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Amaya, DJ, Siler N, Xie SP, Miller AJ.  2018.  The interplay of internal and forced modes of Hadley Cell expansion: lessons from the global warming hiatus. Climate Dynamics. 51:305-319.   10.1007/s00382-017-3921-5   AbstractWebsite

The poleward branches of the Hadley Cells and the edge of the tropics show a robust poleward shift during the satellite era, leading to concerns over the possible encroachment of the globe's subtropical dry zones into currently temperate climates. The extent to which this trend is caused by anthropogenic forcing versus internal variability remains the subject of considerable debate. In this study, we use a Joint EOF method to identify two distinct modes of tropical width variability: (1) an anthropogenically-forced mode, which we identify using a 20-member simulation of the historical climate, and (2) an internal mode, which we identify using a 1000-year pre-industrial control simulation. The forced mode is found to be closely related to the top of the atmosphere radiative imbalance and exhibits a long-term trend since 1860, while the internal mode is essentially indistinguishable from the El Nio Southern Oscillation. Together these two modes explain an average of 70% of the interannual variability seen in model "edge indices" over the historical period. Since 1980, the superposition of forced and internal modes has resulted in a period of accelerated Hadley Cell expansion and decelerated global warming (i.e., the "hiatus"). A comparison of the change in these modes since 1980 indicates that by 2013 the signal has emerged above the noise of internal variability in the Southern Hemisphere, but not in the Northern Hemisphere, with the latter also exhibiting strong zonal asymmetry, particularly in the North Atlantic. Our results highlight the important interplay of internal and forced modes of tropical width change and improve our understanding of the interannual variability and long-term trend seen in observations.

Zhou, ZQ, Xie SP, Zhang GJ, Zhou WY.  2018.  Evaluating AMIP Skill in Simulating Interannual Variability over the Indo-Western Pacific. Journal of Climate. 31:2253-2265.   10.1175/jcli-d-17-0123.1   AbstractWebsite

Local correlation between sea surface temperature (SST) and rainfall is weak or even negative in summer over the Indo-western Pacific warm pool, a fact often taken as indicative of weak ocean feedback on the atmosphere. An Atmospheric Model Intercomparison Project (AMIP) simulation forced by monthly varying SSTs derived from a parallel coupled general circulation model (CGCM) run is used to evaluate AMIP skills in simulating interannual variability of rainfall. Local correlation of rainfall variability between AMIP and CGCMsimulations is used as a direct metric of AMIP skill. This "perfect model'' approach sidesteps the issue of model biases that complicates the traditional skill metric based on the correlation between AMIP and observations. Despite weak local SST-rainfall correlation, the AMIP-CGCM rainfall correlation exceeds a 95% significance level over most of the Indo-western Pacific warm pool, indicating the importance of remote (e.g., El Nino in the equatorial Pacific) rather than local SST forcing. Indeed, the AMIP successfully reproduces large-scale modes of rainfall variability over the Indo-western Pacific warm pool. Compared to the northwest Pacific east of the Philippines, the AMIP-CGCMrainfall correlation is low from the Bay of Bengal through the South China Sea, limited by internal variability of the atmosphere that is damped in CGCM by negative feedback from the ocean. Implications for evaluating AMIP skill in simulating observations are discussed.

Siler, N, Kosaka Y, Xie SP, Li XC.  2017.  Tropical ocean contributions to California's surprisingly dry El Nino of 2015/16. Journal of Climate. 30:10067-10079.   10.1175/jcli-d-17-0177.1   AbstractWebsite

The major El Nino of 2015/16 brought significantly less precipitation to California than previous events of comparable strength, much to the disappointment of residents suffering through the state's fourth consecutive year of severe drought. Here, California's weak precipitation in 2015/16 relative to previous major El Nino events is investigated within a 40-member ensemble of atmosphere-only simulations run with historical sea surface temperatures (SSTs) and constant radiative forcing. The simulations reveal significant differences in both California precipitation and the large-scale atmospheric circulation between 2015/16 and previous strong El Nino events, which are similar to (albeit weaker than) the differences found in observations. Principal component analysis indicates that these ensemble-mean differences were likely related to a pattern of tropical SST variability with a strong signal in the Indian Ocean and western Pacific and a weaker signal in the eastern equatorial Pacific and subtropical North Atlantic. This SST pattern was missed by the majority of forecast models, which could partly explain their erroneous predictions of above-average precipitation in California in 2015/16.

Kamae, Y, Li XC, Xie SP, Ueda H.  2017.  Atlantic effects on recent decadal trends in global monsoon. Climate Dynamics. 49:3443-3455.   10.1007/s00382-017-3522-3   AbstractWebsite

Natural climate variability contributes to recent decadal climate trends. Specifically the trends during the satellite era since 1979 include Atlantic and Indian Ocean warming and Pacific cooling associated with phase shifts of the Atlantic Multidecadal Oscillation and the Pacific Decadal Oscillation, and enhanced global monsoon (GM) circulation and rainfall especially in the Northern Hemisphere. Here we evaluate effects of the oceanic changes on the global and regional monsoon trends by partial ocean temperature restoring experiments in a coupled atmosphere-ocean general circulation model. Via trans-basin atmosphere-ocean teleconnections, the Atlantic warming drives a global pattern of sea surface temperature change that resembles observations, giving rise to the enhanced GM. The tropical Atlantic warming and the resultant Indian Ocean warming favor subtropical deep-tropospheric warming in both hemispheres, resulting in the enhanced monsoon circulations and precipitation over North America, South America and North Africa. The extratropical North Atlantic warming makes an additional contribution to the monsoon enhancement via Eurasian continent warming and resultant land-sea thermal gradient over Asia. The results of this study suggest that the Atlantic multidecadal variability can explain a substantial part of global climate variability including the recent decadal trends of GM.

Hwang, YT, Xie SP, Deser C, Kang SM.  2017.  Connecting tropical climate change with Southern Ocean heat uptake. Geophysical Research Letters. 44:9449-9457.   10.1002/2017gl074972   AbstractWebsite

Under increasing greenhouse gas forcing, climate models project tropical warming that is greater in the Northern than the Southern Hemisphere, accompanied by a reduction in the northeast trade winds and a strengthening of the southeast trades. While the ocean-atmosphere coupling indicates a positive feedback, what triggers the coupled asymmetry and favors greater warming in the northern tropics remains unclear. Far away from the tropics, the Southern Ocean (SO) has been identified as the major region of ocean heat uptake. Beyond its local effect on the magnitude of sea surface warming, we show by idealized modeling experiments in a coupled slab ocean configuration that enhanced SO heat uptake has a profound global impact. This SO-to-tropics connection is consistent with southward atmospheric energy transport across the equator. Enhanced SO heat uptake results in a zonally asymmetric La-Nina-like pattern of sea surface temperature change that not only affects tropical precipitation but also has influences on the Asian and North American monsoons.

Kilpatrick, T, Xie SP, Nasuno T.  2017.  Diurnal Convection-Wind Coupling in the Bay of Bengal. Journal of Geophysical Research-Atmospheres. 122:9705-9720.   10.1002/2017jd027271   AbstractWebsite

Satellite observations of infrared brightness temperature and rainfall have shown offshore propagation of diurnal rainfall signals in some coastal areas of the tropics, suggesting that diurnal rainfall is coupled to land-sea breeze circulations. Here we utilize satellite observations of surface winds and rainfall to show the offshore copropagation of land breeze and diurnal rainfall signals for 300-400 km from the east coast of India into the Bay of Bengal. The wind observations are from the 2003 Quick Scatterometer (QuikSCAT)-SeaWinds "tandem mission" and from 17 years of the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI); the rainfall observations are from the TRMM 3B42 product and from TMI. The surface wind convergence maximum leads the rainfall maximum by 1-2 h in the western part of the bay, implying that the land breeze forces the diurnal cycle of rainfall. The phase speed of the offshore propagation is approximately 18 m s(-1), consistent with a deep hydrostatic gravity wave forced by diurnal heating over India. Comparisons with a cloud system-resolving atmospheric model and the ERA-Interim reanalysis indicate that the models realistically simulate the surface land breeze but greatly underestimate the amplitude of the rainfall diurnal cycle. The satellite observations presented in this study therefore provide a benchmark for model representation of this important atmosphere-ocean-land surface interaction. Plain Language Summary Satellite rainfall observations show a strong diurnal cycle in the Bay of Bengal during the summer monsoon. Here for the first time we utilize concurrent satellite observations of surface winds and rainfall to demonstrate the interaction between the land-sea breeze, forced by the diurnal cycle of solar heating over India, and diurnal rainfall over the Bay of Bengal. The observations are consistent with the land breeze acting as a forcing mechanism for the diurnal cycle of rainfall over the bay and, therefore, illuminate an important atmosphere-ocean-land surface interaction that is poorly represented in many climate models.

Lintner, BR, Langenbrunner B, Neelin JD, Anderson BT, Niznik MJ, Li G, Xie SP.  2016.  Characterizing CMIP5 model spread in simulated rainfall in the Pacific Intertropical Convergence and South Pacific Convergence Zones. Journal of Geophysical Research-Atmospheres. 121:11590-11607.   10.1002/2016jd025284   AbstractWebsite

Current-generation climate models exhibit various errors or biases in both the spatial distribution and intensity of precipitation relative to observations. In this study, empirical orthogonal function analysis is applied to the space-model index domain of precipitation over the Pacific from Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations to explore systematic spread of simulated precipitation characteristics across the ensemble. Two significant modes of spread, generically termed principal uncertainty patterns (PUPs), are identified in the December-January-February precipitation climatology: the leading PUP is associated with the meridional width of deep convection, while the second is associated with tradeoffs in precipitation intensity along the South Pacific Convergence Zone, the Intertropical Convergence Zone (ITCZ), and the spurious Southern Hemisphere ITCZ. An important factor distinguishing PUPs from the analogy to time series analysis is that the modes can reflect either true systematic intermodel variance patterns or internal variability. In order to establish that the PUPS reflect the former, three complementary tests are performed by using preindustrial control simulations: a bootstrap significance test for reproducibility of the intermodel spatial patterns, a check for robustness over very long climatological averages, and a test on the loadings of these patterns relative to interdecadal sampling. Composite analysis based on these PUPs demonstrates physically plausible relationships to CMIP5 ensemble spread in simulated sea surface temperatures (SSTs), circulation, and moisture. Further analysis of atmosphere-only, prescribed SST simulations demonstrates decreased spread in the spatial distribution of precipitation, while substantial spread in intensity remains. Key Points Systematic spread in CMIP5 simulation of Pacific region rainfall is investigated by using empirical mode reduction techniques Two significant modes of model spread are identified for the DJF rainfall climatology These modes are interpreted in terms of spread in simulated patterns of SST and circulation

Wang, H, Xie SP, Liu QY.  2016.  Comparison of climate response to anthropogenic aerosol versus greenhouse gas forcing: Distinct patterns. Journal of Climate. 29:5175-5188.   10.1175/jcli-d-16-0106.1   AbstractWebsite

Spatial patterns of climate response to changes in anthropogenic aerosols and well-mixed greenhouse gases ( GHGs) are investigated using climate model simulations for the twentieth century. The climate response shows both similarities and differences in spatial pattern between aerosol and GHG runs. Common climate response between aerosol and GHG runs tends to be symmetric about the equator. This work focuses on the distinctive patterns that are unique to the anthropogenic aerosol forcing. The tropospheric cooling induced by anthropogenic aerosols is locally enhanced in the midlatitude Northern Hemisphere with a deep vertical structure around 40 degrees N, anchoring a westerly acceleration in thermal wind balance. The aerosol-induced negative radiative forcing in the Northern Hemisphere requires a cross-equatorial Hadley circulation to compensate interhemispheric energy imbalance in the atmosphere. Associated with a southward shift of the intertropical convergence zone, this interhemispheric asymmetric mode is unique to aerosol forcing and absent in GHG runs. Comparison of key climate response pattern indices indicates that the aerosol forcing dominates the interhemispheric asymmetric climate response in historical all-forcing simulations, as well as regional precipitation change such as the drying trend over the East Asian monsoon region. While GHG forcing dominates global mean surface temperature change, its effect is on par with and often opposes the aerosol effect on precipitation, making it difficult to detect anthropogenic change in rainfall from historical observations.

Wang, H, Xie SP, Tokinaga H, Liu Q, Kosaka Y.  2016.  Detecting cross-equatorial wind change as a fingerprint of climate response to anthropogenic aerosol forcing. Geophysical Research Letters. 43:3444-3450.   10.1002/2016gl068521   AbstractWebsite

Anthropogenic aerosols are amajor driver of the twetieth century climate change. In climate models, the aerosol forcing, larger in the Northern than Southern Hemispheres, induces an interhemispheric Hadley circulation. In support of the model result, we detected a robust change in the zonal mean cross-equatorial wind over the past 60 years from ship observations and reanalyses, accompanied by physically consistent changes in atmospheric pressure and marine cloud cover. Single-forcing experiments indicate that the observed change in cross-equatorial wind is a fingerprint of aerosol forcing. This zonal mean mode follows the evolution of global aerosol forcing that is distinct from regional changes in the Atlantic sector. Atmospheric simulations successfully reproduce this interhemispheric mode, indicating the importance of sea surface temperature mediation in response to anthropogenic aerosol forcing. As societies awaken to reduce aerosol emissions, a phase reversal of this interhemispheric mode is expected in the 21st century.

Long, SM, Xie SP.  2016.  Uncertainty in tropical rainfall projections: Atmospheric circulation effect and the ocean coupling. Journal of Climate. 29:2671-2687.   10.1175/jcli-d-15-0601.1   AbstractWebsite

Uncertainty in tropical rainfall projections under increasing radiative forcing is studied by using 26 models from phase 5 of the Coupled Model Intercomparison Project. Intermodel spread in projected rainfall change generally increases with interactive sea surface temperature (SST) warming in coupled models compared to atmospheric models with a common pattern of prescribed SST increase. Moisture budget analyses reveal that much of the model uncertainty in tropical rainfall projections originates from intermodel discrepancies in the dynamical contribution due to atmospheric circulation change. Intermodel singular value decomposition (SVD) analyses further show a tight coupling between the intermodel variations in SST warming pattern and circulation change in the tropics. In the zonal mean, the first SVD mode features an anomalous interhemispheric Hadley circulation, while the second mode displays an SST peak near the equator. The asymmetric mode is accompanied by a coupled pattern of wind-evaporation-SST feedback in the tropics and is further tied to interhemispheric asymmetric change in extratropical shortwave radiative flux at the top of the atmosphere. Intermodel variability in the tropical circulation change exerts a strong control on the spread in tropical cloud cover change and cloud radiative effects among models. The results indicate that understanding the coupling between the anthropogenic changes in SST pattern and atmospheric circulation holds the key to reducing uncertainties in projections of future changes in tropical rainfall and clouds.

Kubota, H, Kosaka Y, Xie SP.  2016.  A 117-year long index of the Pacific-Japan pattern with application to interdecadal variability. International Journal of Climatology. 36:1575-1589.   10.1002/joc.4441   AbstractWebsite

The Pacific-Japan (PJ) pattern affects interannual variability in the East Asian and western North Pacific (WNP) summer monsoons. This teleconnection pattern is characterized by a meridional dipole of anomalous circulation and precipitation between the tropical WNP and the midlatitudes. This study develops a long index of the PJ pattern using station-based atmospheric pressure data to track the PJ variability from 1897 to 2013. This index is correlated with a wide array of climate variables including air temperature, precipitation, Yangtze River flow, Japanese rice yield and the occurrence of tropical cyclones over the WNP (especially those that make landfall on the Chinese and Korean coast). For the recent three decades, the PJ index reproduces well-known correlations with El Nino-Southern Oscillation (ENSO) in the preceding boreal winter and Indian Ocean temperature in the concurrent summer. For the 117-year period, this ENSO-PJ relationship varies on interdecadal time scales, with low correlations in the 1920s and from the 1940s to 1970s, and recurrences of significant correlations at the beginning of the 20th century and the 1930s. In accordance with the modulation, the magnitude and regional climate effect of the PJ variability have changed. These results highlight the importance of interdecadal modulations of climate anomalies in the summer WNP and the need of long-term observations to study such modulations.

Huang, P, Xie SP.  2015.  Mechanisms of change in ENSO-induced tropical Pacific rainfall variability in a warming climate. Nature Geoscience. 8:922-U48.   10.1038/ngeo2571   AbstractWebsite

El Nino/Southern Oscillation (ENSO) is a mode of natural variability that has considerable impacts on global climate and ecosystems(1-4), through rainfall variability in the tropical Pacific and atmospheric teleconnections(5). In response to global warming, ENSO-driven rainfall variability is projected to intensify over the central-eastern Pacific but weaken over the western Pacific, whereas ENSO-related sea surface temperature variability is projected to decrease(6-14). Here, we explore the mechanisms that lead to changes in ENSO-driven rainfall variability in the tropical Pacific in response to global warming, with the help of a moisture budget decomposition for simulations from eighteen state-of-the-art climate models(15). We identify two opposing mechanisms that approximately offset each other: the increase in mean-state moisture content associated with surface warming strengthens ENSO-related rainfall anomalies(7), whereas the projected reduction in ENSO-related variability of sea surface temperatures suppresses rainfall. Two additional effects-spatially non-uniform changes in background sea surface temperatures and structural changes in sea surface temperature related to ENSO-both enhance central-eastern Pacific rainfall variability while dampening variability in the western Pacific, in nearly equal amounts. Our decomposition method may be generalized to investigate how rainfall variability would change owing to nonlinear interactions between background sea surface temperatures and their variability.

Xu, Y, Xie SP.  2015.  Ocean mediation of tropospheric response to reflecting and absorbing aerosols. Atmospheric Chemistry and Physics. 15:5827-5833.   10.5194/acp-15-5827-2015   AbstractWebsite

Radiative forcing by reflecting (e.g., sulfate, SO4) and absorbing (e.g., black carbon, BC) aerosols is distinct: the former cools the planet by reducing solar radiation at the top of the atmosphere and the surface, without largely affecting the atmospheric column, while the latter heats the atmosphere directly. Despite the fundamental difference in forcing, here we show that the structure of the tropospheric response is remarkably similar between the two types of aerosols, featuring a deep vertical structure of temperature change (of opposite sign) at the Northern Hemisphere (NH) mid-latitudes. The deep temperature structure is anchored by the slow response of the ocean, as a large meridional sea surface temperature (SST) gradient drives an anomalous interhemispheric Hadley circulation in the tropics and induces atmospheric eddy adjustments at the NH mid-latitudes. The tropospheric warming in response to projected future decline in reflecting aerosols poses additional threats to the stability of mountain glaciers in the NH. Additionally, robust tropospheric response is unique to aerosol forcing and absent in the CO2 response, which can be exploited for climate change attribution.

Liu, W, Lu J, Xie SP.  2015.  Understanding the Indian Ocean response to double CO2 forcing in a coupled model. Ocean Dynamics. 65:1037-1046.   10.1007/s10236-015-0854-6   AbstractWebsite

This study investigates the roles of multiple ocean-atmospheric feedbacks in the oceanic response to increased carbon dioxide by applying an overriding technique to a coupled climate model. The annual-mean sea surface temperature (SST) response in the Indian Ocean exhibits a zonal-dipolar warming pattern, with a reduced warming in the eastern and enhanced warming in the western tropical Indian Ocean (TIO), reminiscent of the Indian Ocean Dipole (IOD) pattern. The development of the dipole pattern exhibits a pronounced seasonal evolution. The overriding experiments show that the wind-evaporation-sea surface temperature (WES) feedback accounts for most of the enhanced warming in the western and central TIO during May-July with reduced southerly monsoonal wind and contributes partially to the reduced warming in the eastern TIO during June-September. The Bjerknes feedback explains most of the reduced warming in the eastern TIO during August-October, accompanied by a reduction of precipitation, easterly wind anomalies, and a thermocline shoaling along the equator. Both feedbacks facilitate the formation of the dipolar warming pattern in the TIO. The residual from the Bjerknes and WES feedbacks is attributable to the "static" response to increasing CO2. While the static SST response also contributes to the seasonal SST variations, the static precipitation response is relatively uniform in the TIO, appearing as a general increase of precipitation along the equatorial Indian Ocean during June-September.

Zhou, ZQ, Xie SP, Zheng XT, Liu QY, Wang H.  2014.  Global warming-induced changes in El Nino teleconnections over the North Pacific and North America. Journal of Climate. 27:9050-9064.   10.1175/jcli-d-14-00254.1   AbstractWebsite

El Nino-Southern Oscillation (ENSO) induces climate anomalies around the globe. Atmospheric general circulation model simulations are used to investigate how ENSO-induced teleconnection patterns during boreal winter might change in response to global warming in the Pacific-North American sector. As models disagree on changes in the amplitude and spatial pattern of ENSO in response to global warming, for simplicity the same sea surface temperature (SST) pattern of ENSO is prescribed before and after the climate warming. In a warmer climate, precipitation anomalies intensify and move eastward over the equatorial Pacific during El Nino because the enhanced mean SST warming reduces the barrier to deep convection in the eastern basin. Associated with the eastward shift of tropical convective anomalies, the ENSO-forced Pacific-North American (PNA) teleconnection pattern moves eastward and intensifies under the climate warming. By contrast, the PNA mode of atmospheric internal variability remains largely unchanged in pattern, suggesting the importance of tropical convection in shifting atmospheric teleconnections. As the ENSO-induced PNA pattern shifts eastward, rainfall anomalies are expected to intensify on the west coast of North America, and the El Nino-induced surface warming to expand eastward and occupy all of northern North America. The spatial pattern of the mean SST warming affects changes in ENSO teleconnections. The teleconnection changes are larger with patterned mean warming than in an idealized case where the spatially uniform warming is prescribed in the mean state. The results herein suggest that the eastward-shifted PNA pattern is a robust change to be expected in the future, independent of the uncertainty in changes of ENSO itself.

Li, G, Xie SP.  2014.  Tropical Biases in CMIP5 Multimodel Ensemble: The Excessive Equatorial Pacific Cold Tongue and Double ITCZ Problems. Journal of Climate. 27:1765-1780.   10.1175/jcli-d-13-00337.1   AbstractWebsite

Errors of coupled general circulation models (CGCMs) limit their utility for climate prediction and projection. Origins of and feedback for tropical biases are investigated in the historical climate simulations of 18 CGCMs from phase 5 of the Coupled Model Intercomparison Project (CMIP5), together with the available Atmospheric Model Intercomparison Project (AMIP) simulations. Based on an intermodel empirical orthogonal function (EOF) analysis of tropical Pacific precipitation, the excessive equatorial Pacific cold tongue and double intertropical convergence zone (ITCZ) stand out as the most prominent errors of the current generation of CGCMs. The comparison of CMIP-AMIP pairs enables us to identify whether a given type of errors originates from atmospheric models. The equatorial Pacific cold tongue bias is associated with deficient precipitation and surface easterly wind biases in the western half of the basin in CGCMs, but these errors are absent in atmosphere-only models, indicating that the errors arise from the interaction with the ocean via Bjerknes feedback. For the double ITCZ problem, excessive precipitation south of the equator correlates well with excessive downward solar radiation in the Southern Hemisphere (SH) midlatitudes, an error traced back to atmospheric model simulations of cloud during austral spring and summer. This extratropical forcing of the ITCZ displacements is mediated by tropical ocean-atmosphere interaction and is consistent with recent studies of ocean-atmospheric energy transport balance.

Kuwano-Yoshida, A, Taguchi B, Xie SP.  2014.  Baiu rainband termination in atmospheric and coupled atmosphere-ocean models. Journal of Climate. 26:10111-10124.   10.1175/jcli-d-13-00231.1   AbstractWebsite

The baiu rainband is a summer rainband stretching from eastern China through Japan toward the northwestern Pacific. The climatological termination of the baiu rainband is investigated using the Japanese 25-yr Reanalysis (JRA-25), a stand-alone atmospheric general circulation model (GCM) forced with observed sea surface temperature (SST) and an atmosphere-ocean GCM (AOGCM). The baiu rainband over the North Pacific abruptly shifts northward and weakens substantially in early July in the atmospheric GCM (AGCM), too early compared to observations (late July). The midtroposphere westerly jet and its thermal advection explain this meridional shift of the baiu rainband, but the ocean surface evaporation modulates the precipitation intensity. In AGCM, deep convection in the subtropical northwestern Pacific sets in prematurely, displacing the westerly jet northward over the cold ocean surface earlier than in observations. The suppressed surface evaporation over the cold ocean suppresses precipitation even though the midtropospheric warm advection and vertically integrated moisture convergence are similar to those before the westerly jet's northward shift. As a result, the baiu rainband abruptly weakens after the northward shift in JRA-25 and AGCM. In AOGCM, cold SST biases in the subtropics inhibit deep convection, delaying the poleward excursion of the westerly jet. As a result, the upward motion induced by both the strong westerly jet and the rainband persist over the northwestern Pacific through summer in the AOGCM. The results indicate that the westerly jet and the ocean evaporation underneath are important for the baiu rainband, the latter suggesting an oceanic effect on this important phenomenon.

Huang, P, Xie SP, Hu KM, Huang G, Huang RH.  2013.  Patterns of the seasonal response of tropical rainfall to global warming. Nature Geoscience. 6:357-361.   10.1038/ngeo1792   AbstractWebsite

Tropical convection is an important factor in regional climate variability and change around the globe(1,2). The response of regional precipitation to global warming is spatially variable, and state-of-the-art model projections suffer large uncertainties in the geographic distribution of precipitation changes(3-5). Two views exist regarding tropical rainfall change: one predicts increased rainfall in presently rainy regions (wet-get-wetter)(6-8), and the other suggests increased rainfall where the rise in sea surface temperature exceeds the mean surface warming in the tropics (warmer-get-wetter)(9-12). Here we analyse simulations with 18 models from the Coupled Model Intercomparison Project (CMIP5), and present a unifying view for seasonal rainfall change. We find that the pattern of ocean warming induces ascending atmospheric flow at the Equator and subsidence on the flanks, anchoring a band of annual mean rainfall increase near the Equator that reflects the warmer-get-wetter view. However, this climatological ascending motion marches back and forth across the Equator with the Sun, pumping moisture upwards from the boundary layer and causing seasonal rainfall anomalies to follow a wet-get-wetter pattern. The seasonal mean rainfall, which is the sum of the annual mean and seasonal anomalies, thus combines the wet-get-wetter and warmer-get-wetter trends. Given that precipitation climatology is well observed whereas the pattern of ocean surface warming is poorly constrained(13,14), our results suggest that projections of tropical seasonal mean rainfall are more reliable than the annual mean.

Fuckar, NS, Xie SP, Farneti R, Maroon EA, Frierson DMW.  2013.  Influence of the extratropical ocean circulation on the intertropical convergence zone in an idealized coupled general circulation model. Journal of Climate. 26:4612-4629.   10.1175/jcli-d-12-00294.1   AbstractWebsite

The authors present coupled model simulations in which the ocean's meridional overturning circulation (MOC) sets the zonal mean location of the intertropical convergence zone (ITCZ) in the hemisphere with deep-water production. They use a coarse-resolution single-basin sector coupled general circulation model (CGCM) with simplified atmospheric physics and two idealized land-sea distributions.In an equatorially symmetric closed-basin setting, unforced climate asymmetry develops because of the advective circulation-salinity feedback that amplifies the asymmetry of the deep-MOC cell and the upper-ocean meridional salinity transport. It confines the deep-water production and the dominant extratropical ocean heat release to a randomly selected hemisphere. The resultant ocean heat transport (OHT) toward the hemisphere with the deep-water source is partially compensated by the atmospheric heat transport (AHT) across the equator via an asymmetric Hadley circulation, setting the ITCZ in the hemisphere warmed by the ocean.When a circumpolar channel is open at subpolar latitudes, the circumpolar current disrupts the poleward transport of the upper-ocean saline water and suppresses deep-water formation poleward of the channel. The MOC adjusts by lowering the main pycnocline and shifting the deep-water production into the opposite hemisphere from the channel, and the ITCZ location follows the deep-water source again because of the Hadley circulation adjustment to cross-equatorial OHT. The climate response is sensitive to the sill depth of the channel but becomes saturated when the sill is deeper than the main pycnocline depth in subtropics. In simulations with a circumpolar channel, the ITCZ is in the Northern Hemisphere (NH) because of the Southern Hemisphere (SH) circumpolar flow that forces northward OHT.

Xie, SP, Lu B, Xiang BQ.  2013.  Similar spatial patterns of climate responses to aerosol and greenhouse gas changes. Nature Geoscience. 6:828-832.   10.1038/ngeo1931   AbstractWebsite

Spatial variations in ocean warming have been linked to regional changes in tropical cyclones(1), precipitation(2,3) and monsoons(4). But development of reliable regional climate projections for climate change mitigation and adaptation remains challenging(5). The presence of anthropogenic aerosols, which are highly variable in space and time, is thought to induce spatial patterns of climate response that are distinct from those of well-mixed greenhouse gases(4,6-9) Using CMIP5 climate simulations that consider aerosols and greenhouse gases separately, we show that regional responses to changes in greenhouse gases and aerosols are similar over the ocean, as reflected in similar spatial patterns of ocean temperature and precipitation. This similarity suggests that the climate response to radiative changes is relatively insensitive to the spatial distribution of these changes. Although anthropogenic aerosols are largely confined to the Northern Hemisphere, simulations that include aerosol forcing predict decreases in temperature and westerly wind speed that reach the pristine Southern Hemisphere oceans. Over land, the climate response to aerosol forcing is more localized, but larger scale spatial patterns are also evident. We suggest that the climate responses induced by greenhouse gases and aerosols share key ocean-atmosphere feedbacks, leading to a qualitative resemblance in spatial distribution.

Tokinaga, H, Xie SP, Deser C, Kosaka Y, Okumura YM.  2012.  Slowdown of the Walker circulation driven by tropical Indo-Pacific warming. Nature. 491:439-+.   10.1038/nature11576   Abstract

Global mean sea surface temperature (SST) has risen steadily over the past century(1,2), but the overall pattern contains extensive and often uncertain spatial variations, with potentially important effects on regional precipitation(3,4). Observations suggest a slowdown of the zonal atmospheric overturning circulation above the tropical Pacific Ocean (the Walker circulation) over the twentieth century(1,5). Although this change has been attributed to a muted hydrological cycle forced by global warming(5,6), the effect of SST warming patterns has not been explored and quantified(1,7,8). Here we perform experiments using an atmospheric model, and find that SST warming patterns are the main cause of the weakened Walker circulation over the past six decades (1950-2009). The SST trend reconstructed from bucket-sampled SST and night-time marine surface air temperature features a reduced zonal gradient in the tropical Indo-Pacific Ocean, a change consistent with subsurface temperature observations(8). Model experiments with this trend pattern robustly simulate the observed changes, including the Walker circulation slowdown and the eastward shift of atmospheric convection from the Indonesian maritime continent to the central tropical Pacific. Our results cannot establish whether the observed changes are due to natural variability or anthropogenic global warming, but they do show that the observed slowdown in the Walker circulation is presumably driven by oceanic rather than atmospheric processes.