingly large, ranging from 0.39 to 0.75. Models with large surface albedo when these regions are snow-covered will also have a large surface albedo contrast between snow-covered and snow-free regions, and therefore a correspondingly large SAF. Widely-varying treatments of vegetation masking of snow-covered surfaces are probably responsible for the spread in surface albedo where snow occurs, and the persistent spread in SAF in global climate models.

Intensification of decadal and multi-decadal sea level variability in the western tropical Pacific during recent decades

Abstract: Previous studies have linked the rapid sea level rise (SLR) in the western tropical Pacific (WTP) since the early 1990s to the Pacific decadal climate modes, notably the Pacific Decadal Oscillation in the north Pacific or Interdecadal Pacific Oscillation (IPO) considering its basin wide signature. Here, the authors investigate the changing patterns of decadal (10-20 years) and multidecadal (> 20 years) sea level variability (global mean SLR removed) in the Pacific associated with the IPO, by analyzing satellite and in situ observations, together with reconstructed and reanalysis products, and performing ocean and atmosphere model experiments. Robust intensification is detected for both decadal and multidecadal sea level variability in the WTP since the early 1990s. The IPO intensity, however, did not increase and thus cannot explain the faster SLR. The observed, accelerated WTP SLR results from the combined effects of Indian Ocean and WTP warming and central-eastern tropical Pacific cooling associated with the IPO cold transition. The warm Indian Ocean acts in concert with the warm WTP and cold central-eastern tropical Pacific to drive intensified easterlies and negative Ekman pumping velocity in western-central tropical Pacific, thereby enhancing the western tropical Pacific SLR. On decadal timescales, the intensified sea level variability since the late 1980s or early 1990s results from the "out of phase" relationship of sea surface temperature anomalies between the Indian and central-eastern tropical Pacific since 1985, which produces "in phase" effects on the WTP sea level variability.

Bias corrections of global models for regional climate simulations of high-impact weather

Abstract: All global circulation models (GCMs) suffer from some form of bias, which when used as boundary conditions for regional climate models may impact the simulations, perhaps severely Here we present a bias correction method that corrects the mean error in the GCM, but retains the six-hourly weather, longer-period climate-variability and climate change from the GCM We utilize six different bias correction experiments; each correcting different bias components The impact of the full bias correction and the individual components are examined in relation to tropical cyclones, precipitation and temperature We show that correcting of all boundary data provides the greatest improvement

Evaluation of the CORDEX-Africa multi-RCM hindcast: systematic model errors

Abstract: Monthly-mean precipitation, mean (TAVG), maximum (TMAX) and minimum (TMIN) surface air temperatures, and cloudiness from the CORDEX-Africa regional climate model (RCM) hindcast experiment are evaluated for model skill and systematic biases. All RCMs simulate basic climatological features of these variables reasonably, but systematic biases also occur across these models. All RCMs show higher fidelity in simulating precipitation for the west part of Africa than for the east part, and for the tropics than for northern Sahara. Interannual variation in the wet season rainfall is better simulated for the western Sahel than for the Ethiopian Highlands. RCM skill is higher for TAVG and TMAX than for TMIN, and regionally, for the subtropics than for the tropics. RCM skill in simulating cloudiness is generally lower than for precipitation or temperatures. For all variables, multi-model ensemble (ENS) generally outperforms individual models included in ENS. An overarching conclusion in this study is that some model biases vary systematically for regions, variables, and metrics, posing difficulties in defining a single representative index to measure model fidelity, especially for constructing ENS. This is an important concern in climate change impact assessment studies because most assessment models are run for specific regions/sectors with forcing data derived from model outputs. Thus, model evaluation and ENS construction must be performed separately for regions, variables, and metrics as required by specific analysis and/or assessments. Evaluations using multiple reference datasets reveal that cross-examination, quality control, and uncertainty estimates of reference data are crucial in model evaluations.

Equator-to-pole temperature differences and the extra-tropical storm track responses of the CMIP5 climate models

Abstract: This paper aims to understand the physical processes causing the large spread in the storm track projections of the CMIP5 climate models. In particular, the relationship between the climate change responses of the storm tracks, as measured by the 2–6 day mean sea level pressure variance, and the equator-to-pole temperature differences at upper- and lower-tropospheric levels is investigated. In the southern hemisphere the responses of the upper- and lower-tropospheric temperature differences are correlated across the models and as a result they share similar associations with the storm track responses. There are large regions in which the storm track responses are correlated with the temperature difference responses, and a simple linear regression model based on the temperature differences at either level captures the spatial pattern of the mean storm track response as well explaining between 30 and 60 % of the inter-model variance of the storm track responses. In the northern hemisphere the responses of the two temperature differences are not significantly correlated and their associations with the storm track responses are more complicated. In summer, the responses of the lower-tropospheric temperature differences dominate the inter-model spread of the storm track responses. In winter, the responses of the upper- and lower-temperature differences both play a role. The results suggest that there is potential to reduce the spread in storm track responses by constraining the relative magnitudes of the warming in the tropical and polar regions.

Impact of Indian summer monsoon on the South Asian High and its influence on summer rainfall over China

Abstract: By using the monthly ERA-40 reanalysis data and observed rainfall data, we investigated the effect of the Indian summer monsoon (ISM) on the South Asian High (SAH) at 200 hPa, and the role played by the SAH in summer rainfall variation over China. It is found that in the interannual timescale the east–west shift is a prominent feature of the SAH, with its center either over the Iranian Plateau or over the Tibetan Plateau. When the ISM is stronger (weaker) than normal, the SAH shifts westward (eastward) to the Iranian Plateau (Tibetan Plateau). The east–west position of SAH has close relation to the summer rainfall over China. A westward (eastward) location of SAH corresponds to less (more) rainfall in the Yangtze-Huai River Valley and more (less) rainfall in North China and South China. A possible physical process that the ISM affects the summer rainfall over China via the SAH is proposed. A stronger (weaker) ISM associated with more (less) rainfall over India corresponds to more (less) condensation heat release and anomalous heating (cooling) in the upper troposphere over the northern Indian peninsula. The anomalous heating (cooling) stimulates positive (negative) height anomalies to its northwest and negative (positive) height anomalies to its northeast in the upper troposphere, causing a westward (eastward) shift of the SAH with its center over the Iranian Plateau (Tibetan Plateau). As a result, an anomalous cyclone (anticyclone) is formed over the eastern Tibetan Plateau and eastern China in the upper troposphere. The anomalous vertical motions in association with the circulation anomalies are responsible for the rainfall anomalies over China. Our present study reveals that the SAH may play an important role in the effect of ISM on the East Asian summer monsoon.

On the spread of changes in marine low cloud cover in climate model simulations of the 21st century

Abstract: In 36 climate change simulations associated with phases 3 and 5 of the Coupled Model Intercomparison Project (CMIP3 and CMIP5), changes in marine low cloud cover (LCC) exhibit a large spread, and may be either positive or negative. Here we develop a heuristic model to understand the source of the spread. The model’s premise is that simulated LCC changes can be interpreted as a linear combination of contributions from factors shaping the clouds’ large-scale environment. We focus primarily on two factors—the strength of the inversion capping the atmospheric boundary layer (measured by the estimated inversion strength, EIS) and sea surface temperature (SST). For a given global model, the respective contributions of EIS and SST are computed. This is done by multiplying (1) the current-climate’s sensitivity of LCC to EIS or SST variations, by (2) the climate-change signal in EIS or SST. The remaining LCC changes are then attributed to changes in greenhouse gas and aerosol concentrations, and other environmental factors. The heuristic model is remarkably skillful. Its SST term dominates, accounting for nearly two-thirds of the intermodel variance of LCC changes in CMIP3 models, and about half in CMIP5 models. Of the two factors governing the SST term (the SST increase and the sensitivity of LCC to SST perturbations), the SST sensitivity drives the spread in the SST term and hence the spread in the overall LCC changes. This sensitivity varies a great deal from model to model and is strongly linked to the types of cloud and boundary layer parameterizations used in the models. EIS and SST sensitivities are also estimated using observational cloud and meteorological data. The observed sensitivities are generally consistent with the majority of models as well as expectations from prior research. Based on the observed sensitivities and the relative magnitudes of simulated EIS and SST changes (which we argue are also physically reasonable), the heuristic model predicts LCC will decrease over the 21st-century. However, to place a strong constraint, for example on the magnitude of the LCC decrease, will require longer observational records and a careful assessment of other environmental factors producing LCC changes. Meanwhile, addressing biases in simulated EIS and SST sensitivities will clearly be an important step towards reducing intermodel spread in simulated LCC changes.

Soil moisture-temperature feedbacks at meso-scale during summer heat waves over Western Europe

Abstract: This paper investigates the impact of soil moisture-temperature feedback during heatwaves occurring over France between 1989 and 2008. Two simulations of the weather research and forecasting regional model have been analysed, with two different land-surface models. One resolves the hydrology and is able to simulate summer dryness, while the other prescribes constant and high soil moisture and hence no soil moisture deficit. The sensitivity analysis conducted for all heatwave episodes highlights different soil moisture-temperature responses (1) over low-elevation plains, (2) over mountains and (3) over coastal regions. In the plains, soil moisture deficit induces less evapotranspiration and higher sensible heat flux. This has the effect of heating the planetary boundary layer and at the same time of creating a general condition of higher convective instability and a slight increase of shallow cloud cover. A positive feedback is created which increases the temperature anomaly during the heatwaves. In mountainous regions, enhanced heat fluxes over dry soil reinforce upslope winds producing strong vertical motion over the mountain slope, first triggered by thermal convection. This, jointly to the instability conditions, favors convection triggering and produces clouds and precipitation over the mountains, reducing the temperature anomaly. In coastal regions, dry soil enhances land/sea thermal contrast, strengthening sea-breeze circulation and moist cold marine air advection. This damps the magnitude of the heatwave temperature anomaly in coastal areas, expecially near the Mediterranean coast. Hence, along with heating in the plains, soil dryness can also have a significant cooling effect over mountains and coastal regions due to meso-scale circulations.

Evaluation of delta change and bias correction methods for future daily precipitation: intermodel cross-validation using ENSEMBLES simulations

Abstract: Due to inherent limitations in climate models, their output is biased in relation to observed climate and as such does not provide reliable climate projections. In this study, nine methods used to account for biases in daily precipitation are tested. First, cross-validation tests were made using a set of ENSEMBLES regional model simulations to gain insights in the potential performance of the methods in the future climate. The results show that quantile mapping type methods, being able to modify the shape of the precipitation distribution, often outperform other types of methods. Yet, as the performance depends on time of the year, location and part of the distribution considered, it is not possible to distinguish one universally best performing method. In addition, the improvement relative to the projections that would have been obtained assuming unchanged climate is relatively modest, particularly in the early twentyfirst century conditions. Further tests with different method combinations show that the projections could be potentially improved by using several well performing methods in parallel. In the second part of the study, contributions of method and model differences to the overall variation of precipitation projections are assessed. It is shown that although intermodel differences play an important role, uncertainties related to intermethod differences are substantial, particularly in the tails of the distribution. This suggests that method uncertainty should be taken into account when constructing daily precipitation projections, possibly by using several methods in parallel.

Equatorial Atlantic variability and its relation to mean state biases in CMIP5

Abstract: Coupled general circulation model (GCM) simulations participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) are analyzed with respect to their performance in the equatorial Atlantic. In terms of the mean state, 29 out of 33 models examined continue to suffer from serious biases including an annual mean zonal equatorial SST gradient whose sign is opposite to observations. Westerly surface wind biases in boreal spring play an important role in the reversed SST gradient by deepening the thermocline in the eastern equatorial Atlantic and thus reducing upwelling efficiency and SST cooling in the following months. Both magnitude and seasonal evolution of the biases are very similar to what was found previously for CMIP3 models, indicating that improvements have only been modest. The weaker than observed equatorial easterlies are also simulated by atmospheric GCMs forced with observed SST. They are related to both continental convection and the latitudinal position of the intertropical convergence zone (ITCZ). Particularly the latter has a strong influence on equatorial zonal winds in both the seasonal cycle and interannual variability. The dependence of equatorial easterlies on ITCZ latitude shows a marked asymmetry. From the equator to 15°N, the equatorial easterlies intensify approximately linearly with ITCZ latitude. When the ITCZ is south of the equator, on the other hand, the equatorial easterlies are uniformly weak. Despite serious mean state biases, several models are able to capture some aspects of the equatorial mode of interannual SST variability, including amplitude, pattern, phase locking to boreal summer, and duration of events. The latitudinal position of the boreal spring ITCZ, through its influence on equatorial surface winds, appears to play an important role in initiating warm events.

Assessment of future climate change over East Asia due to the RCP scenarios downscaled by GRIMs-RMP

Abstract: This study assesses future climate change over East Asia using the Global/Regional Integrated Model system—Regional Model Program (RMP). The RMP is forced by two types of future climate scenarios produced by the Hadley Center Global Environmental Model version 2 (HG2); the representative concentration pathways (RCP) 4.5 and 8.5 scenarios for the intergovernmental panel on climate change fifth assessment report (AR5). Analyses for the current (1980–2005) climate are performed to evaluate the RMP’s ability to reproduce precipitation and temperature. Two different future (2006–2050) simulations are compared with the current climatology to investigate the climatic change over East Asia centered in Korea. The RMP satisfactorily reproduces the observed seasonal mean and variation of precipitation and temperature. The spatial distribution of the simulated large-scale features and precipitation by the RMP is generally less reflective of current climatic conditions than that is given by the HG2, but their inter-annual variations in East Asia are better captured by the RMP. Furthermore, the RMP shows higher reproducibility of climate extremes including excessive heat wave and precipitation events over South Korea. In the future, strong warming is distinctly coupled with intensified monsoonal precipitation over East Asia. In particular, extreme weather conditions are increased and intensified over South Korea as follows: (1) The frequency of heat wave events with temperature greater than 30 °C is projected to increase by 131 and 111 % in the RCP 8.5 and 4.5 downscaling, relative to the current climate. (2) The RCP 8.5 downscaling shows the frequency and variability of heavy rainfall to increase by 24 and 31.5 %, respectively, while the statistics given by the RCP 4.5 downscaling are similar to those of the current climate.

The role of horizontal resolution in simulating drivers of the global hydrological cycle

Abstract: The role of atmospheric general circulation model (AGCM) horizontal resolution in representing the global energy budget and hydrological cycle is assessed, with the aim of improving the understanding of model uncertainties in simulating the hydrological cycle. We use two AGCMs from the UK Met Office Hadley Centre: HadGEM1-A at resolutions ranging from 270 to 60 km, and HadGEM3-A ranging from 135 to 25 km. The models exhibit a stable hydrological cycle, although too intense compared to reanalyses and observations. This over-intensity is explained by excess surface shortwave radiation, a common error in general circulation models (GCMs). This result is insensitive to resolution. However, as resolution is increased, precipitation decreases over the ocean and increases over the land. This is associated with an increase in atmospheric moisture transport from ocean to land, which changes the partitioning of moisture fluxes that contribute to precipitation over land from less local to more non-local moisture sources. The results start to converge at 60-km resolution, which underlines the excessive reliance of the mean hydrological cycle on physical parametrization (local unresolved processes) versus model dynamics (large-scale resolved processes) in coarser HadGEM1 and HadGEM3 GCMs. This finding may be valid for other GCMs, showing the necessity to analyze other chains of GCMs that may become available in the future with such a range of horizontal resolutions. Our finding supports the hypothesis that heterogeneity in model parametrization is one of the underlying causes of model disagreement in the Coupled Model Intercomparison Project (CMIP) exercises.

Salinity changes in the World Ocean since 1950 in relation to changing surface freshwater fluxes

Abstract: Global hydrographic and air–sea freshwater flux datasets are used to investigate ocean salinity changes over 1950–2010 in relation to surface freshwater flux. On multi-decadal timescales, surface salinity increases (decreases) in evaporation (precipitation) dominated regions, the Atlantic–Pacific salinity contrast increases, and the upper thermocline salinity maximum increases while the salinity minimum of intermediate waters decreases. Potential trends in E–P are examined for 1950–2010 (using two reanalyses) and 1979–2010 (using four reanalyses and two blended products). Large differences in the 1950–2010 E–P trend patterns are evident in several regions, particularly the North Atlantic. For 1979–2010 some coherency in the spatial change patterns is evident but there is still a large spread in trend magnitude and sign between the six E–P products. However, a robust pattern of increased E–P in the southern hemisphere subtropical gyres is seen in all products. There is also some evidence in the tropical Pacific for a link between the spatial change patterns of salinity and E–P associated with ENSO. The water cycle amplification rate over specific regions is subsequently inferred from the observed 3-D salinity change field using a salt conservation equation in variable isopycnal volumes, implicitly accounting for the migration of isopycnal surfaces. Inferred global changes of E–P over 1950–2010 amount to an increase of 1 ± 0.6 % in net evaporation across the subtropics and an increase of 4.2 ± 2 % in net precipitation across subpolar latitudes. Amplification rates are approximately doubled over 1979–2010, consistent with accelerated broad-scale warming but also coincident with much improved salinity sampling over the latter period.

Mixed-phase clouds cause climate model biases in Arctic wintertime temperature inversions

Abstract: Temperature inversions are a common feature of the Arctic wintertime boundary layer. They have important impacts on both radiative and turbulent heat fluxes and partly determine local climate-change feedbacks. Understanding the spread in inversion strength modelled by current global climate models is therefore an important step in better understanding Arctic climate and its present and future changes. Here, we show how the formation of Arctic air masses leads to the emergence of a cloudy and a clear state of the Arctic winter boundary layer. In the cloudy state, cloud liquid water is present, little to no surface radiative cooling occurs and inversions are elevated and relatively weak, whereas surface radiative cooling leads to strong surface-based temperature inversions in the clear state. Comparing model output to observations, we find that most climate models lack a realistic representation of the cloudy state. An idealised single-column model experiment of the formation of Arctic air reveals that this bias is linked to inadequate mixed-phase cloud microphysics, whereas turbulent and conductive heat fluxes control the strength of inversions within the clear state.

Combined effect of El Niño-Southern Oscillation and Pacific Decadal Oscillation on the East Asian winter monsoon

Abstract: Using long-term observational data and numerical model experiments, the combined effect of the El Nino-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) on the variability of the East Asian winter monsoon is examined. In the observations, it is found that when the ENSO and PDO are in-phase combinations (i.e., El Nino/positive PDO phase and La Nina/negative PDO phase), a negative relationship between ENSO and East Asian winter monsoon is significantly intensified. In other words, when El Nino (La Nina) occurs with positive (negative) PDO phase, anomalous warm (cold) temperatures are dominant over the East Asian winter continent. On the other hand, there are no significant temperature anomalies when the ENSO and PDO are out-of-phase combinations (i.e., El Nino/negative PDO phase and La Nina/positive PDO phase). Further analyses indicate that the anticyclone over the western North Pacific including the East Asian marginal seas plays an essential role in modulating the intensity of the East Asian winter monsoon under the changes of ENSO–PDO phase relationship. Long-lasting high pressure and warm sea surface temperature anomalies during the late fall/winter and following spring over the western North Pacific, which appear as the El Nino occurs with positive PDO phase, can lead to a weakened East Asian winter monsoon by transporting warm and wet conditions into the East Asian continent through the southerly wind anomalies along the western flank of the anomalous high pressure, and vice versa as the La Nina occurs with negative PDO phase. In contrast, the anomalous high pressure over the western North Pacific does not show a prominent change under the out-of-phase combinations of ENSO and PDO. Numerical model experiments confirm the observational results, accompanying dominant warm temperature anomalies over East Asia via strong anticyclonic circulation anomalies near the Philippine Sea as the El Nino occurs with positive PDO phase, whereas such warming is weakened as the El Nino occurs with negative PDO phase. This result supports the argument that the changes in the East Asian winter monsoon intensity with ENSO are largely affected by the strength of the anticyclone over the western North Pacific, which significantly changes according to the ENSO–PDO phase relationship.

The eastward shift of the Walker Circulation in response to global warming and its relationship to ENSO variability

Abstract: This study investigates the global warming response of the Walker Circulation and the other zonal circulation cells (represented by the zonal stream function), in CMIP3 and CMIP5 climate models. The changes in the mean state are presented as well as the changes in the modes of variability. The mean zonal circulation weakens in the multi model ensembles nearly everywhere along the equator under both the RCP4.5 and SRES A1B scenarios. Over the Pacific the Walker Circulation also shows a significant eastward shift. These changes in the mean circulation are very similar to the leading mode of interannual variability in the tropical zonal circulation cells, which is dominated by El Nino Southern Oscillation variability. During an El Nino event the circulation weakens and the rising branch over the Maritime Continent shifts to the east in comparison to neutral conditions (vice versa for a La Nina event). Two-thirds of the global warming forced trend of the Walker Circulation can be explained by a long-term trend in this interannual variability pattern, i.e. a shift towards more El Nino-like conditions in the multi-model mean under global warming. Further, interannual variability in the zonal circulation exhibits an asymmetry between El Nino and La Nina events. El Nino anomalies are located more to the east compared with La Nina anomalies. Consistent with this asymmetry we find a shift to the east of the dominant mode of variability of zonal stream function under global warming. All these results vary among the individual models, but the multi model ensembles of CMIP3 and CMIP5 show in nearly all aspects very similar results, which underline the robustness of these results. The observed data (ERA Interim reanalysis) from 1979 to 2012 shows a westward shift and strengthening of the Walker Circulation. This is opposite to what the results in the CMIP models reveal. However, 75 % of the trend of the Walker Circulation can again be explained by a shift of the dominant mode of variability, but here towards more La Nina-like conditions. Thus in both climate change projections and observations the long-term trends of the Walker Circulation seem to follow to a large part the pre-existing dominant mode of internal variability.

An analysis on the physical process of the influence of AO on ENSO

Abstract: The influence of the spring AO on ENSO has been demonstrated in several recent studies. This analysis further explores the physical process of the influence of AO on ENSO using the NCEP/NCAR reanalysis data over the period 1958–2010. We focus on the formation of the westerly wind burst in the tropical western Pacific, and examine the evolution and formation of the atmospheric circulation, atmospheric heating, and SST anomalies in association with the spring AO variability. The spring AO variability is found to be independent from the East Asian winter monsoon activity. The spring AO associated circulation anomalies are supported by the interaction between synoptic-scale eddies and the mean-flow and its associated vorticity transportation. Surface wind changes may affect surface heat fluxes and the oceanic heat transport, resulting in the SST change. The AO associated warming in the equatorial SSTs results primarily from the ocean heat transport in the face of net surface heat flux damping. The tropical SST warming is accompanied by anomalous atmospheric heating in the subtropical north and south Pacific, which sustains the anomalous westerly wind in the equatorial western Pacific through a Gill-like atmospheric response from spring to summer. The anomalous westerly excites an eastward propagating and downwelling equatorial Kelvin wave, leading to SST warming in the tropical central-eastern Pacific in summer-fall. The tropical SST, atmospheric heating, and atmospheric circulation anomalies sustain and develop through the Bjerknes feedback mechanism, which eventually result in an El Nino-like warming in the tropical eastern Pacific in winter.

Potential role of winter rainfall in explaining increased moisture in the Mediterranean and Middle East during periods of maximum orbitally-forced insolation seasonality

Abstract: Precession-related forcing of seasonal insolation changes in the northern hemisphere (NH) alternates between maximum NH seasonality (summer perihelion–increased insolation; winter aphelion–decreased insolation) and minimum NH seasonality (summer aphelion, and winter perihelion). With maximum NH seasonality, climate models simulate stronger NH summer monsoons that bring increased precipitation to North Africa and South and East Asia, in agreement with the in-phase relation of precipitation and NH summer insolation found in many paleoclimatic records. However paleoclimatic records in parts of the Mediterranean, the Middle East, and the interior of Asia also indicate increased moisture at times of maximum NH seasonality, a change not always clearly linked to stronger summer monsoons—either because these regions are at or beyond the boundaries of the present-day monsoon or because the observations allow multiple causal interpretations, or both. This study focuses on the possible role of changes in NH winter climate in explaining these wetter episodes. Using climate model simulations, we show that the ‘NH winter aphelion–decreased NH winter insolation’ orbital configuration is linked to the Mediterranean storm track and increased winter rains in the Mediterranean, the Middle East, and interior Asia. We conclude that wetter periods at precession time scales in these particular regions may have resulted either from increased wintertime storm track precipitation, or from a combination of increased winter and summer rainfall. Given this seasonal ambiguity, both possibilities need to be considered.