A34A-01 INVITED 16:00h
Exploiting Ocean Dynamics in Tropical Atlantic Variability
Ocean dynamical processes that contribute to seasonal-to-interannual variability and predictability of tropical Atlantic sea-surface temperature (SST) anomalies are explored with an atmospheric general circulation model (CCM3) coupled to either a mixed layer ocean (ML) or a Zebiak-Cane type of reduced gravity ocean (RGO). The coupled CCM3-ML model isolates thermodynamic feedbacks from dynamic feedbacks and allows the examination of the joint effect of local thermodynamic feedback and the direct impacts of ENSO on TAV. Ensembles of prediction runs yield the following findings: 1) in the northwestern part of the tropical Atlantic, the positive feedback between the surface heat flux and SST can play an important role in enhancing the predictability of the SST; 2) the remote influence from Pacific ENSO can enhance the SST predictability through constructive interference with the local thermodynamic feedback, but can also make the SST prediction more difficult when the interference is destructive; 3) ocean dynamics play a fundamental role for prediction of SST anomalies in the equatorial and south tropical Atlantic. To further exploit the ocean dynamics, a suite of coupled experiments were conducted with the coupled CCM3-RGO model. These experiments reveal that the entrainment process is the dominant oceanic process contributing to SST variability in the equatorial and south tropical Atlantic. Inclusion of this process in the coupled model substantially enhances the variance of the coupled variability, particularly the Atlantic "meridional mode", and improves the model's ability to simulate Atlantic response to ENSO forcing during the boreal spring and early summer in the equatorial and south tropical Atlantic. The potential role of the ocean dynamics in enhancing SST predictability in these regions will be discussed.
A34A-02 16:20h
The relationship between equatorial Atlantic SST and EL Niña
During the boreal summer SST variability in the tropical Atlantic is governed by a an equatorially confined pattern of variability, with a maximum in the eastern side of the Basin, reminiscent of El Niña. A modeling study of this mode of variability by Zebiak (1993) suggests that it is governed by similar dynamics but that it is not self-sustained. There is evidence that one of the ways by which it is invoked is linked with the evolution of the Pacific El Niña. Here we investigate this link and find that it is tends to change with time on multi-decadal times scales: strong in the first and last few decades of the 20 Century and weak in between. We describe the nature of this link as exhibited by the seasonal behavior of SST and speculate regarding the reasons for its non-stationarity.
A34A-03 16:40h
Land-Ocean Coupling as a Mechanism for Interannual Variability Over West Africa
The development of the rainy season over West Africa is strongly influenced by sea-surface temperatures over the Gulf of Guinea. The connection is linked to a dynamical process that modulates the southwesterly flow over the tropical Atlantic and the Guinea Coast and thereby determines the position of the African Easterly Jet (AEJ) over the continent. Superimposed upon this is the influence of the Tropical Easterly Jet (TEJ), which varies greatly in intensity and location over West Africa. Wet years in the semi-arid zone of West Africa (the Sahel-Soudan) are linked to a strong westerly jet in the lower troposphere, the effect of which is to weaken the AEJ and displace it northward. Along with the northward displacement, strong convective activity is also displaced into the higher latitudes of the semi-arid zone. A strong TEJ in wet years works in tandem with these other systems to enhance convection. During dry years the westerly jet does not develop and the AEJ is stronger, but it lies along a more southward track. This displaces the maximum convective activity equatorward and reduces its intensity, producing abnormally dry conditions in the semi-arid zone. The location and weakened intensity of the TEJ further reduce convection. A comparison with dynamic processes at similar latitudes over Central Africa, where land-ocean contrast plays no role, shows significant differences in the factors regulating precipitation and in the nature of interannual variability. These scenarios described above explain decadal scale variations in precipitation over West Africa.
A34A-04 17:00h
African Aerosol: An Integral Component of the Tropical Atlantic Climate System?
Africa is the largest source of aerosol, including biomass burning aerosol and mineral dust. In the tropical Atlantic, immediately downstream of Africa, the aerosol concentration undergoes seasonal, interannual and decadal variability. Aerosol is known to have climatic impacts through absorption and scattering of radiation and modulation of cloud and precipitation as cloud condensation nuclei and ice nuclei. In the tropical Atlantic, the air-sea coupled climate system also undergoes seasonal, interannual and decadal variability. Tremendous research efforts have been made to understand the climatic impact of aerosol on the one hand, and the dynamics and air-sea coupling of the tropical Atlantic variability on the other hand. There is, however, a striking gap in our knowledge regarding the possible role of African aerosol in the short-term (seasonal, interannual, and decadal) climate variability in the tropical Atlantic. This role is plausible because of the potential effect of African aerosol on cloud and precipitation in the Atlantic marine ITCZ, the coupling between the ITCZ and Atlantic sea surface temperature (SST), influences of Atlantic SST on African rainfall, and connections between African rainfall and African dust. In this presentation, a hypothesis is proposed that African aerosol is an integral component of the climate system in the tropical Atlantic. Knowns and unknowns related to this hypothesis are discussed and possible ways of testing this hypothesis suggested.
A34A-05 INVITED 17:20h
Insights Into Ocean-Land-Atmosphere Interactions In The Tropical Atlantic From The Simulation Of The Boreal Summer African Monsoon In NSIPP1
Analysis of a 9-member ensemble of simulations with a state-of-the-art atmospheric model, forced only by the observed record of sea surface temperature (SST) over 1930-2000, is presented to discuss some aspects of ocean-land-atmosphere interactions in the boreal summer African monsoon. Comparison of observations and model output reveals that the model is capable of capturing the dominant patterns of variability, and successful in separating them into one pattern representing the Gulf of Guinea, between the equator and 10N, and another representing the semi-arid Sahel, between 10N and 20N. While the former is found to connect rainfall over Africa to the Atlantic marine Intertropical Convergence Zone (ITCZ), and to be controlled by local, i.e. eastern equatorial Atlantic, SSTs, the latter is best characterized as a continental pattern, capturing the essence of the African summer monsoon, and displaying high sensitivity to SSTs in the global tropics.
A34A-06 INVITED 17:40h
Role of Air-Sea Heat Fluxes in Seasonal Climate and Variability of the Eastern Tropical Atlantic
The eastern tropical Atlantic exhibits a pronounced annual cycle in SST, in tropical deep convection, and in both zonal and meridional components of surface winds. It has been well known that ocean dynamical processes driven by monsoonal winds play an important role in the annual cooling of the sea surface in the eastern equatorial Atlantic and in influencing air-sea interactions between the Atlantic cold tongue and African monsoon. However, much less have been studied about the effect of the monsoon winds on air-sea heat fluxes and how much air-sea heat fluxes contribute to the seasonal climate and variability of the eastern tropical Atlantic. This study investigates the structure and variability of air-sea heat fluxes in the tropical Atlantic and its different effects on the atmosphere-ocean interactions in the eastern and western basins. The study utilizes an objectively analyzed air-sea heat flux product newly developed at WHOI, the ISCCP surface radiation dataset, and NCEP reanalysis and ECMWF ReAnalysis-40 (ERA40) flux products. The impact of data quality on physical analysis will also be discussed.
http://www.whoi.edu/science/PO/people/lyu/res-flux.html