Teleconnection mechanisms of northeast Brazil droughts: modeling and empirical evidence

Abstract

targeted numerical modelling experimaents are conducted to complement the previous empirical diagnostics of circulation mechanisms leading from sea surface temperature (sst) departures in the equatorial Pacific in January to anomalies in the March-April rainy season of Brazil's Nordeste. A weak interhemispheric northward directed sst gradient in the atlantic favors a more southerly position of the hydrostatically controlled low pressure trough, embedded in which is the intertropical convergence zone (itcz), which is the main rainbearing system for the nordeste. in addition, anomalously warm waters in the equatorial Pacific in January tend to be followed by Nordeste drought. The underlying chain of causalities has been explored by empirical diagnostics and numerical modelling. during El Nino years, an upper-tropospheric wave train extends from the equatorial eastern Pacific to the tropical north atlantic, affecting the patterns of upper-tropospheric topography and divergence, and hence of vertical motion over the Atlantic. This leads to a weaker meridional pressure gradient on the equatorward flank of the North Atlantic subtropical high, weaker North Atlantic tradewinds, an anomalously far northerly itcz position and thus nordeste drought. the previous empirical diagnostics are overall supported by the modelling experiments. Keywords: circulation mechanisms, teleconnection, numerical modeling,equatorial Pacific, tropical atlantic, nordeste rainfall resumo: MecanisMos de teleconeXÕes do noRdeste do BRasil : ModelaGeM nuMÉRica e eVidÊncia ePiRica. Experimentos específicos de modelagem numérica foram conduzidos para complementar diagnósticos empíricos realizados anteriormente dos mecanismos da circulação que relacionam anomalias na temperatura das águas superficiais do Pacífico equatorial em janeiro com as chuvas subsequentes em março-abril no Nordeste. Um gradiente térmico fraco (no sentido norte) no Atlântico favorece a uma posição mais meridional do cavado de baixa pressão, controlado hidrostaticamente, dentro do qual se encontra a Zona de Convergência Inter-Tropical (ITCZ), que é a principal fonte de chuvas para o norte do Nordeste. Além disso, águas anomalamente mais quentes no Pacífico equatorial em janeiro tendem a ser seguidas por secas no Nordeste. Pesquisas de caracter diagnóstico e de modelagem tem buscado estabelecer a causalidade nesta cadeia de eventos que interligam anomalias de temperatura nas águas superficiais do mar e precipitação. Durante anos de El Niño, aparece um trem de ondas na alta atmosfera que se estende do Pacífico equatorial oriental até o Atlântico norte tropical, afetando assim os campos de geopotencial e divergência na alta troposfera e, consequentemente, os movimentos ascendentes sobre o Atlântico. Tudo isto conduz a um fraco gradiente meridional de pressão no lado equatorial da célula de alta pressão do Atlântico norte, a alísios mais fracos no Atlântico norte e a um posicionamento da ITCZ mais ao norte, que são associados à seca no Nordeste. Os diagnósticos empíricos conc...

Fluctuations in the monthly discharge of Guyana Shield rivers, related to Pacific and Atlantic climate variability

“…The configuration of the atmospheric and oceanic anomalies complies with the reinforcement of the meridional Hadley cell between equatorial South America and the northern tropics, and of the zonal Walker cell between equatorial South America and the eastern Pacific. Our SST, geopotential height and wind anomalies are very similar to, though the inverse of, those described by Hastenrath (2006) and Kucharsky et al (2008), who looked for the teleconnection mechanisms of Northeast Brazil droughts, using observations and simulation data. They explained how ENSO events produce an upper tropospheric wave train, extending from the equatorial eastern Pacific to the tropical North Atlantic, affecting the divergence and the vertical motion over the Atlantic.…”

Climate prediction for Brazil's Nordeste by empirical and numerical modeling methods

“…Thus, the northern Nordeste has as main rain-bearing system the intertropical convergence zone (ITCZ) embedded in the near-equatorial low-pressure trough, whose latitude position is controlled by the meridional SST gradient and accompanied by characteristics of the field of meridional wind component in the tropical Atlantic sector. Equatorial Pacific SST may, through well-documented teleconnection mechanisms (Hastenrath, 2006b;Kucharski et al, 2008), affect the temperature of the tropical North Atlantic and thus the meridional SST gradient. The preseason rainfall in the Nordeste is favoured by the precursory ITCZ latitude position, and then may (Paulo Nobre, pers.…”

SPEEDY‐IER: A fast atmospheric GCM with water isotope physics

“…The AGCM has aided studies characterizing global climate modes including nonlinearities of the NAO and tropical Pacific interactions [ Kucharski and Molteni , ; Kucharski et al , ], studies of both forced and internal variability over the Indian Ocean and the Indian monsoon [ Bracco et al , ; Kucharski et al , ; Bracco et al , ; Kucharski et al , ], and El Niño–Southern Oscillation (ENSO) [ Rodríguez‐Fonseca et al , ; Yadav et al , ; Herceg Bulić et al , ]. Finally, SPEEDY has been used to examine the role of sea surface temperature (SST) variation on drought in the Americas and Africa [ Kucharski et al , ]. These studies share a common goal of simulating and explaining low‐frequency variability in the climate system: because of SPEEDY's efficiency, large ensemble runs spanning several decades can be performed at a low computational cost.…”