Eastern Pacific El Nino-Southern Oscillation Research Articles

Regional Characteristics of Summer Precipitation Anomalies in the Northeastern Maritime Continent

Based on the monthly mean reanalysis data from NCEP/NCAR (National Centers for Environmental Prediction/ National Center for Atmospheric Research) and GPCP (Global Precipitation Climatology Project) (1979–2020), the regional characteristics of precipitation in the warm pool side of the Maritime Continent (MC) and the relationships between different precipitation patterns and atmospheric circulations are studied. The results show that there are significant correlations as well as differences between the precipitation in the east of the Philippines (area A) and that in the Pacific Ocean near the Northern Mariana Islands (area B). Precipitation in area A is closely related to the eastern Pacific ENSO (El Nino-Southern Oscillation) and EAP/PJ (East Asia-Pacific/Pacific-Japan) teleconnection pattern, while precipitation in area B is linked to the Indian Ocean basin-wide and the South China Sea summer monsoon. When the precipitation anomaly in area A is positive, the East Asian summer monsoon is weak. A cyclone appears to the northwest of area A at 850 hPa with convergence airflow. After filtering out the effects of precipitation in area B, the cyclone retreats to the west, and an anticyclone appears to the southeast of area A. When the precipitation is above normal in area B, the circulation and water vapor transportation are similar to that in area A but more to the east. The updraft and downdrafts to both north and south sides of area B form two closed meridional vertical circulations. When the influence of area A is moved out, the circulation center in the warm pool area moves eastward. This research contributes to a better understanding of the regional characteristics of the Maritime Continent and the East Asian summer monsoon.

Effects of ENSO diversity and cold tongue bias on seasonal prediction of South China late spring rainfall

Seasonal prediction of South China April to May rainfall is examined based on forecasts by the European Centre for Medium-Range Weather Forecasts (ECMWF) operational model. El-Nino Southern Oscillation (ENSO) is a major source of predictability, conveyed by the lower tropospheric anomalous western Pacific anticyclone and cyclone for El-Nino and La-Nina respectively. By separating ENSO into its diversity of eastern Pacific (EP) and central Pacific (CP) ENSO, different effects on South China rainfall are revealed. From observations, while rainfall is enhanced following EP El-Nino and reduced following EP La-Nina, rainfall remains close to climatology following CP ENSO. However, the seasonal forecast model predicts CP ENSO effect on South China rainfall to be similar to EP ENSO. The model develops excessive westward extension of the eastern Pacific cold tongue within seasonal timescale. While model predicts tropical central Pacific anomalous sea surface temperature (SST) following CP ENSO realistically, the colder mean state is proposed to weaken the anomalous convection, which subsequently induces bias in the western Pacific anomalous rotational flow and hinders South China rainfall prediction. Meanwhile following EP ENSO, the colder mean state is proposed to strengthen the thermocline feedback, inducing stronger tropical eastern Pacific anomalous SST. While bias in the western Pacific anomalous rotational flow is also induced, the bias is far away from South China so rainfall prediction is still realistic. This study highlights the importance of model mean state in the fidelity of model ENSO diversity teleconnections on seasonal timescale.

Seasonal linkage of the Southern Hemisphere extratropical climate variability to two types of ENSO

This study uses the Climate Forecast System Reanalysis (CFSR) to investigate the responses of the Southern Hemisphere (SH) extratropical climate to two types of El Nino-Southern Oscillation (ENSO)—the eastern Pacific (EP) type and the central Pacific (CP) type in different seasons. The responses are denoted by the anomalies of climate variables associated with one-standard-deviation increase in the Nino3 or Nino4 index. The results show that in austral spring the differences in the ENSO-related anomaly (ERA) patterns of atmospheric circulation between the EP ENSO period (1979–1998) and CP ENSO period (1999–2010) are mainly associated with the change in the ENSO-PSA2 relationship. Such differences affect the ERA fields of surface air temperature and mixed layer temperature, and finally result in significant differences in sea-ice concentration anomalies in the Atlantic sector. In austral summer, significant correlation exists between the variations of SAM and both of the variations of Nino3 and Nino4 in 1979–1998, while the correlation between SAM and Nino4 disappears in 1999–2010. For all seasons, the strength of the climate ERAs depend on if there are close relationship between ENSO and the major climate variation modes of the SH extratropics. For the climate variables, the ERA patterns of surface air temperature are generally controlled by surface wind anomalies and mirrored by the mixed layer temperature anomalies. The mixed layer depth anomalies are primarily modulated by surface heat flux anomalies and occasionally by anomalous wind. There are strikingly strong anomalies of surface heat flux in the autumn of 1979–1998 related to the Nino3 variation, the period when there is only significant correlation between ENSO and PSA2. There are no evidence that the SH extratropical climate variability induced by Nino3 variations are stronger in the EP-ENSO period, and that variability induced by Nino4 variations are stronger in the CP-ENSO period.

Diagnosing the representation and causes of the ENSO persistence barrier in CMIP5 simulations

In this study, the persistence barrier (PB) of the El Nino–Southern Oscillation (ENSO) phenomenon is investigated using reanalysis data and historical simulations of the Coupled Model Intercomparison Project Phase 5 (CMIP5) models. Both the timing and intensity of the ENSO PB can be quantified using the maximum gradient of autocorrelation decline of Nino sea surface temperature (SST) anomaly indices. Most of the CMIP5 models were found to reasonably reproduce the observed timing of the ENSO PB that typically occurs during the boreal late spring to early summer, and underestimated the PB intensity compared to observations. Furthermore, the PB properties of the Eastern Pacific (EP) ENSO indices were much better represented by the models than those of the Central Pacific (CP) ENSO indices, probably because CP ENSO events are more challenging to simulate than their counterparts. Approximately half of the models can satisfyingly reflect the intensity and timing of PB for indices of EP ENSO and their distinctions from those of the CP ENSO, with a larger uncertainty for the modeled PB timing than intensity. Further diagnosis has revealed the relationship between the ENSO PB intensity and the factors associated with the tropical Pacific background state. The PB intensity exhibits a stronger relationship with the seasonality of the SST amplitude in CP, compared to the SST amplitude, and the strength of seasonal synchronization of EP SST anomalies is highly correlated with the PB intensity. These results suggest that the seasonality of tropical SST variability may fundamentally contribute to the ENSO PB.

Identification Standard for ENSO Events and Its Application to Climate Monitoring and Prediction in China

The El Nino–Southern Oscillation (ENSO) reflects anomalous variations in the sea surface temperature (SST) and atmospheric circulation over the tropical central–eastern Pacific. It remarkably impacts on weather and climate worldwide, so monitoring and prediction of ENSO draw intensive research. However, there is not yet a unique standard internationally for identifying the timing, intensity, and type of ENSO events. The National Climate Center of China Meteorological Administration (NCC/CMA) has led the effort to establish a national identification standard of ENSO events, which was officially endorsed by the National Standardization Administration of China and implemented operationally in NCC/CMA in 2017. In this paper, two key aspects of this standard are introduced. First, the Nino3.4 SST anomaly index, which is well-recognized in the international ENSO research community and used operationally in the US, has replaced the previous Nino Z index and been used to identify the start, end, and peak times, and intensity of ENSO events. Second, two new indices—the eastern Pacific ENSO (EP) index and the central Pacific ENSO (CP) index, based on the SST conditions in Nino3 and Nino4 region respectively, are calculated to first determine the ENSO type before monitoring and assessing the impacts of ENSO on China’s climate. With this standard, all historical ENSO events since 1950 are consistently re-identified; their distinct properties are diagnosed and presented; and the impacts of ENSO events under different types on China’s climate are re-assessed. This standard is also employed to validate the intensity, grade, and type of the ENSO events predicted by the NCC/CMA operational ENSO prediction system. The new standard and the thus derived unified set of re-analyzed historical ENSO events and associated information provide a good reference for better monitoring and prediction of future ENSO events.

Impacts of ENSO diversity on the western Pacific and North Pacific subtropical highs during boreal summer

This study examines the interannual variability of the North Pacific high during boreal summer of 1979–2008 to understand how its leading modes are related to the two types of El Nino–Southern Oscillation (ENSO). In the observations, the first empirical orthogonal function mode (EOF1) is characterized by an in-phase variation between the western Pacific subtropical high (WPSH) and the northeastern Pacific subtropical high (NPSH), while the second mode (EOF2) is characterized by an out-of-phase WPSH–NPSH variation. The EOF1 mode dominates during the post early-1990s period and is a forced response to sea surface temperature (SST) variations over the maritime continent and tropical central Pacific (CP) regions related to developing CP ENSOs. Its in-phase WPSH–NPSH relationship is established through the ENSO-induced meridional atmospheric circulation, Pacific–North American pattern and eddy–zonal flow interaction over the North Pacific. In contrast, the EOF2 mode dominates prior to the early-1990s and is partially a forced response to tropical Indian Ocean (IO) and eastern Pacific (EP) SST variations related to decaying EP ENSOs and partially a coupled atmosphere–ocean response to western North Pacific SST variations. Of the 28 Atmospheric Model Intercomparison Project models, most (71 %) realistically simulate the EOF1 mode but only a few (14 %) simulate the EOF2 mode. The roughly 50 % underestimation in the strength of the EOF2 mode is due to model deficiencies in properly representing the atmospheric circulation responses to the IO and EP SST variations. This deficiency may be related to underestimations of the strength of the mean Walker circulation in the models.

The impact of basic state on quasi-biennial periodicity of central Pacific ENSO over the past decade

El Nino/Southern Oscillation (ENSO) phenomenon lately appears to have a much fast pace with four warming events in the past decade (2002-2012). Three out of four events have their warming centers confined in the equatorial central Pacific. It is argued that the anomalous zonal sea currents are responsible for the fast transition of the central Pacific warming. Furthermore, based on the heat budget analysis, it is found that zonal advective feedback appears to play an important role in the phase transition of central Pacific ENSO, although the thermocline feedback process that is essential for ENSO, as delineated by the recharge/discharge oscillator theory, also contributes but mainly in the eastern Pacific. A stability analysis is performed using a simple stripped-down coupled model with two different basic state settings derived from the periods over 1982-2001 and 2002-2012, respectively. In the two periods, the basic states show different distribution along the equator and the leading ENSO-like eigen-modes have very different periods and patterns. The mode under the basic state in the earlier period resembles the eastern Pacific ENSO with a period about 5 years, whereas under the basic state in the recent decade, the ENSO mode becomes more like the observed central Pacific ENSO with a period about 2 years. The slow eastern Pacific ENSO mode is dominated by the thermocline feedback and behaves more like the recharge oscillator, whereas the zonal advective feedback plays a significant role in the fast-paced central Pacific ENSO mode. These results are roughly consistent with the broad features of two types of ENSO. In addition, the zonal advective feedback contributes to the phase transitions of both types of ENSO-like modes. In a word, the recent fast-paced ENSO activities in the central Pacific is suggested due to the dominance of zonal advective feedback favored by the tropical Pacific basic state conditions in the past decade.