Wednesday, August 7, 2013

The tropical season so far, and what is yet to come.

The season so far:

So far, there have been four tropical cyclones in the Atlantic Basin. Now while it may seem we're off to a slow start; we're slightly ahead of the historical average curve. The fourth named storm has typically formed past mid August to the end of August. in fact two of these tropical storms were Cape Verde storms. it's rare to have Cape Verde storms before June, must less two. But they do occur.

Cape Verde Season:

Before I get into the Cape Verde season; I want to discuss what a Cape Verde type tropical cyclone is.

Cape Verde storms develop from tropical waves , which form in the African savanna during the wet season and move into the Atlantic Basin and become tropical cyclones within 550 to 620 miles of the Cape Verde Islands.

When we get into the heart of Hurricane season in the Atlantic Basin, we start to watch around the Cape Verde Island in the Western Atlantic just off the coast of Africa. Normally Cape Verde storms occur in August and September, but as was the case this year they can form earlier, or later in the year.

Why are the tropical cyclones having problems developing?

The reason is wind shear and water temperatures, in a nutshell, it's the dry Saharan air layer.


Wind Shear:

wind shear is generally the most essential contributing factor when it comes to tropical cyclone formation. Generally, wind shear refers to any change in wind speed or direction along a straight line. When we're dealing with tropical cyclones vertical wind shear is what we watch . We monitor the difference in wind speed between 200mb layer (around, 40,000 feet) down to the 850mb layer (about 5,000 feet ). You can find these charts on Tropical Page of the Blog/website.

Tropical Cyclones are basically just heat engines powered from the latent heat that is released from water vapor turning into liquid water.

when shear is low, the storms latent heat is focused over a small area of ocean, the lack of strong vertical wind shear allows the storm to grow tall right over the top of this latent heat . But when the wind shear is high (more typical in an El Nino year), the storms latent heat is focused over a much larger area of ocean, the stronger vertical wind shear blows the top of the storm away from its center, make the storm much less efficient in its handling of heat.

So vertical wind shear effects tropical cyclones by removing the heat and moisture they need from the area near their center; this disrupts the inflow and outflow of the tropical cyclone.

Water Temperature:

Several important ingredients are needed for a tropical disturbance to become a tropical cyclone and later strengthen into a tropical storm or hurricane:

1.A tropical disturbance with thunderstorms.

2.A distance of at least 300 miles (500 kilometers) from the equator.

3. Water temperature is the biggest limiting factor in the early part of tropical season If you remember your Earth Science from grade school, then you remember that water warms slower than the land. Therefore, late summer and early fall is the time for the most favorable water temperature for tropical storm development. As most of us are aware, normally Atlantic Basin cyclones form in the area between the West Coast of Africa and the Caribbean, along with the Gulf of Mexico (GOM). Hurricanes thrive over warm water; the warmer the better. Tropical cyclones are fueled by warm water evaporating into the air;  water temperatures 80 degrees (F) or greater will enhance tropical development. Typically you want those warm Ocean temperatures down to a depth of at least 164 feet (50 meters) below the surface.

4.Lots of moisture in the lower and middle part of the atmosphere.

5. And as I just explained low wind shear.

In September Atlantic sea-surface temperatures (SST) typically reach maximums of about 83.3°F (28.5°C) along a band centered at about 7°N

As I said above, we're entering what is typically called Cape Verde season. The reason it's rare to see Cape Verde tropical cyclones before August is It takes a while to warm up the waters off the African Coast to 80 degrees F, deeper than 25 feet.

However, recently, the water temperature has been a few degrees below average near the Antilles.  



The Saharan Air Layer (SAL):

Every year a mass of very dry, dusty air which forms over the Sahara Desert and Sahel regions in Northern Africa during the late spring, summer, and early fall. Normally this dry and dusty air mass moves out over the tropical North Atlantic Ocean every 3-5 days. As this air mass advances westward and emerges over the Atlantic off the northwest African coast, it comes into contact with cool, moist low-level air and becomes the Saharan air layer (SAL). Also, known as Saharan Dust.

The SAL can have a significant negative impact on tropical cyclone intensity and formation.

The inclusion, or drawing in, of dry air into a tropical system. can act to weaken a tropical cyclone by forming downdrafts around the storm.

The surge in the mid-level African easterly jet can substantially increase the vertical wind shear in and around the storm environment.

Since one of the key ingredients for tropical cyclone development is a deep feed of moisture, Saharan Dust often acts to inhibit tropical development.

Because the SAL aids in reversing the trade winds which helps to stabilizes the atmosphere. The more stable atmosphere is the harder it is for tropical convection to develop.

As you can see there is a lot of Saharan Dust over the Atlantic. This will make it hard for tropical cyclones to move west.

The three factors I mentioned: wind shear, water temperatures, and the SAL has prevented the season from really taking off.

Looking ahead:

The global models are hinting that the Atlantic Basin will be become more active around mid month.

Here is a look at the current SST in the Atlantic Basin. The water temperatures are starting to run slightly above normal for this time of year. this would help increase tropical cyclone activity.

sst anomnight_current large

We still have a lot of Saharan Dust out over the Atlantic. And as I just showed this dry air will help to inhibit tropical development   Moisture levels in the Caribbean and western Atlantic are drier than they should be.


ENSO stands for El Nino/ Southern Oscillation. The ENSO cycle refers to the coherent and sometimes very strong year-to-year variations in sea- surface temperatures, convective rainfall, surface air pressure, and atmospheric circulation that occur across the equatorial Pacific Ocean. La Nina and El Nino represent opposite extremes in the ENSO cycle. ENSO is known to affect tropical cyclones in different ways around the globe.

El Nino refers to the above average SST that develop across the east central equatorial Pacific. It is referred to as the warm phase of the ENSO cycle.

La Nina is the below average SST that develop across the east central equatorial Pacific. It's the cold phase of the ENSO cycle.

ENSO neutral is when the SST, rain fall patterns, and wind patterns over the equatorial Pacific Ocean are near the long term mean.

In spite of the recent slight cooling, it does not appear a La Nina — or an El Nino — is underway. The subsurface temperature anomalies under the equatorial Pacific have changed little over the past few months. So the ENSO is currently cool/neutral, and if it remains neutral, an El Niño will not inhibit development of storms in the Atlantic for the rest of this season. Relative to neutral events, the frequency of hurricane landfalls along the East Coast is found to increase (decrease) during ENSO cold (warm) events. This is consistent with previous studies linking ENSO and Atlantic hurricane activity. But there is still a chance El Niño conditions may develop and suppress activity somewhat in the latter portion of the season.

The MJO:

The MJO (Madden Julian Oscillation) is a planetary-scale quasi-periodic oscillation of atmospheric wind and convective cloudiness anomalies that moves slowly eastward along the equator mainly over the tropical Indian and Pacific Oceans with a timescale on the order of 30–60 days.

The MJO is broken into 8 phases which begin over Africa and move eastward.

The MJO phase diagram illustrates the progression of the MJO through different phases. Phase diagram shows the evolution of the last 40 days of observations along with the 15 day forecasts from the constructed analogue (green), autoregressive model (AR), and lagged linear regression (red).
The thick (thin) lines represent the statistical model forecasts for the first 7 days (last 8 days) respectively.

RMM1 and RMM2 are just mathematical algorithms that combine the amount of clouds and winds at various levels of the atmosphere to provide a measure of the strength and location of the MJO (you don't have to worry about it). When the index is within the circle in the center; the MJO is considered weak, meaning it is difficult to discern using the RIMM algorithms. Outside of this circle the index is stronger and will usually move in an anti-clockwise direction as the MJO moves from west to east.  

The Main thing to get out of it is, when the MJO is in phases 1-2, activity in the Atlantic typically picks up, and when it’s in phases 6-7, Atlantic activity decreases. When it's in the other phases, everything more or less averages out (neutral).  If you look at the diagram you can see it is forecasted to head into phases 8 and/or 1. If this does occur, we would see more tropical activity in the Atlantic by mid-August.

   NCPE_phase_21m_small MJO  

Based on all of this, I think we will see an uptick in Atlantic tropical activity as we get around mid month. As far as the season goes I think we will still see above average tropical cyclone activity in the Atlantic Basin.. but perhaps only slightly... My 2013 hurricane outlook might only verify on the lower end.... I guess time will tell.  


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