Evolving anomalous SST patterns leading to ENSO extremes: relations between the tropical Pacific and Atlantic Oceans and the influence on the South American rainfall

Abstract

Several studies have provided observational and numerical evidence that the tropical Pacific and Indian Oceans are influenced by the tropical Atlantic within a one season time scale. The influence of the Atlantic equatorial mode (AEM) in the Pacific El Niño-Southern Oscillation (ENSO) mode is observationally re-examined. The analyses focus on the ENSO-related evolving sea surface temperature (SST) and sea level pressure (SLP) anomalies in the Tropics that follow the occurrences of AEM events and those that are independent of the AEM. The cold (warm) AEM followed by El Niño (La Niña) shows a sequence of maps that might be explained by the mechanism previously outlined on the relationship of the tropical Atlantic and the other tropical Oceans. This mechanism involves an anomalous Atlantic Walker circulation and a Gill-Matsuno-type atmospheric response to anomalous cooling or warming in the tropical Atlantic. The seasonal timing of the relationship studied differs from that of the previous studies. Here, it is noted that the Atlantic SST anomalous conditions are persistent and might be noted 5-6 months before that proposed in previous results. Furthermore, the ENSO extreme conditions are reinforced and maintained by the east-west SST anomalous gradient in the tropical Pacific. Also, the precipitation composites over South America for the ENSO extremes, which are AEM-dependent and AEM-independent cases, are discussed. The AEM-dependent ENSO extremes combine the effects from the tropical Pacific, and equatorial and tropical South Atlantic on the rainfall over South America. The results presented here, to the authors' knowledge, have not been discussed before and might represent a potential for long lead predictability of the climate variations in the tropical Pacific.

Seasonal anomalous rainfall in the central and eastern Amazon and associated anomalous oceanic and atmospheric patterns

“…Thus, the associated east-west anomalous Atlantic Walker circulation, with ascending motion over the eastern equatorial Pacific and descending motion extending over the western equatorial Atlantic and the surrounding central and eastern Amazon, is the main factor contributing to diminish the rainfall over this continental area. This result confirms previous findings by Kayano et al (2010) on the relationship between the inter-Pacific-Atlantic anomalous SST gradient and the South American rainfall. It is worthwhile recalling that the weak positive inter-hemispheric anomalous SST gradient in the tropical Atlantic also contributes to dry conditions in the central and eastern Amazon during its dry season.…”

“…So, both the negative inter-Pacific-Atlantic anomalous SST gradient and the positive inter-hemispheric anomalous SST gradient in the tropical Atlantic contribute to the dry condition in the central and eastern Amazon, through anomalous east-west and north-south direct circulations. Consistent with this result, Kayano et al (2010) found reduced rainfall over the central Amazon due to the action of a negative inter-Pacific-Atlantic anomalous SST gradient; and Souza et al (2000) noted dry conditions over the central and northern Amazon due to the combined effect of El Niño and positive inter-hemispheric anomalous SST gradient. The very wet rainy season case is also strongly modulated by the anomalous SST conditions in both the tropical Pacific and Atlantic Oceans (Figure 3a) and b)).…”

“…While the significant positive SST anomalies weaken, the Atlantic La Niña pattern becomes better established from November/December to December/January. Several authors have shown that an Atlantic La Niña induces the establishment of a Pacific El Niño through an east-west anomalous circulation (Wang, 2006;Kucharski et al, 2007Kucharski et al, , 2008Kayano et al, 2010) during the subsequent months. This is the case here, as a negative inter-PacificAtlantic anomalous SST gradient is established from January/February to March/April (Figure 2a)).…”

“…When the AEM and the ENSO create an inter-basin east-west SST gradient, the Atlantic Walker circulation induces an anomalous east-west flow of air across northern South America and an atmospheric direct circulation between the two oceans. Kayano et al (2010) examining the ENSO-related precipitation patterns over South America found substantial differences in these patterns when the ENSO extremes are associated with the AEM occurrences and AEM independent.…”

Interannual Variability of Precipitation for the Centro de Lançamento de Alcântara in ENSO-Neutral Years

“…The definition of the indices follows Andreoli and Kayano (2007). The precipitation data from the Global Precipitation Climatology Centre (GPCC) have been used in climate studies (Schneider et al 2014) and also in studies focusing on the South America precipitation variability (Kayano et al 2011). For the CLA, GPCC data are compared to the Climate Prediction Center (CPC)/NOAA data, which were used by Pinheiro and Oyama (2013) to obtain the rainy season features for the CLA.…”