Northland: the climate of a world with a hemispheric continent and a
hemispheric ocean
Abstract
Join us in an exploration of the climate of Northland, a world where the
entire northern hemisphere is covered by a continent, and the entire
southern hemisphere is covered by an ocean! On the continent, we will
visit the seasonally moist tropics, the subtropical desert, and the
Great Northern Swamp. We explore the interplay between water, energy,
land, ocean, and atmosphere in this idealized climate model study. We
find that the presence of a continent greatly increases the poleward
extend of the ITCZ over both the land and ocean hemispheres compared to
an aquaplanet, as a result of hemispheric energy imbalances introduced
by (a) the small heat capacity of land and (b) large reductions in
atmospheric water vapor (and thus reduced longwave trapping) over the
continent. A combination of moisture transport from the tropics and
local water recycling results in a polar swamp over the continent. We
explore how the climate state responds to changes in the albedo and
evaporative resistance of the continent. While making the land surface
darker leads to warming, we find that decreasing evaporation from the
land surface leads to global-scale cooling. This is in contrast to past
studies, where reduced terrestrial evaporation leads to warming as a
result of suppressed evaporative cooling of the land surface. In the
case of Northland, the lack of an ocean to provide water to the northern
hemisphere means that decreasing land evaporation leads to large
reductions in water vapor over the northern hemisphere, in turn reducing
strength of the greenhouse effect, resulting in cooling of near-surface
air temperatures. This cooling signal is strongest over the continent,
but cools air temperatures over the ocean hemisphere as well. We
hypothesize that a threshold exists in the temperature response to
reduced terrestrial evaporation: for small decreases in evaporation,
reduced latent cooling dominates and near-surface temperatures warm,
while for large decreases in evaporation, reduced longwave trapping from
reduced atmospheric water vapor dominate, cooling near-surface
temperatures. Through this idealized study of a hypothetical, Earth-like
planet, we gain valuable insight into the connections between water,
energy, land surface properties, and continental distribution in
controlling global-scale climate.