Pacific Sea Surface Temperature Gradient Research Articles

Influence of Natural External Forcings on Interdecadal Variation of Global Land Monsoon over the Last Millennium in CESM-LME

Abstract Interdecadal variations of the global land monsoon have been previously attributed to internal fluctuations of the climate system, but the role of natural external forcings was underexplored. Here, we investigate this issue by using the Community Earth System Model ensemble simulations over the last millennium (LM) (AD 950–1850). Our analysis reveals that the surface temperature, with two dominant structures (global cooling/warming and longitudinal sea surface temperature gradient in the tropical Pacific, which affects the Walker circulation), predominantly shapes the leading forced mode of the global land monsoon. This mode, representing 19% of the total variance, manifests as consistent features across South Asia, the southern part of East Asia, North Australia, South America, and western South Africa, contrasting with other monsoon regions. Under global cooling conditions, the monsoon intensity is enhanced in the northern parts of the East Asian and eastern parts of the North and South African monsoons, but it decreases in the other monsoon regions. Under weak Walker circulation conditions, changes in atmospheric circulation in response to the sea surface temperature gradient in the tropical Pacific are associated with a substantial attenuation of almost all land monsoon regions. It was further shown that the global mean surface temperature and the tropical Pacific temperature gradient jointly account for 75% of the total variance in the leading mode of the global land monsoon, with 29% and 46% as their respective contributions. Furthermore, our results suggest that volcanic eruptions are the dominant external forcing for these variations. These findings provide valuable insights for future research on global monsoon dynamics. Significance Statement Our study using the Community Earth System Model reveals that surface temperature significantly impacts decadal global land monsoon (GLM) variations during the last millennium. Two key factors in relation to global-scale cooling/warming and changes in the tropical Pacific sea surface temperature gradient affect the GLM’s leading forced mode. The leading forced GLM features remain consistent across regions like South Asia, southern East Asia, North Australia, South America, and western South Africa. Under global cooling, monsoon intensities in most land monsoon regions are enhanced, while they are reduced in northern East Asian land monsoon regions. In contrast, weak tropical Pacific temperature gradient conditions cause a decrease in intensities in almost all land monsoon regions. Additionally, the global mean surface temperature and the tropical Pacific temperature gradient account for 75% of the total variance in the leading forced GLM variations, contributing 29% and 46%, respectively. Notably, volcanic eruptions emerge as the key external factor influencing these variations.

Future Projections of the El Niño—Southern Oscillation and Tropical Pacific Mean State in CMIP6

AbstractThe El Niño—Southern Oscillation (ENSO) is an important mode of tropical Pacific atmosphere‐ocean variability that drives teleconnections with weather and climate globally. However, prior studies using state‐of‐the‐art climate models lack consensus regarding future ENSO projections and are often impacted by tropical Pacific sea‐surface temperature (SST) biases. We used 173 simulations from 29 climate models participating in the Coupled Model Intercomparison Project, version 6 (CMIP6) to analyze model biases and future ENSO projections. We analyzed two ENSO indices, namely the ENSO Longitude Index (ELI), which measures zonal shifts in tropical Pacific deep convection and accounts for changes in background SST, and the Niño 3.4 index, which measures SST anomalies in the central‐eastern equatorial Pacific. We found that the warm eastern tropical‐subtropical Pacific SST bias typical of previous generations of climate models persists into many of the CMIP6 models. Future projections of ENSO shift toward more El Niño‐like conditions based on ELI in 48% of simulations and 55% of models, in association with a future weakening of the zonal equatorial Pacific SST gradient. On the other hand, none of the models project a significant shift toward La Niña‐like conditions. The standard deviation of the Niño 3.4 index indicates a lack of consensus on whether an increase or decrease in ENSO variability is expected in the future. Finally, we found a possible relationship between historical SST and low‐level cloud cover biases in the ENSO region and future changes in ELI; however, this result may be impacted by limitations in data availability.

The future of the El Niño–Southern Oscillation: using large ensembles to illuminate time-varying responses and inter-model differences

Abstract. Future changes in the El Niño–Southern Oscillation (ENSO) are uncertain, both because future projections differ between climate models and because the large internal variability of ENSO clouds the diagnosis of forced changes in observations and individual climate model simulations. By leveraging 14 single model initial-condition large ensembles (SMILEs), we robustly isolate the time-evolving response of ENSO sea surface temperature (SST) variability to anthropogenic forcing from internal variability in each SMILE. We find nonlinear changes in time in many models and considerable inter-model differences in projected changes in ENSO and the mean-state tropical Pacific zonal SST gradient. We demonstrate a linear relationship between the change in ENSO SST variability and the tropical Pacific zonal SST gradient, although forced changes in the tropical Pacific SST gradient often occur later in the 21st century than changes in ENSO SST variability, which can lead to departures from the linear relationship. Single-forcing SMILEs show a potential contribution of anthropogenic forcing (aerosols and greenhouse gases) to historical changes in ENSO SST variability, while the observed historical strengthening of the tropical Pacific SST gradient sits on the edge of the model spread for those models for which single-forcing SMILEs are available. Our results highlight the value of SMILEs for investigating time-dependent forced responses and inter-model differences in ENSO projections. The nonlinear changes in ENSO SST variability found in many models demonstrate the importance of characterizing this time-dependent behavior, as it implies that ENSO impacts may vary dramatically throughout the 21st century.

The West Pacific Gradient tracks ENSO and zonal Pacific sea surface temperature gradient during the last Millennium

Small changes in Pacific temperature gradients connected with the El Niño Southern Oscillation (ENSO) influence the Walker Circulation and are related to global climate anomalies. Therefore, it is of paramount importance to develop robust indices of their past behavior. Here, we reconstruct the difference in sea surface temperature between the west and central Pacific during ENSO, coined the West Pacific Gradient (WPG), based on the Last Millennium Paleo Hydrodynamics Data Assimilation. We show that the WPG tracks ENSO variability and strongly co-varies with the zonal gradient in Pacific sea surface temperature. We demonstrate that the WPG strength is related to significant atmospheric circulation and precipitation anomalies during historical El Niño and La Niña events by magnifying or weakening droughts and pluvials across the Indo-Pacific. We show that an extreme negative WPG coupled to a strong zonal Pacific temperature gradient is associated with enhanced megadroughts in North America between 1400 CE and the late sixteenth century. The twentieth century stands out in showing the most extreme swings between positive and negative WPG conditions over the past Millennium. We conclude that the WPG is a robust index together with ENSO indices to reveal past changes in Pacific zonal sea surface temperature gradient variability.

Observational constraint on cloud feedbacks suggests moderate climate sensitivity

Global climate models predict warming in response to increasing GHG concentrations, partly due to decreased tropical low-level cloud cover and reflectance. We use satellite observations that discriminate stratocumulus from shallow cumulus clouds to separately evaluate their sensitivity to warming and constrain the tropical contribution to low-cloud feedback. We find an observationally inferred low-level cloud feedback two times smaller than a previous estimate. Shallow cumulus clouds are insensitive to warming, whereas global climate models exhibit a large positive cloud feedback in shallow cumulus regions. In contrast, stratocumulus clouds show sensitivity to warming and the tropical inversion layer strength, controlled by the tropical Pacific sea surface temperature gradient. Models fail to reproduce the historical sea surface temperature gradient trends and therefore changes in inversion strength, generating an overestimate of the positive stratocumulus cloud feedback. Continued weak east Pacific warming would therefore produce a weaker low-cloud feedback and imply a more moderate climate sensitivity (3.47 ± 0.33 K) than many models predict. The response of low clouds to warming is uncertain among climate models and dominates spread in their projections. Satellite estimates of tropical cumulus and stratocumulus cloud feedbacks, derived using surface warming trends, suggest a more moderate climate sensitivity than many models predict.

Piacenzian Environmental Change and the Onset of Cool and Dry Conditions in Tropical South America

AbstractThe Piacenzian (3.60–2.58 Ma) covers the last stage of the Neogene just before the Earth's climate turned from relatively stable warm conditions to the cooler climate with high amplitude glacial‐interglacial oscillations of the Pleistocene. Even during this period early fluctuations towards cooler conditions occurred, and sea surface temperature (SST) reconstructions show stepwise increasing gradients. The zonal Pacific SST gradient which indicates the strength of the Walker circulation appears to have increased in two steps starting in the Piacenzian. We investigated vegetation and climate change in western equatorial South America under the influence of the Walker circulation and to detect signs for the onset of cooling in the tropics. We studied vegetation changes in western Ecuador using palynological analysis of 88 sediment samples from marine Ocean Drilling Program Site 1239 dated between 3.9 and 2.7 Ma. A general trend towards more open vegetation is observed. The climate changes towards cooler conditions, which is manifested by a lowering of the forest line from 3.3 Ma on. The increase of Amaranthaceae pollen after 3.1 Ma suggests drier conditions along the coast. A comparison with mid‐Piacenzian warm period (mPWP) modeling shows that data and models agree regarding a drier coastal climate during the mPWP. The isochronous occurrence of environmental changes in the presented record, that is, cooling and coastal drying, with the first major pulse of ice‐rafted debris and cooling temperatures in the Northern Hemisphere (between 3.28 and 3.31 Ma) suggests that these changes might have been a precursor of the intensification of the Northern Hemisphere glaciation.

Enhanced warming constrained by past trends in equatorial Pacific sea surface temperature gradient

The equatorial Pacific zonal sea surface temperature (SST) gradient, known to be a pacemaker of global warming, has strengthened since the mid-twentieth century. However, the cause is controversial because a majority of Coupled Model Intercomparison Project Phase 5 (CMIP5) models suggest weakening of the zonal SST gradient from the past to the future. Reconciling this discrepancy is important for the climate change attribution and climate sensitivity assessment. Here we use the CMIP5 ensemble and large ensemble simulations by four climate models to show that the intensifying SST gradient observed during 1951–2010 could arise from internal climate variability. Models and members that simulate historical strengthening of the SST gradient commonly exhibit reversed future trends. Using these models as a constraint, the rate of global-mean temperature rise is amplified by 9–30%, with higher values occurring in low-emission scenarios, because internal variability has a greater impact when the externally forced response is smaller. The east–west gradient in equatorial Pacific sea surface temperature has strengthened, but models suggest the opposite in past and future climates. Model ensembles show that the observed trend can arise from internal variability but their gradient weakens in the long term, causing more climate warming.

Fidelity of CMIP5-simulated teleconnection between Atlantic multidecadal oscillation and Indian summer monsoon rainfall

The present study aims to provide a relevant ground for attaining deeper perception about the teleconnection between the Atlantic multidecadal oscillation (AMO) and the Indian summer monsoon rainfall (ISMR) in observations as well as in 30 models from the World Climate Research Programme (WCRP) Coupled Model Intercomparison Project Phase 5 (CMIP5). Approximately 73% of models reproduce the internal natural variability allied with AMO, but mostly all underestimate the variance. Amongst these, very few replicate the explicit comma-shaped AMO sea surface temperature (SST) pattern, whereas rest illustrates warm SSTs over the sub-polar region and very weak or non-existent AMO’s signature over the sub-tropical North Atlantic. However, only 53% of models emulate the observed AMO–ISMR relationship. The observational analysis bestows the compelling evidence that the AMO influences ISMR through two physical processes: firstly by modulating the El Nino related anomalous Walker and regional Hadley circulations asymmetrically and secondly through the tropospheric response allied with the Rossby wave train. The models that fail to reproduce the AMO–ISMR teleconnection are incompetent in capturing the first physical mechanism correctly, whereas in general all models show limitations in simulating the second physical mechanism. The results divulge a moderate relationship between the quality of reproducing the AMO pattern and the AMO–ISMR teleconnection in models, particularly with respect to the tropical–extratropical Pacific SST gradients during AMO phases. The models, which do show the observed rainfall response over India, also simulate the large-scale features allied with AMO like the cross-equatorial flow, the tropical easterly jets, the anomalous divergence/convergence over the Indian sub-continent at upper/lower levels, the Webster and Yang Monsoon index, and the Monsoon Hadley Circulation index.

Different influences on the tropical Pacific SST gradient from natural and anthropogenic forcing

ABSTRACTThe influences of natural forcings (i.e. solar radiation and volcanic aerosol) and anthropogenic forcings (i.e. greenhouse gases) on the tropical Pacific zonal sea‐surface temperature (SST) gradient are investigated through numerical sensitivity experiments, forced by natural forcing, anthropogenic forcing, or full forcing (both natural and anthropogenic forcing) using the Community Earth System Model (CESM) over the past two millennia. Under full forcing, during the present warming period (PWP; 1901–1999), the Pacific SST shows a larger warming over the tropical Pacific region than the subtropical Pacific region. This pattern is composed of an El Niño‐like SST gradient (due to the greenhouse gas forcing) and a La Niña‐like SST gradient (due to natural forcing). Two sensitivity experiments, the PWP under greenhouse gas forcing and the medieval warming period (MWP; 751–1250) under natural forcing, were used to examine the mechanisms. The results showed that under the greenhouse gas forcing, the larger warming over the eastern tropical Pacific is induced by an increased surface net heat flux, which is mainly caused by short‐wave radiation and long‐wave radiation. Under natural forcing, a larger warming is induced over the western tropical Pacific by changes in the ocean vertical heat transportation, caused by surface wind anomalies.

Circulation background of climate patterns in the past millennium: Uncertainty analysis and re-reconstruction of ENSO-like state

Abstract The question of whether or not global warming has paused since more than ten years ago, namely “warming hiatus”, has attracted the attention of climate science community including the IPCC. Some authors have attributed the “warming hiatus” to the internal changes in the climate system, i.e., the recombination of ocean-atmosphere circulations. Therefore, it is necessary to propose higher requirements on reconstructing circulation background of climate change for the past millennium. However, the analyses of changes in atmospheric circulation over the last millennium as well as the conclusions of related regional climate patterns are so widely different and contradictory, bringing uncertainties to our understanding of regional even global climate change to a great extent. On the other hand, in the last 10 years the high-precision U/Th-dated stalagmite oxygen isotope ratio ( δ 18O) sequences provided an accurate chronological frame for the paleoclimate study of the middle and late Pleistocene, in which all authors from China took the Chinese stalagmite δ 18O as the summer monsoon index without exception. However, this point of view misleads the climate scientists into thinking that the stalagmite δ 18O can be as the proxy of precipitation amount. Nevertheless, it is well known that all of these records have a lot in common in the low frequency trend. However, most sequences cannot be calibrated by instrumental precipitation records, and thus the uncertainty of the climate research framework of China and even of the world has increased. Therefore, it is imperative for climatology to clarify the origin of contradiction and to reduce the uncertainty as early as possible. On the basis of analyzing the significance of stalagmite δ 18O in the monsoon regions of China, the author tries to propose a new circulation proxy in this paper: integrating the Chinese stalagmite oxygen isotope sequence to reconstruct the tropical Pacific sea surface temperature gradient, i.e., the large-scale ENSO-like state over the past millennium. Furthermore, the author speculates that it was warm in the modern times and the Medieval Period, but the circulation recombination was different in both periods. And this inference could be supported by the longer record since Last Glacial Maximum. In other words, the attribution analysis of the identical low-frequency trends of Chinese stalagmite δ 18O on a largescale shows that the ENSO-like state controls the climate change in the monsoon regions of China at different time scales (from interannual to century or even longer time scales). Wherein the important connection of circulations is the western Pacific subtropical high (WPSH), that is to say, besides the interannual anddecadal time scales, the WPSH would possess the circulation mode on longer timescales. For example, we may discuss the change of the WPSH in the whole Holocene epoch, i.e., the half precession period. These discussions could make sense to the study of not only the paleoclimate but also the modern climate.

Climate change and human occupation in the hyperarid core of the Atacama Desert

Significant changes in available moisture have occurred in the hyperarid Atacama Desert throughout the late Quaternary. Paleorecords show that areas in the adjacent high Andes and western Altiplano have experienced significant increases in rainfall, likely the result of intensified summer systems associated with the South American Summer Monsoon, tropical Pacific sea-surface temperature gradients and the mean position of the Intertropical Convergence Zone

Responses of the Western North Pacific Subtropical High to Global Warming under RCP4.5 and RCP8.5 Scenarios Projected by 33 CMIP5 Models: The Dominance of Tropical Indian Ocean–Tropical Western Pacific SST Gradient

Abstract Using the outputs of 33 coupled models that participated in phase 5 of the Coupled Model Intercomparison Project (CMIP5), the changes of the western North Pacific subtropical high (WNPSH) in the 2050–99 period under representative concentration pathway 4.5 and 8.5 (RCP4.5 and RCP8.5) scenarios relative to the 1950–99 period are analyzed. Under both scenarios, the projected changes in the WNPSH intensity are approximately zero in the multimodel ensemble mean (MME), and large intermodel spread is seen. About half of the models project an enhanced WNPSH and about half of the models project a weakened WNPSH under both scenarios. As revealed by both diagnostic studies and numerical simulations, the projected change in the WNPSH intensity is dominated by the change in the zonal sea surface temperature (SST) gradient between the tropical Indian Ocean (TIO) and the tropical western Pacific (TWP). A stronger (weaker) warming in the TIO is in favor of an enhanced (weakened) WNPSH, and a weaker (stronger) warming over the TWP is also in favor of an enhanced (weakened) WNPSH. The projected change of the WNPSH modulates the climate change over eastern China. Under both RCP4.5 and RCP8.5 scenarios, all of the models with a significantly increased (decreased) WNPSH intensity are associated with a significant increase in the precipitation over the northern (southern) part of eastern China and an enhanced (weakened) southerly wind.

The ENSO-Related West Pacific Sea Surface Temperature Gradient

El Niño–Southern Oscillation (ENSO) events are accompanied by an anomalous zonal sea surface temperature (SST) gradient over the west Pacific Ocean, defined here as the west Pacific SST gradient (WPG). The WPG is defined as the standardized difference between area-averaged SST over the central Pacific Ocean (Niño-4 region) and west Pacific Ocean (0°–10°N, 130°–150°E). While the direction of the WPG follows ENSO cycles, the magnitude of the gradient varies considerably between individual El Niño and La Niña events. In this study, El Niño and La Niña events are grouped according to the magnitude of the WPG, and tropical SST, circulations, and precipitation are examined for the period 1948–2011. Until the 1980s the WPG showed little trend as the west and central Pacific warmed at similar rates; however, the west Pacific has recently warmed faster than the central Pacific, which has resulted in an increased WPG during La Niña events.The temporal evolution and distribution of tropical Pacific SST as well as the near-surface tropical Pacific zonal wind, divergence, and vertical velocity are considerably different during ENSO events partitioned according to the strength of the WPG. Modifications to the tropical circulation, resulting in changes to Indo– west Pacific precipitation, are linked to strong and consistent circulation and precipitation modifications throughout the Northern Hemisphere during winter.

Hydroclimate variability in the low-elevation Atacama Desert over the last 2500 yr

Abstract. Paleoclimate reconstructions reveal that Earth system has experienced sub-millennial scale climate changes over the past two millennia in response to internal/external forcing. Although sub-millennial hydroclimate fluctuations have been detected in the central Andes during this interval, the timing, magnitude, extent and direction of change of these events remain poorly defined. Here, we present a reconstruction of hydroclimate variations on the Pacific slope of the central Andes based on exceptionally well-preserved plant macrofossils and associated archaeological remains from a hyperarid drainage (Quebrada Maní, ∼21° S, 1000 m a.s.l.) in the Atacama Desert. During the late Holocene, riparian ecosystems and farming social groups flourished in the hyperarid Atacama core as surface water availability increased throughout this presently sterile landscape. Twenty-six radiocarbon dates indicate that these events occurred between 1050–680, 1615–1350 and 2500–2040 cal yr BP. Regional comparisons with rodent middens and other records suggest that these events were synchronous with pluvial stages detected at higher-elevations in the central Andes over the last 2500 yr. These hydroclimate changes also coincide with periods of pronounced SST gradients in the Tropical Pacific (La Niña-like mode), conditions that are conducive to significantly increased rainfall in the central Andean highlands and flood events in the low-elevation watersheds at inter-annual timescales. Our findings indicate that the positive anomalies in the hyperarid Atacama over the past 2500 yr represent a regional response of the central Andean climate system to changes in the global hydrological cycle at centennial timescales. Furthermore, our results provide support for the role of tropical Pacific sea surface temperature gradient changes as the primary mechanism responsible for climate fluctuations in the central Andes. Finally, our results constitute independent evidence for comprehending the major trends in cultural evolution of prehistoric peoples that inhabited the region.

Asymmetries in tropical rainfall and circulation patterns in idealised CO2 removal experiments

Atmospheric CO2 removal is currently receiving serious consideration as a supplement or even alternative to emissions reduction. However the possible consequences of such a strategy for the climate system, and particularly for regional changes to the hydrological cycle, are not well understood. Two idealised general circulation model experiments are described, where CO2 concentrations are steadily increased, then decreased along the same path. Global mean precipitation continues to increase for several decades after CO2 begins to decrease. The mean tropical circulation shows associated changes due to the constraint on the global circulation imposed by precipitation and water vapour. The patterns of precipitation and circulation change also exhibit asymmetries with regard to changes in both CO2 and global mean temperature, but while the lag in global precipitation can be ascribed to different levels of CO2 at the same temperature state, the regional changes cannot. Instead, ocean memory and heat transfer are important here. In particular the equatorial East Pacific continues to warm relative to the West Pacific during CO2 ramp-down, producing an anomalously large equatorial Pacific sea surface temperature gradient and associated rainfall anomalies. The mechanism is likely to be a lag in response to atmospheric forcing between mixed-layer water in the east Pacific and the sub-thermocline water below, due to transport through the ocean circulation. The implication of this study is that a CO2 pathway of increasing then decreasing atmospheric CO2 concentrations may lead us to climate states during CO2 decrease that have not been experienced during the increase.

Mid-Pliocene equatorial Pacific sea surface temperature reconstruction: a multi-proxy perspective

The Mid-Pliocene is the most recent interval of sustained global warmth, which can be used to examine conditions predicted for the near future. An accurate spatial representation of the low-latitude Mid-Pliocene Pacific surface ocean is necessary to understand past climate change in the light of forecasts of future change. Mid-Pliocene sea surface temperature (SST) anomalies show a strong contrast between the western equatorial Pacific (WEP) and eastern equatorial Pacific (EEP) regardless of proxy (faunal, alkenone and Mg/Ca). All WEP sites show small differences from modern mean annual temperature, but all EEP sites show significant positive deviation from present-day temperatures by as much as 4.4 degrees C. Our reconstruction reflects SSTs similar to modern in the WEP, warmer than modern in the EEP and eastward extension of the WEP warm pool. The east-west equatorial Pacific SST gradient is decreased, but the pole to equator gradient does not change appreciably. We find it improbable that increased greenhouse gases (GHG) alone would cause such a heterogeneous warming and more likely that the cause of Mid-Pliocene warmth is a combination of several forcings including both increased meridional heat transport and increased GHG.

Late Quaternary vegetation and climate history of a perennial river canyon in the Río Salado basin (22°S) of Northern Chile

Plant macrofossils from 33 rodent middens sampled at three sites between 2910 and 3150 m elevation in the main canyon of the Río Salado, northern Chile, yield a unique record of vegetation and climate over the past 22,000 cal yr BP. Presence of low-elevation Prepuna taxa throughout the record suggests that mean annual temperature never cooled by more than 5°C and may have been near-modern at 16,270 cal yr BP. Displacements in the lower limits of Andean steppe and Puna taxa indicate that mean annual rainfall was twice modern at 17,520–16,270 cal yr BP. This pluvial event coincides with infilling of paleolake Tauca on the Bolivian Altiplano, increased ENSO activity inferred from a marine core near Lima, abrupt deglaciation in southern Chile, and Heinrich Event 1. Moderate to large increases in precipitation also occurred at 11,770–9550 (Central Atacama Pluvial Event), 7330–6720, 3490–2320 and at 800 cal yr BP. Desiccation occurred at 14,180, 8910–8640, and 4865 cal yr BP. Compared to other midden sites in the region, early Holocene desiccation seems to have happened progressively earlier farther south. Emerging trends from the cumulative midden record in the central Atacama agree at millennial timescales with improved paleolake chronologies for the Bolivian Altiplano, implying common forcing through changes in equatorial Pacific sea-surface temperature gradients.

A 22,000-Year Record of Monsoonal Precipitation from Northern Chile's Atacama Desert.

Fossil rodent middens and wetland deposits from the central Atacama Desert (22 degrees to 24 degrees S) indicate increasing summer precipitation, grass cover, and groundwater levels from 16.2 to 10.5 calendar kiloyears before present (ky B.P.). Higher elevation shrubs and summer-flowering grasses expanded downslope across what is now the edge of Absolute Desert, a broad expanse now largely devoid of rainfall and vegetation. Paradoxically, this pluvial period coincided with the summer insolation minimum and reduced adiabatic heating over the central Andes. Summer precipitation over the central Andes and central Atacama may depend on remote teleconnections between seasonal insolation forcing in both hemispheres, the Asian monsoon, and Pacific sea surface temperature gradients. A less pronounced episode of higher groundwater levels in the central Atacama from 8 to 3 ky B.P. conflicts with an extreme lowstand of Lake Titicaca, indicating either different climatic forcing or different response times and sensitivities to climatic change.

A Prognostic Cloud Water Parameterization for Global Climate Models

An efficient new prognostic cloud water parameterization designed for use in global climate models is described. The scheme allows for life cycle effects in stratiform clouds and permits cloud optical properties to be determined interactively. The parameterization contains representations of all important microphysical processes, including autoconversion, accretion, Bergeron–Findeisen diffusional growth, and cloud/rain water evaporation. Small-scale dynamical processes, including detrainment of convective condensate, cloud-top entrainment instability, and stability-dependent cloud physical thickness variations, are also taken into account. Cloud optical thickness is calculated from the predicted liquid/ice water path and a variable droplet effective radius estimated by assuming constant droplet number concentration. Microphysical and radiative properties are assumed to be different for liquid and ice clouds, and for liquid clouds over land and ocean. The parameterization is validated in several simulations using the Goddard Institute for Space Studies (GISS) general circulation model (GCM). Comparisons are made with a variety of datasets, including ERBE radiative fluxes and cloud forcing, ISCCP and surface-observed cloud properties, SSM/I liquid water path, and SAGE II thin cirrus cover. Validation is judged on the basis of the model's depiction of both the mean state; diurnal, seasonal, and interannual variability; and the temperature dependence of cloud properties. Relative to the diagnostic cloud scheme used in the previous GISS GCM, the prognostic parameterization strengthens the model's hydrologic cycle and general circulation, both directly and indirectly (via increased cumulus heating). Sea surface temperature (SST) perturbation experiments produce low climate sensitivity and slightly negative cloud feedback for globally uniform SST changes, but high sensitivity and positive cloud feedback when tropical Pacific SST gradients weaken with warming. Changes in the extent and optical thickness of tropical cumulus anvils appear to be the primary factor determining the sensitivity. This suggests that correct simulations of upward transport of convective condensate and of Walker circulation changes are of the highest priority for a realistic estimate of cloud feedback in actual greenhouse gas increase scenarios.