The Timing of the Baroclinicity Maximum Along the East and Gulf Coasts of the United States

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1 The Timing of the Baroclinicity Maximum Along the East and Gulf Coasts of the United States Brent Walker Graduate Student Robert Straughan 2 Graduate Student Brent R. Sheeter 2 Professor 'ndiana State University Terre Haute, ndiana Salisbury State University Salisbury, Maryland ABSTRACT Certain mid-latitude coastal areas are zones of extreme baroclinicity during the coldest months of the year. Because midlatitude cyclones often tend to travel along these zones of coastal baroclinicity, the exact timing of the variations in the degree of baroclinicity along these coasts is important with regard to the forecasting of cyclonic storm tracks and their associated weather. This study examines the variations in the timing and magnitude of the baroclinicity along the east coast and the Gulf of Mexico of the United States. Near coastal land surface temperatures and sea surface temperatures along both coasts are compared for the months of October through April. Along each coast baroclinicity is found to increase steadily throughout the fall into early winter, with January having the greatest baroclinicity. The January maximum is followed by a steady decrease throughout the remaining months of the study period. KEY WORDS: baroclinicity, cyclonic storm tracks, land surface temperatures, sea surface temperatures, climate. NTRODUCTON Baroclinicity is the state of the atmosphere in which surfaces of constant pressure intersect surfaces of constant density (Ahrens, 1988). The atmosphere will be baroclinic whenever a sharp contrast in surface temperatures exists between two adjacent regions. Byers (1974) notes that the natural atmosphere is primarily baroclinic. As a result of their large temperature gradients, baroclinic zones are areas of very steep pressure gradients, especially aloft (Ahrens, 1988). Baroclinic zones are important since they are primary areas of cyclogenesis (Sanders and Bluestein, 1987) and because cyclonic systems tend to travel along baroclinic zones (Keshishian and Bosart, 1987). ndeed, the strongest flow aloft (e.g. the Polar Front Jet Stream) is located over the area of greatest baroclinicity (Holton, 1972). Numerous factors can cause the atmosphere to be baroclinic (StUll, 1988). 17

2 The most common and persistent type of baroclinicity is the baroclinic zone which occurs along many coastal areas. One of the most fundamental concepts in climatology is that water surfaces (and the air above them) heat and cool much more slowly than do land surfaces (and the air above them) (Critchfield, 1983). Extreme baroclinicity exists whenever a strong outbreak of polar air occurs over the land. However, even without a cold wave, conditions are normally somewhat baroclinic along many coastal areas during the winter. The normal more rapid wintertime cooling of land compared to that of water results in baroclinicity along coastal areas during the cold season. Major baroclinic zones resulting from land surface temperature (LST)/sea surface temperature (SST) differences are found along the east and Gulf Coasts of the United States during the cold season. The combination of a colder continent adjacent to a relatively warm ocean (made even warmer by the Gulf Stream) creates a strongly baroclinic zone (Wayland and Raman, 1989). A similar situation exists along the eastern seaboard of Asia and is aided by the northward flowing Kuroshio Current (Wells, 1986). Wells (1986), Keshishian and Bosart (1987), and Wayland and Raman (1989) state that the period of greatest baroclinicity along the United States' east and Gulf coasts occurs during the winter months. However, several questions remain regarding the timing of this baroclinic maximum: 1) during what month is the baroclinicity along the coast greatest? For example, is the baroclinicity greatest during January when the continent is coldest, or is it sometime prior to or after January? 2) Are there latitudinal differences in the timing of the coastal baroclinicity maximum along the East Coast? 3) How does the magnitude of the baroclinicity maximum vary latitudinally along the east coast and longitudinally along the Gulf coast? This study investigates each of these questions. DATA AND METHODS Both monthly LST and SST data were obtained to measure the degree of coastal baroclinicity. The SST data were acquired via the monthly map series entitled Oceanographic Monthly Summary (U.S. Department of Commerce, ). These maps show gridded monthly mean SSTs adjacent to the United States for the northwest Atlantic Ocean and the Gulf of Mexico (Fig. 1 and Fig. 2). The temperature data are in Celsius to the nearest tenth of a degree. The LSTs were acquired from the CD-ROM World WeatherDisc (WeatherDisc Associates, 1988). The years were selected for analysis due to the availability of both the SST and LST data. Also, only the months of October through April were analyzed since the period of greatest baroclinicity does not occur during the warmer months due to the reversed temperature gradient between land and water during the warm season. Six coastal temperature sites were selected along the east coast: Boston, Massachusetts; Atlantic City, New Jersey; Lewes, Delaware; Cape Hatteras, North Carolina; Charleston, South Carolina; and Jacksonville, Florida (Fig. 3). Six interior temperature sites were also selected in order to construct a transect with each coastal site for the purpose of obtaining a suitable land temperature mean. The six interior eastern temperature sites were: Blue Hill Observatory, Massachusetts; Wilmington, Delaware; Washington, D.C.; Raleigh-Durham, North Carolina; Macon, Georgia; and Tallahassee, Florida. The selection of the interior sites were based on data availability. Each of these interior sites was along a roughly perpendicular transect with a coastal site. The transects between coastal and interior points were mostly between one and two hundred miles apart. t was necessary to have a distance of no more than 2 miles between the coastal and interior sites to insure an accurate mean, while at the same time avoid the much lower temperatures associated with the higher elevations of the mountains. The two monthly land temperatures for each transect were averaged to form a monthly land temperature mean. The same technique was used for selecting the Gulf Coast sites and computing the means. The four coastal tem- 18

3 4SN 4N /',../ > " J / ". 3SN 3N 259 Miles Z) N ~J h FGURE 1. Monthly mean sea surface temperature for the NW Atlantic Ocean during January (Source: Oceanographic Monthly Summary, U.S. Dept. of Commerce.) perature sites were Galveston, Texas; New Orleans, Louisiana; Mobile, Alabama ; and Pensacola, Florida. The four interior temperature sites were Houston, Texas; Jackson, Mississippi; Meridian, Mississippi; and Montgomery, Alabama. Along the east coast the offshore waters are actually composed of two very different zones. Cold shelf waters associated with the Labrador Current which extends from the coast to about 75 to 1 miles offshore. At that point the colder shelf water meets the warmer water of the more dominant Gulf Stream (Sethuraman et al., 1986). Along the Gulf Coast there are no major contrasting currents. Therefore, the offshore water temperatures are much more uniform. To obtain a single representative monthly SST value to compare with each LST mean the following procedure was used. First, a 2-mile line was extended perpendicular to the coast adjacent to the coastal temperature site. Temperatures were recorded for each of four evenly spaced points along each line (j.e., at 5, 1, 15 and 2 miles from the coast). n many cases it was necessary to interpolate between temperature readings on the maps (Fig. 1 and Fig. 2). The SSTs for the four points were averaged so as to yield a mean monthly SST value representative of the coastal waters. The difference between the LST mean and the SST mean was then calculated for each of the paired land and sea means. 19

4 f ' ( '"' -.." '.. "'" ",of", --,.. J.., (.J r- - 35N 3N ZSN O:..-~~... 2_5 ~ Miles '. J b.712b.s 2b. S 2b.7 2b.! 1" 2~~7>7.3!U! 2U :lbj!4t ~: : 2b{; ~l~>b'! i 2b.!.2~.2b ' ~r3, b. S lb. S 2b. Silb.7 2b.7 l b.! lb.b / lb.3 lb.4. S lb.7 lb.9 1W 9SW 9W 8SW 8W ZON lsn FGURE 2. Monthly mean sea surface temperatures for the Gulf of Mexico during January (Source: Oceanographic Monthly Summary, U.S. Dept. of Commerce.) This difference served as the baroclinicity index and was then averaged for each month for each pair of means. RESULTS AND CONCLUSONS Tables 1 and 2 and Figures 4 and 5 summarize the monthly variations in baroclinicity along the east and Gulf Coasts. Along the east coast, January was clearly the most baroclinic month. All six land/ sea mean differences ranked January as most baroclinic, with no other month being even close. December was ranked second in all six cases. The Cape Hatteras/Raleigh transect was the most baroclinic east coast site. This was likely in large part due to the proximity of the Gulf Stream due to the eastward extension of North Carolina. Boston/Blue Hill on the other hand, was on average the least baroclinic. Along the Gulf coast. January was found to be the most baroclinic month 11

5 95 (,.,....!.'- --, ' , ' '.,, 1 5 '. ';(,,. Blue Hill MA Boston -- --: ~~: - - -,-- 't A tl'rtic City NJ!, '-~;'h~ng;i~-'~~:~nsg:: DE (, _...: National,..,' { (... './ " - - " --.." l. (... Raieigh-OUrhim Cape Hatteras 7 ~, 1- r :,,-, ~ _ N C /-NC ", _ ! --:...', ~ ' - -,- " it,! Macon, Charleston c 3 -- J Meri~ian MS - '.. - GA -'f- SC -, -.1 Montgomery,----'. _, AL 3- ', _ - - _ - - -v-_ Jackson MS Jacksonville FL.' ~~~ile,~~ ~Tallahassee FL Houston _ ~ Pensacoi"a FL ~ - "~~eans LA f Galveston TX ~ '" 95 GULF OF MEXCO o o " 5 Km. 4 M. FGURE 3. Coastal and interior temperature sites used for determining land temperature means along the east and Gulf coasts. for all of the sites except one. The exception was the Houston and Galveston, Texas transect which was most baroclinic during both December and January. There are several possible explanations for this exception. Houston and Galveston are only approximately 5 miles apart. Therefore, Houston's climate has a somewhat marine influence as a result of its proximity to Galveston Bay and the Gulf of Mexico. All of the sites for the other transects are located much farther apart. Also, Houston and Galveston have similar topography, with both being located on the Gulf Coastal Plain. All of the other sites differ topographically, with one being on the coastal plain and the other being in close prox- 111

6 TABLE 1 All Sites Composite Summary-East Coast Land/Sea Surface Temperature (OC) Mean Difference (and Rank) Location Jan Feb Mar Apr Oct Nov Dec Boston, MAl Blue Hill, MA (1 ) (3) (6) (5) (6) (4) (2) Atlantic City, NJ Wilmington, DE (1 ) (4) (7) (6) (5) (3) (2) Lewes, DE Washington, D.C. (1 ) (3) (7) (6) (5) (4) (2) Cape Hatteras, NC Raleigh-Durham, NC (1 ) (3) (5) (7) (6) (4) (2) Charleston, SC Macon, GA (1 ) (4) (6) (7) (5) (3) (2) Jacksonville, FL Tallahassee, FL (1 ) (3) (5) (7) (6) (4) (2) Average For Six Stations (1 ) (3) (6) (7) (5) (4) (2) TABLE 2 All Sites Composite Summary-Gulf Coast Land/Sea Surface Temperature (OC) Mean Difference (and Rank) Location Jan Feb Mar Apr Oct Nov Dec Houston, TX Galveston, TX (1 ) (4) (6) (7) (5) (3) (1 ) Jackson, MS New Orleans, LA (1 ) (3) (6) (7) (5) (4) (2) Meridian, MS Mobile, AL (1 ) (4) (6) (7) (5) (3) (2) Montgomery, AL Pensacola, FL (1 ) (3) (5) (7) (5) (4) (2) Average For Four Stations (1 ) (3) (6) (7) (5) (4) (2) 112

7 14 12 <> 1 '"" X W z 8 >- to- ~ z ::::i 6 () a: c( CO a o ~------~------~------~------~----~~----~ OCT NOV DEC JAN FEB MAR APR -- CAPE HATTERAS. NC RALEGH-DURHAM. NC - - JACKSONVLLE. FL TALLAHASSEE. FL CHARLESTON. SC MACON. GA ATLANTC CTY. NJ WLMNGTON. DE ---- LEWES. DE WASHNGTON. D.C. BOSTON. MA BLUE HLL. MA - AVERAGE FOR SX STATONS FGURE 4. Monthly variations in baroclinicity along the U.S. East Coast for the period (Source : Oceanographic Monthly Summary, U.S. Dept. of Commerce; World WeatherDisc.) 113

8 14 12 G 1 "" X W C z 8 >- ~ (3 z :J 6 () a: «ED 4 2 o OCT NOV DEC JAN FEB MAR APR JACKSON, MS NEW ORLEANS, LA MERDAN, MS MOBLE, AL ". MONTGOMERY, AL PENSACOLA, FL -- HOUSTON, TX GALVESTON, TX - AVERAGE FOR FOUR STATONS FGURE 5. Monthly variations in baroclinicity along the U.S. Gulf Coast for the period (Source: Oceanographic Monthly Summary, U.S. Dept. of Commerce; World WeatherDisc.) 114

9 imity to the mountains and the highlands. Finally, the Houston-Galveston transect is adjacent to an area of shallow water in relation to the other three transects. Galveston Bay is much shallower than is the Gulf of Mexico. Therefore, the waters near Galveston are warmer. This translates into a smaller temperature difference between the land and the water. This may also explain why the baroclinicity along the Houston and Galveston transect is small in comparison to the other transects, although this particular transect may warrant further research. n general, baroclinicity along the Gulf of Mexico and east coast regions is found to increase steadily throughout the fall and early winter, reaching a peak in January. This is followed by a steady decrease throughout the remainder of the study period. The LSTs decrease dramatically in winter while the SSTs decrease at a much slower rate. This results in a lag of SST change as compared to the LST changes. From the late winter throughout the remainder of the study period, the reverse situation is true, with the land heating much more quickly than the water. All transects demonstrate that by the month of April baroclinicity is minimal. AREAS OF FUTURE RESEARCH The methodology used in this study could be extended to other areas to determine if similar baroclinicity trends occur elsewhere. One natural area for the extension of similar research is the coast of southeast Asia because the area displays many of the characteristics that are found along the southeastern United States. Another topic which warrants future research is the relationship between the zones of coastal baroclinicity and the positioning of cyclonic storm tracks. Research should be carried out to determine to what extent cyclonic storm tracks follow along a coast during periods of differing degrees of baroclinicity. Further advancing the understanding of the relationship between storm tracks and areas of coastal baroclinicity may allow for more accurate prediction of storm tracks and therefore improve the reliability of forecasting major winter storms along the Gulf of Mexico and east coasts of the United States. REFERENCES Ahrens, Donald C Meteorology Today. St. Paul, MN. 3d ed. West Publishing Co. Byers, R. B General Meteorology. New York. 4th ed. McGraw-Hil. Critchfield, H. J General Climatology. Englewood Cliffs, N.J. 4th ed. Prentice Hall. Holton, J. R An ntroduction to Dynamic Meteorology. Academic Press, New York. Keshishian, L. G. and H. B. Bluestein A Case Study of Extended East Coast Frantogenesis. Monthly Weather Review, 115: Sanders, F. and H. B. Bluestein Baraclinic field. n S. P. Parker (Ed.) Meteorology Source Book. McGraw-Hili Book Co. New York. Sethuraman, S., A. J. Riordan, T. Holt, M. Stunder, and J. Hinman Observation of the Marine Boundary Layer Thermal Structure Over the Gulf Stream During a Cold Air Outbreak. Journal of Climate and Applied Meteorology, 25 : Stull, R. B An ntroduction to Boundary Layer Meteorology. Dordrecht, The Netherlands. Kluwer Academic Publishers. U.S. Department of Commerce National Oceanic and Atmospheric Administration. National Weather Service/ Nat ional Earth Satellite Service Oceanographic Monthly Summary. (Washington, D.C.): U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service/ National Earth Satellite Service, Volume 2-7, Number 1-4, Wayland, R. J. and S. Raman Mean and Turbulent Structure of a Baroclinic Marine Boundary Layer During the 28 January 1986 Cold-Air Outbreak (gale 86). Boundary-Layer Meteorology, 48: Wells, N The Atmosphere and Ocean : A Physical ntroduction. Taylor and Francis, London. World WeatherDisc, Version 1., WeatherDisc Associates, ncorporated, Seattle, Washington. 115