This paper examines surface atmospheric and oceanic conditions associated with anomalous rainfall at the Guinea coast. For time-scales shorter than 5 years, rainfall in the Guinea coast is practically not correlated with rainfall in the Sahel, although both time series show a well-defined drying trend in the last 30 years or so. While interannual variability of Sahel rainfall is associated with a sea-surface temperature (SST) pattern anti-symmetric about the Equator, anomalous rainfall near the Guinea coast in associated with an SST pattern roughly symmetric about the Equator. During wet years near the Guinea coast this SST pattern corresponds to a less developed cold tougue in the boreal summer.Using the Comprehensive Ocean-Atmosphere Data Set surface fields we examine key terms of the oceanic heat budget. Anomalous shortwave radiation and latent heat fluxes seem to play an important role in the decaying of the SST anomaly. During wet years anomalous atmospheric winds act to relax the trade winds, which probably reduces the equatorial thermocline slope and contributes to the observed warming of the equatorial waters in the east.We also investigate the relationship between the surface winds and the underlying SST using numerical and analytical solutions of a simple model. The anomalous winds are shown to be consistent with the hydrostatic adjustment of the tropical boundary layer.

The meridional gradient of sea surface temperature (SST) in the tropical Atlantic hydrostatically controls the atmospheric pressure gradient, thereby steering the latitude position of the Intertropical Convergence Zone (ITCZ), which affects rainfall in adjacent land areas. This study investigates the mechanisms controlling long-term changes in meridional SST gradients, using surface and subsurface ship observations from 1951-1990. SST forcing from surface heat fluxes and entrainment are scaled by monthly climatic-mean mixed-layer depth. Observed SST tendency and contributions from various forcing mechanisms are computed for the 1951-1990 mean and trend. A substantial warming trend in the surface waters of the tropical South Atlantic is concentrated in the austral summer months of February-March, while SST in the tropical North Atlantic increased in boreal summer (August-September) and decreased in winter. As a consequence of the strengthened interhemispheric SST gradient in February-March, the position of the ITCZ was displaced southward, and rainfall in northeast Brazil increased over 1951-1990. A southward displacement of the ITCZ during boreal summer, accompanying warming between the equator and the ITCZ contributed to the decrease of rainfall over the west African Sahel. Strongest warming trends in the respective summer hemisphere imply an amplification of both seasonal cooling during winter and warming during summer in each hemisphere. During September-February a trend toward intensified northeast trade winds results in increased seasonal cooling due to latent heat transfer, whereas weakened southeast trades reduce latent heat loss, intensifying seasonal warming. For March-August a dominant mechanism forcing enhanced seasonal cooling in the South Atlantic is associated with entrainment as mixed-layer depth increases during the transition to winter; this process acts to dilute warm anomalies developed during summer. 16,683 16,684 WAGNER: DECADAL-SCALE TRENDS IN THE TROPICAL ATLANTIC 60øW . 30øW _ ,, --, _20oS , , • 60:W ,/ 30øW I

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How much of politics is specific to its actors and how much is the reflection of an established structure is a perennial concern of political analysts, one that becomes especially intense with the candidacy and then the presidency of Donald Trump. In order to have a template for assigning the outcomes of politics to structure rather than idiosyncrasy, we begin with party balance, ideological polarization, substantive content, and a resulting process of policy-making drawn from the immediate postwar period. The analysis then jumps forward with that same template to the modern world, dropping first the Trump candidacy and then the Trump presidency into this framework. What emerges is a modern electoral world with increased prospects for what might be called off-diagonal candidacies and a policy-making process that gathers Bill Clinton, George W. Bush, Barack Obama, and Donald Trump together as the modern presidents.