The PDO and south FL winters
Posted: Sun Nov 02, 2008 4:19 pm
The PDO exerts a significant influence on south Florida winters, as evidenced by this study. The correlation between the magnitude/phase of the PDO and cold outbreaks is vividly illustrated on the Florida peninsula, where temperature variations are significant from north to south. The absolute locations of places on the peninsula (relative to the cold air advection) and the peninsula's topography greatly influences these temperature variations. The angle of cold air advection and the orientation of the surface ridge often determines the outcome in southern Florida, which can make the difference between a significant, severe, persistent cold event or a much milder outcome (unlike areas farther north in central/northern Florida). Therefore, it is probable that certain patterns can influence the synoptic/thermodynamic environment and provide clues in regards to the general evolution of winter temperatures in southern Florida. The author's research indicates one of the most significant factors is the PDO.
It has been revealed that southern Florida has periodically experienced severe, persistent, and devastating cold events. For instance, some of the region's most severe events occurred in the late 1800s, the first half of the 20th century, and the 1970s-1990s. These events were nearly always associated with mean +PDO regimes during the DJFM (December/January/February/March) period. Conversely, cold events were less severe/persistent during DJFM -PDO regimes, including the 1950s-1960s and 2000s.
Study
Total -PDO events from 1948-present: 34
In all, 13 (32.8 percent) of the 34 events were -1.00 or lower.
1948-1949: -2.2
1949-1950: -1.45
1950-1951: -1.01
1951-1952: -1.44
1955-1956: -2.09
1961-1962: -1.44
1970-1971: -1.30
1971-1972: -1.84
1975-1976: -1.00
1990-1991: -1.35
1999-2000: -1.34
2001-2002: -1.04
2007-2008: -1.33
Data originates from ftp://ftp.ncdc.noaa.gov/pub/data/ersst-v2/pdo.1854.latest.ts
Here is the December/January/February/March 500 mbar mean data for the samples.
DJFM:
http://i33.tinypic.com/mvspk8.png
December:
http://i33.tinypic.com/ngxesi.png
January:
http://i34.tinypic.com/2vaxn40.png
February:
http://i34.tinypic.com/34ywe53.png
March:
http://i33.tinypic.com/zimydz.png
Here are the average 850 mb wind vectors.
December:
http://i36.tinypic.com/14dn11f.png
January:
http://i37.tinypic.com/2mg0efb.jpg
February:
http://i37.tinypic.com/2mg0efb.png
March:
http://i37.tinypic.com/rk5vr8.png
Mean temperatures in south FL:
http://i33.tinypic.com/2hxaq9z.png
Conclusions
1. Note that the samples featured mean heights above average (denoting a mean ridge) over the Florida peninsula and Southeast from December through February. In all cases, the strongest ridging was present in January and February. Interestingly, many of these -PDO samples featured a common characteristic: the most significant cold event (or one of the most significant events) of the meteorological winter in south Florida occurred in December, although the December event was typically brief and short lived. Later, many of the samples featured a secondary March/early April cold event in southern Florida, although it was typically less severe than the December event. The vast majority of the samples featured no significant south Florida cold event in January and February, and most of the events during this period were short lived and weak. In recent -PDO examples (such as 2007-2008), the NWS Miami office frequently overestimated the magnitude of January/February cold events in their CWA (south Florida).
2.The low level 850 mb easterlies were considerably more prevalent in DJFM -PDO events, including events that featured mean PDO values below -1.00. Obviously, the low level ridging was stronger over the Florida peninsula during the -PDO events. Additionally, shortwave troughs were typically not as deep in -PDO seasons, unlike +PDO seasons. In addition, the orientation of the polar highs were less conducive for severe, sustained cold events south of Lake Okeechobee. In +PDO examples, the 850 mb easterly wind vectors were more transient, while northwesterly wind vectors and cold air advection was more dominant over south Florida (as seen in 1976-1977, 1988-1989, and other significant cold seasons with persistent/severe cold outbreaks).
3. The majority of the mean DJFM -PDO events featured a colder March with lower 500 mb heights and greater cold air advection/cP intrusions, especially across the northern tier of the CONUS. Seasons with stronger -PDO values during March exhibited stronger cold air intrusions, aided by a mean -NAO and blocking ridge over Greenland and eastern Canada. This trend can be seen in the composite of the <-1.00 March PDO events.
It has been revealed that southern Florida has periodically experienced severe, persistent, and devastating cold events. For instance, some of the region's most severe events occurred in the late 1800s, the first half of the 20th century, and the 1970s-1990s. These events were nearly always associated with mean +PDO regimes during the DJFM (December/January/February/March) period. Conversely, cold events were less severe/persistent during DJFM -PDO regimes, including the 1950s-1960s and 2000s.
Study
Total -PDO events from 1948-present: 34
In all, 13 (32.8 percent) of the 34 events were -1.00 or lower.
1948-1949: -2.2
1949-1950: -1.45
1950-1951: -1.01
1951-1952: -1.44
1955-1956: -2.09
1961-1962: -1.44
1970-1971: -1.30
1971-1972: -1.84
1975-1976: -1.00
1990-1991: -1.35
1999-2000: -1.34
2001-2002: -1.04
2007-2008: -1.33
Data originates from ftp://ftp.ncdc.noaa.gov/pub/data/ersst-v2/pdo.1854.latest.ts
Here is the December/January/February/March 500 mbar mean data for the samples.
DJFM:
http://i33.tinypic.com/mvspk8.png
December:
http://i33.tinypic.com/ngxesi.png
January:
http://i34.tinypic.com/2vaxn40.png
February:
http://i34.tinypic.com/34ywe53.png
March:
http://i33.tinypic.com/zimydz.png
Here are the average 850 mb wind vectors.
December:
http://i36.tinypic.com/14dn11f.png
January:
http://i37.tinypic.com/2mg0efb.jpg
February:
http://i37.tinypic.com/2mg0efb.png
March:
http://i37.tinypic.com/rk5vr8.png
Mean temperatures in south FL:
http://i33.tinypic.com/2hxaq9z.png
Conclusions
1. Note that the samples featured mean heights above average (denoting a mean ridge) over the Florida peninsula and Southeast from December through February. In all cases, the strongest ridging was present in January and February. Interestingly, many of these -PDO samples featured a common characteristic: the most significant cold event (or one of the most significant events) of the meteorological winter in south Florida occurred in December, although the December event was typically brief and short lived. Later, many of the samples featured a secondary March/early April cold event in southern Florida, although it was typically less severe than the December event. The vast majority of the samples featured no significant south Florida cold event in January and February, and most of the events during this period were short lived and weak. In recent -PDO examples (such as 2007-2008), the NWS Miami office frequently overestimated the magnitude of January/February cold events in their CWA (south Florida).
2.The low level 850 mb easterlies were considerably more prevalent in DJFM -PDO events, including events that featured mean PDO values below -1.00. Obviously, the low level ridging was stronger over the Florida peninsula during the -PDO events. Additionally, shortwave troughs were typically not as deep in -PDO seasons, unlike +PDO seasons. In addition, the orientation of the polar highs were less conducive for severe, sustained cold events south of Lake Okeechobee. In +PDO examples, the 850 mb easterly wind vectors were more transient, while northwesterly wind vectors and cold air advection was more dominant over south Florida (as seen in 1976-1977, 1988-1989, and other significant cold seasons with persistent/severe cold outbreaks).
3. The majority of the mean DJFM -PDO events featured a colder March with lower 500 mb heights and greater cold air advection/cP intrusions, especially across the northern tier of the CONUS. Seasons with stronger -PDO values during March exhibited stronger cold air intrusions, aided by a mean -NAO and blocking ridge over Greenland and eastern Canada. This trend can be seen in the composite of the <-1.00 March PDO events.
