1. Introduction
The percentage of available upper-ocean energy extracted by a tropical cyclone (QH_util) as a function of inner-core surface enthalpy flux (Hcore) and storm speed (TCspeed). In this illustration, Hcore was initially held constant at 1300 W m−2, while TCspeed varied between 2.5 and 10 m s−1. Then, TCspeed was held constant at 5 m s−1, while Hcore varied between 650 and 2600 W m−2. In all cases hurricane heat potential (aka “heat content”) was 75 kJ cm−1. [Note: Illustration initially published in Cione and Uhlhorn (2003).]
Citation: Monthly Weather Review 143, 3; 10.1175/MWR-D-13-00380.1
This study will utilize in situ observations obtained from 62 hurricanes spanning 33 seasons to analyze how near-surface atmospheric and oceanic variability potentially impact the “26°C minimum threshold” often linked to hurricane maintenance. In addition, the relative roles the ocean and atmosphere each have in determining inner-core Δq, as well as possible meridional linkages associated with Δq variability will be investigated.
2. 26°C: An important threshold for hurricane maintenance
As previously mentioned, correlations between SST at or exceeding 26°C and hurricane formation and maintenance have been noted since the 1940s (e.g., Palmen 1948). For the large majority of cases, this relationship still holds true today. The widely held presumption associated with this finding, as evidenced by numerous recent TC OHC and/or ocean warm core ring studies (Jacob et al. 2000; Mao et al. 2000; Chan et al. 2001; Goni and Trinanes 2003; Jacob and Shay 2003; Lin et al. 2005, 2008, 2011; Mainelli et al. 2008; Yablonsky and Ginis 2013) is that below 26°C, the amount of stored available energy in the upper ocean is insufficient to maintain a storm at hurricane intensity. As will be shown, results from this study do not support that conclusion and instead suggest that the criticality of 26°C is much more closely tied to near-surface atmospheric conditions typically found within the inner core of most hurricanes.
It should be noted that observations used in this study were obtained from the Tropical Cyclone Buoy Database (TCBD). The TCBD uses buoy measurements and other storm-specific information associated with Atlantic, Caribbean, and Gulf of Mexico TCs that occurred between 1975 and 2007. In all, the TCBD includes tens of thousands of near-surface observations in a storm-relative framework for over 62 Atlantic, Gulf of Mexico, and Caribbean hurricanes. Many of the observations included in the TCBD were originally obtained from the National Data Buoy Center’s (NDBC) quality-controlled online archive buoy database (Gilhousen 1988, 1998). (Details related to platform locations, configurations, sensor descriptions, data accuracy, averaging techniques, and quality control can be found at http://www.ndbc.noaa.gov/.) In addition to information directly obtained from NDBC, the TCBD also incorporates non-air–sea specific data such as storm location, intensity, speed, large-scale shear magnitude, and shear direction for each buoy observation in the database (Cione et al. 2013).
Figure 2a depicts inner-core TCBD air–sea measurements for 22 hurricanes observed between 1975 and 2007, south of 32.5°N. To provide additional context, the location of the buoy platforms associated with these 245 individual observations is shown in Fig. 2b. A representative SST analysis for September (1971–2000) is included in this illustration. Observations from Fig. 2a show that SSTs within the hurricane core (<111 km) exceeded 26°C 94% of the time. This figure also depicts no instances whereby SST was observed to be lower than the 10-m air temperature despite the fact that 15 hurricane cases (~6%) exhibited inner-core SSTs at or below 26°C.
(a) SST vs SST − Ta10m (red) and qSST − qa10m (blue) for Tropical Cyclone Buoy Database (TCBD) hurricanes observed between 1975 and 2007, south of 32.5°N. Only observations inside 1° radius (<111 km) of the analyzed storm center were utilized. SST, Ta10m, and qa10m had to be simultaneously available in order to be included in this comparative analysis. The black dashed vertical line represents the 26°C inner-core SST threshold. Statistical information as it relates to specific SST, SST − Ta10m (ΔT), and qSST − qa10m (Δq) thresholds is also provided. SST and SST − Ta10m are measured in °C, while qSST − qa10m is given in g kg−1. (b) Average SST analysis in °C representative for the 30-yr period including 1971–2000. Buoy platform locations for individual observations included in (a) are also shown (blue boxes). This figure is provided courtesy of the NOAA/Environmental Science Research Laboratory/Physical Sciences Division.
Citation: Monthly Weather Review 143, 3; 10.1175/MWR-D-13-00380.1
In Table 1, the 245 individual inner-core hurricane observations shown in Fig. 2a are stratified into latitudinal bands. It should be noted that latitude was chosen as the primary stratifying parameter (instead of SST) because of subsequent analyses designed to investigate the relative contributions of atmospheric and oceanic forcing on a key thermodynamic parameter controlling surface moisture flux as tropical systems move from the deep tropics northward. If results were instead sorted by inner-core SST, the variability of the oceanic component would have been explicitly and artificially constrained.
Latitude-stratified storm sample statistics. Inner core (<111 km from the storm center; no “eye” observations) vs ambient environment (444–555 km from the storm center). Rows 1, 4, 7, etc., represent buoy observations sampled between 15° and 25.84°N. Rows 2, 5, 8, etc., represent buoy observations sampled between 26° and 28.95°N. Rows 3, 6, 9, etc., represent buoy observations sampled between 29.18° and 32.5°N. Boldface font denotes statistically significant mean differences at the 95% level or higher between inner-core and ambient samples.
In addition to SST and 10-m air temperature shown in Table 1, information on inner-core surface dewpoint (10-m Td), specific humidity (qSST, 10-m qa), relative humidity (10-m RH), air–sea contrast (SST − 10-m Ta), specific humidity gradient (qSST − 10-m qa), and buoy latitude are provided. Table 1 also includes information on these same parameters over the same latitudinal bands within the ambient hurricane environment (defined to be 4°–5° latitude from the storm center, or 444–555 km). Using these additional data, estimates for inner-core SST cooling (SSTIC − SSTamb) were included when available. Here, “IC” represents inner-core SSTs (inside 111 km) while “amb” refers to ambient SST values 444–555 km from the storm center. Values notated in rows 1, 4, 7, 10, etc. in Table 1 are associated with buoy observations between 15° and 25.84°N. Similarly, rows 2, 5, 8, 11, etc. represent statistics associated with observations between 26° and 28.95°N while rows 3, 6, 9, 12, etc. depict buoy statistics for hurricanes sampled between 29.18° and 32.5°N. Boldface values in Table 1 denote statistically significant mean differences at the 95% level or higher between inner-core and ambient samples. The “b” footnotes represent statistically significant mean differences at the 95% level or higher between the lowest latitude [i.e., deep tropic (DT)] and midlatitude [i.e., subtropic (ST)] samples, while the use of “a” depicts statistically significant mean differences at or above the 95% level between the DT and the highest latitude [i.e., lower midlatitude (LML)] hurricane sample. The use of “c” highlights statistically significant mean differences at the 95% level or higher between the ST and LML samples.
Using information from Table 1, Fig. 3a illustrates ambient (444–555 km) and inner-core (<111 km) SST (blue), 10-m air temperature (Ta10m, red), and 10-m dewpoint temperature (Td10m, green) conditions for each latitudinal band depicted in Table 1. Figure 3a converts radial distance into dimensionless R/Rmax, which is defined as the radius of the observation to the storm center divided by the storm radius of maximum wind. Information on Rmax is included in the TCBD. For this illustration, average inner-core R/Rmax is ~2 while ambient values range between 12 and 14.
(a) SST (blue), 10-m air temperature (Ta10m, red), and 10-m dewpoint temperature (Td10m, green) as a function of radius to the storm center divided by the storm radius of maximum wind (R/Rmax). Both ambient inner-core mean values (with 95% confidence intervals) are illustrated. SST, Td, and Ta ambient-core pairings are shown for observations south of 26°N (solid line), between 26° and 28.9°N (dotted), and between 29° and 32.5°N (dashed). SST, Ta10m, and Td10m are given in °C. (b) qSST − qa10m (light blue) and SST − Ta10m (purple) as a function of radius to the storm center divided by the storm radius of maximum wind (R/Rmax). Both ambient inner-core mean values (with corresponding 95% confidence intervals) are illustrated. qSST − qa10m (Δq) and SST − Ta10m (ΔT) ambient-core pairings are shown for observations south of 26°N (solid line) and between 29° and 32.5°N (dashed). As noted in the illustration, average core Δq values were found to be 33% greater for the lower-latitude sample. Here Δq is measured in g kg−1 while the units for ΔT are in °C.
Citation: Monthly Weather Review 143, 3; 10.1175/MWR-D-13-00380.1
3. The meridional variability of near-surface temperature and moisture differences in hurricanes
a. Radial structure of SST, surface moisture, and Δq
As previously mentioned, inner-core Td10m exhibits very little variability as a function of latitude for the storms sampled in this study (see Table 1; Fig. 3a). As noted by the 95% confidence intervals shown in Fig. 3a, statistically significant differences between average Td10m for DT, ST, and LML systems were not observed within the hurricane inner core. In addition, Fig. 3a and Table 1 also illustrate that statistically significant differences in mean Td10m were not found between ST and DT ambient samples or between ambient and core ST and DT samples, respectively. These results, which were not found when ambient SST and Ta10m samples were compared with inner-core mean conditions, suggest that on average, near-surface moistening that typically occurs during inflow (e.g., ocean evaporation, sea spray interactions, precipitation) is likely to be largely counterbalanced by drying processes associated with convective downdrafts outside the hurricane inner-core environment. These results are consistent with earlier findings (e.g., Barnes et al. 1983; Powell 1990; Cione et al. 2000, 2013).
In contrast, low-latitude SST near the storm center was found to be statistically significantly greater than inner-core SST observed for higher-latitude hurricanes. This finding is not surprising since most storms tend to experience (relatively) cooler upper ocean conditions as they travel northward (e.g., Palmen 1948; Price 1981; Lloyd and Vecchi 2011). Nevertheless, the results found in Table 1 and Fig. 3a are significant since they help quantify the impact latitude can have on inner-core SST (SSTIC). It is also worth noting that average ambient SSTs (SSTamb) were not found to vary appreciably as a function of latitude. In fact, unlike inner-core SSTs, statistically significant differences between average ambient SST samples were not found. These results suggest that ocean cooling (i.e., SSTIC − SSTamb) resulting from wind-driven turbulent mixing is likely to exhibit a strong latitudinal dependence, which is consistent with the notion that shallower mixed layers and stronger stratification is associated with higher-latitude systems.
Using values presented in Table 1, Fig. 3b depicts Δq (and ΔT) for the lowest and highest latitude hurricane samples. This figure illustrates how the magnitude of Δq decreases as a function of radial distance to the storm. This trend, which is especially noticeable for higher-latitude storms, is due to the combined effects of SST cooling (and hence reductions in qSST) as well as a radially constant (or increasing) qa10m term. The statistically significant differences (33%) in inner-core Δq depicted in Fig. 3b are largely a result of cooler (warmer) ocean conditions found within the inner core for higher- (lower-) latitude storm systems. All other factors being equal, these findings suggest that the average magnitude of surface moisture flux is approximately ⅓ greater for DT hurricanes (<26°N) relative to conditions found for LML storms (29°–32.5°N).
b. Radial structure of surface air temperature and ΔT
While Δq is the dominant thermodynamic parameter that impacts the magnitude of surface enthalpy fluxes in hurricanes, it is also of interest to examine the behavior of ΔT within the tropical cyclone environment. From Table 1 and Fig. 3a, we see that comparisons between ambient and inner-core Ta10m values generally resemble the ambient-to-core cooling patterns observed for SST. However, depending on storm latitude, the average ambient-to-inner-core differences in Ta10m are 1.5–6 times greater than SSTIC − SSTamb values shown in Table 1. As a result of this factor, ambient-to-core ΔT values increase as observations approach the storm center (see Fig. 3b). In addition, and unlike statistically significant results found for inner-core Δq, average inner-core ΔT for low- and high-latitude storm samples were not found to be appreciably different due to compensating cooling trends observed for both SST and Ta10m. It is also worth noting that differences between ambient and inner-core ΔT increase as a function of storm latitude, while the opposite trend is found for Δq samples used in this study. In addition, results highlighted in Table 1 show that ambient-to-core mean RH differences are statistically significant for each of the three latitude-stratified storm samples. However, ambient-to-core mean Td10m and qa10m differences were observed to be negligible for deep-tropic and subtropic hurricanes. This finding highlights the danger of using RH to ascertain (absolute) moisture content given the parameter’s dependence on temperature. These findings, in addition to results illustrated in Fig. 3a, should provide value for future efforts attempting to evaluate the physical performance of regional- and global-scale forecast models.
4. Thermodynamic controls on inner-core surface latent heat flux: Ocean versus atmosphere
The qSST − qa10m or “Δq” term illustrated in (2) represents the primary thermodynamic parameter controlling surface latent heat flux (LH). In this section, we investigate how observations of qSST and qa10m near the storm center individually correlate with Δq as a function of storm latitude. A primary objective is to quantify the relative roles the atmosphere (qa10m) and ocean (qSST) each have on Δq variability within the high wind hurricane inner-core environment.
Figure 4a depicts qSST (blue) and qa10m (green) as a function of Δq for all LML hurricane events between 29° and 32.5°N. Here, all TCBD observations illustrated were within 111 km from the analyzed storm center. From this figure we see that the explained variance (r2) for inner-core qSST is slightly greater than that of qa10m (0.345 vs 0.284). Figure 4b is similar to Fig. 4a except that observations shown are valid for DT hurricanes south of 26°N. Here, the explained variance for Δq associated with the near-surface atmosphere (qa10m; 0.570) is far greater than the observed r2 found for the ocean (qSST; 0.084). Figure 4c highlights the relationship found between Δq explained variance and latitude for both qSST (ocean) and for qa10m (atmosphere) for DT (<26°N), ST (26°–28.9°N), and LML (29°–32.5°N) hurricanes. This illustration clearly depicts the reduced (increased) influence inner-core qSST (qa10m) has on the critical Δq term for DT and ST storm samples. At higher latitudes, contributions from oceanic and atmospheric forcing are observed to be similar. An additional factor worth noting in Fig. 4b is the reduced scatter found for both qSST and qa10m (relative to Fig. 4a). This reduction is quantified in Table 1 and illustrated in Fig. 4d. Here, inner-core oceanic and atmospheric air–sea thermodynamic variability is shown to be noticeably reduced for lower-latitude hurricanes. Figure 4d also illustrates that the ratio of atmospheric to oceanic variability dramatically increases with decreasing latitude. This is likely an important factor responsible for the relatively high (low) Δq explained variance found for qa10m (qSST) for the DT hurricane sample (Fig. 4c). These results suggest that for hurricanes south of 29°N, near-surface atmospheric specific humidity is much more likely to influence Δq, and, therefore, the magnitude of surface latent heat flux observed within the hurricane inner-core environment.
(a) Surface specific humidity (qSST, blue) and 10-m specific humidity (qa10m, green) variability as a function of qSST − qa10m (Δq) for Tropical Cyclone Buoy Database (TCBD) hurricanes between 29.0° and 32.5°N. Only qSST and qa10m values within 1° radius (<111 km) of the storm center were included for this analysis. The solid lines represent least squares best-fit estimates for both samples. Information on slope, intercept, and explained variance (r2) are also provided for each curve. Both qSST and qa10m are given in g kg−1. (b) As in (a), but for Tropical Cyclone Buoy Database (TCBD) hurricane observations south of 26°N. (c) The explained variance (r2) of inner-core Δq as a function of latitude for both surface specific humidity (qSST, blue) and 10-m specific humidity (qa10m, green) for deep-tropic (<26°N), subtropic (26°–28.9°N), and lower midlatitude (29°–32.5°N) hurricanes. As in (a) and (b), only qSST and qa10m values within 1° radius (<111 km) of the storm center were included for this analysis. Both qSST and qa10m are given in g kg−1. (d) The variability (standard deviation) of inner-core qSST (blue) and qa10m (green) as a function of latitude. The ratio of atmospheric variability (SD qa10m) divided by oceanic variability (SD qSST) is also shown (dashed black curve). As in (a)–(c), only qSST and qa10m values within 1° radius (<111 km) of the storm center were included for this analysis. SD qSST and SD qa10m are given in g kg−1, while the ratio of atmospheric to oceanic variability is illustrated as a percentage. Percentage values in blue (green) depict average conditions where oceanic (atmospheric) variability is greatest.
Citation: Monthly Weather Review 143, 3; 10.1175/MWR-D-13-00380.1
5. Summary
Analyses from this study reconfirm the decades-old, empirically driven linkage between 26°C and hurricane activity. However, the assumption that SSTs of 26°C or greater are required in order to supply the energy necessary to maintain hurricane intensity is not supported by observations used in this study. As illustrated here [and in Cione and Uhlhorn (2003)] the energy required to maintain a nonstationary hurricane amounts to approximately 2%–8% (depending on storm speed and intensity) of the ocean heat content available to the system. Results from this study also illustrate that hurricanes can (and occasionally do) maintain hurricane intensity even when SSTs are at or below 26°C. This occurs, despite the fact that OHC is less than zero in cases where surface ocean temperatures do not exceed 26°C.
Instead of focusing on absolute SST thresholds and/or incomplete upper-ocean thermal energy storage diagnostic terms, this research utilizes a more direct approach to assess how near-surface thermodynamic conditions affect storm maintenance by monitoring the relative difference between the near-surface dewpoint temperature and the SST within the high wind hurricane environment. When inner-core SST is lower than the atmospheric dewpoint temperature, surface enthalpy flux would, by definition, “shut down” since SST − Ta10m (ΔT) and qSST − qa10m (Δq) would both be negative. Even if other factors remained favorable for hurricane maintenance, extended periods of zero surface enthalpy flux would ultimately end up significantly weakening the system. Observations from this study show that the maximum inner-core surface dewpoint temperature (observed at any latitude for any storm) was 26.5°C. As such, when SSTs exceed 26°C, in the overwhelming majority of cases, both Δq and ΔT would be greater than zero, which in turn would ensure positive surface enthalpy flux conditions within the hurricane inner-core environment. This conclusion regarding the criticality of 26°C runs counter to the widely held OHC assertion that the available thermal energy stored in the upper ocean is insufficient to maintain hurricane intensity when inner-core SSTs fall below 26°C. Observations from this multidecadal hurricane study do not support this contention given the 15 sub-26°C hurricane events that have been documented. Instead, the results presented support the “positive enthalpy flux condition” given the fact that in each of the 245 cases sampled, positive values for ΔT and Δq were observed.
Results show that inner-core SST, on average, ranged between 28.3°C (for storms south of 26°N) and 27.2°C (for systems between 29° and 32.5°N). In contrast, mean inner-core 10-m surface dewpoint temperatures were found to be largely insensitive to changes in latitude, and varied between 24.2° and 24.5°C. As a result of these combined effects, the magnitude of inner-core Δq exhibits a significant latitude signal that is largely driven by inner-core SST cooling. As depicted in earlier illustrations, the net effect is that deep-tropic hurricanes (south of 26°N) on average exhibit Δq values 33% higher relative to systems observed at higher latitudes (29°–32.5°N).
For storm systems observed between 29° and 32.5°N, the explained variance associated with Δq directly attributable to the ocean (qSST) and atmosphere (qa10m) was found to be roughly similar. However, for hurricanes south of 29°N, atmospheric forcing was observed to dominate. Analyses from this study also illustrate that the ratio of atmospheric variability (i.e., std dev qa10m) to oceanic variability (i.e., std dev qSST) increased with decreasing latitude. Combined, these results suggest that for hurricanes south of 29°N, the atmosphere, not the ocean is much more likely to influence the key thermodynamic parameter (Δq) controlling surface moisture flux within the hurricane high wind environment.
Acknowledgments
The author would like to thank University of Colorado Ph.D. candidate, Evan Kalina, for his assistance reviewing and editing the manuscript as well as the external peer reviewers for their thoughtful comments and helpful insights and suggestions.
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