Subtle but important differences are identified between the 1997/1998 and 2015/2016 extreme El Niños that reflect fundamental differences in their underlying dynamics. The 1997/1998 event is found to ...evolve following the eastern Pacific El Niño dynamics that relies on basin‐wide thermocline variations, whereas the 2015/2016 event involves additionally the central Pacific (CP) El Niño dynamics that depends on subtropical forcing. The stronger CP dynamics during the 2015/2016 event resulted in its sea surface temperature (SST) anomalies lingering around the International Date Line during the decaying phase, which is in contrast to the retreat of the anomalies toward the South American Coast during the decaying phase of the 1997/1998 event. The different SST evolution excited different wave trains resulting in the western U.S. not receiving the same above‐normal rainfall during the 2015/2016 El Niño as it did during the 1997/1998 El Niño. Ensemble model experiments are conducted to confirm the different climate impacts of the two El Niños.
Key Points
The 1997/1998 event is the strongest EP El Niño, while the 2015/2016 event is the strongest mixed EP and CP El Niño ever recorded
The two events exhibit subtle differences in their equatorial SST evolution that reflects fundamental differences in the underlying dynamics
The SST differences led to large differences in tropical convection, resulting in different impacts on North American climate
In this study, we evaluate the intensity of the Central‐Pacific (CP) and Eastern‐Pacific (EP) types of El Niño‐Southern Oscillation (ENSO) simulated in the pre‐industrial, historical, and the ...Representative Concentration Pathways (RCP) 4.5 experiments of the Coupled Model Intercomparison Project Phase 5 (CMIP5). Compared to the CMIP3 models, the pre‐industrial simulations of the CMIP5 models are found to (1) better simulate the observed spatial patterns of the two types of ENSO and (2) have a significantly smaller inter‐model diversity in ENSO intensities. The decrease in the CMIP5 model discrepancies is particularly obvious in the simulation of the EP ENSO intensity, although it is still more difficult for the models to reproduce the observed EP ENSO intensity than the observed CP ENSO intensity. Ensemble means of the CMIP5 models indicate that the intensity of the CP ENSO increases steadily from the pre‐industrial to the historical and the RCP4.5 simulations, but the intensity of the EP ENSO increases from the pre‐industrial to the historical simulations and then decreases in the RCP4.5 projections. The CP‐to‐EP ENSO intensity ratio, as a result, is almost the same in the pre‐industrial and historical simulations but increases in the RCP4.5 simulation.
Key Points
Smaller inter‐model diversity of ENSO intensities in CMIP5 than in CMIP3
Decrease in the diversity is particularly significant for the simulated EP ENSO
Different response of EP and CP ENSO to global warming
The composite analyses during 1950–2016 show that the impacts of El Niño on the western Pacific subtropical high (WPSH) are different among the Eastern Pacific type, Central Pacific type‐I (CP‐I), ...and Central Pacific type‐II (CP‐II). The three types of El Niño produce distinct impacts on WPSH due to the varying importance of the Northwestern Pacific coupling, Indian Ocean capacitor, and Maritime Continent mechanisms. The different enhancements and cancellations among these three mechanisms are related to differences in SST anomaly locations and Indian Ocean conditions among the El Niño types. The CP‐II El Niño becomes the most influential type of El Niño, while the CP‐I El Niño becomes the least influential type. The different impacts and mechanisms for the CP‐I and CP‐II types of El Niño imply that these two subtypes of CP El Niño may involve different forcing from the Indian Ocean and extratropical Pacific for their generation.
Plain Language Summary
It is now well recognized that there exists a conventional Eastern Pacific type and an emerging Central Pacific type of El Niño. Recent studies have suggested that the CP El Niño should be further separated into two subtypes. It is shown here that these three El Niño–Southern Oscillation types produce distinct impacts on the western Pacific subtropical high (WPSH). The conventional views of El Niño's impacts on the WPSH and their underlying mechanisms need to be revised to take into account of the El Niño diversity. This study also offers new insights into the generation dynamics of the two subtypes of the CP El Niño. We find that the two subtypes of CP El Niño differ not only in their sea surface temperature anomalies in the northeastern Pacific but also in the Indian Ocean.
Key Points
El Niño diversity invokes NW Pacific coupling, Indian Ocean capacitor, and MC mechanism to produce distinct impacts on WPSH
The changing importance of these mechanisms is caused by different SST anomaly in Pacific and Indian Oceans during various El Niño types
The different impacts indicate that CP‐I and CP‐II El Niños are forced by Indian Ocean and extratropical Pacific, respectively
A 2,200‐year CESM1 pre‐industrial simulation is used to contrast Antarctic sea ice concentration (SIC) variations between the first and second austral winters of multi‐year La Niñas. The typical SIC ...anomaly pattern induced by single‐year La Niñas appears only during the second austral winter of multi‐year La Niñas. A similar pattern, but zonally shifted compared to the typical one, is found during the first winter and exhibits a tripolar pattern with anomaly centers over the Ross, Amundsen‐Bellingshausen, and Weddell Seas. The shift is a result of the pre‐onset conditions associated with multi‐year La Niñas that excites unique atmospheric circulation modes during the first winter. The distinct zonally‐shifted SIC anomaly pattern is observed in four of the six multi‐year La Niña events during the period 1979–2020. These results suggest that it is helpful to separate La Niñas into single and multi‐year events to better understand the La Niña impacts on Antarctic climate.
Plain Language Summary
La Niña events are characterized by abnormal cooling of sea surface waters in the tropical Pacific Ocean. Recently, more La Niña events have been observed to persist for multiple year. In this study, we analyze a long‐term climate model simulation to show that multi‐year La Niñas produce a different impact on Antarctic sea ice concentrations (SIC) than single‐year La Niñas. The typical SIC impact produced by single‐year La Niñas appears only during the second austral winter of multi‐year La Niñas. In contrast, during the first winter of multi‐year La Niñas the pattern is shifted westward and is characterized by a distinct tripolar pattern with anomaly centers over the Ross, Amundsen‐Bellingshausen, Weddell Seas. The different impact pattern during the first winter is caused by the different atmospheric wave trains excited by the different Indian Ocean conditions during the first and second winters of multi‐year events. The distinct impacts and impact mechanisms found in the climate model are also found in the observations. Four out of the six multi‐year La Niña events observed during 1979–2020 exhibit this zonally‐shifted SIC anomaly pattern during their first winters. The increasing occurrence of multi‐year La Niña events may affect Antarctic sea ice patterns in new ways.
Key Points
The typical sea ice anomaly pattern appears during the second winter of multi‐year La Niñas but was zonally shifted during the first winter
The shifted pattern results from an unique pre‐onset Indian ocean condition of multi‐year La Niña and atmospheric teleconnections it excites
The different sea ice impacts revealed in the CESM1 simulation appear in four of the six observed multi‐year La Niña events during 1979–2020
The El Niño-Southern Oscillation (ENSO) and the variability in the Pacific subtropical highs (PSHs) have major impacts on social and ecological systems. Here we present an Atlantic capacitor effect ...mechanism to suggest that the Atlantic is a key pacemaker of the biennial variability in the Pacific including that in ENSO and the PSHs during recent decades. The 'charging' (that is, ENSO imprinting the North Tropical Atlantic (NTA) sea surface temperature (SST) via an atmospheric bridge mechanism) and 'discharging' (that is, the NTA SST triggering the following ENSO via a subtropical teleconnection mechanism) processes alternate, generating the biennial rhythmic changes in the Pacific. Since the early 1990s, a warmer Atlantic due to the positive phase of Atlantic multidecadal oscillation and global warming trend has provided more favourable background state for the Atlantic capacitor effect, giving rise to enhanced biennial variability in the Pacific that may increase the occurrence frequency of severe natural hazard events.
This study finds the seasonal footprinting (SF) mechanism to be a key source of El Niño–Southern Oscillation (ENSO) complexity, whereas the charged‐discharged (CD) mechanism acts to reduce ...complexity. The CD mechanism forces El Niño and La Niña to follow each other, resulting in a more cyclic and less complex ENSO evolution, while the SF mechanism involves subtropical forcing and results in an ENSO evolution that is more episodic and irregular. The SF mechanism also has a tendency to produce multiyear La Niña events but not multiyear El Niño events, contributing to El Niño‐La Niña asymmetries. The strength of CD mechanism has been steady, but SF mechanism has intensified during the past two decades, making ENSO more complicated. Most Climate Model Intercomparison Project version 5 models overestimate the strength of the CD mechanism but underestimate the strength of the SF mechanism, causing their simulated ENSOs to be too regular and symmetric.
Plain Language Summary
El Niño–Southern Oscillation (ENSO) is known to have profound climate impacts worldwide, but the causes of its complex behaviors are still not fully understood. In this study, we show that the subtropical atmosphere‐ocean coupled forcing is a key source of ENSO complexity, whereas the tropical ocean heat content variation acts to reduce ENSO complexity. The subtropical forcing also has a tendency to produce multiyear La Niña events but not multiyear El Niño events, contributing to El Niño‐La Niña asymmetries. In contrast to the steady strength of the tropical variation throughout the past six decades, the strength of the subtropical forcing has increased since the early 1990s. This may have made ENSO more complex recently and, if this trend does not reverse, possibly into the coming decades. Contemporary climate models overestimate the strength of the tropical ocean heat content variation but underestimate the strength of the subtropical forcing, which may be a reason why contemporary models produce ENSO behavior that is too regular.
Key Points
The seasonal footprinting (SF) mechanism is a key to ENSO complexity and asymmetry, but the charged‐discharged (CD) mechanism reduces them
The CMIP5 models overestimate (underestimate) the strength of the CD (SF) mechanism, causing their simulated ENSOs to be too regular
The strength of CD mechanism has been steady, but SF mechanism has intensified during the past two decades, making ENSO more complicated
El Niño–Southern Oscillation (ENSO) transitions from one event to another in complex ways. Using observational analyses and forced atmospheric model experiments, we show that a preceding ENSO event ...can activate a subtropical Pacific forcing mechanism to trigger another ENSO event during the following year. These tropical‐subtropical Pacific interactions result in a cyclic ENSO transition if the two ENSO events are of opposite signs or a multiyear ENSO transition if they are of the same sign. The preceding ENSO event should excite deep convections in the tropical Pacific in order to activate the subtropical Pacific mechanism. This requirement enables mean temperatures in the cold tongue and warm pool to respectively control how easily the cyclic and multiyear transitions can occur. A future warmer tropical Pacific is projected to decrease the frequency of occurrence of multiyear ENSO transitions but increase the occurrence of cyclic ENSO transitions.
Plain Language Summary
El Niño–Southern Oscillation (ENSO) is one of the strongest climate variation phenomena in Earth's climate system, causing regional climate extremes and massive ecosystem impacts. An ENSO event can transition from one event to another in complex ways. An El Niño (La Niña) event can be preceded by a La Niña (El Niño) event to become a cyclic ENSO, by a neutral event to become an episodic ENSO, or by another El Niño (La Niña) event to become a multiyear ENSO. The complex nature of ENSO transition challenges our understanding of ENSO dynamics and its future responses to greenhouse warming. Here we show, using observational analyses, climate model simulations, and a novel framework focusing specifically on the onset processes of ENSO, that multiyear ENSO events related to the subtropical forcing are projected to decrease and cyclic ENSO events to increase as the climate warms. These changes in ENSO transition complexity are linked to the warming of the tropical Pacific mean state, which is a key factor controlling ENSO transitions through a series of tropical‐subtropical interactions.
Key Points
An ENSO event can activate a subtropical Pacific mechanism to onset another event resulting in a cyclic or multiyear ENSO transition
Mean SSTs in the cold tongue and warm pool respectively control how easily cyclic and multiyear transitions can occur via this process
A future warming of the tropical Pacific is projected to reduce (increase) the occurrence of the multiyear (cyclic) ENSO transitions
This study examines the linkages between leading patterns of interannual sea level pressure (SLP) variability over the extratropical Pacific (20°–60°N) and the eastern Pacific (EP) and central ...Pacific (CP) types of El Niño–Southern Oscillation (ENSO). The first empirical orthogonal function (EOF) mode of the extratropical SLP anomalies represents variations of the Aleutian low, and the second EOF mode represents the North Pacific Oscillation (NPO) and is characterized by a meridional SLP anomaly dipole with a nodal point near 50°N. It is shown that a fraction of the first SLP mode can be excited by both the EP and CP types of ENSO. The SLP response to the EP type is stronger and more immediate. The tropical–extratropical teleconnection appears to act more slowly for the CP ENSO. During the decay phase of EP events, the associated extratropical SLP anomalies shift from the first SLP mode to the second SLP mode. As the second SLP mode grows, subtropical SST anomalies are induced beneath via surface heat flux anomalies. The SST anomalies persist after the peak in strength of the second SLP mode, likely because of the seasonal footprinting mechanism, and lead to the development of the CP type of ENSO. This study shows that the CP ENSO is an extratropically excited mode of tropical Pacific variability and also suggests that the decay of an EP type of ENSO can lead to the onset of a CP type of ENSO with the aid of the NPO. This extratropical linking mechanism appears to be at work during the 1972, 1982, and 1997 strong El Niño events, which were all EP events and were all followed by strong CP La Niña events after the NPO was excited in the extratropics. This study concludes that extratropical SLP variations play an important role in exciting the CP type of ENSO and in linking the transitions from the EP to CP events.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Surface observations and subsurface ocean assimilation datasets are examined to contrast two distinct types of El Niño–Southern Oscillation (ENSO) in the tropical Pacific: an eastern-Pacific (EP) ...type and a central-Pacific (CP) type. An analysis method combining empirical orthogonal function (EOF) analysis and linear regression is used to separate these two types. Correlation and composite analyses based on the principal components of the EOF were performed to examine the structure, evolution, and teleconnection of these two ENSO types. The EP type of ENSO is found to have its SST anomaly center located in the eastern equatorial Pacific attached to the coast of South America. This type of ENSO is associated with basinwide thermocline and surface wind variations and shows a strong teleconnection with the tropical Indian Ocean. In contrast, the CP type of ENSO has most of its surface wind, SST, and subsurface anomalies confined in the central Pacific and tends to onset, develop, and decay in situ. This type of ENSO appears less related to the thermocline variations and may be influenced more by atmospheric forcing. It has a stronger teleconnection with the southern Indian Ocean. Phase-reversal signatures can be identified in the anomaly evolutions of the EP-ENSO but not for the CP-ENSO. This implies that the CP-ENSO may occur more as events or epochs than as a cycle. The EP-ENSO has experienced a stronger interdecadal change with the dominant period of its SST anomalies shifted from 2 to 4 yr near 1976/77, while the dominant period for the CP-ENSO stayed near the 2-yr band. The different onset times of these two types of ENSO imply that the difference between the EP and CP types of ENSO could be caused by the timing of the mechanisms that trigger the ENSO events.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
In around 1990, significant shifts occurred in the spatial pattern and temporal evolution of the El Niño‐Southern Oscillation (ENSO), with these shifts showing asymmetry between El Niño and La Niña ...phases. El Niño transitioned from the Eastern Pacific (EP) to the Central Pacific (CP) type, while La Niña's multi‐year (MY) events increased. These changes correlated with shifts in ENSO dynamics. Before 1990, El Niño was influenced by the Tropical Pacific (TP) ENSO dynamic, shifting to the Subtropical Pacific (SP) ENSO dynamic afterward, altering its spatial pattern. La Niña was influenced by the SP ENSO dynamic both before and after 1990 and has maintained the CP type. The strengthened SP ENSO dynamic since 1990, accompanied by enhanced precipitation efficiency during La Niña, make it easier for La Niña to transition into MY events. In contrast, there is no observed increase in precipitation efficiency during El Niño.
Plain Language Summary
In this study, we explored changes in the El Niño‐Southern Oscillation (ENSO) phenomenon from 1950 to 2022. We discovered significant shifts in ENSO complexity, particularly after 1990, affecting where ENSO events occur and how long they last. Notably, these changes differed between El Niño and La Niña phases. El Niño's location shifted from the Eastern Pacific to the Central Pacific, while La Niña extended its duration, leading to more multi‐year events. These complexities relate to shifts in El Niño and La Niña dynamics. El Niño changed from a Tropical Pacific dynamic to a Subtropical Pacific dynamic, influencing its shift to the central Pacific. La Niña dynamics remained constant, causing La Niña's central location to remain unchanged. After 1990, the tropical precipitation efficiency showed an asymmetric change between El Niño and La Niña phases. The intensified atmospheric response to La Niña cooling enabled more frequent activations of the SP ENSO dynamic, thus increasing the frequency of multi‐year La Niña. These findings advance our understanding of ENSO and can enhance ENSO prediction.
Key Points
El Niño‐Southern Oscillation complexity underwent significant asymmetric changes around 1990, especially in spatial pattern and temporal evolution
El Niño's pattern shifted from Eastern Pacific to Central Pacific, while La Niña's timing transitioned from single‐year to multi‐year
El Niño's primary dynamic shifted from Tropical Pacific to Subtropical Pacific (SP) whereas La Niña consistently remained in the SP
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