The phrase "latitudinal gradient" means that something changes or varies as you move from the north to the south or vice versa on the Earth's surface. Full definition
«Connecting the proximate mechanisms responsible for organismal diversity to the ultimate causes of latitudinal gradients in species richness.» (uchospitals.edu)
The observed annual mean latitudinal gradient of atmospheric CO2 concentration during the last 20 years is relatively large (about 3 to 4 ppm) compared with current measurement accuracy. (ipcc.ch)
There are opposite latitudinal gradients in these threats to reefs, meaning that areas projected to experience annual bleaching later are exposed to the effects of acidification for longer. (reefresilience.org)