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.

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.

The ENSO:

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.


    


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.  



Rebecca

2013 hurricane outlook

I meant to post this a few weeks ago, but do the active weather and other personal concerns, it took a backseat. Anyway without no farther postponement, here is my tropical cyclone activity outlook for 2103.

Thoughts On The 2013 Atlantic Hurricane Season.

We are a little more than a week before the official start to the 2013 Hurricane season. Here is my thoughts on this year's hurricane season.

Here's the list of names for the 2013 hurricane season. If my personally is any indication, I hope we don't make it to R ;)

Andrea
Barry
Chantal
Dorian
Erin
Fernand
Gabrielle
Humberto
Ingrid
Jerry
Karen
Lorenzo
Melissa
Nestor
Olga
Pablo
Rebekah
Sebastien
Tanya
Van
Wendy

As my last blog post noted, we are seeing a similar pattern setup that was predominate in the 1950's. These same ocean oscillations and Sea Surface Temperatures (SST's) will have a major effect on Atlantic Basin tropical cyclone activity in 2013.

The last few years have seen major storms impact the Northeast and Mid Atlantic, Irene, Lee, and Sandy are most likely the first to come to mind. So let's take a look at this year's setup.

Part one can be found here.

Part two can be found here.  

Oscillations:

The Pacific Decadal Oscillation (PDO):

The PDO is in a negative (cool phase) which means cool water temperatures off the Pacific Northwest coast. The affect this also has is it helps the Atlantic warm.

The North Atlantic Oscillation (NAO):

This oscillation is in a negative cool phase. The NAO has a strong relationship to temperature, precipitation, and weather patterns along the Eastern United States. The traditional definition of the NAO is the difference of normalized sea level pressure anomaly between Iceland and the subtropical eastern North Atlantic.

During a negative NAO unusually high geopotential height occurs across the high latitude North Atlantic. This is typically found by looking at the 500 mb level and is referred to as blocking. This blocking ridge across the high latitude North Atlantic causes both upstream and downstream troughs across Eastern North America. Which help steer systems on the East Coast.

Recent studies at the NC State Climate Office indicate an increased potential for wintry weather in NC due to the position and availability of cold air, and a more favorable upper level pattern conducive to coastal storm tracks.  And as we saw with hurricane Sandy, blocking in the North Atlantic can have dire consequences for the Mid Atlantic and Northeast.

Atlantic Multidecadal Oscillation (AMO):

The current state of the AMO is positive (warm phase) which means warm water temperatures off the East Coast. When the AMO is in a positive phase there is melting of sea ice in the Arctic. The recent news of the Greenland ice sheet and sea ice melting is the result of the AMO being in a warm phase.

When the AMO is in a positive phase Greenland blocking is more likely, which means cold air pretty much locks in place, in the East; during last winter the AMO being in a positive phase, in conjunction with the NAO being in a negative phase, was the reason New England saw so many nor'easters, with many seeing near record to record breaking snow amounts.

Sea Surface Temperature  (SST):

 I've outlined an area in the Atlantic basin where temps are above average for this time of year.... The sea surface temps off the west coast of Africa over into the Caribbean / Gulf and just off the East Coast, are above normal for this time of year.

    

Sea surface temperature in the equatorial Pacific Ocean are also important to Atlantic Basin hurricanes.  El Niño is characterized by unusually warm temperatures. La Niña by unusually cool temperatures in the equatorial Pacific. Anomalies. So far, it's been in a neutral phase. and with slight variations this should continue.

All of these ocean temperatures and ocean currents have an important influence on weather. Warmer temperatures will add more moisture to the air. Changes in ocean temperatures will change the wind patterns aloft. Changing the wind pattern will change how moisture is distributed. El Nino and La Nina are examples of how changing ocean temperatures can influence the weather pattern. 

The PDO and AMO help anchor the jet streams which in turn amplify or de-amplify temperatures and moisture distribution. For example, A warm AMO means less arctic ice and with a cold PDO more Atlantic hurricanes.

One more thing I want to mention before I start concluding:

The Tropical North Atlantic Index (TNA), which measures the anomaly of the average monthly sea surface temperatures of the tropical Atlantic Ocean, is in a positive phase and the pattern of the index is eerily similar to that of the active seasons of 2004, 2005. The setup also resembles 2007.

2004 had 15 named storms.

2005 was the most active Atlantic hurricane season in history. They had so many that they had to use the Greek alphabet; when more than 21 storms are named the additional storms are given names from the Greek alphabet.

2005 saw 30 named and one unnamed tropical cyclones (This unnamed subtropical cyclone was identified during NHC's post-season re-analysis) for a total of 31 tropical cyclones.

Tropical Storm Zeta became the final storm of the season when it formed on December 30, six hours short of tying the record of Hurricane Alice of 1954 as the latest-forming named storm in a season (retying back to my post on "Are We Returning To The Weather Patterns Of Past Decades").

2007 was another active Atlantic hurricane season that produced 17 tropical cyclones.

What does all this mean? 

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; as I said above, water temperatures 80 degrees (F) or greater will enhance tropical development. So with the area off the west African coast warmer than average, one can conclude that it will help intensify tropical waves moving off the West Coast of Africa.

The NAO has been principally on the negative side during the Winter and into Spring 2013, it looks to say this way for the foreseeable future. Research has shown that negative NAO patterns tend to weaken trade winds. One thing the trades winds do is help mix water along the surface of the ocean. Because the trades are weaker it will help support the warmer water over the tropical Atlantic.  Also with the NAO being negative we will see more in the way of Greenland blocking. And as I alluded to above, blocking helps keep tropical systems closer to the US East Coast. And as was the case with Sandy, blocking can help force tropical cyclones into the East Coast.

As I said above, temperatures in the Pacific have influences on the Atlantic. One of these is wind shear. (if you've followed my weather page or read this blog) you most likely know wind shear is simply a change of wind speed and direction with height. Wind shear can kill a tropical system in its tracks.  With the current neutral to weak La Nina in the Pacific, we should see the less wind shear and warmer SST in the tropical Atlantic condition continue.

What does this year's Atlantic season look like?

Based on the factors I listed above, statistical analysis, along with a few others. Also based on what I posted in part one and part two of the "Are We Returning To The Weather Patterns Of Past Decades" This season will be quite active with 12-18 named systems, with 8-10 becoming hurricanes, four or five (perhaps more) of which will make landfall somewhere in the U.S. Also, I feel 2-4 of these will strike or impact the Northeast and northern Mid Atlantic states. Of the 12-18 named storms 3-4 will be major. Once we get toward the end of July and onward I think the season will become quite active.

last year the SST off the west African coast was cool, which reduced the number of Cape Verde Hurricanes (an Atlantic hurricane that develops near the Cape Verde islands, off the west coast of Africa). So many of the tropical cyclones we saw formed in the GOM or the Caribbean. With cooler SST  not looking to be an issue this season, we should see a return of Cape Verde Hurricanes.

Well that's it... hope you enjoyed reading it......

My next blog post will be on meteorological terms and phases. 


Rebecca.