An interesting phenomena has been observed in the wake of Hurricane Irma as it moves along the west Florida coastline.  Several images have been put on social media of water drastically receding away from the coastline, whereas tropical storms usually result in surge as they make landfall.

This phenomenon is largely due to 2 factors.  First, the winds ahead of Irma are westward and offshore.  This has the effect of pushing water offshore.  Second, the bathymetry on the West Florida Shelf is characterized by shallow sloping, meaning that a 1 or 2 meter decrease in sea level would result in long stretches of dry seabed.

As this is dynamically driven, it can be observed in our CNAPS model.  The included animation shows offshore winds in advance of Irma’s eye which quickly whip around to onshore.  Offshore winds result in a modeled 2+ meter decrease in sea level, but once the eye passes, sea level rises with a combination of returning coastal water and storm surge.  The National Hurricane Center has been sending out dire warnings to people in the path of the returning surge to stay away temporarily dry seabeds.  Further up the coast, in Tampa Bay, water levels are rising rapidly and surge is already exceeding the Mean Higher High Water (MHHW) levels.

Original text taken from the Ocean Observing and Modeling Group (OOMG) blog.

Update: Parts of this story were published on Sept. 12th in the scientific blog Live Science.

Blog post submitted by Mark Luther

Hurricane Irma caused only a minimal storm surge in Tampa Bay. Storm surge is the difference between predicted (or astronomical) tidal water level and the observed or actual water level (called storm tide) and is what causes the majority of damage in a land falling hurricane.  In relatively shallow coastal waters like those of the Tampa Bay region, wind blowing across the water surface drags water in the direction of the wind.  If it piles up against the coastline, it causes a storm surge.  We operate a network of water level gauges and wind measurement sites around the bay in cooperation with the NOAA National Ocean Service called the Tampa Bay Physical Oceanographic Real Time System (TBPORTS; see http://tbports.org/).  Here is what our observations from TBPORTS tell us about the surge from Irma.

As Irma approached the southwest coast of Florida, forecasters were unsure if the center of the storm would stay just offshore or would move inland. The predictions of storm surge at that time assumed a worst case scenario where the eye of the storm stayed just offshore of Tampa Bay, with possible storm surge of 8 to 12 feet above predicted tide or 5 to 9 feet above ground level.  Had that happened, Tampa Bay would have been devastated.  Fortunately for us, the storm moved inland and weakened quickly once it went over land.

The eye of the storm tracked well to the east of the bay.  By the time that the eye of the storm passed to our north and winds turned to the west (about 5 am EDT), we were only seeing 30 to 34 knot sustained wind speeds from the WNW with gusts to just under 40 knots. By 10 am the winds were 25 to 28 knots from WNW with occasional gusts to 32 knots.  By 5 pm wind speeds were less than 20 knots, still from W to WNW. The wind never went south of west. If the wind had come from the southwest, the surge would have been greater, as wind would be pushing water straight up the bay. The storm also moved quickly through the area so that the winds didn’t have time to push as much water toward the coast and up the bay. For comparison, in Hurricane Francis in 2004, the eye of the storm stalled just to the northwest of the bay region and we saw sustained winds of 50+ knots from the southwest for approximately 12 to 18 hours. That drove a surge of about 6 ft. into the St. Petersburg area (flooding my garage) and an even larger surge into downtown Tampa.

The most striking aspect of the storm surge from Irma was the large negative surge seen as the storm approached. Strong winds from the east to northeast pushed water out of the bay, leading to water levels in the northern parts of the bay that were more than 6 feet lower than the predicted tide level.  In Irma, the strongest wind speed from the north at our mid-bay site was 56 knots from the NNE gusting to 65 knots. Those NE to N winds drove the water out of the bay as expected. That factor may have delayed the return of water into the bay but not by much. Water flows downhill pretty fast.  In the plots below, you see the water level rises to predicted tide level very quickly once the winds turned west of north, and then first goes above predicted water level at 4 am at the Port Manatee gauge.  Maximum storm surge occurred at Port Manatee of 2.17 ft at 11:18 am, at Old Port Tampa (south of Gandy Bridge) of 2.35 ft at 11:54 am, at St. Petersburg of 2.17 ft at 12:36 pm, and at McKay Bay (Port of Tampa) of 3.08 ft at 1:24 pm, all very nearly coincident with low tide. If the maximum surge had occurred at or near high tide, coastal flooding would have been 2 to 2.5 feet greater than what we observed.

We were very lucky to have escaped major damage.

 

Contributed by Nick Shay (nshay@rsmas.miami.edu)

Given favorable atmospheric conditions, Irma has intensified to category 5 status in the past few days. Central to this intensity increase is the relatively high ocean heat content values (80-120 kJ cm-2) as shown in the image relative to the projected track of Irma based on satellite remote sensing. As she approaches the Florida Straits, the high heat content water coming through the Yucatan Straits into the Gulf is essentially taking a quick right turn out of the Gulf of Mexico since the Loop Current is now retracted after separating a large warm core eddy in the eastern Gulf. The exiting water from the Gulf of Mexico forms the core of the Florida Current. Given the large current speeds of 2 m s-1 as observed with High Frequency radar measurements, this current advects the warm water through the Straits forming the core of the Gulf Stream further north in the SECOORA footprint. Given both the current strength and depth of the warm water, any ocean mixing induced by hurricane force winds is arrested.  Thus, there is very little sea surface temperature cooling in the remotely sensed signals and the air-sea fluxes are allowing Irma the storm to intensify (i.e. there is more deep warm water to help intensify Irma via the air-sea fluxes).

Note the white space is the figure is Cay Sal Bank which is very shallow to calculate ocean heat content from our 2.5 layer model. The water is quite warm over that feature and the ocean heat content is not zero.

An ocean experiment is planned using a combination of drifting buoys and APEX-EM floats to measure the ocean response to Irma in collaboration with SIO and AOML scientists. Details will be forthcoming.  However, the predicted track continues to be problematic at the present time due to the weakening of the subtropical ridge over the Atlantic and the possible influence of a short wave trough that is currently digging southward. The bottom line is it is a race to see what happens with the track of this very powerful hurricane that has approached its maximum potential intensity over high ocean heat content water.

The above Irma figure is updated daily with altimetry data and track information on the Atlantic Ocean website:  http://isotherm.rsmas.miami.edu/heat/weba/atlantic.php

Hurricane Irma, now being called an Extreme Category 5 hurricane, is making headway towards Florida.  Many SECOORA members are bracing for impact while others are well on their way towards executing their evacuation plans.  Several news articles, including one from NPR, are calling Hurricane Irma a “monster”. Make no mistake, this is not a storm where heroics are in order – winds speeds of 185 mph make the go/no go decision for you.  You have to go.

As we sit and watch the news, and learn about the devastation in the Caribbean, we hope and pray that our CARICOOS colleagues, their friends and family, co-workers, and students are safe.  Within SECOORA, we are reaching out to our members and asking them to stay safe.  We are also taking stock of our deployed assets so that we can better assess damages after the storm. Throughout the course of Irma, SECOORA will maintain a website that provides access to regional ocean observing data, hurricane tracking and storm surge models, information resources, and a blog post.  We are encouraging you to submit blog posts and photos so that we can share them with our members and stakeholders.  Since the track of Irma is still uncertain for the southeast, please send your blogs and photos to Debra (debra@secoora.org), Megan (mlee@secoora.org), Jennifer (jdorton@secoora.org) or Abbey (abbey@secoora.org) so that we can share your experiences during this unprecedented storm.  At least one of us should be able to keep the information flowing.