We present a study that is motivated by a population of water ice clouds in the Hellas Basin that has been observed by the MARs Color Imager (MARCI) instrument on the Mars Reconnaissance Orbiter (MRO) to persist throughout the majority of Northern Hemisphere (NH) summer. Although water ice clouds are present in Hellas at very low opacities throughout NH spring, they noticeably thicken after Ls 60° and continue to do so until their peak optical thickness is attained at Ls ~120°. They dissipate rapidly from Ls 120° to 150°, and by Ls 150°, the only clouds in Hellas are on the southern side of the basin and are indistinguishable from the polar hood clouds. We use the NASA/Ames Legacy Mars Global Climate Model (GCM), which is supported by the Agency's Mars Climate Modeling Center, to investigate the dynamical and microphysical mechanisms that control the formation and evolution of Hellas water ice clouds. We show that water is transported from the North Polar Residual Cap (NPRC) southward across the equator and into the Hellas region. Water vapor is confined down low by cloud formation in the aphelion cloud belt, thus limiting the effectiveness of transport by the zonal mean overturning circulation (i.e., the Hadley cell). Thus, contrary to the commonly held conceptual understanding that the Hadley cell controls cross-equatorial water transport during this season, we show that the southward transport of water is done primarily in the vapor phase by stationary eddies at nearly all latitudes, including across the equator. Clouds form near the surface in the basin as moist air mixes with cold polar air on the western side of Hellas and are subsequently transported clockwise around the north side of the basin by the low-level cyclonic circulation. The simulated Hellas clouds have small particle sizes (~3–5 μm), are very low, and exist as long as the NPRC is exposed. •Clouds observed by MRO/MARCI in the Hellas Basin during NH summer form when water vapor travels from the North Polar Residual Cap (NPRC) across the equator and condenses when it mixes with the cold near-surface air in the Hellas basin.•Tropical aphelion clouds confine water vapor to low levels such that the cross-equatorial transport of total water occurs mainly in the vapor phase by stationary eddies rather than the Hadley cell.•The simulated Hellas clouds have small particle sizes (~ 3-5 microns), are very low, and exist as long as the NPRC is exposed.