Binary Cyclones and Atmospheric Rivers: Unraveling the Fujiwhara Effect's Impact on Pacific Northwest Weather

This week, a remarkable meteorological phenomenon unfolded off the Pacific Northwest coast near the USA-Canada border. A binary pair of cyclones developed in close proximity, engaging in the Fujiwhara effect—a process where cyclones orbit around one another due to their mutual interaction. This interaction significantly enhanced a powerful atmospheric river, resulting in intense moisture transport, prolonged precipitation, and heightened risks of flooding and other hydrometeorological hazards. Here is an in-depth look at what happened, supported by research and evidence.

The Formation of Binary Cyclones and the Fujiwhara Effect

Two cyclones formed over the northeastern Pacific Ocean in close proximity to one another. Their mutual interaction caused them to rotate around a common center, effectively altering their paths and intensifying their circulation. This phenomenon, known as the Fujiwhara effect, has been observed in both tropical and mid-latitude systems, and its influence on atmospheric rivers is an area of growing scientific focus.

In "Rotation of Mid-Latitude Binary Cyclones: A Potential Vorticity Approach" by Ziv and Alpert (2003), the Fujiwhara effect is described as a dynamic interaction driven by upper-level potential vorticity (PV) anomalies. These anomalies dominate the rotational behavior and energy exchange between cyclones, making them a key factor in amplifying the intensity of binary systems. The Pacific Northwest event closely followed these dynamics, with PV anomalies significantly influencing the cyclones’ paths and intensity.

Enhanced Moisture Transport and Prolonged Precipitation

The binary system acted like a moisture pump, pulling vast amounts of water vapor from the Pacific Ocean and channeling it into a concentrated corridor—the atmospheric river. This moisture transport resulted in heavy precipitation across the Pacific Northwest, including significant rainfall in coastal areas and heavy snowfall in higher elevations. The Fujiwhara effect slowed the progression of the system, extending the duration of precipitation and increasing its impacts.

These dynamics are consistent with findings in "Binary Interactions Between Polar Lows" by Renfrew et al. (1997). The study demonstrates that binary interactions between cyclones can cause unusual tracks and intensification of weather systems, particularly through enhanced vorticity and moisture convergence. While the Pacific Northwest event occurred in mid-latitudes, the underlying mechanisms are similar, underscoring the role of cyclone interactions in shaping regional weather patterns.

Lessons from Twin Cyclones in Other Regions

The Pacific Northwest event closely mirrors the interactions observed in "Rapid Merger and Recyclogenesis of Twin Extratropical Cyclones Leading to Heavy Precipitation Around Japan" by Yamamoto et al. (2011). This study examined the interactions of twin cyclones near Japan, demonstrating how these systems can rapidly reorganize atmospheric energy and moisture, leading to enhanced precipitation. Yamamoto’s work highlights the importance of latent heat release and topographical effects in shaping cyclone dynamics. Similarly, in the Pacific Northwest, the interaction of the binary cyclones amplified the atmospheric river’s intensity and precipitation, particularly as moist air masses were lifted over the region’s mountainous terrain.

The Role of Baroclinicity and Potential Vorticity

Baroclinicity, or the variation of temperature with height, played a significant role in this event. The 1995 study "Rotation of Mid-Latitude Binary Cyclones: Observational and Theoretical Perspectives" explains how baroclinic zones provide the necessary conditions for cyclone formation and intensification. This framework helps to understand how the Pacific Northwest cyclones developed and interacted so dynamically. In particular, the interaction of upper-level PV anomalies with surface cyclones created a feedback loop, accelerating rotational motion and increasing the intensity of the atmospheric river.

WRF Model Simulations

To analyze this meteorological event in detail, I utilized the Weather Research and Forecasting (WRF) Model developed by NSF NCAR. This high-resolution numerical model is well-suited for capturing the complex atmospheric processes associated with binary cyclones and their interaction with an atmospheric river. The simulation provided a comprehensive depiction of the development, interaction, and impacts of the binary cyclones, along with the associated atmospheric river. Key atmospheric layers and features were analyzed to understand the vertical and horizontal dynamics driving this event. Below is an expanded analysis of the key outputs from the WRF simulation:

250 hPa Winds and Height Fields

• The upper-level jet stream dynamics at 250 hPa played a critical role in steering the binary cyclones and intensifying their circulation.
• The jet streaks, or localized regions of stronger winds within the jet stream, were positioned to provide divergence aloft, enhancing vertical motion and fueling surface cyclogenesis. This upper-level support allowed the cyclones to deepen and contributed to the enhancement of the atmospheric river.
• The alignment of the jet stream with the atmospheric river acted as a conveyor belt for moisture transport, channeling water vapor directly toward the Pacific Northwest.

500 hPa Vorticity and Winds

• At 500 hPa, the mid-level vorticity fields showed enhanced rotation in the binary cyclones due to the Fujiwhara effect. These vorticity maxima indicated the interaction and mutual intensification of the cyclones as they rotated around a common center.
• The interaction amplified upward motion within the atmospheric river, aiding in moisture condensation and precipitation processes.
• This layer also highlighted the influence of baroclinic forcing, where temperature gradients contributed to the cyclones’ continued development.

700 hPa Chart: Relative Humidity, Mean Sea Level Pressure, and Thickness

• At 700 hPa, relative humidity values approached saturation (80-100%) across the core of the atmospheric river. This layer highlighted the concentration of moisture along the river’s axis, indicating robust moisture transport and condensation processes.
• The mean sea level pressure patterns in the 700 hPa analysis aligned well with the surface cyclones’ positions, clearly showing the Fujiwhara effect. The cyclones rotated around a common center, reinforcing their interaction and sustaining the atmospheric river’s intensity.
• Thickness fields at this level indicated strong thermal gradients, reflecting the presence of a well-defined baroclinic zone. This gradient was a key driver of the cyclones’ intensification, as it fueled the transport of energy and moisture into the atmospheric river.

850 hPa Temperature and Wind Fields

• The simulation at 850 hPa provided critical insights into the transport of warm, moist air into the atmospheric river. Strong low-level winds, often referred to as the low-level jet, were aligned with the atmospheric river’s axis, effectively funneling moisture from the Pacific Ocean toward the Pacific Northwest.
• Warm air advection was evident, particularly associated with the southern cyclone. This warm air provided additional buoyancy, enhancing uplift and intensifying precipitation within the atmospheric river.
• The temperature and wind fields at this level also demonstrated how the cyclones modified the surrounding environment, enhancing the flow of moisture into the atmospheric river.

950 hPa Chart: Wind, Temperature, and Pressure

• At 950 hPa, the WRF simulation captured significant low-level wind fields directly contributing to the atmospheric river’s strength. The winds were aligned with the river’s axis, accelerating the flow of moisture from the Pacific Ocean toward the Pacific Northwest.
• Temperature fields at this level highlighted warm advection associated with the southern cyclone. This warm air provided critical instability, enhancing vertical motion and precipitation processes as it interacted with the colder air associated with the northern cyclone.
• The proximity of the two cyclones was especially apparent in this layer, with strong low-level winds and directional convergence near their centers. This interaction further reinforced the cyclones’ mutual influence and their impact on moisture transport.

Sea Level Pressure and Winds

• The sea level pressure fields captured the Fujiwhara effect in action. The binary cyclones rotated around their shared center, intensifying the circulation and creating a robust mechanism for drawing in moisture from the Pacific Ocean.
• The surface winds converged into the cyclones, enhancing the vertical motion required for moisture condensation and precipitation. The cyclonic rotation also helped sustain the atmospheric river’s alignment and moisture flow, contributing to prolonged precipitation over the Pacific Northwest.

Precipitable Water Fields

• Precipitable water fields showed concentrated moisture plumes extending from the central Pacific Ocean into the Pacific Northwest. The simulation captured the alignment of the atmospheric river with the cyclones’ low-level wind fields, illustrating the river’s structure and intensity.
• The highest precipitable water values were located along the atmospheric river’s core, exceeding climatological averages for the region and indicating a significant moisture anomaly. This moisture surge was a direct result of the binary cyclones’ interaction and their ability to act as a conveyor of atmospheric moisture.
• The precipitable water values over land demonstrated how the atmospheric river maintained its strength as it interacted with the complex topography of the Pacific Northwest, leading to enhanced precipitation, particularly along windward slopes of the Cascade and Coast Ranges.

High-Resolution Modeling: Grid and Domain

The WRF model simulation utilized a 300 by 300 grid at 9 km resolution, providing detailed spatial and temporal coverage of the event. This resolution was critical for capturing the mesoscale features of the binary cyclones and the atmospheric river. The high-resolution output allowed for a detailed analysis of the vertical and horizontal interactions driving the event’s intensity and prolonged impacts.

Summary of WRF Insights

The WRF simulation offered a detailed view of the dynamic interactions between the binary cyclones and the atmospheric river. The key atmospheric layers—from the upper-level jet dynamics at 250 hPa to the low-level moisture transport at 950 hPa—revealed a vertically integrated system of processes that sustained and enhanced the atmospheric river. The alignment of wind fields, temperature advection, and moisture convergence across multiple levels demonstrated how these cyclones acted as an efficient mechanism for moisture transport and precipitation generation. This simulation provides a valuable tool for understanding and forecasting the impacts of similar events in the future.

This event highlights the complexity of interactions between binary cyclones and atmospheric rivers. The amplification of moisture transport, combined with prolonged precipitation, underscores the importance of studying mesoscale dynamics in weather systems. Events like these demonstrate how localized interactions can have broader impacts, affecting regions far inland with heavy precipitation and associated risks such as flooding.

From a forecasting perspective, this Pacific Northwest system emphasizes the need for high-resolution models and detailed observations to predict and mitigate the impacts of similar events in the future. The insights from this event, combined with the body of research on binary cyclone dynamics, provide valuable tools for advancing our understanding of atmospheric processes and improving weather prediction capabilities.