Dried out

The MJO has moved into phase 4 or even maybe 5 by now, with the active convection over the "Maritime Continent" (Indonesia) and making its way towards the Pacific. The Indian ocean has been suppressed for around a week now. Here's the current infrared satellite image:

There is some deep convection in the IO, but it's spotty. In the east, it's focused way in the southern hemisphere, in the form of two tropical cyclones. The eastern one has been named "Alenga", and is the stronger of the two (though still a tropical storm, below category 1 on the Saffir-Simpson scale used in the US), while the weaker western one has been dubbed, poetically "Two" - it will get a better name if it gets intense enough to deserve it. North of the equator, there is a well-defined but not particularly intense ITCZ in the west, off the east African coast. The northeast quadrant is pretty quiet up until Sumatra. Convection in the Bay of Bengal and Arabian Sea has completely shut down, perhaps for the season as we head into northern winter and the sea surface temperatures there are cooling rapidly. The southeastern IO is also completely dry. This can't be ascribed to the season, but perhaps can be blamed in part on the suppressed phase of the MJO.

A lot of us working on DYNAMO have become fond of the "morphed" precipitable water product called "MIMIC" produced at the University of Wisconsin. This is a product based on passive microwave satellite observations. ("Passive" means that the radiation measured is just what's naturally emitted from the planet, as opposed to e.g. a radar that sends out its own signal; "microwave" refers to the low frequency of the radiation measured.) The quantity shown is total column-integrated water vapor. That is, if you took all the water vapor in the atmosphere overhead at any given point and condensed it to liquid, this is how deep a layer of liquid water it would make. If the whole column were saturated - 100% relative humidity from the surface into the stratosphere - this depth would be a little over 70 mm, typically, in the tropics. (This maximum saturation value depends on the temperature, and so is less at higher latitudes.) Not much water in the atmosphere, right?

In any case, the atmosphere is never completely saturated, so the values you see are less everywhere than 70 mm (see the color scale on the right) though they get close in some places. A couple interesting things to notice in this picture:

1. The two tropical cyclones, Alenga and Two, show up as round red blobs swirling gracefully in the south, wrapping dry air around them on their western flanks.

2. The tongue of yellow and green pushing into the orange and red area - just south of the equator in the western IO, and extending about as far east 80E, roughly parallel with the tip of India. This is dry subtropical and continental air that has been pulled in by low-level westerlies associated in part with the late active and early suppressed phases of the MJO. It seems that the active phase to some extent puts an end to itself because the convection causes the westerlies (by fluid dynamics I'm not explaining today) but then the westerlies bring in dry air which shuts down the convection.

Going briefly to the local picture, here is a recent radiosonde trace from Gan plotted as a "skew-T log-P diagram". This is a very strange graph used only by meteorologists. I will not explain it in detail today, we'll leave that for another post. For now I just want you to notice that the green and red traces are relatively far from each other for most of the way up. This means that the dew point is much less than the temperature, which means the relative humidity is low. Not that low really, by the standards of the earth as a whole - the column water vapor is 45 mm (4.5 cm) as shown in the inset plot on the upper right. Notice, incidentally, that this value derived from in situ balloon measurements is in pretty good agreement with the satellite-retrieved value. In the MIMIC picture above, look at 73E, 1S - the location of Addu Atoll - and notice the color is yellowish there, which is around 45 mm on the scale at the right.

45 mm of column water vapor would be a large value - a humid atmosphere - in some places, for example New York City in winter. But for Addu Atoll in November, this is a relatively low value, lower than any observed during the 17 days I was just there during the buildup and active phase.