Does global climate change have an influence on tropical cyclone activity?

NOAA Satellite image of Major Hurricane Dorian

Does global climate change have an influence on tropical cyclone activity and can these influences be detected?  This is important given the high risk to populations along coastal regions. This question was tackled in a recent assessment published in The Bulletin of the American Meteorological Society (BAMS):  Tropical Cyclones and Climate Change Assessment (1)

 

In this assessment, the authors focused on the question: “Can an anthropogenic influence on tropical cyclone activity be detected in past data?”  The paper reviewed a number of published case studies about possible detectable anthropogenic influence on tropical cyclones and concluded that there was:

at least a low to medium confidence that the observed poleward migration of the latitude of maximum intensity in the western North Pacific is detectable, or highly unusual compared to expected natural variability.”

The opinions on the team were divided, however, on whether any observed tropical cyclone changes could be attributed directly to anthropogenic influence and these opinions are summarized here:  https://journals.ametsoc.org/view-large/12351541

Storm Surge and Extreme Rainfall 

A storm surge causes tides to quickly rise while rough waves pound the concrete seawall along the shores of Lake Pontchartrain. Hurricane Isaac made landfall along the Gulf Coast and now threatens New Orleans.

Regarding storm surge, the paper indicating that:

a widespread worsening of total inundation levels during storms is occurring because of the global mean sea level rise associated with anthropogenic warming, assuming all other factors equal, although we note that no TC climate change signal has been convincingly detected in sea level extremes data. To date, there is not convincing evidence of a detectable anthropogenic influence on hurricane precipitation rates, in contrast to the case for extreme precipitation in general, where some anthropogenic influence has been detected.”

Reference

  1. Knutson, T., and Coauthors, 2019: Tropical Cyclones and Climate Change       Assessment:    Part I: Detection and Attribution. Bull. Amer. Meteor. Soc., 100, 1987–2007, https://doi.org/10.1175/BAMS-D-18-0189.1.

 

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Tampa Bay Hurricane Storm Surge Scenarios

Hurricane Isaac Storm Surge along the Gulf Coast. Image Credit NOAA

The 2020 hurricane season is off to an early start so it is not too early to prepare for the possibility of the Tampa Bay area experiencing a direct hurricane hit. Tampa has experienced strong storm surges in the past and the question is when will it happen again?

Will History Repeat? 

Since Tampa was a small town associated with a US Army post, Fort Brooke in the 1820s, there have been only two major hurricanes that directly impacted the Tampa Bay area with high winds and storm surges.  The highest reported storm tide in Tampa Bay occurred in September of 1848 during the “Great Tampa Gale” when the tide was reported to have been 15 feet above low tide or about 12.75 feet above MHHW.  During the October Hurricane of 1921 the tide was reported to have reached 10.5 feet above low water or about 8.25 feet above MHHW.  Both of these storms were category 3 hurricanes and so far, Tampa Bay has escaped a direct hit from any category 4 or 5 storms.   Hurricane Elena,  in August of 1985, although not directly hitting Tampa Bay, did create a storm surge that was 4.0 feet above Mean Higher High Water (MHHW) or 6.26 feet above Mean Lower Low Water (MLLW) as Elena hovered offshore over the Gulf of Mexico to the northwest of Tampa Bay. 

Although these catestrophic events are rare, they can and will happen again.  I would estimate that there is a 1-2% risk for a major hurricane impacting the Tampa Bay area in any given year and the question is will it be this year?  Even though there has never been a Category 4 or 5 hurricane to directly hit Tampa Bay, it is not impossible and the devastation would be extreme.

Storm Surge Simulations

In the paper:  Hurricane Storm Surge Simulations for Tampa Bay (1), authors Robert H. Weisberg and Lianyuan Zheng  took a close look at the flooding potential caused by wind stress and atmospheric pressure induced storm surge for a category 2 hurricane and for a catastrophic category 4 hurricane where storm surges of 4-6 meters (13-20 ft) are likely. The paper describes 11 different scenarios with 8 different tracks.  Storm tracks E1, E2, E3, and E4 are for eastward moving hurricanes making landfall at Indian Rocks Beach, Sarasota, Tampa Bay mouth, and Tarpon Springs, respectively. Tracks D2, D3, D4, and D5 are for hurricanes paralleling the axis of the bay or paralleling the coastline from the northwest or the southeast, respectively.

Storm surge hurricane tracks

Proposed Hurricane Track Scenarios Image: (Weisberg and Zheng (1))

Most people believe that a hurricane moving northeastward directly up Tampa Bay would be the “worst case scenario”, however, this is not the case. The paper, in fact, shows that a storm transiting up the bay axis from southwest to northeast actually results in the smallest surge!  

Storm Surge (meters) associated with a category 2 storm moving up the axis of Tampa Bay.  Image: (Weisberg and Zheng (1))

While no storm scenario is a good one for the Tampa Bay area, the worst case is when the hurricane center is located north of the Tampa Bay entrance such that the maximum winds associated with the eye wall occur at the mouth of the bay with storms making landfall farther north result in lower surges.  Storms that make landfall to the south of the bay tend to reduce storm surge along Pinellas County beaches, although localized surges can still occur within the bay.  

Storm surge is also sensitive to the direction that storms approach the bay.  Storms that approach from the south produce lower surges than those approaching from the north. The speed at which the hurricane approaches is also important.  If a storm moves fast enough, the surge may not have enough time to reach the full storm surge potential.  Wind speed is also key as surge height tends to increase with the square of the wind speed.  A category 2 storm might produce limited flooding, while the flooding potential for a category 4 storm could be “catastrophic”. 

Additional factors in surge development include the effects of tides, rivers, and waves. Tide ranges for Tampa Bay tend to be small (0.5 m to 1 m) when compared to the surge potential by winds and pressure. Flooding by heavy rains can be locally important with additional inundation and damage caused by waves.  

Below is the storm surge expected by a Category 2 hurricane, approaching from the west at 5 m/s (10 knots)  and making landfall at Indian Rocks Beach. The asterisk denotes the landfall location while the filled circles show the storm center. The bold lines are surge elevation contours at 1-m intervals.

Cat 2 Hurricane landfall Indian Beach

Storm surge expected by a Category 2 hurricane making landfall at Indian Rocks Beach  Image: (Weisberg & Zheng (1))

 

Cat. 4 Hurricane Scenari0

Below is the storm surge associated with a Cat 4 hurricane making landfall at Indian Rocks Beach. The asterisk denotes the landfall location while the filled circles show the storm center. The bold lines are surge elevation contours at 1-m intervals.

Cat. 4 Hurricane Storm Surge

Storm surge associated with a Cat 4 hurricane making landfall at Indian Rocks Beach  Image: (Weisberg & Zheng (1))

How do Waves add to Storm Surge?

During hurricanes Ivan in 2004 (Alabama) and Katrina in 2005 (SE Louisiana and Mississippi) heavy waves combined with storm surge destroyed sections of bridges. The contributions of the waves to the storm surge and the increased water elevation by storm surge to the waves was described in another paper: Coupling of surge and waves for an Ivan‐like hurricane impacting the Tampa Bay, Florida region (2)

The paper describes the important  interaction between waves and storm surge in shoaling water where the surge height will be elevated by the wave‐induced forces and where changes in water level by storm surge will greatly impact the wave field. It was found that storm surge is increased by wave stress by 0.3–0.5 m.  In addition, the greatest surge enhancement by waves occurs in advance of the actual peak in the storm surge because the wave speed exceeds the storm speed of advance. 

On the other hand, there is also an effect of storm surge on waves. The Significant Wave Height  (the average of the ⅓ highest waves) can increase with surge elevation by about a 1.0–1.5 m and hence perhaps as much as a 2–3 m increase in the maximum wave height.  Such enhancement of waves for a major hurricane could be sufficient to initiate large‐scale damage within the Tampa Bay region prior to the onset of hurricane strength winds. About 3 hours prior to landfall, both the inundation and the significant wave heights would be high enough to begin causing massive damage to low-lying buildings that are open water exposure. 

How Vulnerable is Tampa Bay?

Particularly vulnerable would be the northeast section of St. Petersburg and the northern sections of Old Tampa Bay, including the Courtney Campbell Causeway, the Howard Frankland Bridge, and the Gandy Bridge. As the storm makes landfall, the situation worsens as the inundation continues to increase along with the significant wave heights, even until the  center passes Tampa Bay. 

Therefore, it is not only the wind speed (Saffir‐Simpson category) that matters, rather it is the complex mix of wind, tide, surge and waves that combine to cause major destruction. The entire coastline of Tampa Bay with low elevation and open to water exposure is susceptible to such damage. The eastern shore of Tampa Bay is at risk, even after the storm center passes beyond the Bay since the bay would already be filled with storm surge water and waves which would then shift direction to attack the eastern shore.  

It is also very likely that the wave forces acting vertically beneath bridge spans would destroy spans of the Courtney Campbell Causeway, the Howard Frankland Bridge, and the Gandy Bridge that are closest to the water. McDill Air Force Base and Tampa International Airport would be damaged along with Tampa General Hospital, as well as some other critical areas of infrastructure. 

References:

  1. Weisberg, R.H., Zheng, L. Hurricane storm surge simulations for Tampa Bay. Estuaries and Coasts: J ERF 29, 899–913 (2006).
    https://www.researchgate.net/publication/226176569_Hurricane_storm_surge_simulation_for_Tampa_Bay
  2. Huang, Yong & Weisberg, Robert & Zheng, Lianyuan. (2010). Coupling of surge and waves for an Ivan‐like hurricane impacting the Tampa Bay, Florida region. J. Geophys. Res. 115. 0.1029/2009JC006090.
    https://www.researchgate.net/publication/228973196_Coupling_of_surge_and_waves_for_an_Ivan-like_hurricane_impacting_the_Tampa_Bay_Florida_region

Other Links:

Hurricane Storm Surge Simulations for Florida’s Tampa Bay Region   http://ocgweb.marine.usf.edu/Posters/TBHurricane.pdf

Mariner’s Wave Guide  http://oceanweatherservices.com/featured_blog_posts/a_mariners_guide_to_wavesStorm

Surge Overview, NHC:
https://www.nhc.noaa.gov/surge

NOAA Blog Inside the Eye
Storm Surge–Plain and Simple (Part 1)

Ocean Weather Services
http://www.oceanweatherservices.com

 

 

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What is a Storm Surge?

Storm Surge NOAA NWS

Storm Surge is the abnormal sea level elevations (or depressions) caused by winds and atmospheric pressure. The components are:

1. Coastal set up (down) by the along shore wind stress. 
In deep water, the Earth’s rotation causes a water to move at a right angle the wind stress. This sets up a sea level slope against the coast and an alongshore current in geostrophic balance. With the current limited by friction the sea level set up is less than a meter.
2. Coastal set up (down) by atmospheric pressure.
Atmospheric pressure operates like an inverted barometer. Each mb of pressure drop (increase) raises (lowers) sea level by 1 cm. The largest hurricanes with pressure drops of 100 mb can cause a 1 m surge by this mechanism.
3. Coastal set up (down) by the across shore wind stress. 
In shallow water, and because of friction, the wind stress drives water downwind and piles it up against the coastline. The resulting sea surface slope (tending to balance the across shore wind stress) is the largest contributor to coastal storm surge and can exceed several m.

Other Factors

4. Coastal geometry.
By varying fetch and direction relative to a hurricane the embayment geometry is very important, as are the water depths and land elevations.
5. Continental shelf width.
In shallow water the sea surface slope required to balance the across shelf wind stress is inversely proportional to water depth. Hence wide, shallow shelves are prone to larger storm surges.
6. Tides.
Water level will be higher (lower) at high (low) tide. Since tides in Tampa Bay are about plus and minus 1.5’ this is small relative to the storm surge.
7. Water density.
By being lighter, warmer water in summer stands higher than colder water in winter. This can amount to about 1’.
8. Waves.
Waves are additive to surge. Theoretically a solitary wave can be 1.8 times the water depth. While this is not naturally realized, waves can have a huge impact. Imagine the surf zone on a very rough day displaced to Gulf Blvd.
Sources:
R.H. Weisberg and L. Zheng (College of Marine Science at USF St. Petersburg, Ocean Circulation Group)
http://ocgweb.marine.usf.edu/
National Hurricane Center: Storm Surge Overview https://www.nhc.noaa.gov/surge/
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Tropical Cyclone Wind Speed probability graphic

Tropical Cyclone Wind Speed probability graphic

In addition to the familiar National Hurricane Center’s (NHC) hurricane track forecast with it’s iconic cone, the NHC also produces a wind speed probabilities graphic which provides probabilities (in percent) that wind speeds of at least 34 kt (39 mph, tropical storm force), 50 kt (58 mph), or 64 kt (74 mph, hurricane force) will occur during cumulative time periods at each specific point on the map. These cumulative probabilities indicate the overall chances that the indicated wind speed will occur at any specific location on the map during the period between hour 0 and the forecast hour.

These cumulative probabilities tell decision-makers the chances that the event will happen at any point on the map within the forecast time period stated.  The wind speed probabilities also help users to understand forecast uncertainties, such that they are not surprised by any relatively small changes in the track. This graphic also shows why it is crucial to make proper preparations when a watch or warning is issued for your area, even if the exact track forecast does not go over your area.

Read full explanation here: https://www.nhc.noaa.gov/aboutnhcprobs3.shtml

NOAA NHC Wind Probabilities for 50 knots or higher for Hurricane Katrina

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2020 Atlantic Hurricane Outlook issued by Colorado State University

Hurricane Dorian at its peak Image credit NOAA

The Department of Atmospheric Science Colorado State University has issued its “Extended Range Forecast for the 2020 Atlantic Hurricane Season.  

The outlook anticipates that the 2020 Atlantic basin hurricane season will have above-normal activity. The report points to the current warm neutral ENSO conditions to likely transition to cool neutral ENSO or potentially even weak La Niña conditions by this summer and/or /fall with sea surface temperatures over most of the tropical Atlantic warmer than normal. 

 

 

As a result, an above-average probability for major hurricanes making landfall along the continental United States coastline and in the Caribbean is anticipated. Read full report https://tropical.colostate.edu/media/sites/111/2020/04/2020-04.pdf

 

Colorado State 2020 Hurricane Outlook

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Hurricane Storm Surges in Tampa Bay

Hurricane Elena NOAA Image

The highest storm surges in Tampa Bay are usually associated with hurricanes, with the three highest tides occurring during Cat 3 storms in 1848, 1921 and 1985.

Hurricane Elena in late August of 1985 caused the highest storm surge recorded at the St. Petersburg tide gauge which has been in operation since Dec. 1946.  Late on the 31st of August the gauge recorded a water level some 4.0 feet above Mean Higher High Water (MHHW) or 6.26 feet above Mean Lower Low Water (MLLW) as Hurricane Elena hovered offshore over the Gulf of Mexico. 

 

The day prior, Hurricane Elena was about 200 miles southeast of the mouth of the Mississippi when it turned eastward. During the next 24-36 hours, the storm moved closer to the west coast of Florida but then stalled about 50 miles off Cedar Key.  At its closest point to Tampa Bay, late on the evening of the 31st, Elena was packing winds to 100 knots making it a Cat. 3 storm. 

Hurricane Elena Track

Due to the slow and erratic motion of this storm, warnings were issued along the Gulf from Sarasota to Grand Island, LA.  All told, 1.5 million people, including about 300,00 in the Tampa Bay area were evacuated. Residents of the Tampa Bay area endured heavy rains and winds, evacuations were ordered and many chose hold-up in shelters. On September 1st, Elena started to move westward again and finally made landfall near Biloxi, Mississippi on Sept. 2nd as a category 3 hurricane.

Earlier Storms
Prior to the tide gauge being installed at St. Petersburg, there were at least 2 incidences of higher surges.  The highest reported storm tide in Tampa Bay occurred in September of 1848 during the “Great Tampa Gale” when the tide was reported to have been 15 feet above low tide or about 12.75 feet above MHHW.  During the October Hurricane of 1921 the tide was reported to have reached 10.5 feet above low water or about 8.25 feet above MHHW.

What about the future?
There have been two Cat 3 hurricanes that have directly impacted the Tampa Bay area since 1848 with the last event in October of 1921, nearly 100 years ago. Although these events are rare they can and will happen again. Based on the above, I would estimate that there is a 1-2% risk for a major hurricane impacting the Tampa Bay area in any given year.  Hurricane Elena, although it did not directly hit Tampa Bay, created a storm surge that was significant. Even though there has never been a Cat. 5 storm to hit Tampa Bay, it is not impossible and the devastation would be extreme.

Project Phoenix
A 2010 study called Project Phoenix funded by FEMA took a look at the Tampa Bay area as if we were hit by a Cat 5 hurricane. In this “worst case” scenario, power outages would be widespread with all of Pinellas County being without power with Pasco and Hillsborough Counties experiencing nearly total power outages as well. The associated storm surge would be as high as 26 feet in Tampa, 24 feet in Apollo Beach, and 20 feet in St. Petersburg. 

Hurricane Phoenix track scenario

All told, the storm could result in up to 2,000 people dead, 2 million injured, and almost 500,000 homes and businesses destroyed.  All three bridges between Tampa Bay and the Courtney Campbell Causeway would “sustain either structural damage or have their approaches washed away by water and waves”

 

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Two Major North Atlantic Hurricane-force Storms Expected

Two major hurricane-force storm lows are forecast to develop and move across the North Atlantic during the next 72 hours producing winds up to 80 knots and significant wave heights to 17 meters (over 55 feet)!  This will be a significant challenge to shipping between ports in Northern Europe and the USEC ports.  

Rapidly deepening storm south of Newfoundland 12Z 12 Feb 2020:

By 12Z 13th Surface forecast shows a deepening storm of 944 mb with winds to 80 knots over the main shipping lanes. In addition a second storm is expected to develop along the US East Coast.  

Surface Forecast 12Z 13 Feb 2020

NOAA OPC 12z 13 Feb Significant wave forecasts showing waves to 15.5 meters (over 50 feet)!  

NOAA OPC Significant Wave Height Forecast

NOAA OPC Surface forecast for 12Z 14th Feb shows the deep 936 mb low becoming semi-stationary southwest of Iceland while a second rapidly deepening low is moving northeastward well east of Newfoundland deepening rapidly to 924 mb by 15th. 

NOAA OPC Surface Forecast 12Z 14 Feb 2020

By 12Z 14th significant wave heights are forecast to 17 meters (over 55 feet) west of Ireland. 

NOAA OPC Significant wave height forecast 12Z 14 Feb 2020

Learn more about these winter hurricane storms

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Hurricane-Force Storm expected over Gulf of Alaska

NOAA OPC Surface Forecast 12Z 28 Jan 2020

A rapidly deepening storm low is forecast over the east-central North Pacific during the next 48 hours then moving northeastward over the Gulf of Alaska. This storm will produce storm to hurricane force winds (50-65 knots) and seas building to 14 meters (45 feet).

NOAA OPC 12Z 29 Jan Surface Forecast

NOAA OPC 48 hr Significant Wave Height Forecast

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Hurricane-Force Storm NW of Ireland

An intense winter storm over the eastern North Atlantic northwest of Ireland has been producing hurricane-force winds with significant wave heights to 16 meters (52 feet) west of Ireland.

NOAA OPC Significant Wave Height Analysis 12Z 13 Jan 2020feet).

NOAA OPC Surface Analysis Jan 13, 2020 1200Z

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Capsizing and Sinking of Barge Dredge200 and Loss of Workboat R.E. Pierson 2 Pushed by Tugboat Big Jake

The National Transportation Safety Board (NTSB) has determined that the probable cause of the capsizing and sinking of the Dredge200 and the R.E. Pierson 2  in December 2, 2018 was the decision by the tow captain and owner to attempt a transit in forecasted wind and waves that exceeded their original plan for the voyage. A workboat and barge sank, resulting in nearly $2 million in losses.

Although the captain and owner had a discussion regarding a plan for the tow configuration and set a limit of 4-foot seas for the voyage, they proceeded through the canal and into Cape Cod Bay despite knowing that the weather forecast called for seas that exceeded the limits they established for the voyage. 

NOAA Surface Analysis Dec. 2, 2018 1800 UTC

 

 

 

 

 

 

 

 

Read the entire NTSB report Capsizing and Sinking of Barge Dredge200 and Loss of Workboat R.E. Pierson 2 Pushed by Tugboat Big Jake

Marine casualty

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