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Tokinaga, H, Xie SP, Mukougawa H.  2017.  Early 20th-century Arctic warming intensified by Pacific and Atlantic multidecadal variability. Proceedings of the National Academy of Sciences of the United States of America. 114:6227-6232.   10.1073/pnas.1615880114   AbstractWebsite

With amplified warming and record sea ice loss, the Arctic is the canary of global warming. The historical Arctic warming is poorly understood, limiting our confidence in model projections. Specifically, Arctic surface air temperature increased rapidly over the early 20th century, at rates comparable to those of recent decades despite much weaker greenhouse gas forcing. Here, we show that the concurrent phase shift of Pacific and Atlantic interdecadal variability modes is the major driver for the rapid early 20th-century Arctic warming. Atmospheric model simulations successfully reproduce the early Arctic warming when the interdecadal variability of sea surface temperature (SST) is properly prescribed. The early 20th-century Arctic warming is associated with positive SST anomalies over the tropical and North Atlantic and a Pacific SST pattern reminiscent of the positive phase of the Pacific decadal oscillation. Atmospheric circulation changes are important for the early 20th-century Arctic warming. The equatorial Pacific warming deepens the Aleutian low, advecting warm air into the North American Arctic. The extratropical North Atlantic and North Pacific SST warming strengthens surface westerly winds over northern Eurasia, intensifying the warming there. Coupled ocean-atmosphere simulations support the constructive intensification of Arctic warming by a concurrent, negative-to-positive phase shift of the Pacific and Atlantic interdecadal modes. Our results aid attributing the historical Arctic warming and thereby constrain the amplified warming projected for this important region.

Zhou, WY, Xie SP.  2017.  Intermodel spread around the Kuroshio-Oyashio Extension region in coupled GCMs caused by meridional variation of the westerly jet from atmospheric GCMs. Journal of Climate. 30:4589-4599.   10.1175/jcli-d-16-0831.1   AbstractWebsite

The Kuroshio-Oyashio Extension (KOE) is a region of energetic oceanic mesoscale eddies and vigorous air-sea interaction that can influence climate variability over the northwest Pacific and East Asia. General circulation models (GCMs) exhibit considerable differences in their simulated climatology around the KOE region. Specifically, there are substantial intermodel spreads in both sea surface temperature (SST) and the upper-level westerly jet. In this study, the cause for such large spreads is studied by analyzing 21 pairs of coupled and atmospheric GCMs from phase 5 of the Coupled Model Intercomparison Project (CMIP5). It is found that the intermodel spread of the climatological westerly jet among coupled GCMs is largely inherited from their atmospheric models rather than being due to their SST difference as previously thought. An anomalous equatorward shift in the simulated westerly jet can give rise to a cold SST bias around the KOE region as follows. The equatorward jet shift induces cyclonic surface wind anomalies over the North Pacific, which not only enhance the turbulent heat fluxes out of the ocean south of the KOE but also drive an anomalous cyclonic ocean circulation that brings colder (warmer) water into the north (south) of the KOE. The KOE region is consequently cooled due to both the atmospheric and oceanic effects. Such processes are demonstrated through idealized perturbation experiments using an ocean model. The results herein point to reducing atmospheric model errors in the westerly jet as the way forward to improve the coupled simulations around the KOE region.

Richter, I, Xie SP, Morioka Y, Doi T, Taguchi B, Behera S.  2017.  Phase locking of equatorial Atlantic variability through the seasonal migration of the ITCZ. Climate Dynamics. 48:3615-3629.   10.1007/s00382-016-3289-y   AbstractWebsite

The equatorial Atlantic is marked by significant interannual variability in sea-surface temperature (SST) that is phase-locked to late boreal spring and early summer. The role of the atmosphere in this phase locking is examined using observations, reanalysis data, and model output. The results show that equatorial zonal surface wind anomalies, which are a main driver of warm and cold events, typically start decreasing in June, despite SST and sea-level pressure gradient anomalies being at their peak during this month. This behavior is explained by the seasonal northward migration of the intertropical convergence zone (ITCZ) in early summer. The north-equatorial position of the Atlantic ITCZ contributes to the decay of wind anomalies in three ways: (1) horizontal advection associated with the cross-equatorial winds transports air masses of comparatively low zonal momentum anomalies from the southeast toward the equator. (2) The absence of deep convection leads to changes in vertical momentum transport that reduce the equatorial wind anomalies at the surface, while anomalies aloft remain relatively strong. (3) The cross-equatorial flow is associated with increased total wind speed, which increases surface drag and deposit of momentum into the ocean. Previous studies have shown that convection enhances the surface wind response to SST anomalies. The present study indicates that convection also amplifies the surface zonal wind response to sea-level pressure gradients in the western equatorial Atlantic, where SST anomalies are small. This introduces a new element into coupled air-sea interaction of the tropical Atlantic.

Ma, J, Xie SP, Xu HM.  2017.  Intermember variability of the summer northwest Pacific subtropical anticyclone in the ensemble forecast. Journal of Climate. 30:3927-3941.   10.1175/jcli-d-16-0638.1   AbstractWebsite

The accurate prediction of the East Asian summer monsoon (EASM) remains a major challenge for the climate research community. The northwest Pacific (NWP) subtropical anticyclone (NWPSA) is the dominant feature of the EASM low-level circulation variability. This study identifies two coupled modes between intermember anomalies of the NWPSA and sea surface temperature (SST). The first mode features SST anomalies over the tropical Pacific. This tropical Pacific mode has little impact on East Asian climate. The second mode features a strong coupling between SST in the north Indian Ocean (NIO)-NWP and NWPSA, with large impacts on East Asia. This resembles the Indo-western Pacific Ocean capacitor (IPOC) mode of interannual variability. Major differences exist in temporal evolution of the intermember SST spread between the equatorial Pacific and NIO. In the equatorial Pacific, the intermember SST spread grows gradually with lead time, while the spread of SST and low-level zonal wind grow rapidly from May to June in the NIO. The rapid growth over the NIO is due to positive feedback arising from the coupling between intermember anomalies of SST and winds. In post-El Nino summer, the intermember spread in equatorial Pacific SST forecast represents the variations in the timing of the El Nino phase transition. The late decay of El Nino relates to SST cooling and an anomalous cyclonic circulation over the South China Sea (SCS) but with little impact on East Asian climate. Thus, a better representation of the IPOC mode of regional ocean-atmosphere interaction over the NIO-NWP holds the key to improving the reliability of seasonal forecast of East Asian climate.

Xie, SP, Zhou ZQ.  2017.  Seasonal modulations of El Nino-related atmospheric variability: Indo-Western Pacific Ocean feedback. Journal of Climate. 30:3461-3472.   10.1175/jcli-d-16-0713.1   AbstractWebsite

The spatial structure of atmospheric anomalies associated with El Nino-Southern Oscillation varies with season because of the seasonal variations in sea surface temperature (SST) anomaly pattern and in the climatological basic state. The latter effect is demonstrated using an atmospheric model forced with a time-invariant pattern of El Nino warming over the equatorial Pacific. The seasonal modulation is most pronounced over the north Indian Ocean to northwest Pacific where the monsoonal winds vary from northeasterly in winter to southwesterly in summer. Specifically, the constant El Nino run captures the abrupt transition from a summer cyclonic to winter anticyclonic anomalous circulation over the northwest Pacific, in support of the combination mode idea that emphasizes nonlinear interactions of equatorial Pacific SST forcing and the climatological seasonal cycle. In post-El Nino summers when equatorial Pacific warming has dissipated, SST anomalies over the Indo-northwest Pacific Oceans dominate and anchor the coherent persisting anomalous anticyclonic circulation. A conceptual model is presented that incorporates the combination mode in the existing framework of regional Indo-western Pacific Ocean coupling.

Wang, CY, Xie SP, Kosaka Y, Liu QY, Zheng XT.  2017.  Global influence of tropical Pacific variability with implications for global warming slowdown. Journal of Climate. 30:2679-2695.   10.1175/jcli-d-15-0496.1   AbstractWebsite

The impact of internal tropical Pacific variability on global mean surface temperature (GMST) is quantified using a multimodel ensemble. A tropical Pacific index (TPI) is defined to track tropical Pacific sea surface temperature (SST) variability. The simulated GMST is highly correlated with TPI on the interannual time scale but this correlation weakens on the decadal time scale. The time-scale dependency is such that the GMST regression equation derived from the observations, which are dominated by interannual variability, would underestimate the magnitude of decadal GMST response to tropical Pacific variability. The surface air temperature response to tropical Pacific variability is strong in the tropics but weakens in the extratropics. The regression coefficient of GMST against TPI shows considerable intermodel variations, primarily because of differences in high latitudes. The results have important implications for the planned intercomparison of pacemaker experiments that force Pacific variability to follow the observed evolution. The model dependency of the GMST regression suggests that in pacemaker experiments-model performance in simulating the recent "slowdown'' in global warming-will vary substantially among models. It also highlights the need to develop observational constraints and to quantify the TPI effect on the decadal variability of GMST. Compared to GMST, the correlation between global mean tropospheric temperature and TPI is high on both interannual and decadal time scales because of a common structure in the tropical tropospheric temperature response that is upward amplified and meridionally broad.

Xu, LX, Xie SP, Jing Z, Wu LX, Liu QY, Li PL, Du Y.  2017.  Observing subsurface changes of two anticyclonic eddies passing over the Izu-Ogasawara Ridge. Geophysical Research Letters. 44:1857-1865.   10.1002/2016gl072163   AbstractWebsite

Eddy-bathymetry interactions are common in the ocean, but the full evolution of the interaction is difficult to observe below the surface. Using 17 Iridium Argo floats, we continually track two anticyclonic eddies (AEs) in the North Pacific that migrate westward and encounter the Izu-Ogasawara Ridge. Based on over 5000 Argo profiles following the two AEs, this study presents the first detailed descriptions of changes in eddy vertical structure and diapycnal mixing as the two AEs pass the Ridge. There, we find that isopycnals dome up and the eddy diameter increases, while the diapycnal mixing is enhanced-to the order of 10(-4) m(2) s(-1) or larger, in comparison with an ambient of 10(-5) m(2) s(-1). The enhanced mixing around the AE center in the upper -1000m appears where the underlying bathymetry is shallower than -4000m and is mainly sustained by tidally generated internal waves.

Liu, W, Xie S-P, Liu Z, Zhu J.  2017.  Overlooked possibility of a collapsed Atlantic Meridional Overturning Circulation in warming climate. Science Advances. 3   10.1126/sciadv.1601666   Abstract

Changes in the Atlantic Meridional Overturning Circulation (AMOC) are moderate in most climate model projections under increasing greenhouse gas forcing. This intermodel consensus may be an artifact of common model biases that favor a stable AMOC. Observationally based freshwater budget analyses suggest that the AMOC is in an unstable regime susceptible for large changes in response to perturbations. By correcting the model biases, we show that the AMOC collapses 300 years after the atmospheric CO2 concentration is abruptly doubled from the 1990 level. Compared to an uncorrected model, the AMOC collapse brings about large, markedly different climate responses: a prominent cooling over the northern North Atlantic and neighboring areas, sea ice increases over the Greenland-Iceland-Norwegian seas and to the south of Greenland, and a significant southward rain-belt migration over the tropical Atlantic. Our results highlight the need to develop dynamical metrics to constrain models and the importance of reducing model biases in long-term climate projection.

Li, G, Xie SP, Du Y, Luo YY.  2016.  Effects of excessive equatorial cold tongue bias on the projections of tropical Pacific climate change. Part I: the warming pattern in CMIP5 multi-model ensemble. Climate Dynamics. 47:3817-3831.   10.1007/s00382-016-3043-5   AbstractWebsite

The excessive cold tongue error in the equatorial Pacific has persisted in several generations of climate models. Based on the historical simulations and Representative Concentration Pathway (RCP) 8.5 experiments in the Coupled Model Intercomparison Project phase 5 (CMIP5) multi-model ensemble (MME), this study finds that models with an excessive westward extension of cold tongue and insufficient equatorial western Pacific precipitation tend to project a weaker east-minus-west gradient of sea surface temperature (SST) warming along the equatorial Pacific under increased greenhouse gas (GHG) forcing. This La Nia-like error of tropical Pacific SST warming is consistent with our understanding of negative SST-convective feedback over the western Pacific warm pool. Based on this relationship between the present simulations and future projections, the present study applies an "observational constraint" of equatorial western Pacific precipitation to calibrate the projections of tropical Pacific climate change. After the corrections, CMIP5 models robustly project an El Nio-like warming pattern, with a MME mean increase by a factor of 2.3 in east-minus-west gradient of equatorial Pacific SST warming and reduced inter-model uncertainty. Corrections in projected changes in tropical precipitation and atmospheric circulation are physically consistent. This study suggests that a realistic cold tongue simulation would lead to a more reliable tropical Pacific climate projection.

Meehl, GA, Hu AX, Santer BD, Xie SP.  2016.  Contribution of the Interdecadal Pacific Oscillation to twentieth-century global surface temperature trends. Nature Climate Change. 6:1005-1008.   10.1038/nclimate3107   AbstractWebsite

Longer-term externally forced trends in global mean surface temperatures (GMSTs) are embedded in the background noise of internally generated multidecadal variability(1). A key mode of internal variability is the Interdecadal Pacific Oscillation (IPO), which contributed to a reduced GMST trend during the early 2000s(1-3). We use a novel, physical phenomenon-based approach to quantify the contribution from a source of internally generated multidecadal variability-the IPO-to multidecadal GMST trends. Here we show that the largest IPO contributions occurred in its positive phase during the rapidwarming periods from 1910-1941 and 1971-1995, with the IPO contributing 71% and 75%, respectively, to the difference between the median values of the externally forced trends and observed trends. The IPO transition from positive to negative in the late-1990s contributed 27% of the discrepancy between model median estimates of the forced part of the GMST trend and the observed trend from 1995 to 2013, with additional contributions that are probably due to internal variability outside of the Pacific(4) and an externally forced response from small volcanic eruptions(5). Understanding and quantifying the contribution of a specific source of internally generated variability-the IPO-to GMST trends is necessary to improve decadal climate prediction skill.

Yan, XH, Boyer T, Trenberth K, Karl TR, Xie SP, Nieves V, Tung KK, Roemmich D.  2016.  The global warming hiatus: Slowdown or redistribution? Earths Future. 4:472-482.   10.1002/2016ef000417   AbstractWebsite

Global mean surface temperatures (GMST) exhibited a smaller rate of warming during 1998-2013, compared to the warming in the latter half of the 20th Century. Although, not a "true" hiatus in the strict definition of the word, this has been termed the "global warming hiatus" by IPCC (2013). There have been other periods that have also been defined as the "hiatus" depending on the analysis. There are a number of uncertainties and knowledge gaps regarding the "hiatus." This report reviews these issues and also posits insights from a collective set of diverse information that helps us understand what we do and do not know. One salient insight is that the GMST phenomenon is a surface characteristic that does not represent a slowdown in warming of the climate system but rather is an energy redistribution within the oceans. Improved understanding of the ocean distribution and redistribution of heat will help better monitor Earth's energy budget and its consequences. A review of recent scientific publications on the "hiatus" shows the difficulty and complexities in pinpointing the oceanic sink of the "missing heat" from the atmosphere and the upper layer of the oceans, which defines the "hiatus." Advances in "hiatus" research and outlooks (recommendations) are given in this report.

Zhou, WY, Xie SP, Zhou ZQ.  2016.  Slow preconditioning for the abrupt convective jump over the Northwest Pacific during summer. Journal of Climate. 29:8103-8113.   10.1175/jcli-d-16-0342.1   AbstractWebsite

The rapid intensification of convective activity in mid-July over the northwest Pacific marks the final stage of the Asian summer monsoon, accompanied by major shifts in regional rainfall and circulation patterns. An entraining plume model is used to investigate the physical processes underlying the abrupt convective jump. Despite little change in sea surface temperature (SST), gradual lower-troposphere mixing leads to a threshold transition in the model as follows. Before mid-July, although SST is already high (29 degrees C), the convective plume is inhibited by the capping inversion above the trade cumulus boundary layer. As the lower troposphere is gradually mixed, the boundary layer top rises with reduced atmospheric stability and increased humidity in the lower troposphere. These factors weaken the inhibition effect of the inversion on the entraining plume. As soon as the plume is able to overcome the inversion barrier, it can rise all the way to the upper troposphere. This marks an abrupt threshold transition to a deep convection regime with heavy rainfall. The convective available potential energy (CAPE) of the entraining plume is found to be a better indicator of the rainfall intensity compared to the conventional undiluted CAPE. The latter fails to capture the onset by neglecting interactions between convective clouds and the environment. Current general circulation models (GCMs) fail to capture the abrupt convective jump and instead simulate a rather smooth seasonal evolution of rainfall. Compared to observations, GCMs simulate a higher trade cumulus top with excessive mixing in the lower troposphere. Convection is no longer inhibited by the inversion barrier, and rainfall simply follows the smooth variation of SST.

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

Mei, W, Xie SP.  2016.  Intensification of landfalling typhoons over the northwest Pacific since the late 1970s. Nature Geoscience. 9:753-+.   10.1038/ngeo2792   AbstractWebsite

Intensity changes in landfalling typhoons are of great concern to East and Southeast Asian countries(1). Regional changes in typhoon intensity, however, are poorly known owing to inconsistencies among different data sets(2-8). Here, we apply cluster analysis to bias-corrected data and show that, over the past 37 years, typhoons that strike East and Southeast Asia have intensified by 12-15%, with the proportion of storms of categories 4 and 5 having doubled or even tripled. In contrast, typhoons that stay over the open ocean have experienced only modest changes. These regional changes are consistent between operational data sets. To identify the physical mechanisms, we decompose intensity changes into contributions from intensification rate and intensification duration. We find that the increased intensity of landfalling typhoons is due to strengthened intensification rates, which in turn are tied to locally enhanced ocean surface warming on the rim of East and Southeast Asia. The projected ocean surface warming pattern under increasing greenhouse gas forcing suggests that typhoons striking eastern mainland China, Taiwan, Korea and Japan will intensify further. Given disproportionate damages by intense typhoons(1), this represents a heightened threat to people and properties in the region.

Zheng, XT, Xie SP, Lv LH, Zhou ZQ.  2016.  Intermodel uncertainty in ENSO amplitude change tied to Pacific Ocean warming pattern. Journal of Climate. 29:7265-7279.   10.1175/jcli-d-16-0039.1   AbstractWebsite

How El Nino-Southern Oscillation (ENSO) will change under global warming affects changes in extreme events around the world. The change of ENSO amplitude is investigated based on the historical simulations and representative concentration pathway (RCP) 8.5 experiments in phase 5 of the Coupled Model Intercomparison Project (CMIP5). The projected change in ENSO amplitude is highly uncertain with large intermodel uncertainty. By using the relative sea surface temperature (SST) as a measure of convective instability, this study finds that the spatial pattern of tropical Pacific surface warming is the major source of intermodel uncertainty in ENSO amplitude change. In models with an enhanced mean warming in the eastern equatorial Pacific, the barrier to deep convection is reduced, and the intensified rainfall anomalies of ENSO amplify the wind response and hence SST variability. In models with a reduced eastern Pacific warming, conversely, ENSO amplitude decreases. Corroborating the mean SST pattern effect, intermodel uncertainty in changes of ENSO-induced rainfall variability decreases substantially in atmospheric simulations forced by a common ocean warming pattern. Thus, reducing the uncertainty in the Pacific surface warming pattern helps improve the reliability of ENSO projections. To the extent that correcting model biases favors an El Nino-like mean warming pattern, this study suggests an increase in ENSO-related SST variance likely under global warming.

Kilpatrick, TJ, Xie SP.  2016.  Circumventing rain-related errors in scatterometer wind observations. Journal of Geophysical Research-Atmospheres. 121:9422-9440.   10.1002/2016jd025105   AbstractWebsite

Satellite scatterometer observations of surface winds over the global oceans are critical for climate research and applications like weather forecasting. However, rain-related errors remain an important limitation, largely precluding satellite study of winds in rainy areas. Here we utilize a novel technique to compute divergence and curl from satellite observations of surface winds and surface wind stress in rainy areas. This technique circumvents rain-related errors by computing line integrals around rainy patches, using valid wind vector observations that border the rainy patches. The area-averaged divergence and wind stress curl inside each rainy patch are recovered via the divergence and curl theorems. We process the 10 year Quick Scatterometer (QuikSCAT) data set and show that the line-integral method brings the QuikSCAT winds into better agreement with an atmospheric reanalysis, largely removing both the "divergence bias" and "anticyclonic curl bias" in rainy areas noted in previous studies. The corrected QuikSCAT wind stress curl reduces the North Pacific midlatitude Sverdrup transport by 20-30%. We test several methods of computing divergence and curl on winds from an atmospheric model simulation and show that the line-integral method has the smallest errors. We anticipate that scatterometer winds processed with the line-integral method will improve ocean model simulations and help illuminate the coupling between atmospheric convection and circulation.

Li, G, Xie SP, Du Y.  2016.  A robust but spurious pattern of climate change in model projections over the tropical Indian Ocean. Journal of Climate. 29:5589-5608.   10.1175/jcli-d-15-0565.1   AbstractWebsite

Climate models consistently project reduced surface warming over the eastern equatorial Indian Ocean (IO) under increased greenhouse gas (GHG) forcing. This IO dipole (IOD)-like warming pattern, regarded as robust based on consistency among models by the new Intergovernmental Panel on Climate Change (IPCC) report, results in a large increase in the frequency of extreme positive IOD (pIOD) events, elevating the risk of climate and weather disasters in the future over IO rim countries. These projections, however, do not consider large model biases in both the mean state and interannual IOD variance. In particular, a "present-future relationship" is identified between the historical simulations and representative concentration pathway (RCP) 8.5 experiments from phase 5 of the Coupled Model Intercomparison Project (CMIP5) multimodel ensemble: models with an excessive IOD amplitude bias tend to project a strong IOD-like warming pattern in themean and a large increase in extreme pIOD occurrences under increased GHG forcing. This relationship links the present simulation errors to future climate projections, and is also consistent with our understanding of Bjerknes ocean-atmosphere feedback. This study calibrates regional climate projections by using this present-future relationship and observed IOD amplitude. The results show that the projected IOD-like pattern of mean changes and frequency increase of extreme pIOD events are largely artifacts of model errors and unlikely to emerge in the future. These results illustrate that a robust projection may still be biased and it is important to consider the model bias effect.

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.

Merrifield, AL, Xie SP.  2016.  Summer US surface air temperature variability: controlling factors and AMIP simulation biases. Journal of Climate. 29:5123-5139.   10.1175/jcli-d-15-0705.1   AbstractWebsite

This study documents and investigates biases in simulating summer surface air temperature (SAT) variability over the continental United States in the Atmospheric Model Intercomparison Project (AMIP) experiment from phase 5 of the Coupled Model Intercomparison Project (CMIP5). Empirical orthogonal function (EOF) and multivariate regression analyses are used to assess the relative importance of circulation and the land surface feedback at setting summer SAT over a 30-yr period (1979-2008). Regions of high SAT variability are closely associated with midtropospheric highs, subsidence, and radiative heating accompanying clear-sky conditions. The land surface exerts a spatially variable influence on SAT through the sensible heat flux and is a second-order effect in the high-variability centers of action (COAs) in observational estimates. The majority of the AMIP models feature high SAT variability over the central United States, displaced south and/or west of observed COAs. SAT COAs in models tend to be concomitant and strongly coupled with regions of high sensible heat flux variability, suggesting that excessive land-atmosphere interaction in these models modulates U.S. summer SAT. In the central United States, models with climatological warm biases also feature less evapotranspiration than ERA-Interim but reasonably reproduce observed SAT variability in the region. Models that overestimate SAT variability tend to reproduce ERA-Interim SAT and evapotranspiration climatology. In light of potential model biases, this analysis calls for careful evaluation of the land-atmosphere interaction hot spot region identified in the central United States. Additionally, tropical sea surface temperatures play a role in forcing the leading EOF mode for summer SAT in models. This relationship is not apparent in observations.

Kosaka, Y, Xie S-P.  2016.  The tropical Pacific as a key pacemaker of the variable rates of global warming. Nature Geosci. advance online publication: Nature Publishing Group   10.1038/ngeo2770   Abstract

Global mean surface temperature change over the past 120 years resembles a rising staircase: the overall warming trend was interrupted by the mid-twentieth-century big hiatus and the warming slowdown since about 1998. The Interdecadal Pacific Oscillation has been implicated in modulations of global mean surface temperatures, but which part of the mode drives the variability in warming rates is unclear. Here we present a successful simulation of the global warming staircase since 1900 with a global ocean–atmosphere coupled model where tropical Pacific sea surface temperatures are forced to follow the observed evolution. Without prescribed tropical Pacific variability, the same model, on average, produces a continual warming trend that accelerates after the 1960s. We identify four events where the tropical Pacific decadal cooling markedly slowed down the warming trend. Matching the observed spatial and seasonal fingerprints we identify the tropical Pacific as a key pacemaker of the warming staircase, with radiative forcing driving the overall warming trend. Specifically, tropical Pacific variability amplifies the first warming epoch of the 1910s–1940s and determines the timing when the big hiatus starts and ends. Our method of removing internal variability from the observed record can be used for real-time monitoring of anthropogenic warming.

Liu, JW, Xie SP, Yang S, Zhang SP.  2016.  Low-cloud transitions across the Kuroshio Front in the East China Sea. Journal of Climate. 29:4429-4443.   10.1175/jcli-d-15-0589.1   AbstractWebsite

The East China Sea Kuroshio (ECSK) flows in the East Asian monsoon region where the background atmospheric circulation varies significantly with season. A sea surface temperature (SST) front associated with the ECSK becomes narrower and sharper from winter to spring. The present study investigates how low clouds respond to the ECSK front in different seasons by synthesizing spaceborne lidar and surface visual observations. The results reveal prominent cross-frontal transitions in low clouds, which exhibit distinct behavior between winter and spring. In winter, cloud responses are generally confined below 4 km by the strong background descending motion and feature a gradual cloud-top elevation from the cold to the warm flank of the front. The ice clouds on the cold flank of the ECSK front transform into liquid water clouds and rain on the warm flank. The springtime clouds, by contrast, are characterized by a sharp cross-frontal transition with deep clouds reaching up to 7 km over the ECSK. In both winter and spring, the low-cloud morphology exhibits a large transformation from the cold to the warm flank of the ECSK front, including increases in cloud-top height, a decline in smoothness of cloud top, and the transition from stratiform to convective clouds. All this along with the atmospheric soundings indicates that the decoupling of the marine atmospheric boundary layer (MABL) is more prevalent on the warm flank of the front. Thus, long-term observations reveal prominent cross-frontal low-cloud transitions in morphology associated with MABL decoupling that resemble a large-scale cloud-regime transition over the eastern subtropical Pacific.

Cheng, XH, Xie SP, Du Y, Wang J, Chen X, Wang J.  2016.  Interannual-to-decadal variability and trends of sea level in the South China Sea. Climate Dynamics. 46:3113-3126.   10.1007/s00382-015-2756-1   AbstractWebsite

Interannual-to-decadal variability and trends of sea level in the South China Sea (SCS) are studied using altimetric data during 1993-2012 and reconstructed sea level data from 1950-2009. The interannual variability shows a strong seasonality. Surface wind anomalies associated with El Nio-Southern Oscillation explain the sea-level anomaly pattern in the interior SCS, while Rossby waves radiated from the eastern boundary dominate the sea-level variability in the eastern SCS. Decadal variability of sea level in the SCS follows that in the western tropical Pacific, with large variance found west of Luzon Island. Local atmospheric forcing makes a negative contribution to decadal variability in the central SCS, and Rossby waves radiated from the eastern boundary appear to be important. During 1993-2012, decadal sea level averaged in the SCS is significantly correlated with the Pacific Decadal Oscillation (PDO) (r = -0.96). The decadal variability associated with the PDO accounts for most part of sea-level trends in the SCS in the last two decades.

Zhang, RS, Xie SP, Xu LX, Liu QY.  2016.  Changes in mixed layer depth and spring bloom in the Kuroshio extension under global warming. Advances in Atmospheric Sciences. 33:452-461.   10.1007/s00376-015-5113-8   AbstractWebsite

The mixed layer is deep in January-April in the Kuroshio Extension region. This paper investigates the response in this region of mixed layer depth (MLD) and the spring bloom initiation to global warming using the output of 15 models from CMIP5. The models indicate that in the late 21st century the mixed layer will shoal, and the MLD reduction will be most pronounced in spring at about 33A degrees N on the southern edge of the present deep-MLD region. The advection of temperature change in the upper 100 m by the mean eastward flow explains the spatial pattern of MLD shoaling in the models. Associated with the shoaling mixed layer, the onset of spring bloom inception is projected to advance due to the strengthened stratification in the warming climate.

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.

Xie, SP, Kosaka Y, Du Y, Hu KM, Chowdary J, Huang G.  2016.  Indo-western Pacific ocean capacitor and coherent climate anomalies in post-ENSO summer: A review. Advances in Atmospheric Sciences. 33:411-432.   10.1007/s00376-015-5192-6   AbstractWebsite

ENSO induces coherent climate anomalies over the Indo-western Pacific, but these anomalies outlast SST anomalies of the equatorial Pacific by a season, with major effects on the Asian summer monsoon. This review provides historical accounts of major milestones and synthesizes recent advances in the endeavor to understand summer variability over the Indo-Northwest Pacific region. Specifically, a large-scale anomalous anticyclone (AAC) is a recurrent pattern in post-El NiEeno summers, spanning the tropical Northwest Pacific and North Indian oceans. Regarding the ocean memory that anchors the summer AAC, competing hypotheses emphasize either SST cooling in the easterly trade wind regime of the Northwest Pacific or SST warming in the westerly monsoon regime of the North Indian Ocean. Our synthesis reveals a coupled ocean-atmosphere mode that builds on both mechanisms in a two-stage evolution. In spring, when the northeast trades prevail, the AAC and Northwest Pacific cooling are coupled via wind-evaporation-SST feedback. The Northwest Pacific cooling persists to trigger a summer feedback that arises from the interaction of the AAC and North Indian Ocean warming, enabled by the westerly monsoon wind regime. This Indo-western Pacific ocean capacitor (IPOC) effect explains why El Nino stages its last act over the monsoonal Indo-Northwest Pacific and casts the Indian Ocean warming and AAC in leading roles. The IPOC displays interdecadal modulations by the ENSO variance cycle, significantly correlated with ENSO at the turn of the 20th century and after the 1970s, but not in between. Outstanding issues, including future climate projections, are also discussed.