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Zhou, WY, Xie SP.  2018.  A hierarchy of idealized monsoons in an intermediate GCM. Journal of Climate. 31:9021-9036.   10.1175/jcli-d-18-0084.1   AbstractWebsite

A hierarchy of idealized monsoons with increased degrees of complexity is built using an intermediate model with simplified physics and idealized land-sea geometry. This monsoon hierarchy helps formulate a basic understanding about the distribution of the surface equivalent potential temperature (e), which proves to provide a general guide on the monsoon rainfall. The zonally uniform monsoon in the simplest aquaplanet simulations is explained by a linearized model of the meridional distribution of (e), which is driven by the seasonally varying solar insolation and damped by both the monsoon overturning circulation and the local negative feedback. The heat capacities of the surface and the atmosphere give rise to an intrinsic time scale that causes the monsoon migration to lag behind the sun and reduces the monsoon extent and intensity. Monsoons with a zonally confined continent can be understood based on the zonally uniform monsoon by considering the ocean influence on the land through the westerly jet advection, which reduces the monsoon extent and induces zonal asymmetry. Monsoon responses to more realistic factors such as land geometry, albedo, and ocean heat flux are consistently predicted by their impacts on the surface (e) distribution. The soil moisture effect, however, does not fully fit into the surface (e) argument and provides additional control on monsoon rainfall by inducing regional circulation and rainfall patterns.

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.

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.

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.

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.

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.

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

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

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.

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.

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.

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.

Li, XC, Xie SP, Gille ST, Yoo C.  2016.  Atlantic-induced pan-tropical climate change over the past three decades. Nature Climate Change. 6:275-+.   10.1038/nclimate2840   AbstractWebsite

During the past three decades, tropical sea surface temperature (SST) has shown dipole-like trends, with warming over the tropical Atlantic and Indo-western Pacific but cooling over the eastern Pacific. Competing hypotheses relate this cooling, identified as a driver of the global warming hiatus(1,2), to the warming trends in either the Atlantic(3,4) or Indian Ocean(5). However, the mechanisms, the relative importance and the interactions between these teleconnections remain unclear. Using a state-of-the-art climate model, we show that the Atlantic plays a key role in initiating the tropical-wide teleconnection, and the Atlantic-induced anomalies contribute similar to 55-75% of the tropical SST and circulation changes during the satellite era. The Atlantic warming drives easterly wind anomalies over the Indo-western Pacific as Kelvin waves and westerly anomalies over the eastern Pacific as Rossby waves. The wind changes induce an Indo-western Pacific warming through the wind-evaporation-SST effect(6,7), and this warming intensifies the La Nina-type response in the tropical Pacific by enhancing the easterly trade winds and through the Bjerknes ocean dynamical processes(8). The teleconnection develops into a tropical-wide SST dipole pattern. This mechanism, supported by observations and a hierarchy of climate models, reveals that the tropical ocean basins are more tightly connected than previously thought.

Gao, WD, Li PL, Xie SP, Xu LX, Liu C.  2016.  Multicore structure of the North Pacific subtropical mode water from enhanced Argo observations. Geophysical Research Letters. 43:1249-1255.   10.1002/2015gl067495   AbstractWebsite

Seventeen Argo profiling floats with enhanced vertical and temporal sampling were deployed in the Kuroshio recirculation gyre in the western North Pacific in late March 2014. The Subtropical Mode Water (STMW) observed in many profiles displays a multicore structure with more than one minima in potential vorticity (PV), corroborated by vertical covariations in apparent oxygen utilization (AOU). These cores are classified into four submodes according to density and AOU. The submode waters are typically 100m thick, in which PV varies by 1x10(-10)m(-1)s(-1) and AOU by 10 mu mole/kg. The STMW multicore structure is most frequently observed in spring, gradually taken over by single-core profiles into summer. The seasonal evolution is suggestive of vertical mixing, especially in STMW of lower density.

Kwon, EY, Deutsch C, Xie SP, Schmidtko S, Cho YK.  2016.  The North Pacific Oxygen uptake rates over the past half century. Journal of Climate. 29:61-76.   10.1175/jcli-d-14-00157.1   AbstractWebsite

The transport of dissolved oxygen (O-2) from the surface ocean into the interior is a critical process sustaining aerobic life in mesopelagic ecosystems, but its rates and sensitivity to climate variations are poorly understood. Using a circulation model constrained to historical variability by assimilation of observations, the study shows that the North Pacific thermocline effectively takes up O-2 primarily by expanding the area through which O-2-rich mixed layer water is detrained into the thermocline. The outcrop area during the critical winter season varies in concert with the Pacific decadal oscillation (PDO). When the central North Pacific Ocean is in a cold phase, the winter outcrop window for the central mode water class (CMW; a neutral density range of = 25.6-26.6) expands southward, allowing more O-2-rich surface water to enter the ocean's interior. An increase in volume flux of water to the CMW density class is partly compensated by a reduced supply to the shallower densities of subtropical mode water ( = 24.0-25.5). The thermocline has become better oxygenated since the 1980s partly because of strong O-2 uptake. Positive O-2 anomalies appear first near the outcrop and subsequently downstream in the subtropical gyre. In contrast to the O-2 variations within the ventilated thermocline, observed O-2 in intermediate water (density range of = 26.7-27.2) shows a declining trend over the past half century, a trend not explained by the open ocean water mass formation rate.

Zhou, ZQ, Xie SP.  2015.  Effects of climatological model biases on the projection of tropical climate change. Journal of Climate. 28:9909-9917.   10.1175/jcli-d-15-0243.1   AbstractWebsite

Climate models suffer from long-standing biases, including the double intertropical convergence zone (ITCZ) problem and the excessive westward extension of the equatorial Pacific cold tongue. An atmospheric general circulation model is used to investigate how model biases in the mean state affect the projection of tropical climate change. The model is forced with a pattern of sea surface temperature (SST) increase derived from a coupled simulation of global warming but uses an SST climatology derived from either observations or a coupled historical simulation. The comparison of the experiments reveals that the climatological biases have important impacts on projected changes in the tropics. Specifically, during February-April when the climatological ITCZ displaces spuriously into the Southern Hemisphere, the model overestimates (underestimates) the projected rainfall increase in the warmer climate south (north) of the equator over the eastern Pacific. Furthermore, the global warming-induced Walker circulation slowdown is biased weak in the projection using coupled model climatology, suggesting that the projection of the reduced equatorial Pacific trade winds may also be underestimated. This is related to the bias that the climatological Walker circulation is too weak in the model, which is in turn due to a too-weak mean SST gradient in the zonal direction. The results highlight the importance of improving the climatological simulation for more reliable projections of regional climate change.

Wang, GH, Xie SP, Huang RX, Chen CL.  2015.  Robust warming pattern of global subtropical oceans and its mechanism. Journal of Climate. 28:8574-8584.   10.1175/jcli-d-14-00809.1   AbstractWebsite

The subsurface ocean response to anthropogenic climate forcing remains poorly characterized. From the Coupled Model Intercomparison Project (CMIP), a robust response of the lower thermocline is identified, where the warming is considerably weaker in the subtropics than in the tropics and high latitudes. The lower thermocline change is inversely proportional to the thermocline depth in the present climatology. Ocean general circulation model (OGCM) experiments show that sea surface warming is the dominant forcing for the subtropical gyre change in contrast to natural variability for which wind dominates, and the ocean response is insensitive to the spatial pattern of surface warming. An analysis based on a ventilated thermocline model shows that the pattern of the lower thermocline change can be interpreted in terms of the dynamic response to the strengthened stratification and downward heat mixing. Consequently, the subtropical gyres become intensified at the surface but weakened in the lower thermcline, consistent with results from CMIP experiments.

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.

Xie, SP, Deser C, Vecchi GA, Collins M, Delworth TL, Hall A, Hawkins E, Johnson NC, Cassou C, Giannini A, Watanabe M.  2015.  Towards predictive understanding of regional climate change. Nature Climate Change. 5:921-930.   10.1038/nclimate2689   AbstractWebsite

Regional information on climate change is urgently needed but often deemed unreliable. To achieve credible regional climate projections, it is essential to understand underlying physical processes, reduce model biases and evaluate their impact on projections, and adequately account for internal variability. In the tropics, where atmospheric internal variability is small compared with the forced change, advancing our understanding of the coupling between long-term changes in upper-ocean temperature and the atmospheric circulation will help most to narrow the uncertainty. In the extratropics, relatively large internal variability introduces substantial uncertainty, while exacerbating risks associated with extreme events. Large ensemble simulations are essential to estimate the probabilistic distribution of climate change on regional scales. Regional models inherit atmospheric circulation uncertainty from global models and do not automatically solve the problem of regional climate change. We conclude that the current priority is to understand and reduce uncertainties on scales greater than 100 km to aid assessments at finer scales.

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

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

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

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

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.

Mei, W, Xie SP, Zhao M, Wang YQ.  2015.  Forced and internal vriability of tropical cyclone track density in the Western North Pacific. Journal of Climate. 28:143-167.   10.1175/jcli-d-14-00164.1   AbstractWebsite

Forced interannual-to-decadal variability of annual tropical cyclone (TC) track density in the western North Pacific between 1979 and 2008 is studied using TC tracks from observations and simulations by a 25-km-resolution version of the GFDL High-Resolution Atmospheric Model (HiRAM) that is forced by observed sea surface temperatures (SSTs). Two modes dominate the decadal variability: a nearly basinwide mode, and a dipole mode between the subtropics and lower latitudes. The former mode links to variations in TC number and is forced by SST variations over the off-equatorial tropical central North Pacific, whereas the latter might be associated with the Atlantic multidecadal oscillation. The interannual variability is also controlled by two modes: a basinwide mode driven by SST anomalies of opposite signs located in the tropical central Pacific and eastern Indian Ocean, and a southeast-northwest dipole mode connected to the conventional eastern Pacific ENSO. The seasonal evolution of the ENSO effect on TC activity is further explored via a joint empirical orthogonal function analysis using TC track density of consecutive seasons, and the analysis reveals that two types of ENSO are at work. Internal variability in TC track density is then examined using ensemble simulations from both HiRAM and a regional atmospheric model. It exhibits prominent spatial and seasonal patterns, and it is particularly strong in the South China Sea and along the coast of East Asia. This makes an accurate prediction and projection of TC landfall extremely challenging in these regions. In contrast, basin-integrated metrics (e.g., total TC counts and TC days) are more predictable.

Liu, L, Xie SP, Zheng XT, Li T, Du Y, Huang G, Yu WD.  2014.  Indian Ocean variability in the CMIP5 multi-model ensemble: the zonal dipole mode. Climate Dynamics. 43:1715-1730.   10.1007/s00382-013-2000-9   AbstractWebsite

The performance of 21 Coupled Model Intercomparison Project Phase 5 (CMIP5) models in the simulation of the Indian Ocean Dipole (IOD) mode is evaluated. Compared to CMIP3, CMIP5 models exhibit a similar spread in IOD intensity. A detailed diagnosis was carried out to understand whether CMIP5 models have shown improvement in their representation of the important dynamical and thermodynamical feedbacks in the tropical Indian Ocean. These include the Bjerknes dynamic air-sea feedback, which includes the equatorial zonal wind response to sea surface temperature (SST) anomaly, the thermocline response to equatorial zonal wind forcing, the ocean subsurface temperature response to the thermocline variations, and the thermodynamic air-sea coupling that includes the wind-evaporation-SST and cloud-radiation-SST feedback. Compared to CMIP3, the CMIP5 ensemble produces a more realistic positive wind-evaporation-SST feedback during the IOD developing phase, while the simulation of Bjerknes dynamic feedback is more unrealistic especially with regard to the wind response to SST forcing and the thermocline response to surface wind forcing. The overall CMIP5 performance in the IOD simulation does not show remarkable improvements compared to CMIP3. It is further noted that the El Nio-Southern Oscillation (ENSO) and IOD amplitudes are closely related, if a model generates a strong ENSO, it is likely that this model also simulates a strong IOD.

Xu, LX, Xie SP, McClean JL, Liu QY, Sasaki H.  2014.  Mesoscale eddy effects on the subduction of North Pacific mode waters. Journal of Geophysical Research-Oceans. 119:4867-4886.   10.1002/2014jc009861   AbstractWebsite

Mesoscale eddy effects on the subduction of North Pacific mode waters are investigated by comparing observations and ocean general circulation models where eddies are either parameterized or resolved. The eddy-resolving models produce results closer to observations than the noneddy-resolving model. There are large discrepancies in subduction patterns between eddy-resolving and noneddy-resolving models. In the noneddy-resolving model, subduction on a given isopycnal is limited to the cross point between the mixed layer depth (MLD) front and the outcrop line whereas in eddy-resolving models and observations, subduction takes place in a broader, zonally elongated band within the deep mixed layer region. Mesoscale eddies significantly enhance the total subduction rate, helping create remarkable peaks in the volume histogram that correspond to North Pacific subtropical mode water (STMW) and central mode water (CMW). Eddy-enhanced subduction preferentially occurs south of the winter mean outcrop. With an anticyclonic eddy to the west and a cyclonic eddy to the east, the outcrop line meanders south, and the thermocline/MLD shoals eastward. As eddies propagate westward, the MLD shoals, shielding the water of low potential vorticity from the atmosphere. The southward eddy flow then carries the subducted water mass into the thermocline. The eddy subduction processes revealed here have important implications for designing field observations and improving models.