When air encounters the mountains along the west coast of the United States, it is forced upwards. Precipitation over the ocean falls along the feature’s cold front on its northern side.Īnother way that air can rise and condense into precipitation is through orographic lift. Much of the moisture stays close to the surface but the rising motion of the low pressure to the north results in the air cooling, condensing the water vapor into a liquid. Extreme rainfall has also been associated with the more intense gradients. The more intense the pressure gradient is, the stronger the winds are that transport the water vapor. The pressure gradient between the clockwise flow of the Californian high and the counterclockwise flow of the Aleutian low funnel the atmospheric moisture into a narrow corridor. The atmospheric river is guided by the semi-permanent sub-tropical high pressure off the coast of California and the Baja Peninsula as well as the Aleutian low in the Gulf of Alaska. The feature then travels towards the west coast of the United States as a sub-class of atmospheric rivers commonly referred to as the “pineapple express” due to its origin near Hawai’i. Tropical moisture is pulled in from the ITCZ and in this example, converges with other moisture sources to form an atmospheric river. The blue shading shown here gives a three-dimensional view of the water vapor transport. The American Meteorological Society defines an atmospheric river as “a long, narrow, and transient corridor of strong horizontal water vapor transport that is typically associated with a low-level jet stream ahead of the cold front of an extratropical cyclone.” A common measure for the strength of an atmospheric river is the integrated water vapor transport, or the amount of moisture that is moved from one place to another by the flow of the atmosphere. Studies have shown that atmospheric rivers account for the vast majority of the poleward transport of water vapor. Disturbances in its flow transport immense amounts of moisture and energy from the tropics to the poles. Within the tropics easterly trade winds converge along the equator to create a moisture rich cluster of clouds, convection, and precipitation called the intertropical convergence zone, or ITCZ. Within the mid-latitudes, winds move clouds from west to east. Clouds and precipitation shown here are from NASA’s MERRA-2 reanalysis, a retrospective blend of a weather model and conventional and satellite observations. Features in Earth’s atmosphere, spawned by the heat of the Sun and the rotation of the Earth, transport water and energy around the globe.
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