ENSO Modoki Research Articles

Dipolar redistribution of summertime tropical cyclone genesis between the Philippine Sea and the northern South China Sea and its possible mechanisms

Recent observational records show that the dipole oscillation between the Philippine Sea (PS) and the northern South China Sea (nSCS) is a leading empirical orthogonal function (EOF) of summertime tropical cyclone (TC) genesis in the western North Pacific (WNP). This PS‐nSCS oscillation is characterized by a distinguished decadal variability in addition to an interannual variability. Meanwhile, the typical El Niño–Southern Oscillation (ENSO)‐related mode is found in the second EOF mode, which is predominantly interannual. With regard to the PS‐nSCS oscillation, its interannual component appears to be linked with the previous wintertime ENSO event, but the linkage is not so robust in the sense that about half of its significant events are classified as the ENSO‐related case, whereas its decadal component is coupled to a sea surface temperature anomaly (SSTA) in the central Pacific with its equatorial core near the Niño‐4 region, which is flanked by an opposite SSTA on both sides along the latitudinal belt. Interestingly, this SSTA pattern resembles that related to the ENSO Modoki, and equivalently, the transitional phase of ENSO. The ENSO Modoki is regionally manifested by the anomalous zonal SST gradient between the equatorial western and central Pacific; thus the decadal modulation of this anomalous zonal SST gradient can be regarded as an actual forcing that forms and modulates the decadal PS‐nSCS oscillation. In addition, the decadal PS‐nSCS oscillation is compared with the decadal basin‐wide TC genesis variation in terms of the Pacific SSTA pattern and the associated anomalous large‐scale environments.

A Monsoon-Like Southwest Australian Circulation and Its Relation with Rainfall in Southwest Western Australia

Abstract Using the NCEP–NCAR reanalysis, the 40-yr ECMWF Re-Analysis (ERA-40), and precipitation data from the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) and the Australian Bureau of Meteorology, the variability and circulation features influencing southwest Western Australia (SWWA) winter rainfall are investigated. It is found that the climate of southwest Australia bears a strong seasonality in the annual cycle and exhibits a monsoon-like atmospheric circulation, which is called the southwest Australian circulation (SWAC) because of its several distinct features characterizing a monsoonal circulation: the seasonal reversal of winds, alternate wet and dry seasons, and an evident land–sea thermal contrast. The seasonal march of the SWAC in extended winter (May–October) is demonstrated by pentad data. An index based on the dynamics’ normalized seasonality was introduced to describe the behavior and variation of the winter SWAC. It is found that the winter rainfall over SWWA has a significant positive correlation with the SWAC index in both early (May–July) and late (August–October) winter. In weaker winter SWAC years, there is an anticyclonic anomaly over the southern Indian Ocean resulting in weaker westerlies and northerlies, which are not favorable for more rainfall over SWWA, and the opposite combination is true in the stronger winter SWAC years. The SWAC explains not only a large portion of the interannual variability of SWWA rainfall in both early and late winter but also the long-term drying trend over SWWA in early winter. The well-coupled SWAC–SWWA rainfall relationship seems to be largely independent of the well-known effects of large-scale atmospheric circulations such as the southern annular mode (SAM), El Niño–Southern Oscillation (ENSO), Indian Ocean dipole (IOD), and ENSO Modoki (EM). The result offers qualified support for the argument that the monsoon-like circulation may contribute to the rainfall decline in early winter over SWWA. The external forcing of the SWAC is also explored in this study.

Different impacts of two kinds of Pacific Ocean warming on tropical cyclone frequency over the western North Pacific

The present study examines the relationship of ENSO Modoki and canonical ENSO, respectively, with the tropical cyclone (TC) frequency over the western North Pacific (WNP) for the period 1960–2008. The TC frequency is significantly positively correlated with ENSO Modoki index. The Niño‐3 index has a markedly negative (positive) correlation with the TC frequency in the northern (southeastern) portion of the WNP. In response to heating source related to El Niño Modoki, a large‐scale cyclonic anomaly forms over the WNP. In contrast, during the canonical El Niño years, zonally‐elongated heating source and sink exhibit a meridional dipole pattern, which induces an anticyclonic anomaly in the subtropics and a cyclonic anomaly near the equatorial central Pacific. Numerical experiments under the realistic mean state and heating profiles validate that the anomalous circulation responses to heating play essential roles in different modulations of two kinds of Pacific Ocean warming on the TC frequency.

Rainfall Teleconnections with Indo-Pacific Variability in the WCRP CMIP3 Models

Abstract The present study assesses the ability of climate models to simulate rainfall teleconnections with the El Niño–Southern Oscillation (ENSO) and the Indian Ocean dipole (IOD). An assessment is provided on 24 climate models that constitute phase 3 of the World Climate Research Programme’s Coupled Model Intercomparison Project (WCRP CMIP3), used in the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC). The strength of the ENSO–rainfall teleconnection, defined as the correlation between rainfall and Niño-3.4, is overwhelmingly controlled by the amplitude of ENSO signals relative to stochastic noise, highlighting the importance of realistically simulating this parameter. Because ENSO influences arise from the movement of convergence zones from their mean positions, the well-known equatorial Pacific climatological sea surface temperature (SST) and ENSO cold tongue anomaly biases lead to systematic errors. The climatological SSTs, which are far too cold along the Pacific equator, lead to a complete “nonresponse to ENSO” along the central and/or eastern equatorial Pacific in the majority of models. ENSO anomalies are also too equatorially confined and extend too far west, with linkages to a weakness in the teleconnection with Hawaii boreal winter rainfall and an inducement of a teleconnection with rainfall over west Papua New Guinea in austral summer. Another consequence of the ENSO cold tongue bias is that the majority of models produce too strong a coherence between SST anomalies in the west, central, and eastern equatorial Pacific. Consequently, the models’ ability in terms of producing differences in the impacts by ENSO from those by ENSO Modoki is reduced. Similarly, the IOD–rainfall teleconnection strengthens with an intensification of the IOD relative to the stochastic noise. A significant relationship exists between intermodel variations of IOD–ENSO coherence and intermodel variations of the ENSO amplitude in a small subset of models in which the ENSO anomaly structure and ENSO signal transmission to the Indian Ocean are better simulated. However, using all but one model (defined as an outlier) there is no systematic linkage between ENSO amplitude and IOD–ENSO coherence. Indeed, the majority of models produce an ENSO–IOD coherence lower than the observed, supporting the notion that the Indian Ocean has the ability to generate independent variability and that ENSO is not the only trigger of the IOD. Although models with a stronger IOD amplitude and rainfall teleconnection tend to have a greater ENSO amplitude, there is no causal relationship; instead this feature reflects a commensurate strength of the Bjerknes feedback in both the Indian and Pacific Oceans.

ENSO Modoki impact on the Southern Hemisphere storm track activity during extended austral winter

Impacts of the recently discovered ENSO Modoki phenomena on extended winter storm track activity in the Southern Hemisphere are examined using the observed rainfall, sea surface temperature, and reanalyzed upper air circulation data for the period 1979–2004. The partial correlation technique is utilized to distinguish the impact of ENSO Modoki events from those of the ENSO and Indian Ocean Dipole (IOD). El Niño Modoki events introduce an anomalous blocking over central eastern Australia, which suppresses the storm track activity from southwest till centraleast, reducing the storm‐associated rainfall in southeastern tip and portions of the southeast. On the other hand, the storm track activity in central Argentina is enhanced owing to the strengthened upper air westerlies in this region. The impacts from the ENSO Modoki events are apparently stronger than the individual impacts from the ENSO and IOD events.

La Niña Modoki impacts Australia autumn rainfall variability

Both El Niño‐Southern Oscillation (ENSO) and ENSO Modoki affect Australian rainfall but the commonalities and contrasts of their impacts have not been fully explored. We show that both types feature a strong asymmetry between impacts of La Niña and El Niño in austral autumn (March–May); the La Niña‐Australian rainfall teleconnection is statistically significant, whereas the El Niño‐Australian rainfall relationship is not. A La Niña Modoki cold anomaly near the Dateline is effective in shifting convection westward, causing an autumn rainfall increase over northwestern Australia extending to the northern Murray‐Darling Basin, rather than over the east as in a conventional La Niña. During an El Niño Modoki, the tendency for lower Australian rainfall is far weaker. The asymmetry explains the strong inter‐ENSO variations in rainfall anomalies, including 1983, when a strong El Niño residual was associated with a wet autumn. Our results highlight the importance of considering the influence from La Niña Modoki in predicting ENSO's impacts.

El Niño Modoki and its possible teleconnection

Using observed data sets mainly for the period 1979–2005, we find that anomalous warming events different from conventional El Niño events occur in the central equatorial Pacific. This unique warming in the central equatorial Pacific associated with a horseshoe pattern is flanked by a colder sea surface temperature anomaly (SSTA) on both sides along the equator. empirical orthogonal function (EOF) analysis of monthly tropical Pacific SSTA shows that these events are represented by the second mode that explains 12% of the variance. Since a majority of such events are not part of El Niño evolution, the phenomenon is named as El Niño Modoki (pseudo‐El Niño) (“Modoki” is a classical Japanese word, which means “a similar but different thing”). The El Niño Modoki involves ocean‐atmosphere coupled processes which include a unique tripolar sea level pressure pattern during the evolution, analogous to the Southern Oscillation in the case of El Niño. Hence the total entity is named as El Niño–Southern Oscillation (ENSO) Modoki. The ENSO Modoki events significantly influence the temperature and precipitation over many parts of the globe. Depending on the season, the impacts over regions such as the Far East including Japan, New Zealand, western coast of United States, etc., are opposite to those of the conventional ENSO. The difference maps between the two periods of 1979–2004 and 1958–1978 for various oceanic/atmospheric variables suggest that the recent weakening of equatorial easterlies related to weakened zonal sea surface temperature gradient led to more flattening of the thermocline. This appears to be a cause of more frequent and persistent occurrence of the ENSO Modoki event during recent decades.