El Nino Types Research Articles

Evaluating spatial patterns of Asian meteorological drought variations and associated SST anomalies in CMIP6 models

This study evaluates the spatial patterns of the Asian summer drought variations and the associated sea surface temperature (SST) anomalies in 42 Coupled Model Intercomparison Project (CMIP6) models during 1950–2014. The analysis is focused on the meteorological drought measured using the standardized precipitation index (SPI). The evaluation is conducted for short-term, medium-term, and long-term droughts represented by 3-month, 9-month, and 24-month SPI, respectively. Most of the 42 models are able to capture the observed leading spatial pattern of short-term and medium-term drought variations, characterized by a north-south dipole structure. In contrast, most models fail to simulate the observed leading spatial pattern of long-term drought variations, featuring a southwest-northeast oriented tripole distribution. Further analysis shows that most models can represent the spatial pattern of interannual variation of long-term drought with a north-south dipole structure, but cannot produce the spatial pattern of interdecadal variation and trend of long-term drought. In most of the models, the dipole pattern of short-term and medium-term drought variations is associated with an El Nino–type SST anomaly pattern in the tropical Indo-Pacific region, which is similar to the observations, so is the dipole pattern of interannual variation of long-term droughts. This is attributed to the ability of most models to capture the tropica Indo-Pacific SST-related large-scale atmospheric circulation anomaly pattern.

A study of climate model responses of the western Pacific subtropical high to El Niño diversity

This study examines a climate model hindcast of the responses of the western Pacific subtropical high (WPSH) to three types of El Nino events: the Eastern Pacific (EP) El Nino and the types I and II of Central Pacific (CP-I and CP-II) El Nino. These El Nino types differ from each other in the central locations and patterns of their sea surface temperature (SST) anomalies. They invoke four different mechanisms to impact the WPSH. Hindcasts produced by the Taiwan Central Weather Bureau (CWB) Climate Forecast System 1-tiered model (TCWB1T1) are analyzed. These hindcasts realistically simulate the impacts on the WPSH during the CP-I El Nino, but overestimate the impacts during the decaying summer of the EP El Nino and during the developing autumn and early winter of the CP-II El Nino. The overestimates are mainly the result of an overly strong Maritime Continent regional circulation mechanism during EP El Ninos and an overly strong Indian Ocean capacitor mechanism during CP-II El Ninos. Further analyses show that these model biases are related to biases in the simulations of El Nino SST anomalies and the Walker circulation. Both model deficiencies are common to contemporary coupled climate models.

Effects of the Pacific Decadal Oscillation on the Characteristics of Two Types of El Niño under Possible Climate Change

The effect of the Pacific Decadal Oscillation (PDO) phase on the intensity and localization of anomalies in sea surface temperature and meteorological parameters associated with two types of El Nino is analyzed. The analysis is based on reanalysis and CMIP5 model data for modern and future climate (RCP 8.5 scenario). It is found that El Nino does not exhibit robust intensification during the positive PDO phase, as mentioned in the previous studies, but the PDO influence is highly dependent on the El Nino type. The canonical El Nino is intensified during the positive PDO phase, while, on the contrary, El Nino Modoki weakens. In future climate the interaction between PDO and El Nino is modified, but the significant intermodel spread does not allow an unambiguous determination of trends.

Intensifying effects of El Niño events on winter precipitation extremes in southeastern China

This paper studies the impacts of El Nino-Southern Oscillation (ENSO) on wintertime extreme precipitation in China from 1961 to 2017, and possibly different influences of two El Nino types are also examined. We find that ENSO poses profound influences on extreme precipitation in many portions of China. El Nino (La Nina) intensifies (weakens) the winter precipitation extremes in southeastern China (SEC) and slightly weakens (intensifies) those in central-north China, mainly by changing the frequency rather than the intensity of extreme precipitation. Further analysis suggests that the East Asian winter monsoon tends to be weaker during El Nino winters, suppressing the southward invasion of cold dry air. Via the weakened Walker circulation, El Nino also triggers anomalously descending motion and anticyclone over the western North Pacific (WNP), which exhibits southwesterly anomalies over SEC and is thus conducive to the transport of sufficient moisture into that area. The changes over WNP are also accompanied by ascending motions over East Asia via a local meridional circulation alike the Pacific–Japan pattern. Meanwhile, El Nino induces a southward-displaced East Asian jet stream, as characterized by intensified westerly over southern China, corresponding to anomalously ascending motion there. Together, these changes facilitate the updrafts of circulations and the condensation of water vapor, thus increasing the occurrence of extreme precipitation events in SEC. Moreover, we find that different Nino types exhibit distinct subregional influences. In particular, the canonical eastern Pacific El Nino significantly increases the frequency of extreme precipitation in South China, whereas the Modoki central Pacific El Nino mainly impacts that in East China.

ENSO diversity and the recent appearance of Central Pacific ENSO

ENSO diversity refers to the appearance in recent decades of different El Nino types in the tropical Pacific: In addition to the canonical El Nino, the newly discovered type, called Central Pacific (CP) El Nino, has its center of warming shifted more to the central equatorial Pacific. Whether these ENSO types are really distinct or are different manifestations of a continuum is currently under debate. The mechanisms for the recent appearance of the additional type of ENSO have not been clarified. While previously it was claimed that the increased CP El Nino occurrence in recent decades were due to tropical trade winds weakened by global warming, the observed trade winds actually intensified. We systematically study the variability of the Pacific sea-surface temperature (SST) using the new method of Rotated Principal Component Analysis (RPCA). The essential statistical characteristics of these two El Nino types are found to be describable by only three fundamental modes of pan-Pacific SST with different proportions: the ENSO-cycle, the pan-Pacific counterpart of the regionally defined Pacific Decadal Oscillation (PDO), and a new climate mode we call the Central Pacific Variability (CPV). It is shown that CP El Nino can be described by the PDO and the CPV. The latter has a horseshoe-shaped warm SST pattern in the Central Pacific flanked by cold SST to the east, roughly similar to the original definition of ENSO Modoki by Ashok et al. (J Geophys Res Oceans 112:C11007, 2007) as the second Empirical Orthogonal Function in a narrow tropical domain decomposition. The Principal Component (PC) of this mode, called Modoki PC (MPC) was thought to characterize the occurrence of this new phenomenon, but it has been shown to be unable to separate it from the canonical ENSO. In contrast, we demonstrate that Central Pacific El Nino occurs when the CPV index is larger than 0.5. The intensity of the CPV mode has increased dramatically since 1970s, likely as a result of the intensification of the easterly trade winds in the tropical Pacific, which tilts the thermocline along the tropical Pacific and raises the SST in the central equatorial Pacific. Moderate El Ninos are more prone to stalling in the Central Pacific, becoming CP El Ninos.

Decadal modulation of the relationship between intraseasonal tropical variability and ENSO

The El Nino Southern Oscillation (ENSO) amplitude is modulated at decadal timescales, which, over the last decades, has been related to the low-frequency changes in the frequency of occurrence of the two types of El Nino events, that is the Eastern Pacific (EP) and Central Pacific (CP) El Ninos. Meanwhile ENSO is tightly linked to the intraseasonal tropical variability (ITV) that is generally enhanced prior to El Nino development and can act as a trigger of the event. Here we revisit the ITV/ENSO relationship taking into account changes in ENSO properties over the last six decades. The focus is on two main components of ITV, the Madden–Julian Oscillation (MJO) and convectively coupled equatorial Rossby waves (ER). We show that the ITV/ENSO relationship exhibits a decadal modulation that is not related in a straight-forward manner to the change in occurrence of El Nino types and Pacific decadal modes. While enhanced MJO activity associated to EP El Nino development mostly took place over the period 1985–2000, the ER activity is enhanced prior to El Nino development over the whole period with a tendency to relate more to CP El Nino than to EP El Nino. In particular the relationship between ER activity and ENSO was particularly strong for the period 2000–2015, which results in a significant positive long-term trend of the predictive value of ER activity. The statistics of the MJO and ER activity is consistent with the hypothesis that they can be considered a state-dependent noise for ENSO linked to distinct lower frequency climate modes.

The carbon cycle response to two El Nino types: an observational study

We analyze monthly tropical near surface air temperature and Mauna Loa Observatory carbon dioxide (CO2) data within 1960–2016 to identify different carbon cycle responses for two El Nino types: El Ninos originating in the central tropical Pacific (CP El Nino) and El Ninos originating in the eastern tropical Pacific (EP El Nino). We find significant differences between the two types of El Nino events with respect to time delay of the CO2 rise rate that follows the increase in tropical near surface air temperatures caused by El Nino events. The average time lag of the CP El Nino is 4.0 ± 1.7 months, while the mean time lag of EP El Nino is found to be 8.5 ± 2.3 months. The average lag of all considered 1960–2016 El Ninos is 5.2 ± 2.7 months. In contrast the sensitivity of the CO2 growth rate to tropical near surface air temperature increase is determined to be about the same for both El Nino types equal to 2.8 ± 0.9 ppm yr−1 K−1 (or 5.9 ± 1.9 GtC yr−1 K−1). Our results should be useful for the understanding of the carbon cycle and constraining it in climate models.

Predictability of two types of El Niño and their climate impacts in boreal spring to summer in coupled models

The predictability of the two El Nino types and their different impacts on the East Asian climate from boreal spring to summer have been studied, based on coupled general circulation models (CGCM) simulations from the APEC Climate Center (APCC) multi-model ensemble (MME) hindcast experiments. It was found that both the spatial pattern and temporal persistence of canonical (eastern Pacific type) El Nino sea surface temperature (SST) are much better simulated than those for El Nino Modoki (central Pacific type). In particular, most models tend to have El Nino Modoki events that decay too quickly, in comparison to those observed. The ability of these models in distinguishing between the two types of ENSO has also been assessed. Based on the MME average, the two ENSO types become less and less differentiated in the model environment as the forecast leadtime increases. Regarding the climate impact of ENSO, in spring during canonical El Nino, coupled models can reasonably capture the anomalous low-level anticyclone over the western north Pacific (WNP)/Philippine Sea area, as well as rainfall over coastal East Asia. However, most models have difficulties in predicting the springtime dry signal over Indochina to South China Sea (SCS) when El Nino Modoki occurs. This is related to the location of the simulated anomalous anticyclone in this region, which is displaced eastward over SCS relative to the observed. In boreal summer, coupled models still exhibit some skills in predicting the East Asian rainfall during canonical El Nino, but not for El Nino Modoki. Overall, models’ performance in spring to summer precipitation forecasts is dictated by their ability in capturing the low-level anticyclonic feature over the WNP/SCS area. The latter in turn is likely to be affected by the realism of the time mean monsoon circulation in models.

Современная классификация Эль-Ниньо и сопоставление соответствующих климатических откликов в Атлантико-Евразийском регионе

Comparative study of El Nino classification after different authors results and approaches. The preferences of objective spatio-temporal classification which done earlier by the authors of present paper were shown for climate manifestation study over the Atlanic-Eurasian region. Using of NCEP/NCAR reanalysis data on sea level pressure in 1948-2016 the El-Nino types manifestations were estimated in Azor high, Iceland low and Siberian anticyclone. On this basis, appropriate prognostic estimates of typical climate anomalies in the Atlantic-Eurasian region are made. Next, the previous predictions of typical climate anomalies in the Atlantic-Eurasian region associated with El Nino types were done in the paper.

Analyzing the influences of two types of El Niño on tropical cyclone genesis with a modified genesis potential index

To understand the impacts of large-scale circulation during the evolution of El Nino cycle on tropical cyclones (TC) is important and useful for TC forecast. Based on best-track data from the Joint Typhoon Warning Center and reanalysis data from National Centers for Environmental Prediction for the period 1975-2014, we investigated the influences of two types of El Nino, the eastern Pacific El Nino (EP-El Nino) and central Pacific El Nino (CP-El Nino), on global TC genesis. We also examined how various environmental factors contribute to these influences using a modified genesis potential index (MGPI). The composites reproduced for two types of El Nino, from their developing to decaying phases, were able to qualitatively replicate observed cyclogenesis in several basins except for the Arabian Sea. Certain factors of MGPI with more influence than others in various regions are identified. Over the western North Pacific, five variables were all important in the two El Nino types during developing summer (July-August-September) and fall (OctoberNovember-December), and decaying spring (April-May-June) and summer. In the eastern Pacific, vertical shear and relative vorticity are the crucial factors for the two types of El Nino during developing and decaying summers. In the Atlantic, vertical shear, potential intensity and relative humidity are important for the opposite variation of EP- and CP-El Ninos during decaying summers. In the Southern Hemisphere, the five variables have varying contributions to TC genesis variation during peak season (January-February-March) for the two types of El Nino. In the Bay of Bengal, relative vorticity, humidity and omega may be responsible for clearly reduced TC genesis during developing fall for the two types and slightly suppressed TC cyclogenesis during EP-El Nino decaying spring. In the Arabian Sea, the EP-El Nino generates a slightly positive anomaly of TC genesis during developing falls and decaying springs, but the MGPI failed to capture this variation.

Blocking Anticyclones in the European Region and Their Variability Associated with El Nino Events

Parameters of blocking anticyclones (their number, duration and frequency) in the European region, calculated by NCEP/NCAR re-analysis data by seasons for 1948–2012, were analyzed. It is shown that the maximal intensity of blocking anticyclones are observed mainly in winter and spring, and the minimum intensity – in autumn. Their linear trends in 1948–2012 are characterized by positive sign in winterspring period and negative in summer. Change of parameters of blocking anticyclones and air temperature anomalies in the European region associated with two El Nino types (differentiated by spatial and temporal dynamics) are analyzed. Comparison of analyzed El Nino types shows that their manifestations are different significantly by the number of blocking events in autumn-winter, and by the sign of air temperature anomalies in spring-summer

Impacts of variations in the strength and structure of El Niño events on Pacific rainfall in CMIP5 models

El Nino-Southern Oscillation (ENSO) plays a dominant role in interannual climate variability in Pacific island countries, directly affecting lives there. Many countries show different rainfall responses depending on the sea surface temperature (SST) structure of different types of El Nino events. El Nino events are classified into three types based on previous studies: those with strongest SST anomalies in the eastern Pacific Cold Tongue region (CTE), in the Western Pacific Warm Pool region (WPE), and those in between, a “Mixed” El Nino (MxE), and results from 30 CMIP5 models are investigated. These models accurately reproduce observed SST and precipitation anomalies for the three El Nino types and La Nina. CMIP5 models simulate much larger ranges in the strength of ENSO events than observed. Results clarify the roles of both the different structures of El Nino SST anomalies and their magnitudes on rainfall in the Pacific, and demonstrate that each of the three El Nino types has different impacts on rainfall in the region. These impacts vary with location, with WPE and CTE producing very different impacts in most Pacific island countries. There is a linear intensification of both the mean and maximum rainfall anomalies in the equatorial Pacific as the events become stronger. Equatorial rainfall shifts eastward in CTE and MxE, westward in La Nina. Both the South Pacific and Intertropical Convergence Zones (SPCZ and ITCZ) shift equatorward in El Nino and poleward in La Nina, the shifts increasing as events strengthen. WPE show different behaviour to other events, with little east-west shift in equatorial rainfall, and the orientation angle of the convergence zones increases. Identification of models with no erroneous westward bias in SST anomalies has clarified the effect of strong CTE events producing “zonal” SPCZ and shifting rainfall away to the east from western equatorial countries.

Altered atmospheric responses to eastern Pacific and central Pacific El Niños over the North Atlantic region due to stratospheric interference

The two types of El Nino that have been identified, namely the eastern Pacific (EP) and central Pacific (CP) El Ninos, are known to exert different climatic impacts on the North Atlantic region during winter. Here, we investigate the characteristics of the teleconnection of the two El Nino types with a focus on the stratosphere-troposphere coupling. During the EP El Nino, polar stratospheric warming and polar vortex weakening frequently occur with a strong tendency for downward propagation near the tropopause. Consequently, the atmospheric pattern within the troposphere over the North Atlantic sector during midwinter closely resembles the negative North Atlantic Oscillation pattern. In contrast, during CP El Nino events stratospheric warming events exhibit a much weaker downward propagation tendency. This difference in the stratospheric circulation response arises from the different seasonal evolution of the tropospheric wave response to the two El Nino types. For the EP El Nino, the Aleutian Low begins growing during December and is sustained throughout the entire winter (December to February), which provides favorable conditions for the continuous downward propagation of the stratospheric warming. We also discuss the origin of the difference in the teleconnections from the two types of El Nino associated with the distinct longitudinal position of the warm SST anomaly that determines troposphere-stratosphere coupling.

The role of mean state on changes in El Niño’s flavor

Recently, many studies have argued for the existence of two types of El Nino phenomena based on different spatial distributions: the conventional El Nino [or Eastern Pacific (EP) El Nino], and the Central Pacific (CP) El Nino. Here, we investigate the decadal modulation of CP El Nino occurrences using a long-term coupled general circulation model simulation, focusing, in particular, on the role of climate state in the regime change between more and fewer CP El Nino events. The higher occurrence regime of the CP El Nino coincides with the lower occurrence regime of EP El Nino, and vice versa. The climate states associated with these two opposite regimes resemble the leading principal component analysis (PCA) modes of tropical Pacific decadal variability, indicating that decadal change in climate state may lead to regime change in terms of two different types of El Nino. In particular, the higher occurrence regime of CP El Nino is associated with a strong zonal gradient of mean surface temperature in the equatorial Pacific, along with a strong equatorial Trade wind over the area east of the dateline. In addition, the oceanic variables—the mixed layer depth and the thermocline depth—show values indicating increased depth over the western-to-central Pacific. The aforementioned climate states obviously intensify zonal advective feedback, which promotes increased generation of the CP El Nino. Frequent CP El Nino occurrences are not fully described by oceanic subsurface dynamics, and dynamical or thermodynamical processes in the ocean mixed layer and air–sea interaction are important contributors to the generation of the CP El Nino. Furthermore, the atmospheric response with respect to the SSTA tends to move toward the west, which leads to a weak air–sea coupling over the eastern Pacific. These features could be regarded as evidence that the climate state can provide a selection mechanism of the El Nino type.

Modeling of El Niño events using a simple model

In this paper, we present the results of modeling of El Nino events using a simple model of a classical oscillator with decay under external forcing. The sea surface temperature in the East Pacific and the mean thermocline depth in the Equatorial Pacific correspond to the roles of the momentum and position, respectively. The external forcing of the system is determined by two factors: the short-period meridional mass fluctuations in the Pacific sector of the Southern Ocean due to the joint effect of the atmospheric variability over the Antarctic Circumpolar Current, bottom topography, and coastlines; and the variability of western winds in the tropics. Under such conditions, oscillations of El Nino type arise in the model as a result of propagation of signals generated in the Southern Ocean (due to the fluctuations of meridional transport fluxes and the variability of western winds in the tropics). These signals propagate over the Equatorial Pacific as fast wave processes. It is shown that external forcing is the main factor in establishing the oscillation pattern of the model characteristics variability.

Numerical simulation by a stretched-coordinate Ocean General Circulation Model: A preliminary results

¶A three-dimensional Ocean General Circulation Model has been developed in stretched coordinate from scratch. The same model has been used to perform some numerical experiments to simulate the basic circulation pattern and the model variability to atmospheric forcing. For numerical simulations 72 × 25 grid points in the horizontal directions and nine (10, 30, 75, 250, 500, 1000, 1500, 2000 and 3000 m) vertical levels are considered. The lateral boundaries are set at 60° N and 60° S. The basic focus of the paper is on the demonstration of the performance of the model and its assessment by employing appropriate forcing from the outputs of an atmospheric general circulation model. Hence, the model was forced with the forcing (wind and thermodynamic) derived from the ECMWF runs from the AMIP archives. The preliminary results show the realistic simulation of basic pattern of different fields. The model simulations show that the model is able to reproduce some of the general features of the ocean, such as surface currents, surface temperature and salinity, mass transport and meridional heat transport. It is also to be noted that the model is capable to capture the El-Nino and La-Nina type events.

METEOROLOGICAL STRUCTURE OF THE 1992/93 DROUGHT OVER EASTERN SOUTH AFRICA FROM ECMWF AND SATELLITE OLR ANALYSES

ABSTRACT An analysis of the 1992/93 drought in eastern South Africa is undertaken for various climatic variables from ECMWF data. Increased westerly winds with surface marine lows and continental highs over southern Africa are prominent features that sustained drought during this period. Anomalous low-level divergence and mid-level subsidence were coupled with reduced tropical moist inflows. An area of reduced water vapour flux extended from 15–33°S and 15–35°E. Anticyclonic vorticity and upper level convergence suppressed convection over most parts of South Africa. Mid-latitude winds played a significant role in emphasising the signature of the drought through a northward (southwards) movement of the subpolar (subtropical) jet streams giving rise to a strongly diffluent kinematic forcing over eastern South Africa. Low-level westerly anomalies and upper-level easterly anomalies were prominent over east Africa. A previous characterisation of the 1992/93 drought at antecedent SON and peak summer season and at a larger spatial domain by Dube and Jury (2001) has indicated that the 1992/93 summer carried a non-ENSO signal though a few parameters analysed showed an El Nino-type of influence. This study is a discussion of the circulation structure specific to the eastern parts of South Africa, particularly KwaZulu-Natal.

Enso and Sunspot Cycles in Varved Eocene Oil Shales from Image Analysis

ABSTRACT On the premise that sequential changes in varve thickness offer a proxy for climatic variations, we investigated varve thickness in three core segments from the distal lacustrine oil shales (Tipton and Laney members) of the Green River Formation, by means of an image analysis program. Of two strong bimodal periodicities one, at 4.8-5.6 years, is interpreted as an El Nino type (ENSO) phenomenon of atmospheric dynamics, while the other, at 10.4-14.7 years, is interpreted as the sunspot cycle, originally recognized in this formation by Bradley (1929, 1931). Weaker periodicities may exist at ca. 8 and 33 years--the latter also recognized by Bradley. Taken in conjunction with the work of Bradley (1929, 1931) and of Crowley et al. (1986), this suggests that some but not all of the oil shale of the Green River Formation is truly varved and can be used to infer climatic time-series. Automated instrumental scans have greatly extended the possibilities of varve studies as approaches to climatic variations in time, but careful selection of facies in their geological framework is necessary.

Possible association of stratospheric aerosols and El Nino type events

To determine whether the 1982/83 El Nino event and the eruption of El Chichon in April 1982 are related, eleven similar eruptions at low latitudes (<20°, tropical) between 1868 and 1980 were analyzed for subsequent warm sea surface temperature anomalies in the eastern tropical Pacific Ocean. The composite result using superposed epoch analysis show a significant warming of the sea surface temperatures in the region 0°‐10°S, 90° ‐180°W. The results are also consistent with the statistical dynamical model calculations of MacCracken and Luther which indicate atmospheric circulation changes similar to those thought to lead to an El Nino type event within a few seasons after the establishment of an El Chichon type aerosol. In addition the composite behavior of twenty eruptions at high latitudes (extratropical, >20°) show just the opposite behavior. They show significant cooling of the sea surface temperatures in this region especially in the fourth and fifth season after the eruption. The association is found to be significant at the 95% level.

A study of the dynamics of drought initiation using a global general circulation model

A global circulation model has been used to explore observed relationships between cold sea surface temperature anomalies in the Australian region and drought over the Australian continent. The model employed a comprehensive physics package, was set up for fixed January conditions, and used climatological sea surface temperatures. Two experiments were performed: one involving a large cold sea surface temperature anomaly located north of Australia but extending well into the Pacific and Indian Oceans, together with a much smaller cold anomaly in the tropical Atlantic; the other added a warm anomaly in the tropical eastern Pacific to the previous anomalies. The overall results were very similar in the two experiments. Droughtlike conditions were produced over parts of Australia, southern Africa, South America, and North America. By relating variations in evaporation and precipitation rates to changes in the synoptic meridional velocity distribution, a reasonably coherent understanding of the model's responses to the anomalies was obtained. The dominant response was in the Australian region, with secondary responses being produced over southern Africa and South America. This might indicate some form of synchronous behavior over the continents of the southern hemisphere to such sea surface temperature anomalies. The tropical easterlies were enhanced by the SST anomalies, resulting in a single westerly duct remaining between the two hemispheres. Difference charts of the 200‐mbar height field for the anomaly and control experiments revealed a weak wave train that originated in the duct region and propagated to North America, where it apparently caused large changes in the synoptic conditions over North America. Overall the experiments help to establish a cause‐and‐effect relationship between cold sea surface temperature anomalies and drought in the adjacent Australian region together with a re‐inforcement of these effects when an El Nino type warm anomaly is located in the east Pacific. Nevertheless, the need is highlighted for much further research before the requirements for forecasting drought can be assessed.