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Alexander, MA, Seo H, Xie SP, Scott JD.  2012.  ENSO's Impact on the Gap Wind Regions of the Eastern Tropical Pacific Ocean. Journal of Climate. 25:3549-3565.   10.1175/jcli-d-11-00320.1   Abstract

The recently released NCEP Climate Forecast System Reanalysis (CFSR) is used to examine the response to ENSO in the northeast tropical Pacific Ocean (NETP) during 1979-2009. The normally cool Pacific sea surface temperatures (SSTs) associated with wind jets through the gaps in the Central American mountains at Tehuantepec, Papagayo, and Panama are substantially warmer (colder) than the surrounding ocean during El Nino (La Nina) events. Ocean dynamics generate the ENSO-related SST anomalies in the gap wind regions as the surface fluxes damp the SSTs anomalies, while the Ekman heat transport is generally in quadrature with the anomalies. The ENSO-driven warming is associated with large-scale deepening of the thermocline; with the cold thermocline water at greater depths during El Nino in the NETP, it is less likely to be vertically mixed to the surface, particularly in the gap wind regions where the thermocline is normally very close to the surface. The thermocline deepening is enhanced to the south of the Costa Rica Dome in the Papagayo region, which contributes to the local ENSO-driven SST anomalies. The NETP thermocline changes are due to coastal Kelvin waves that initiate westward-propagating Rossby waves, and possibly ocean eddies, rather than by local Ekman pumping. These findings were confirmed with regional ocean model experiments: only integrations that included interannually varying ocean boundary conditions were able to simulate the thermocline deepening and localized warming in the NETP during El Nino events; the simulation with variable surface fluxes, but boundary conditions that repeated the seasonal cycle, did not.

Amaya, DJ, Xie SP, Miller AJ, McPhaden MJ.  2015.  Seasonality of tropical Pacific decadal trends associated with the 21st century global warming hiatus. Journal of Geophysical Research-Oceans. 120:6782-6798.   10.1002/2015jc010906   AbstractWebsite

Equatorial Pacific changes during the transition from a nonhiatus period (pre-1999) to the present global warming hiatus period (post-1999) are identified using a combination of reanalysis and observed data sets. Results show increased surface wind forcing has excited significant changes in wind-driven circulation. Over the last two decades, the core of the Equatorial Undercurrent intensified at a rate of 6.9 cm s(-1) decade(-1). Similarly, equatorial upwelling associated with the shallow meridional overturning circulation increased at a rate of 2.0 x 10(-4) cm s(-1) decade(-1) in the central Pacific. Further, a seasonal dependence is identified in the sea surface temperature trends and in subsurface dynamics. Seasonal variations are evident in reversals of equatorial surface flow trends, changes in subsurface circulation, and seasonal deepening/shoaling of the thermocline. Anomalous westward surface flow drives cold-water zonal advection from November to February, leading to surface cooling from December through May. Conversely, eastward surface current anomalies in June-July drive warm-water zonal advection producing surface warming from July to November. An improved dynamical understanding of how the tropical Pacific Ocean responds during transitions into hiatus events, including its seasonal structure, may help to improve future predictability of decadal climate variations.

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.

Annamalai, H, Murtugudde R, Potemra J, Xie SP, Liu P, Wang B.  2003.  Coupled dynamics over the Indian Ocean: spring initiation of the Zonal Mode. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 50:2305-2330.   10.1016/s0967-0645(03)00058-4   Abstract
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Annamalai, H, Xie SP, McCreary JP, Murtugudde R.  2005.  Impact of Indian Ocean sea surface temperature on developing El Nino. Journal of Climate. 18:302-319. Abstract
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Annamalai, H, Liu P, Xie SP.  2005.  Southwest Indian Ocean SST variability: Its local effect and remote influence on Asian monsoons. Journal of Climate. 18:4150-4167. Abstract
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Biasutti, M, Voigt A, Boos WR, Braconnot P, Hargreaves JC, Harrison SP, Kang SM, Mapes BE, Scheff J, Schumacher C, Sobel AH, Xie SP.  2018.  Global energetics and local physics as drivers of past, present and future monsoons. Nature Geoscience. 11:392-+.   10.1038/s41561-018-0137-1   AbstractWebsite

Global constraints on momentum and energy govern the variability of the rainfall belt in the intertropical convergence zone and the structure of the zonal mean tropical circulation. The continental-scale monsoon systems are also facets of a momentumand energy-constrained global circulation, but their modern and palaeo variability deviates substantially from that of the intertropical convergence zone. The mechanisms underlying deviations from expectations based on the longitudinal mean budgets are neither fully understood nor simulated accurately. We argue that a framework grounded in global constraints on energy and momentum yet encompassing the complexities of monsoon dynamics is needed to identify the causes of the mismatch between theory, models and observations, and ultimately to improve regional climate projections. In a first step towards this goal, disparate regional processes must be distilled into gross measures of energy flow in and out of continents and between the surface and the tropopause, so that monsoon dynamics may be coherently diagnosed across modern and palaeo observations and across idealized and comprehensive simulations. Accounting for zonal asymmetries in the circulation, land/ocean differences in surface fluxes, and the character of convective systems, such a monsoon framework would integrate our understanding at all relevant scales: from the fine details of how moisture and energy are lifted in the updrafts of thunderclouds, up to the global circulations.

Brown, PT, Li W, Xie S-P.  2015.  Regions of significant influence on unforced global mean surface air temperature variability in climate models. Journal of Geophysical Research: Atmospheres.   10.1002/2014JD022576   Abstract

We document the geographic regions where local variability is most associated with unforced global mean surface air temperature (GMT) variability in Coupled Model Intercomparison Project Phase 5 coupled global climate models (GCMs) at both the subdecadal and interdecadal timescales. For this purpose, Regions of Significant Influence on GMT are defined as locations that have a statistically significant correlation between local surface air temperature (SAT) and GMT (with a regression slope greater than 1), and where local SAT variation leads GMT variation in time. In both GCMs and observations, subdecadal timescale GMT variability is most associated with SAT variation over the eastern equatorial Pacific. At the interdecadal timescale, GMT variability is also linked with SAT variation over the Pacific in many GCMs, but the particular spatial patterns are GCM dependent, and several GCMs indicate a primary association between GMT and SAT over the Southern Ocean. We find that it is difficult to validate GCM behavior at the interdecadal timescale because the pattern derived from observations is highly depended on the method used to remove the forced variability from the record. The magnitude of observed GMT variability is near the ensemble median at the subdecadal timescale but well above the median at the interdecadal timescale. GCMs with a stronger subdecadal relationship between GMT and SAT over the Pacific tend to have more variable subdecadal GMT while GCMs with a stronger interdecadal relationship between GMT and SAT over parts of the Southern Ocean tend to have more variable GMT.

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Chang, CH, Xie SP, Schneider N, Qiu B, Small J, Zhuang W, Taguchi B, Sasaki H, Lin XP.  2012.  East Pacific ocean eddies and their relationship to subseasonal variability in Central American wind jets. Journal of Geophysical Research-Oceans. 117   10.1029/2011jc007315   Abstract

Subseasonal variability in sea surface height (SSH) over the East Pacific warm pool off Central America is investigated using satellite observations and an eddy-resolving ocean general circulation model. SSH variability is organized into two southwest-tilted bands on the northwest flank of the Tehuantepec and Papagayo wind jets and collocated with the thermocline troughs. Eddy-like features of wavelength similar to 600 km propagate southwestward along the high-variance bands at a speed of 9-13 cm/s. Wind fluctuations are important for eddy formation in the Gulf of Tehuantepec, with a recurring interval of 40-90 days. When forced by satellite wind observations, the model reproduces the two high-variance bands and the phase propagation of the Tehuantepec eddies. Our observational analysis and model simulation suggest the following evolution of the Tehuantepec eddies. On the subseasonal timescale, in response to the gap wind intensification, a coastal anticyclonic eddy forms on the northwest flank of the wind jet and strengthens as it propagates offshore in the following two to three weeks. An energetics analysis based on the model simulation indicates that besides wind work, barotropic and baroclinic instabilities of the mean flow are important for the eddy growth. Both observational and model results suggest a re-intensification of the anticyclonic eddy in response to the subsequent wind jet event. Off Papagayo, ocean eddy formation is not well correlated with local wind jet variability. In both the Gulfs of Tehuantepec and Papagayo, subseasonal SSH variability is preferentially excited on the northwest flank of the wind jet. Factors for this asymmetry about the wind jet axis as well as the origins of wind jet variability are discussed.

Chelton, DB, Xie S-P.  2010.  COUPLED OCEAN-ATMOSPHERE INTERACTION AT OCEANIC MESOSCALES. Oceanography. 23:52-69. Abstract
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Cheng, X, Xie S-P, Tokinaga H, Du Y.  2011.  Interannual Variability of High-Wind Occurrence over the North Atlantic. Journal of Climate. 24:6515-6527.   10.1175/2011jcli4147.1   Abstract
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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.

Chikamoto, Y, Timmermann A, Luo JJ, Mochizuki T, Kimoto M, Watanabe M, Ishii M, Xie SP, Jin FF.  2015.  Skilful multi-year predictions of tropical trans-basin climate variability. Nature Communications. 6   10.1038/ncomms7869   AbstractWebsite

Tropical Pacific sea surface temperature anomalies influence the atmospheric circulation, impacting climate far beyond the tropics. The predictability of the corresponding atmospheric signals is typically limited to less than 1 year lead time. Here we present observational and modelling evidence for multi-year predictability of coherent trans-basin climate variations that are characterized by a zonal seesaw in tropical sea surface temperature and sea-level pressure between the Pacific and the other two ocean basins. State-of-the-art climate model forecasts initialized from a realistic ocean state show that the low-frequency trans-basin climate variability, which explains part of the El Nino Southern Oscillation flavours, can be predicted up to 3 years ahead, thus exceeding the predictive skill of current tropical climate forecasts for natural variability. This low-frequency variability emerges from the synchronization of ocean anomalies in all basins via global reorganizations of the atmospheric Walker Circulation.

Chow, CH, Liu QY, Xie SP.  2015.  Effects of Kuroshio Intrusions on the atmosphere northeast of Taiwan Island. Geophysical Research Letters. 42:1465-1470.   10.1002/2014gl062796   AbstractWebsite

The Kuroshio loses bathymetric support off northeast Taiwan Island, causing large variability in its path. The resultant covariability of sea surface temperature (SST) and the lower atmosphere is investigated using satellite observations. In winter and spring off northeast Taiwan Island, the intrusions of warm Kuroshio water onto the continental shelf cause a large increase in local SST, intensify the northeasterly monsoonal winds, and lead to the increases in water vapor and rainfall. Key to this air-sea interaction is the existence of anomalous heat advection by the Kuroshio intrusions. The Kuroshio intrusions are partly due to westward propagating ocean eddies east of Taiwan Island with a lead time of 3weeks, hinting at the possibility of improved weather prediction near northeast Taiwan Island by considering ocean variability east of Taiwan Island.

Chowdary, JS, Xie S-P, Luo J-J, Hafner J, Behera S, Masumoto Y, Yamagata T.  2011.  Predictability of Northwest Pacific climate during summer and the role of the tropical Indian Ocean. Climate Dynamics. 36:607-621.   10.1007/s00382-009-0686-5   Abstract
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Chowdary, JS, Xie S-P, Lee J-Y, Kosaka Y, Wang B.  2010.  Predictability of summer northwest Pacific climate in 11 coupled model hindcasts: Local and remote forcing. Journal of Geophysical Research-Atmospheres. 115   10.1029/2010jd014595   Abstract
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Chowdary, JS, Xie S-P, Tokinaga H, Okumura YM, Kubota H, Johnson N, Zheng X-T.  2012.  Interdecadal Variations in ENSO Teleconnection to the Indo-Western Pacific for 1870-2007. Journal of Climate. 25:1722-1744.   10.1175/jcli-d-11-00070.1   Abstract
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Collins, M, Minobe S, Barreiro M, Bordoni S, Kaspi Y, Kuwano-Yoshida A, Keenlyside N, Manzini E, O'Reilly CH, Sutton R, Xie SP, Zolina O.  2018.  Challenges and opportunities for improved understanding of regional climate dynamics. Nature Climate Change. 8:101-108.   10.1038/s41558-017-0059-8   AbstractWebsite

Dynamical processes in the atmosphere and ocean are central to determining the large-scale drivers of regional climate change, yet their predictive understanding is poor. Here, we identify three frontline challenges in climate dynamics where significant progress can be made to inform adaptation: response of storms, blocks and jet streams to external forcing; basin-to-basin and tropical-extratropical teleconnections; and the development of non-linear predictive theory. We highlight opportunities and techniques for making immediate progress in these areas, which critically involve the development of high-resolution coupled model simulations, partial coupling or pacemaker experiments, as well as the development and use of dynamical metrics and exploitation of hierarchies of models.

Cronin, MF, Xie SP, Hashizume H.  2003.  Barometric pressure variations associated with Eastern Pacific tropical instability waves. Journal of Climate. 16:3050-3057. Abstract
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Dai, A, Fyfe JC, Xie S-P, Dai X.  2015.  Decadal modulation of global surface temperature by internal climate variability. Nature Clim. Change. advance online publication: Nature Publishing Group   10.1038/nclimate2605   Abstract

Despite a steady increase in atmospheric greenhouse gases (GHGs), global-mean surface temperature (T) has shown no discernible warming since about 2000, in sharp contrast to model simulations, which on average project strong warming1, 2, 3. The recent slowdown in observed surface warming has been attributed to decadal cooling in the tropical Pacific1, 4, 5, intensifying trade winds5, changes in El Niño activity6, 7, increasing volcanic activity8, 9, 10 and decreasing solar irradiance7. Earlier periods of arrested warming have been observed but received much less attention than the recent period, and their causes are poorly understood. Here we analyse observed and model-simulated global T fields to quantify the contributions of internal climate variability (ICV) to decadal changes in global-mean T since 1920. We show that the Interdecadal Pacific Oscillation (IPO) has been associated with large T anomalies over both ocean and land. Combined with another leading mode of ICV, the IPO explains most of the difference between observed and model-simulated rates of decadal change in global-mean T since 1920, and particularly over the so-called ‘hiatus’ period since about 2000. We conclude that ICV, mainly through the IPO, was largely responsible for the recent slowdown, as well as for earlier slowdowns and accelerations in global-mean T since 1920, with preferred spatial patterns different from those associated with GHG-induced warming or aerosol-induced cooling. Recent history suggests that the IPO could reverse course and lead to accelerated global warming in the coming decades.

Deser, C, Alexander MA, Xie S-P, Phillips AS.  2010.  Sea Surface Temperature Variability: Patterns and Mechanisms. Annual Review of Marine Science. 2:115-143.   10.1146/annurev-marine-120408-151453   Abstract
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Diao, Y, Xie SP, Luo DH.  2015.  Asymmetry of winter European surface air temperature extremes and the North Atlantic Oscillation. Journal of Climate. 28:517-530.   10.1175/jcli-d-13-00642.1   AbstractWebsite

Interannual variations of winter warm and cold extremes in Europe are investigated. It is found that the variations are closely connected to the phase of the North Atlantic Oscillation (NAO). The leading EOF of the winter cold (warm) surface air temperature (SAT) extreme frequency shows an enhanced occurrence over western (eastern) Europe. The SAT probability distribution function of the cold extreme winter exhibits both a decrease of the mean SAT and a marked increase in SAT variance, whereas it shows only a shift of the mean SAT to the warmer side for extreme warm winters. This study reveals an asymmetry in location between the cold and warm extremes, caused by the NAO modulations of blocking events and other submonthly variations. Winters with frequent cold extremes are mainly accompanied by the eastern Atlantic blocking. The circulation causes not only marked local cooling but also increased SAT gradient, resulting in both enhanced SAT variance and increased occurrence of cold extremes. By contrast, winters with frequent warm extremes are associated with the northeast-southwest tilted positive NAO pattern. The warm advection by the submonthly perturbations is responsible for the development of warm extremes. The reduced SAT gradient due to enhanced warm advection weakens SAT variance over northern Europe. Thus, the cold extremes are larger in terms of deviations from the monthly mean than the warm extremes.

Du, Y, Xie SP, Huang G, Hu KM.  2009.  Role of Air-Sea Interaction in the Long Persistence of El Nino-Induced North Indian Ocean Warming. Journal of Climate. 22:2023-2038.   10.1175/2008jcli2590.1   Abstract
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