Recent studies suggest the existence of a global atmospheric teleconnection of extratropical cooling to the tropical rainfall climate, mediated through the development of a thermal contrast between the hemispheres-an interhemispheric thermal gradient. This teleconnection has been largely motivated by studies that show a global synchronization of rapid climate change during abrupt climate changes of the last glacial period, in addition to attribution studies of twentieth-century Sahel drought and studies that examined the climate impacts of anthropogenic aerosols. This research has led to interesting developments in atmospheric dynamics of the underlying mechanisms and in applications toward understanding past and present tropical climate change. The emerging teleconnection hypothesis promises to offer new insights into understanding future patterns of tropical rainfall changes due to interhemispheric thermal gradients from greenhouse warming, aerosols, and land-use change.

The temperature contrast between the Northern and Southern Hemispheres—the interhemispheric temperature asymmetry (ITA)—is an emerging indicator of global climate change, potentially relevant to the Hadley circulation and tropical rainfall. The authors examine the ITA in historical observations and in phases 3 and 5 of the Coupled Model Intercomparison Project (CMIP3 and CMIP5) simulations. The observed annual-mean ITA (north minus south) has varied within a 0.8°C range and features a significant positive trend since 1980. The CMIP multimodel ensembles simulate this trend, with a stronger and more realistic signal in CMIP5. Both ensembles project a continued increase in the ITA over the twenty-first century, well outside the twentieth-century range. The authors mainly attribute this increase to the uneven spatial impacts of greenhouse forcing, which result in amplified warming in the Arctic and northern landmasses. The CMIP5 specific-forcing simulations indicate that, before 1980, the greenhouse-forced ITA trend was primarily countered by anthropogenic aerosols. The authors also identify an abrupt decrease in the observed ITA in the late 1960s, which is generally not present in the CMIP simulations; it suggests that the observed drop was caused by internal variability. The difference in the strengths of the northern and southern Hadley cells covaries with the ITA in the CMIP5 simulations, in accordance with previous findings; the authors also find an association with the hemispheric asymmetry in tropical rainfall. These relationships imply a northward shift in tropical rainfall with increasing ITA in the twenty-first century, though this result is difficult to separate from the response to global-mean temperature change.

The tropical Atlantic interhemispheric gradient in sea surface temperature significantly influences the rainfall climate of the tropical Atlantic sector, including droughts over West Africa and Northeast Brazil. This gradient exhibits a secular trend from the beginning of the twentieth century until the 1980s, with stronger warming in the south relative to the north. This trend behavior is on top of a multidecadal variation associated with the Atlantic multidecadal oscillation. A similar long-term forced trend is found in a multimodel ensemble of forced twentieth-century climate simulations. Through examining the distribution of the trend slopes in the multimodel twentieth-century and preindustrial models, the authors conclude that the observed trend in the gradient is unlikely to arise purely from natural variations; this study suggests that at least half the observed trend is a forced response to twentieth-century climate forcings. Further analysis using twentiethcentury single-forcing runs indicates that sulfate aerosol forcing is the predominant cause of the multimodel trend. The authors conclude that anthropogenic sulfate aerosol emissions, originating predominantly from the Northern Hemisphere, may have significantly altered the tropical Atlantic rainfall climate over the twentieth century.

Sea surface salinity (SSS) is a major ocean circulation component and indicator of the hydrological cycle. Here we investigate an unprecedented Atlantic SSS compilation from 1896 to 2013 and analyze the main modes of SSS decadal variability. Using principal component analysis, we find that the low‐latitude (tropical and subtropical) Atlantic and the subpolar Atlantic have distinct variability. Subpolar and low‐latitude SSS are negatively correlated, with subpolar anomalies leading low‐latitude anomalies by about a decade. Subpolar SSS varies in phase with the Atlantic Multidecadal Oscillation (AMO), whereas low‐latitude SSS varies in phase with the North Atlantic Oscillation (NAO). Additionally, northern tropical SSS is anticorrelated with Sahel rainfall, suggesting that SSS reflects the Intertropical Convergence Zone latitude. The 1896–2013 SSS trend shows amplification of the mean SSS field, with subpolar freshening and low‐latitude salinification. The AMO and NAO have little effect on the long‐term trend but contribute to the trend since 1970.