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(T. F. “Storm” Walsh)
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Greetings to everyone!
Please be aware, even though I do not post every night, rest assured I am continuously monitoring various areas for any significant weather. I will be taking Sundays off (family time), unless we have active systems that may be posing a threat (i.e. Tropical, Winter Weather, Coastal Storms, etc.).
This post is the sixth Hurricane Tutorial, and will cover basically, how El Nino and La Nina come to fruition, and the effects each has on the Atlantic Hurricane Season.
I wanted to begin with how El Nino basically develops. One indicator I like to use, is the SOI (Southern Oscillation Index) graph.
The Southern Oscillation Index (SOI) is a standardized index based on the observed sea level pressure differences between Tahiti and Darwin, Australia. The SOI is one measure of the large-scale fluctuations in air pressure occurring between the western and eastern tropical Pacific (i.e., the state of the Southern Oscillation) during El Niño and La Niña episodes. In general, smoothed time series of the SOI correspond very well with changes in ocean temperatures across the eastern tropical Pacific. The negative phase of the SOI represents below-normal air pressure at Tahiti and above-normal air pressure at Darwin. Prolonged periods of negative (positive) SOI values coincide with abnormally warm (cold) ocean waters across the eastern tropical Pacific typical of El Niño (La Niña) episodes. The methodology used to calculate SOI is available by clicking here. More information can be found at the Climate Prediction Center SOI page.
CURRENT SOI CHART (LINKED)
So, when the graph is in the NEGATIVE region, this implies below normal air pressure at Tahiti, and higher pressures over Darwin Australia. Now remember, in this instance of higher pressure over Darwin, Darwin is SOUTH of the Equator, so the flow from high pressure goes COUNTER-CLOCKWISE. This flow, combined with the CLOCKWISE flow from lower pressure over Tahiti (also south of the Equator), allows for warmer sub-surface SST’s (warmer water) to travel east toward the coast of South America, and does not allow for upwelling of cooler water along the coast of South America associated with the Humboldt current. This setup is common with downward motion pulses of the MJO (Madden Julian Oscillation) when they occur.
PRESSURE CELLS DURING EL NINO AND LA NINA (BASICALLY)
In the following graphic, red and orange indicate downward motion or sinking air from the 200 mb level, and blue/green is upward motion. During the above mentioned setup, red and orange would be near or over Australia, and neutral to green/blue over Tahiti and/or the Eastern Pacific.
ECMWF 200 MB VELOCITY POTENTIAL MAP FROM JUNE 2019
This downward motion (higher pressure), also has a the tendency to “push” on the ocean’s surface. This, combined with the west to east flow, allows the warmer water to travel west to east in the sub-surface. This warmer water continues to travel east, until encountering the South American west coast in the sub-surface. Once this occurs, the warmer water then “ricochets” back toward the west, and rises to the surface. Hence, the warm tongue of water associated with El Nino.
During a La Nina phase, the OPPOSITE of what I have just explained occurs. The SOI graph/chart rises into the positive portion of the graph. When this occurs, and is strong enough in the positive phase for approximately 3 months, La Nina takes form. The positive SOI phase indicates a reversal of the trade winds and pressure patterns, causing a reverse wind flow so the wind circulation is from east to west. This accomplishes 2 things, it pushes the warmer water toward the west, and allows for the upwelling of colder water from the Humboldt or “Peruvian” current, creating the “cold” tongue of water we are familiar with in a La Nina pattern.
THE HUMBOLDT CURRENT
When this occurs, generally the 200 mb CHI pattern is different, with sinking air over Tahiti and EPAC, and rising air near Darwin and WPAC. Again, refer to the pressure cells graphic above.
Regarding the effects of each on our Atlantic Hurricane Season, El Nino has the tendency to “suppress” activity in the Atlantic, and La Nina has the tendency to “enhance” activity in the Atlantic, with both affecting the Equatorial Walker Circulation.
In an El Nino event, the waters along the South American coast display much warmer anomalies, and extend sometimes well into the Western Pacific. This warmer water places a good amount of heat energy into the atmosphere in the Eastern Pacific, which affects the Walker circulation and our Jetstream pattern in the U.S. Warm air rises from this water creating lower pressures in the EPAC. As the air rises, it diverges around the 200 mb level, and spreads out. This air cools and begins to sink over the Caribbean and Atlantic basins. As the air sinks or “subsides”, it warms and dries, creating a stable atmosphere over the Atlantic (seen as drier air in water vapor images). Clouds cannot really develop vertically in this stability, hence it’s hard to get any thunderstorm activity to develop. El Nino also leads to increased wind shear over the Caribbean Sea and Atlantic ocean. This is from NOAA as well, explaining how shear occurs:
During El Niño, the area of tropical Pacific convection and its associated Hadley circulation expand eastward from the western Pacific, sometimes extending to the west coast of South America. (A tutorial on El Niño and La Niña can be found at the NOAA Climate Prediction Center website.) At the same time, the equatorial Walker circulation is weaker than average.
These conditions produce an anomalous upper-level, ridge-trough pattern in the subtropics, with an amplified ridge over the subtropical Pacific in the area north of the enhanced convection, and a downstream trough over the Caribbean Sea and western tropical Atlantic. Over the central and eastern Pacific, the enhanced subtropical ridge is associated with weaker upper-level winds and reduced vertical wind shear, which favors more hurricane activity.
Over the Atlantic basin, the amplified trough is associated with stronger upper-level westerly winds and stronger lower-level easterly trade winds, both of which increase the vertical wind shear and suppress hurricane activity. In addition to enhanced vertical wind shear, El Niño suppresses Atlantic hurricane activity by increasing the amount of sinking motion and increasing the atmospheric stability.
During a La Nina event, the opposite is true. Due to higher pressure near S. America, and the colder water from the Peruvian current, the pattern is reversed, and wind shear over the Atlantic is absent. This is the explanation from NOAA, regarding La Nina:
La Niña has opposite impacts across the Pacific and Atlantic basins. During La Niña, the area of tropical convection and its Hadley circulation is retracted westward to the western Pacific and Indonesia, and the equatorial Walker circulation is enhanced. Convection is typically absent across the eastern half of the equatorial Pacific.
In the upper atmosphere, these conditions produce an amplified trough over the subtropical Pacific in the area north of the suppressed convection, and a downstream ridge over the Caribbean Sea and western tropical Atlantic. Over the central and eastern subtropical Pacific, the enhanced trough is associated with stronger upper-level winds and stronger vertical wind shear, which suppress hurricane activity. Over the Atlantic basin, the anomalous upper-level ridge is associated with weaker upper- and lower- level winds, both of which reduce the vertical wind shear and increased hurricane activity. La Niña also favors increased Atlantic hurricane activity by decreasing the amount of sinking motion and decreasing the atmospheric stability.
The following video link gives a good explanation of the Walker circulation:
WIND SHEAR AND HURRICANES
Wind shear is defined as the change in direction, or the change in direction and/or speed of the wind with height. Wind shear affects tropical storms and hurricanes by pretty much blowing the tops off the storms, and/or separating pretty much the mid and upper level portions of the storm from the low level circulation. This basically “tilts” the system, usually west to east. This affects the circulation of the storm. Think of it as a child’s toy top that is slowing down…the top tilts, and starts to wobble. Also, wind shear spreads the moisture and heat energy the system needs to maintain, or strengthen, over a larger area. In order for a system to even develop, let alone strengthen, heat and moisture MUST be focused in a column in order for moisture and heat energy to continue to rise through the atmosphere to continue the formation of clouds and the release of latent heat, which fuels a hurricane. The following 2 videos, which were made during Hurricane Lane which affected Hawaii, give somewhat of an explanation of shear affecting a hurricane. I wasn’t that impressed with either, but they serve their purpose. Stop both videos at the 2:00 minute mark:
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Have a blessed evening!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST /SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS
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