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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.

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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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.

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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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Inatsu, M, Mukougawa H, Xie SP.  2002.  Stationary eddy response to surface boundary forcing: Idealized GCM experiments. Journal of the Atmospheric Sciences. 59:1898-1915. Abstract
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Kilpatrick, T, Xie S-P, Miller AJ, Schneider N.  2018.  Satellite observations of enhanced chlorophyll variability in the Southern California Bight. Journal of Geophysical Research: Oceans. 123:7550-7563.   10.1029/2018JC014248   Abstract

Satellite observations from the Moderate Resolution Imaging Spectroradiometer and Sea-viewing Wide Field-of-view Sensor reveal a “tongue” of elevated near-surface chlorophyll that extends into the Southern California Bight from Point Conception. A local chlorophyll maximum at the western edge of the bight, near the Santa Rosa Ridge, indicates that the chlorophyll is not solely due to advection from Point Conception but is enhanced by local upwelling. Chlorophyll in the bight peaks in May and June, in phase with the seasonal cycle of wind stress curl. The spatial structure and seasonal variability suggest that the local chlorophyll maximum is due to a combination of bathymetric influence from the Santa Rosa Ridge and orographic influence from the coastline bend at Point Conception, which causes sharp wind stress curl in the bight. High-resolution glider observations show thermocline doming in May–June, in support of the local upwelling effect. Despite the evidence for local wind stress curl-forced upwelling in the bight, we cannot rule out alternative mechanisms for the local chlorophyll maximum, such as iron supply from the ridge. Covariability between chlorophyll, surface wind stress, and sea surface temperature (SST) indicates that nonseasonal chlorophyll variability in the bight is closely related to SST, but the spatial patterns of SST influence vary by time scale: Subannual chlorophyll variability is linked to local wind-forced upwelling, while interannual chlorophyll variability is linked to large-scale SST variations over the northeast Pacific. This suggests a greater role for nonlocal processes in the bight's low-frequency chlorophyll variability.

Kubokawa, A, Xie SP.  2002.  Steady response of a ventilated thermocline to enhanced Ekman pumping. Journal of Oceanography. 58:565-575. Abstract
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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.

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Maloney, ED, Xie SP.  2013.  Sensitivity of tropical intraseasonal variability to the pattern of climate warming. Journal of Advances in Modeling Earth Systems. 5:32-47.   10.1029/2012ms000171   AbstractWebsite

An aquaplanet general circulation model is used to assess the sensitivity of intraseasonal variability to the pattern of sea surface temperature (SST) warming. Three warming patterns are used. Projected SST warming at the end of the 21st century from the Geophysical Fluid Dynamics Laboratory Climate Model 2.1 is one pattern, and zonally symmetric and globally uniform versions of this warming perturbation that have the same global mean SST change are the other two. Changes in intraseasonal variability are sensitive to the pattern of SST warming, with significant decreases in Madden-Julian oscillation (MJO)-timescale precipitation and wind variability for a zonally symmetric warming, and significant increases in MJO precipitation amplitude for a globally uniform warming. The amplitude of the wind variability change does not scale directly with precipitation, but is instead mediated by increased tropical dry static stability associated with SST warming. The patterned SST simulations have a zonal mean SST warming that maximizes on the equator, which fosters increased equatorial boundary layer convergence and also increases equatorial SST relative to the rest of the tropics. Both factors support increased convection, reflected in reduced gross moist stability (GMS). Mean precipitation is decreased and GMS is increased in the off-equatorial Eastern Hemisphere near 10 degrees S in the patterned warming simulations, where the strongest MJO-related intraseasonal precipitation variability is preferred in both the model and observations. It is argued that future changes in MJO activity may be sensitive to the pattern of SST warming, although these results should not be interpreted as a prediction of how MJO activity will change in future climate.

Matsumura, S, Huang G, Xie S-P, Yamazaki K.  2010.  SST-Forced and Internal Variability of the Atmosphere in an Ensemble GCM Simulation. Journal of the Meteorological Society of Japan. 88:43-62.   10.2151/jmsj.2010-104   Abstract
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Merrifield, AL, Xie SP.  2016.  Summer US surface air temperature variability: controlling factors and AMIP simulation biases. Journal of Climate. 29:5123-5139.   10.1175/jcli-d-15-0705.1   AbstractWebsite

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

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Ogata, T, Xie S-P.  2011.  Semiannual Cycle in Zonal Wind over the Equatorial Indian Ocean. Journal of Climate. 24:6471-6485.   10.1175/2011jcli4243.1   Abstract
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Okumura, Y, Xie SP.  2006.  Some overlooked features of tropical Atlantic climate leading to a new Nino-like phenomenon. Journal of Climate. 19:5859-5874. Abstract
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Qu, TD, Xie SP, Mitsudera H, Ishida A.  2002.  Subduction of the north pacific mode waters in a global high-resolution GCM. Journal of Physical Oceanography. 32:746-763. Abstract
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Sasaki, H, Xie S-P, Taguchi B, Nonaka M, Masumoto Y.  2010.  Seasonal variations of the Hawaiian Lee Countercurrent induced by the meridional migration of the trade winds. Ocean Dynamics. 60:705-715.   10.1007/s10236-009-0258-6   Abstract
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Tanimoto, Y, Kanenari T, Tokinaga H, Xie S-P.  2011.  Sea Level Pressure Minimum along the Kuroshio and Its Extension. Journal of Climate. 24:4419-4434.   10.1175/2011jcli4062.1   Abstract
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Tokinaga, H, Xie SP, Deser C, Kosaka Y, Okumura YM.  2012.  Slowdown of the Walker circulation driven by tropical Indo-Pacific warming. Nature. 491:439-+.   10.1038/nature11576   Abstract

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

Tokinaga, H, Tanimoto Y, Xie SP.  2005.  SST-induced surface wind variations over the Brazil-Malvinas confluence: Satellite and in situ observations. Journal of Climate. 18:3470-3482. Abstract
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Xie, SP, Xie Q, Wang DX, Liu WT.  2003.  Summer upwelling in the South China Sea and its role in regional climate variations. Journal of Geophysical Research-Oceans. 108   10.1029/2003cj001867   Abstract
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Xie, SP, Lu B, Xiang BQ.  2013.  Similar spatial patterns of climate responses to aerosol and greenhouse gas changes. Nature Geoscience. 6:828-832.   10.1038/ngeo1931   AbstractWebsite

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

Xie, SP, Annamalai H, Schott FA, McCreary JP.  2002.  Structure and mechanisms of South Indian Ocean climate variability. Journal of Climate. 15:864-878. Abstract
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