Canonical El Nino-Southern Oscillation Research Articles

Recent changes in the major modes of Asian summer monsoon rainfall: influence of ENSO-IOD relationship

Indo-Pacific Ocean sea surface temperature (SST) anomalies in the form of Indian Ocean Dipole (IOD) and El Nino-Southern Oscillation (ENSO) have a major role in determining the climate of the tropics and extratropics. The tropical Pacific variability was dominated by canonical ENSO (EOF1) and El Nino Modoki (EOF2) during the 1980–1998 period (period 1), while during the 1999–2017 period (period 2), both the flavors of ENSO contributed to EOF1. This Pacific Ocean variability weakened the ENSO-IOD relationship also. The present study estimates the changes that occurred to the major modes of Asian summer monsoon (ASM) rainfall during these two periods and addresses the role of the recent changes in SST boundary forcing in ASM interannual variability. During period 1, the first mode (EOF1) of ASM rainfall was associated with ENSO and IOD with 32% variability. But after 1998, EOF1 depends only on the ENSO flavors (canonical and Modoki ENSO) and has slightly reduced variability (26%). The second mode was controlled by the decay phase of ENSO during the 1980–1998 period (12%), while the eastern Indian Ocean and central-northeast Pacific SST variability contributed to EOF2 (15%) during period 2. EOF3 is controlled by IOD in both periods. Further analysis indicates that the extended warming pattern from central to eastern Pacific of ENSO during period 2 weakened ENSO-IOD co-occurrence through the modification of tropical circulation. In the absence of IOD, ENSO-monsoon teleconnection becomes stronger during period 2 than in the previous period. Indian summer monsoon rainfall (ISMR) relationship with the developing phase of ENSO became stronger after 1998, and that of the decay phase of ENSO is decreased. The ISMR-ENSO Modoki relationship has a phase shift from summer maxima to winter maxima. Meanwhile, the ISMR-IOD relationship in the absence of ENSO became slightly stronger in the 1999–2017 period. Thus, the Pacific variability in these two periods influenced ASM rainfall variability through the modulation of its relationship with tropical Indo-Pacific SST anomalies.

An analysis of high cloud variability: imprints from the El Niño–Southern Oscillation

Using data from the International Satellite Cloud Climatology Project (ISCCP), we examine how near-global (60°N–60°S) high cloud fraction varies over time in the past three decades. Our focus is on identifying dominant modes of variability and associated spatial patterns, and how they are related to sea surface temperature. By performing the principal component analysis, we find that the first two principal modes of high cloud distribution show strong imprints of the two types of El Nino–Southern Oscillation (ENSO)—the canonical ENSO and the ENSO Modoki. Comparisons between ISCCP data and 14 models from the Atmospheric Model Intercomparison Project Phase 5 (AMIP5) show that models simulate the spatial pattern and the temporal variations of high cloud fraction associated with the canonical ENSO very well but the magnitudes of the canonical ENSO vary among the models. Furthermore, the multi-model mean of the second principal mode in the AMIP5 simulations appears to capture the temporal behavior of the second mode but individual AMIP5 models show large discrepancies in capturing observed temporal variations. A new metric, defined by the relative variances of the first two principal components, suggests that most of the AMIP5 models overestimate the second principal mode of high clouds.

Decadal and interannual variability of the Indian Ocean SST

The variability of the Indian Ocean on interannual and decadal timescales is investigated in observations, coupled model simulation and model experiment. The Indian Ocean Dipole (IOD) mode was specifically analyzed using a data-adaptive method. This study reveals one decadal mode and two interannual modes in the sea surface temperature (SST) of the IOD. The decadal mode in the IOD is associated with the Pacific Decadal Oscillation (PDO) of the North Pacific SST. The two interannual modes are related to the biennial and canonical components of El Nino-Southern Oscillation (ENSO), consistent with previous studies. This study hypothesizes that the relation between the Indian Ocean and the North Pacific on decadal scale may be through the northerly winds from the western North Pacific. The long simulation of Community Climate System Model version 4 also indicates the presence of IOD modes associated with the decadal PDO and canonical ENSO modes. However, the model fails to simulate the biennial ENSO mode in the Indian Ocean. The relation between the Indian Ocean and North Pacific Ocean is further supported by the regionally de-coupled model experiment.

Possible North Atlantic origin for changes in ENSO properties during the 1970s

The most intense El Nino episodes in more than a century occurred after the 1970s climate shift. Previous studies show that the characteristics of the El Nino-Southern Oscillation (ENSO) phenomenon changed synchronously with the shift, but the associated causes are not fully understood. An analysis of the observed tropical Pacific sea surface temperature (SST) anomalies shows that their increase in the eastern part of the basin after the 1970s is not related to the canonical ENSO pattern, but to the tropical Pacific meridional mode (TPMM). We present observational evidence which supports the hypothesis that the change in the TPMM was triggered by the great salinity anomaly (GSA), which manifested in the North Atlantic during the late 1960s. The GSA induced a weak Labrador convection and a SST dipole south of Greenland. The associated atmospheric structure includes a North Pacific Oscillation sea level pressure dipole in the Pacific sector. This excites the TPMM which contributes to the intense El Nino events and to the enhanced ENSO’s asymmetry, observed after the shift. Our results imply that, if the GSA has not an anthropic origin, as was suggested, then the tropical Pacific climate shift has a natural origin. This is supported by the end of the North Atlantic regime in the 1990s and by the rebound of the tropical Pacific after 1998.

Interdecadal change of the controlling mechanisms for East Asian early summer rainfall variation around the mid-1990s

East Asian (EA) summer monsoon shows considerable differences in the mean state and principal modes of interannual variation between early summer (May–June, MJ) and late summer (July–August, JA). The present study focuses on the early summer (MJ) precipitation variability. We find that the interannual variation of the MJ precipitation and the processes controlling the variation have been changed abruptly around the mid-1990s. The rainfall anomaly represented by the leading empirical orthogonal function has changed from a dipole-like pattern in pre-95 epoch (1979–1994) to a tripole-like pattern in post-95 epoch (1995–2010); the prevailing period of the corresponding principal component has also changed from 3–5 to 2–3 years. These changes are concurrent with the changes of the corresponding El Nino-Southern Oscillation (ENSO) evolutions. During the pre-95 epoch, the MJ EA rainfall anomaly is coupled to a slow decay of canonical ENSO events signified by an eastern Pacific warming, which induces a dipole rainfall feature over EA. On the other hand, during the post-95 epoch the anomalous MJ EA rainfall is significantly linked to a rapid decay of a central Pacific warming and a distinct tripolar sea surface temperature (SST) in North Atlantic. The central Pacific warming-induced Philippine Sea anticyclone induces an increased rainfall in southern China and decreased rainfall in central eastern China. The North Atlantic Oscillation-related tripolar North Atlantic SST anomaly induces a wave train that is responsible for the increase northern EA rainfall. Those two impacts form the tripole-like rainfall pattern over EA. Understanding such changes is important for improving seasonal to decadal predictions and long-term climate change in EA.

Interannual and interdecadal variability of ocean temperature along the equatorial Pacific in conjunction with ENSO

In this paper, the leading modes of ocean temperature anomalies (OTA) along the equatorial Pacific Ocean are analyzed and their connection with El Nino-Southern Oscillation (ENSO) and interdecadal variation is investigated. The first two leading modes of OTA are connected with the different phases of the canonical ENSO and display asymmetric features of ENSO evolution. The third leading mode depicts a tripole pattern with opposite variation of OTA above the thermocline in the central Pacific to that along the thermocline in the eastern and western Pacific. This mode is found to be associated with so-called ENSO-Modoki. Insignificant correlations of this mode with the first two leading modes suggest that ENSO-Modoki may be a mode that is independent to the canonical ENSO and also has longer time scales compared with the canonical ENSO. The fourth mode reflects a warming (cooling) tendency above (below) the thermocline since 2000. Both the first and second modes have a large contribution to the interdecadal change in thermocline during 1979–2012. Also, the analysis also documents that both ENSO and OTA shifted into higher frequency since 2000 compared with that during 1979–1999. Interestingly, the ENSO-Modoki related OTA mode does not have any trend or significant interdecadal shift during 1979–2012. In addition, it is shown that first four EOF modes seem robust before and after 1999/2000, suggesting that the interdecadal shift of the climate system in the tropical Pacific is mainly a frequency shift and the changes in spatial pattern are relatively small, although the mean states over two periods experienced some significant changes.

ENSO Regime Change since the Late 1970s as Manifested by Two Types of ENSO

During the late 1970s, the El Nino-Southern Oscillation (ENSO) experienced a notable regime change, manifested by a change in amplitude, dominant ENSO period, and sea surface temperature anomaly (SSTA) propagation characteristics. The present study shows that these features of the ENSO regime change are associated with property changes of the canonical ENSO, i.e., cold-tongue (CT) type ENSO. Another signature of the ENSO regime change is manifested in the frequent occurrence of a warm-pool (WP) type ENSO that accompanies SSTAs centered over the central Pacific near the WP edge and exhibits characteristics differing from those of the CT ENSO. The distinct manifestations of the two types of ENSO detected in this ENSO regime change are clearly identifiable with the removal of the strong background decadal signal. Since the late 1970s, the WP ENSO has featured a weak eastward (westward) propagation of the SSTA center in the developing (decaying) phase, which makes no net contribution to the observed eastward propagation, and a 2 3y r period compared toth e 4 5 yr period of the CT ENSO. Observations strongly suggest that the WP and CT ENSO are independent quasi-biennial and quasi-quadrennial modes, respectively, of the tropical Pacific climate variability. Our observations also suggest that these two ENSO modes have coexisted actively since the late 1970s when either El Nino or La Nina can be separated into the two types.

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.

The relationship between the canonical ENSO and the phase transition of the Antarctic oscillation at the quasi-quadrennial timescale

A correlation analysis is performed to investigate the relationship between El Nino-Southern Oscillation (ENSO) and the Antarctic oscillation (AAO) at the quasi-quadrennial (QQ) timescale. It is found that the cold tongue index (CTI) and the AAO index (AAOI) are negatively correlated with about a 7-month lead-time, while they are positively correlated with about a 15-month lag-time. To further explore this relationship, complex empirical orthogonal function analysis is employed in the QQ sea level pressure (SLP) anomalies from 1951 to 2002. The results indicate that, during the ENSO cycle, there exists one kind of global tropical wave of wavenumber 1 (GTW1) propagating eastward. With the traveling of GTW1, the tropical SLP anomaly tends to intrude into the southern mid-latitudes. Accordingly, three strong signals travel synchronously along the circum-South-Pacific path, and a relatively weak signal extends eastward and poleward over the South Ocean in the Atlantic-Indian Ocean sector. Following the propagation of these signals, the AAO phase tends to be reversed progressively. As a result, there exists an evident lead-lag correlation between CTI and AAOI. It can be concluded that ENSO plays a key role in the phase transition of AAO at the QQ timescale. It is also noticed that this regular relationship is only evident in the canonical ENSO events, for which sea surface temperature (SST) anomalies extend westward from the tropical eastern Pacific. On the other hand, the similar relationships are not found among those atypical ENSO events for which SST anomalies spread eastward from the central Pacific, such as the 1982–1983 ENSO event.

Eastern Equatorial Pacific SST Variability: ENSO and Non-ENSO Components and Their Climatic Associations

Using an updated Kaplan et al. global SST anomaly (SSTA) dataset (1870-1999) a canonical representation of El Nino-Southern Oscillation (ENSO) is constructed. When this canonical ENSO is subtracted from the data, a residual (non-ENSO) dataset for SSTA is left that includes interseasonal to multidecadal variability. Over the eastern equatorial Pacific (Nino-3) the canonical ENSO accounts for about 79% of the total SSTA variability, and the residual, dominated by decadal timescales, accounts for the rest. In particular, about 40%-50% of the amplitudes of the strong 1982-83 and 1997-98 El Nino events were accounted for by the residual variability. The non-ENSO variability is characterized by the known shift from cold to warm eastern tropical Pacific in the mid- to late 1970s as well as by a (nonstationary) interannual variance increase during the 1980s and 1990s. Composite maps of surface (SST, sea level pressure, and winds) and tropospheric (divergent winds, velocity potential, and vertical velocity) variables are used to compare the spatial patterns characterizing the canonical ENSO and the residual components of the Nino-3 variability. The residual composites are found to only share large-amplitude fluctuations of SST anomalies in the equatorial Pacific east of the date line. When these com- posites are separated into decadal and interannual components, the decadal part closely resembles the structure of the Pacific decadal oscillation. The major patterns of tropospheric variability associated with the ENSO and decadal non-ENSO components are very different. At low latitudes, they imply nearly opposite impacts on far- field regional climates, based on their respective warming (or cooling) phases within the Nino-3 region. This unexpected result for low-latitude climate associations runs contrary to the naive expectation (recently shown to be true for North America) that a decadally warm tropical east Pacific will reinforce the climate effects associated with ENSO alone. This indicates that, in the Tropics, climate outlooks may be more accurate if based on separately analyzed relationships between these SSTA components and their associated climate fluctuations.