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2018
Kang, SM, Shin Y, Xie S-P.  2018.  Extratropical forcing and tropical rainfall distribution: energetics framework and ocean Ekman advection. npj Climate and Atmospheric Science. 1:2.   10.1038/s41612-017-0004-6   Abstract

Intense tropical rainfall occurs in a narrow belt near the equator, called the inter-tropical convergence zone (ITCZ). In the past decade, the atmospheric energy budget has been used to explain changes in the zonal-mean ITCZ position. The energetics framework provides a mechanism for extratropics-to-tropics teleconnections, which have been postulated from paleoclimate records. In atmosphere models coupled with a motionless slab ocean, the ITCZ shifts toward the warmed hemisphere in order for the Hadley circulation to transport energy toward the colder hemisphere. However, recent studies using fully coupled models show that tropical rainfall can be rather insensitive to extratropical forcing when ocean dynamics is included. Here, we explore the effect of meridional Ekman heat advection while neglecting the upwelling effect on the ITCZ response to prescribed extratropical thermal forcing. The tropical component of Ekman advection is a negative feedback that partially compensates the prescribed forcing, whereas the extratropical component is a positive feedback that amplifies the prescribed forcing. Overall, the tropical negative feedback dominates over the extratropical positive feedback. Thus, including Ekman advection reduces the need for atmospheric energy transport, dampening the ITCZ response. We propose to build a hierarchy of ocean models to systematically explore the full dynamical response of the coupled climate system.

2017
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.

Kamae, Y, Mei W, Xie SP.  2017.  Climatological relationship between warm season atmospheric rivers and heavy rainfall over East Asia. Journal of the Meteorological Society of Japan. 95:411-431.   10.2151/jmsj.2017-027   AbstractWebsite

Eddy transport of atmospheric,ater vapor from the tropics is important for rainfall and related natural disasters in the middle latitudes. Atmospheric rivers (ARs), intense moisture plumes that are typically associated with extratropical cyclones, often produce heavy precipitation upon encountering topography on the west coasts of mid-latitude North America and Europe. ARs also occur over the northwestern Pacific and sometimes cause floods and landslides over East Asia, but the climatological relationship between ARs and heavy rainfall in this region remains unclear. Here we evaluate the contribution of ARs to the hydrological cycle over East Asia using high-resolution daily rainfall observations and an atmospheric reanalysis during 1958-2007. Despite their low occurrence, ARs account for 14-44 % of the total rainfall and 20-90 % of extreme heavy-rainfall events during spring, summer, and autumn. AR-related extreme rainfall is especially pronounced over western-to-southeastern slopes of terrains over the Korean Peninsula and Japan, owing to strong orographic effects and a stable direction of low-level moisture flows. A strong relationship between warm-season AR heavy rainfall and preceding-winter El Nino is identified since the 1970s, suggesting the potential of predicting heavy-rainfall risk over Korea and Japan at seasonal leads.

Ma, J, Xie SP, Xu HM.  2017.  Contributions of the North Pacific Meridional Mode to Ensemble Spread of ENSO Prediction. Journal of Climate. 30:9167-9181.   10.1175/jcli-d-17-0182.1   AbstractWebsite

Seasonal prediction of El Nino-Southern Oscillation (ENSO) employs the ensemble method, which samples the uncertainty in initial conditions. While much attention has been given to the ensemble mean, the ensemble spread limits the reliability of the forecast. Spatiotemporal coevolution of intermember anomalies of sea surface temperature (SST) and low-level winds over the Pacific is examined in ensemble hindcasts. Two types of evolution of intermember SST anomalies in the equatorial Pacific are identified. The first features an apparent southwestward propagation of the SST spread from the subtropical northeastern Pacific southeast of Hawaii to the central equatorial Pacific in boreal winter-spring, indicative of the precursor effect of the North Pacific meridional mode (NPMM) on ENSO variability. Extratropical atmospheric variability generates ensemble spread in ENSO through wind-evaporation-SST (WES) in the subtropical northeastern Pacific and then Bjerknes feedback on the equator. In the second type, ensemble spread grows in the equatorial Pacific with a weak contribution from the subtropical southeastern Pacific in summer. Thus, the extratropical influence on ENSO evolution is much stronger in the Northern Hemisphere than in the Southern Hemisphere. The growth of Nino-4 SST ensemble spread shows a strong seasonality. In hindcasts initialized in September-March, the Nino-4 SST spread grows rapidly in January-April, stabilizes in May-June, and grows again in July-September. The rapid growth of the Nino-4 SST spread in January-April is due to the arrival of NPMM, while the slowdown in May-June and rapid growth in July-September are attributable primarily to the seasonality of equatorial ocean-atmosphere interaction. NPMM contributes to the ensemble spread in equatorial Pacific SST, limiting the reliability of ENSO prediction.

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.

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.

Kilpatrick, T, Xie SP, Nasuno T.  2017.  Diurnal Convection-Wind Coupling in the Bay of Bengal. Journal of Geophysical Research-Atmospheres. 122:9705-9720.   10.1002/2017jd027271   AbstractWebsite

Satellite observations of infrared brightness temperature and rainfall have shown offshore propagation of diurnal rainfall signals in some coastal areas of the tropics, suggesting that diurnal rainfall is coupled to land-sea breeze circulations. Here we utilize satellite observations of surface winds and rainfall to show the offshore copropagation of land breeze and diurnal rainfall signals for 300-400 km from the east coast of India into the Bay of Bengal. The wind observations are from the 2003 Quick Scatterometer (QuikSCAT)-SeaWinds "tandem mission" and from 17 years of the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI); the rainfall observations are from the TRMM 3B42 product and from TMI. The surface wind convergence maximum leads the rainfall maximum by 1-2 h in the western part of the bay, implying that the land breeze forces the diurnal cycle of rainfall. The phase speed of the offshore propagation is approximately 18 m s(-1), consistent with a deep hydrostatic gravity wave forced by diurnal heating over India. Comparisons with a cloud system-resolving atmospheric model and the ERA-Interim reanalysis indicate that the models realistically simulate the surface land breeze but greatly underestimate the amplitude of the rainfall diurnal cycle. The satellite observations presented in this study therefore provide a benchmark for model representation of this important atmosphere-ocean-land surface interaction. Plain Language Summary Satellite rainfall observations show a strong diurnal cycle in the Bay of Bengal during the summer monsoon. Here for the first time we utilize concurrent satellite observations of surface winds and rainfall to demonstrate the interaction between the land-sea breeze, forced by the diurnal cycle of solar heating over India, and diurnal rainfall over the Bay of Bengal. The observations are consistent with the land breeze acting as a forcing mechanism for the diurnal cycle of rainfall over the bay and, therefore, illuminate an important atmosphere-ocean-land surface interaction that is poorly represented in many climate models.

Sugimoto, S, Hanawa K, Watanabe T, Suga T, Xie SP.  2017.  Enhanced warming of the subtropical mode water in the North Pacific and North Atlantic. Nature Climate Change. 7:656-+.   10.1038/nclimate3371   AbstractWebsite
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Kamae, Y, Mei W, Xie SP, Naoi M, Ueda H.  2017.  Atmospheric Rivers over the Northwestern Pacific: Climatology and Interannual Variability. Journal of Climate. 30:5605-5619.   10.1175/jcli-d-16-0875.1   AbstractWebsite

Atmospheric rivers (ARs), conduits of intense water vapor transport in the midlatitudes, are critically important for water resources and heavy rainfall events over the west coast of North America, Europe, and Africa. ARs are also frequently observed over the northwestern Pacific (NWP) during boreal summer but have not been studied comprehensively. Here the climatology, seasonal variation, interannual variability, and predictability of NWPARs (NWPARs) are examined by using a large ensemble, high-resolution atmospheric general circulation model (AGCM) simulation and a global atmospheric reanalysis. The AGCM captures general characteristics of climatology and variability compared to the reanalysis, suggesting a strong sea surface temperature (SST) effect on NWPARs. The summertime NWPAR occurrences are tightly related to El Ni (n) over tildeo-Southern Oscillation (ENSO) in the preceding winter through Indo-western Pacific Ocean capacitor (IPOC) effects. An enhanced East Asian summer monsoon and a low-level anticyclonic anomaly over the tropical western North Pacific in the post-El Ni (n) over tildeo summer reinforce low-level water vapor transport from the tropics with increased occurrence of NWPARs. The strong coupling with ENSO and IPOC indicates a high predictability of anomalous summertime NWPAR activity.

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.

Kamae, Y, Shiogama H, Imada Y, Mori M, Arakawa O, Mizuta R, Yoshida K, Takahashi C, Arai M, Ishii M, Watanabe M, Kimoto M, Xie SP, Ueda H.  2017.  Forced response and internal variability of summer climate over western North America. Climate Dynamics. 49:403-417.   10.1007/s00382-016-3350-x   AbstractWebsite

Over the past decade, anomalously hot summers and persistent droughts frequented over the western United States (wUS), the condition similar to the 1950s and 1960s. While atmospheric internal variability is important for mid-latitude interannual climate variability, it has been suggested that anthropogenic external forcing and multidecadal modes of variability in sea surface temperature, namely, the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO), also affect the occurrence of droughts and hot summers. In this study, 100-member ensemble simulations for 1951-2010 by an atmospheric general circulation model were used to explore relative contributions of anthropogenic warming, atmospheric internal variability, and atmospheric response to PDO and AMO to the decadal anomalies over the wUS. By comparing historical and sensitivity simulations driven by observed sea surface temperature, sea ice, historical forcing agents, and non-warming counterfactual climate forcing, we found that large portions of recent increases in mean temperature and frequency of hot summers (66 and 82 %) over the wUS can be attributed to the anthropogenic global warming. In contrast, multidecadal change in the wUS precipitation is explained by a combination of the negative PDO and the positive AMO after the 2000s. Diagnostics using a linear baroclinic model indicate that AMO- and PDO-related diabatic heating anomalies over the tropics contribute to the anomalous atmospheric circulation associated with the droughts and hot summers over wUS on multidecadal timescale. Those anomalies are not robust during the periods when PDO and AMO are in phase. The prolonged PDO-AMO antiphase period since the late twentieth century resulted in the substantial component of multidecadal anomalies in temperature and precipitation over the wUS.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.