Archive for July 20th, 2008

Final thoughts on Bertha and praise for Hurricane Researchers

Sunday, July 20th, 2008

Scott showed some beautiful imagery showing the big interial currents associated with big hurricanes.  The large events are also known to have impacts on the chemistry and biology of the ocean, even if we can rarely sample during these violent events.  In the deep ocean during hurricane season,  warm waters are straitified above the colder waters at depth.  These warm waters generally have low nutrients, as they have been used up by phytoplankton at a previous time.  The large storms can disrupt this straitification, and the net result is that the nutrient rich water at depth can be supplied to the surface.  These nutrients that can stimulate new blooms of phytoplankton.  Thus the storms as they pass can leave a trail of enhanced ocean phytoplankton productivity in its wake.  Some of the images below show this effect, and once the clouds clear we will assess if we can see this effect has happened with Bertha.  The figure below was produced by Babin et al. (2004).  The upper back panel shows the track of Hurricane Bonnie and the sea surface temperature.  Along the track (black line) there are green colors, indicating lower temperature water then the surrounding waters which are red/orange.  The lower front panel, shows the ocean image, and coincident with the low temperature waters along the path of Hurricane Bonnie are enhanced phytoplankton concentrations. With luck clear imagery will allow us to see what the biological impact of Bertha was. 

Babin, S.M., J.A. Carton, T.D. Dickey, and J.D. Wiggert, 2004,  Journal Geophysical Research, 109, C3, C03043, doi:1029/2003JC001938.

Waiting for the global dataset.

Sunday, July 20th, 2008

Wow, strong currents to the north. That was a nice surprise this morning.  The sea surface temperature map is blank - all white because of the cloud cover from Bertha.  The global data lags by a day - a good reason to have a local acquisition site where you get instintaneous access.  We should have that near 35 W, the western most extent of the Canary Island satellite receiver.  But we should have a good global SST maps sometime tomorrow.

But the current time series is confused due to the inertial waves from Bertha (see previous post).

The Altimetry product is still waiting for a recent overpass in our region.  Based on the altimetry, we have already crossed the strongest currents associated with the cold ring (blue color in the Sea surface height map below).

So we move on to the Navy forecast product below, here plotted as speed.  The eddy is just to our west.

Below we plot the northward component of the velocity as green, with the brighest green being the fastest velocities.  The stronger currents are a mere 20 km to our west in this forecast.

Our plan today is to follow the Navy forecast, fly to the northwest, into the forecast region for the stronger currents, and wait for the next round of global Sea Surface Temperature data to come in.

Bertha’s Inertial Tail

Sunday, July 20th, 2008

The cloud image below overlaid on the Sea Surface temperature image shows that Bertha no longer looks like a tropical storm.  It is over cold water and the clear eye is gone. Wikipedia lists it as the longest lived tropical storm in July on record.

The NOAA National Hurricane Center has declared Bertha "Extratropical" over the far North Atlantic, officially ending its record breaking run. Tropical storms have that well defined eye and a nearly symmetric circulation.  Extratrpoical storms look more like the storm fronts we get with Northeasters on the east coast of the U.S., very assymetric. NOAA NHC sent out their last advisory this morning.

Below is the wind history along its path.  The strengthening back into a hurricane after passing over Bermuda was what we were watching as it approached RU17.

Below is the last advisory for Bertha as a tropcial storm.  Heading northeast.

Below is a snapshot of the present ocean surface waves from Oceanweather.  The larger waves are to our northeast, and are declining in the vicinity of RU17. We are down to about 10 ft waves in the vicinity of the glider.

Below is what a 6 foot glider looks like climbing a 10 foot wave.  But its not RU17 in the North Atlantic. Its RU05 in the Pacific right off Diamond Head in Hawaii.  The photo was taken from a 32 foot boat right after it was deployed.  Just Chip and I went on this deployment.  I left my kids on shore.  We took two waves over the bow that came over the cabin and drenched us on the back deck. The strong currents opposing the waves really stack up the seas - very steep waves and a bumpy wet ride was the result for the humans. The robots don't seem to care. One reason we liked RU17's position during Bertha on the eastern side of the eddy was the currents were running to the north with the waves instead of against them.  This reduces the steepness of the waves.


 Below is a time series plot of the currents observed by RU17 after the passage of Bertha.  It looks very confused.  Thats because the currents we report are averaged over a 6 hour subsurface segment.  As the storm is passing, there are three main contributors to the current, all of which are changing in space and time as RU17 flies through the ocean.  One component is the geostrophic currents we see in the altimetric sea surface height plots from Colorado.  The second is the wind driven currents from the tropical storm itself.  The third component is called the inertial currents.  Inertial currents are the ocean responding at its natural frequency to a strong change in forcing.   Just like a bell rings at its natural frequency when you bang it with a hammer, a tropical storm is a mighty big hammer, and when it strikes the ocean, the ocean rings. The period of the ringing at this latitude is something like 18 hours. The ringing response is the currents rotating around in a circle. They turn clockwise with time.   In the time series below, we see all those effects, but they are not well resolved by the glider's sampling.

Surprisingly, Wikipedia has no entry for Inertial Currents.  Inertial currents are one of the most commonly observed types of currents in the world ocean, and we did not even make Wikipedia.  Hmmm. The top return from google is

The best example of inertial currents I have ever seen are from Tommy Dickey. Below is his poster of the inertial ringing of the ocean by BTM, the Bermuda Testbed Mooring, as Hurricane Fabian passed overhead.  With the time series data from this mooring, you can sample much more often in time, and dramatically resolve the clockwise rotation of the inertial currents as the ocean rings from Fabian's hammer.