Mumbai and Chennai

Last week I had an op-ed piece in the Times of India. It’s about the risk to the city of Mumbai from a tropical cyclone landfall and storm surge.

Mumbai is on the Arabian Sea, where tropical cyclones are relatively rare, and the city hasn’t been struck by a serious one in a very long time – well beyond living memory. If it were, though, the results could be catastrophic, as it’s both very low-lying and heavily developed right to the shore. This is exactly the recipe for an awful disaster: high vulnerability, low awareness of the risk.

I had the opportunity to write this thanks to an invitation from the great writer Amitav Ghosh, who was the guest editor for this issue of ToI. I was thrilled to get an email this past summer from Amitav, in which he said that he had enjoyed my book Storm Surge and wanted to talk to me about climate change and the risk to Mumbai. This was related to a series of lectures he was to give (now has given) at the University of Chicago in which climate change is a central theme, and which will be turned into a book in due time.

Amitav and I subsequently met to discuss the science of climate change and tropical cyclones in my apartment in New York as he was making his way to Chicago. By this time I had begun making my way through his books. They are wonderful. Themes of history, environment, ecology, and climate are woven through them, and cyclones figure prominently as plot elements in at least a couple. I got several copies signed.

So now the op-ed has come out. (The issue also has an interview with Amitav himself, and a piece by Christian Parenti on the relationship between climate and Naxalite violence in India.) Coincidentally, my piece on hypothetical flooding of Mumbai appeared at the same time as a new burst of extreme rainfall was causing very real flooding in Chennai, on India’s opposite coast.

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Infrared satellite image from November 17, showing a disturbance that one of the earlier waves of flooding in Chennai.

The Chennai flooding had begun a couple of weeks earlier, when cyclone BOB3 (“BOB” stands for “Bay of Bengal” – this storm was never strong enough to get a proper name) came ashore. This is the storm that had formed in the Bay of Bengal while cyclone Megh was in the Arabian Sea, heading for Yemen. Some forecast models predicted that BOB3 would cross the subcontinent and reach the Arabian Sea itself. But it didn’t do that. It hung around over land as a disorganized disturbance, dumping lots of rain.

Then another major burst on December 1 – a foot of rain fell on that one day. This after 40 inches in November (from BOB3 and a couple of other storms) – three times the average in Chennai for that month, and about equal to New York City’s annual average.

While it was a tremendous amount of rain, some argue that the flooding was made much more severe by careless development of Chennai in recent years, as floodplains and drainage channels have been paved over. The story is similar in Mumbai, where destruction of mangroves and river channelization have been blamed for similarly exacerbating the flood which occurred there in 2005 (also driven by rain, not storm surge, nor related to a cyclone) – and will similarly make things worse if, someday, a cyclone causes a major storm surge there. And to New York City, for that matter, where real estate development is now hot again in some of the areas worst hit by Sandy. I don’t know that op-eds or books can make much difference in the face of such indifference to risk, but it’s worth trying.

Review of Storm Surge by Brian Mapes in BAMS

In current issue of the Bulletin of the America Meteorological Society is a review of Storm Surge by Brian Mapes. It’s a beautiful, thoughtful, insightful review (I say from my totally unbiased perspective). Brian saw everything I had hoped the reader would see in the book. Now, BAMS is not the New York Times; this won’t sell thousands of books. But it means a great deal to get this kind of approval, from such a brilliant colleague, in a core publication of my own field. I didn’t write the book for other meteorologists, really, but if they didn’t like it, I’d know I had done something wrong. Thanks Brian!

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A Foggy Day

(Note: I updated this post several times, after initially posting it, by accident, sooner than I had meant to.)

After a weekend that really started to feel like summer, it was cold this morning in New York City this morning, with a thick fog. Here is a photo I took from the George Washington Bridge at about 11:30 AM, looking south along the Hudson. See the boat in there?

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The weather service’s forecast discussion calls it an “advection fog”, which means it happens when warm, moist air moves over a cooler surface, and ascribes it to a “back door cold front”. The modifier “back door” refers to the fact that while most cold fronts – like most weather of any kind at our latitude – come from the west, this one came from the east. You can see it in this image, a map of the flow this morning. The arrows show the flow at 1000 hPa (near the surface), and the colors show the temperature there. You can see the cool air blowing into NYC from offshore:

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The definition of advection fog is that it forms when relatively warm, moist air moves over a cooler surface, so that it cools by contact with that surface and eventually reaches saturation. I’m not sure if that’s happening here, at least not in the short term. It looks as if the air was already cool before it blew onshore. But the key thing is that the layer of cool, moist air over the sea is very shallow and topped by a temperature inversion, so that warmer air overlies it. This means the boundary layer is very stable, and the air near the surface won’t easily mix with that above. Thus while over the ocean, the air took on more and more water vapor and couldn’t get rid of it, and eventually reached saturation. The sounding from this morning at Upton, NY in Long Island shows it very clearly:

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Look at the bottom: there is only a single white curve until somewhere above 950 hPa (see the blue numbers at left which give the pressure) This means the temperature and dew point are the same, which means the air is saturated. I.e., fog. Then the steep jump to the right means a temperature inversion; it’s a good couple of degrees C warmer at 900 hPa (roughly 1 km up) than at the surface. Now look at the wind barbs at bottom right, showing the easterly flow just right near the surface, taking that cool foggy air in from offshore, while just a little ways up we have west to northwesterlies.

To get even nerdier:

Actually, there are at least four distinct layers in this sounding. 1. The cool fog layer at the surface. 2. Above the inversion, just above 950 hPa, a layer that is close to saturated, but not quite (the two white lines are separate, but near each other, indicating temperature is just a little greater than dew point. The lapse rate is a little steeper than moist adiabatic (temperature angling to the left of the white dashed curve) which, given how close to saturation, suggests this air is almost unstable to a little elevated convection? 3. Atop that, between around 700-600 hPa, a roughly isothermal layer – very stable, close to being another inversion in that there is a slight temperature increase – in which the humidity drops steeply. 4. Above that, an atmosphere that is close to moist adiabatic in its temperature structure, but very dry.

I won’t try to do a whole analysis of this structure here, but it’s fascinating!

Tropical Pacific goes wild

Here in NYC, it’s cool and wet, while the ice slowly finishes melting and spring struggles to take hold. No big excitement, really. In the tropical Pacific, on the other hand, everything is happening at once.

At this moment there are two tropical cyclones (TCs – the generic name for hurricanes, typhoons etc.) in the southern hemisphere, both in the Pacific. Tropical Cyclone Pam just wreaked havoc in Vanuatu, ripping through that small island state as a category 5 storm. Here’s a visible satellite closeup:

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And Nathan is offshore of northern Australia right now. Briefly, yesterday, there had been three at once in the southern hemisphere, with Olwyn making landfall in Western Australia (coming from the Indian ocean, that is, rather than the Pacific) in addition to Pam and Nathan. Australia had seen several cyclones already this season before that; a couple of them, Lam and Marcia, were quite intense. It is around the normal peak TC season in the southern hemisphere now, but even so this moment is exceptionally active. At the same time, there is even a weak one in the northern hemisphere, Tropical Storm Bavi in the Western North Pacific, where this is normally about the deadest part of the year.

Why is the Pacific going so nuts? A proximate factor seems to be that the Madden-Julian oscillation (MJO) is nearly off-the-charts strong right now, with its active phase right in the central Pacific. (If you have never heard of the MJO, but are interested to know about the most important mode of weather and climate variability in the couple-weeks time scale range, you can start with my old blog posts here, here, and here.) The MJO tends to spin off TCs as it moves slowly eastward. So we can hold it partly responsible for some of the Australian activity as it was there around 7-10 days ago, and now the Pacific storms.

There’s also now an El Niño officially underway, which can help to jack up TC activity in the Pacific. It is probably temporarily helping to amp up the MJO as well, as the signals associated with the two are briefly in phase. It’s a weak El Niño event by standard metrics, but it’s possible for that to be the case while still its impact on TCs is strong. In fact, when it was hurricane and typhoon season in the northern hemisphere, last northern summer and fall, the whole season looked very El Niño-like even though an El Niño had not yet been declared to exist by most forecasters, but was limping along just below their thresholds for calling it. The Atlantic was quiet, the Eastern Pacific was gangbusters, and the Western Pacific had a large number of very powerful typhoons, all typical of El Niño years.

Apart from Pam’s destruction in Vanuatu, these storms have been doing relatively little damage – compared to what they could have done, given their intensities. The Australian landfalls, in particular, have largely spared population centers. But it’s been an impressive display of atmospheric power.