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I was just reading this about how Hurricane Ethel could have merged with Hurricane Dora in 1964.

Has such a merge ever happened before in history? If so, what was the result? Would storms become twice as powerful? Or would they disrupt and dissipate each other?

They don't have to be hurricanes or typhoons per se, just large storms. I would think the low pressure regions of two storms would tend to attract each other if they were nearby, but it's apparently rare or unheard of because a quick google search showed nothing.

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    $\begingroup$ @casey - Isn't it the en.wikipedia.org/wiki/Fujiwhara_effect ? $\endgroup$ – gansub Sep 7 '15 at 2:01
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    $\begingroup$ @gansub great reference, you should turn it into an answer :) $\endgroup$ – casey Sep 7 '15 at 2:07
  • $\begingroup$ Early tropical cycylones generally form as localized vortices merge (look up vortical hot towers). But for large storms to merge, their structures would have to merge, and generally that would disrupt their rather fragile cores, and almost certainly weaker a TC for a long while. In theory the increased warm core\decreased pressure could lead a stronger storm eventually. But TCs can grow quite rapidly on their own in favorable conditions, such that the disruption a merger would cause would probably not be the extraordinary event you'd think. $\endgroup$ – JeopardyTempest Oct 21 '16 at 2:10
  • $\begingroup$ Now, on the other hand, baroclinic systems can and regularly do merge and intensify, as they are driven by their temperature gradients, not the warm core at the center of a TC. The warm core needs continual nearby vigorous updrafts, and secondary pressure areas typically disrupt the necessary convergence to maintain this. Extratropical lows just need the temperature gradient, and so doesn't require continual convection to sustain... generally mergers can combine their gradients easily without much disruption to the storm. $\endgroup$ – JeopardyTempest Oct 21 '16 at 2:18
  • $\begingroup$ So 2 TCs merging don't generally occur favorably. $\endgroup$ – JeopardyTempest Oct 21 '16 at 2:29
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Yes two hurricanes/tropical cyclones/typhoons can merge with each other and the effect is known as Fujiwhara effect- Fujiwhara effect.

The National Weather Service defines the Fujiwhara effect as "Binary Interaction where tropical cyclones within a certain distance(300-375 nautical miles depending on the size of the cyclones) of each other begin to rotate about a common midpoint". What really happens is that centers of both systems begin to orbit in a counter clockwise direction about a midpoint that is determined by the relative mass and cyclone intensity. Eventually the smaller cyclone may merge into the larger cyclone. There are several examples of the Fujiwhara effect and one example would be Hurricane Connie Hurricane Connie and Diane Hurricane Diane way back in 1955. Shimokawa et al Fujiwhara Effect Types talk about the various kinds of interactions that can take place among various typhoons(Please note that the Fujiwhara effect is not restricted to two systems). The various kinds of interactions are

  • Complete Merger
  • Partial Merger
  • Complete Straining Out
  • Partial Straining Out
  • Elastic Straining Out

Complete straining and complete merger interactions lead to destruction of one of the vortices. Partial merger and partial straining lead to partial destruction of one vortex and elastic straining is an interaction in which both vortices survive with their initial circulation. Partial merger and partial straining out have received less attention in the literature on binary tropical cyclone interactions as the interactions are extremely complex. Prieto et al claim that during a partial merger repeated mass exchanges occur between vortices. As these are nonlinear effects a quantification is only possible by a direct numerical integration and precise initial condition Binary TC Vortex Like Interactions

The period of orbit maybe as small as one day or there are others such as Cyclone Kathy and Cyclone Marie Kathy/Marie Fujiwhara orbited for a period of 5 days prior to merging into each other as pointed out by Lander et al. If the period of orbit is longer then there is a greater probability of a merger.

Region wise binary cyclones are more common in the Western North Pacific than the North Atlantic as pointed by Dong et al Relative Motion Of Binary Tropical Cyclones Regarding the predictability of the track of binary cyclones Dong et al. state that prediction of steering forces of a single tropical cyclone are replete with numerical forecasting uncertainties and the problem is accentuated by the presence of another tropical cyclone in close proximity. Those who followed the progress of Hurricane Sandy in late October 2012 (an instance of the Fujiwhara effect but in this case a tropical cyclone merged with an extra tropical storm) will remember the ECMWF model correctly predicted the landfall location.

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  • $\begingroup$ Are there any observational studies for intensity results following Fujiwara interaction? $\endgroup$ – JeopardyTempest Mar 24 '17 at 18:05
  • $\begingroup$ Your answer is definitely the most solid and useful from a science perspective. I do wonder if we might be able to edit it to make it a little more approachable to everyday folks who may not understand some of the terminology or details you touch on. The choice is certainly yours... I'll give it a try if you wish or you're welcome to or we can leave it be. Whatever is of help to people getting their information the best possible :-) $\endgroup$ – JeopardyTempest Mar 24 '17 at 18:21
  • $\begingroup$ @JeopardyTempest - Go ahead and edit and I will look at it and then accept the changes if appropriate. $\endgroup$ – gansub Mar 25 '17 at 1:04
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Yes, two or even more rarely, three tropical storms or hurricanes / typhoons can merge, and have done so in nearly all the major ocean basins where sea surface temperatures exceed 27 deg C. As far as I know this has never been observed in the South Atlantic (can anyone confirm this?). The result of a tropical storm merger is initially determined by 'conservation of angular momentum'. As the storm progresses, it either weakens or strengthens in the usual way, That is, according to (a) the available latent heat of evaporation, which in turn is a function of sea surface temperature beneath the storm footprint, and (b) whether kinetic energy is dissipated as friction over land.

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    $\begingroup$ can you add a few references to your answer? $\endgroup$ – gansub Oct 12 '15 at 4:57

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