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Wang, CY, Xie SP, Kosaka Y.  2018.  Indo-Western Pacific Climate Variability: ENSO Forcing and Internal Dynamics in a Tropical Pacific Pacemaker Simulation. Journal of Climate. 31:10123-10139.   10.1175/jcli-d-18-0203.1   AbstractWebsite

El Nino-Southern Oscillation (ENSO) peaks in boreal winter but its impact on Indo-western Pacific climate persists for another two seasons. Key ocean-atmosphere interaction processes for the ENSO effect are investigated using the Pacific Ocean-Global Atmosphere (POGA) experiment with a coupled general circulation model, where tropical Pacific sea surface temperature (SST) anomalies are restored to follow observations while the atmosphere and oceans are fully coupled elsewhere. The POGA shows skills in simulating the ENSO-forced warming of the tropical Indian Ocean and an anomalous anticyclonic circulation pattern over the northwestern tropical Pacific in the post-El Nino spring and summer. The 10-member POGA ensemble allows decomposing Indo-western Pacific variability into the ENSO forced and ENSO-unrelated (internal) components. Internal variability is comparable to the ENSO forcing in magnitude and independent of ENSO amplitude and phase. Random internal variability causes apparent decadal modulations of ENSO correlations over the Indo-western Pacific, which are high during epochs of high ENSO variance. This is broadly consistent with instrumental observations over the past 130 years as documented in recent studies. Internal variability features a sea level pressure pattern that extends into the north Indian Ocean and is associated with coherent SST anomalies from the Arabian Sea to the western Pacific, suggestive of ocean-atmosphere coupling.

Xie, SP, Peng QH, Kamae Y, Zheng XT, Tokinaga H, Wang DX.  2018.  Eastern Pacific ITCZ Dipole and ENSO Diversity. Journal of Climate. 31:4449-4462.   10.1175/jcli-d-17-0905.1   AbstractWebsite

The eastern tropical Pacific features strong climatic asymmetry across the equator, with the intertropical convergence zone (ITCZ) displaced north of the equator most of time. In February- April (FMA), the seasonal warming in the Southern Hemisphere and cooling in the Northern Hemisphere weaken the climatic asymmetry, and a double ITCZ appears with a zonal rainband on either side of the equator. Results from an analysis of precipitation variability reveal that the relative strength between the northern and southern ITCZ varies from one year to another and this meridional seesaw results from ocean-atmosphere coupling. Surprisingly this meridional seesaw is triggered by an El Nino-Southern Oscillation (ENSO) of moderate amplitudes. Although ENSO is originally symmetric about the equator, the asymmetry in the mean climate in the preceding season introduces asymmetric perturbations, which are then preferentially amplified by coupled ocean-atmosphere feedback in FMA when deep convection is sensitive to small changes in cross-equatorial gradient of sea surface temperature. This study shows that moderate ENSO follows a distinct decay trajectory in FMA and southeasterly cross-equatorial wind anomalies cause moderate El Nino to dissipate rapidly as southeasterly cross-equatorial wind anomalies intensify ocean upwelling south of the equator. In contrast, extreme El Nino remains strong through FMA as enhanced deep convection causes westerly wind anomalies to intrude and suppress ocean upwelling in the eastern equatorial Pacific.

Hu, KM, Xie SP, Huang G.  2017.  Orographically Anchored El Nino Effect on Summer Rainfall in Central China. Journal of Climate. 30:10037-10045.   10.1175/jcli-d-17-0312.1   AbstractWebsite

Year-to-year variations in summer precipitation have great socioeconomic impacts on China. Historical rainfall variability over China is investigated using a newly released high-resolution dataset. The results reveal summer-mean rainfall anomalies associated with ENSO that are anchored by mountains in central China east of the Tibetan Plateau. These orographically anchored hot spots of ENSO influence are poorly represented in coarse-resolution datasets so far in use. In post-El Nino summers, an anomalous anticyclone forms over the tropical northwest Pacific, and the anomalous southwesterlies on the northwest flank cause rainfall to increase in mountainous central China through orographic lift. At upper levels, the winds induce additional adiabatic updraft by increasing the eastward advection of warm air from Tibet. In post-El Nino summers, large-scale moisture convergence induces rainfall anomalies elsewhere over flat eastern China, which move northward from June to August and amount to little in the seasonal mean.

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.

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.

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.

Hu, KM, Huang G, Zheng XT, Xie SP, Qu X, Du Y, Liu L.  2014.  Interdecadal variations in ENSO influences on Northwest Pacific-East Asian early summertime climate simulated in CMIP5 models. Journal of Climate. 27:5982-5998.   10.1175/jcli-d-13-00268.1   AbstractWebsite

The present study investigates interdecadal modulations of the El Nino-Southern Oscillation (ENSO) influence on the climate of the northwest Pacific (NWP) and East Asia (EA) in early boreal summer following a winter ENSO event, based on 19 simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5). In the historical run, 8 out of 19 models capture a realistic relationship between ENSO and NWP early summer climate-an anomalous anticyclone develops over the NWP following a winter El Nino event- and the interdecadal modulations of this correlation. During periods when the association between ENSO and NWP early summer climate is strong, ENSO variance and ENSO-induced anomalies of summer sea surface temperature (SST) and tropospheric temperature over the tropical Indian Ocean (TIO) all strengthen relative to periods when the association is weak. In future projections with representative concentration pathways 4.5 and 8.5, the response of TIO SST, tropospheric temperature, and NWP anomalous anticyclone to ENSO all strengthen regardless of ENSO amplitude change. In a warmer climate, low-level specific humidity response to interannual SST variability strengthens following the Clausius-Clapeyron equation. The resultant intensification of tropospheric temperature response to interannual TIO warming is suggested as the mechanism for the strengthened ENSO effect on NWP-EA summer climate.

Zinke, J, Rountrey A, Feng M, Xie SP, Dissard D, Rankenburg K, Lough JM, McCulloch MT.  2014.  Corals record long-term Leeuwin current variability including Ningaloo Nino/Nina since 1795. Nature Communications. 5   10.1038/ncomms4607   AbstractWebsite

Variability of the Leeuwin current (LC) off Western Australia is a footprint of interannual and decadal climate variations in the tropical Indo-Pacific. La Nina events often result in a strengthened LC, high coastal sea levels and unusually warm sea surface temperatures (SSTs), termed Ningaloo Nino. The rarity of such extreme events and the response of the southeastern Indian Ocean to regional and remote climate forcing are poorly understood owing to the lack of long-term records. Here we use well-replicated coral SST records from within the path of the LC, together with a reconstruction of the El Nino-Southern Oscillation to hindcast historical SST and LC strength from 1795 to 2010. We show that interannual and decadal variations in SST and LC strength characterized the past 215 years and that the most extreme sea level and SST anomalies occurred post 1980. These recent events were unprecedented in severity and are likely aided by accelerated global ocean warming and sea-level rise.

Feng, M, McPhaden MJ, Xie SP, Hafner J.  2013.  La Nina forces unprecedented Leeuwin Current warming in 2011. Scientific Reports. 3   10.1038/srep01277   AbstractWebsite

Unprecedented warm sea surface temperature (SST) anomalies were observed off the west coast of Australia in February-March 2011. Peak SST during a 2-week period were 5 degrees C warmer than normal, causing widespread coral bleaching and fish kills. Understanding the climatic drivers of this extreme event, which we dub "Ningaloo Nino", is crucial for predicting similar events under the influence of global warming. Here we use observational data and numerical models to demonstrate that the extreme warming was mostly driven by an unseasonable surge of the poleward-flowing Leeuwin Current in austral summer, which transported anomalously warm water southward along the coast. The unusual intensification of the Leeuwin Current was forced remotely by oceanic and atmospheric teleconnections associated with the extraordinary 2010-2011 La Nina. The amplitude of the warming was boosted by both multi-decadal trends in the Pacific toward more La Nina-like conditions and intraseasonal variations in the Indian Ocean.

Kosaka, Y, Xie SP, Lau NC, Vecchi GA.  2013.  Origin of seasonal predictability for summer climate over the Northwestern Pacific. Proceedings of the National Academy of Sciences of the United States of America. 110:7574-7579.   10.1073/pnas.1215582110   AbstractWebsite

Summer climate in the Northwestern Pacific (NWP) displays large year-to-year variability, affecting densely populated Southeast and East Asia by impacting precipitation, temperature, and tropical cyclones. The Pacific-Japan (PJ) teleconnection pattern provides a crucial link of high predictability from the tropics to East Asia. Using coupled climate model experiments, we show that the PJ pattern is the atmospheric manifestation of an air-sea coupled mode spanning the Indo-NWP warm pool. The PJ pattern forces the Indian Ocean (IO) via a westward propagating atmospheric Rossby wave. In response, IO sea surface temperature feeds back and reinforces the PJ pattern via a tropospheric Kelvin wave. Ocean coupling increases both the amplitude and temporal persistence of the PJ pattern. Cross-correlation of ocean-atmospheric anomalies confirms the coupled nature of this PJIO mode. The ocean-atmosphere feedback explains why the last echoes of El Nino-Southern Oscillation are found in the IO-NWP in the form of the PJIO mode. We demonstrate that the PJIO mode is indeed highly predictable; a characteristic that can enable benefits to society.