Annual to Interannual Equatorial Atlantic Variability: Mechanisms and Tropical Impacts.

Ding, Hui (2010) Annual to Interannual Equatorial Atlantic Variability: Mechanisms and Tropical Impacts. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, Germany, 92 pp.

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Abstract

In this thesis, I investigate the annual to interannual variability in the equatorial Atlantic and its impacts on other tropical oceans. There are three parts to this work. The first part (chapter 2) is to understand the seasonal cycle of ocean dynamics, which has been published in 2009. The second part (chapter 3) is to understand the mechanisms of interannual variability in the equatorial Atlantic, which is mainly associated with Atlantic zonal mode. This part has been accepted recently for publication and is in press. The last part (chapter 4) investigates the influence of Atlantic zonal mode on El Niño Southern Oscillation in a coupled climate model. This work will be submitted for publication. In chapter 2, the dynamics of the seasonal cycle in the upper equatorial Atlantic ocean are studied using observations and a hierarchy of ocean models. Distinctive features of the seasonal cycle are strong annual and semi-annual components; eastward (westward) propagating sea surface height (SSH) and thermocline depth at the equator (off the equator); and westward propagating surface zonal currents at the equator. Modelling results show that linear theory can explain the seasonal cycle in thermocline depth and SSH. While to first order linear theory can also explain the structure of the seasonal cycle of surface zonal currents at the equator, nonlinear terms are required. The linear solution is essentially determined by the four gravest baroclinic modes, and Kelvin and first meridional mode Rossby waves. The semi-annual cycle in zonal winds although much weaker than the annual component forces a strong semi-annual component in SSH and surface zonal currents, because it excites the basin mode of the second baroclinc mode. The second part of the thesis investigates the dynamics of the Atlantic zonal mode using observed sea surface height (SSH), sea surface temperature (SST), and heat flux and reanalysis wind stress and upper ocean temperature. Principal oscillation pattern (POP) analysis shows that the zonal mode is an oscillatory normal-mode of the observed coupled system, obeying the delayed-action/recharge oscillator paradigm for ENSO. Net surface heat flux anomalies generally act to damp SST anomalies. The zonal mode explains a large amount (70%) of SST variability in the east and a significant fraction (19%) of equatorial variability. Thus, the predictability potential in the Equatorial Atlantic on seasonal time scales may be considerably higher than currently thought. In the third part, a coupled climate model is employed to investigate the influence of Atlantic zonal mode on ENSO in the Pacific. Five ensemble member simulations forced with observed sea surface temperature in the Tropical Atlantic, but with full air-sea coupling allowed elsewhere, were performed for the period 1950-2005. Model results show that a warm phase of Atlantic zonal mode during boreal summer favours the development of a La Niña event during the boreal winter and vice versa. A warm SST anomaly in the Atlantic affects the Walker Circulation with anomalous rising and subsiding motions in the Tropical Atlantic and eastern Tropical Pacific, respectively. This causes an easterly anomaly to the west of subsiding region in the Pacific that is amplified by the Bjerknes positive feedback, favouring the following development of a La Niña event. In general, the coupled model has revealed the same mechanisms of how Atlantic zonal mode influence ENSO as found in observations and reanalysis. This is the first convincing evidence from a model for the existence of a physically robust influence of the Atlantic Zonal mode on Pacific interannual variability.

Document Type: Thesis (PhD/ Doctoral thesis)
Thesis Advisor: Latif, Mojib and Keenlyside, Noel S.
Keywords: Meteorology; Tropical Atlantic Variability, seasonal variations, teleconnection
Research affiliation: OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-ME Maritime Meteorology
Open Access Journal?: Yes
Date Deposited: 03 Aug 2010 10:38
Last Modified: 06 Jul 2012 15:05
URI: http://oceanrep.geomar.de/id/eprint/8737

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