Wetter and drier

Increase in the range between wet and dry season precipitation : Nature Geoscience 

The abstract I'll reproduce in full, because this topic is something that climate change "skeptics" just continually can't seem to get their head around :

Global temperatures have risen over the past few decades. The water vapour content of the atmosphere has increased as a result, strengthening the global hydrological cycle^{1, 2, 3, 4}. This, in turn, has led to wet regions getting wetter, and dry regions drier^{1, 2, 3, 4, 5, 6}. Climate model simulations suggest that a similar intensification of existing patterns may also apply to the seasonal cycle of rainfall⁷. Here, we analyse regional and global trends in seasonal precipitation extremes over the past three decades, using a number of global and land-alone observational data sets. We show that globally the annual range of precipitation has increased, largely because wet seasons have become wetter. Although the magnitude of the shift is uncertain, largely owing to limitations inherent in the data sets used, the sign of the tendency is robust. On a regional scale, the tendency for wet seasons to get wetter occurs over climatologically rainier regions. Similarly, the tendency for dry season to get drier is seen in drier regions. Even if the total amount of annual rainfall does not change significantly, the enhancement in the seasonal precipitation cycle could have marked consequences for the frequency of droughts and floods.

A journalist explanation of the study adds some more detail:

...the gap between wet and dry seasons was widening at a rate of 1.47 millimeters per day per century. All three trends, they report, “are significant at the 99 percent confidence level.”

In the real world most of us experience, it’s hard to be sure that rainy spells are rainier, and dry seasons are drier: that is because, as the authors concede, global rainfall patterns are “spatially complex.”

But there is general agreement that such changes are taking place, with good physical reasons for doing so: a warmer world means more evaporation, and more precipitation. Furthermore, the authors say, simulations predict such a pattern and observations confirm it.