Frank Strait

Why Our Tropics Are Quiet And Will Be For A While

6/28/2015, 3:50:05 PM

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Due to a little technical glitch, I am going to pass on a video for today. Not much has changed in the forecast since yesterday, anyway.

Instead, we'll talk about the tropics. I have said lately that it's quiet in the tropics on our side of the world and will stay quiet for a while. There are several reasons for this. First, let's look at some satellite pictures to see what it looks like down there.

First, the East Pacific:


There are some thunderstorms along the Intertropical Convergence Zone but nothing of any development concern.

Now, for the Atlantic Basin:




Nowhere is there anything that looks even remotely of any concern! Now, for the why. First, and I think foremost, there is sinking air all over the place on this side of the world. That's due to the current state of the Madden-Julian Oscillation, which is one of many wave patterns meteorologists watch to help diagnose the weather. This one drifts eastward mostly near the equator and has a large impact on regulating thunderstorms in the tropics. The best way to visualize it is to look at what is called velocity potential anomalies at the upper troposphere. That's complicated to describe and so I won't go into that, but looking at a plot of that tells you where you have either convergence, air coming together, or divergence, air spreading apart, at the top of the troposphere. Where you have strong convergence aloft, there will be sinking air beneath it. Where you have divergence aloft, air will be rising beneath it. The plot shows very strong convergence aloft over the Atlantic and especially into Africa:


Brown contours show the convergence while green contours show convergence. This tells us that air in generally is subsiding over the Atlantic Basin and very strongly so over the eastern Atlantic.

There is another consequence of this to consider as well. Sinking air tends to produce very dry air masses. The strongest sinking motion right now is found over Africa and the eastern Atlantic. All this sinking air can sink only so far; once it gets close to the surface, it has to spread out. The result is higher surface pressure where the sinking air occurs, but also, since the strongest sinking air is over Africa, the trade winds are stronger than usual. Sure enough, pressures are higher than usual:


... by more than 5 millibars or so, and the Bermuda-Azores high ridge extends farther south than usual. And yes, the winds are strong across the trade wind regions as well. That's especially true over the Caribbean, where other factors are in play. First, that area is in between where the sinking is occurring over Africa and rising air is occurring farther west. The rising motion found over the Pacific, particularly the West Pacific, means pressures are lower there and air at the surface will move from high to low pressure. So, the flow across the Caribbean will be faster than usual because surface pressures east are higher than usual and pressures west are lower than usual. There is also the usual factor that pressures are higher to the north normally and normally lower to the south over Central America. This speeds up the flow even more across the Caribbean and also deflects it a little to make the flow more from east-northeast to west-southwest.

There are four important consequences for the Atlantic. First, the winds are going to be reaching gale-force over parts of the Caribbean the next few days, affecting shipping, pleasure cruises and other travel. Second, tropical waves are going to move fast, generally 10 or more degrees of longitude per day. They typically only drift west at 7-8 degrees per day. This is important because fast-moving tropical waves struggle to develop further. Finally, and perhaps most importantly, the very dry and dusty air that is present over the Saharan Desert will continue to get pulled strongly westward over the Atlantic. That has been going on for a while and it's going to be several days before this changes. Finally, with the easterly flow rather fast, it is contributing further to the strong wind shear present over the Atlantic. By wind shear with respect to tropical systems, it means that the winds at one level of the atmosphere are moving at a different speed and direction compared to the winds aloft. Check out this analysis from the folks at CIMMS at the University of Wisconsin (producers of my favorite tropical data product, MIMIC):


The green wind barbs are at lower levels, 18,500 feet up to around 25,000 feet. Notice they show easterly winds, similar to what is happening below and close to the surface. Meanwhile, the blue wind barbs, showing the flow at 35,000 to around 50.000 feet, are showing westerly flow aloft. It's actually even stronger than usual. This is caused by ongoing El Nino conditions in the Pacific. This means over the Caribbean, wind shear is tremendous, over 100 knots (that is, to say, the difference of flow between the low levels and well aloft is over 100 knots, 100 knots is about 115 mph). The shear is strong over the tropical Atlantic farther east is well.

So, everything that can go wrong for tropical developments is going wrong right now. The air mass over the tropics is very dry with many pockets of Saharan dust. Winds are about as unfavorable as it can get in the Atlantic due to the shear. If this wasn't enough, there's one other thing:


Sea surface temperatures are cooler than usual all over the main development region (10-20 degrees north longitude) of the Atlantic. So, with all of these strongly negative factors, mostly due to the Madden-Julian Oscillation being in an unfavorable phase, we have very quiet tropics on our side o the world.

Now, things are not quite as bad in the East Pacific but some of the issues the Atlantic has is bleeding over into the East Pacific. The stronger than usual easterly flow is no help. This is pulling some of the dry air from the Atlantic in. Second, this easterly flow is causing some upwelling west of Central America and that is cooling the surface waters somewhat:


With an El Nino going on, you would expect to see the waters here warmer than usual, and near the equator, they are. But for the moment, the upwelling is leading to cooler water. Well, the upwelling is part of it. The fact that there have been three tropical cyclones pass through the area south of the Mexican Riviera so far this season has an impact as well. Tropical cyclones some of the heat out of the water, too, it's their reason for being, really, to take heat from the tropics, including some of the heat in the ocean, and sent it northward.

You also will see some shear in the East Pacific looking at winds. It's not as strong, but it's still there:


Yes, the wind barbs are pointed in the same direction for the most part, but notice the blue ones show fast flow, 50 knots or more, much stronger than what's happening below with the green wind barbs. That results in some speed shear even though the flow is in the same direction. Also, keep in mind that this doesn't show the surface flow. The surface flow is actually light over the area south of Mexico right now because there is also a surface high west of California that results in similar pressures over the Gulf of Mexico and the other side of Mexico, but still easterly. South of the high, the surface flow is much more strongly easterly, so shear is very strong there (out along 140 west longitude).

So, conditions are rather hostile to tropical systems on our side of the world. But it's a different story on the other side of the world:


As the sun is rising over the West Pacific, look at all the clouds and thunderstorms out there! Currently there are no developed tropical cyclones, but there are three different areas that have the potential to become one, two north of the equator and one south. The computer models are actually showing an area well southeast of Guam becoming a potentially significant typhoon over the next several days. Notice on the velocity potential anomaly map above that this area is under strong divergence aloft, meaning air is rising in this area, a setup much more conducive to thunderstorms.

You're probably wondering when things are going to change and it might get more active again on this side of the world. Well, these waves of the Madden-Julian Oscillation typically drift eastward, but slowly. A wave, if it totally circumnavigates the world, might take 45 days to do so. My understanding is that the stronger waves like the one we currently see move a little slower. Being that the part of the wave favorable for rising air is almost totally on the opposite side of the world from us, that means we're probably three weeks from the Atlantic having more favorable conditions. With less distance for the wave to travel, it may be more like two weeks before the East Pacific perks up some.

I don't know about y'all, but I'm not going to complain about having less weather to deal with over the next few weeks than I might have otherwise!

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