Jingzhi Su

Causes of the El Niño and La Niña Amplitude Asymmetry in the Equatorial Eastern Pacific

Abstract The amplitude asymmetry between El Nino and La Nina is investigated by diagnosing the mixed-layer heat budget during the ENSO developing phase by using the three ocean assimilation products: Simple Ocean Data Assimilation (SODA) 2.0.2, SODA 1.4.2, and the Global Ocean Data Assimilation System (GODAS). It is found that the nonlinear zonal and meridional ocean temperature advections are essential to cause the asymmetry in the far eastern Pacific, whereas the vertical nonlinear advection has the opposite effect. The zonal current anomaly is dominated by the geostrophic current in association with the thermocline depth variation. The meridional current anomaly is primarily attributed to the Ekman current driven by wind stress forcing. The resulting induced anomalous horizontal currents lead to warm nonlinear advection during both El Nino and La Nina episodes and thus strengthen (weaken) the El Nino (La Nina) amplitude. The convergence (divergence) of the anomalous geostrophic mixed-layer currents dur...

What hindered the El Niño pattern in 2014

At the beginning of 2014, an El Nino event was predicted to occur in the following winter. However, the El Nino that started to develop in 2014 was hindered in the boreal summer, and only the ocean reached a weak El Nino condition. This outcome was largely attributed to a suppressed ocean-atmosphere interaction caused by anomalous easterly winds in the eastern equatorial Pacific. These winds were related to negative sea surface temperature anomalies (SSTAs) in the southeastern subtropical Pacific (SESP). The negative phase of the Interdecadal Pacific Oscillation (IPO) laid the foundation for the persistence of cooler SSTAs and enhanced trade winds in the SESP after the year 2000. As the recent IPO downward trend continued, the SSTAs in SESP reached an extremely low value in the boreal summer of 2014 and imposed a serious obstacle to the evolution of a warming event.

The Initiation and Developing Mechanisms of Central Pacific El Niños

AbstractThe initiation and developing mechanisms of four major central Pacific (CP) El Nino events in 1994, 2002, 2004, and 2009 were investigated by analyzing oceanic and atmospheric reanalysis data. A mixed layer heat budget analysis was conducted and the result shows that the initiation mechanism of the 1994 CP El Nino is very different from other CP El Ninos in 2000s, while the developing mechanisms are similar among these events. The initial sea surface temperature (SST) warming of the 1994 El Nino was caused by enhanced solar radiation, which was related to atmospheric meridional overturning circulation in association with positive SST anomaly forcing in the subtropical Pacific. The subtropical SST anomalies also induced anticyclonic surface wind stress curl anomalies, which caused the formation of subsurface warmer waters in the off-equatorial regions. The off-equatorial subsurface warmer waters were transported farther equatorward by the mean subsurface ocean currents, leading to the subsurface wa...

On the Relationship between Winter Sea Ice and Summer Atmospheric Circulation over Eurasia

AbstractUsing NCEP–NCAR reanalysis and Japanese 25-yr Reanalysis (JRA-25) data, this paper investigates the association between winter sea ice concentration (SIC) in Baffin Bay southward to the eastern coast of Newfoundland, and the ensuing summer atmospheric circulation over the mid- to high latitudes of Eurasia. It is found that winter SIC anomalies are significantly correlated with the ensuing summer 500-hPa height anomalies that dynamically correspond to the Eurasian pattern of 850-hPa wind variability and significantly influence summer rainfall variability over northern Eurasia. Spring atmospheric circulation anomalies south of Newfoundland, associated with persistent winter–spring SIC and a horseshoe-like pattern of sea surface temperature (SST) anomalies in the North Atlantic, act as a bridge linking winter SIC and the ensuing summer atmospheric circulation anomalies over northern Eurasia. Indeed, this study only reveals the association based on observations and simple simulation experiments with S...

Abrupt termination of the 2012 Pacific warming and its implication on ENSO prediction

In the summer of 2012, there was a clear signal of the developing El Nino over the equatorial Pacific, and many climate models forecasted the occurrence of El Nino with a peak phase in the subsequent winter. However, the warming was aborted abruptly in late fall. Here we show that the abrupt termination of the 2012 Pacific warming was largely attributed to the anomalous sea surface temperature (SST) cooling in the northeastern and southeastern subtropical Pacific. The anomalous SST cooling induced strong easterly and low-level divergence anomalies, suppressing the development of westerly and convection anomalies over the equatorial central Pacific. Thus, the surface warming over the equatorial Pacific was decoupled from the surface wind forcing and subsurface thermocline variability, inhibiting its further development into a mature El Nino in the winter of 2012–2013. This study highlights the importance of the SST anomaly in the subtropical Pacific in El Nino prediction.

Impact of El Niño on atmospheric circulations over East Asia and rainfall in China: Role of the anomalous western North Pacific anticyclone

This paper presents a review on the impact of El Niño on the interannual variability of atmospheric circulations over East Asia and rainfall in China through the anomalous anticyclone over western North Pacific (WNPAC). It explains the formation mechanisms of the WNPAC and physical processes by which the WNPAC affects the rainfall in China. During the mature phase of El Niño, the convective cooling anomalies over western tropical Pacific caused by the weakened convections trigger up an atmospheric Rossby wave response, resulting in the generation of the WNPAC. The WNPAC can persist from the winter when the El Niño is in its peak to subsequent summer, which is maintained by multiple factors including the sustained presence of convective cooling anomalies and the local air-sea interaction over western tropical Pacific, and the persistence of sea surface temperature anomalies (SSTA) in tropical Indian and tropical North Atlantic. The WNPAC can influence the atmospheric circulations over East Asia and rainfall in China not only simultaneously, but also in the subsequent summer after an El Niño year, leading to more rainfall over southern China. The current paper also points out that significant anomalies of atmospheric circulations over East Asia and rainfall over southern China occur in El Niño winter but not in La Niña winter, suggesting that El Niño and La Niña have an asymmetric effect. Other issues, including the impact of El Niño diversity and its impact as well as the relations of the factors affecting the persistence of the WNPAC with summer rainfall anomalies in China, are also discussed. At the end of this paper some issues calling for further investigation are discussed.

Sea Surface Temperature in the Subtropical Pacific Boosted the 2015 El Niño and Hindered the 2016 La Niña

AbstractAfter the quick decaying of the 2015 super El Nino, the predicted La Nina unexpectedly failed to materialize to the anticipated standard in 2016. Diagnostic analyses, as well as numerical experiments, showed that this ENSO evolution of the 2015 super El Nino and the hindered 2016 La Nina may be essentially caused by sea surface temperature anomalies (SSTAs) in the subtropical Pacific. The self-sustaining SSTAs in the subtropical Pacific tend to weaken the trade winds during boreal spring–summer, leading to anomalous westerlies along the equatorial region over a period of more than one season. Such long-lasting wind anomalies provide an essential requirement for ENSO formation, particularly before a positive Bjerknes feedback is thoroughly built up between the oceanic and atmospheric states. Besides the 2015 super El Nino and the hindered La Nina in 2016, there were several other El Nino and La Nina events that cannot be explained only by the oceanic heat content in the equatorial Pacific. However,...

Impact of the South and North Pacific Meridional Modes on the El Niño–Southern Oscillation: Observational Analysis and Comparison

AbstractAn interannual variability mode in the southeast Pacific with a physical interpretation similar to that of the Pacific meridional mode (PMM) in the North Pacific was recently identified. Both modes have been shown to influence the subsequent development of El Nino–Southern Oscillation (ENSO) events. This study investigates the relationship between ENSO and the two PMMs using observational and reanalysis data. The results show that the South Pacific meridional mode (SPMM) mainly favors the development of sea surface temperature anomalies (SSTAs) in the eastern equatorial Pacific, whereas the North Pacific meridional mode (NPMM) mainly favors the development of SSTAs in the central equatorial Pacific. Both of the meridional modes are considered to be analogous in terms of their physical interpretation and can be important predictors of ENSO when considering different flavors of ENSO. Neither the NPMM nor the SPMM can be precluded as accurate indicators when forecasting particular flavors of ENSO.

Impact of atmospheric model resolution on simulation of ENSO feedback processes: a coupled model study

This study examines El Niño-Southern Oscillation (ENSO)-related air-sea feedback processes in a coupled general circulation model (CGCM) to gauge model errors and pin down their sources in ENSO simulation. Three horizontal resolutions of the atmospheric component (T42, T63 and T106) of the CGCM are used to investigate how the simulated ENSO behaviors are affected by the resolution. We find that air-sea feedback processes in the three experiments mainly differ in terms of both thermodynamic and dynamic feedbacks. We also find that these processes are simulated more reasonably in the highest resolution version than in the other two lower resolution versions. The difference in the thermodynamic feedback arises from the difference in the shortwave-radiation (SW) feedback. Due to the severely (mildly) excessive cold tongue in the lower (higher) resolution version, the SW feedback is severely (mildly) underestimated. The main difference in the dynamic feedback processes lies in the thermocline feedback and the zonal-advection feedback, both of which are caused by the difference in the anomalous thermocline response to anomalous zonal wind stress. The difference in representing the anomalous thermocline response is attributed to the difference in meridional structure of zonal wind stress anomaly in the three simulations, which is linked to meridional resolution.