Spatial and Temporal Extent of Sea Surface Temperature Modifications

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1364 MONTHLY WEATHER REVIEW VOLUME 126

Spatial and Temporal Extent of Sea Surface Temperature Modi®cations by Hurricanes in the Sargasso Sea during the 1995 Season*

NORMAN B. NELSON Bermuda Biological Station for Research, Inc., St. George's, Bermuda 9 September 1996 and 13 May 1997

ABSTRACT Sea surface temperature anomalies in the central and western Sargasso Sea resulting from tropical cyclones were investigated during the 1995 hurricane season. High-resolution image data from Advanced Very High Resolution Radiometer instruments on board the NOAA-12 and NOAA-14 satellites were used to make 3-day composite sea surface temperature maps covering 22Њ±40ЊN and 50Њ±82.5ЊW. Ten tropical cyclones passed through this region in 1995, six at hurricane strength (winds greater than 33 m sϪ1). Four hurricanes (Felix, Iris, Luis, and Marilyn) caused signi®cant cooling of the sea surface (up to 4ЊC) along their tracks. The largest surface area impacted by these four hurricanes at any one time was at least 4.8 ϫ 105 km2, or 6% of the study area. Restoration of the ocean surface to prehurricane conditions occurred on the order of l0 days, except where successive hurricanes passed through a previously in¯uenced area. Hurricanes Felix, Luis, and Marilyn all passed through an area northwest of Bermuda where signi®cant sea surface temperature anomalies (greater than Ϫ1ЊC) persisted in this region for two-and-one-half months after the passage of Felix.

1. Introduction farther into the central Sargasso Sea than can be ac- quired from the United States east coast. Of the major Strong tropical cyclones are known to depress sea tropical cyclones in the Atlantic basin (excluding the surface temperature up to 6ЊC along their tracks due to Gulf of Mexico), only Hurricane Humberto (22 August± a combination of upwelling, turbulent mixing, and heat 1 September 1995) failed to pass through the region transport (e.g., Stramma et al. 1986; Cornillon et al. observed at the Bermuda Biological Station for Re- 1987). These cool water wakes can be important for the search (BBSR) HRPT site. Estimates of sea surface tem- development of tropical storms because they limit the perature derived from AVHRR thermal infrared data are heat available at the surface (e.g., Emanuel 1986, 1995; here used to estimate the spatial and temporal extent of Ginis 1995). Also, ocean response to tropical cyclones surface ocean modi®cations caused by tropical cyclones has implications for mesoscale oceanographic process- in the central and western Sargasso Sea during 1995. es, in particular mesoscale circulation (e.g., Price et al. 1994; Dickey et al. 1998) and biogeochemical processes (e.g., Malone et al. 1993). 2. Methods The 1995 Atlantic hurricane season produced a near- record number of tropical cyclones. Many of these trop- A TeraScan HRPT ground station (SeaSpace Inc., San ical storms and hurricanes passed within the radio ho- Diego) was used to capture AVHRR data from two daily rizon of an HRPT (high-resolution picture transmission) passes (each) of the NOAA-12 and NOAA-14 satellites. receiving site located in Bermuda (32.375ЊN, 64.7ЊW; Sea surface temperature was computed from each pass Nelson 1996). At this site Advanced Very High Reso- using the multichannel sea surface temperature lution Radiometer (AVHRR) high-resolution (1.1 km at (MCSST) algorithm (McClain et al. 1985). In this ap- nadir) data can be collected out to 50ЊW, which is much proach, the data were subjected to various tests to identify cloudy areas. Brightness temperature in each AVHRR infrared channel was computed from raw counts according to instrument-speci®c calibrations *Bermuda Biological Station for Research Contribution Number (Kidwell 1991). Three separate tests were then used to 1462. identify clouds. Each pixel in the image was examined as the center pixel in a 3 ϫ 3 array of pixels. First, the difference in AVHRR channel 4 (10.7-␮m center wave- Corresponding author address: Dr. Norman B. Nelson, Bermuda Biological Station for Research, Inc., Ferry Reach, St. George's, Ber- length) brightness temperature between the warmest and muda. coolest elements in the array was computed. If this dif- E-mail: [email protected] ference was greater than 0.3ЊC, the center pixel was

᭧ 1998 American Meteorological Society

Unauthenticated | Downloaded 09/26/21 01:58 PM UTC MAY 1998 NOTES AND CORRESPONDENCE 1365 marked as contaminated by subpixel clouds. In daytime Center (Lawrence 1996; May®eld and Beven 1996; images, pixels with near-infrared albedo (AVHRR chan- Rappaport 1996a,b; Jarvinen et al. 1984). These esti- nel 2) greater than 3% were excluded as being mostly mates relied primarily on satellite data, but surface ob- covered with bright clouds or contaminated by sun glint. servations and aircraft reconnaissance observations This test cannot be used at night, so a test was used that were also incorporated. Surface wind ®elds in the vi- compared the average brightness temperature of cinity of selected hurricanes were estimated from data AVHRR channel 3 (3.7-␮m center wavelength) to the collected on air force reconnaissance missions (P. Black average brightness temperature of AVHRR channel 4. 1996, personal communication). These surface wind If this difference was less than 1.5ЊC, the pixel was ®elds were inspected to estimate the radius of maximum marked cloudy. Finally, for the remaining pixels, sea wind. surface temperature was computed as a linear weighted Sea surface temperature anomalies were judged to be sum of the brightness temperatures of the infrared chan- signi®cant if their magnitude was greater than Ϫ1ЊC. nels, using empirically derived coef®cients speci®c to This criterion was chosen to try to exclude natural (i.e., each AVHRR instrument (e.g., Kidwell 1991). These not storm-caused) SST anomalies, and to exclude pos- methods have been shown to result in sea surface tem- sible variations in retrieved sea surface temperature due perature estimates with an rms error of approximately to the limited accuracy of the MCSST algorithm. Es- 0.7ЊC as compared to concurrently collected data from timates of the surface area covered by SST anomalies moored platforms (McClain et al. 1985). were prepared by summing the surface area of each Cloud-contaminated pixels that remained after the individual pixel with a signi®cant anomaly. Since sig- MCSST cloud-identi®cation tests were occasionally ni®cant cloud cover was present in the 3-day composite found in nighttime images processed in the previously images (40%±90%, on average 75%), estimates of the described manner. The contaminated pixels were mostly area impacted by the storms are conservative underes- discarded by discarding all values below 17ЊC, which timates. Areas with signi®cant SST anomalies that were is below the annual lowest temperature in the north- not associated with a storm track were masked. These western Sargasso Sea (Michaels and Knap 1996). anomalies appeared near 40ЊN, 60ЊW,and were probably Cloud-free sea surface temperature images were then artifacts caused by the changing position of the Gulf gridded using the nearest-neighbor algorithm to an 800- Stream relative to the climatological maps. line-by-1024-pixel Mercator projection (approximately 3.5-km resolution). Composite images were prepared by 3. Results computing the simple mean of four or more valid pixels; where fewer than four pixels were found the mean was Tropical Cyclones Barry, Chantal, Erin, Felix, Iris, not calculated and the pixel was excluded from the im- Jerry, Karen, Luis, Marilyn, and Tanya all passed into age. Three days of data (12 images) were used to make or through the study area (20Њ±40ЊN, 50Њ±82.5ЊW) dur- each composite image. Estimates of SST from com- ing the 1995 hurricane season. Of these, four (Felix, posite images prepared in this manner have been found Iris, Luis, and Marilyn) caused sea surface temperature to have an rms error of 0.5ЊC when compared to ship- anomalies greater than Ϫ1ЊC that lasted longer than 10 board SST measurements (N. Nelson 1996, unpublished days. Hurricane Erin may have caused a slight anomaly data). north of the Bahamas that persisted for less than one Sea surface temperature anomalies were computed by week. Hurricane Tanya also may have caused small sea subtracting monthly sea surface temperature climatol- surface temperature modi®cations while passing out of ogies (1Њϫ1Њ gridded) (Levitus et al. 1994; Levitus the study area in late October, but persistent cloudiness 1982) from composite temperature images. Climatolog- prevented us from resolving these effects with satellite ical SST maps for each 3-day period at the same res- data. Tropical Storms Barry, Chantal, Jerry, and Karen olution as the SST maps were prepared by (a) linear left no noticeable SST anomalies, but any effect of Kar- interpolation in time, and (b) Barnes objective analysis en may have been hidden by persistent clouds. interpolation (Barnes 1964; Koch et al. 1983; Seaman Figure 1 shows the regions where Ϫ1ЊC or greater 1989) in the spatial dimensions. Linear interpolation in anomalies existed subsequent to the passages of hurri- time was chosen because a quadratic ®t to monthly data canes Felix and Iris. Hurricane Felix passed through the at a single point did not accurately represent the data study area between 11 and 22 August 1995, with sus- points. The Barnes algorithm was used with an effective tained wind speeds from 30 to 60 m sϪ1 (May®eld and radius parameter of 1.43. First-guess datasets for the Bevan 1996). The eyewall of Felix passed over the Ber- objective analysis were prepared by using an inverse- muda Testbed Mooring (31Њ44ЈN, 64Њ10ЈW) on 15 Au- distance squared interpolation, using a search radius of gust, and sustained wind speeds greater than 30 m sϪ1 22 pixels. were recorded, along with temperature and current at Hurricane estimated track and wind speed data were varying depths (Dickey et al. 1998, their Fig. 2). The obtained from the preliminary reports of the National largest extent of the region in¯uenced by Felix was at Oceanic and Atmospheric Administration±National least 3.8 ϫ 105 km2 (Fig. 1). The largest temperature Hurricane Center (NOAA±NHC) Tropical Prediction anomaly (approximately Ϫ4ЊC) occurred southwest of

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FIG. 1. Sea surface temperature anomaly ®elds in the Sargasso Sea along the tracks of Hurricanes Felix (11±22 August 1995) and Iris (29 August±4 September 1995). Contours shown are the Ϫ1ЊC sea surface anomaly contours, estimated from sea surface temperature anomaly images. Anomalies were calculated as the difference between 3-day composite AVHRR sea surface temperatures and interpolated World Ocean Atlas 1994 monthly climatological SSTs (Lev- itus et al. 1994). Thick contour: 21 August. Thin contour: 30 August. Dotted lines are the estimated tracks from the NOAA National Hur- ricane Center for each hurricane.

Bermuda, near the mooring location. Felix remained between Bermuda and the Gulf Stream for nearly 6 days, FIG. 3. Time series of SST anomalies, for two regions in the Sar- gasso Sea in¯uenced by hurricanes in 1995. Anomalies were cal- resulting in a large (approximately 300-km diameter) culated as the difference between 3-day composite AVHRR sea sur- temperature anomaly centered around 35ЊN, 75ЊW (Fig. face temperatures and interpolated World Ocean Atlas 1994 monthly 1). Iris crossed Felix's path near 24ЊN, 61ЊW, on 30 climatological SSTs (Levitus et al. 1994). Values are mean of all valid August (Fig. 1). Iris also produced a noticeable tem- pixels within 1Њ squares centered on (a) 25ЊN, 60ЊW (in¯uenced by perature depression in this region extending up to 30ЊN, Felix and Iris, Fig. 1). Vertical dashed lines show the time of passage of Felix (13 August) and Iris (31 August). (b) 35ЊN, 70ЊW (in¯uenced with temperature anomalies near Ϫ4ЊC (Fig. 1). The by Felix, Luis, and Marilyn, Fig. 2). Dashed lines show the times of passage of Felix (15 August), Luis (10 September), and Marilyn (20 September).

region near 27.5ЊN, 60ЊW in¯uenced by both Iris and Felix stayed colder than average until 15 September. Figure 2 shows the additional areas in¯uenced by Hurricanes Luis and Marilyn, as well as residual anomalies from the passage of Felix and Iris. Luis was an intense hurricane, with 46 m sϪ1 sustained winds during its passage west of Bermuda (Lawrence 1996). Marilyn was less intense while passing Bermuda. Luis and Mar- ilyn followed very similar tracks through the western Sargasso Sea (Fig. 2) and crossed the path of Hurricane Felix west of Bermuda. This region stayed cooler than normal for approximately 30 days, probably due to the multiple passage of the hurricanes. Sea surface temperature anomaly time series for two areas in¯uenced by multiple hurricanes are shown in FIG. 2. Sea surface temperature anomaly ®elds in the Sargasso Sea Fig. 3. Figure 3a shows the SST anomaly time series (as Fig. 1), showing the residual anomalies caused by Hurricanes Felix and Iris (Fig. 1) and the additional areas in¯uenced by Hurri- for 25ЊN, 60ЊW, near the point where the paths of Felix canes Luis and Marilyn. Thick line: 14 September. Thin line: 23 and Iris crossed. Felix produced an approximate Ϫ1.5ЊC September. temperature anomaly, which appeared to be enhanced

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TABLE 1. Location and magnitude of maximum observed SST anomalies for three 1995 hurricanes (excluding areas where successive hurricanes passed) and related variables. Observed SST anomalies are the mean of four or more satellite observations taken over days after the passage of the hurricane and do not re¯ect ocean conditions during the hurricanes' passage. ``Width of wake'' refers to the distance between Ϫ1ЊC sea surface temperature anomaly contours along a line perpendicular to the track (Fig. 1, Fig. 2). Felix Iris Luis Date of passage 14 Aug 1995 1 Sep 1995 08 Sep 1995 Central date of composite 18 Aug 1995 5 Sep 1995 11 Sep 1995 Location 31ЊN, 63.3ЊW 25.3ЊN, 58.8ЊW 27.3ЊN, 69.1ЊW SST anomaly (ЊC) Ϫ3.8 Ϫ4.1 Ϫ2.9 Width of wake (km) 440 232 325 Sustained wind (m sϪ1) 56 48.5 59 Radius of max wind (km) 165 n/a 96 Forward speed (m sϪ1) 7 0.3 6 by Iris's passage, to a Ϫ2ЊC anomaly, which persisted hurricane track because of interaction between the hur- until 15 September (Fig. 3a). A similar pattern appeared ricane's forward motion and the anticlockwise rotation in the temperature anomaly time series (Fig. 3b) near of the wind stress (Stramma et al. 1986; Cornillon et 35ЊN, 60ЊW, where Felix, Luis, and Marilyn passed al. 1987; Price et al. 1994). The satellite observations (Figs. 1 and 2). After Felix's passage a trend toward presented here are retrospective means, and thus are return to normal SST appears to have been interrupted underestimates of the magnitude of the SST anomaly. by the passages of Luis and Marilyn, leading to a Ϫ2Њ Nevertheless some qualitative comparisons are possible. to Ϫ2.5ЊC anomaly, which lasted into November. In all Table 1 provides a summary of the maximum ob- observed hurricanes the greatest width and magnitude served SST anomaly near the track of each hurricane of SST anomaly was found to the right of the track (excluding areas where hurricane wakes intersected, (Figs. 1 and 2), as has been observed in prior studies which excluded the wake of Hurricane Marilyn), and (e.g., Stramma et al. 1986; Cornillon et al. 1987). the presumed controlling variables. The slowest moving hurricane (Iris on 1 September) produced the largest SST anomaly (Ϫ4.1ЊC). The width of Felix's wake and 4. Discussion the magnitude of the SST anomaly (14 August) were Hurricane-force tropical cyclones passing through the greater than those of Luis (8 September) despite Felix's central and western Sargasso Sea produced signi®cant lower sustained wind speed and greater forward speed depressions of the sea surface temperature (up to Ϫ4ЊC) (Table 1). These differences may have been due to for a time period extending from 16 August to 2 No- Felix's greater radius of maximum wind. vember 1995 in an area northwest of Bermuda, and for The two regions with persistent anomalies extending shorter periods over a much wider area. Tropical cy- long beyond the time of hurricane passage were near clones that passed through the study area (20Њ±40ЊN, places where multiple hurricane tracks intersected. Iris 50Њ±82.5ЊW) with sustained wind speeds less than 33 crossed the path of Felix near 25ЊN, 60ЊW, 18 days after msϪ1 did not generate sea surface temperature anom- Felix's passage. Luis and Marilyn both crossed Felix's alies greater than Ϫ1ЊC that were visible on timescales path near 32.5ЊN, 67.5ЊW on 8 September and 19 Sep- of greater than 3 days. The area covered by these sea tember, 24 and 35 days after Felix, respectively (Fig. surface modi®cations was greater than 4.8 ϫ 105 km2, 3). Away from these two regions the persistence time or at least 6.5% of the ocean surface of the study area. of surface ocean modi®cations caused by hurricanes The actual area in¯uenced was probably greater than were similar to those observed by Stramma et al. (1986), this estimate, because persistent cloud cover masked from 3 to 16 days. some disturbed areas from infrared satellite sensors. In some cases the sea surface temperature depression Factors controlling the extent of the observed sea caused by the passage of a hurricane appeared to limit surface temperature anomalies were also considered. the intensity of subsequent hurricanes crossing the The magnitude and areal extent of sea surface temper- wake. Iris's estimated central pressure remained near ature anomalies caused by cyclones has been shown by 971 mb while crossing Felix's wake, but declined to its theoretical, model, and observational results to be a minimum pressure of 965 mb after exiting the area in- function of the wind statistics, radius of maximum wind, ¯uenced by Felix (central pressure estimates from NHC and forward speed of the storm center (e.g., Stramma preliminary reports). Luis's estimated central pressure et al. 1986; Cornillon et al. 1987; Price et al. 1994; increased from 941 to 945 mb while crossing Felix's Ginis 1995). Brie¯y, stronger storms with a slower wake, but Marilyn's estimated central pressure dropped translation speed will produce greater SST anomalies, 2 mb (from 976 to 974 mb, then returned to 976) while and broader storms will in¯uence a greater area of the crossing Felix's path. However, at this time Marilyn was sea surface (Price et al. 1994; Ginis 1995). Also, most already moving through waters in¯uenced by Luis (Fig. of the affected area will be found to the right of the 2). These observations are largely consistent with the

Unauthenticated | Downloaded 09/26/21 01:58 PM UTC 1368 MONTHLY WEATHER REVIEW VOLUME 126 concept that reduced sea surface temperature due to pri- sponse to Hurricane Felix as measured by the Bermuda testbed or hurricane passage can in¯uence the development of mooring. Mon. Wea. Rev., 126, 1195±1201. Emanuel, K. A., 1988: An air±sea interaction theory for tropical subsequent hurricanes (e.g., Emanuel 1995; Ginis cyclones. J. Atmos. Sci., 42, 1062±1071. 1995). , 1995: Sensitivity of tropical cyclones to surface exchange co- Results of the present study have shown that passage ef®cients and a revised steady-state model incorporating eye of strong tropical cyclones in 1995 had a measurable dynamics. J. Atmos. Sci., 52, 3969±3976. Ginis, I., 1995: Ocean response to tropical cyclone. Global perspec- and persistent effect on surface ocean properties, and tives on tropical cyclones, WMO Tech. Document 693, 198± the persistence of these temperature anomalies was en- 260. [Available from World Meteorological Organization, Case hanced by the passage of subsequent cyclones. Consis- Postale 2300, CH-1211 Geneva 2, Switzerland.] tent with earlier observations (e.g., Stramma et al. Jarvinnen, B. R., C. J. Neumann, and M. A. S. Davis, 1984: A tropical cyclone data tape for the North Atlantic Basin, 1886±1983: con- Ϫ1 1986), tropical storms (winds less than 33 m s ) did tents, limitations, and uses. NOAA. Tech. Memo. NWS NHC not create similar intense or persistent anomalies. Anal- 22, 21 pp. [Available from NODC, User Services Branch, ysis or forecasting models of tropical cyclone devel- NOAA/NESDIS E/OC21, Washington, DC 20235.] opment may bene®t by explicitly accounting for the Kidwell, K., 1991: NOAA Polar Orbiter Users Guide. NOAA/NES- DIS/NCDC Satellite Data Services Division, 233 pp. effects of sea surface temperature modi®cations caused Koch, S. E., M. DesJardins, and P. J. Kochin, 1983: An interactive by previous passage of large storms. Furthermore, the Barnes objective map analysis scheme for use with satellite and area affected by tropical cyclones in 1995 was suf®cient conventional data. J. Appl. Meteor., 22, 1487±1503. Lawrence, M. B., cited 1996: Preliminary report: Hurricane Luis (27 to have a major impact on ocean biogeochemical pro- August±11 September 1995). National Hurricane Center. [Avail- cesses such as air±sea gas exchange (Bates et al. 1996) able online at http://www.nhc.noaa.gov/luis.html.] and primary productivity (Malone et al. 1993). Levitus, S., 1982: Climatological Atlas of the World Ocean. National Oceanographic and Atmospheric Administration, 173 pp. , T. P. Boyer, and J. Antonov, 1994: World Ocean Atlas 1994. Acknowledgments. This study was supported by the Vol. 4, Temperature, NOAA Atlas NESDIS 4, 129 pp. NASA Ocean Biogeochemistry Program (NASA Malone, T. C., S. E. Pike, and D. J. Conley, 1993: Transient variations NAGW-4637). 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