Indian Ocean cyclones

We’re in the middle of a very active period for tropical cyclones over the north Indian ocean. Here is a brief post on the State of the Planet.


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!



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?


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:


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:


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:


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.

This harsh winter: the improbable occurs

It’s 10 degrees Fahrenheit in New York City as I write this, with an overnight low that will get close to zero (Fahrenheit). It has been cold in the northeast US for weeks, and is forecast to stay cold for weeks more. Boston is being choked by snow from multiple major storms – two of which have been in their all-time historical top ten for snow accumulation – and stands a good chance to break the all-time record for most snow in a season.

I wrote a post back in October about the coming winter. The main point of the post was that seasonal forecasts in general, and for the northeast in particular, have large uncertainty and not a huge amount of skill.

I concluded by saying that

“Without looking at any weather data or models, one can say pretty confidently that it is very unlikely that this winter will be as cold as last winter was in the eastern US. Last winter was very extreme by historical standards, so a winter that extreme is – basically by definition – improbable in *any* year. No information currently available (including the state of El Nino), or that will be available ahead of time, is strong enough to change that. Again this is a probabilistic statement: it’s not impossible that this winter will be as cold or colder than last, it’s just very unlikely.”

I haven’t seen the statistics to prove it yet, but I think it may well turn out that for the northeast at least, this winter in fact is going to turn out more severe than last. In terms of snow in much of New England, it certainly is already, and I think with the current cold snap – projected to last another two weeks in virtually the whole eastern US, as per NOAA’s latest 8-14 day forecast (shown below) – we are headed to beat last year in terms of cold as well.

I stand by my claim that this was improbable, as far as the scientific information available in October would have told us. But the improbable seems to be occurring.