GOMEX SST/heat content- effect & frequency and instensity?

[Ed Mahmoud]

On the now dying Caribbean thread, someone noted with a ridge overhead, near the time of maximum sun angle and day length, with few clouds, little rain and generally light winds, albedo should be minimized by generally calm seas, and the Gulf should get warm. Very warm.

Of course, as regular readers know, there is heat content, which, best I can tell, is based on integrating the energy content, a function of temperature, from the surface down to the a depth where a certain isotherm occurs, about 26º

Now, it becomes plain from the image below that insolation on the Gulf is not the only factor, and may not even be the primary factor, in energy content.



The Caribbean which gets much higher sun angles in Winter, and far fewer polar front intrusions, seems to be the main factor.

Now, currents are affected by winds (recall seeing a link that current and its relation to wind actually involves a variation of the Coriolis effect, but I digress), so weather patterns besides lots of sunshine and light winds may effect Gulf energy content.

Now, I would argue, that "heat content" alone, assuming similarly favorable conditions of upper level shear, outflow and moisture content, is not as critical to faster moving cyclones. A slow moving Cat 5 Hurricane Mitch storm in the Gulf would upwell cooler water and weaken, but I'd think a faster moving strong storm in the Gulf would be more affected by the heat content well above above the 26ºC isotherm, as it would be, to some degree, staying ahead of the maximum upwelling.

I think we generally agree high SSTs and heat content, all else equal, means stronger storms.

But what about frequency?

I used to see posts on the Wunderground threads like "wow, the Gulf is boiling, something has to pop soon".

1996 was a busy Gulf season, 1997, the El Nino year, had only Danny. Were SSTs that different? I don't know, actually.


Anyway, just wanted to get opinions from the folks, pros and amateurs alike, on my theory that heat content as currently defined is not as important as surface SST for fast moving systems, and whether frequency is a function of Gulf SSTs.

[poof121]

The warmer water at the top will stay at the top because it is lighter than the water below. But if there are moderate winds, say 15-20 kts, the warmer water can mix down, causing a deeper warm layer, which should then increase the TCHP.

[Ed Mahmoud]

The warmer water at the top will stay at the top because it is lighter than the water below. But if there are moderate winds, say 15-20 kts, the warmer water can mix down, causing a deeper warm layer, which should then increase the TCHP.

But higher winds make waves, and IIRC flat, calm seas have the lowest albedo. But I can see that the warm water won't sink on its own.


Semi-related, in Vermillion parish, rice farmers can actually irrigate their fields with pumps pulling from the Gulf.

The Atchafalaya river carries fresh water, which is less dense, and floats above the denser salt water when it first spills into the Gulf.

I can also say, having flown many a helicopter from Louisiana offshore, there are two sharp discontinuities. The first is from brown to greenish water, and then an even sharper break from green, opaque water, to crystal clear blue water, further offshore.

Not sure what causes that, exactly.

[poof121]

The warmer water at the top will stay at the top because it is lighter than the water below. But if there are moderate winds, say 15-20 kts, the warmer water can mix down, causing a deeper warm layer, which should then increase the TCHP.

But higher winds make waves, and IIRC flat, calm seas have the lowest albedo. But I can see that the warm water won't sink on its own.

Yeah, without winds, the surface layer will get really hot. Conduction will slowly raise the temperature below, but that is much slower than mixing.

Semi-related, in Vermillion parish, rice farmers can actually irrigate their fields with pumps pulling from the Gulf.

The Atchafalaya river carries fresh water, which is less dense, and floats above the denser salt water when it first spills into the Gulf.

I can also say, having flown many a helicopter from Louisiana offshore, there are two sharp discontinuities. The first is from brown to greenish water, and then an even sharper break from green, opaque water, to crystal clear blue water, further offshore.

Not sure what causes that, exactly.

Those transitions you see are due to river water full of silt and marine life. The crystal blue water is water with no life. When water gets upwelled, which doesn't happen too much in the deep gulf, the deeper water with greater oxygen content is brought to the surface, which allows more habitable enviroment for life to grow. I'd be willing to bet that after a decent size hurricane, the water color wouldn't be as blue.

[Ed Mahmoud]

Nah, in the clear blue water, out on the rigs, you'll sometimes see schools of rays. Pretty cool, actually.

[poof121]

I was referring to microsopic life. That's what causes the color differences.

[clfenwi]

Some initial comments and links:

The paper that established the idea of heat potential is very readable as far as those things go: Hurricane Heat Potential in the Gulf of Mexico (PDF)

- I haven't read through this one, but it could have some interesting insights:
Application of Oceanic Heat Content Estimation to Operational Forecasting of Recent
Atlantic Category 5 Hurricanes

ftp://ftp.aoml.noaa.gov/pub/phod/goni/w ... lietal.pdf

- Hurricane Alley SST Variability in 2005 and 2006

http://staff.washington.edu/chiodi/publ ... on2008.pdf

Again haven't read through this one yet, but it makes a pertinent point in the intro:

By their description, the 2005 North Atlantic hurricanes may be divided into two groups: those that form in the eastern tropics and circulate clockwise around the ocean basin east of 65°W, and those, of interest here, that form over the warm waters of the WAC region and the Gulf of Mexico. Five of these hurricanes made landfall over the Gulf Coast and the eastern seaboard of the United States in 2005; the first of these landfalls occurred in early July 2005. In 2006, however, there were no hurricanes in the WAC region and the Gulf of Mexico. Although SST in the Gulf of Mexico was warmer in 2006 than in 2005, over the WAC region (study area 15°–30°N, 70°–40°W; see Fig. 1) the SST was significantly cooler in July, August, and September (JAS) 2006 than in JAS 2005

(WAC = Western Atlantic and Caribbean)

As far as sunshine goes, it probably accounts for a fair amount of short-term variability for areas that the loop current does not wander into. However, movement of the loop current has much greater effect than sunshine.

Should be able to find SST data for '96 and '97. One thing to note when looking at previous years from the same source, be careful to make sure the scales are the same. Three years ago, I kind of got burned comparing SST charts. I was looking at just the color shades and implicitly assuming that same shade=same temperature across the charts. Wasn't the case has the scale in later years had been expanded, so what was orange in newer charts was actually warmer than the orange in older charts.

More later....

[tailgater]

The warmer water at the top will stay at the top because it is lighter than the water below. But if there are moderate winds, say 15-20 kts, the warmer water can mix down, causing a deeper warm layer, which should then increase the TCHP.

But higher winds make waves, and IIRC flat, calm seas have the lowest albedo. But I can see that the warm water won't sink on its own.

Yeah, without winds, the surface layer will get really hot. Conduction will slowly raise the temperature below, but that is much slower than mixing.

Semi-related, in Vermillion parish, rice farmers can actually irrigate their fields with pumps pulling from the Gulf.

The Atchafalaya river carries fresh water, which is less dense, and floats above the denser salt water when it first spills into the Gulf.

I can also say, having flown many a helicopter from Louisiana offshore, there are two sharp discontinuities. The first is from brown to greenish water, and then an even sharper break from green, opaque water, to crystal clear blue water, further offshore.

Not sure what causes that, exactly.

Those transitions you see are due to river water full of silt and marine life. The crystal blue water is water with no life. When water gets upwelled, which doesn't happen too much in the deep gulf, the deeper water with greater oxygen content is brought to the surface, which allows more habitable enviroment for life to grow. I'd be willing to bet that after a decent size hurricane, the water color wouldn't be as blue.

Cold water is denser, so it would drop. Molecules not moving as fast if I remember chemistry correctly.
Yes I'd say that's correct about speed of a storm being a determining factor as to which is more important. TCHP vs SST.

[Ed Mahmoud]

OK, the Caribbean and currents seem to dominate where the best heat content can be found, but the image loop would indicated boatloads of sunshine do warm the surface away from the Loop.

[poof121]

Just FYI, here is a link to forecasted currents and SST at NCEP.

http://www.opc.ncep.noaa.gov/GlobalOceanStart.shtml

[MGC]

No matter, the GOM in August and September will support a major hurricane. All depends on UL winds....MGC