Saturday, November 7, 2015
2:13 pm
Less than a week ago, Cyclone Chapala slammed into Socotra, a small Yemeni Island some 150 miles east of the Horn of Africa, and Yemen, a small country on the southwestern tip of the Arabian Peninsula currently embroiled in a brutal civil war, becoming the first hurricane-force cyclone to hit Socotra since 1922
and the only hurricane-force cyclone to hit Yemen in their 124-year record.
Chapala was the strongest cyclone ever recorded in the Gulf of Aden, and, with
130 mph sustained winds and a 940 hPa central pressure, the second-strongest
cyclone ever recorded in the entire Arabian Sea. That's equivalent to a
category 4 hurricane!
While singular events like this are very impressive, it's even more impressive when a similar thing happens less than a week later.
Right after Chapala wreaked havoc on Yemen, Cyclone Megh
formed. Megh followed an extremely similar path to Chapala, first striking
Socotra and then weakening as it headed towards Yemen and encountered dry air
coming off the Arabian Peninsula. Although not as deep as Chapala, it was still
equivalent to a category 3 hurricane. It directly hit Socotra near peak
strength and ravaged the small Yemeni island with sustained winds of 127 mph.
Even though it made landfall in Yemen, it encountered dry air from the Arabian
Peninsula as it did so and weakened substantially, sparing Yemen of further
damage.
Take a look at a graphic that
Stu Ostro of The Weather Channel tweeted. The storms
look strikingly similar, and are in a very similar region.
The paths have been very similar too, as illustrated by a
graphic showing the tracks and strengths of cyclones in the North Indian basin
this year. You can clearly see Chapala heading straight into Yemen, and Megh is
the "unfinished" track just to the south of Chalapa and nearly
paralleling it, making a direct hit on Socotra and later making landfall in
Yemen.
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| Cyclone Tracks in the North Indian Ocean. Credit: Keith Edkins |
When you compare these paths to the paths of cyclones in the
North Indian Ocean from 1970-2005, it's easy to see that these tracks were
pretty abnormal. No systems struck Yemen in that time period. Suddenly, we have
two landfalls in one week!
Chalapa was the stronger of the two cyclones, and it was
able to keep its general structure before making landfall in Yemen, delivering
heavy rain to the area. In fact, some coastal regions got as much as 24 inches
of rain in two days... which is equivalent to 7 years’ worth of rainfall for
them! In fact, take a look at the two satellite pictures below of Yemen before
and after the storm. The difference is startling!
However, Megh wasn't so lucky, as the entrainment of hot,
dry air from the Arabian Desert into the cyclone made the storm all but evaporate. The satellite
picture below is hilariously choppy, but it still shows how the dry,
continental air utterly destroyed Megh. Still, when you think about how dry
that air is, it is very impressive that any storm could even approach the
region, much less one of hurricane strength, as was the case with Chapala. Megh
landed as a tropical storm.
Why did all of this happen? Why had Yemen never recorded a
tropical cyclone and then suddenly get two in one week? Why did Socotra get
pounded by two major storms in a week when the last hurricane strength cyclones
were in 1922 and 1885 and were only equivalent to a category 1 hurricane?
My first thought was that this might have something to do
with El Niño. After all, that's all we've really been talking about for
the past six months. It turns out that El Niño does have something to do
with it, but it's not the sole reason. There are other factors at play,
including some that had never even crossed my mind. I ended up digging pretty
deep into the scientific literature on the subject, and came to some interesting
conclusions. If you don’t like science, I’d suggest you turn back now.
First, I discovered that the Arabian Sea is experiencing
record-high sea-surface temperatures.
Much of this has to do with a lack of upwelling off the Somali Coast. Upwelling is the process by which deep, cool, nutrient-rich water rises to the surface, so a lack of upwelling means that the water in that area will be warmer. Most zones of upwelling are on the
eastern boundaries of ocean basins, including the upwelling that occurs right off our coast in the Northeast Pacific and the upwelling off the coast of Peru in the Eastern Tropical Pacific. The upwelling off the Somali Coast is unique in that it is on a
western boundary of an ocean basin. But why does this lack of upwelling occur in the first place?
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| A chronological sequence of atmospheric and oceanic events in the Indian Ocean that are often set off by an El Niño or a weak Indonesian-Australian monsoon. Credit: Izuma et al. (2010) |
According to Izuma et al.'s "
Role of the Somalia-Oman upwelling and ENSO on Indian monsoon rainfall
variability," there are two reasons for the initial existence of warm
water: (1) El Niño and/or (2) a weak Indonesian-Australian monsoon from the
previous winter. These two phenomena often go hand-in-hand. This is
because El Niños often weaken the Indonesian-Australian monsoon by
creating anomalously high pressure in the western Pacific and dry conditions
over Indonesia, causing devastating forest fires in regions that practice slash
& burn agriculture. When one/both of these events occur, they change the
pattern of winds in the South Indian Ocean, and these changes can cause
downwelling,
or the sinking of water from the surface to depth. As a result, you get a large
mass of warmer-than-normal water that slowly propagates westward from the
Central Indian Ocean to the Southwest Indian Ocean off the eastern coast of
Africa.
This mass of water propagates as a Rossby Wave,
which is a very long wave driven by wind stress. Rossby waves can be hundreds
of kilometers long, but only a few meters tall at the thermocline, tapering to
a few centimeters at the ocean surface. These particular Rossby waves take 4-5
months to travel to the eastern coast of Africa, but Rossby waves can take
years to cross a large ocean basin such as the Pacific! Rossby waves are very
prevalent in the atmosphere as well, but that is for a different blog.
Anyway, this warm water causes a temporary increase in
precipitation in the Southwest Indian Ocean. This increase in precipitation
leads to weaker northeasterly winds over the Arabian Sea, and because these
winds are weaker and less water is being pushed away from the Arabian Coast,
less upwelling occurs, and water temperatures are warmer. Izuma et al. found
that this actually corresponded with a stronger monsoon for the west coast of
India (particularly the Western Ghats, a large mountain range adjacent to the
Arabian Sea) due to increased evaporation and moisture transport from the Arabian
Sea due to the warmer sea-surface temperatures there.
These findings went against the prevailing idea is
that once the water in the Arabian Sea is warmer; the temperature
difference between the hot Indian subcontinent and warm Arabian Sea is less,
leading to a weaker monsoon, which in turn further reduces upwelling. This
shows that there is still a lot of uncertainty in how El Niños affect the
Indian monsoon. On the other hand, Izuma et al.'s findings about decreased
upwelling off the Somali Coast were consistent with most scientific literature
and, more importantly, observations, so there appears to be more certainty on
that front at this point.
The decrease in upwelling off the Somali Coast causes
enhanced sea surface temperatures in that location, but it turns of that sea
surface temperatures have been steadily increasing across the entire Indian
Ocean basin over the past decade.
Lee et al. (2015) found that ocean heat content
between the surface and 700 meters has increased substantially in the Indian
Ocean from 2003 to 2012. They hypothesized that the increased frequency of La Niñas
during that period created anomalously high sea-surface-heights over the warm
waters of the western tropical Pacific, and that some of this warm water
traveled to the Indian Ocean through passages between islands in Indonesia in
an attempt to reduce the difference in high sea surface heights in the Western
Tropical Pacific and low sea surface heights in the Indian Ocean.
Take a look at the change in heat content of the upper 700 meters of the Indian Ocean over time (the "observation" line). Notice how it stays relatively constant with some variability (including a big drop in the mid-late 90s), but that after 2003, it dramatically increases.
Compare this chart to the "multivariate ENSO
index" below, which is an index used to measure the strength of an El
Niño or La Niña. As the chart shows, the year-to-year El Niño/La Niña
oscillations are superimposed on longer-scale warm and cold oscillations
lasting several decades. The heat content in the Indian Ocean started rising
from its minimum value around 1997 or 1998 (the year of the largest El
Niño on record), and the unofficial "cold" cycle (as marked by the
graphic below) began that year as well. Coincidence? Probably not.
But if heat content is rising now, why didn't heat content
rise back in the "cold" phase before 1975?
The answer to this question is likely global warming, and the massive heat intake by the ocean over the past few decades. Even though atmospheric temperatures have risen dramatically since 1970, only 2.3% of heat added due to fossil fuel emissions actually ends up in the atmosphere. 93.4 percent of it ends up in the ocean, with most of it residing in the upper 700 meters. To put it more bluntly, the ocean has absorbed 40 times more heat than the atmosphere, and considering how fast the atmosphere has warmed up, that's saying something. The ocean will be much slower to respond because of the much higher heat capacity of water, but still, the amount of heat they have absorbed is staggering. Since 1990, the oceans have absorbed more than 2 x 10
23 joules of energy. That's equivalent to five Hiroshima-sized bombs every
second.
Lee et al.’s model above shows how there are higher than
normal sea surface heights in the Western Tropical Pacific, and the arrows show
how this warm water flows through channels between the Indonesian islands and
works its way into the Indian Ocean. This, added with decreased upwelling,
is why the Arabian Sea is experiencing such high temperatures.
But it takes more than just a warm ocean to create a tropical cyclone.
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| Credit: National Center for Environmental Prediction. Retrieved from Forbes Magazine |
Dry air is a hurricane's kryptonite. However, even though
air around the Saudi Peninsula and Gulf of Amen is very dry, it is still moister
than usual, assisting hurricane formation. This may be related to what was discussed
in Izuma et al.'s paper - warm ocean temperatures and decreased upwelling
correspond with increased evaporation and surface moisture transport to India
- but it is more directly attributable
to the current state of the Madden-Julian Oscillation (again, that's for
another blog).
Additionally, we are currently in between the summer monsoon
season (the wet southwesterly monsoon from the ocean to the land) and the
winter monsoon season (the dry northeasterly monsoon from the land to the
ocean). Due to this, wind shear is lower, making cyclone formation conditions
more favorable. Although this happens every year, the existence of
record-breaking sea surface temperatures and a moister than normal atmosphere
made conditions more favorable than usual for tropical cyclone formation.
As for why the cyclones took such a similar track into a
place that hardly ever experiences any tropical activity, your guess is as good
as mine. Thankfully, there are some very smart scientists on the case, and I'm
sure we will learn more in the coming months! But I hoped you still
enjoyed my detective work. These posts are fun, albeit very time consuming!
On a more local note, there is a threat of major flooding on
Western Washington rivers over the weekend. I’ll have a blog on that tomorrow!
Charlie :)
Sources:
Birchard, G. (2015,
October 30). 155 mph Cyclone Chapala, 2nd strongest ever in Arabian sea,
Unprecedented Threat to Yemen. Retrieved November 10, 2015, from
Izumo, T., Montégut,
C., Luo, J., Behera, S., Masson, S., & Yamagata, T. (2008). The Role of the
Western Arabian Sea Upwelling in Indian Monsoon Rainfall Variability. Journal
of Climate, 5603-5623. doi:10.1175/2008JCLI2158.1
Lee, S., Park, W.,
Baringer, M., Gordon, A., Huber, B., & Liu, Y. (2015). Pacific origin of
the abrupt increase in Indian Ocean heat content during the warming
hiatus. Nature Geoscience, 8, 445-449.
doi:10.1038/ngeo2438
Molion, L., &
Lucio, P. (2013). A Note on Pacific Decadal Oscillation, El Nino Southern
Oscillation, Atlantic Multidecadal Oscillation and the Intertropical Front in
Sahel, Africa. Atmospheric and Climate Sciences,3(3), 269-274.
doi:10.4236/acs.2013.33028
