Tag Archives: Brian McNoldy

What Happens Underwater During a Hurricane?

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We think we’re pretty familiar with hurricanes – strong winds, storm surge, flooding rains, ominous satellite images from space, and radar loops when they get near land. But what goes on at and below the ocean’s surface when a hurricane passes overhead? Quite a lot, actually!

Effects on the ocean properties

The upper levels of the ocean are typically strongly stratified by temperature and by salinity. That is, colder, saltier water lies below the warmer, fresher water near the surface. When a hurricane comes by, it mixes everything up, resulting in a muddled and more homogeneous upper ocean. That means the surface water is cooler and saltier than it was previously was, and deeper water is warmer and less salty than it previously was. However, in very shallow coastal areas, the copious amount of fresh cold rain water from the hurricane can actually reduce the temperature and salinity of the near-surface water.

Time series of the vertical profile of temperature and salinity from the ocean’s surface down to 200m, and spanning one day prior to the hurricane’s passage through 2.5 days after the passage. The dramatic mixing down to approximately 150m is evident. Time in days relative to the passage is listed along the horizontal axis. This particular case is from Hurricane Frances (2004) on 1 September. (Sanford et al., 2007)

The colder surface water upwelled by the hurricane can actually be a fairly significant player in controlling the hurricane’s intensity. A strong slow-moving hurricane will upwell cold water much more effectively than a weaker and/or fast-moving hurricane. And since hurricanes require warm ocean water to fuel their “engine”, that upwelling can end up weakening the storm. The trail of upwelled cooler water left behind a storm is called a “cold wake”, and shows up clearly on maps of sea surface temperature.

Map of sea surface temperature before (left) and after (right) Hurricane Isabel in 2003. Isabel’s track from the eastern Atlantic all the way into the mid-Atlantic coast is evident by the cold wake left behind. (NASA/GSFC)

Intense hurricanes can generate 60′+ waves, and at the ocean surface, the boundary between the water and the air becomes nebulous. Amidst the formidable waves, sea spray and foam streak horizontally across the surface at high speed, blurring the view of the ocean’s surface in this photo from an aircraft flying through a hurricane.

Photo of the sea state under Category 4 Hurricane Isabel taken from 400 feet above the surface. Note that the aircraft was not in or near the eyewall at this time or altitude. (Will Drennan, RSMAS)

But below the ocean’s surface, the currents and turbulence beneath those waves can also be quite destructive. Unlike places above the surface, the ocean doesn’t “forget” about the storm very quickly… strong currents and turbulence have been known to exist up to a week after the storm passes overhead. Damaging currents can extend down to at least 300 feet below the surface, capable of dismantling coral reefs, relocating ship wrecks, breaking oil pipelines, and displacing huge volumes of sand on the seabed.

Simplified schematic showing the parts of an ocean wave. At the surface, there are crests and troughs. Crests are separated by a wavelength. The depth to which a wave’s effects can be felt depends on the wavelength and wave height.

Effects on marine life

Some studies conducted in the Caribbean Sea have shown that in the year following a hurricane, coral cover is reduced by 15-20% (more or less, depending on the intensity of the hurricane) in the affected areas. There are several factors that go into the negative effect on coral: 1) the turbulent water breaks it, 2) the days of muddied water reduces the amount of sunlight reaching the algae in coral tissue, 3) the fine suspended particles clog the pores, and 4) the tremendous amount of rain reduces the salinity of the shallow ocean in the immediate area which can stress coral.

Large self-propelled marine animals such as sharks seem to be minimally affected, since they can detect tiny changes in pressure as larger waves at the surface approach, as well as the reduced surface pressure associated with the storm itself, and go deeper or leave the area. However, hurricanes have been known to result in tremendous numbers of dead fish, crabs, sea turtles, oysters, etc due to reduced amounts of dissolved oxygen in the water, rapid salinity changes, and violent surf.

Just like us up here on the surface, marine life suffers for months to several years from the death and destruction following a hurricane.

Brian McNoldy
Senior Research Associate
& Author of Tropical Atlantic Update
Follow Brian on Twitter: @BMcNoldy

October Ushers in Highest Likelihood of Hurricanes in South Florida

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In the 2012 Atlantic hurricane season, we have had 14 tropical storms so far, 8 of which became hurricanes, and just 1 of those became a major (Category 3+) hurricane. An extremely unusual aspect of the season is that essentially all of the hurricane activity occurred north of 25N! Only Ernesto briefly reached minimal hurricane intensity in the tropics just prior to hitting the Yucatan peninsula in early August. All other hurricanes were in the subtropics or mid-latitudes. The season’s only major hurricane so far (Michael) formed from an upper-level cold low pressure system, not from an easterly wave or anything connected to the tropics.

As we head into October, the fifth month of the official Atlantic hurricane season, it’s very important for us in south Florida to realize that this is the greatest hurricane risk month. More hurricanes directly hit or affect southern Florida in October than in any other month. In the graphic shown below, the gray circle is 300 miles across and centered on far western Broward County – designed to include all of southern Florida and immediate surrounding ocean. Any storm of hurricane intensity (sustained winds of 75mph+) whose center passed within that circle is shown in the colored lines, and the legend in the lower right corner associates the color with a category on the Saffir-Simpson scale (yellow is Category 1, orange is Category 2, etc). Finally, the coastal counties are shaded by historic landfall frequency, with darker reds corresponding to more frequent, and pale reds corresponding to less frequent. The monthly tally of tracks passing through the circle is indicated in parentheses below the month. Keep in mind that all of these storms were hurricanes – tropical storms and depressions are not included; and most importantly, never focus on exactly where the center of the track is. Destructive winds, tornadoes, flooding rains, and inundating storm surges can and do occur for hundreds of miles away from the center; so even tracks on the fringe of the circle likely brought severe weather conditions to the mainland.

Another interesting aspect of these maps is that in August and September, southern Florida is most likely to get struck by a storm coming from the southeast. But in October, the dominant direction is from the southwest… due to storms coming from the Gulf of Mexico and western Caribbean, the more favored areas for hurricane formation later in the season.

On the topic of landfalls, do you remember the last time a major hurricane made landfall on the U.S.? It was Hurricane Wilma, on the morning of October 24, 2005, and it hit southern Florida at Category 3 intensity. That was 2,535 days ago, an utterly unprecedented span between major U.S. hurricane landfalls. Streaks that even approach this long are very rare, and only two other spans of over 2,000 days have occurred since 1900. Why has it been so long? Pure luck. There have been several major hurricane landfalls since 2005 in other countries (Nicaragua, Mexico, Cuba, Virgin Islands, Bahamas, and close encounters in Jamaica and Belize), so it definitely isn’t correct to say that seasons since 2005 have been quiet … far from it. No one knows exactly when our lucky streak will end, but I can say with 100% certainty that it will end eventually!

Finally, we are still tracking Hurricane Nadine, which has been around since September 11 (the wave that would become Nadine actually left the African coast on September 7). It’s currently about 550 miles west of the Azores, and is a Category 1 hurricane with 75mph sustained winds. To accurately compare its longevity to historical storms, we need to discount the extra-tropical portions of its lifetime; doing so yields a total of 19.00 days as of this morning at 8am, which is certainly quite long, but not long enough to set any records just yet. This table puts Nadine’s lifetime in perspective among the record holders:

1st 28.00 days Hurricane #3* (1899)
2nd 27.25 days Hurricane Ginger (1971)
3rd 24.75 days Hurricane Inga (1969)
4th 22.00 days Hurricane Kyle (2002)
5th 21.00 days Hurricane #4* (1926)
… (four other storms) …
10th 19.00 days Hurricane Nadine (2012)

* These storms were prior to the satellite era when it was very easy to miss some of a storm’s existence, so these totals are likely underestimates.

Brian McNoldy
Senior Research Associate
& Author of Tropical Atlantic Update
Follow Brian on Twitter: @BMcNoldy

Hurricane Isaac Makes Landfall – Tropical Storm Kirk is Born

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Isaac was upgraded to a hurricane just before landfall, and actually continued to strengthen as it got closer and closer to the coast. Not only that, it also stalled, and is sitting in basically the same place for 12 hours and counting. The full radar loop of Isaac’s approach from the New Orleans radar is very illustrative for both the intensification and the stalling. It first clipped the Mississippi delta on Tuesday evening, then the center moved offshore just a little, and came ashore again about 70 miles west several hours later. However, hurricanes are large, and damaging effects are always felt very far from the exact center.

As expected, the storm has caused massive power outages (half a million and increasing) and substantial storm surge. The surge was almost perfectly forecast by the National Hurricane Center, and peaked at about 11′ in Shell Beach LA, 8′ in Bay St. Louis MS, 4′ in Mobile Bay AL, 3′ in Pensacola FL, and reports of significant surge in Destin FL. The storm surge will again be a major contributor to the damage, even for a low-end Category 1 hurricane. Storm surge is intentionally no longer a part of the Saffir-Simpson intensity scale.

The rainfall is another big factor, as was very well forecast by HPC. Below is the 24-hour rainfall estimate ending at 6am CDT this morning (so obviously the final values will be higher).

In what seems to be a Circle of Life in the tropics, just as Hurricane Isaac makes landfall and is destined to dissipate into nothing, Tropical Storm Kirk is born far to the east with no threat to land. For more on Kirk’s current position and development, visit Tropical Atlantic Update.

Brian McNoldy
Senior Research Associate
& Author of Tropical Atlantic Update
Follow Brian on Twitter: @BMcNoldy

Quick Look: Hurricane Isaac vs Hurricane Katrina

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Much to the relief of everyone in southern Florida, Isaac never did get too organized or intense after passing by Haiti and Cuba. It maintained a steady tropical storm intensity as it skimmed by Key West, as it made the journey across the Gulf of Mexico. It has just been upgraded to a hurricane off the Louisiana coast, but it could have been much, much worse.

During the early morning hours on August 29, 2005, Hurricane Katrina made landfall in eastern Louisiana as an enormous Category 3 storm. It intensified from a tropical storm to a minimal hurricane as it passed over Miami and the southern Florida peninsula on August 25th, then took full advantage of ideal conditions in the eastern Gulf of Mexico and dramatically intensified to a monster Category 5 storm on August 28th. Something similar was certainly possible with Isaac, and haunting similarities were everywhere – the dates, the tracks, the size, the landfall location, but one key difference remained: the intensity.

Hurricane Katrina (2005 – top) vs. Hurricane Isaac (2012 – bottom) at exactly 10:15am on August 28th.

I also made a comparison image of the two storms as they appeared on satellite at exactly the same date and time, just seven years apart (10:15am on August 28th). At this time, Katrina had 165mph sustained winds, while Isaac had 70mph sustained winds.

As far as Isaac goes, it is now a hurricane with 75mph winds as of 11:20am this morning. This is the first time that it has reached hurricane intensity during its entire 12-day journey across the Atlantic. It is just hours from landfall, and just hours from the exact landfall time of Hurricane Katrina seven years ago. You can monitor the storm with a long radar loop from New Orleans. Rainfall totals along the northern Gulf coast are expected to be in the 12-18” range, and the storm surge could be significant between the center of the storm and places for hundreds of miles east of the center as its circulation pushes the ocean out ahead of it and onto the coastline.

Luckily, we aren’t looking at a repeat of one of our country’s largest natural disasters, but it acts to keep us vigilant and prepared.

Brian McNoldy
Senior Research Associate
& Author of Tropical Atlantic Update
Follow Brian on Twitter: @BMcNoldy

Isaac Making Closest Approach to South Florida Later Today

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At 11am this morning, Isaac was still a tropical storm, with maximum sustained winds of 65mph. The center of the storm is easily within radar range, and I have a very long loop from Key West showing the passage of multiple rainbands and now the proto-eyewall. A single frame lets you see the center quickly approaching the southern Keys.

This is a relatively large storm, with tropical storm force winds extending 205 miles from the center. A recent visible satellite image puts the size of the storm with its rainbands in perspective.

A detailed hourly forecast from the NWS shows the brunt of the storm will be upon us later this afternoon. Sites in our general area have received 3-5” of rain so far, and could see another few inches before the storm moves away later on Monday. Aside from the danger of flooding, there is a risk of tornadoes associated with hurricanes near/over land.

Isaac will have another couple of days over the warm eastern and central Gulf of Mexico, and will very likely intensify to a hurricane as it travels to the northwest then north toward the north-central Gulf coast. As it looks now, it should make its final landfall somewhere in the north-central Gulf coast as at least a Category 2 hurricane – on the exact 7-year anniversary of Katrina’s infamous landfall. I will continue to watch the situation closely.

Brian McNoldy
Senior Research Associate
& Author of Tropical Atlantic Update
Follow Brian on Twitter: @BMcNoldy

Isaac’s impacts to be felt early next week in South Florida

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At 11am this morning, Tropical Storm Isaac’s intensity was increased to 60mph; still a tropical storm, but the strongest it’s been so far. It’s located south of the Dominican Republic and Haiti, and moving toward the west-northwest. The circulation is large, so locations hundreds of miles from the center are experiencing tropical storm conditions. Although, as of this post we are not under a watch, it’s very likely that a tropical storm watch will be issued for us later this evening.

The big two factors of interest to everyone here are wind and rain. The latest tropical storm force wind probabilities for the next five days are shown in this graphic. According to the official forecast from the National Hurricane Center, Miami has a 26% chance of experiencing tropical storm force winds (focused almost entirely between Sunday morning and Monday morning). It is not completely out of the question that we could experience hurricane conditions in that timeframe either… it will depend on how quickly the storm recovers after passing over Cuba. You will notice the weather turning for the worse on Sunday morning, so it would be best to do the bulk of your preparations on Saturday if it still looks like a threat.

And for rain, HPC has predicted about 7-8” of rain for us, but that’s still 2-3 days out, so the exact forecast will change as the storm gets closer and the track is better known. Even 3-4” is a lot of rain, however. I have several radar loops available that will cover Isaac’s path over the next few days to help track the center as well as the outer rainbands. Again, even if the center passes west of the Florida peninsula, we would still feel some effects here, including strong winds and flooding.

To summarize the local aspects, it currently looks like we can expect the rain and stronger winds to pick up on Sunday morning, getting worse throughout the day. Plan for very heavy rain from midday Sunday into midday Monday as well as tropical storm force winds, with a very slight possibility of winds nudging into Category 1 hurricane force range. Conditions should start improving and clearing later on Monday.

Brian McNoldy
Senior Research Associate
& Author of Tropical Atlantic Update
Follow Brian on Twitter: @BMcNoldy