Cyclone Pathways and Intensification of Extreme Northeast U.S. Atmospheric Rivers

Atmospheric rivers (ARs) are narrow channels of water vapor transport that occur in

conjunction with an extratropical cyclone (ETC), and are measured by the vertically integrated

water vapor transport (IVT) within that channel. ETCs can follow several pathways, and IVT

values are highest when a strong pressure gradient is created between the ETC and a high-

pressure system. This study aims to understand how the interaction between a high-pressure

system over the Atlantic Ocean and ETCs of various pathways impact the intensification of ARs

in the northeast U.S.. A case study of three northeast ARs was conducted using archived GFS

maps. A stationary high-pressure system over the Atlantic Ocean is essential to create a pressure

gradient over the northeast U.S. that will increase wind speeds and transport water vapor from

subtropical moisture sources. When the ETC follows an Alberta Clipper pathway, moisture

values are low due to the far proximity from the Gulf of Mexico and Atlantic Ocean. The

pressure gradient which is created between the ETC and the high-pressure enhances southerly

winds that draw moisture north and increase IVT values. When the ETC follows a Gulf Low

pathway, the cyclone forms in a moisture-rich region that already features high IVT values.

However, the presence of a ridge off the northeast U.S. coast results in strong southeasterly

winds as the ETC moves closer to the high-pressure center and the pressure gradient intensifies,

bringing the AR into the Northeast. When the ridge does not remain stationary, wind speed

decreases as the gradient weakens. The direction of moisture transport moves away from the

northeast quickly as southwesterly winds result from the high-pressure moving east. These

findings suggest that without a stationary high-pressure system over the Atlantic Ocean, ARs

with extreme IVT values would be less likely in the northeast U.S..