What is an atmospheric river? A large and narrow stream of high intensity moisture extending from tropical influences pushed by the jet stream across mid-latitude land masses. The North Pacific atmospheric river is the most famous (and effective). What causes an atmospheric river to form? The best way to create an atmospheric river is the phasing (or connection) between the storm track propelled by the subtropical jet stream and increased heavy convection in the deep tropics enhanced by presence of large areas of strong vertical currents known as the positive (or convection) phase of the Madden Julian oscillation (MJO).