Interannual variability of the tropical Atlantic independent of and associated with ENSO: Part I. The North Tropical Atlantic

The interannual variability of the tropical Atlantic ocean-atmosphere system is examined using 50 years of sea-surface temperature (SST) and re-analysis data, and satellite data when available. A singular value decomposition analysis of 12- to 72-month bandpass filtered SST and zonal wind stress reveals two dominant modes of interannual variability. The SST anomalies are confined to the North Tropical Atlantic (NTA) in the first mode and extend over the equatorial and South Tropical Atlantic in the second mode. No evidence is found for an Atlantic SST dipole. The structure of the first (NTA) mode is examined in detail here, while the second mode has been described in a companion paper. In particular, the relationship of the NTA mode with El Nino-Southern Oscillation (ENSO) is investigated. There are 12 NTA events (seven warm and five cold) that are associated with ENSO, and 18 NTA events (seven warm and 11 cold) that are independent of ENSO. The ENSO-associated NTA events appear to be a passive response to remote ENSO forcing, mainly via a Pacific-North America (PNA)-like wave train that induces SST anomalies over the NTA through changes in the surface wind and latent heat flux. The NTA anomalies peak four months after ENSO. There does not appear to be an atmospheric response to the NTA SST anomalies as convection over the Atlantic is suppressed by the anomalous Walker circulation due to ENSO. The ENSO-independent NTA events also appear to be induced by an extratropical wave train from the Pacific sector (but one that is independent of Pacific SST), and forcing by the North Atlantic Oscillation (NAO) also contributes. As the event matures, the atmosphere does respond to the NTA SST anomalies, with enhanced convection over the Caribbean and a wave train that propagates northeastward to Europe

Interannual variability of the Tropical Atlantic independent of and associated with ENSO: Part II. The South Tropical Atlantic

Two dominant ocean-atmosphere modes of variability on interannual timescales were defined in Part I of this work, namely, the North Tropical Atlantic (NTA) and South Tropical Atlantic (STA) modes. In this paper we focus on the STA mode that covers the equatorial and sub-tropical South Atlantic. We show that STA events occurring in conjunction with ENSO have a preference for the southern summer season and seem to be forced by an atmospheric wave train emanating from the central tropical Pacific and travelling via South America, in addition to the more direct ENSO-induced change in the Walker circulation. They are lagged by one season from the peak of ENSO. These events show little evidence for other-than-localised coupled ocean-atmosphere interaction. In contrast, STA events occurring in the absence of ENSO favour the southern winter season. They appear to be triggered by a Southern Hemisphere wave train emanating from the Pacific sector, and then exhibit features of a self-sustaining climate mode in the tropical Atlantic. The southward shift of the inter tropical convergence zone that occurs during the warm phase of such an event triggers an extra tropical wave train that propagates downstream in the Southern Hemisphere. We present a unified view of the NTA and STA modes through our observational analysis of the interannnual tropical Atlantic variability

Methane seepage in a Cretaceous greenhouse world recorded by an unusual carbonate deposit from the Tarfaya Basin, Morocco

During the Cretaceous major episodes of oceanic anoxic conditions triggered large scale deposition of marine black shales rich in organic carbon. Several oceanic anoxic events (OAEs) have been documented including the Cenomanian to Turonian OAE 2, which is among the best studied examples to date. This study reports on a large limestone body that occurs within a black shale succession exposed in a coastal section of the Tarfaya Basin, Morocco. The black shales were deposited in the aftermath of OAE 2 in a shallow continental sea. To decipher the mode and causes of carbonate formation in black shales, a combination of element geochemistry, palaeontology, thin section petrography, carbon and oxygen stable isotope geochemistry and lipid biomarkers are used. The ¹³C-depleted biphytanic diacids reveal that the carbonate deposit resulted, at least in part, from microbially-mediated anaerobic oxidation of methane in the shallow subseafloor at a hydrocarbon seep. The lowest obtained δ¹³Ccarbonate values of −23.5‰ are not low enough to exclude other carbon sources than methane apart from admixed marine carbonate, indicating a potential contribution from in situ remineralization of organic matter contained in the black shales. Nannofossil and trace metal inventories of the black shales and the macrofaunal assemblage of the carbonate body reveal that environmental conditions became less reducing during the deposition of the background shales that enclose the carbonate body, but the palaeoenvironment was overall mostly characterized by high productivity and episodically euxinic bottom waters. This study reconstructs the evolution of a hydrocarbon seep that was situated within a shallow continental sea in the aftermath of OAE 2, and sheds light on how these environmental factors influenced carbonate formation and the ecology at the seep site

ENSO Atmospheric Teleconnections and Their Response to Greenhouse Gas Forcing

This is the final version of the article. Available from AGU via the DOI in this record.El Niño and Southern Oscillation (ENSO) is the most prominent year-to-year climate fluctuation on Earth, alternating between anomalously warm (El Niño) and cold (La Niña) sea surface temperature (SST) conditions in the tropical Pacific. ENSO exerts its impacts on remote regions of the globe through atmospheric teleconnections, affecting extreme weather events worldwide. However, these teleconnections are inherently nonlinear and sensitive to ENSO SST anomaly patterns and amplitudes. In addition, teleconnections are modulated by variability in the oceanic and atmopsheric mean state outside the tropics and by land and sea ice extent. The character of ENSO as well as the ocean mean state have changed since the 1990s, which might be due to either natural variability or anthropogenic forcing, or their combined influences. This has resulted in changes in ENSO atmospheric teleconnections in terms of precipitation and temperature in various parts of the globe. In addition, changes in ENSO teleconnection patterns have affected their predictability and the statistics of extreme events. However, the short observational record does not allow us to clearly distinguish which changes are robust and which are not. Climate models suggest that ENSO teleconnections will change because the mean atmospheric circulation will change due to anthropogenic forcing in the 21st century, which is independent of whether ENSO properties change or not. However, future ENSO teleconnection changes do not currently show strong intermodel agreement from region to region, highlighting the importance of identifying factors that affect uncertainty in future model projections.S. W. Y. is supported by the KoreaMeteorological Administration Researchand Development Program under grant KMIPA2015-2112. Wenju Cai is supported by Earth System and Climate Change Hub of the Australia National Environmental Science Programme, and Centre for Southern Hemisphere Oceans Research, an international collaboration between CSIRO and Qingdao National Laboratory for Marine Sciences and Technology. B. Dewitte acknowledges supports from FONDECYT(1151185) and from LEFE-GMMC. Dietmar Dommenget is supported by ARC Centre of Excellence for Climate System Science (CE110001028)