The Effect of the South Pacific Convergence Zone on the Termination of El Niño Events and the Meridional Asymmetry of ENSO*

2012 ◽  
Vol 25 (16) ◽  
pp. 5566-5586 ◽  
Author(s):  
Shayne McGregor ◽  
Axel Timmermann ◽  
Niklas Schneider ◽  
Malte F. Stuecker ◽  
Matthew H. England
Keyword(s):  
Boundary Layer ◽  
El Niño ◽  
El Nino ◽  
South Pacific ◽  
South Pacific Convergence Zone ◽  
Convergence Zone ◽  
Wind Speeds ◽  
Wind Response ◽  
El Niño Events ◽  
El Nino Events

Abstract During large El Niño events the westerly wind response to the eastern equatorial Pacific sea surface temperature anomalies (SSTAs) shifts southward during boreal winter and early spring, reaching latitudes of 5°–7°S. The resulting meridional asymmetry, along with a related seasonal weakening of wind anomalies on the equator are key elements in the termination of strong El Niño events. Using an intermediate complexity atmosphere model it is demonstrated that these features result from a weakening of the climatological wind speeds south of the equator toward the end of the calendar year. The reduced climatological wind speeds, which are associated with the seasonal intensification of the South Pacific convergence zone (SPCZ), lead to anomalous boundary layer Ekman pumping and a reduced surface momentum damping of the combined boundary layer/lower-troposphere surface wind response to El Niño. This allows the associated zonal wind anomalies to shift south of the equator. Furthermore, using a linear shallow-water ocean model it is demonstrated that this southward wind shift plays a prominent role in changing zonal mean equatorial heat content and is solely responsible for establishing the meridional asymmetry of thermocline depth in the turnaround (recharge/discharge) phase of ENSO. This result calls into question the sole role of oceanic Rossby waves in the phase synchronized termination of El Niño events and suggests that the development of a realistic climatological SPCZ in December–February/March–May (DJF/MAM) is one of the key factors in the seasonal termination of strong El Niño events.

scholarly journals Stronger Increase in the Frequency of Extreme Convective than Extreme Warm El Niño Events under Greenhouse Warming

2020 ◽  
Vol 33 (2) ◽  
pp. 675-690 ◽  
Author(s):  
Guojian Wang ◽  
Wenju Cai ◽  
Agus Santoso
Keyword(s):  
El Niño ◽  
El Nino ◽  
South Pacific Convergence Zone ◽  
Boreal Winter ◽  
Greenhouse Warming ◽  
Convergence Zone ◽  
Central Pacific ◽  
Mixed Regime ◽  
El Niño Events ◽  
El Nino Events

AbstractSince 1979, three extreme El Niño events occurred, in 1982/83, 1997/98, and 2015/16, with pronounced impacts that disrupted global weather patterns, agriculture, fisheries, and ecosystems. Although all three episodes are referred to as strong equatorial eastern Pacific (EP) El Niño events, the 2015/16 event is considered a mixed regime of both EP and central Pacific (CP) El Niño. During such extreme events, sea surface temperature (SST) anomalies peak over the EP region, hereafter referred to as an extreme warm El Niño (ExtWarmEN) event. Simultaneously, the intertropical convergence zone (ITCZ) moves southward to the usually dry and cold Niño-3 region, resulting in dramatic rainfall increases to more than 5 mm day−1 averaged over boreal winter, referred to as an extreme convective El Niño (ExtConEN) event. However, in climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) that are able to simulate both types of events, ExtConEN events are found not to always coincide with ExtWarmEN events and the disassociation becomes more distinct under greenhouse warming when the increased frequency of ExtConEN events is notably larger than that of ExtWarmEN events. The disassociation highlights the role of eastward migration of western Pacific convection and equatorward shift of the South Pacific convergence zone associated with the faster warming over the EP region as a result of greenhouse warming.

The Contribution of Boreal Spring South Pacific Atmospheric Variability to El Niño Occurrence

2020 ◽  
Vol 33 (19) ◽  
pp. 8301-8313
Author(s):  
Qingye Min ◽  
Renhe Zhang
Keyword(s):  
El Niño ◽  
El Nino ◽  
South Pacific ◽  
Tropical Pacific ◽  
The South ◽  
Atmospheric Variability ◽  
Boreal Spring ◽  
El Niño Events ◽  
El Nino Events ◽  
The Tropical Pacific

AbstractDespite the fact that great efforts have been made to improve the prediction of El Niño events, it remains challenging because of limited understanding of El Niño and its precursors. This research focuses on the influence of South Pacific atmospheric variability on the development of the sea surface temperature anomaly (SSTA) in the tropical Pacific. It is found that as early as in the boreal spring of El Niño years, the sea level pressure anomaly (SLPA) shows a configuration characterized by two significant negative anomaly centers in the north and a positive anomaly center in the south between the subtropics and high latitudes in South Pacific. Such an anomalous SLPA pattern becomes stronger in the following late boreal spring and summer associated with the strengthening of westerly anomalies in the tropical Pacific, weakening the southeasterly trade winds and promoting the warming of tropical eastern Pacific, which is conducive to the development of El Niño events. It is demonstrated that the SLPA pattern in boreal spring revealed in this study is closely associated with boreal summer South Pacific Oscillation (SPO) and South Pacific meridional mode (SPMM). As a precursor in boreal spring, the prediction skill of the South Pacific SLPA in boreal spring for the SSTA in the eastern equatorial Pacific is better than that of the SPMM. This study is helpful to deepen our understanding of the contribution of South Pacific extratropical atmospheric variability to El Niño occurrence.

scholarly journals On the role of the south Pacific subtropical high at the onset of El Niño events

2018 ◽  
Author(s):  
Youjia Zou ◽  
Xiangying Xi
Keyword(s):  
El Niño ◽  
El Nino ◽  
South Pacific ◽  
Warm Pool ◽  
Equatorial Pacific ◽  
Subtropical High ◽  
Trade Winds ◽  
Southward Shift ◽  
El Niño Events ◽  
El Nino Events

Abstract. Previous studies have suggested that an eastward propagation of the warm pool in the western Pacific during El Niño events may be induced by a weakening of the easterly Trade Winds (Alexander et al., 2002; Bjerknes, 1969). However, the dynamic mechanism of the Trade Winds weakening is not well understood. Here we use a model and other published proxy records to demonstrate that the anomalous southward shift of the south Pacific subtropical high (SPSH) may play a crucial role at the onset of El Niño events. By analyzing the relationship between the Trade Winds, the Equatorial Currents, the Eastern Boundary Currents and the SPSH, we find that an anomalous southward shift of the SPSH can result in a weakening of the SE Trade Winds and a southward intrusion of the NE Trade Winds, leading to a southward migration of the Trade Wind-induced Equatorial Currents, including the Equatorial Countercurrent (from ~5°–8° N to ~0°). The warm pool in the western equatorial Pacific is therefore forced to propagate eastward by the enhanced Equatorial Countercurrent and, thus, a warm phase in the central or the eastern equatorial Pacific. Moreover, the equatorward upwelling in the eastern South Pacific, usually recurving along the equator, shifts southward along with the SPSH, in turn diverts towards the west at ~15° S to feed the westward South Equatorial Currents, resulting in a failure of cooling sea surface in the eastern tropical Pacific, thus a flattening of the thermocline. The model experiments indicate that the meridional position and intensity of the Equatorial Countercurrent in the Pacific are some of the determining factors in giving rise to El Niño diversity, suggesting that there should be more frequent warm events due to a meridional expansion of the warm pool under global warming.

scholarly journals Characterization of the Variability of the South Pacific Convergence Zone Using Satellite and Reanalysis Wind Products

2016 ◽  
Vol 29 (5) ◽  
pp. 1717-1732 ◽  
Author(s):  
Autumn Kidwell ◽  
Tong Lee ◽  
Young-Heon Jo ◽  
Xiao-Hai Yan
Keyword(s):  
Time Scales ◽  
El Niño ◽  
El Nino ◽  
South Pacific ◽  
South Pacific Convergence Zone ◽  
Multiple Time Scales ◽  
Convergence Zone ◽  
Multiple Time ◽  
The South ◽  
Central Pacific

Abstract The variability of the South Pacific convergence zone (SPCZ) is evaluated using ocean surface wind products derived from the atmospheric reanalysis ERA-Interim for the period of 1981–2014 and QuickSCAT for the period of 1999–2009. From these products, indices were developed to represent the SPCZ strength, area, and centroid location. Excellent agreement is found between the indices derived from the two wind products during the QuikSCAT period in terms of the spatiotemporal structures of the SPCZ. The longer ERA-Interim product is used to study the variations of SPCZ properties on intraseasonal, seasonal, interannual, and decadal time scales. The SPCZ strength, area, and centroid latitude have a dominant seasonal cycle. In contrast, the SPCZ centroid longitude is dominated by intraseasonal variability due to MJO influence. The SPCZ indices are all correlated with El Niño–Southern Oscillation indices. Interannual and intraseasonal variations of SPCZ strength during strong El Niño are approximately twice as large as the respective seasonal variations. SPCZ strength depends more on the intensity of El Niño rather than the central-Pacific versus eastern-Pacific type. The change from positive to negative Pacific decadal oscillation (PDO) around 1999 results in a westward shift of the SPCZ centroid longitude, a much smaller interannual swing in centroid latitude, and a decrease in SPCZ area. This study improves the understanding of the variations of the SPCZ on multiple time scales and reveals the variations of SPCZ strength not reported previously. The diagnostics analyses can be used to evaluate climate models to gauge their fidelity.

scholarly journals Optimally growing initial errors of El Niño events in the CESM

2021 ◽  
Author(s):  
Hui Xu ◽  
Lei Chen ◽  
Wansuo Duan
Keyword(s):  
El Niño ◽  
El Nino ◽  
Equatorial Pacific ◽  
Initial Error ◽  
Initial Errors ◽  
El Niño Events ◽  
Type 3 ◽  
El Nino Events

AbstractThe optimally growing initial errors (OGEs) of El Niño events are found in the Community Earth System Model (CESM) by the conditional nonlinear optimal perturbation (CNOP) method. Based on the characteristics of low-dimensional attractors for ENSO (El Niño Southern Oscillation) systems, we apply singular vector decomposition (SVD) to reduce the dimensions of optimization problems and calculate the CNOP in a truncated phase space by the differential evolution (DE) algorithm. In the CESM, we obtain three types of OGEs of El Niño events with different intensities and diversities and call them type-1, type-2 and type-3 initial errors. Among them, the type-1 initial error is characterized by negative SSTA errors in the equatorial Pacific accompanied by a negative west–east slope of subsurface temperature from the subsurface to the surface in the equatorial central-eastern Pacific. The type-2 initial error is similar to the type-1 initial error but with the opposite sign. The type-3 initial error behaves as a basin-wide dipolar pattern of tropical sea temperature errors from the sea surface to the subsurface, with positive errors in the upper layers of the equatorial eastern Pacific and negative errors in the lower layers of the equatorial western Pacific. For the type-1 (type-2) initial error, the negative (positive) temperature errors in the eastern equatorial Pacific develop locally into a mature La Niña (El Niño)-like mode. For the type-3 initial error, the negative errors in the lower layers of the western equatorial Pacific propagate eastward with Kelvin waves and are intensified in the eastern equatorial Pacific. Although the type-1 and type-3 initial errors have different spatial patterns and dynamic growing mechanisms, both cause El Niño events to be underpredicted as neutral states or La Niña events. However, the type-2 initial error makes a moderate El Niño event to be predicted as an extremely strong event.

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