Tuesday, August 30, 2011
Sunday, August 28, 2011
Saturday, August 27, 2011
This is supposed to be a blog about the MJO, but as I am sitting in NYC during the onset of a weather event that's nearly unprecedented in living memory for this city, I'm hijacking it to put up a little log of that.
I have been following Irene since about Wednesday night. During that time the forecast has been remarkably stable. As the various models have moved the track either a little inland or a little further out to sea, the estimates of Irene’s intensity upon landfall in the NYC region have fluctuated between category 1 Hurricane and Tropical Storm, with the passage through our latitude occurring somewhere from a little west of the city to eastern Long Island. Tracks either further inland or far enough east that Long Island and New England are spared have looked unlikely from the start. The intensity forecast for us remains in the tropical storm - category 1 range (despite the fact that the intensity with which it recently made landfall in North Carolina has ended up lower than had been forecast a day or so before).
In any case we are almost certain to get an awful lot of rain. It is already raining now (11:30 AM, when the storm center is still down in North Carolina), from what looks to be a far outer band of Irene.
The storm surge risk is real as well. Even if the intensity stays on the lower side of the forecast range, the large size and slow motion of Irene mean that the wind will be pushing on a large piece of oceanic real estate for a long time, which means more water pushed up into New York harbor. Because we have a spring tide now, the high tide will be about as high as it ever gets. So the magnitude of the storm surge depends a lot on its timing - whether it arrives at high tide or low. Some detailed accounts of the evolving forecast of this are here. In any case it seems to me that the city is doing the right thing evacuating zone A (but not B). Stopping the subways seems the right move too, though doing so at noon seems a little over-conservative. The first concern as I understand it is wind on the elevated lines, but it seems pretty likely that there will be flooding in some underground lines from rain, storm surge, or both.
Our local grocery store very crowded, with long lines and some staples running out. The whole neighborhood seems pretty much out of batteries.
Tuesday, August 23, 2011
The words Madden-Julian Oscillation (MJO) refer to a tendency of the weather in some regions of the tropics to fluctuate between rainier and drier states a few times over the course of a season. In the rainy state, the wind close to the earth’s surface tends to blow from west to east (“westerly”), while in the dry state it blows from east to west (“easterly”). A given state tends to last a month or two, and to cover a region of size several thousand km in the east-west direction. Changes from rainy to dry (or vice versa) tend to progress slowly from west to east so that – for example - if it is rainy in one place, it will probably become rainy soon in places nearby to the east of there. The MJO most strongly affects the weather over the tropical Indian and western Pacific oceans, but also influences places far from there, including at higher latitudes.
In the scientific literature you will find statements like this. “The MJO is a mode of natural variability in the tropical climate system characterized by planetary spatial scale, intraseasonal (30-60 day) time scale, and eastward propagation.” Translating that into English one bit at a time:
“A mode of natural variability” has two parts, “mode of variability” and “natural”. “Natural” means “not caused by humans”. The MJO would be there in the absence of human perturbations to the climate. It is not a result of global warming (though it is quite possible that some of its properties will change as the climate warms). “Mode of variability” means that it is a tendency for some aspects of the climate to change in a particular ways. The changes are generally back and forth between two states, rather than permanent. In this case the aspects that change most dramatically include rain, clouds, and wind.
“In the tropical climate system” means that the MJO occurs primarily in the tropics; “climate system” means in this context that both the atmosphere and ocean are involved (although for the MJO we believe that the most important action is in the atmosphere, while the ocean mainly responds to that).
“Planetary spatial scale” means that if a given state of the MJO (i.e., rainy or dry, or in jargon, “active” or “suppressed”) is in place somewhere, the same state tends to be in place everywhere around there for a long distance. “Long” here means at least thousands of km. Really the term “planetary spatial scale” means “not many times smaller than the circumference of the earth”. The circumference of the earth at the equator is 40,000 km, so planetary scale means maybe at least 5,000-10,000 km.
“Intraseasonal (30-60 day) time scale” means that 30-60 days is the typical “period”, or time it takes for a given state of the MJO (rainy or dry) to repeat itself after having first switched to the other state. The time to switch from one state to the other would then be half that period. The range 30-60 days, rather than just a single number, indicates that the switching back in forth is not that regular. In mathematical terms we would say the MJO is not truly “periodic”, but only “quasi-periodic”. (This is typical of nearly all natural climate and weather fluctuations, and is what makes them difficult to predict far in advance; truly periodic fluctuations are the diurnal cycle (day-night) and the seasonal cycle, both of which are easy to predict because their periods are known exactly and never change.) The word “intraseasonal” refers to the fact that 30-60 days is a little shorter than a season (especially in tropical regions where one might say there are really only two seasons in a year) so that within a given season there is time for a few MJO cycles.
“Eastward propagation” means that if a rainy MJO phase is happening somewhere, it will happen a little later in places east of there. In particular, MJO events tend to start in the Indian ocean and move across Indonesia to the western Pacific. The speed at which they move is around 5 meters per second. After reaching the western Pacific, MJO events continue moving eastward, into the eastern Pacific, then across the Americas into the Atlantic; but in those parts of the world the change in the weather that accompanies the MJO is often not as strong as it was over the Indian and western Pacific oceans.
Tuesday, August 16, 2011
This blog will be about the Madden-Julian Oscillation (MJO), and the upcoming field campaign in the Indian ocean to study it called Dynamics of the Madden-Julian Oscillation, acronym DYNAMO. The authors will include a team of scientists from Columbia University, Colorado State University, and Harvard University who will be (along with many others) involved in this campaign. For anyone reading this who doesn’t know what the MJO is, you might start here and maybe here, but we’ll explain it more in the next few weeks.
We have several purposes in putting up this blog, and don’t know yet which it will turn out to serve most. One is just as a record of our activities and what happens in the field, from a science point of view. Another is for our colleagues in climate science to be apprised of what is going on. Yet another is to do some public education for people outside the field who are interested in it.
The last will be travelogue. Each of us will go to the Maldives for a couple of weeks or so (at different times) to participate in the field campaign. We'll be on Gan island; looks like a nice place. Some scientists in our field do a lot of field work and go to exotic places often. We’re considered “modelers”, which means we work almost exclusively on computers and don’t need to travel (except sometimes to go to conferences). Only one of us, Sobel, has participated in a field campaign before, and him not many times. It will be a new and unusual experience for us, so we’re going to document it.
The team will consist of at least the following people:
Adam Sobel (Columbia)
Zhiming Kuang (Harvard)
Eric Maloney (Colorado State)
Daehyun Kim (Columbia)