عنوان مقاله [English]
The El Niño-Southern Oscillation (ENSO) cycle of alternating warm El Niño and cold La Niña events occurs when the tropical Pacific Ocean and its overlying atmosphere change from its natural state for at least several consecutive months (Neelin et al., 1998). The neutral phase of the El Niño-Southern Oscillation is derived by the strong zonally asymmetric state of the equatorial Pacific and is characterized by surface easterly trade winds along the equatorial Pacific, rising motion, deep convection and heavy rainfall over the western equatorial Pacific, westerly winds at upper levels and sinking motion over the eastern equatorial Pacific (Bjerknes, 1969). El Niño is characterized by weak and La Niña by strong zonal SST gradients, accompanied respectively by weakening and strengthening of the trade winds across the equatorial Pacific (McPhaden et al. 2006). As a result, compared to the neutral phase of the El Niño-Southern Oscillation, convective systems intensify in the western tropical Pacific and slightly shift to the west during La Niña events, but shift to the central and eastern tropical Pacific during El Niño events. Since this early recognition of the coupling between the atmosphere and the Pacific ocean by Bjerknes (1966) and Bjerknes (1969), major advances have beenmade toward a comprehensive understanding of the physics of the El Niño-Southern Oscillation. This is particularly achived through development of complex climatemodels for realistic simulation of the El Niño-Southern Oscillation cycle (Bellenger et al., 2014), and great observational advances that have been made during the international Tropical Ocean-Global Atmosphere (TOGA) program conducted between 1985 and 1994 (McPhaden et al., 1998). El Niño or the warm phase of the El Niño-Southern Oscillation is a quasi-periodic natural phenomenon that occurs in the tropical Pacific Ocean. The El Niño-Southern Oscillation not only influences the climate of nearby regions, but it is the most important natural climate factor that contributes to the interannual climate variability over many regions across the globe, including North America (e.g. Yu et al., 2015; Guo et al., 2017), the Middle East (e.g. Alizadeh-choobari, 2017; Alizaeh-Choobari et al., 2018a, Alizaeh-Choobari et al., 2018b), East Asia (e.g. Feng and Li, 2011), Southeast Asia (e.g. Lee et al., 2017) and the Indian subcontinent (e.g. Kumar et al., 2006). Depending on the location of the maximum sea surface temperature in the eastern or central equatorial Pacific, the eastern Pacific El Niño or the central Pacific El Niño are identified, while a mixed event of the eastern and central Pacific El Niño events has been also diagnosed.
Materials and methods
In this study, using the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis Interim (ERA-Interim) monthly dataset with a horizontal resolution of 0.75º 0.75º, and the Extended Reconstructed Sea Surface Temperature version 5 (ERSSTv5) dataset, the phase of ENSO and the type of El Niño events during the period 1979-2016 are determined. In addition, characteristics of the 1997-98 eastern Pacific El Niño, the 2009-10 central Pacific El Niño and the 2015-16 El Niño, which is a mixed event of the eastern and central Pacific El Niño, are investigated.
Results and discussion
Analysis indicateed that during the period 1979-2016, 1979-80, 1982-83, 1986-87, 1987-88, 1991-92, 1994-95, 1997-98, 2002-03, 2004-05, 2006-07, 2009-10, 2014-15 and 2015-16 have been the years for which three-month running means of the Oceanic Niño Index (ONI) for 5 consecutive periods became greater or equal to 0.5 degree Celcius, indicating the occurrence of El Niño in these years. Using the empirical orthogonal function (EOF) and by examining spatial correlation between sea surface temperature anomalies in the equatorial Pacific Ocean and results of the empirical orthogonal function, the eastern and central Pacific El Niños during the period 1979-2016 are determined. The 1997-98 and 2015-16 El Niño events are both categorized as extreme El Niño events. The 2009-10 El Niño is weaker than the other two events, but over the last century, it has been the strongest central Pacific El Niño event. Results indicated that the onset of all these three events was in June, while some differences are found between termination of the 1997-98 and 2015-16 El Niños, including different time of dissipation for these events. All these three events have shown characteristics of classic El Niño events, such that anomalous positive and negative sea surface temperature are seen in the eastern and western equatorial Pacific, respectively. Nevertheless, maximum positive sea surface temperature is formed in the eastern equatorial Pacific during the 1997-98 El Niño, which is different from the 2009-10 El Niño event with the maximum sea surface temperature in the central (near the dateline) equatorial Pacific. In fact, maximum positive sea surface temperature anomalies are located in the eastern and central equatorial Pacific during the 1997-98 and 2009-10 El Niño events, respectively, while it extends from central to eastern equatorial Pacific during the 2015-16 El Niño. Intensities of the maximum sea surface temperature anomalies and mean sea level pressure have been greater during the 1997-98 El Niño compared to those during the 2009-10 event, indicating that central Pacific El Niños are generally less intense than eastern Pacific El Niño events. It is shown that positive sea surface temperature anomalies in the 2015-16 El Niño event cover a larger area, extending from the central to the eastern equatorial Pacific. It is found that both sea surface temperature and mean sea level pressure anomalies in the equatorial Pacific were larger during the 1997-98 eastern Pacific El Niño than that of the 2009-10 central Pacific El Niño. This suggests that the central Pacific El Niño events are generally weaker than the eastern Pacific El Niño events. In all of the three El Niño events, positive (negative) geopotential height anomalies at 300 hPa pressure level in the equatorial Pacific are collocated with positive (negative) sea surface temperature anomalies. Geopotential height anomalies in the upper levels over the tropical Pacific influence weather patterns of other regions.It is discussed and shown that different geopotential height anomalies at upper levels of the equatorial Pacific during the three El Niño events have led to different teleconnections across the globe. For example, temperature anomalies in the Antarctic during the 2009-10 El Niño were opposite to those during the 1997-98 and 2015-16 El Niño events.
Analysis of the ERA-Interim dataset with the horizontal resolution of 0.75º 0.75º for the period 1979-2016 indicated that the eastern, central and mixed El Niño events have generally different characteristics in the equatorial Pacific. As a result, teleconnection patterns of these events across the globe are also found to be different.