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 228±408N and 508±82.58W. Ten tropical cyclones passed through this region in 1995, six at hurricane strength (winds greater than 33 m s21). Four hurricanes (Felix, Iris, Luis, and Marilyn) caused signi®cant cooling of the sea surface (up to 48C) along their tracks. The largest surface area impacted by these four hurricanes at any one time was at least 4.8 3 105 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 218C) 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 68C 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 (32.3758N, 64.78W; 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 508W, which is much proach, the data were subjected to various tests to iden- tify 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 3 3 array of pixels. First, the difference in AVHRR channel 4 (10.7-mm 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 0.38C, the center pixel was q 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-mm 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 1.58C, 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 218C. 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- 0.78C 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 178C, 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 408N, 608W,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 (208±408N, 508±82.58W) 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 0.58C when compared to ship- anomalies greater than 218C 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 (18318 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 218C 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 s21 (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 (318449N, 648109W) on 15 Au- distance squared interpolation, using a search radius of gust, and sustained wind speeds greater than 30 m s21 22 pixels.