Tropical Storm Gonzalo travelled to the south of Grenada this weekend. Gonzalo was small and there was a lot of uncertainty over the peak intensity of the storm, but forecasters seemed more confident with respect to the storm’s track. The forecast track for Gonzalo was predominantly westerly with a gradual drift north. For most of its duration, the forecast suggested that Gonzalo would pass over Grenada or the Grenadines; however, the storm maintained a due westerly track over Trinidad and then quickly dissipated.
Uncertainty with respect to wind speed and Gonzalo’s failure to drift north as forecast caused a fair amount of stress for cruisers from Trinidad to Martinique, and it was a welcome relief that the storm dissipated without much incident. However, the events of the weekend raised some questions. Why didn’t the storm track north? What steers hurricane motion? Here are some thoughts on hurricane tracks and the conditions that drive them.
Hurricane motion is driven by two main factors: environmental conditions and beta drift.
Beta Drift
Beta drift is a consequence of the Coriolis force which acts on a rotating storm system (cyclone), pushing it westward and away from the equator. So, in the absence of any external environmental factors, the natural tendency for a cyclone in the North Atlantic is to move in a north westerly direction. The Coriolis force increases the vorticity (tendency to spin) of north–south traveling winds as their distance from the equator increases. Therefore, vorticity is increasing on the eastern margin of a counter clockwise rotating system, and decreasing on the western margin. Because the absolute vorticity of the cyclone is conserved, the system will move westwards to compensate for this disparity. Furthermore, differences in relative vorticity between the western and eastern sides of the cyclone create their own smaller circulations, the net result being a poleward wind which provides the tendency for Atlantic hurricanes to drift north. Larger hurricanes experience more deflection of their north–south wind field, so beta drift is more pronounced.
Environmental Conditions
Environmental conditions that can impact hurricane tracks include the wind field, blocking highs, and low pressure systems.
In the main development region of the Atlantic, the prevailing easterly trade winds are the biggest influence on hurricane track. At higher latitudes, mid latitude westerlies will tend to steer systems to the east. The position of the jet stream can also impact the path of a tropical cyclone. For example, in 1999 Hurricane Lenny developed south of Cuba and travelled in an easterly direction, hitting the Virgin islands as a category 4 hurricane. Lenny’s unusual path was attributed to the southerly position of the jet stream at the time.
High pressure systems and ridges can block and channel cyclone movement. For the Atlantic basin, the winds around the subtropical ridge (the Azores high), tend to steer hurricanes in a north-westerly direction. In fact, the persistence of the Azores High is one of the main reasons why hurricanes from the main development region tend to follow a path towards the Caribbean and/or the Bahamas and then shift towards the US eastern seaboard.
Low pressure systems have the opposite effect on hurricane tracks and tend to attract weather systems. This was the case for hurricane Humberto (2013) which deviated northward in the central north Atlantic towards a trough that was present in the mid level atmosphere. The tendency for cyclonic systems to attract each other is known as the Fujiwhara effect, where two cyclonic systems start to converge and orbit one another (clockwise in the northern hemisphere and counter clockwise in the southern hemisphere), eventually merging into a single, much larger system. The Fujiwhara effect is more commonly seen between low pressure systems outside of the tropics; however, interaction between tropical cyclones does occur, particularly in the Pacific. Multiple tropical cyclones in the Atlantic basin are rare, but they do happen. In late September 1998, four tropical systems were active in the Atlantic at the same time: Georges, Ivan, Jeanne, and Carl.
The extent to which the track of a hurricane will be steered by these factors is determined by the relative strength of the influences and hurricane structure. Hurricanes can vary in size, depth, asymmetry, and intensity, and these features will impact the way in which the system interacts with its environment. For example, a shallow system will not be as heavily influenced by the jet stream as a deeper system, and a larger system will more readily resist the pull from surrounding regions of low pressure.
So, why didn’t Gonzalo turn north as forecast?
In my opinion, a few factors worked together to keep Gonzalo heading due west. Firstly, the Azores high was very broad while Gonzalo was active so the prevailing winds did not begin to shift the system northerly as they usually do. Secondly, Gonzalo had a very small radius of rotation so it wasn’t really impacted by beta drift. Finally, as the storm began to dissipate faster than forecast, the steering influences acting on the system became been weaker relative to the forecast.
The forecast intensity of Gonzalo was very uncertain throughout the duration of the system and this may have also had an impact on the accuracy of the forecast track. A study (https://journals.ametsoc.org/mwr/article/147/6/2231/344520) published in 2019 on the erratic movement of Hurricane Joaquin (2015) suggested that errors in the intensity forecast resulted in errors modeling the depth of the storm, which in turn impacted how the system interacted with the steering winds aloft. So, it seems probably that uncertainties in Gonzalo’s intensity also contributed to the storm remaining south of its forecast track.
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