oceanography, weather

A major source of a hurricane’s energy

Everyone knows something about tropical cyclones, the broad class of tropical low-pressure systems that includes hurricanes and typhoons (Fig. 1). Many of us know when and where they are more likely to form and how they are named. If you were asked about the most favorable conditions under which these tropical systems develop, among the first things you would probably say would be sea surface temperature. Some of you may also know that a sea surface water temperature above about 80oF is a very favorable factor. No wonder that hurricanes form over low-latitude ocean areas.

A hurricane or its antecedent weather system could be thought as an engine, drawing heat energy from the water body underneath it. The system does this in two ways. First, air touching warm water is warmed by contact (conduction), and air heated from below wants to rise (convection)*. Second, water evaporates from the warm ocean surface (with latent heat trapped in the water vapor) and is then carried upward by rising air currents (updrafts). The latent heat is released during the phase change from water vapor to water droplets (condensation and cloud formation) and that heat is then distributed vertically within the updraft (Fig. 2).

Now, think about a weather situation modeled in your kitchen. Imagine that you have a bowl with hot, steaming soup. Would you rather cool it down by blowing on the whole bowl or a small portion of the soup in a spoon? Almost everybody would choose the second option. Well, now think about the force of the winds in a hurricane. Don’t you think they are strong enough to lower the water temperature? And if you take also into account the large amounts of chilled precipitation that falls on the ocean surface, then you would realize that we need something else to keep providing the necessary energy for hurricanes.

The depth of the isothermal layer (a layer in the ocean that has an almost constant temperature) plays an important role in sustaining and intensifying hurricanes and tropical cyclones. Considering its depth and its temperature, we could then estimate the available potential energy for the hurricane’s heat engine. It is important to note that the hurricane circulation is strong enough and its pressure low enough to actually lift the surface of the ocean, causing deeper layers with a lower temperature to rise, thereby squeezing this isothermal layer somewhat. Depending on the strength of the storm’s circulation, how low the atmospheric pressure is in the center of the storm, and the storm’s speed of motion, the effect can be quite strong. It could possibly even generate a cool surface water “footprint.”  Obviously, such cool “footprints” can weaken a hurricane.

Conversely, we can say that there must be certain areas where a tropical cyclone could intensify. Tropical meteorologists call these “warm pools.” Forecasters can monitor these and ascertain how they affect a hurricane’s growth and/or intensification.

*Sensible heat is almost negligible, since it depends of the temperature difference between the water surface and the air, which in the tropics is usually less than 1oC ( Emanuel, K: The theory of hurricanes, 1991).

© 2017 Mayguen Ojeda

Originally posted 6/17/17

biology, geography, learning, oceanography, weather

Red tide still affecting many southwest Florida beaches (H. Michael Mogil, CCM, CBM, DMS)

For weeks, a “red tide” (such tides are also referred to as a “harmful algal bloom” or HAB) has affected many beaches from Lee County, FL northward. With the departure of Hurricane Matthew a little over a week ago, the HAB was pushed southward and eastward, finally reaching beaches in Collier County. A day earlier (Fri., Oct. 6, 2016), news reports from Sarasota, FL indicated that with increasing winds and winds blowing from land to water (the approach of Matthew), the tide in its area would likely lessen. Unfortunately, the tide had to go somewhere. On Sat., Oct. 7, 2016, the HAB’s smelly signature arrived in Naples, FL. The tide has since returned to Sarasota area beaches, as well.

According to Florida’s Fish and Wildlife Conservation Commission, a “red tide is a naturally-occurring microscopic alga that has been documented along Florida’s Gulf Coast since the 1840’s and occurs nearly every year. Blooms, or higher-than-normal concentrations, of the Florida red tide alga, Karenia brevis, frequently occur in the Gulf of Mexico at this time of year (late summer or early fall). Red tide begins in the Gulf of Mexico 10 to 40 miles offshore and can be transported in (to) shore by winds and currents.”

fig001-san-diego-area-algal-bloom-imageAs the name “red tide” suggests, this bloom of algae often turns the water red (Fig. 1).

A HAB has two potential components. First, according to NOAA’s National Ocean Service (NOS), are the harmful toxins that some (not all) algal blooms create. Florida’s “red tide” bloom is of the type that can kill fish and make shellfish dangerous to eat. These same toxins may also make the surrounding air difficult to breathe. Both of these can have local economic impacts (fishing and tourism).

Another way the algal bloom (even without the presence of harmful toxins) can harm marine life involves the natural end-of-life of the bloom. According to NOS, “…when masses of algae die and decompose, the decaying process can deplete oxygen in the water, causing the water to become so low in oxygen that animals either leave the area or die.”

fig002-dead-fish-at-vanderbilt-beach-naples-161007Either way, with onshore winds and/or an ocean current with a coastward component, there is the potential for dead fish to wash up on the beach (as I personally witnessed 10 days ago on a north Naples beach). The presence of hundreds to tens of thousands of dead fish makes a beach a very unattractive and very odiferous place to visit (Fig. 2). This setting can easily ruin vacations and impact a local area’s tourism industry.

Local agencies and volunteers often work quickly to clean up the dead fish. However, as long as the tide sits nearby, fish deposition on the shore often continues.

fig003-sfl-watch-warning-map-161017-0630amedtCurrently, the National Weather Service (NWS) offices in Miami and Tampa Bay have posted “beach hazards statements” advising of the HAB’s potential human impacts through this Monday evening (Oct. 17, 2016) along many southwest Florida beaches (including those in Collier, Pineallis, Sarasota and Manatee counties). See Fig. 3 for affected areas in southwest Florida.  These statements note that, the red tide affecting the area may cause, “…possible respiratory irritation in some coastal areas. The irritation can include coughing, sneezing, and/or tearing eyes.” The statements further note that, “…people with respiratory conditions such as asthma, emphysema and other pulmonary diseases may be more sensitive (to these conditions).” The statements also urge people to check out current “red tide” conditions at the following web sites:

http://tidesandcurrents,noaa.gov/hab

http://mote.org/beaches

http://myfwc.com/redtidestatus and consider visiting nearby unaffected beaches.

For health information, people should check with their physician, as appropriate. People can also get medical information at the floridahealth.gov web site or by calling the Poison Control Center at 1-800-222-1222

For links to “red tide” conditions across other coastal states, visit NOAA’s “red tide” web site.

© 2016 H. Michael Mogil

Originally posted 10/17/16

"Life", astronomy, biology, education, geology, hydrology, learning, nature, oceanography, photography

Bing’s homepage images return excitement to learning

There’s probably not a better way to get a brain engaged first thing in the morning than to use the Bing home page. Each day, Bing (a Microsoft-based search engine) features an incredible homepage image involving something from nature and the Earth, the human sphere, architecture, and other categories. Images range from…To read the entire feature, click here.

Originally published 3/12/15

oceanography, weather

A new perspective on ocean and lake currents

For the past few days, Hurricane (now extra-tropical storm) Arthur brought concerns to East Coast residents about storm surge and rip currents. Today, Jul. 6, 2014, the focus shifts to the eastern and western shores of Lake Michigan where an array of wind-driven currents could create problems for boaters, swimmers and lifeguards…To read the entire feature, click here.

Originally published 07/06/14