Amazon and Orinoco River Plumes and NBC Rings: Bystanders Or Participants in Hurricane Events?

316 JOURNAL OF CLIMATE VOLUME 20

Amazon and Orinoco River Plumes and NBC Rings: Bystanders or Participants in Hurricane Events?

AMY FFIELD Earth and Space Research, Upper Grandview, New York

(Manuscript received 25 July 2005, in final form 5 May 2006)

ABTRACT

The Amazon and Orinoco River plumes and North Brazil Current (NBC) rings are investigated during the 1 June through 30 November Atlantic hurricane season to identify their impact on upper-ocean temperatures in the region and to draw attention to their potential role in hurricane maintenance and intensification. The analysis uses ocean temperature and salinity stratification data, infrared and microwave satellite-derived sea surface temperature (SST) data, and Atlantic tropical storm and hurricane tracks data. The Amazon–Orinoco River plume spreads into the western equatorial Atlantic Ocean forming an extensive (0°–20°N, 78°–33°W) 10–60-m-thick buoyant surface layer associated with the warmest surface temperatures (up to ϩ3°C) in the region due to the freshwater barrier layer effect. At times the warm Amazon–Orinoco River plume is bisected by cool-surface NBC rings. For the 1960 to 2000 time period, 68% of all category 5 hurricanes passed directly over the historical region of the plume, revealing that most of the most destructive hurricanes may be influenced by ocean–atmosphere interaction with the warm plume just prior to reaching the Caribbean. Statistical analyses of tropical Atlantic SSTs and tropical cyclone wind speeds reveal a significant and unique relationship between warm (cool) SSTs in the Amazon– Orinoco River plume and stronger (weaker) tropical cyclone wind speeds between 35° and 55°W. This implies that warmer (cooler) plume SSTs due to increased (decreased) river discharge may directly contribute to a more (less) vigorous hurricane season.

1. Introduction per-ocean heat content on tropical cyclones continues to be studied from many perspectives (Free et al. 2004; Energy supplied by underlying warm ocean water is Goni et al. 2004; Perrie et al. 2004; Shay et al. 2004; Lin fundamental to the development of deep atmospheric et al. 2003). Underscoring the potential importance of convection of tropical cyclones and at times is possibly the processes controlling ocean surface temperatures a pivotal factor in the maintenance and intensification during a tropical cyclone event, a statistical study of 23 of hurricanes [tropical cyclones that attain wind speeds Atlantic hurricanes correlated the change of the SST greater than 33 m sϪ1 (64 kt or 74 mph)]. Yet, there are from before an event to during an event, to the change many oceanic and atmospheric processes controlling in hurricane intensity (Cione and Uhlhorn 2003). ocean surface temperatures, before and during a tropi- In this paper, two distinct ocean processes are pre- cal cyclone event, that if better understood might help sented from the perspective of how they define the advance our ability to forecast the evolution of tropical early southernmost oceanic conditions experienced by cyclones, or more specifically, the likelihood of hurri- more than half of all Atlantic hurricanes just prior to cane intensification. For example, in the Atlantic reaching the Caribbean Sea: 1) the spreading of fresh- Ocean upper-ocean warm core rings are thought to water discharges from the Orinoco and Amazon Rivers have played a role in hurricane intensification (Goni out into the western equatorial Atlantic Ocean, and 2) and Trinanes 2003; Bosart et al. 2000; Shay et al. 2000). the periodic movement of North Brazil Current (NBC) The impact of sea surface temperature (SST) and up- rings through the river plumes. At the sea surface the freshwater discharges from the Amazon and Orinoco Rivers spread outward, forming extensive low-salinity Corresponding author address: Dr. Amy Ffield, Earth and ϳ ϫ 6 3 Ϫ1 Space Research, 290 Clausland Mountain Road, Upper Grand- plumes. The 0.2 10 m s discharge from the view, NY 10960-4113. Amazon River is the world’s largest and the ϳ0.03 ϫ Ϫ E-mail: [email protected] 106 m3 s 1 discharge from the Orinoco River is the

DOI: 10.1175/JCLI3985.1

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JCLI3985 15 JANUARY 2007 F F I E L D 317 world’s third largest (Dagg et al. 2004; Perry et al. ity to penetrate clouds, whereas the more traditional 1996). Pailler et al. (1999) map the freshwater barrier infrared-based satellite SST measurements are blocked layer of the Amazon and Orinoco River plumes that by cloud cover. In general the 0.25° ϫ 0.25° gridded can trap heat in the surface layer (0–30 m) of the ocean, TRMM daily dataset has at least one measurement resulting in warm SST anomalies, especially in August listed every 3 days at each grid point, and often there through October, and they discuss associated ocean– are two measurements listed per day at each grid point, atmosphere exchange climate implications. In the same although the true resolution is actually less than this. region, Ffield (2005) describes the SST signatures of Satellite sampling limitations of the TRMM data are anticyclonic NBC rings as they pass northwestward primarily associated with the aliasing of the diurnal so- through the Amazon–Orinoco River plume with rela- lar heating cycle. While there is typically an SST mea- tively cool SSTs during July through December in com- surement listed at least once every 3 days in the TRMM parison to the warm SSTs associated with the plume. dataset at each grid point, it actually takes about 46.7 The Amazon–Orinoco River plume and NBC rings days to repeat an SST measurement at approximately are assessed in this paper specifically during the 1 June the same time of day due to the orbit of the satellite. through 30 November Atlantic hurricane season, to Therefore, the diurnal SST cycle is manifested within identify their impact on the upper-ocean temperatures the SST time series as a 46.7-day signal. In this region in the region, as well as to determine their proximity to the highest aliasing (and therefore the largest diurnal passing hurricanes. The goal of this study is to provide cycles) is contained along the coast (Ffield 2005). the foundation for future oceanic boundary layer hur- Infrared satellite–derived SST measurements are ricane modeling studies, as well as for more systematic from the 7-day averaged Reynolds optimal interpola- observational studies. Ultimately, the goal is to deter- tion SST dataset beginning in November 1981 and mine if the Amazon–Orinoco River plume and NBC provided by the National Oceanic and Atmospheric rings are active participants, rather than passive by- Administration–Cooperative Institute for Research in standers, to the evolution of hurricanes, either by con- Environmental Sciences (NOAA–CIRES) Climate Di- tributing to their maintenance or by impacting changes agnostics Center (Boulder, Colorado; www.cdc.noaa. in their intensity. gov). In general, in situ “SSTs” may be cooler than the satellite-derived SSTs, as a result of the in situ data 2. Data incorporating the top meter or so of the ocean, whereas Historical in-situ ocean temperature and salinity pro- the satellite data measures the top centimeter or so of file data are provided by the National Oceanographic the ocean. Data Center (NODC, released in 2002). “Atlantic Tropical Storm and Hurricane Tracks” data are from the National Hurricane Center (http:// 3. Background and analysis www.nhc.noaa.gov/). The dataset includes tropical a. Amazon and Orinoco River plumes storms [tropical cyclones that attain wind speeds greater than 17 m sϪ1 (34 kt or 39 mph)] and hurricanes The freshwater discharges from the Amazon and [as noted above, tropical cyclones that attain wind Orinoco Rivers spread outward into the western equa- speeds greater than 33 m sϪ1 (64 kt or 74 mph)], but torial Atlantic Ocean while continually mixing with sur- does not include tropical depression data [tropical rounding salty ocean surface water. The geographical cyclones with wind speeds less than 17 m sϪ1 (34 kt or distribution of the low-salinity signatures of the Ama- 39 mph)]. zon and Orinoco River plumes can be revealed with Microwave satellite–derived SST measurements are historical in situ surface salinity data. The low salinity from the Tropical Rainfall Measuring Mission from the Amazon River freshwater plume flows to the (TRMM) Microwave Imager (TMI) beginning in early northwest along the coast (Ou 1989) from January to December 1997. TMI data are produced by Remote June (Ffield 2005) until it reaches the eastern edge of Sensing Systems and sponsored by the National Aero- the Guiana Plateau (52°W), where it spreads broadly nautics and Space Administration (NASA) Earth Sci- northward. From July to December (Ffield 2005) the ence Research, Education and Applications Solution Amazon River freshwater signal is also observed to Network (REASoN) Project (data are available online flow to the northwest along the coast, but then at the at www.remss.com). The advantage of the TRMM Mi- eastern edge of the Guiana Plateau it extends north- crowave Imager (Wentz and Meissner 2000) is its abil- ward, and then both westward (merging with the Ori-

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FIG. 1. The June through November hurricane season: (a) average surface salinity highlighting the Amazon–Orinoco River plume signature, (b) surface temperature for only profiles with surface salinities less than 33.5 to isolate the temperature signal of the Amazon–Orinoco River plume, (c) average surface temperature, and (d) surface temperature extremes with only temperatures greater than 29°C (primarily located in the plume region) and less than 26°C (found in the NBC and the northeastern quadrant) plotted. noco River plume) and eastward (Lentz and Lime- tropical convection (Wang and Enfield 2001). The an- burner 1995; Lentz 1995). nual cycle of tropical Western Hemisphere warm pool The spreading of the Amazon and Orinoco River intensity and areal extent is associated with the annual plumes into the western equatorial Atlantic Ocean is development of tropical storms and hurricanes (Wang the topic of numerous papers (e.g., Gonzalez-Silvera et and Enfield 2001). al. 2004; Astor et al. 2003; Hellweger and Gordon 2002; Kelly et al. 2000; Signorini et al. 1999; Muller-Karger et 1) SALINITY SIGNATURE OF THE PLUME al. 1988), and the plumes are pertinent to a host of chemistry (Koertzinger 2003; Cutter et al. 2001; Ternon Coinciding with the Atlantic hurricane season, the et al. 2000), biology (Carpenter et al. 2004), fisheries plumes from both the Amazon and Orinoco Rivers (de Moura et al. 2001), and geology (Warne et al. 2002) combine and spread to their average maximum histori- studies. The SST in the region is linked to Amazon cal extent during 1 June through 30 November, as re- Basin rainfall (Fu et al. 2004; Labat et al. 2004; Ron- vealed by the 35.4 salinity contour of the averaged his- chail et al. 2002; Wang and Fu 2002; Fu et al. 2001; torical surface salinity data (Fig. 1a) reaching north- Liebmann and Marengo 2001) and the intertropical ward (22°N, 58°W), westward (15°N, 77°W), and convergence zone (Chang et al. 2000). In addition, the eastward (7°N, 34°W). The average surface salinity in- plumes spread into the region classified as the tropical side this 35.4 salinity contour is less than 35.4 and is Western Hemisphere warm pool, defined by SSTs freshest closest to the river outlets. The 35.4 salinity greater than 28.5°C, and associated with increased contour is subsequently used in this study to define the

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Fig 1 live 4/C 15 JANUARY 2007 F F I E L D 319 historical average extent of the Amazon–Orinoco River anomalous salinity signature, in this case 35.4 salinity. plume during hurricane season. Or considered in a different way, 0.23 Sv [1 Sverdrup The Amazon–Orinoco River plume is farthest east- (Sv) ϵ 1 ϫ 106 m3 sϪ1] of Amazon and Orinoco fresh- ward simultaneous with the height of the hurricane sea- water river discharges mixed with 36 salinity ocean son from mid-August through mid-October. The maxi- water can “produce” 13.6 Sv “flow” of 35.4 salinity mum monthly Amazon River discharge is in May and plume water, a volume transport comparable to the June (Hellweger and Gordon 2002). The maximum upper 50-m-layer flows of the ocean’s major western monthly Orinoco River discharge is in August (two to boundary currents. three months after the Amazon; note that the 2) TEMPERATURE SIGNATURE OF THE PLUME Orinoco’s maximum flow is less than the seasonal minimum flow of the Amazon) (Hellweger and Gordon The Amazon and Orinoco River plumes are poten- 2002). In the eastern extension of the plume (7°N, tially important to tropical cyclone evolution, because 34°W), the seasonal cycle of the river-influenced, low- the plume is associated with the warmest temperatures salinity ocean surface waters is from July to October, in the region (Pailler et al. 1999; Ffield 2005). Pailler et with a minimum salinity of 32.8 (an extremely low al. (1999) reveal the dynamics of the temperature asso- open-ocean value) in August, which is two to three ciation by mapping the barrier layer connected with the months after the maximum in the Amazon River dis- freshwater discharge of the Amazon River as it spreads charge and coincides with the height of the hurricane into the western equatorial Atlantic Ocean, inhibiting season; during the rest of the year, November to June, mixing between surface and subsurface waters and the salinity averages 36.0 in that region (a typical open- trapping solar heating within the 3–30-m ocean surface ocean value), indicating that there is no freshwater layer due to the sharp vertical contrast in density. Here, river discharge influence at that location during those the temperature signal associated with the low-salinity months (Ffield 2005). For the westward extension of waters of the plume is isolated by averaging the surface the plume (15°N, 77°W), low salinities are observed temperatures of only stations with surface salinities less entering the Caribbean, and a high inverse correlation than 33.5 (a relatively restrictive cutoff because the av- (R2 ϭ 0.92) is found between the annual cycle of ocean erage plume extent was defined above using the saltier surface salinity near Barbados (approximately located 35.4 salinity contour). This restricts nearly all the re- at 13°N, 59°W) and the 2-month lagged Amazon River maining data to the region within the plume boundaries discharge, which is concluded to predominate over the (as defined by the 35.4 salinity contour), and the core of Orinoco River discharge (Hellweger and Gordon warmest temperatures associated with the plume is re- 2002). The ocean surface salinity near Barbados is mini- vealed by the 28.0°C contour (Fig. 1b) reaching north- mum in June (34.24), the beginning of the Atlantic hur- ward (15°N, 64°W), westward (14°N, 66°W), and east- ricane season, and maximum in January (35.48), two ward (7°N, 34°W) during the June through November months after hurricane season is finished (Hellweger hurricane season months. Inside the 28.0°C tempera- and Gordon 2002). ture contour, average plume surface temperatures The large size of the plume is the result of the tre- greater than 28.5°C are revealed in the eastern exten- mendous freshwater outflows from the Amazon and sion of the plume. The mapping of the river plume Orinoco Rivers that discharge almost 20% of the total barrier layer (Pailler et al. 1999) is similar to this map- annual global freshwater river outflow directly to the ping of the core of warmest temperatures associated surface waters of the western equatorial Atlantic and with the plume. (Note that even higher SSTs are ob- Caribbean. While the Amazon River influence is prob- served in the Caribbean Sea, which is not included in ably most important—its discharge is nearly 10 times this study.) larger than the Orinoco’s discharge—in this study both In comparison to the warm temperatures associated rivers are generally referred to because signatures from with the plume, the ocean surface temperatures to the each river are clearly observed just offshore of their north and to the northeast are much cooler, averaging respective deltas, but it is beyond the scope of this pa- between 25° and 27°C during hurricane season (Fig. per to distinguish between the two signals farther off- 1c). By extracting only the extreme temperature values shore where they appear merged. The combined fresh- during hurricane season from the dataset, the locations water river discharge can affect a considerable ocean of the potentially warmest versus coolest regions are volume, because, as in this case, the regional ocean with exposed: ocean surface temperatures warmer than 29°C salinities generally greater than 36 can be diluted by are found in the plume region versus cooler than 26°C just a tiny portion of zero-salinity river water to form both to the northeast and to the southeast (in the modified plume waters with a dynamically significant NBC), a contrast of more than 3°C (Fig. 1d).

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FIG. 2. The 1960 to 1965 hurricane tracks for tropical cyclones that attained maximum wind speeds of 83 kt or greater (category 2 hurricane or higher). The historical average extent of the Amazon–Orinoco River plume during hurricane season (Fig. 1a) is shown in green. Hurricane tracks passing through (outside) the plume are indicated by thick yellow (thin blue) lines. The tropical cyclones initially travel westward. The southernmost hurricane track is Hurricane Flora, 1963, a category 4 hurricane. The historical average extent of the warmest temperatures of the Amazon–Orinoco River plume during hurricane season (Fig. 1b) is shown in red. Schematic positions of the NBC, the NBC retroflected flow (RTFL), and a recently shed NBC ring (R) are indicated on the map.

3) BUOYANCY SIGNATURE OF THE PLUME sive ϳ33-m-thick plume. Therefore, seasonal and inter- The plume’s pool of buoyancy contributes to its per- annual variability in the temperature, stability, area, sistence to remain as a distinct surface layer and inhibits and depth of the plume are probably important factors the potential for deep-reaching wind mixing due to its to consider, but all are outside the scope of this paper. high stability. The average salinity profile for the east- 4) HISTORICAL TROPICAL CYCLONES TRACKS ern extension of the plume (not shown) reveals the low-salinity signature reaching to 50-m depth in con- Examples of individual hurricane tracks reveal that trast to the much higher salinities in the upper 50 m in hurricanes often pass directly through the northern sec- the neighboring regions. Viewed in terms of density tion of the Amazon–Orinoco River plume. In Fig. 2, the (combining the buoyancy effects of temperature and tracks for tropical cyclones that at some point attain salinity; not shown), even in the far offshore northern maximum wind speeds of 83 kt or greater (i.e., category section of the plume the density in the surface (22.3 2 hurricanes and greater) for the 1960 to 1965 time sigma theta) and in the upper 50 m (22.9 sigma theta) is period are shown, as an example, revealing that 14 of far more buoyant (and therefore stable) than the sur- the hurricanes pass over the historical region of the rounding waters (surface and upper 50-m densities all plume (yellow lines) and 12 do not (thin blue lines). In 23.7 sigma theta and denser). During hurricane season fact, as hurricane strength increases, an increasing per- the plume varies from 10- to 60-m depth, with an aver- centage of hurricanes pass through the plume region: age of 33 m, defined by restricting salinities to less than for the 1960 to 2000 time period 68% of the 19 hurri- 35.4 at the sea surface and less than 35.8 at depth; the canes that attained maximum wind speeds greater than average minimum (maximum) plume depths are 10 to 135 kt (i.e., category 5 hurricanes) passed directly over 20 m (20 to 60 m) deep. However, it is likely that the the historical region of the plume, whereas only 32% plume is patchy at times, both in ocean–atmosphere never passed over the plume (Table 1). This reveals area contact and in depth, varying from thin patches of that most of the most destructive hurricanes may be insignificant ϳ2- to 5-m slicks, to a continuous, exten- directly influenced by ocean–atmosphere interaction

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TABLE 1. The distribution of 1960–2000 hurricanes by location. TABLE 2. The distribution of 1960–2000 hurricanes by category With increasing category (hurricane strength), an increasing (de- within each location. With increasing category (hurricane creasing) percentage of hurricanes pass through (outside) the strength), the percentage of hurricanes passing through (outside) plume region. For example, for category 5 hurricanes, 68% passed the plume region remains relatively similar (decreases). For ex- through the plume region, while only 32% passed outside the ample, for hurricanes passing through the plume region, 16% are plume region. category 5 and relatively similarly, 22% are category 1, whereas for hurricanes passing outside the plume region, only 4% are Hurricanes Through plume Outside plume All hurricanes category 5 and significantly more, 54%, are category 1. 1960–2000 No. No./total No. No./total Total Hurricanes Through plume Outside plume All hurricanes Category 1 17 17% 84 83% 101 1960–2000 No. No./total No. No./total No. No./total Category 2 13 29% 32 71% 45 Category 3 18 45% 22 55% 40 Category 1 17 22% 84 54% 101 43% Category 4 18 60% 12 40% 30 Category 2 13 16% 32 21% 45 19% Category 5 13 68% 6 32% 19 Category 3 18 23% 22 14% 40 17% Category 4 18 23% 12 8% 30 13% Category 5 13 16% 6 4% 19 8% with the plume just prior to reaching the Caribbean. Total 79 156 235 Tallied within subregions, of all 1960 to 2000 hurricanes, 16% of those passing over the warm plume ulti- much warmer ocean water when they reach the Ama- mately attain category 5 strength, whereas only 4% of zon–Orinoco River plume. Warm SSTs in the northern- those passing outside of the plume ultimately attain most reach of the plume (22°N, 58°W), north of the category 5 strength (Table 2). In contrast, the weakest core of warmest plume temperatures, are also linked to hurricanes (category 1) typically do not pass over the the freshwater discharge of the rivers, but a less restric- plume (only 17%), and instead most (83%) miss the tive salinity cutoff must be used in order to allow for the warm plume temperatures and pass outside (i.e., north) reduced intensity of the freshwater river signal with of the plume (Table 1). Hurricanes are cyclonic and distance from shore due to vertical mixing with subsur- typically have 300- to 1000-km diameters; therefore the face saltier waters as well as horizontal mixing with ocean–atmosphere contact between the hurricanes and regional saltier surface waters (evaporation may also be the plume is much larger than the one-dimensional hur- a factor). Pailler et al. (1999) show this temperature ricane track lines convey. The southernmost hurricanes, association with a temperature–salinity diagram. In a those with tracks south of 14°N, are likely to be most similar fashion, by contrasting stations with surface sa- directly influenced by the warmest core temperatures linities less than 35.8 versus those with surface salinities of the plume. greater than 35.8 during the June through November Using all the hurricane track data reveals that Atlan- hurricane season, the temperature–salinity scatterplot tic tropical cyclones historically pass just north of the for data within the 12° to 20°N latitude band indepen- core of the warmest surface temperatures of the Ama- dently reveals the warm temperature signal and geo- zon–Orinoco River plume and directly over the north- graphical position of the Amazon–Orinoco River ernmost portion of the low-salinity reach of the plume. plume at these more northern latitudes (Fig. 4). In this By binning all tropical storm and hurricane tracks from case, the average surface temperatures first encoun- 1950 through 2003 in one-degree squares, the historical tered by tropical cyclones moving westward between tropical cyclone corridor is revealed along approxi- 12° and 20°N is only 26°C (Fig. 4, blue symbols), but mately 12° to 20°N (Fig. 3). As suggested above by the upon reaching the Amazon–Orinoco River plume, hurricane track examples (Fig. 2), this corridor passes identified by the lower salinities, the average surface through the northernmost portion of the average his- temperature encountered by tropical cyclones is 2°C torical position of the Amazon Plume defined by the warmer, 28°C (Fig. 4, red symbols). Seasonal and inter- 35.4 salinity contour (Fig. 3, green contour) and asso- annual variability in the northward spreading of the ciated with 2°C on average warmer surface tempera- Amazon–Orinoco River plume may also be an impor- tures (Fig. 4, red circle symbols). The corridor also tant factor controlling the ocean temperatures encoun- passes alongside, just north of, the Ͼ28°C maximum tered by hurricanes in the 12° to 20°N latitude band at temperatures associated with the plume for a distance this location. of about 3300 km (Fig. 3, red contour). 6) UPPER-OCEAN TEMPERATURES AND SALINITIES 5) SSTS ENCOUNTERED BY HURRICANES ENCOUNTERED BY HURRICANES As hurricanes move westward across the Atlantic in In outlining the SST conditions during hurricane sea- the 12° to 20°N latitude band they abruptly encounter son, the ocean characteristics below the air–sea bound-

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FIG. 3. The number of 1950 through 2003 “best track” tropical storms and hurricanes per one degree square (smoothed by a 3° ϫ 3° block average). The tropical cyclones initially travel westward. ary also need to be considered, because warm SSTs northwesterly along the coast of Brazil, retroflects near during a tropical cyclone event can be cooled quickly by 6°N, 48°W, and then flows easterly; at the retroflection vertical mixing with colder subsurface water. In general the NBC periodically sheds 300–500-km-diameter anti- the subsurface temperatures mirror the SST trends, cyclonic rings that translate toward the Caribbean Sea such that regions with warmer (cooler) SSTs also have (Goes et al. 2005; Fratantoni and Richardson 2006; Fra- relatively warmer (cooler) subsurface temperatures. tantoni and Glickson 2002; Garraffo et al. 2003; Garzoli In the zonal strip that is frequently traversed by et al. 2003; Goni and Johns 2003; Johns et al. 2003; tropical cyclones (same zonal strip as in Fig. 4), the Wilson et al. 2002; Limeburner et al. 1995). Historical northern plume temperatures are consistently very salinity data during hurricane season highlight the jux- warm during hurricane season throughout the upper taposition between the NBC retroflection, with high ocean, averaging from 28°C at the sea surface down to surface salinities, and the Amazon–Orinoco River 25.5°C at 100-m (ϳdb) depth, with a narrow standard plume, with low surface salinities (35.4 salinity contour; deviation envelope (Fig. 5a, red). These warm tempera- 6°N, 48°W; Fig. 1a). During hurricane season, the NBC tures are again associated with the river-influenced low- rings carry relatively cool ocean surface temperatures salinity signature, which can be easily discerned from from the NBC (down to less than 26°C) directly the ocean surface down to 50 m with salinities less than through the middle of the historical location of the 36 (Fig. 5b, red). In contrast, to the east in the strip warm Amazon–Orinoco River plume (up to more than frequently traversed by tropical cyclones, as well as to 29°C; Fig. 1d). This is opposite to 1) the January to June the north of the strip, the ocean temperatures through- time period when the SSTs of the rings and retroflec- out the upper ocean during hurricane season are much tion are relatively warm in comparison to the regional cooler, averaging between 25° and 26°C at the surface, ocean SSTs (Ffield 2005), and 2) the relatively warm down to around 22°C at 100 m, with a large standard SSTs associated with NBC rings when they are in the deviation envelope (Fig. 5a, blue and green), and the Caribbean Sea (Goni et al. 2004; Goni and Trinanes salinities are all higher than 36 (Fig. 5b, blue and 2003). The NBC rings are on the order of half the di- green). Consequently, even with deep-reaching mixing ameter of Atlantic hurricanes, and they are slow mov- down to 100-m depth into the ocean, hurricanes passing ing, taking a month or so to move through the plume over the plume region will abruptly encounter signifi- region. Therefore, the up to 3°C cooler surface tem- cantly warmer, and more stable, ocean temperatures. peratures of the rings may cover a sufficiently large ocean surface area, for a long enough time, to diminish b. NBC retroflection and rings the warm temperature effect of the much larger and To fully assess the impact of the warm Amazon– more visible pool of warm temperatures associated with Orinoco River plume on hurricanes, or even to just the Amazon–Orinoco River plume. properly interpret satellite SST images in the region, It is of particular importance to note that it takes the role of NBC rings that periodically move through extra effort to identify the cool surface temperatures of the plume region must be considered. The NBC flows NBC rings when using satellite SST data, because at the

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FIG. 4. Scatterplot of the June–November SST vs sea surface salinity for a zonal strip frequently traversed by tropical cyclones. All points with salinities greater than 35.8 are colored blue, and all points with salinities less than 35.8 are colored red. The saltier salinities (blue) are found to the east (map inset) and on average they are 2°C cooler than the fresher salinities, which are located to the west, associated with the Amazon–Orinoco River plume. Tropical cyclones traversing this zonal strip may abruptly encounter considerably warmer plume SSTs as they initially move westward. The historical average extent of the Amazon– Orinoco River plume during hurricane season (Fig. 1a) is shown in green. The historical average extent of the warmest temperatures of the Amazon–Orinoco River plume during hurricane season (Fig. 1b) is contoured in red. Schematic positions of the NBC, the NBC retroflected flow (RTFL), and a recently shed NBC ring (R) are indicated on the map. sea surface warm Amazon–Orinoco River plume wa- spinning ring. At deeper levels the spinning ring carries ters can partially or completely hide their presence. the cooler and saltier South Atlantic water character- Compounding the difficulty, there are only a few de- istic of the NBC. In this situation concurrent satellite tailed oceanographic surveys of NBC rings, especially SST observations will not reveal the presence of the during hurricane season, which could help guide proper cool near-surface NBC ring waters, because they are interpretation of satellite data. In the following ex- hidden just underneath the warm plume. [However, the ample, Amazon–Orinoco River plume water is ob- ring can be successfully monitored by continuous track- served to completely cover the surface of an NBC ring. ing of the SST fields (“TMI-98-7”; Ffield 2005).] There- In December 1998 (Fig. 6a), a ring was surveyed (Wil- fore, the influence of the warm Amazon–Orinoco River son et al. 2002) that probably shed from the NBC ret- plume on hurricanes cannot be solely determined by a roflection around October 1998 during hurricane sea- quick view of satellite SST observations, as relatively son, and therefore may be characteristic of rings during cool NBC rings hiding just underneath the warm plume hurricane season. The ring displays warm (Ͼ28.4°C) can contribute to much faster cooling of SST as hurri- and low-salinity, river-influenced (Ͻ35.4) plume water canes pass by. in the top-most 30–40 m, centered over the core of the Strong winds from the 300–1000-km-diameter cy-

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FIG. 5. Average hurricane season (a) temperature and (b) salinity ocean profiles, with one standard deviation envelopes, for a zonal strip that is frequently traversed by tropical cyclones (same zonal strip as used in Fig. 4), and a region to the north (colored green). Within the zonal strip, all profiles with surface salinities greater than 35.8 are colored blue, and all profiles with surface salinities less than 35.8 are colored red (river-influenced plume water). The plume waters (red) are significantly warmer throughout the upper 100 db (ϳm) and clearly fresher down to 50 m than the cooler and saltier waters to the east (blue) and to the north (green). Therefore, tropical cyclones can abruptly encounter considerably warmer upper-ocean temperatures in the plume, even if the ocean becomes well mixed down to 100 m. On the inset station map, the historical average extent of the Amazon–Orinoco River plume during hurricane season (Fig. 1a) is shown in green, and the historical average extent of the warmest temperatures of the Amazon–Orinoco River plume during hurricane season (Fig. 1b) is contoured in red.

clonic hurricanes might quickly obliterate a thin plume, retroflection around December 1998, just after hurri- exposing several degrees cooler NBC ring water di- cane season ended. The ring reveals high-salinity rectly to the atmosphere midstorm. Possibly there is a (Ͼ36.2) South Atlantic water throughout its upper lay- relationship dependent on the translation speed of hur- ers, with low-salinity (Ͻ35.4) plume waters caught only ricanes with slow and vigorous hurricanes most likely to in the top-most 30 m at the edges of the spinning ring. mix through to the cool temperatures of rings. Alter- The ring carries temperatures around 27°C throughout natively, it is plausible that instead a slow hurricane the upper layers (which are actually warmer than the may be able to intensify by continually drawing energy surrounding waters as this example is not during hur- from a plume that is sufficiently deep and stable enough ricane season). SSTs associated with this ring are easily to shield the hurricane from any effect of cool ring observed in satellite SST observations (“TMI-99-1”; waters hidden underneath. Ffield 2005), in contrast to the previous example, dem- A second example of an NBC ring is shown to point onstrating the variability in the SST signature of NBC. out how Amazon–Orinoco River plume water can also Therefore, while the potentially cooling effect of NBC just partially hide the presence of a ring. In this case the rings on the SSTs during hurricane season in the Ama- plume water is entrained just at a ring’s edges, so con- zon–Orinoco River plume region should be considered, current satellite SST observations should detect most of it is not necessarily an easy task, because a thin layer of the near-surface temperature signal associated with the warm plume water can partially or completely hide the ring. In February 1999 (Fig. 6b), a ring was surveyed potentially more substantive cool near-surface tem- (Wilson et al. 2002) that probably shed from the NBC peratures of a ring.

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FIG. 6. Two contrasting examples of the anticyclonic (clockwise) NBC rings are shown with depth vs distance (km; northern end at 0 km) sections for temperature (°C), salinity, and velocity (cm sϪ1; westward: Ϫ; eastward: ϩ). (a) A thermocline-intensified ring observed in December 1998 that reveals maximum swirl velocities below 100 m, and (b) a surface-intensified ring observed in February 1999 that reveals maximum swirl velocities above 75 m. In (a) warm (Ͼ28.4°C) and low-salinity (Ͻ35.4) Amazon–Orinoco River plume water is revealed in the topmost 30–40 m centered over the core of the spinning ring. In (b), high-salinity (Ͼ36.2) South Atlantic water is observed throughout the ring’s upper layers with low-salinity (Ͻ35.4) plume waters caught only on the edges of the ring in the topmost 30 m. c. Two hurricane case examples sider in future, dedicated studies on the demise, in the first case, and the maintenance/intensification, in the Two hurricane case examples are shown to illustrate second case, of hurricanes passing through the plume the hypothesis of the possible role of the Amazon– region. Orinoco River plume ocean region as a participant in the evolution of hurricanes. Microwave (cloud- 1) HURRICANE JOYCE, 2000 penetrating) satellite-derived SST data are used in jux- taposition with wind-speed coded hurricane tracks. In The Amazon–Orinoco River plume SST conditions a both examples, the warm SSTs of the plume and the few days prior to Hurricane Joyce were not particularly intruding cool SSTs of the NBC retroflection from the extensive or warm for the time of year, and the cool south are evident. However, the contrasting sizes and NBC retroflection had pushed relatively far northwest temperatures of the plume and the presence of an NBC into the plume. In September–October of 2000, Hurri- ring differentiate the two cases, and these factors are cane Joyce became a hurricane after passing west of presented as examples of the possible influences to con- 38°W, attaining an 80-kt wind speed (category 1) at

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relatively cold (Ͻ28.5°C) NBC retroflection had pushed northwestward to 8°N, 50°W, about 220 km south of the future path of the center of the hurricane (in the southern part of the white box in Fig. 7a). From 8 yr of altimetry data, the average northerly position of the NBC retroflection is 6.7°N Ϯ 1.8° (Fonseca et al. 2004). Therefore, the NBC retroflection just prior to Hurricane Joyce is more northerly than average. Hurricanes passing through the western tropical At- lantic Ocean can expose cool NBC rings concealed just underneath the warm surface waters of the Amazon– Orinoco River plume or they can contribute to keeping the cool SSTs of a newly shed NBC ring exposed. The SST image a few days after Hurricane Joyce passed reveals that, between 52° and 48°W, the warm SSTs of the plume were obliterated and a broad meridional swath of cool SSTs was exposed (48°–53°W; Fig. 7b). In addition, there is now, with SSTs less than 28°C, a characteristic circular surface signature of a cool NBC ring isolated within the warm surface waters of the Ama- zon–Orinoco River plume (southern half of white box, Fig. 7b). The cool SST features coincide, whether coin- cidentally or causally, with the hurricane winds drop- ping suddenly from their maximum levels, and subsequently dissipating altogether just to the west. While directly over the swath of cool SSTs, the winds dropped from 70 to 55 kt. The maximum SST contrast between the warm plume (Ͼ29.5°C) and the ring feature (Ͻ27.5°C) is 2°C. The SST difference map, calculated as the difference between the “after” and “before” SST images of Hurricane Joyce, and expanded for the NBC ring generation region, clearly reveals the sharply delineated ring feature with SSTs Ϫ0.9° to Ϫ1.5°C cooler FIG. 7. Two microwave satellite-derived SST composite images than before the hurricane (Fig. 8c). of the Amazon plume region revealing the SST conditions (a) before and (b) after the passing of Hurricane Joyce, a category 1 Hurricane Joyce was a relatively slow moving hurri- hurricane that attained wind speeds of 80 kt in September– cane in comparison to the next case example, and this October 2000. Color-coded circles mark the 6-h positions and may have enabled sufficient time for the cool SSTs of wind speeds of the hurricane. A white outlined box centered at the NBC ring to be exposed to the surface by the hur- 10°N, 50°W highlights the NBC ring generation region and is ricane-strength winds. The westward translation speed shown for easier cross comparison. Five days of data, centered on 23 Sep 2000 in (a) and 2 Oct 2000 in (b), have been averaged to of Hurricane Joyce averaged around 5 degrees of lon- construct the SST images, which are smoothed by a 2° ϫ 2° block gitude per day (Fig. 8a). In addition, the relative cool- average. ness, and therefore probably weak stability, of the plume just prior to the hurricane passing may also have 12.2°N, 42.5°W, but then surprisingly dropped to lower contributed to the potential ease in exposing the NBC wind speeds (Ϫ5 kt over 6 h) after passing west of ring to the atmosphere. It is concluded that the pres- 10.9°N, 46.1°W (Lawrence 2000; color-coded circles, ence of a large, cool NBC ring, suddenly exposed within Fig. 7a). Before the hurricane, the warmest plume SSTs the warm plume waters during Hurricane Joyce, is an were Ͼ29.5°C, and were just to the south of the immi- important factor to consider in studies of the hurri- nent center track path of Hurricane Joyce (Fig. 7a). In cane’s demise. the region bounded by 5°–15°N, 55°–35°W, only 40% of 2) HURRICANE IVAN, 2004 the SSTs were warmer than 29.0°C, but 13% were cooler than 28.0°C, quite cool conditions in comparison The Amazon–Orinoco River plume SST conditions a to the next case example. Also before the hurricane, the few days before Hurricane Ivan were quite extensive

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FIG. 8. The westward translation speed time series of (a) relatively slow-moving Hurricane Joyce and (b) relatively fast-moving Hurricane Ivan, calculated as degrees of longitude per day; the averages are indicated by the straight solid lines. (c), (d) The SST(°C) difference maps, calculated as the difference in SST between the “SST after” and “SST before” maps of the corresponding figures (Fig. 7 and Fig. 9). (c) For Hurricane Joyce, the SST difference map reveals the sharply delineated cool NBC ring (Ϫ0.9° to Ϫ1.5°C), and (d) for Hurricane Ivan, the SST difference map reveals unstructured, yet overall cool temperatures (down to Ϫ1.0°C). In (c) and (d), the white outlined boxes centered at 10°N, 50°W highlight the NBC ring generation region and are shown for easier cross comparison. and warm, and the NBC retroflection was also warmer. However in this case the NBC retroflection is signifi- In September 2004, Hurricane Ivan, ultimately a cat- cantly warmer, with its northernmost tip 1.0°C warmer egory 5 hurricane and the strongest hurricane to be (29.5°C) than it was for Hurricane Joyce (28.5°C). SSTs recorded so far south, continually intensified as it cooler than 28.5°C in the NBC retroflection are more passed through the plume attaining a 115-kt wind speed than 660 km away from the future center track of Hur- (category 4) at 10.6°N, 48.5°W, at which point its wind ricane Ivan, outside of its direct ocean–atmosphere speeds dropped (Ϫ2.5 kt over 6 h), briefly decreasing to contact, whereas they were only 220 km away, most 90 kt at 11.3°N, 54.4°W, from whence they began to likely in direct ocean–atmosphere contact of Hurricane increase again (Stewart 2005; Fig. 9a). Before the hur- Joyce. ricane, the warmest SSTs (Ͼ29.5°C) in the core of the Depending on seasonal and interannual variability, plume were directly in the center of the future path of the Amazon–Orinoco River plume may be warm, deep, Hurricane Ivan (Fig. 9a). The size of the region with stable, and extensive enough such that the plume and temperatures Ͼ28.5°C was about twice as large (Fig. its associated warm ocean surface temperatures are sus- 9a) as it had been for Hurricane Joyce (Fig. 7a). In the tained as a hurricane passes. After Hurricane Ivan region bounded by 5°–15°N, 55°–35°W, 78% of the passed, the SSTs in the region indicate that despite the SSTs were warmer than 29.0°C and none were cooler hurricane-force winds, the Amazon–Orinoco River than 28.0°C just before Hurricane Ivan, a much warmer plume retained its size and characteristically warm tem- regime than it had been for Hurricane Joyce. Before peratures, with SSTs greater than 29.5°C (Fig. 9b). The the hurricane, the NBC retroflection is also at about warm plume SST features coincide, whether coinciden- 8°N, 50°W (southern part of the white box, Fig. 9a). tally or causally, with the hurricane winds intensifying

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FIG. 10. (a) The time series of the June–November average tropical cyclone wind speed (“ϩ” symbols) with the time series of the June–November average infrared satellite-derived SST (“᭺” symbols) from 7.5°N, 52.5°W. The wind speed time series is averaged from all the tropical cyclone track data in the 7.9°–25.0°N, 55.0°–35.0°W block. The SST time series is from the location at the northern edge of the Guiana Plateau where the highest cross- correlation coefficient, 0.67, is observed in Fig. 11. (b) The 1-yr block filtered Niño-3 (5°N–5°S, 150°–90°W) SST index plotted to show ENSO variability for comparison.

course of Hurricane Ivan, as there was for Hurricane Joyce. Hurricane Ivan was a relatively fast moving hurricane, and this may have prohibited sufficient time for cool ocean temperatures underneath the warm Ama- zon–Orinoco River plume, if there were any, to ever be exposed to the atmosphere. The westward translation speed of Hurricane Ivan averaged around 7 degrees of longitude per day (Fig. 8b). In addition, the extreme FIG. 9. Two microwave satellite-derived SST composite images warmness, and therefore probably strong stability, of of the Amazon plume region revealing the SST conditions (a) before and (b) after the passing of Hurricane Ivan, a category 5 the plume just prior to the hurricane passing may also hurricane that attained wind speeds of 140 kt in September 2004. have contributed to the persistence of the warm plume Color-coded circles mark the 6-h positions and wind speeds of the SSTs despite strong hurricane-force winds. It is con- hurricane. A white outlined box centered at 10°N, 50°W high- cluded that the presence of a large, warm plume di- lights the NBC ring generation region and is shown for easier rectly under the path of Hurricane Ivan for five days is cross comparison. Five days of data, centered on (a) 1 Sep 2004 and (b) 8 Sep 2004 have been averaged to construct the SST an important factor to consider in future studies on the images, which are smoothed by a 2° ϫ 2° block average. maintenance/intensification of Hurricane Ivan. d. SST and tropical cyclone wind speed statistics over the plume. While directly over the 30°C (Fig. 9a) Creating hurricane season time series with data just core of warmest plume SSTs, the winds increased from from June through November months for each year 50 to 110 kt. The SST difference map, calculated as the reveals the known ENSO variability associated with difference between the after and before SST images of both hurricanes and regional SSTs. All tropical cyclone Hurricane Ivan, and expanded for the NBC ring gen- wind speed values falling within the 7.9°–25.0°N, 55.0°– eration region, reveals unstructured, yet overall cool 35.0°W block are averaged together for each June temperatures with SSTs down to Ϫ1.0°C cooler than through November hurricane season, producing a before the hurricane (Fig. 8d). This leads to the con- single time series with a single average wind speed clusion that there is no evidence of an NBC ring im- value for each year (Fig. 10a, “ϩ” symbols). For tem- pacting ocean–atmosphere interaction during the perature, infrared satellite-derived SSTs are also aver-

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FIG. 11. Map of the cross-correlation coefficient between the 1982–2003 June–November averaged (one value per year) tropical cyclone wind speed time series for the region outlined by the black rectangle and each 1982–2003 June–November averaged (one value per year) infrared satellite-derived SST time series for each one-degree square. The solid black arrow is the average position of the tropical cyclone track positions within the region used to create the wind speed time series (7.9°–25.0°N, 55.0°– 35.0°W; southernmost tropical cyclone track is at 7.9°N; the time series is shown in Fig. 10a; “ϩ” symbols). The historical average extent of the warmest temperatures of the Amazon–Orinoco River plume during hurricane season (Fig. 1b) is contoured in red. The higher cross-correlation coefficients, Ͼ0.5, are almost all located within the historical position of the plume. The highest cross-correlation coefficient, 0.67, is located at 7.5°N, 52.5°W (marked with a white star), in the region where recently shed NBC rings typically pass through. aged for each June through November hurricane sea- correlations (Ͼ0.50) are all generally within the histori- son, producing a time series at each one-degree square cal position of the plume (as defined previously by the with a single average SST value for each year in 1988 average in situ 28.0°C surface temperatures; Fig. 1b), through 2003. A single example of an SST time series and the highest correlations (Ͼ0.60) correspond to the from within the Plume is shown in Fig. 10a (“᭺” sym- two locations associated with the freshest plume waters bols). Both time series reveal the known association of stemming from the Orinoco River and the Amazon hurricanes and regional SSTs with ENSO (Fig. 10b), River mouths (Fig. 1a, two separate 34.7 salinity con- such that, in this rendition, stronger-than-average tropi- tours). In contrast, the correlations are quite low cal cyclone wind speeds and warmer-than-usual SSTs (Ͻ0.25) to the north of the plume even though the wind are observed during La Niña years (e.g., 1988, 1996, and speed data from that region are included in the average 1999) and weaker-than-average wind speeds and wind speed time series (northern half of the large cooler-than-usual SSTs during El Niño (e.g., 1987 and “square” on Fig. 11). The region for the wind speed 1997). time series was chosen to simply encompass the corri- A more general assessment of the association be- dor of historical tropical cyclones. Consequently, the tween tropical cyclone wind speeds and SSTs in the geographic pattern of cross correlations with SST inde- region of the Amazon–Orinoco River plume also iso- pendently, and with remarkably similar boundaries, lates and identifies the plume as the most characteristic highlights the importance of the SST in the region of feature defining the ocean surface conditions in the re- the historical Amazon–Orinoco River plume. In addi- gion. The cross correlations between the hurricane sea- tion, the high correlations in the plume region imply son time series of tropical cyclone wind speed (Fig. 10a) that warm plume SSTs may directly contribute to stron- and SST at each one-degree square are calculated. The ger hurricane wind speeds. (A causal linkage cannot be cross-correlation coefficients (Fig. 11) reveal that high proven with this type of analysis.) La Niña is associated

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Fig 11 live 4/C 330 JOURNAL OF CLIMATE VOLUME 20 with increased rainfall over the nearby continent, which through river discharge is likely to correlate to a warmer (and/or more extensive) plume in the same year. La Niña is also associated with a more vigorous hurricane season. Therefore, warmer (cooler) plume SSTs due to increased (decreased) river discharge during La Niña (El Niño) may directly contribute to a more (less) vigorous hurricane season. The highest cross correlation between the hurricane season time series of tropical cyclone wind speed and SSTs locates the center of the Amazon low-salinity plume water, which also happens to be the within the FIG. 12. The correlation coefficients for the linear regressions core of the northwestward translation pathway of NBC (not shown) of all the lagged sets of 52 pairs of values of the rings just after they shed from the NBC retroflection. average wind speed for each individual tropical storm and hurri- The highest cross-correlation coefficient (0.67) is at cane passing through the 7.9°–25.0°N, 55.0°–35.0°W block for the 7.5°N, 52.5°W (Fig. 11), where the lowest sea surface years 1982 through 2003, and the infrared satellite-derived SST salinities stemming from the Amazon River are ob- from 7.5°N, 52.5°W identified at the same time as each event, and as a function of lagged time, for each event. The largest correla- served as noted before (Fig. 1a, rightmost 34.7 con- tion coefficients are associated with SSTs 14.6 days earlier than tour). The hurricane season SST time series for this wind speeds, possibly associated with the fortnightly tides and location was shown as the SST example in Fig. 10a (“᭺” with SSTs 51.1–54.8 days earlier than the wind speeds, and pos- symbols). The location is at the northern edge of the sibly associated with NBC rings. Guiana Plateau, a prominent feature reaching out from the coastline. The Guiana Plateau is commonly men- gestive of the role of NBC rings, because for a 1.6-yr tioned in connection with the shedding of NBC rings, observation period, the characteristic number of days and this connection is substantiated by all NBC ring between NBC retroflection shedding events was be- drifter tracks for 10 rings clearly passing through the tween 45 and 55 days, with an average rate of 49.4 days location (Fratantoni and Richardson 2006). (Ffield 2005). In addition, a high-resolution general cir- More in-depth analysis of the tropical cyclone wind culation model with variable winds shows that the speeds and SST at the northern edge of the Guiana North Equatorial Counter Current radiates Rossby Plateau (7.5°N, 52.5°W) suggests that 53-day earlier waves with a period of 50 days that reflect at the Bra- SSTs, possibly associated with the shedding of NBC zilian coast, producing NBC rings (Jochum and Malan- rings, are most significantly related to passing tropical otte-Rizzoli 2003). So perhaps because of the similar cyclones farther to the north. In this calculation, the timing the processes are related in some (not obvious) average wind speed for each individual tropical cyclone way to the tropical cyclone wind speeds. Alternatively, (whereas before all the cyclones for a year were aver- observations from a current meter mooring indicate aged together) passing through the corridor of histori- that the NBC rings take about 1 month for the leading cally intensifying tropical cyclones (the 7.9°–25.0°N, and trailing edges to pass across a longitude with an 55.0°–35.0°W block) is determined for the years 1982 average 12.5 km dayϪ1 translation speed (Johns et al. through 2003. For each of the 52 resulting values, the 2003). Therefore, over 53 days a new NBC ring might infrared satellite-derived SST from 7.5°N, 52.5°Wis move 634 km (equivalent to about 6°) northwestward identified at the same time as each tropical cyclone, and into the more direct path of a westward-moving tropical as a function of lagged time, for each tropical cyclone. cyclone, where the ring’s cool surface temperature Linear regressions (not shown) for all the lagged sets of could more directly impact the evolution of the tropical 52 pairs of values are determined. The results (Fig. 12) cyclone on its southern half. However by that time the reveal the most significant linear regression (r ϭ 0.48) SST signal of the NBC ring could be masked by a thin with SSTs that are 51.1 to 54.8 days earlier, such that— layer of warm plume water, possibly explaining why it as with the annually averaged hurricane season time would be the more southern, earlier, and unadulterated series—stronger (weaker) tropical cyclone wind speeds SST that shows the strongest linkage to the tropical are associated with warmer (cooler) SSTs. (The second- cyclone wind speeds. For this reason, simply including most significant linear regression is with SSTs that are the concurrent “skin-layer” SST field into hurricane 14.6 days earlier, suggestive of fortnightly tides with a prediction models might not be sufficient, as a hidden 14.76-day period, possibly linked to tidal currents re- ring with a few degrees cooler temperatures, over an sponding to the Guiana Plateau.) The ϳ53 days is sug- area on order of 71 000 to 196 000 km2 just below the

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Most importantly, a future study should consider the size and exact position of the tropical cyclones, distin- guishing between cases with large (small) diameter tropical cyclones that may potentially have more (less) direct contact with the warm Amazon–Orinoco River plume and any cool-surface NBC rings in their path.

4. Conclusions The freshwater discharges from the Amazon and Orinoco Rivers spread into the western equatorial At- lantic Ocean combining to form an extensive buoyant plume at the ocean surface with the lowest salinities and warmest temperatures in the region. The spreading of the Amazon–Orinoco River plume exhibits a seasonal cycle coinciding with the Atlantic hurricane season (1 June through 30 November) with river- influenced minimum salinities observed farthest eastward in August during the height of the hurricane season (mid-August to mid-October). For the 1960 to FIG. 13. The four-year running Gaussian mean is removed from 2000 time period 68% of all category 5 hurricanes the 7.5°N, 52.5°W infrared satellite-derived SST time series and passed directly over the historical region of the plume, the values from 53 days earlier are paired with average tropical revealing that most of the most destructive hurricanes cyclone track wind speed anomaly values from the 7.9°–25.0°N, 55.0°–35.0°W region. The scatterplot of the anomaly pairs reveals may be influenced by ocean–atmosphere interaction that earlier, warm SSTs at the northern edge of the Guiana Pla- with the plume just prior to reaching the Caribbean. teau are associated with both weak and strong tropical cyclone Average ocean surface temperatures first encountered wind speeds, but earlier, cool SSTs—possibly from recently shed by tropical cyclones moving westward between 12° and NBC rings—are almost always (never) associated with weak 20°N is only 26°C, but upon reaching the northern (strong) average tropical cyclone wind speeds. reaches of the Amazon–Orinoco River plume, identified by its river-influenced low salinities, the average ocean’s surface, with stabilities governed by both tem- ocean surface temperatures encountered by tropical cy- perature and salinity, might be quickly reexposed with clones are 2°C warmer: 28°C. (These results are from in the passing of the hurricane. situ data measuring the top few meters of the ocean; Removing the interannual variability from the time satellite-derived SST values are generally warmer as series presents an additional perspective to view the they measure the upper-most “skin” of the ocean.) On data enabling a hypothesis on the impact of SSTs in the average, low salinities persist down to 80 m in the ocean Amazon–Orinoco region on the evolution of passing and are associated with warmer temperatures up to tropical cyclones. When the 4-yr Gaussian running ϩ4°C at depth. As warm ocean surface temperatures mean is removed from the average wind speed for each may play a role in hurricane maintenance and intensi- individual tropical cyclone passing through the corridor fication, understanding the processes and variability of historically intensifying tropical cyclones (the 7.9°– controlling the temperatures of the plume may be im- 25.0°N, 55.0°–35.0°W block) and plotted as a function portant. In addition, the buoyant, and therefore stable, of the 4-yr anomaly 7.5°N, 52.5°W SSTs from 53 days 10- to 60-m-thick layer of the plume can mask other earlier, it is revealed that earlier, warm SSTs at the ocean processes and features just below the surface, in northern edge of the Guiana Plateau are associated particular cool (during hurricane season) surface tem- with both weak and strong tropical cyclone wind peratures carried by NBC rings. After shedding from speeds, but earlier, cool SSTs—possibly from recently the NBC retroflection, the 300–500-km-diameter anti- shed NBC rings—are almost always associated with cyclonic (clockwise) NBC rings pass northwestward weak tropical cyclone wind speeds and never associated through the Amazon–Orinoco River plume toward the with strong tropical cyclone wind speeds (Fig. 13). This Caribbean. The limited observations reveal that at result is not robust, as the plot using 51-day earlier SSTs times the cool upper-layer temperatures of the NBC (not shown) does not show as convincing a relationship rings are exposed to the atmosphere at the sea surface (three points contradict). However, it does suggest the and at other times they are hidden just underneath linkages that a more in-depth study might explore. warm plume water. Strong winds from the 300–1000-

Unauthenticated | Downloaded 09/26/21 01:02 PM UTC 332 JOURNAL OF CLIMATE VOLUME 20 km-diameter cyclonic (counterclockwise) hurricanes formation of dissolved and particulate materials on continen- might quickly obliterate a thin plume, exposing several- tal shelves influenced by large rivers: Plume processes. Cont. degrees-cooler NBC ring water to the surface, and po- Shelf Res., 24, 833–858. de Moura, R. L., J. L. de Figueiredo, and I. Sazima, 2001: A new tentially contributing to rapid cooling of SST midstorm. parrotfish (Scaridae) from Brazil, and revalidation of Spari- In this study, the warm temperatures associated with soma amplum (Ranzani, 1842), Sparisoma frondosum (Agas- the low-salinity Amazon–Orinoco River plume and the siz, 1831), Sparisoma axillare (Steindachner, 1878), and relatively cool temperatures associated with NBC rings Scarus trispinosus Valenciennes, 1840. Bull. Mar. Sci., 68, during hurricane season are shown to be in close prox- 505–524. Ffield, A., 2005: North Brazil Current rings viewed by TRMM imity to hurricanes passing over the region. Their prox- Microwave Imager SST and the influence of the Amazon imity draws attention to their possible role as active Plume. Deep-Sea Res. I, 52, 137–160. participants in hurricane maintenance and intensifica- Fonseca, C. A., G. J. Goni, W. E. Johns, and E. J. D. Campos, tion. Any modeling study on the role of the oceanic 2004: Investigation of the North Brazil Current retroflection boundary layer on hurricane maintenance and intensi- and North Equatorial Countercurrent variability. Geophys. Res. Lett., 31, L21304, doi:10.1029/2004GL020054. fication in this region must consider the roles of the Fratantoni, D. M., and D. A. Glickson, 2002: North Brazil Current Amazon–Orinoco River plume and NBC rings on the ring generation and evolution observed with SeaWiFS. J. upper-ocean temperature, depth, and stability (a func- Phys. Oceanogr., 32, 1058–1074. tion of temperature and salinity), as these processes are ——, and P. L. Richardson, 2006: The evolution and demise of probably important and cannot be simply represented North Brazil Current Rings. J. Phys. Oceanogr., 36, 1241– 1264. by including satellite-derived skin SSTs into models. In Free, M., M. Bister, and K. Emanuel, 2004: Potential intensity of fact, the highest cross correlations between SSTs in the tropical cyclones: Comparison of results from radiosonde and tropical Atlantic Ocean and average tropical cyclone reanalysis data. J. Climate, 17, 1722–1727. wind speeds between 35° and 55°W are found only in Fu, R., R. E. Dickinson, M. Chen, and H. Wang, 2001: How do the Amazon–Orinoco River plume region. This implies tropical sea surface temperatures influence the seasonal distribution of precipitation in the equatorial Amazon? J. Cli- a possible causal relationship between warmer plume mate, 14, 4003–4026. temperatures and stronger hurricane wind speeds and ——, M. Chen, and H. Wang, 2004: Can continental rainfall in- may be relevant to the finding that 68% of all category fluence climate variability over tropical Atlantic Ocean 5 hurricanes pass through the plume region. (2004–13AIRSEA). Preprints, 13th Conf. on Interactions of the Sea and Atmosphere, Portland, ME, Amer. Meteor. Soc., Acknowledgments. The author wishes to thank the CD-ROM, 6.4. Garraffo, Z. D., W. E. Johns, E. P. Chassignet, G. J. Goni, 2003: three anonymous reviewers for their insightful com- North Brazil Current rings and transport of southern waters ments and the participants of the NBCR experiment for in a high resolution numerical simulation of the North At- the success of the project. lantic. Interhemispheric Water Exchange in the Atlantic Ocean, G. J. Goni and P. 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