Tropics fairly quiet, though pattern supports activity for the next 10 days

By: Levi32 , 3:01 PM GMT on July 06, 2011

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The tropical Atlantic (and really the global tropics) remain fairly quiet today. As expected, despite all the thunderstorm activity near and just north of the Caribbean over the 4th of July holiday, the area of energy was too broad to allow anything significant to develop out of it. The only feature I would watch for any possibility of development would be a disturbance within the ITCZ, currently in the central Atlantic near 40W, which will be moving towards the Trinidad area in about 3 days, and into the Caribbean from there. Right now it is at a fairly low latitude, and will thus have a hard time gaining significant spin, but once it starts to gain latitude, it may be worth watching. Such features following behind tropical waves can be sneaky at times. It is not an immediate threat, but may be worth watching next week in the Caribbean, as conditions could allow something to brew.

Other than that, there are no other current threats. The Atlantic will be favored for activity by the MJO for about the next 10 days before the models show this current upward motion pulse moving on into the Indian Ocean, allowing a greater portion of sinking air to return to the Atlantic, suppressing activity for the latter portion of July. This month should be a fairly quiet month overall, though I could see us getting one development out of it. Folks should keep in mind that June and July are generally not great indicators of what the peak of the hurricane season (August-October) will be like. Last year, we had a deadly quiet July, with only Tropical (Rainstorm) Bonnie forming, and yet we went on to have 19 named storms when it was all said and done. A quiet July should be no reason to let down your guard for this hurricane season. We have a long way to go, so let's enjoy the quiet while it lasts.

We shall see what happens!



Caribbean/East Pacific Visible/IR2 Satellite (click image for loop):



Central Atlantic Visible/IR2 Satellite (click image for loop):



Atlantic Tropical Surface Analysis:



200mb Vertical Velocity (green areas represent upward motion associated with the MJO):






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9. Levi32
12:14 AM GMT on July 07, 2011
Quoting TropicalTracker2011:
Hey Levi, I got a couple of questions.
1: Why is shear very high in the carribean and gulf
2: Will it be decreasing anytime soon
3: The steering flow would take the central atlantic wave into the carribean but from there where would it go
Thanks.


An upper low is causing the wind shear. It may lower some over the next few days. Current model projections would take the wave into central America as a ridge holds strong over Texas again.
Member Since: November 24, 2005 Posts: 635 Comments: 26754
8. Levi32
12:13 AM GMT on July 07, 2011
Quoting TomTaylor:
Wow great post!

The prediction of shear only increasing in the Atlantic and perhaps a few other regions makes sense, rather than increasing shear over the entire tropical region. It is interesting to note that the climate model is favoring the idea of a somewhat semi permanent El Nino and warm PDO. Make for a much warmer climate down here in San Diego...not to be selfish but that does sound nice lol. Also the extreme warming off of south America is interesting.

About the cooling of the gulf...So the cooler gulf doesn't just affect the lower atmosphere then? And since it affects the mid atmosphere as well (~600mb) it strengthens the AEJ by inducing a stronger pressure gradient? That makes sense. And I'm guessing a stronger AEJ increases vorticity, instability and some other more complicated stuff

About the models, that makes perfect sense.

Thanks for all your help levi, I appreciate it.


Oh it affects everything. If you're thinking about thermal conduction all the way up to the 600mb level, then indeed that is not going to be all that big of a difference if the gulf cools by a couple degrees. However, cooling of the lower troposphere makes the air column denser there, and thus the entire tropospheric column compacts closer to the ground overall. This lowers the 600mb isosurface, thereby lowering the 600mb height, and creating a stronger pressure gradient at that level.

Thanks for all your help levi, I appreciate it.

My pleasure. Glad I could help.
Member Since: November 24, 2005 Posts: 635 Comments: 26754
6. TomTaylor
10:41 PM GMT on July 06, 2011
Wow great post!

The prediction of shear only increasing in the Atlantic and perhaps a few other regions makes sense, rather than increasing shear over the entire tropical region. It is interesting to note that the climate model is favoring the idea of a somewhat semi permanent El Nino and warm PDO. Make for a much warmer climate down here in San Diego...not to be selfish but that does sound nice lol. Also the extreme warming off of south America is interesting.

About the cooling of the gulf...So the cooler gulf doesn't just affect the lower atmosphere then? And since it affects the mid atmosphere as well (~600mb) it strengthens the AEJ by inducing a stronger pressure gradient? That makes sense. And I'm guessing a stronger AEJ increases vorticity, instability and some other more complicated stuff

About the models, that makes perfect sense.

Thanks for all your help levi, I appreciate it.
Member Since: August 24, 2010 Posts: 19 Comments: 4358
5. Levi32
7:30 PM GMT on July 06, 2011
Quoting TomTaylor:
hey levi, thanks for the update

got a few questions though...sorry they're pretty random

1. I commonly hear people mention that a potential side effect of a warmer planet would be increased wind shear over the tropics which would inhibit development. What is the reasoning behind the increased shear? My guess is that they assume there will be increased heat in the tropics creating more convection and therefore stronger shear outside of the ITCZ as a result of all the convection. However, (assuming my assumption in the last sentence was correct) what doesn't make sense is that it's not just the equatorial region that is warming, the entire planet is warming. In fact, the equator region is warming the slowest of all regions. Therefore, I would expect that a decrease in temperature gradients would lower shear. Anyway, that's just my train of thought behind that topic

2. How does cooling of the Gulf of Guinea promote a stronger AEJ? I agree cooling the region creates a stronger temperature gradient, allowing for stronger winds, meaning greater convergence and vorticity associated with tropical waves, as well as a further north exit region. However, I do not understand how this effects the AEJ. The AEJ is a result of the Saharan High located above the heat low and is located (the jet) between 500-700mb and is strongest around 600mb (from what I've heard). So, I don't understand how the cooler Gulf of Guinea strengthens the AEJ.

3. Last but not least, the green and purple regions on this ECMWF 500mb ensemble mean map represent areas of disagreement among the ensemble members, correct? What are they disagreeing on? And what specifically do the green regions mean? and the purple?




thanks a ton in advance, have a nice day!


I love questions =)

1. So firstly, I have generally heard of the Atlantic being cited as the basin to suffer from increased wind shear, not necessarily the entire tropical belt. That wouldn't make sense anyway, for the reasons you mentioned. Secondly, when climatologists say that they expect wind shear to increase, what they really mean is that their climate computer models simulate an increase in wind shear. The main reason why they show increased wind shear over the tropical Atlantic is because most of the simulations I have seen show a semi-permanent El Nino condition develop in the equatorial Pacific. For example, here is the IPCC-AR4 A1B scenario multi-model ensemble mean for the change in 700mb relative humidity and 500mb velocity pressure per degree Celsius of global warming:



Notice the dramatic increase in relative humidity and stronger magnitudes of negative velocity pressures at the 500mb level in the equatorial Pacific, off the west coast of North America, and off the west coast of Peru. Also notice the opposite changes occurring in the SW Atlantic and Caribbean. This is a classic El Nino mode being portrayed by this model mean, and it would make sense for wind shear in the tropical Atlantic to increase under this scenario.

2.

I think you are confused about at what heights the pressure gradients are affected. They are affected throughout the troposphere. Below is a crossection of August-September climatological zonal wind over Africa from 10N to 20N (the crossection is along the averaged longitude from -15W to 10E, so looking into the image is looking westward). The boundary between the African monsoonal low and the Saharan High is at about 850mb. The monsoonal low is quite shallow due to the sinking air motions within the subtropical high pressure cell, which create a strong inversion layer above 850mb. This is why convection north of 20N within the monsoonal low (but beneath the Saharan High) is usually rare.



Notice that as we move up into the Saharan High, the core maximum of easterly winds (negative values, purple colors) occurs between 600mb and 650mb. That is the African Easterly Jet. Once we can see this structure, it is apparent that you answered your own question. It is simply the temperature gradient between the Gulf of Guinea and the Saharan High which is increased, thus increasing the pressure gradient, which increases the AEJ speed.

It can be slightly confusing thinking of how the temperature changes will affect each level of the atmosphere. A cooler Gulf of Guinea also strengthens the African monsoonal low beneath the Saharan High, which increases the monsoonal surface flow into the Sahel beneath the AEJ, which also aids in tropical wave development and strength.

3.

The colors represent the average number of standard deviations of the parameter specified (I showed 500mb geopotential height today) of disagreement between the ECMWF ensemble members. The color scale on the right is such that the green colors reflect very low disagreement, and high confidence between the members. The purple and pink colors indicate higher disagreement and thus lower confidence within the ensemble members. Ironically, when tracking tropical cyclone development on ensemble means, the greater the disagreement between ensemble members, the greater the overall confidence of tropical formation. This is because of the fact that the average disagreement between the ensemble members is usually directly proportional to the number of members that develop an area of strong low pressure. In a less stable situation, they will disagree more. The more that they disagree, the more strongly they agree that there will be an unstable situation.

I hope that cleared some of those things up for you. Let me know if it didn't.
Member Since: November 24, 2005 Posts: 635 Comments: 26754
4. TomTaylor
6:35 PM GMT on July 06, 2011
hey levi, thanks for the update

got a few questions though...sorry they're pretty random

1. I commonly hear people mention that a potential side effect of a warmer planet would be increased wind shear over the tropics which would inhibit development. What is the reasoning behind the increased shear? My guess is that they assume there will be increased heat in the tropics creating more convection and therefore stronger shear outside of the ITCZ as a result of all the convection. However, (assuming my assumption in the last sentence was correct) what doesn't make sense is that it's not just the equatorial region that is warming, the entire planet is warming. In fact, the equator region is warming the slowest of all regions. Therefore, I would expect that a decrease in temperature gradients would lower shear. Anyway, that's just my train of thought behind that topic

2. How does cooling of the Gulf of Guinea promote a stronger AEJ? I agree cooling the region creates a stronger temperature gradient, allowing for stronger winds, meaning greater convergence and vorticity associated with tropical waves, as well as a further north exit region. However, I do not understand how this effects the AEJ. The AEJ is a result of the Saharan High located above the heat low and is located (the jet) between 500-700mb and is strongest around 600mb (from what I've heard). So, I don't understand how the cooler Gulf of Guinea strengthens the AEJ.

3. Last but not least, the green and purple regions on this ECMWF 500mb ensemble mean map represent areas of disagreement among the ensemble members, correct? What are they disagreeing on? And what specifically do the green regions mean? and the purple?




thanks a ton in advance, have a nice day!
Member Since: August 24, 2010 Posts: 19 Comments: 4358
3. InTheCone
3:32 PM GMT on July 06, 2011
Thanks Levi, great update!
Member Since: September 1, 2007 Posts: 0 Comments: 1990
2. TropicalAnalystwx13
3:15 PM GMT on July 06, 2011
Thanks Levi!
Member Since: July 6, 2010 Posts: 113 Comments: 34164

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