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The changing interhemispheric temperature difference: mechanisms and impacts


The surface temperature difference between the northern and southern hemispheres is the simplest climate change indicator following global mean temperature, and reveals unique information about the state of the global climate, in particular regarding tropical atmospheric circulation and rainfall. This dissertation examines the historical behavior, future projections, and tropical hydrologic impacts of this interhemispheric (north-south) temperature difference.

Historically, most of the variability in the interhemispheric temperature difference is from the northern hemisphere. Investigation of specific-forcing simulations shows that globally uniform radiative forcing from well-mixed greenhouse gases causes asymmetric northern warming due to the hemispheric land-ocean contrast and Arctic amplification. However, sulfate aerosols, which were disproportionately emitted in the northern hemisphere, caused cooling that masked the northern warming until the mid 1970s. Air pollution regulations in North America and Europe combined with sustained emissions of greenhouse gases have resulted in a positive trend in the interhemispheric temperature difference in the past few decades.

Future simulations of phases 3 and 5 of the Coupled Model Intercomparison Project (CMIP3 and CMIP5) project that this recent asymmetric northern hemispheric warming will continue in the 21st century. The projected increase is well outside the range of historical variability in both moderate and business-as-usual scenarios. There is also a projected multimodel mean northward shift in the Hadley circulation and tropical rainfall, though the multimodel spread is much larger than for temperature.

A prominent feature in the interhemispheric temperature record is an abrupt decrease around 1970, which was most pronounced in sea surface temperature (SST). Examination of the shift using surface and subsurface ocean datasets reveals that there were pronounced cooling and freshening at depth in the subpolar North Atlantic north of 50°N, which coincided with opposing warming and salting in the western mid-latitude North Atlantic between 35° and 45°N. These combined features cannot be completely accounted for by atmospheric forcing alone. Rather, they point to a discrete subpolar North Atlantic freshening known as the Great Salinity Anomaly and the corresponding weakening of the North Atlantic thermohaline circulation as causes of the 1970 interhemispheric shift.

Spatially, the strongest surface temperature correlations with the interhemispheric SST difference are found in the extratropical North Atlantic and southern hemisphere oceans, which are key regions for the shift around 1970. Additionally, the correlations with low-latitude rainfall and the flow of major rivers are examined. Positive rainfall correlations are found in tropical North Africa, South and Southeast Asia; and negative correlations are found in Australia, southern and eastern South America, and northern Mexico. Among the rivers examined, the Niger, which flows through the Sahel, is most strongly correlated with the interhemispheric SST difference. The Indus is also significantly positively correlated, while the Mississippi and Parana are significantly negatively correlated. Based on this assessment, these regions are likely to experience impacts in response to future variations in the interhemispheric temperature difference from the extratropical Atlantic and southern hemisphere oceans, such as from the meridional overturning circulation or aerosol forcing.

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