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2019
Eddebbar, YA, Rodgers KB, Long MC, Subramanian AC, Xie SP, Keeling RF.  2019.  El Nino-like physical and biogeochemical ocean response to tropical eruptions. Journal of Climate. 32:2627-2649.   10.1175/jcli-d-18-0458.1   AbstractWebsite

The oceanic response to recent tropical eruptions is examined in Large Ensemble (LE) experiments from two fully coupled global climate models, the Community Earth System Model (CESM) and the Geophysical Fluid Dynamics Laboratory Earth System Model (ESM2M), each forced by a distinct volcanic forcing dataset. Following the simulated eruptions of Agung, El Chichon, and Pinatubo, the ocean loses heat and gains oxygen and carbon, in general agreement with available observations. In both models, substantial global surface cooling is accompanied by El Nino-like equatorial Pacific surface warming a year after the volcanic forcing peaks. A mechanistic analysis of the CESM and ESM2M responses to Pinatubo identifies remote wind forcing from the western Pacific as a major driver of this El Nino-like response. Following eruption, faster cooling over the Maritime Continent than adjacent oceans suppresses convection and leads to persistent westerly wind anomalies over the western tropical Pacific. These wind anomalies excite equatorial downwelling Kelvin waves and the upwelling of warm subsurface anomalies in the eastern Pacific, promoting the development of El Nino conditions through Bjerknes feedbacks a year after eruption. This El Nino-like response drives further ocean heat loss through enhanced equatorial cloud albedo, and dominates global carbon uptake as upwelling of carbon-rich waters is suppressed in the tropical Pacific. Oxygen uptake occurs primarily at high latitudes, where surface cooling intensifies the ventilation of subtropical thermocline waters. These volcanically forced ocean responses are large enough to contribute to the observed decadal variability in oceanic heat, carbon, and oxygen.

Cai, WJ, Wu LX, Lengaigne M, Li T, McGregor S, Kug JS, Yu JY, Stuecker MF, Santoso A, Li XC, Ham YG, Chikamoto Y, Ng B, McPhaden MJ, Du Y, Dommenget D, Jia F, Kajtar JB, Keenlyside N, Lin XP, Luo JJ, Martin-Rey M, Ruprich-Robert Y, Wang GJ, Xie SP, Yang Y, Kang SM, Choi JY, Gan BL, Kim GI, Kim CE, Kim S, Kim JH, Chang P.  2019.  Pantropical climate interactions. Science. 363:944-+.   10.1126/science.aav4236   AbstractWebsite

The El Nino-Southern Oscillation (ENSO), which originates in the Pacific, is the strongest and most well-known mode of tropical climate variability. Its reach is global, and it can force climate variations of the tropical Atlantic and Indian Oceans by perturbing the global atmospheric circulation. Less appreciated is how the tropical Atlantic and Indian Oceans affect the Pacific. Especially noteworthy is the multidecadal Atlantic warming that began in the late 1990s, because recent research suggests that it has influenced Indo-Pacific climate, the character of the ENSO cycle, and the hiatus in global surface warming. Discovery of these pantropical interactions provides a pathway forward for improving predictions of climate variability in the current climate and for refining projections of future climate under different anthropogenic forcing scenarios.

Wang, H, Xie SP, Kosaka Y, Liu QY, Du Y.  2019.  Dynamics of Asian summer monsoon response to anthropogenic aerosol forcing. Journal of Climate. 32:843-858.   10.1175/jcli-d-18-0386.1   AbstractWebsite

Anthropogenic aerosols partially mask the greenhouse warming and cause the reduction in Asian summer monsoon precipitation and circulation. By decomposing the atmospheric change into the direct atmospheric response to radiative forcing and sea surface temperature (SST)-mediated change, the physical mechanisms for anthropogenic-aerosol-induced changes in the East Asian summer monsoon (EASM) and South Asian summer monsoon (SASM) are diagnosed. Using coupled and atmospheric general circulation models, this study shows that the aerosol-induced troposphere cooling over Asian land regions generates anomalous sinking motion between 20 degrees and 40 degrees N and weakens the EASM north of 20 degrees N without SST change. The decreased EASM precipitation and the attendant wind changes are largely due to this direct atmospheric response to radiative forcing, although the aerosol-induced North Pacific SST cooling also contributes. The SST-mediated change dominates the aerosol-induced SASM response, with contributions from both the north-south interhemispheric SST gradient and the local SST cooling pattern over the tropical Indian Ocean. Specifically, with large meridional gradient, the zonal-mean SST cooling pattern is most important for the Asian summer monsoon response to anthropogenic aerosol forcing, resulting in a reorganization of the regional meridional atmospheric overturning circulation. While uncertainty in aerosol radiative forcing has been emphasized in the literature, our results show that the intermodel spread is as large in the SST effect on summer monsoon rainfall, calling for more research into the ocean-atmosphere coupling.

Zheng, XT, Hui C, Xie SP, Cai WJ, Long SM.  2019.  Intensification of El Nino Rainfall Variability Over the Tropical Pacific in the Slow Oceanic Response to Global Warming. Geophysical Research Letters. 46:2253-2260.   10.1029/2018gl081414   AbstractWebsite

Changes in rainfall variability of El Nino-Southern Oscillation (ENSO) are investigated under scenarios where the greenhouse gases increase and then stabilize. During the period of increasing greenhouse forcing, the ocean mixed layer warms rapidly. After the forcing stabilizes, the deeper ocean continues to warm the surface (the slow response). We show that ENSO rainfall variability over the tropical Pacific intensifies in both periods but the rate of increase per degree global mean surface temperature (GMST) warming is larger for the slow response because of greater relative warming in the base state as the mean upwelling changes from a damping to a driver of the surface warming. Our results have important implications for climate extremes under GMST stabilization that the Paris Agreement calls for. To stabilize GMST, the fast surface cooling offsets the slow warming from the prior greenhouse gas increase, while ENSO rainfall variability would continue to increase. Plain Language Summary The Paris Agreement calls for limiting global mean surface temperature increase to well below 2 degrees at the end of the 21st century. This requires the greenhouse gas (GHG) concentration to peak and subsequently decline in the next few decades. After the GHG concentration peak, the heat accumulated in the ocean surface layer continues to penetrate to the deeper ocean. This deeper ocean warming leads to a slow response of surface warming, further influencing the climate system. This study examines scenarios where GHGs increase and then stabilize to isolate the fast and slow responses of El Nino-Southern Oscillation (ENSO) rainfall variability. We find intensification of ENSO rainfall variability both during the increase and after stabilization of GHG concentrations due to a persistent El Nino-like mean warming pattern in the tropical Pacific. Furthermore, for unit global mean surface temperature increase, the changes in the mean state temperature and ENSO rainfall variability in the eastern equatorial Pacific is larger during the slow response. These results imply that there is a need for GHG emission reduction in the near future to avoid more extreme tropical rainfall during El Nino.

Li, JB, Xie SP, Cook ER, Chen FH, Shi JF, Zhang DD, Fang KY, Gou XH, Li T, Peng JF, Shi SY, Zhao YS.  2019.  Deciphering human contributions to Yellow River flow reductions and downstream drying using centuries-long tree ring records. Geophysical Research Letters. 46:898-905.   10.1029/2018gl081090   AbstractWebsite

The Yellow River flow has decreased substantially in recent decades, and the river often dried up in the lower reach and failed to reach the sea. Climate change and human disruption have been suggested as major causes of the flow reduction, but quantification of their relative contribution is challenging due to limited instrumental records and disturbance by dams. Here we use a basin-wide tree ring network to reconstruct the Yellow River flow for the past 1,200 years and show that the flow exhibits marked amplitude variations that are closely coupled to the hydrological mean state swings at multidecadal to centennial timescales. Recent flow should have increased to the highest level of the past 1,200 years if there were no human disruption. However, human activities have caused a loss of nearly half of natural flow since the late 1960s and are the main culprit for recent downstream flow reduction. Plain Language Summary Recent Yellow River flow reductions have had major impacts on China's economy and water policy. The short and heavily human-modified gauge records are unable to reveal natural flow variability now and in the past. Here we use tree rings to reconstruct long-term Yellow River flow, which enables an assessment of natural flow variability and the detection of human contributions to recent flow reductions. Our 1,200-year reconstruction reveals that under natural conditions the Yellow River flow should have increased markedly since the early twentieth century. However, the observed flow decreased since the late 1960s and such a decrease must be predominately caused by human interventions instead of climate change.

2018
Lin, L, Xu YY, Wang ZL, Diao CR, Dong WJ, Xie SP.  2018.  Changes in extreme rainfall over India and China attributed to regional aerosol-cloud interaction during the late 20th century rapid industrialization. Geophysical Research Letters. 45:7857-7865.   10.1029/2018gl078308   AbstractWebsite

Both mean and extreme rainfall decreased over India and Northern China during 1979-2005 at a rate of 0.2%/decade. The aerosol dampening effects on rainfall has also been suggested as a main driver of mean rainfall shift in India and China. Conflicting views, however, exist on whether aerosols enhance or suppress hazardous extreme heavy rainfall. Using Coupled Model Intercomparison Project phase 5 (CMIP5) multimodel ensemble, here we show that only a subset of models realistically reproduces the late-20th-century trend of extreme rainfall for the three major regions in Asia: drying in India and Northern China and wetting in Southern China, all consistent with mean rainfall change. As a common feature, this subset of models includes an explicit treatment of the complex physical processes of aerosol-cloud interaction (i.e., both cloud-albedo and cloud-lifetime effects), while simulation performance deteriorates in models that include only aerosol direct effect or cloud-albedo effect. The enhanced aerosol pollution during this rapid industrialization era is the leading cause of the spatially heterogeneous extreme rainfall change by dimming surface solar radiation, cooling adjacent ocean water, and weakening moisture transport into the continental region, while GHG warming or natural variability alone cannot explain the observed changes. Our results indicate that the projected intensification of regional extreme rainfall during the early-to-mid 21st-century, in response to the anticipated aerosol reduction, may be underestimated in global climate models without detailed treatment of complex aerosol-cloud interaction. Plain Language Summary Over Asia, a robust pattern of drying-wetting-drying trend over three most populated regions (India, South China, and North China, respectively) have been observed in the past few decades. Yet the cause of the 30-year trend is rather unclear, with conflicting arguments on the importance of natural variability, the greenhouse gas, land cover, and aerosols. Most of the previous studies, however, fail to provide a holistic explanation for all three major regions simultaneously. The aerosol-cloud interaction-induced oceanic cooling, as we show here, provides a critical piece in reproducing the past trend. Only a fraction of climate models with complex treatment of aerosol-cloud interaction capture the observed pattern; thus, unconstrained model data set provides biased outlook of extreme rainfall in this region.

Liu, W, Xie SP.  2018.  An ocean view of the global surface warming hiatus. Oceanography. 31:72-79.   10.5670/oceanog.2018.217   AbstractWebsite

The rate of global mean surface temperature increase slowed during 1998-2012. We review oceanic changes during this global warming hiatus from different but related perspectives. In one perspective, we explore the physical mechanisms for sea surface temperature patterns and highlight the role of natural variability, particularly the Interdecadal Pacific Oscillation (IPO) and the Atlantic Multidecadal Oscillation (AMO) that both have chaotic/random phases. In the other perspective, we investigate how the hiatus relates to changes in energy fluxes at the top of the atmosphere and to the three-dimensional distribution of ocean heat content change on decadal timescales. We find that the recent surface warming hiatus is associated with a transition of the IPO from a positive to negative phase and with heat redistribution between the tropical Pacific and Indian Oceans. The AMO has shifted to a positive phase since the late 1990s, inducing a La Nina-type response over the tropical Pacific via a tropic-wide teleconnection, contributing to the global warming hiatus.

Liu, W, Lu J, Xie SP, Fedorov A.  2018.  Southern Ocean heat uptake, redistribution, and storage in a warming climate: The role of meridional overturning circulation. Journal of Climate. 31:4727-4743.   10.1175/jcli-d-17-0761.1   AbstractWebsite

Climate models show that most of the anthropogenic heat resulting from increased atmospheric CO2 enters the Southern Ocean near 60 degrees S and is stored around 45 degrees S. This heat is transported to the ocean interior by the meridional overturning circulation (MOC) with wind changes playing an important role in the process. To isolate and quantify the latter effect, we apply an overriding technique to a climate model and decompose the total ocean response to CO2 increase into two major components: one due to wind changes and the other due to direct CO2 effect. We find that the poleward-intensified zonal surface winds tend to shift and strengthen the ocean Deacon cell and hence the residual MOC, leading to anomalous divergence of ocean meridional heat transport around 60 degrees S coupled to a surface heat flux increase. In contrast, at 45 degrees S we see anomalous convergence of ocean heat transport and heat loss at the surface. As a result, the wind-induced ocean heat storage (OHS) peaks at 46 degrees S at a rate of 0.07 ZJ yr(-1) (degrees lat)(-1) (1 ZJ = 10(21) J), contributing 20% to the total OHS maximum. The direct CO2 effect, on the other hand, very slightly alters the residual MOC but primarily warms the ocean. It induces a small but nonnegligible change in eddy heat transport and causes OHS to peak at 42 degrees S at a rate of 0.30 ZJ yr(-1) (degrees lat)(-1), accounting for 80% of the OHS maximum. We also find that the eddy-induced MOC weakens, primarily caused by a buoyancy flux change as a result of the direct CO2 effect, and does not compensate the intensified Deacon cell.

Yang, L, Liu JW, Ren ZP, Xie SP, Zhang SP, Gao SH.  2018.  Atmospheric conditions for advection-radiation fog over the western Yellow Sea. Journal of Geophysical Research-Atmospheres. 123:5455-5468.   10.1029/2017jd028088   AbstractWebsite

Advection fog occurs usually when warm and moist air flows over cold sea surface. It is occasionally reported that the fog air temperature falls below sea surface temperature (called here the sea fog with sea surface heating [ssH]) due to longwave radiation cooling at fog top. Using 8-year buoy observations, this study reveals that about 33% of the time, the advection fog is with ssH in the western Yellow Sea. By synthesizing long-term observations from meteorological stations, atmospheric soundings, and offshore buoys, this study further investigates the marine atmospheric boundary layer (MABL) structure and atmospheric circulation associated with the ssH sea fog. Composite analysis shows that a local anomalous high pressure favors widespread formation of the ssH sea fog. The subsidence in the high pressure intensifies the thermal and moist stratification between the MABL and free atmosphere through adiabatic warming. The dry air above helps cool the fog layer by enhancing the longwave radiative cooling at the fog top and the vertical mixing beneath, causing air temperature to drop below sea surface temperature. The ratio of sea fog with ssH to total sea fog decreases from spring to summer as the descending motion and MABL stratification both weaken. This study highlights the importance of longwave radiative cooling at the advection fog top and suggests a way to improve sea fog forecast in the Yellow Sea.

Johnson, NC, Xie SP, Kosaka Y, Li XC.  2018.  Increasing occurrence of cold and warm extremes during the recent global warming slowdown. Nature Communications. 9   10.1038/s41467-018-04040-y   AbstractWebsite

The recent levelling of global mean temperatures after the late 1990s, the so-called global warming hiatus or slowdown, ignited a surge of scientific interest into natural global mean surface temperature variability, observed temperature biases, and climate communication, but many questions remain about how these findings relate to variations in more societally relevant temperature extremes. Here we show that both summertime warm and wintertime cold extreme occurrences increased over land during the so-called hiatus period, and that these increases occurred for distinct reasons. The increase in cold extremes is associated with an atmospheric circulation pattern resembling the warm Arctic-cold continents pattern, whereas the increase in warm extremes is tied to a pattern of sea surface temperatures resembling the Atlantic Multidecadal Oscillation. These findings indicate that large-scale factors responsible for the most societally relevant temperature variations over continents are distinct from those of global mean surface temperature.

2017
Xu, LX, Xie SP, Liu QY, Liu C, Li PL, Lin XP.  2017.  Evolution of the North Pacific subtropical mode water in anticyclonic eddies. Journal of Geophysical Research-Oceans. 122:10118-10130.   10.1002/2017jc013450   AbstractWebsite

Anticyclonic eddies (AEs) trap and transport the North Pacific subtropical mode water (STMW), but the evolution of the STMW trapped in AEs has not been fully studied due to the lack of eddy-tracking subsurface observations. Here we analyze profiles from special-designed Argo floats that follow two STMW-trapping AEs for more than a year. The enhanced daily sampling by these Argo floats swirling around the eddies enables an unprecedented investigation into the structure and evolution of the trapped STMW. In the AEs, the upper (lower) thermocline domes up ( concaves downward), and this lens-shaped double thermocline encompasses the thick STMW within the eddy core. The lighter STMW (25.0 similar to 25.2 sigma(theta)) trapped in AEs dissipates quickly after the formation in winter because of the deepening seasonal thermocline, but the denser STMW (25.2 similar to 25.4 sigma(theta)) remains largely unchanged except when the AE passes across the Izu Ridge. The enhanced diapycnal mixing over the ridge weakens the denser STMW appreciably. While many AEs decay upon hitting the ridge, some pass through a bathymetric gap between the Hachijojima and Bonin Islands, forming a cross- ridge pathway for STMW transport. By contrast, the North Pacific Intermediate Water (NPIW) underneath is deeper than the eddy trapping depth (600 m), and hence left behind east of the Izu Ridge. In Argo climatology, the shallow STMW (< 400 m) intrudes through the gap westward because of the eddy transport, while the NPIW (800 m) is blocked by the Izu Ridge.

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.

Merrifield, A, Lehner F, Xie SP, Deser C.  2017.  Removing Circulation Effects to Assess Central US Land-Atmosphere Interactions in the CESM Large Ensemble. Geophysical Research Letters. 44:9938-9946.   10.1002/2017gl074831   AbstractWebsite

Interannual variability of summer surface air temperature (SAT) in the central United States (U.S.) is influenced by atmospheric circulation and land surface feedbacks. Here a method of dynamical adjustment is used to remove the effects of circulation on summer SAT variability over North America in the Community Earth System Model Large Ensemble. The residual SAT variability is shown to reflect thermodynamic feedbacks associated with land surface conditions. In particular, the central U.S. is a hot spot of land-atmosphere interaction, with residual SAT accounting for more than half of the total SAT variability. Within the hot spot, residual SAT anomalies show higher month-to-month persistence through the warm season and a redder spectrum than dynamically induced SAT anomalies. Residual SAT variability in this region is also shown to be related to preseason soil moisture conditions, surface flux variability, and local atmospheric pressure anomalies.

Yang, JC, Lin XP, Xie SP.  2017.  A Transbasin Mode of Interannual Variability of the Central American Gap Winds: Seasonality and Large-Scale Forcing. Journal of Climate. 30:8223-8235.   10.1175/jcli-d-17-0021.1   AbstractWebsite

A transbasin mode (TBM) is identified as the leading mode of interannual surface wind variability over the Intra-Americas Seas across Central America based on empirical orthogonal function analysis. The TBM is associated with variability in Central American gap winds, most closely with the Papagayo jet but with considerable signals over the Gulfs of Tehuantepec and Panama. Although El Nino-Southern Oscillation (ENSO) is the main large-scale forcing, the TBM features a distinct seasonality due to sea level pressure (SLP) adjustments across the Pacific and Atlantic. During July-September, ENSO causes meridional SLP gradient anomalies across Central America, intensifying anomalous geostrophic winds funneling through Papagayo to form the TBM. During wintertime, ENSO peaks but imparts little anomalous SLP gradient across Central America with a weak projection on the TBM because of the competing effects of the Pacific-North American teleconnection and tropospheric Kelvin waves. Besides ENSO, tropical Atlantic sea surface temperature anomalies make a weak contribution to the TBM in boreal summer by strengthening the cross-basin gradient. ENSO and the Atlantic forcing constitute a cross-basin seesaw pattern in SLP, manifested as an anomalous Walker circulation across the tropical Americas. The TBM appears to be part of the low-level branch of the anomalous Walker circulation, which modulates Central American wind jets by orographic effect. This study highlights the seasonality of gap wind variability, and calls for further research into its influence on regional climate.

Li, G, Xie SP, He C, Chen ZS.  2017.  Western Pacific emergent constraint lowers projected increase in Indian summer monsoon rainfall. Nature Climate Change. 7:708-+.   10.1038/nclimate3387   AbstractWebsite

The agrarian-based socioeconomic livelihood of densely populated South Asian countries is vulnerable to modest changes in Indian summer monsoon (ISM) rainfall(1-3). How the ISM rainfall will evolve is a question of broad scientific and socioeconomic importance(3-9). In response to increased greenhouse gas (GHG) forcing, climate models commonly project an increase in ISM rainfall(4-9). This wetter ISM projection, however, does not consider large model errors in both the mean state and ocean warming pattern(9-11). Here we identify a relationship between biases in simulated present climate and future ISM projections in a multi-model ensemble: models with excessive present-day precipitation over the tropical western Pacific tend to project a larger increase in ISM rainfall under GHG forcing because of too strong a negative cloud-radiation feedback on sea surface temperature. The excessive negative feedback suppresses the local ocean surface warming, strengthening ISM rainfall projections via atmospheric circulation. We calibrate the ISM rainfall projections using this 'present-future relationship' and observed western Pacific precipitation. The correction reduces by about 50% of the projected rainfall increase over the broad ISM region. Our study identifies an improved simulation of western Pacific convection as a priority for reliable ISM projections.

Liu, C, Xie SP, Li PL, Xu LX, Gao WD.  2017.  Climatology and decadal variations in multicore structure of the North Pacific subtropical mode water. Journal of Geophysical Research-Oceans. 122:7506-7520.   10.1002/2017jc013071   AbstractWebsite

The pycnostad of the North Pacific subtropical mode water (STMW) often displays multiple vertical minima in the potential vorticity profile. Argo profile data from 2004 to 2015 are used to investigate interannual to decadal variations of the multicore structure. Climatologically, about 24% pycostads of STMW have multicore structure, and most of them distribute in the region west of 150 degrees E. STMW cores are classified into three submodes-the cold, middle, and warm ones with potential temperatures of 16.0-17 degrees C, 17-18 degrees C, and 18-19.5 degrees C, respectively. The Kuroshio Extension (KE) varies between stable and unstable states. The unstable KE with large meanders increases the subsurface stratification and shoals the winter mixed layer east of 150 degrees E with warmer temperatures. There, the dominant STMW type varies from the cold single type in stable KE years (making up 72% of the profiles with STMW) to the middle single one (53%) in unstable years. The variation of the dominant STMW type in the region east of 150 degrees E subsequently affects the multicore structure of STMW west of 150 degrees E. In a broad region between 130 degrees E and 180 degrees E, profiles with STMW are fewer in unstable years but the proportion of multicore profiles increases among STMW profiles. This might be due to the split recirculation gyre with a chaotic KE.

Yang, Y, Xie SP, Wu LX, Kosaka Y, Li JP.  2017.  Causes of enhanced sst variability over the equatorial atlantic and its relationship to the Atlantic Zonal Mode in CMIP5. Journal of Climate. 30:6171-6182.   10.1175/jcli-d-16-0866.1   AbstractWebsite

A spurious band of enhanced sea surface temperature (SST) variance (SBEV) is identified over the northern equatorial Atlantic in the Geophysical Fluid Dynamics Laboratory (GFDL) Climate Model, version 2.1. The SBEV is especially pronounced in boreal spring owing to the combined effect of both anomalous atmospheric thermal forcing and oceanic vertical upwelling. The SBEV is a common bias in phase 5 of the Coupled Model Intercomparison Project (CMIP5), found in 14 out of 23 models. The SBEV in CMIP5 is associated with the atmospheric thermal forcing and the oceanic vertical upwelling, similar to GFDL CM2.1. While the tropical North Atlantic variability is only weakly correlated with the Atlantic zonal mode (AZM) in observations, the SBEV in CMIP5 produces conditions that drive and intensify the AZM variability via triggering the Bjerknes feedback. This partially explains why AZM is strong in some CMIP5 models even though the equatorial cold tongue and easterly trades are biased low.

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.

2016
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.

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

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.