ENSO Modoki Research Articles

Impacts of Two-Type ENSO on Rainfall over Taiwan

Impacts of two-type ENSO (El Niño/Southern Oscillation), canonical ENSO and ENSO Modoki, on rainfall over Taiwan are investigated by the monthly mean rainfall data accessed from Taiwan Central Weather Bureau. The periods of the two-type ENSO are distinguished by Niño 3.4 index and ENSO Modoki index (EMI). The rainfall data in variously geographical regions are analyzed with the values of Niño 3.4 and EMI by correlation method. Results show that the seasonal rainfalls over Taiwan are different depending on the effects of two-type ENSO. In canonical El Niño episode, the rainfall increases in winter and spring while it reduces in summer and autumn. On the contrary, the rainfall increases in summer and autumn but reduces in winter and spring in El Niño Modoki episode. Nevertheless, two types of La Niña cause similar effects on the rainfall over Taiwan. It increases in autumn only. The rainfall variations in different types of ENSO are mainly caused by the monsoon and topography.

Indo-Pacific–Induced Wave Trains during Austral Autumn and Their Effect on Australian Rainfall

Abstract During austral winter and spring, the El Niño–Southern Oscillation (ENSO) and the Indian Ocean dipole (IOD), individually or in combination, induce equivalent-barotropic Rossby wave trains, affecting midlatitude Australian rainfall. In autumn, ENSO is at its annual minimum, and the IOD has usually not developed. However, there is still a strong equivalent-barotropic Rossby wave train associated with tropical Indian Ocean sea surface temperature (SST) variability, with a pressure anomaly to the south of Australia. This wave train is similar in position, but opposite in sign, to the IOD-induced wave train in winter and spring and has little effect on Australian rainfall. This study shows that the SST in the southeastern tropical Indian Ocean (SETIO) displays a high variance during austral autumn, with a strong influence on southeast and eastern Australian rainfall. However, this influence is slightly weaker than that associated with SST to the north of Australia, which shares fluctuations with SST in the SETIO region. The SST north of Australia is coherent with a convective dipole in the tropical Pacific Ocean, which is the source of a wave train to the east of Australia influencing rainfall in eastern Australia. ENSO Modoki is a contributor to the convective dipole and as a result it exerts a weak influence on eastern Australian rainfall through the connecting north Australian SST relationship. Thus, SST to the north of Australia acts as the main agent for delivering the impact of tropical Indo-Pacific variability to eastern Australia.

Impacts of four types of ENSO events on tropical cyclones making landfall over mainland china based on three best-track datasets

Impacts of El Nino Modoki (ENM), La Nina Modoki (LNM), canonical El Nino (CEN) and canonical La Nina (CLN) on tropical cyclones (TCs) that made landfall over mainland China during 1951–2011 are analysed using best-track data from China, the USA and Japan. Relative to cold phase years (LNM and CLN), landfalling TCs in warm years (ENM and CEN) have a farther east genesis location, as well as longer track lengths and durations, both in total and before landfall. ENM demonstrates the highest landfall frequency, most northerly mean landfall position, and shortest after-landfall sustainability (track length and duration), which indicate a more frequent and extensive coverage of mainland China by TCs, but with shorter after-landfall influence. CEN has low landfall frequency and the most southerly mean landfall location. LNM has the most westerly genesis location, being significantly farther west than the 1951–2011 average and leading to short mean track lengths and durations both in total or before landfall, all of which are significantly shorter than the 1951–2011 average. Variations in the low-level wind anomaly, vertical wind shear, mid-level relative humidity, steering flow, the monsoon trough and the western Pacific subtropical high (WPSH) can to some extent account for the features of frequency, location, track length and duration of landfalling TCs. Since ENSO Modoki is expected to become more frequent in the near future, the results for ENSO Modoki presented in this paper are of particular significance.

Modulation of tropical cyclogenesis over the South China Sea by ENSO Modoki during boreal summer

This study examines the modulation of tropical cyclogenesis over the South China Sea (SCS) by the El Nino-Southern Oscillation (ENSO) Modoki during the boreal summer. Results reveal that there were more tropical cyclones (TCs) formed over the SCS during central Pacific warming years and less TC frequency during central Pacific cooling years. How different environmental factors (including low-level relative vorticity, mid-level relative humidity, vertical wind shear, and potential intensity) contribute to this influence is investigated, using a genesis potential (GP) index developed by Emanuel and Nolan. Composite anomalies of the GP index are produced for central Pacific warming and cooling years separately, which could account for the changes of TC frequency over the SCS in different ENSO Modoki phases. The degree of contribution by each factor is determined quantitatively by producing composites of modified indices in which only one of the contributing factors varies, with the others set to climatology. The results suggest that the vertical wind shear and low-level relative vorticity, which are associated with the ENSO Modoki-induced anomalous circulations in Matsuno-Gill patterns, make the largest contributions to the ENSO Modoki modulation of tropical cyclogenesis over the SCS as implied by the GP index. These results highlight the important roles of dynamic factors in the modulation of TC frequency over the SCS by the ENSO Modoki during the boreal summer.

Decadal changes of Meiyu rainfall around 1991 and its relationship with two types of ENSO

AbstractThis study investigates Meiyu (June‐July) rainfall from 1980 to 2000 over Yangtze‐Huaihe River valley (YHRV) and reveals a decadal change around 1991 with a seesaw‐like distribution. This decadal change is closely associated with negative phase of preceding winter El Niño–Southern Oscillation (ENSO) Modoki (EM) in the pre‐1991 epoch and positive phase of conventional ENSO (CE) in the post‐1991 epoch. In response to negative EM (pre‐1991 epoch), the source of water vapor is western North Pacific, associated with the eastward retreat of western Pacific subtropical high (WPSH) and the northward shift of East Asian westerly jet (EAWJ). On the other hand, in response to positive CE (post‐1991 epoch), water vapor generally comes from the South China Sea, associated with the enhancement and westward advance of the WPSH and southward shift of the EAWJ. Two possible mechanisms are proposed to explain the relationship between preceding winter sea surface temperature (SST) and Meiyu rainfall over YHRV in the two epochs. In the pre‐1991 epoch, due to enhanced precipitation, solar radiation decreases, resulting in SST cooling over western North Pacific in Meiyu period. An anomalous anticyclone associated with the SST cooling suppresses precipitation over northern part of YHRV. Meanwhile, an anomalous cyclone accompanied with the anticyclone locates over East China Sea and brings more precipitation to the southern part of YHRV. In the post‐1991 epoch, the persistent simultaneous warming in eastern equatorial Pacific and Indian Ocean leads to abundant moisture brought to southern China. Further analysis suggests that the two types of ENSO have asymmetric features with respect to the impact of their positive and negative phases on the seesaw‐like Meiyu rainfall, which is closely related with EM La Niña (CE El Niño) in the pre‐1991 epoch (post‐1991 epoch).

Dynamical seasonal prediction of Southern African summer precipitation

Prediction skill for southern African (16°–33°E, 22°–35°S) summer precipitation in the Scale Interaction Experiment-Frontier coupled model is assessed for the period of 1982–2008. Using three different observation datasets, deterministic forecasts are evaluated by anomaly correlation coefficients, whereas scores of relative operating characteristic and relative operating level are used to evaluate probabilistic forecasts. We have found that these scores for December–February precipitation forecasts initialized on October 1st are significant at 95 % confidence level. On a local scale, the level of prediction skill in the northwestern and central parts of southern Africa is higher than that in northeastern South Africa. El Nino/Southern Oscillation (ENSO) provides the major source of predictability, but the relationship with ENSO is too strong in the model. The Benguela Nino, the basin mode in the tropical Indian Ocean, the subtropical dipole modes in the South Atlantic and the southern Indian Oceans and ENSO Modoki may provide additional sources of predictability. Within the wet season from October to the following April, the precipitation anomalies in December-February are the most predictable. This study presents promising results for seasonal prediction of precipitation anomaly in the extratropics, where seasonal prediction has been considered a difficult task.

Contrasting Impacts of Two Types of ENSO on the Boreal Spring Hadley Circulation

Abstract The possible influences of two types of ENSO [i.e., the canonical ENSO and ENSO Modoki (EM)] on Hadley circulation (HC) during the boreal spring are investigated during 1979–2010. El Niño events are featured with a symmetric pattern in equatorial zonal-mean sea surface temperature anomalies (SSTA), with a maximum around the equator. In contrast, the zonal-mean SSTA associated with El Niño Modoki events shows an asymmetric structure with a maximum around 10°N. The contrasting underlying thermal structures corresponding with ENSO and EM have opposite impacts on the simultaneous HC. In El Niño years, a symmetric anomalous meridional circulation is seen, with enhanced rising around the equator and anomalous descent at about 15°N and 20°S. In contrast, an asymmetric equatorial meridional circulation is observed for El Niño Modoki years, with anomalous ascent around 10°N and descent at about 10°S and 20°N. The contrasting meridional circulation anomalies within ENSO and EM are caused by their different meridional SSTA structure. This result is theoretically explained, indicating that anomalous meridional circulation is subject to the meridional SSTA gradient. Moreover, the observed results are reproduced in numerical experiments driven by anomalous warming in the eastern and central Pacific Ocean. Thus, the authors conclude that the anomalous HC linked to ENSO and EM is induced by the accompanying meridional gradient in zonal-mean SSTA.

Revisiting the Indian summer monsoon–ENSO links in the IPCC AR4 projections: A cautionary outlook

The climate change experiments under the fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC), namely the twentieth century simulations (20C3M) and Special Report on Emissions Scenarios (SRES) A1B, are revisited to study whether these models can reproduce the ENSO and ENSO Modoki patterns as the two important modes from statistical linear analysis as observed. The capability of the models in simulating realistic ENSO/ENSO Modoki teleconnections with the Indian summer monsoon, and also the implications for the future are also explored. Results from the study indicate that only ~1/4th of the models from 20C3M capture either ENSO or ENSO Modoki pattern in JJAS. Of this 1/4th, only two models simulate both ENSO and ENSO Modoki as important modes. Again, out of these two, only one model simulates both ENSO and ENSO Modoki as important modes during both summer and winter.It is also shown that the two models that demonstrate ENSO Modoki as well as ENSO associated variance in both 20C3M and SRESA1B represent the links of the ISMR with ENSO reasonably in 20C3M, but indicate opposite type of impacts in SREA1B. With the limited skills of the models in reproducing the monsoon, the ENSO and ENSO Modoki, it is difficult to reconcile that the teleconnections of a tropical driver can change like that. All these indicate the challenges associated with the limitations of the models in reproducing the variability of the monsoons and ENSO flavors, not to speak of failing in capturing the potential impacts of global warming as they are expected to. More research in improving the current day simulations, improving model capacity to simulate better by improving the Green House Gases (GHG) and aerosols in the models are some of the important and immediate steps that are necessary.

Influences of two types of ENSO on South American precipitation

AbstractSea surface temperature (SST) anomalies in the Pacific Ocean during El Niño‐Southern Oscillation (ENSO) episodes exhibit different spatial patterns from year to year. In most ENSO studies, SST anomalies have been analysed in the 3.4 or 3.0 El Niño regions. Recent analyses have considered different SST anomalies areas in the Pacific, such as ENSO Modoki, which takes into account anomalies in the central Pacific which are bounded by opposite anomalies in the eastern and western Pacific. In order to analyse the influence of different Pacific spatial patterns on South American precipitation and on the Southern Hemisphere atmospheric circulation, composites obtained from Canonical ENSO were compared with those from ENSO Modoki in cases when strong anomalies were present in the Central Pacific. During the Canonical El Niño (La Niña), there tends to be a precipitation increase (decrease) in the La Plata Basin (LPB, 45°W–65°W, 15°S–35°S) and a decrease (increase) over northern South America during all seasons. In ENSO Modoki exhibiting strong anomalies in the Central Pacific, these typical patterns are not observed, and in some regions the anomalies even show opposite signs. Precipitation anomaly differences or similarities over South America between the two cases occur in different areas and different seasons. In both cases, differences in tropical South American precipitation during both ENSO types are related to differences in the Walker circulation. In extra‐tropical South America, the precipitation differences are due to differences in the Pacific wavetrains and differences in moisture flux intensity over the continent. Copyright © 2012 Royal Meteorological Society

Assessment of the APCC coupled MME suite in predicting the distinctive climate impacts of two flavors of ENSO during boreal winter

Forecast skill of the APEC Climate Center (APCC) Multi-Model Ensemble (MME) seasonal forecast system in predicting two main types of El Nino-Southern Oscillation (ENSO), namely canonical (or cold tongue) and Modoki ENSO, and their regional climate impacts is assessed for boreal winter. The APCC MME is constructed by simple composite of ensemble forecasts from five independent coupled ocean-atmosphere climate models. Based on a hindcast set targeting boreal winter prediction for the period 1982–2004, we show that the MME can predict and discern the important differences in the patterns of tropical Pacific sea surface temperature anomaly between the canonical and Modoki ENSO one and four month ahead. Importantly, the four month lead MME beats the persistent forecast. The MME reasonably predicts the distinct impacts of the canonical ENSO, including the strong winter monsoon rainfall over East Asia, the below normal rainfall and above normal temperature over Australia, the anomalously wet conditions across the south and cold conditions over the whole area of USA, and the anomalously dry conditions over South America. However, there are some limitations in capturing its regional impacts, especially, over Australasia and tropical South America at a lead time of one and four months. Nonetheless, forecast skills for rainfall and temperature over East Asia and North America during ENSO Modoki are comparable to or slightly higher than those during canonical ENSO events.

Remote Influence of the Tropical Atlantic on the Variability and Trend in North West Australia Summer Rainfall

Abstract Rainfall in North West Australia (NWA) has been increasing over the past decades, occurring mainly in the austral summer season (December–March). A range of factors such as decreased land albedo in Australia and increasing anthropogenic aerosols in the Northern Hemisphere, identified using simulations from climate models, have been implicated in this wetting trend. However, the impact of land albedo and aerosols on Australian rainfall remains unclear. In addition, previous studies showed that dominant sea surface temperature (SST) signals in the Pacific–Indian Ocean including El Niño–Southern Oscillation (ENSO), ENSO Modoki, and the Indian Ocean dipole mode have no significant impact on the NWA rainfall trend. The present study proposes another viewpoint on the remote influence of tropical Atlantic atmospheric vertical motion on the observed rainfall variability and trend in NWA. It is found that, with the atmospheric ascent instigated by the warming of SST over the tropical Atlantic, a Rossby wave train is emanating southeastward from off the west coast of subtropical South America to the midlatitudes of the South Atlantic Ocean. It then travels eastward embedded in the westerly jet waveguide over the South Atlantic and South Indian Oceans. The eastward-propagated Rossby wave induces an anticyclonic anomaly in the upper troposphere over Australia, which is at the exit of the westerly jet waveguide. This leads to an in situ upper-tropospheric divergence, ascending motion and a lower-tropospheric convergence, and the associated increase in rainfall in NWA. Thus, the increasing trend in atmospheric upward motion induced by the warming trend of SST in the tropical Atlantic may partially explain the observed rainfall trend in NWA.

Linear trends in sea surface temperature of the tropical Pacific Ocean and implications for the El Niño-Southern Oscillation

A principal component decomposition of monthly sea surface temperature (SST) variability in the tropical Pacific Ocean demonstrates that nearly all of the linear trends during 1950–2010 are found in two leading patterns. The first SST pattern is strongly related to the canonical El Niño-Southern Oscillation (ENSO) pattern. The second pattern shares characteristics with the first pattern and its existence solely depends on the presence of linear trends across the tropical Pacific Ocean. The decomposition also uncovers a third pattern, often referred to as ENSO Modoki, but the linear trend is small and dataset dependent over the full 61-year record and is insignificant within each season. ENSO Modoki is also reflected in the equatorial zonal SST gradient between the Niño-4 region, located in the west-central Pacific, and the Niño-3 region in the eastern Pacific. It is only in this zonal SST gradient that a marginally significant trend arises early in the Northern Hemisphere spring (March–May) during El Niño and La Niña and also in the late summer (July–September) during El Niño. Yet these SST trends in the zonal gradient do not unequivocally represent an ENSO Modoki-like dipole because they are exclusively associated with significant positive SST trends in either the eastern or western Pacific, with no corresponding significant negative trends. Insignificant trends in the zonal SST gradient are evident during the boreal wintertime months when ENSO events typically mature. Given the presence of positive SST trends across much of the equatorial Pacific Ocean, using fixed SST anomaly thresholds to define ENSO events likely needs to be reconsidered.

The El Niño–Southern Oscillation (ENSO) Modoki signal in the stratosphere

El Nino–Southern Oscillation (ENSO) is known to be the largest source of interannual variability in the tropical troposphere. However, the variability in the tropical Pacific since 1979 seems to be associated not only with “canonical” ENSO events but also with a variation thereof known as ENSO Modoki, which is characterized by warm anomalies in the central Pacific, west from those occurring during a typical ENSO. This works analyzes the signal of ENSO Modoki in the stratosphere and compares it to canonical ENSO by using the chemistry‐climate Whole Atmosphere Community Climate Model (WACCM3.5). The results reveal a significant warming in the Southern Hemisphere polar stratosphere during boreal winter months, which propagates downward in early spring; this is absent during canonical warm ENSO events. On the other hand, in the Northern Hemisphere stratosphere, the anomalous warming typical of canonical El Niño episodes during boreal winter is not statistically significant during El Niño Modoki events. These differences are related in WACCM3.5 to changes in tropical convection and tropospheric teleconnections associated with each type of event. In particular, an enhancement and westward displacement of the anomalous convective area during El Niño Modoki episodes is related to an intensification of the Pacific South American teleconnection pattern and a weakening of the Aleutian Low. During cold ENSO Modoki events a significant anomalous cooling is present in the model simulations.

Observation of first and second baroclinic mode Yanai waves in the ocean

AbstractThe structure and propagation of oceanic Yanai waves (also known as mixed Rossby–gravity waves) are investigated by analysis of satellite‐derived sea‐surface‐height (SSH) data. Significant spectral peaks along the dispersion curves of first and second baroclinic mode Yanai waves are identified by the wavenumber–frequency spectral analysis of SSH for the period 10–17 days. The spatial structure and propagation of these modes are described by an analysis of SSH time series filtered in the frequency–wavenumber domain that includes strong signals along the dispersion curves but excludes those of 17‐day tropical instability waves. The difference in meridional structure between the first and second baroclinic modes, which is consistent with theory, is evident. The slow eastward propagation of first and second baroclinic mode Yanai wave packets, which is consistent with the group velocity derived from theory, is also isolated from the SSH data. Year‐to‐year variability of Yanai wave activity is compared with that of the El Niño–Southern Oscillation (ENSO), showing a moderate correlation with ENSO Modoki. © Published in 2011 by John Wiley & Sons Ltd.

Influence of El Niño Modoki on spring rainfall over south China

[1] Using observed data sets from 1979 to 2006, the relationship between El Niño Modoki and spring rainfall over south China (SC) is investigated. Of particular interest is the difference in the influence on spring rainfall of typical El Niño events and the recently recognized El Niño Modoki events, which are characterized by distinct warm sea surface temperature anomalies (SSTA) in the central Pacific and weaker cold anomalies in the western and eastern parts of the basin. Associated with the SSTA, anomalous ascent occurs over the central Pacific and downward flow is observed over the eastern and western Pacific. The anomalous flow is associated with anomalous convergence in the upper troposphere over the western Pacific. SC is influenced by an anomalous anticyclonic circulation with prevailing northeasterly anomalies. The convective activity in SC becomes weaker, resulting in reduced rainfall. However, the situation is different in the case of El Niño, in terms of the influence on rainfall over SC. While El Niño Modoki events are accompanied by a significant reduction in rainfall over SC, there is enhanced rainfall associated with El Niño events. Moreover, there exists a strong asymmetry in the relationship between SC spring rainfall, typical El Niño–Southern Oscillation (ENSO) and ENSO Modoki events. It appears that these relationships are only statistically significant for positive events. The asymmetric influence of positive and negative in two ENSO phenomena may explain the difference in their respective relationships with spring rainfall over SC.

Evidence for strengthening of the tropical Pacific Ocean surface wind speed during 1979–2001

Using multiple surface wind speed (SWS) data sets and trend empirical orthogonal function analysis, we have explored the trend in SWS associated with the large-scale tropical Pacific atmospheric circulation for the period 1979–2001. The present research provides a robust evidence of strengthening of the tropical Pacific Ocean SWS during this period and the magnitude is generally in line with the finding of Wentz et al. The strengthening in SWS is closely associated with the so-called La Nina-like sea surface temperature (SST) trend pattern rather than the changes in the ENSO, ENSO Modoki, or PDO. The present results, together with those from some recent climate model simulations, suggest that global warming forcing may have caused an intensification of SWS in the tropical Pacific Ocean by inducing the La Nina-like SST trend pattern due to ocean dynamics. Meanwhile, the strengthening in the tropical Pacific Ocean surface trade winds may also feedback to enhance the La Nina-like SST trend pattern under the positive wind-upwelling dynamic feedback mechanism.

Different impacts of two types of Pacific Ocean warming on Southeast Asian rainfall during boreal winter

The impacts of conventional El Niño‐Southern Oscillation (ENSO) and ENSO Modoki on wintertime Southeast Asian rainfall and related mechanisms are studied using the method of partial regression and correlation and numerical simulations of a simple baroclinic model. Results show that the Southeast Asian rainfall associated with these two kinds of ENSO exhibits different spatial distributions. In the case of El Niño, wet conditions are observed over south China, and dry conditions are seen over the Philippines, Borneo, Celebes, and Sulawesi. In contrast, for El Niño Modoki, the negative rainfall anomalies around the Philippines are weaker and are located more northward compared to the El Niño counterpart. The different Southeast Asian rainfalls that are related to ENSO and ENSO Modoki are attributed to the different anomalous Walker circulation and low‐level anticyclone around the Philippines. Both the Philippine anticyclone and the descending branch center of the Walker circulation over the western North Pacific occupy a smaller domain and are located more northward during El Niño Modoki than during El Niño. All of these factors favor the difference in the Southeast Asian rainfall anomalies between the two events. Numerical experiments also suggest that the different low‐level atmospheric responses are mainly induced by different diabatic cooling over the western North Pacific related to El Niño and El Niño Modoki.

Net air–sea surface heat flux during 1984–2004 over the North Pacific and North Atlantic oceans (10°N–50°N): annual mean climatology and trend

Using the Objectively Analyzed air–sea Fluxes dataset (and also the National Oceanography Centre Southampton Flux Dataset v2.0), we examined both the annual mean climatology and trend of net air–sea surface heat flux (Qnet) for 1984–2004 over the North Pacific and North Atlantic oceans (10°N–50°N). The annual mean Qnet climatology shows that oceans obtain the positive Qnet over much of the North Pacific and North Atlantic oceans. Exceptions are the regions of western boundary currents (WBCs) including the Kuroshio and its extension off Japan and the Gulf Stream off the USA and its extension, where oceans release lots of heat into the atmosphere, mainly ascribed to the large surface turbulent heat loss. The statistically significant negative Qnet trends occurred in the WBCs, while the statistically significant positive Qnet trends appeared in the central basins of Northern Subtropical Oceans (CNSOs) including the central basin of Northern Subtropical Pacific and the central basin of Northern Subtropical Atlantic. These indentified Qnet trends, which are independent of both El Nino-Southern Oscillation (ENSO) and ENSO Modoki but closely related to global warming forcing, are predominately due to the statistically significant surface latent heat (LH) trends. Over the WBCs, the positive LH trends are mainly induced by the sea surface temperature increasing, indicating the ocean forcing upon overlying atmosphere. In contrast, over the CNSOs, the negative LH trends are mainly caused by the near-surface air specific humidity increasing, indicative of an oceanic response to overlying atmospheric forcing.

Asymmetry in ENSO Teleconnection with Regional Rainfall, Its Multidecadal Variability, and Impact

Abstract An asymmetry, and its multidecadal variability, in a rainfall teleconnection with the El Niño–Southern Oscillation (ENSO) are described. Further, the breakdown of this relationship since 1980 is offered as a cause for a rainfall reduction in an ENSO-affected region, southeast Queensland (SEQ). There, austral summer rainfall has been declining since around the 1980s, but the associated process is not understood. It is demonstrated that the rainfall reduction is not simulated by the majority of current climate models forced with anthropogenic forcing factors. Examination shows that ENSO is a rainfall-generating mechanism for the region because of an asymmetry in its impact: the La Niña–rainfall relationship is statistically significant, as SEQ summer rainfall increases with La Niña amplitude; by contrast, the El Niño–induced rainfall reductions do not have a statistically significant relationship with El Niño amplitude. Since 1980, this asymmetry no longer operates, and La Niña events no longer induce a rainfall increase, leading to the observed SEQ rainfall reduction. A similar asymmetric rainfall teleconnection with ENSO Modoki exists and shares the same temporal evolutions. This breakdown is caused by an eastward shift in the Walker circulation and the convection center near Australia’s east coast, in association with a post-1980 positive phase of the interdecadal Pacific oscillation (IPO). Such a breakdown occurred before 1950, indicating that multidecadal variability alone could potentially be responsible for the recent SEQ rainfall decline. An aggregation of outputs from climate models to distill the impact of climate change suggests that the asymmetry and the breakdown may not be generated by climate change, although most models do not perform well in simulating the ENSO–rainfall teleconnection over the SEQ region.

Forcing of Tropical SST Anomalies by Wintertime AO-like Variability

Abstract A lagged maximum covariance analysis (MCA) is utilized to investigate large-scale patterns of covariability between sea surface temperature (SST) in the global tropics and 500-mb geopotential height (Z500) in the extratropics at monthly to interannual time scales distinct from the conventional El Niño–Southern Oscillation (ENSO) signal during the Northern Hemisphere (NH) winter. The first MCA mode indicates a strong impact of tropical SST anomalies associated with ENSO on the extratropical atmosphere. The second MCA mode corresponds with coupling between Arctic Oscillation (AO)-like atmospheric variations and tropical SST anomalies. An AO-like MCA mode appears to depict an atmosphere-to-ocean forcing, in which the tropical ocean responds to the higher extratropical AO-like atmospheric anomalies with an intraseasonal time lag. In winter, AO-like atmospheric variability is associated with the northern tropical Atlantic mode and the tropical Pacific ENSO Modoki mode through enhanced or weakened trade winds. The above forced SST anomalies by the AO-like variability may play a role in the subsequent evolution of the conventional ENSO phenomena.