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Kubota, H, Kosaka Y, Xie SP.  2016.  A 117-year long index of the Pacific-Japan pattern with application to interdecadal variability. International Journal of Climatology. 36:1575-1589.   10.1002/joc.4441   AbstractWebsite

The Pacific-Japan (PJ) pattern affects interannual variability in the East Asian and western North Pacific (WNP) summer monsoons. This teleconnection pattern is characterized by a meridional dipole of anomalous circulation and precipitation between the tropical WNP and the midlatitudes. This study develops a long index of the PJ pattern using station-based atmospheric pressure data to track the PJ variability from 1897 to 2013. This index is correlated with a wide array of climate variables including air temperature, precipitation, Yangtze River flow, Japanese rice yield and the occurrence of tropical cyclones over the WNP (especially those that make landfall on the Chinese and Korean coast). For the recent three decades, the PJ index reproduces well-known correlations with El Nino-Southern Oscillation (ENSO) in the preceding boreal winter and Indian Ocean temperature in the concurrent summer. For the 117-year period, this ENSO-PJ relationship varies on interdecadal time scales, with low correlations in the 1920s and from the 1940s to 1970s, and recurrences of significant correlations at the beginning of the 20th century and the 1930s. In accordance with the modulation, the magnitude and regional climate effect of the PJ variability have changed. These results highlight the importance of interdecadal modulations of climate anomalies in the summer WNP and the need of long-term observations to study such modulations.

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Xie, SP, Xu HM, Kessler WS, Nonaka M.  2005.  Air-sea interaction over the eastern Pacific warm pool: Gap winds, thermocline dome, and atmospheric convection. Journal of Climate. 18:5-20. Abstract
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Kilpatrick, TJ, Xie S-P.  2015.  ASCAT observations of downdrafts from mesoscale convective systems. Geophysical Research Letters.   10.1002/2015GL063025   AbstractWebsite

Downdrafts of air cooled by evaporating raindrops are an essential component of mesoscale convective systems (MCSs). Here we use surface wind observations from the Advanced SCATterometer (ASCAT) to identify MCS downdrafts over the western equatorial Pacific Ocean as regions of horizontal wind divergence exceeding 10-4 s-1. More than 1300 downdrafts are identified over the observation period (2009–2014). The downdraft signal in the surface winds is validated with satellite measurements of brightness temperature and rainfall rate, and surface buoy measurements of air temperature; composite analysis with these measurements indicates ASCAT detects downdrafts that lag the peak convection by 8–12 h. While ASCAT resolves mesoscale downdrafts in regions of light rain, a composite against buoy air temperature indicates that ASCAT fails to resolve the stronger convective-scale downdrafts associated with heavy rainfall at squall fronts. Nevertheless, the global observations by the satellite scatterometer open a new avenue for studying MCSs.

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.

Minobe, S, Miyashita M, Kuwano-Yoshida A, Tokinaga H, Xie S-P.  2010.  Atmospheric Response to the Gulf Stream: Seasonal Variations. Journal of Climate. 23:3699-3719.   10.1175/2010jcli3359.1   Abstract
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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.

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Kuwano-Yoshida, A, Taguchi B, Xie SP.  2014.  Baiu rainband termination in atmospheric and coupled atmosphere-ocean models. Journal of Climate. 26:10111-10124.   10.1175/jcli-d-13-00231.1   AbstractWebsite

The baiu rainband is a summer rainband stretching from eastern China through Japan toward the northwestern Pacific. The climatological termination of the baiu rainband is investigated using the Japanese 25-yr Reanalysis (JRA-25), a stand-alone atmospheric general circulation model (GCM) forced with observed sea surface temperature (SST) and an atmosphere-ocean GCM (AOGCM). The baiu rainband over the North Pacific abruptly shifts northward and weakens substantially in early July in the atmospheric GCM (AGCM), too early compared to observations (late July). The midtroposphere westerly jet and its thermal advection explain this meridional shift of the baiu rainband, but the ocean surface evaporation modulates the precipitation intensity. In AGCM, deep convection in the subtropical northwestern Pacific sets in prematurely, displacing the westerly jet northward over the cold ocean surface earlier than in observations. The suppressed surface evaporation over the cold ocean suppresses precipitation even though the midtropospheric warm advection and vertically integrated moisture convergence are similar to those before the westerly jet's northward shift. As a result, the baiu rainband abruptly weakens after the northward shift in JRA-25 and AGCM. In AOGCM, cold SST biases in the subtropics inhibit deep convection, delaying the poleward excursion of the westerly jet. As a result, the upward motion induced by both the strong westerly jet and the rainband persist over the northwestern Pacific through summer in the AOGCM. The results indicate that the westerly jet and the ocean evaporation underneath are important for the baiu rainband, the latter suggesting an oceanic effect on this important phenomenon.

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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.

Collins, M, Minobe S, Barreiro M, Bordoni S, Kaspi Y, Kuwano-Yoshida A, Keenlyside N, Manzini E, O'Reilly CH, Sutton R, Xie SP, Zolina O.  2018.  Challenges and opportunities for improved understanding of regional climate dynamics. Nature Climate Change. 8:101-108.   10.1038/s41558-017-0059-8   AbstractWebsite

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

Kilpatrick, TJ, Xie SP.  2016.  Circumventing rain-related errors in scatterometer wind observations. Journal of Geophysical Research-Atmospheres. 121:9422-9440.   10.1002/2016jd025105   AbstractWebsite

Satellite scatterometer observations of surface winds over the global oceans are critical for climate research and applications like weather forecasting. However, rain-related errors remain an important limitation, largely precluding satellite study of winds in rainy areas. Here we utilize a novel technique to compute divergence and curl from satellite observations of surface winds and surface wind stress in rainy areas. This technique circumvents rain-related errors by computing line integrals around rainy patches, using valid wind vector observations that border the rainy patches. The area-averaged divergence and wind stress curl inside each rainy patch are recovered via the divergence and curl theorems. We process the 10 year Quick Scatterometer (QuikSCAT) data set and show that the line-integral method brings the QuikSCAT winds into better agreement with an atmospheric reanalysis, largely removing both the "divergence bias" and "anticyclonic curl bias" in rainy areas noted in previous studies. The corrected QuikSCAT wind stress curl reduces the North Pacific midlatitude Sverdrup transport by 20-30%. We test several methods of computing divergence and curl on winds from an atmospheric model simulation and show that the line-integral method has the smallest errors. We anticipate that scatterometer winds processed with the line-integral method will improve ocean model simulations and help illuminate the coupling between atmospheric convection and circulation.

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.

Tomita, H, Xie SP, Tokinaga H, Kawai Y.  2013.  Cloud response to the meandering Kuroshio extension front. Journal of Climate. 26:9393-9398.   10.1175/jcli-d-13-00133.1   AbstractWebsite

A unique set of observations on board research vessel (R/V) Mirai in April 2010 captured a striking cloud hole over a cold meander of the Kuroshio Extension (KE) east of Japan as corroborated by atmospheric soundings, ceilometer, shipboard radiation data, and satellite cloud images. Distinct differences were also observed between the warm meander farther to the north and warm water south of the KE. The atmosphere is highly unstable over the warm meander, promoting a well-mixed marine atmospheric boundary layer (MABL) and a layer of solid stratocumulus clouds capped by a strong inversion. Over the warm water south of the KE, MABL deepens and is decoupled from the ocean surface. Scattered cumulus clouds develop as captured by rapid variations in ceilometer-derived cloud base. The results show that the meandering KE front affects the entire MABL and the clouds. Such atmospheric response can potentially intensify the baroclinicity in the lower atmosphere.

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.

Kang, SM, Held IM, Xie SP.  2014.  Contrasting the tropical responses to zonally asymmetric extratropical and tropical thermal forcing. Climate Dynamics. 42:2033-2043.   10.1007/s00382-013-1863-0   AbstractWebsite

The mechanism is investigated by which extratropical thermal forcing with a finite zonal extent produces global impact. The goal is to understand the near-global response to a weakened Atlantic meridional overturning circulation suggested by paleoclimate data and modeling studies. An atmospheric model coupled to an aquaplanet slab mixed layer ocean, in which the unperturbed climate is zonally symmetric, is perturbed by prescribing cooling of the mixed layer in the Northern Hemisphere and heating of equal magnitude in the Southern Hemisphere, over some finite range of longitudes. In the case of heating/cooling confined to the extratropics, the zonally asymmetric forcing is homogenized by midlatitude westerlies and extratropical eddies before passing on to the tropics, inducing a zonally symmetric tropical response. In addition, the zonal mean responses vary little as the zonal extent of the forced region is changed, holding the zonal mean heating fixed, implying little impact of stationary eddies on the zonal mean. In contrast, when the heating/cooling is confined to the tropics, the zonally asymmetric forcing produces a highly localized response with slight westward extension, due to advection by mean easterly trade winds. Regardless of the forcing location, neither the spatial structure nor the zonal mean responses are strongly affected by wind-evaporation-sea surface temperature feedback.

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Feng, M, Hendon HH, Xie SP, Marshall AG, Schiller A, Kosaka Y, Caputi N, Pearce A.  2015.  Decadal increase in Ningaloo Nino since the late 1990s. Geophysical Research Letters. 42:104-112.   10.1002/2014gl062509   AbstractWebsite

Ningaloo Nino refers to the episodic occurrence of anomalously warm ocean conditions along the subtropical coast of Western Australia (WA). Ningaloo Nino typically develops in austral spring, peaks in summer, and decays in autumn, and it often occurs in conjunction with La Nina conditions in the Pacific which promote poleward transport of warm tropical waters by the Leeuwin Current. Since the late 1990s, there has been a marked increase in the occurrence of Ningaloo Nino, which is likely related to the recent swing to the negative phase of the Interdecadal Pacific Oscillation (IPO) and enhanced El Nino-Southern Oscillation variance since 1970s. The swing to the negative IPO sustains positive heat content anomalies and initiates more frequent cyclonic wind anomalies off the WA coast so favoring enhanced poleward heat transport by the Leeuwin Current. The anthropogenically forced global warming has made it easier for natural variability to drive extreme ocean temperatures in the region.

Kobashi, F, Xie SP, Iwasaka N, Sakamoto TT.  2008.  Deep Atmospheric Response to the North Pacific Oceanic Subtropical Front in Spring. Journal of Climate. 21:5960-5975.   10.1175/2008jcli2311.1   Abstract
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Kang, SM, Xie SP.  2014.  Dependence of climate response on meridional structure of external thermal forcing. Journal of Climate. 27:5593-5600.   10.1175/jcli-d-13-00622.1   AbstractWebsite

This study shows that the magnitude of global surface warming greatly depends on the meridional distribution of surface thermal forcing. An atmospheric model coupled to an aquaplanet slab mixed layer ocean is perturbed by prescribing heating to the ocean mixed layer. The heating is distributed uniformly globally or confined to narrow tropical or polar bands, and the amplitude is adjusted to ensure that the global mean remains the same for all cases. Since the tropical temperature is close to a moist adiabat, the prescribed heating leads to a maximized warming near the tropopause, whereas the polar warming is trapped near the surface because of strong atmospheric stability. Hence, the surface warming is more effectively damped by radiation in the tropics than in the polar region. As a result, the global surface temperature increase is weak (strong) when the given amount of heating is confined to the tropical (polar) band. The degree of this contrast is shown to depend on water vapor- and cloud-radiative feedbacks that alter the effective strength of prescribed thermal forcing.

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.

Xie, SP, Kosaka Y, Okumura YM.  2016.  Distinct energy budgets for anthropogenic and natural changes during global warming hiatus. Nature Geoscience. 9:29-+.   10.1038/ngeo2581   AbstractWebsite

The Earth's energy budget for the past four decades can now be closed(1), and it supports anthropogenic greenhouse forcing as the cause for climate warming. However, closure depends on invoking an unrealistically large increase in aerosol cooling(2) during the so-called global warming hiatus since the late 1990s (refs 3,4) that was due partly to tropical Pacific Ocean cooling(5-7). The difficulty with this closure lies in the assumption that the same climate feedback applies to both anthropogenic warming and natural cooling. Here we analyse climate model simulations with and without anthropogenic increases in greenhouse gas concentrations, and show that top-of-the-atmosphere radiation and global mean surface temperature are much less tightly coupled for natural decadal variability than for the greenhouse-gas-induced response, implying distinct climate feedback between anthropogenic warming and natural variability. In addition, we identify a phase difference between top-of-the-atmosphere radiation and global mean surface temperature such that ocean heat uptake tends to slow down during the surface warming hiatus. This result deviates from existing energy theory but we find that it is broadly consistent with observations. Our study highlights the importance of developing metrics that distinguish anthropogenic change from natural variations to attribute climate variability and to estimate climate sensitivity from observations.

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.

Wang, H, Xie SP, Kosaka Y, Liu QY, Du Y.  2019.  Dynamics of Asian summer monsoon response to anthropogenic aerosol forcing. Journal of Climate. 32:843-858.   10.1175/jcli-d-18-0386.1   AbstractWebsite

Anthropogenic aerosols partially mask the greenhouse warming and cause the reduction in Asian summer monsoon precipitation and circulation. By decomposing the atmospheric change into the direct atmospheric response to radiative forcing and sea surface temperature (SST)-mediated change, the physical mechanisms for anthropogenic-aerosol-induced changes in the East Asian summer monsoon (EASM) and South Asian summer monsoon (SASM) are diagnosed. Using coupled and atmospheric general circulation models, this study shows that the aerosol-induced troposphere cooling over Asian land regions generates anomalous sinking motion between 20 degrees and 40 degrees N and weakens the EASM north of 20 degrees N without SST change. The decreased EASM precipitation and the attendant wind changes are largely due to this direct atmospheric response to radiative forcing, although the aerosol-induced North Pacific SST cooling also contributes. The SST-mediated change dominates the aerosol-induced SASM response, with contributions from both the north-south interhemispheric SST gradient and the local SST cooling pattern over the tropical Indian Ocean. Specifically, with large meridional gradient, the zonal-mean SST cooling pattern is most important for the Asian summer monsoon response to anthropogenic aerosol forcing, resulting in a reorganization of the regional meridional atmospheric overturning circulation. While uncertainty in aerosol radiative forcing has been emphasized in the literature, our results show that the intermodel spread is as large in the SST effect on summer monsoon rainfall, calling for more research into the ocean-atmosphere coupling.

Kosaka, Y, Xie S-P, Nakamura H.  2011.  Dynamics of Interannual Variability in Summer Precipitation over East Asia. Journal of Climate. 24:5435-5453.   10.1175/2011jcli4099.1   Abstract
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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.

Pahnke, K, Sachs JP, Keigwin L, Timmermann A, Xie SP.  2007.  Eastern tropical Pacific hydrologic changes during the past 27,000 years from D/H ratios in alkenones. Paleoceanography. 22   10.1029/2007pa001468   Abstract
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