The season is indicated by Ls, which means solar longitude. Ls is given in degrees 0 to 360, with northern Spring being 0 to 90, Summer, 90 to 180, Fall, 180 to 270, and Winter, 270-360. It appears along the horizontal axis in the graphs in figure 1. The geographical latitude appears along the vertical axis in these graphs.
I'll be very interested to see how they break down their findings further according to geographical longitude. Then we could correlate the increase to low altitude locations where liquid water could persist for significant periods.
For more info on interpreting their findings, see these refs on their observations on the northern polar regions:

Mars probes find more frozen water
Northern ice boosts search for ancient life.
WASHINGTON, June 26, 2003
http://msnbc.msn.com/id/3077661/

CO2 Snow Depth and Subsurface Water-Ice Abundance in the Northern Hemisphere of Mars.
Science, Vol 300, Issue 5628, 2081-2084 , 27 June 2003
http://www.sciencemag.org/cgi/content/full/300/5628/2081

Bob Clark