Dierk Polzin

Diagnosing the 2005 Drought in Equatorial East Africa

Abstract Equatorial East Africa suffered severe drought during its 2005 “short rains,” centered on October–November. The circulation mechanisms of such precipitation anomalies are examined, using long-term upper-air and surface datasets, and based on diagnostic findings from earlier empirical investigations. The steep eastward pressure gradient is conducive to fast westerlies over the central-equatorial Indian Ocean, surface manifestation of a powerful zonal circulation cell with subsidence over East Africa, and ascending motion over Indonesia. With fast westerlies, rainfall in East Africa is deficient and they tend to be accompanied by anomalously cold waters in the northwestern and warm anomalies in the southeastern extremity of the equatorial Indian Ocean Basin, without any seesaw between these domains. In October–November 2005, pressure in the west was anomalously high, entailing a steep eastward pressure gradient along the equator, conducive to fast westerlies and, further symptomatic of the zonal ci...

Assimilation of seasonal chlorophyll and nutrient data into an adjoint three-dimensional ocean carbon cycle model : Sensitivity analysis and ecosystem parameter optimization

[1] An adjoint method is applied to a three-dimensional global ocean biogeochemical cycle model to optimize the ecosystem parameters on the basis of SeaWiFS surface chlorophyll observation. We showed with identical twin experiments that the model simulated chlorophyll concentration is sensitive to perturbation of phytoplankton and zooplankton exudation, herbivore egestion as fecal pellets, zooplankton grazing, and the assimilation efficiency parameters. The assimilation of SeaWiFS chlorophyll data significantly improved the prediction of chlorophyll concentration, especially in the high-latitude regions. Experiments that considered regional variations of parameters yielded a high seasonal variance of ecosystem parameters in the high latitudes, but a low variance in the tropical regions. These experiments indicate that the adjoint model is, despite the many uncertainties, generally capable to optimize sensitive parameters and carbon fluxes in the euphotic zone. The best fit regional parameters predict a global net primary production of 36 Pg C yr -1 , which lies within the range suggested by Antoine et al. (1996). Additional constraints of nutrient data from the World Ocean Atlas showed further reduction in the model-data misfit and that assimilation with extensive data sets is necessary.

Diagnosing the droughts and floods in equatorial East Africa during boreal autumn 2005-08.

Abstract Building on an earlier report on the 2005 drought in equatorial East Africa, this short note examines the circulation mechanisms of the anomalies in the boreal autumn “short rains” season in the subsequent three years. Westerlies during this season are the surface manifestation of a powerful zonal–vertical circulation cell along the Indian Ocean equator. The surface equatorial westerlies were fast during the 2005 and 2008 droughts, near average during the near-average 2007 short rains, and slack during the 2006 floods, consistent with the known circulation diagnostics.

Circulation Mechanisms of Kenya Rainfall Anomalies

Abstract Expanding earlier studies on the boreal spring and autumn rainy seasons in equatorial East Africa, pending challenges on the mechanisms of rainfall variability, are investigated. Eastward pressure gradient and slack south Indian Ocean trade winds allow surface equatorial westerlies in spring and autumn. Complementing that, upper-tropospheric easterlies are required for the development of a zonal vertical circulation cell along the Indian Ocean equator. Because of the summer warming and high stand of upper-tropospheric topography over South Asia, strong upper-tropospheric easterlies over the tropical northern and equatorial Indian Ocean persist from summer into autumn, thus allowing the development of a zonal vertical circulation cell. By contrast, the winter cooling entails low stand of upper-tropospheric topography in the north, thus hindering easterlies over the equator. Consequently, an equatorial zonal circulation cell does not develop in boreal spring. The equatorial zonal circulation cell, ...

Circulation mechanisms of climate anomalies in the equatorial Indian Ocean

The circulation mechanisms of climatic anomalies in the equatorial Indian Ocean are diagnosed from long-term surface and upper-air data for the October-November core of the East African Short Rains and season of zonal circulation cell along the Indian Ocean Equator. Climatic variability is dominated by variations in the intensity of the zonal circulation cell, manifested in subsidence in the West, ascending motion in the East, divergent westward flow in the upper troposphere, and equatorial surface westerlies (UEQ). The UEQ requires steep zonal pressure gradient, with low pressure in the East. Disastrous floods in East Africa and drought in Indonesia occur in years with weak development of the zonal circulation cell, characterized by high surface pressure anomaly in the East, which is common but not limited to the low phase of the Southern Oscillation (SO). A deficient Indian summer monsoon, common in the low SO phase, leaves behind an anomalously warm western Indian Ocean. With high pressure anomaly in the East during the months preceding October-November the Indonesian waters tend to be anomalously cold. Accordingly, with weak zonal circulation cell and in the low SO phase, the westward gradient of sea surface temperature in the equatorial Indian Ocean tends to be reduced. Weak equatorial zonal circulation in October-November is preceded by flow departures of same sense in the near-equatorial atmosphere evolving since boreal summer, and this is evidenced in the 1958-97 record as a whole, as well as for the individual extreme years 1961, 1994 and 1997. Peculiar to the 1961 extreme event is that it was preceded by an extremely good Indian summer monsoon yet anomalously warm waters in the West of the basin, and that the functionally important surface high pressure anomaly in the East did not coincide with the low SO phase.

Teleconnection mechanisms of northeast Brazil droughts: modeling and empirical evidence

Experimentos especificos de modelagem numerica foram conduzidos para complementar diagnosticos empiricos realizados anteriormente dos mecanismos da circulacao que relacionam anomalias na temperatura das aguas superficiais do Pacifico equatorial em janeiro com as chuvas subsequentes em marco-abril no Nordeste. Um gradiente termico fraco (no sentido norte) no Atlântico favorece a uma posicao mais meridional do cavado de baixa pressao, controlado hidrostaticamente, dentro do qual se encontra a Zona de Convergencia Inter-Tropical (ITCZ), que e a principal fonte de chuvas para o norte do Nordeste. Alem disso, aguas anomalamente mais quentes no Pacifico equatorial em janeiro tendem a ser seguidas por secas no Nordeste. Pesquisas de caracter diagnostico e de modelagem tem buscado estabelecer a causalidade nesta cadeia de eventos que interligam anomalias de temperatura nas aguas superficiais do mar e precipitacao. Durante anos de El Nino, aparece um trem de ondas na alta atmosfera que se estende do Pacifico equatorial oriental ate o Atlântico norte tropical, afetando assim os campos de geopotencial e divergencia na alta troposfera e, consequentemente, os movimentos ascendentes sobre o Atlântico. Tudo isto conduz a um fraco gradiente meridional de pressao no lado equatorial da celula de alta pressao do Atlântico norte, a alisios mais fracos no Atlântico norte e a um posicionamento da ITCZ mais ao norte, que sao associados a seca no Nordeste. Os diagnosticos empiricos concordam com os experimentos de modelagem numerica.

Diagnosing Kenya Rainfall in Boreal Autumn: Further Exploration

AbstractA powerful zonal vertical circulation cell along the Indian Ocean equator controls the boreal autumn rains in Kenya, with a tight negative correlation between surface westerlies (UEQ) and rainfall. UEQ is favored by a steep eastward pressure gradient (PWE) and slow winds in the downstream portion of the South Indian Ocean trade winds (SIW). The high phase of the Southern Oscillation (SO) favors weak SIW, lower pressure in the east, and thus steeper PWE, but that is also affected by pressure in the west. In 1958–97 circulation causalities were most distinct in the regime of abundant rain and slow UEQ, with the SO in the low phase. In the regime of deficient rain and fast UEQ, relationships were less distinct, although the SO was in the high phase almost throughout. In the decade 2001–10, UEQ had a weaker (stronger) correlation with PWE (SIW), the SO was more active and contributing in the proper sense to pressure in the east, but PWE was mainly controlled by pressure in the west. UEQ strongly contr...

Climate of Brazil's nordeste and tropical atlantic sector: preferred time scales of variability

Resuming earlier research, this study explores rainfall variability in Brazils Nordeste and underlying circulation mechanisms. The semi-arid northern Nordeste has its short rainy season centered around March-April-May, when temperature maximum, low pressure trough and wind confluence reach their southernmost position. Interannual variability can be understood as departures from the average annual cycle. Based on novel long-term datasets, the present study explores the preferred time scales of variability. In Nordeste rainfall and pertinent circulation indices in the tropical Atlantic sector most prominent are frequencies of 13.2, 9.9 and 5.6 years. Frequency peak of 13.1 years appears also in the record of Southern Oscillation, and of 5.6 years in North Atlantic Oscillation, indicative of causality chain.