Due to better atmospheric studies from observations and simulations during the past several decades, our understanding of the Martian atmosphere has significantly expanded. Water has been acknowledged to play a key influence in regulating the features of the Martian atmosphere (Smith, 2002). One of the most studied impacts of the water cycle on Mars’s climate is the direct radiative forcing of water ice clouds (Haberle et al., 2011; Madeleine et al., 2012; Navarro et al., 2014; Wilson et al., 2008; Wilson et al., 2007). Moreover, an even more important climatic effect of clouds may arise from their interactions with suspended dust. As such, the water cycle occupies a nodal position in Martian climate, connecting all major climatic variables in an exponential way. Much effort has been put into understanding the repeatable seasonal activity of water on Mars. During spring and summer, when water vapor is predicted and observed to be at maximum at either pole due to local sublimation, a sluggish meridional mixing is taking place that allows moisture to be advected from the pole and to be exported towards the equator within three specific longitudinal corridors. From the end of summer until next early spring, mid-latitude and polar region climate is driven by the latitudinal and seasonal wandering of the polar vortex, whose latitudinal excursion is at maximum around the winter solstice. The three significant characteristics of the water cycle collected from observations are the sublimation peak of water vapor during the northern summer solstice, the zonal mean of the amount of atmospheric water vapor in the tropics during northern fall, and the mean aphelion cloud belt opacity (Navarro et al., 2014). The essence of the seasonal cycle of water is that vast and exposed reservoirs of ice communicate with the atmosphere whose circulation is vigorous enough to transport water from pole to pole and back, thereby closing the water budget on an annual basis(Haberle et al., 2017). Thus, there is thus a vital need to reveal the features and mechanisms by which the water crosses the equator.              

The water cycle crossing the equator, also known as cross-equatorial transport, is dominated by the overturning Hadley cell (Haberle et al., 2017). There are 10­¹² kg water transferred from north to south and back every year through the solstitial Hadley cells, half of the mass of water subliming from the NPC in spring and summer (Houben et al., 1997; Montmessin et al., 2004). The Clancy effect indicates that the Martian elliptical orbit is a key factor of the hemispheric asymmetry of water on Mars, since the cold aphelion climate impose a saturation altitude of water near or below the retuning branch of the northern summer Hadley cell (Clancy et al., 1996). Another factor is the north-to-south topography dichotomy of Mars, which forces a more intense southern summer Hadley circulation compared to that of the northern summer and favors the accumulation of water in the northern hemisphere (Richardson and Wilson, 2002).

The water cycle and dust cycle and their interaction are dominated features defining the present Martian climate (Smith, 2002), where there are lots of research focusing on impact of water on dust cycle. However, only a few studies have dealt with the effects of dust on the water cycle, let alone the effect of dust on cross-equatorial water transport. Investigations by Haberle et al. (1982) and Murphy et al. (1993) showed that the dust entering the rising branch of the Hadley cell heats the atmosphere, which results in intensifying and increasing the meridional extent of the Hadley cell (Basu et al., 2006; Forget et al., 1999; Guzewich et al., 2013; Haberle et al., 1982; Haberle et al., 1993; James et al., 1999; Kahre et al., 2005; Liu et al., 2003; Martin and Kieffer, 1979; Newman et al., 2002a; Newman et al., 2002b; Smith et al., 2001; Wang et al., 2003; Wang et al., 2005; Wilson, 1997; Zurek et al., 1992). The correlation of dust and water is strong positive during the low-dust period but it switch signs during the high-dust seasons because of the dust lifting and the associated temperature change (Guha et al., 2021). High dust loading can also lead to more mesospheric water ice cloud (Liuzzi et al., 2020). Dust can alter the meridional transport of water by impacting the hygropause altitude. During the low dust scenario, the atmosphere remains below saturation and water is carried equatorward without cloud limitations. However, the north polar atmosphere saturates at low levels in summer, leading to cloud formation and water confinement near the surface in the high dust scenario (Montmessin et al., 2007).

Previous studies of the effect of dust on water cycle mainly focused on the observation data(Smith et al., 2001; Smith, 2002), comparing the low dust seasons with high dust seasons. Some studies use observational dust data and water data from simulation to couple the water and dust cycle imprecisely to dig their correlation. This study attempts to elucidate how water transport across the equator and how dust influence the cross-equatorial water transport by using a more sophisticated analysis of the numerical modeling. The model is LMD Martian atmospheric general circulation model, which includes water cycle with water vapor, ice, and “double mode” dust transport. The study compares the different water cycle pattern in low-dust and high dust scenarios by altering the dust opacity.
 

Mars， Water Cycle