In recent days, the formation of a cyclone in the Bay of Bengal has already been announced. Today, the system has now shown clear structures for the first time and reached storm strength – criteria required to name a tropical storm. The storm has been christened Mocha and will move northward in the coming days, reaching the coast of Myanmar and Bangladesh by the turn of the week.
The disaster takes its course...
It has been looming over the past few days, now the disaster is imminent. Cyclone Mocha (pronounced as Mocha) is heading towards the border region of Bangladesh and Myanmar in these hours and will hit land during Sunday. The storm system is both impressive and frightening. The image below shows a well-developed eye of what is now considered a Category 5 (out of 5) tropical storm.
Fig. 1: Infrared image of Mocha, low temperatures show a high cloud top; Source: CIMSS
Currently, Mocha is exhibiting wind speeds of 240 km/h, with 1-minute gust peaks even reaching nearly 300 km/h. It is expected to maintain this intensity for the next several hours before weakening somewhat ahead of landfall due to high wind shear and interaction with land surfaces. The eye is expected to make landfall near Sittwe on Sunday afternoon/evening (early Sunday afternoon in Switzerland). Currently, a third or fourth category tropical storm is expected at the time of landfall – near the coast, meaning maximum winds of over 200 km/h. In addition to devastating damage from the winds, there are other dangers. On the one hand, an extreme storm surge with heights of several meters and heavy rainfall with locally several hundred liters of rain per square meter must be expected. This is a particularly fatal combination because the coastal area is relatively flat, densely populated and in a river delta with marshes. In addition, landslides are imminent in the relatively nearby hills. Even though evacuations are currently in full swing, it will unfortunately not be possible to bring everyone to safety. Great suffering with many deaths must be feared.
Fig. 2: This is how the wind field might look just before shore leave; Source: Tropical Tidbits
A sad look into the history books
Since 1970, three different cyclones have claimed more than 100,000 lives in this region. The most globally deadly cyclone struck the Ganges delta farther west on Nov. 12, 1970, causing an extreme storm surge that claimed between 300,000 and 500,000 lives. The 1991 Bangladesh cyclone is also among the world's deadliest tropical storms, killing 140,000 people. Fifteen years ago, Cyclone Nargis – hit land a little further south in Myanmar compared to Mocha –, destroying thousands of homes and killing around 100,000 people.
The list of those storms shows the great vulnerability of this region – and this time will probably not be much different...
Tropical Storm Mocha in the Bay of Bengal
Already more than a week ago, global weather models indicated a potentially strong cyclone in the Bay of Bengal for the end of this week. Between the models and also the individual calculations of respective models, an exact track and thus an estimate of cyclone strength was not clear. This is not unusual, because as long as the system does not yet have a defined center, a slightly different core and starting point is used for the calculations, depending on the model. Over the period of time, sometimes well over a week, the initially small differences of the individual models thus become larger and larger differences. Now, as the time period progresses and develops into Tropical Storm Mocha, more and more concrete approaches emerge. The following satellite image shows the current situation over the Bay of Bengal.
Fig. 1: Tropical Storm Mocha over the Bay of Bengal, imaged by the Himawari-9 satellite.; Source: Tropical Tidbits
What do the weather models forecast?
The models agree on one thing – the current tropical storm will strengthen rapidly over the warm ocean surface and soon reach cyclone status, probably even today. A cyclone is a tropical cyclone which has 1-minute average winds of at least 119 km/h. Depending on the region, it is also referred to as a hurricane or typhoon.
Fig. 2: Surface temperatures above 30 degrees promote rapid reinforcement of mocha; Source: CIMSS
Where and when the cyclone will make landfall is still subject to some uncertainty. There are currently many indications that the cyclone will hit the region between Sittwe and Pathein in the Myanmar state of Rakhaing. However, the extreme east of Bangladesh also remains in the possible danger zone. According to most models, landfall is expected between Sunday and Monday.
Fig. 3: Possible trajectories and strengths of the cyclone according to ensembles from the GFS model; Source: Tropical Tidbits
How strong will Mocha be?
This question is currently being hotly debated because several factors play a decisive role. The surface temperature of the ocean is one, where values of 27 degrees and above are typically conducive to the formation and strengthening of a tropical storm. As can be seen in the second figure, the cyclone moves over much warmer waters. This suggests rapid intensification. One of the largest antagonists of a vertically extended system (as Mocha is) is wind shear. This describes the different wind directions and strengths at different altitudes in the atmosphere. The more the winds differ, the more likely the storm system will be pulled apart and thus disturbed. At the moment, the wind shear is rather low and thus hardly hindering a further strengthening of Mocha (drawn as green lines in the following figure, momentary location of Mocha in orange). However, this will change in the coming days. The more northerly the cyclone comes, the higher the wind shear becomes (drawn as red lines in the figure below). This could weaken Mocha a bit before landfall. At the moment of landfall, a second or third category cyclone is assumed with wind speeds of about 200 km/h.
Fig. 4: Wind shear over the Bay of Bengal. Green indicates little, red indicates strong shear; Source: CIMSS
The strong winds in combination with a lot of rain in a short period of time as well as the densely populated coastal areas make this region particularly vulnerable. In addition, high wave action must be expected. The European model predicts combined flood and wave heights of around 10 meters in the directly affected area. So there will be major damage and suffering in some areas.
Fig. 5: Combined tide and wave height at the time of shore leave according to ECMWF.; Source: Tropical Tidbits