Evolution of (a) sea surface temperature anomalies (SSTAs) from the Optimum Interpolation Sea Surface Temperature (OISST) v2 high‐resolution daily data set, (b) mixed‐layer temperature anomalies (shading) and thermocline depth anomalies (contour line) from the Global Ocean Data Assimilation System (GODAS), (c) zonal ocean current anomalies from GODAS, and (d) anomalous daily zonal wind at a height of 10 m along the equator (within 5°N–5°S) from ERA‐Interim. The fields in Figures 1a and 1d are smoothed with a 7 day running mean. The pause phase is indicated by gray shading. The y axis ticks label the first day of each month.

Although each El Niño event has unique features, the warm event in 2014 showed an unprecedented development process. At the beginning of 2014, an El Niño event was predicted to develop in the winter of 2014/2015 [ Ludeschera et al ., 2014 ]. This forecast was supported by the results of all climate models monitored both by the United States National Oceanic and Atmospheric Administration's (NOAA's) Climate Prediction Center and the Australian Bureau of Meteorology (BoM). In addition, several models predicted a stronger event with an amplitude of approximately 2°C ( http://www.cpc.ncep.noaa.gov/products/CDB/CDB_Archive_html/bulletin_032014/Forecast/figf4.shtml;http://www.bom.gov.au/climate/enso/archive/ensowrap_20140422.pdf ). Concern regarding the possibility of the development of a super El Niño was exacerbated by the news media and attracted considerable public attention in the first half of 2014. However, the warming already established in the central‐eastern equatorial Pacific dramatically paused during the boreal summer. Although the oceanic conditions recovered and escalated to an El Niño level after November 2014 (Figure 1 ), the combined atmosphere and oceanic conditions monitored by the NOAA's Climate Prediction Center and the BoM remained in an ENSO‐neutral status at the end of 2014. As a result, El Niño failed to occur until the end of 2014. What factors hindered the warming event in 2014 from developing into a mature El Niño? Why was the El Niño pattern hindered during the boreal summer of 2014? This study focuses on the hindered development of the 2014 El Niño that failed to develop, especially in the central equatorial Pacific during the boreal summer of 2014.

The El Niño–Southern Oscillation (ENSO) can influence Earth's climate over most regions of the globe. Benefiting from continuous improvements in the coverage of observational networks and the availability of ENSO physics models, considerable progress has been made in ENSO monitoring and forecasting [ McPhaden et al ., 1998 ; Latif et al ., 1998 ; Barnston et al ., 2012 ]. However, the reliability of ENSO predictions is often limited to approximately 6 months [e.g., Jin et al ., 2008 ]. Similarly, no essential improvements to real‐time ENSO prediction have been achieved in the last few decades [ Barnston et al ., 2012 ]. Moreover, El Niño events display a wide variety of formations, such as the classical eastern Pacific (EP) El Niño and the central Pacific (CP) El Niño [ Kao and Yu , 2009 ] or El Niño Modoki [ Ashok et al ., 2007 ]. The latter type of formation has occurred more frequently in recent decades [ Yeh et al ., 2009 ; Lee and McPhaden , 2010 ]. Wang et al . [ 2012 ] suggested that ENSO features might be changing and that preparation for the possible emergence of a new dominant type of ENSO might be required. Therefore, the existing modeling and prediction strategies that were developed primarily for the conventional EP type of ENSO must be revised.

To understand the mechanism that causes the variation in the SST anomalies (SSTAs), a mixed‐layer heat budget analysis following Su et al . [ 2010 ] was diagnosed in the Niño 3.4 region (5°S–5°N, 170°W–120°W). The mixed‐layer depth is based on climatological values [ de Boyer Montégut et al ., 2004 ]. In the following discussion, V = ( u , v , w ) represents the three‐dimensional ocean current, ∇ = (∂/∂x, ∂/∂ y , ∂/∂z) denotes the three‐dimensional gradient operator, a bar ( ) represents the climatologic annual cycle variables, and a prime (′) represents the anomaly variables.

The second Japanese global atmospheric reanalysis project (JRA55) provided monthly data on wind speed at a height of 10 m and convective precipitation [ Ebita et al ., 2011 ]. The NOAA Optimum Interpolation Sea Surface Temperature version 2 (OISST v2) monthly data set and a high‐resolution daily data set provided sea surface temperatures (SSTs) [ Reynolds et al ., 2002 , 2007 ]. The Global Ocean Data Assimilation System (GODAS) provided the ocean monthly mean data set reanalysis [ Saha et al ., 2006 ]. The National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) Reanalysis 1 provided surface heat flux data [ Kalnay et al ., 1996 ]. The ERA‐Interim Reanalysis provided the daily means of wind speed at a height of 10 m [ Dee and Uppala , 2009 ]. The Global Precipitation Climatology Project (GPCP) provided daily precipitation data [ Huffman et al ., 2001 ]. The climatological mixed‐layer depth data from the French Research Institute for Exploitation of the Sea with a fixed threshold criterion (0.03 kg m −3 ) was used to define the mixed‐layer depth [ de Boyer Montégut et al ., 2004 ]. Unless noted otherwise, all anomalies are defined as the departure from 1981 to 2010 climatological values of each month.

3 Results

3.1 Observational Features for the 2014 El Niño Pattern Positive SSTAs first appeared in the eastern equatorial Pacific during March 2014, and they exceeded 1.0°C in June (Figure 1a). In the central equatorial Pacific, the SSTAs became positive in April but failed to increase continuously after June. Instead, the SSTAs in the Niño 3.4 region even decreased from 0.5°C in June to 0.1°C in July. After November 2014, the SSTAs in both central and eastern equatorial Pacific began to increase again and reached approximately 0.8°C in December 2014. Hence, the El Niño pattern was hindered during the boreal summer of 2014, particularly from June to July. Similarly, the SSTAs in the eastern equatorial Pacific also experienced conditions that hindered their development from June to September. The initial conditions at the beginning of 2014 implied that a strong El Niño would occur that year. In February 2014, a strong warm Kelvin wave was formed in the western equatorial Pacific and was later transported eastward (Figure 1b). This Kelvin wave enhanced the heat content in the equatorial Pacific. The integrated warm water volume (WWV) above the 20°C isotherm within 5°N–5°S and 120°E–80°W was selected to represent the equatorial Pacific heat content [Meinen and McPhaden, 2000]. The amplitude of the WWV anomaly in 2014 was comparable with that of the 1997 and 1982 El Niño events (Figure S1 in the supporting information). However, the warming signal in the Niño 3.4 region after its initiation in 2014 was relatively weak compared with the El Niño events of recent decades (Figure S1). The unachieved amplitude of the 2014 warm event was mainly caused by its pause during the boreal summer. Because the SSTAs in the central equatorial Pacific decreased from June to July, we defined March–May 2014 as the initiation phase and June–July 2014 as the pause phase. Based on the results of the heat budget analysis, the 2014 warm event showed unusual signatures during the pause phase when compared to other El Niño events during 1981–2010 (Figures S2b and S2d). However, there were few differences between them from March to May (Figures S2a and S2c). For El Niño events during 1981–2010, the SSTA trends in the Niño 3.4 region were primarily determined by the ocean advection terms after initiation. For the 2014 event, despite the normal evolution in the initial phase, the SSTA trend of the Niño 3.4 region decreased in an abnormal manner. This unexpected change was primarily caused by the ocean advection term, which was mainly resulted in by the mean zonal advection of an anomalous zonal temperature gradient. The zonal advection process actively contributes to the evolution of an El Niño event [Picaut et al., 1997; Kug et al., 2009; Santoso et al., 2013]. However, it is interesting to examine what factors made the zonal advection feedback of 2014 warming so different from other El Niño events. We will investigate the dynamics that controlled the development of the warming of 2014 in the Niño 3.4 region and focus on the mechanisms that hindered such warming.

3.2 Causes of the Pause of the 2014 Warm Event During the pause period of June–July, particularly in July, anomalous westward ocean currents and weak upwelling were observed in the central equatorial Pacific regions (approximately 155°W–180°W, 0–100 m), associated with an upwelling Kelvin wave pattern (Figures 3c and 3d). The eastward propagation of the upwelling Kelvin wave further cooled the subsurface temperatures within approximately 150°W–120°W at depths of 100–150 m. Hence, the anomalous zonal advection term associated with the mean zonal temperature gradient became negative ( ), which was largely responsible for the SST cooling in the Niño 3.4 region after May 2014 (Figures 2c and 2d and Figure S2). Furthermore, the formation of the anomalous upwelling Kelvin wave in the central equatorial Pacific was closely related to local easterly wind anomalies (Figures 1d, 3c, and 3d). Figure 2 Open in figure viewer PowerPoint SSTAs (shading, with intervals of 0.25°C), anomalous precipitation (contours, blue for positive and red for negative, with intervals of 4 mm d−1), and anomalous wind at a height of 10 m (vector, in m s−1) during each month of 2014. These wind and precipitation data were obtained from JRA55, and the SST data were obtained from OISST v2. Figure 3 Open in figure viewer PowerPoint Mean temperature anomalies (contour, with intervals of 0.5°C), anomalous ocean zones, vertical current (vectors), and wind stress along the equator (averaged within 5°N–5°S) during 2014. These data were obtained from GODAS. The anomalous easterlies in the central equatorial Pacific can be further analyzed on an intraseasonal time scale. The westerly wind events (WWEs) that began approximately during February and March 2014 (Figure 1d) favored the formation of significantly warm Kelvin waves (Figure 1b), as discussed above. The WWEs were weakened in April and nearly disappeared from May to mid‐June. After comparing the 2014 case with the event in 1997, Menkes et al. [2014] inferred that the absence of WWEs caused the warm event to fade in the boreal summer of 2014. However, the SSTAs in the central equatorial Pacific continued to warm from April to mid‐June (Figure 1a). The warm SSTAs began to decline only after June 10, when an easterly wind event occurred (Figure 1d). Therefore, we believe that the anomalous easterly winds reversed the warming tendency during June and July 2014, not the absence of WWEs. Several anomalous easterlies occurred in the central equatorial Pacific during June and July (e.g., 5–10 June, 1–6 July, and 11–16 July) and provoked upwelling Kelvin waves. The anomalous easterlies in the equatorial Pacific during the pause period were related to the negative SSTAs in the southeastern subtropical Pacific (SESP) that persisted from the winter of 2013/2014 (not shown). Following the mechanism of wind‐evaporation‐SST feedback [Xie and Philander, 1994], the cold SSTAs in the SESP could sustain themselves and penetrate into the equatorial Pacific [Chiang and Vimont, 2004; Vimont et al., 2001; Zhang et al., 2014], which further induced easterly wind anomalies over the equatorial Pacific. The contribution of the negative SSTAs in the SESP for those easterly wind anomalies can be easily observed on even an intraseasonal time scale (Figure S3). No significant movement of the precipitation anomaly centers moving eastward from the Indian Ocean to the Pacific was observed during the pause period, indicating that no Madden‐Julian oscillation was active. Figures S3b–S3e show that the easterly wind anomalies from the SESP gradually penetrated into the central equatorial Pacific during 5–10 June, which played an important role in reversing the former westerly winds in the central equatorial Pacific. A similar situation also emerged during 25 June to 6 July and 11–16 July (not shown). The anomalous easterlies resulted in upwelling Kelvin waves and led to a pause in the development of the SSTAs in the central‐eastern equatorial Pacific. No significant precipitation anomalies occurred in the central equatorial Pacific during this period. However, the maximum positive SSTAs in the eastern equatorial Pacific (5°N–5°S, 100°W–90°W) reached approximately 1.0°C during June and July. The evaporation rate decreased because the SSTs cooled in the SESP. As a result, the southeast trade winds brought less moisture to the equator, which favored decreased precipitation rates in the central equatorial Pacific. Hence, the ocean‐atmosphere interaction in the central equatorial Pacific was suppressed by enhanced trade winds, which caused a pause in the development of an El Niño event during June and July 2014.