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Atlantic Tropical Action 2012 - Part II


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There are some possible explanations for the unusually high static stability and suppressed anticyclonic flow over the basin this year: 1) the very high sunspot activity, which through ultraviolet radiation has, through warmer mid-tropospheric temperatures in the equatorial region, partially worked to suppress seasonal AAM feedback and MJO activity; 2) the suppressed surface flux in the Atlantic, partially as a result of the former; and 3) an unfavorable pattern of reduced 250hPa zonal and potential velocities across the Western Atlantic and the Caribbean. These factors also bolstered the suppressed anticyclonic flow and along with low soil moisture (long-term, decadal drought conditions) kept in place the Western North American ridge / Eastern United States trough set-up that has caused so many recurves during 2010-2012. I would imagine that a very complex interplay of factors resulted in the rather unfavorable Atlantic regimen despite the Niña-like atmosphere globally. The high mid-tropospheric temperatures and the tropical circulation pattern has been behaving like that of a developing Niño since late July / early August despite the absence of the Brewer-Dobson circulation and the existence of a low AAM / suppressed MJO. (These factors also explain why the East Pacific tropical-cyclone season has greatly underperformed relative to past Niño seasons.) This discussion touches on some of these ideas and is a great learning tool. I think a serious discussion is in order as to how and why certain factors made this Atlantic season--and even the past two seasons--so lackluster in terms of intense hurricane days and U.S. major landfalls.

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I don't think that I would have bet anyone before the season started that we would make it to the P storm

with only one major hurricane day.

For what it's worth Kirk may have very well been a major hurricane and there's a small possibility Gordon was as well.

It isn't really worth anything.

That 98L track reminds me of the Great Hurricane of 1780, and how it's been 232 years since a storm managed to become great in that location/time.

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There are some possible explanations for the unusually high static stability and suppressed anticyclonic flow over the basin this year: 1) the very high sunspot activity, which through ultraviolet radiation has, through warmer mid-tropospheric temperatures in the equatorial region, partially worked to suppress seasonal AAM feedback and MJO activity; 2) the suppressed surface flux in the Atlantic, partially as a result of the former; and 3) an unfavorable pattern of reduced 250hPa zonal and potential velocities across the Western Atlantic and the Caribbean. These factors also bolstered the suppressed anticyclonic flow and along with low soil moisture (long-term, decadal drought conditions) kept in place the Western North American ridge / Eastern United States trough set-up that has caused so many recurves during 2010-2012. I would imagine that a very complex interplay of factors resulted in the rather unfavorable Atlantic regimen despite the Niña-like atmosphere globally. The high mid-tropospheric temperatures and the tropical circulation pattern has been behaving like that of a developing Niño since late July / early August despite the absence of the Brewer-Dobson circulation and the existence of a low AAM / suppressed MJO. (These factors also explain why the East Pacific tropical-cyclone season has greatly underperformed relative to past Niño seasons.) This discussion touches on some of these ideas and is a great learning tool. I think a serious discussion is in order as to how and why certain factors made this Atlantic season--and even the past two seasons--so lackluster in terms of intense hurricane days and U.S. major landfalls.

Nice post. I have also noticed more of a suppressed convective signal from 2006-2012 compared to 1999-2005 in the Atlantic.

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There are some possible explanations for the unusually high static stability and suppressed anticyclonic flow over the basin this year: 1) the very high sunspot activity, which through ultraviolet radiation has, through warmer mid-tropospheric temperatures in the equatorial region, partially worked to suppress seasonal AAM feedback and MJO activity; 2) the suppressed surface flux in the Atlantic, partially as a result of the former; and 3) an unfavorable pattern of reduced 250hPa zonal and potential velocities across the Western Atlantic and the Caribbean. These factors also bolstered the suppressed anticyclonic flow and along with low soil moisture (long-term, decadal drought conditions) kept in place the Western North American ridge / Eastern United States trough set-up that has caused so many recurves during 2010-2012. I would imagine that a very complex interplay of factors resulted in the rather unfavorable Atlantic regimen despite the Niña-like atmosphere globally. The high mid-tropospheric temperatures and the tropical circulation pattern has been behaving like that of a developing Niño since late July / early August despite the absence of the Brewer-Dobson circulation and the existence of a low AAM / suppressed MJO. (These factors also explain why the East Pacific tropical-cyclone season has greatly underperformed relative to past Niño seasons.) This discussion touches on some of these ideas and is a great learning tool. I think a serious discussion is in order as to how and why certain factors made this Atlantic season--and even the past two seasons--so lackluster in terms of intense hurricane days and U.S. major landfalls.

Interesting post... I just have one question.. Are there any papers out there that discuss the impact of solar activity on MJO activity? If there were a way to forecast MJO activity at seasonal, or even interannual timescales, that would be something.

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I did a three way model blend for the medium range pressures today for days 3-5, and still got a system with a pressure of 1000 hPa on Monday morning, 996 hPa Tuesday morning, 992 hPa Wednesday morning, and 980 hPa Thursday morning (extratropical Wednesday and Thursday). It wouldn't be surprised if it became a hurricane...give it time. Major hurricane? Probably not, but you never know. Michael became one.

Two ugly ducklings in a row.

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After Rafael, I'd watch the remnant vortcity from Patty once it retrogrades into the Caribbean. It could fester there for a few days and possibly become enhanced a bit by the passing CCKW. I wouldn't be surprised at all if we saw at least an invest come out of that region over the next 10 days.

Some of the experimental genesis models like the 'Carla Cradle' over the next two weeks...

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I did a three way model blend for the medium range pressures today for days 3-5, and still got a system with a pressure of 1000 hPa on Monday morning, 996 hPa Tuesday morning, 992 hPa Wednesday morning, and 980 hPa Thursday morning (extratropical Wednesday and Thursday). It wouldn't be surprised if it became a hurricane...give it time. Major hurricane? Probably not, but you never know. Michael became one.

But where? In the middle of nowhere? :(

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Nice post. I have also noticed more of a suppressed convective signal from 2006-2012 compared to 1999-2005 in the Atlantic.

Yes, indeed...and I suspect that the warming of the mid-troposphere during 2006-2012 may have played a significant role. For some reason, the Atlantic, in my experience, seems to be more sensitive to planetary factors than does either the East or West Pacific basins...perhaps, in the case of the Caribbean, because of the topographical influence of South America and its effect on the Walker circulation. (Note that the Atlantic is enclosed by large land masses and is therefore more readily influenced by synoptic and land-sea thermal differences/air masses than is the North Pacific, which is a much larger basin that is not so readily influenced by large land masses except north of 30-35°N or so.) Note also that the Caribbean has generally become more hostile for development in 2006-2012 as the South American heat low has become more dominant.

Another interesting detail is that, relative to historical patterns, including inactive cycles, while the average number of landfalling U.S. majors does not seem to have changed significantly, the intensity of U.S. major hurricanes, as well as the frequency of Category-4/5 hits, appears to have declined during 1995-2012. While data on active and inactive Atlantic cycles is somewhat unreliable before 1926--and while more reliable intensity data on storms begins from 1900 onward--the numbers from 1851-1900 are still reasonably consistent with data from similar periods after 1900. Of course, some storms may have also been underestimated in intensity as most coastal areas were sparsely populated before 1900. Obviously, most major strikes, and most Category-4/5 hits, tended to come in concentrated spurts or clusters, so I included the median clusters or intervals in which the Cat.-4/5 hits tended to arrive.

1851-1900: 26 major strikes,* avg. intensity 108 kt, Cat. 4/5 once every 9.8 yrs. (median cluster w/in 5 yrs.)

1901-1950: 31 major strikes (19 during active cycle 1926-1950),* avg. intensity 111 kt (avg. intensity 113 kt during active cycle 1926-1950), Cat. 4/5 once every 4.4 yrs. (median cluster w/in 3 yrs.)

1951-1994: 24 major strikes (11 during inactive cycle 1969-1994),* avg. intensity 112 kt (avg. intensity 115 kt during inactive cycle 1969-1994), Cat. 4/5 once every 5.1 yrs. (median cluster w/in 3 yrs.)

1995-2012: 10 major strikes,* avg. intensity 106 kt, Cat. 4/5 once every 9 yrs. (median cluster w/in >9 yrs.)

*Includes major hurricanes that did not make landfall in the United States but produced major-hurricane conditions on a section of the U.S. coastline.

Note: After 1935--the latest year reanalyzed by the Best Track Committee--I usually used the official U.S. hurricane designations and took the lowest possible major (>100 kt) wind speed supportable if HURDAT listed a lower value than 100 kt. If a reanalyzed value was available--i.e., the 105 kt (Strahan et al., 2010) for the 1938 Long Island Express / Great New England hurricane--I used that value instead. After 1943, I utilized the intensities from the 1944-1953 reanalysis of Hagen et al. (2012). Obviously, such values are preliminary and my list will not be exact, but it should give a rough idea as to long-term trends.

Some noted highlights from the findings I made:

  • The intensity of major hurricanes and the frequency Cat.-4/5 strikes were actually about the same as in previous periods during the last reliably documented inactive cycle (1969-1994), but the active cycle since 1995 has featured a decline in both the intensity of majors and the frequency of Cat.-4/5 hits relative to those which occurred in previously documented active cycles/periods (1926-1950, 1950-1968).
  • Gulf Coast Cat.-4/5 hits were much more frequent historically: 5 in 1851-1900, 4 in 1901-1950 (at least 1 in 1926-1950), and at least 3 (perhaps higher if Celia 1970 and Frederic 1979 are reanalyzed as Cat. 4) in 1951-1994 (all in 1951-1969; this may be higher or stay the same if Betsy 1965 is reanalyzed as Cat. 4 in LA). None has occurred in 1995-2012, even though the average return rate in previous active cycles was once per 8.4 yrs. (with a median cluster w/in 6 yrs.).

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