Ocean Temperature Observations in Hurricane Dorian (2019)

Casey R. Densmore aMassachusetts Institute of Technology–Woods Hole Oceanographic Institution Joint Program in Oceanography, Woods Hole, Massachusetts

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Elizabeth R. Sanabia bU.S. Naval Academy, Annapolis, Maryland

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Steven R. Jayne cWoods Hole Oceanographic Institution, Woods Hole, Massachusetts

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Abstract

Upper-ocean temperatures from 72 airborne expendable bathythermographs (AXBTs) collected during U.S. Air Force Hurricane Hunter flights into Hurricane Dorian (2019) over a 72-h period are examined. Three transects collected behind the storm reveal increased cross-track sea surface temperature gradient magnitudes as Dorian intensified to a category-5 hurricane and slowed while approaching the Bahamas. The cold wake, evident in vertical and horizontal cross sections from in situ and satellite sensors, appears as an expected response to tropical cyclone passage. Atypical, however, is the 2°C surface cooling observed over 36 h in a pair of transects ahead of hurricane force winds in Dorian, likely due to changes in the tropical cyclone’s translation speed and direction and/or proximity to the Gulf Stream and continental shelf. Collocated AXBT pairs document a dynamical regime shift from mixing to upwelling as Dorian slows and turns. Relationships between time-integrated wind stress and sea surface temperature indicate track-relative differences varying with storm translation speed and heading changes, paralleling the shift in cooling dynamics.

Significance Statement

We studied in situ and satellite ocean temperature observations beneath Hurricane Dorian (2019) as the storm moved slowly, turned north, and weakened near Grand Bahama Island. We found a distinct change in the spatial distribution of cool upper-ocean temperatures beneath the storm, which indicated a shift in the primary cooling mechanism from ocean mixing to upwelling. This mechanism shift is important because hurricanes depend on warm ocean temperatures for energy, and upwelling roughly doubles the area of cooling beneath the storm. Our results highlight the effects of large heading changes on the upper-ocean response beneath tropical cyclones, especially in tandem with slow translation speeds.

© 2023 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Casey Densmore, densmore@alum.mit.edu

Abstract

Upper-ocean temperatures from 72 airborne expendable bathythermographs (AXBTs) collected during U.S. Air Force Hurricane Hunter flights into Hurricane Dorian (2019) over a 72-h period are examined. Three transects collected behind the storm reveal increased cross-track sea surface temperature gradient magnitudes as Dorian intensified to a category-5 hurricane and slowed while approaching the Bahamas. The cold wake, evident in vertical and horizontal cross sections from in situ and satellite sensors, appears as an expected response to tropical cyclone passage. Atypical, however, is the 2°C surface cooling observed over 36 h in a pair of transects ahead of hurricane force winds in Dorian, likely due to changes in the tropical cyclone’s translation speed and direction and/or proximity to the Gulf Stream and continental shelf. Collocated AXBT pairs document a dynamical regime shift from mixing to upwelling as Dorian slows and turns. Relationships between time-integrated wind stress and sea surface temperature indicate track-relative differences varying with storm translation speed and heading changes, paralleling the shift in cooling dynamics.

Significance Statement

We studied in situ and satellite ocean temperature observations beneath Hurricane Dorian (2019) as the storm moved slowly, turned north, and weakened near Grand Bahama Island. We found a distinct change in the spatial distribution of cool upper-ocean temperatures beneath the storm, which indicated a shift in the primary cooling mechanism from ocean mixing to upwelling. This mechanism shift is important because hurricanes depend on warm ocean temperatures for energy, and upwelling roughly doubles the area of cooling beneath the storm. Our results highlight the effects of large heading changes on the upper-ocean response beneath tropical cyclones, especially in tandem with slow translation speeds.

© 2023 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Casey Densmore, densmore@alum.mit.edu

1. Introduction

Tropical cyclone (TC) formation and intensification relies upon energy supplied via sea surface latent and sensible heat fluxes from the ocean to the atmosphere under the developing storm (Malkus and Riehl 1960; Emanuel 1988). A sea surface temperature (SST) minimum of 26°C has long been identified as a lower bound for TC formation (Palmén 1948; Malkus and Riehl 1960; Gray 1968; Dare and McBride 2011; Cione 2015; McTaggart-Cowan et al. 2015); however, it has also been shown that 2.5°C of SST cooling in the storm environment is sufficient to induce weakening or inhibit further intensification (Emanuel 1999). Increasing surface wind speeds result in shear-induced mixing of the upper ocean beneath the storm, which (depending on the upper-ocean structure) can cool and deepen the ocean mixed layer and thereby reduce the heat transfer across the air–sea interface and weaken the TC (Chang and Anthes 1978; Chen et al. 2017). This negative feedback process, which can reduce TC intensity by more than 50% (Schade and Emanuel 1999; Karnauskas et al. 2021), necessitates understanding the mechanisms by which TC surface wind forcing drives an upper-ocean response to improve TC intensity forecasts (Emanuel 1999; Schade and Emanuel 1999).

The first known investigation into tropical cyclone upper-ocean responses was published by Hidaka and Akiba (1955), who demonstrated that circular cyclonic wind systems should induce upwelling. Nine years later, Jordan (1964) proposed that mixing of deeper, colder water to the surface was a cause of observed SST cooling induced by typhoons in the western Pacific Ocean. Since then, a range of studies have observed upper-ocean responses to TC passage via in situ measurements (e.g., Leipper 1967; McFadden 1967; Wright 1969; Pudov et al. 1978; Federov et al. 1979; Schramm 1979; Pudov 1980; Sanford et al. 2007; Black et al. 2007; D’Asaro et al. 2007; Black and Dickey 2008; Sanabia et al. 2013; Domingues et al. 2015; Sanabia and Jayne 2020) and remote sensors (e.g., Jansen et al. 2010; Mei and Pasquero 2013; Zhang et al. 2019; Karnauskas et al. 2021; Avila-Alonso et al. 2021). The response structure (e.g., magnitude and extent) is sensitive to upper-ocean temperatures prior to storm passage, as well as to TC translation speed, intensity, and size (Lin et al. 2009; Haakman et al. 2019). Moreover, the presence of ocean features, such as warm core eddies in the Gulf of Mexico (Shay et al. 2000), barrier layers from major freshwater sources such as the Amazon–Orinoco River plume (Rudzin et al. 2020), or western boundary currents such as the Kuroshio (He et al. 2022), can further modulate the response. Cold SSTs in the storm wake often persist for more than a month (Ivanov and Pudov 1977; Hart et al. 2007; Haakman et al. 2019) and subsurface effects have been noted to decay twice as slowly as SSTs (Mrvaljevic et al. 2013). Cold wakes are important to TC forecasting as they have been shown to limit TC intensity (Walker et al. 2014) and inhibit development of subsequent storms passing through the same region (Balaguru et al. 2014; Karnauskas et al. 2021).

Two of the primary mechanisms known to force unique upper-ocean responses to TC passage include shear-driven mixing and upwelling (Geisler 1970; Price 1981). In the former, wind-driven ocean surface currents slow and turn with depth, shearing the subsurface currents in the waters below. With sufficiently high surface wind speeds and/or a sufficiently shallow mixed layer, shear-driven mixing entrains cool thermocline water upward into the ocean mixed layer. The subsequent mixing simultaneously cools and warms the water above and below the original mixed layer depth, respectively, as the mixed layer deepens. In a TC, mixing-driven surface cooling occurs in the storm wake, as changes in wind speed and direction during hurricane passage exacerbate shear in the upper-ocean currents. For mixing-dominant ocean responses, cooling is greatest to the right of the storm track in the Northern Hemisphere (left in the Southern Hemisphere), where winds during tropical cyclone passage rotate in the same direction as inertial oscillations and thus resonantly enhance surface currents (Price 1981; Gill 1984; Samson et al. 2009).

In slow-moving TCs, however, persistent cyclonic atmospheric forcing generates divergent ocean surface currents through Ekman transport and subsequently results in upwelling beneath and around the storm center. In this case, the entire column is cooled as water shoals upward to replace that lost in the divergent surface flow. Price (1981) defined a translation speed threshold below which the dominant ocean response mechanism shifts from shear-driven mixing to upwelling as 4 m s−1. This shift is important because upwelling azimuthally expands cooling from the wake (during shear-driven mixing) to the area ahead of the storm and shifts the maximum cooling toward the TC center. This increased areal extent of surface cooling correspondingly reduces enthalpy fluxes across the air–sea interface, which can then reduce TC intensity (Jaimes de la Cruz et al. 2021).

Subsurface structure changes due to TC passage may include mixed layer deepening and cooling (and corresponding upper thermocline warming) during a mixing-dominant response that is greatest right of track (in the Northern Hemisphere, left of track in the Southern Hemisphere) or may feature upwelling-induced shoaling with storm track-centered cooling throughout the water column (Samson et al. 2009). Following TC passage, mixed layer depths oscillate due to internal waves triggered by a sudden mixed layer depth displacement downward (due to mixing) or upward (due to upwelling) and propagate outward (Sanabia and Jayne 2020). Anomalous temperatures and mixed layer depths due to TC upper-ocean responses then decay with an e-folding time scale on the order of days to weeks (Chen et al. 2013). Since mixing and upwelling result in different SST cooling distributions, understanding when and how each of these ocean response mechanisms become dominant is necessary to accurately quantify the energy available to a TC and to accurately forecast TC intensity. Here we present an analysis of in situ and satellite ocean observations collected during Hurricane Dorian (2019) to further explore conditions that facilitate these upper-ocean response mechanisms.

Hurricane Dorian developed in August 2019 over the Atlantic Ocean and made landfall first over St. Croix as a category-1 storm (on the Saffir–Simpson scale), then over the Bahamas in early September as a category-5 hurricane (Figs. 1a,b) and again over Cape Hatteras as a category-1 hurricane before recurving out to sea south of New England (Avila et al. 2020). From 25 August to 6 September 2019, sustained weather reconnaissance missions were conducted by both U.S. Air Force Reserve Command (AFRC) WC-130J and National Oceanic and Atmospheric Administration (NOAA) WP-3D Hurricane Hunter aircraft.

Fig. 1.
Fig. 1.

(a) Hurricane Dorian track and intensity, pre-TC 9-km Optimum Interpolation SSTs [OISSTs (shaded) for 30 Aug 2019; Wentz et al. 2000], ⅓° OSCAR currents (arrows, for the 27 Aug 2019 pentad; ESR 2009), and AXBT deployment locations colored by flight. Gulf Stream currents are capped at 1 m s−1 for readability (marked with red vectors; mean velocity in the included segment of the Gulf Stream is 1.2 m s−1). Black solid and dotted lines indicate best-track hurricane- and tropical storm–force wind swaths, respectively. Colored lines denote locations of wake (solid blue, red, and green; poststorm) and Gulf Stream (dashed blue and magenta; prestorm) transects. (b) Dorian intensity, translation speed, and heading for the period in (a). Vertical lines denote the time Dorian that crossed each wake transect, colored as in (a), and the horizontal line marks a 4 m s−1 translation speed. (c) Storm-relative AXBT transect positions, colored as in (a) and (b). The arrow indicates storm direction.

Citation: Monthly Weather Review 151, 6; 10.1175/MWR-D-22-0271.1

Dorian became a major hurricane east of the Bahamas on 30 August 2019 and subsequently underwent rapid intensification, reaching category-5 strength, slowing, and making landfall on Elbow Cay in the Abacos on 1 September 2019 with sustained surface winds of 160 kt (1 kt ≈ 0.51 m s−1) (Avila et al. 2020; Figs. 1a,b). During this period, Dorian’s translation speed decreased below the 4 m s−1 threshold (Price 1981) for upwelling to become the dominant ocean response mechanism, and remained slower than this threshold for nearly 72 h (Figs. 1a,b). Changes in Dorian’s translation speed and its 90° right turn both modified the surface wind rotation rate, an important factor in eliciting an upper-ocean response from a passing storm. Increasing proximity to the Gulf Stream also likely complicated air–sea interactions beneath Dorian. The upper-ocean temperatures observed beneath Hurricane Dorian and the modulation of the ocean response by these factors are the subject of this study.

Since 2011, personnel from the Training and Research in Oceanic and atmospheric Processes In tropical Cyclones (TROPIC) field program have collected upper-ocean observations in the tropical cyclone (TC) environment during missions with the AFRC 53rd Weather Reconnaissance Squadron (Sanabia et al. 2013). The ocean data are processed and then transmitted in near–real time via the Global Telecommunications System (GTS) and are assimilated into coupled numerical models to improve hurricane forecasts. From 31 August to 3 September 2019, 72 airborne expendable bathythermograph (AXBT; Sessions et al. 1975; Gent 1982) ocean temperature profiles were collected during seven missions in Hurricane Dorian. Those profiles, and corresponding satellite SST and current observations, are analyzed in two parts: first introducing temperature observations collected in Hurricane Dorian, and second examining the dynamical upper-ocean response through the lens of those measurements.

2. Data and methods

The AXBT profiles were acquired using either a Sippican MK21 Oceanographic Data Acquisition System (Sanabia et al. 2013) or an AXBT Realtime Editing System (ARES) Data Acquisition System (Densmore et al. 2021), and all were quality controlled using the ARES Profile Editing System (Densmore et al. 2021). The 72 AXBTs (which have a rated temperature error of 0.56°C; Sessions et al. 1975) analyzed here were deployed within and around the storm environment along the WC-130J flight path. Per standard practice, AXBT observation positions and times were recorded at deployment since the instrument is not GPS enabled. The AXBT spacing and timing in this study were limited by a combination of geographic and operational constraints and occasional equipment failure.

Daily 9-km microwave-infrared Optimum Interpolation SST data (OISST; Wentz et al. 2000) were analyzed to identify prestorm (30 August 2019) SST spatial variability (Fig. 1) and to assess storm-driven SST changes. Because AXBTs provide single-profile observations and thus cannot individually measure temperature changes associated with TC passage, these storm-induced changes were diagnosed in two ways. First, individual AXBT temperature profiles were compared if they were deployed within 10 km of one another and at least 9 h apart. A total of 13 AXBT pairs met these criteria. Second, OISSTs were used to calculate surface cooling for 2-day periods with start dates spanning from 31 August to 5 September 2019, and the maximum 2-day cooling across that time span was examined (with the understanding that limitations in the OISST dataset may prevent the full spatial and/or temporal upper-ocean responses from being captured). A validation of OISST data against AXBT, Argo, and National Data Buoy Center (NDBC) observations in the vicinity of Hurricane Dorian is provided in the appendix. In addition, ⅓° Ocean Surface Current Analyses Real-time (OSCAR; Bonjean and Lagerloef 2002; ESR 2009) current data for the 27 August 2019 pentad were examined to identify prestorm spatial variability.

Stepped Frequency Microwave Radiometer (SFMR; Uhlhorn and Black 2003; Uhlhorn et al. 2007) data from 12 AFRC missions and 6 NOAA missions between 30 August and 5 September 2019 were used to provide surface wind context for AXBT observations and to interpolate a surface wind field for Dorian over time. To accomplish the latter, SFMR 10-m wind data from 74 aircraft passes through the TC center were split into 148 radial transects (from the storm center outward) and then divided by storm quadrant to facilitate interpolation since aircraft inbound and outbound headings during eyewall penetrations often differed. Storm centers associated with each transect were defined by corresponding weather reconnaissance vortex data messages. Surface winds at locations between each radial transect were then interpolated along lines of constant radius as a function of bearing from storm center to generate a sequence of 31 surface wind fields centered around Dorian. Time between successive wind fields averaged 5 h, depending on the temporal distribution of SFMR transects through the TC center, and ranged between 15 min (as a result of concurrent AFRC and NOAA operations) and 16 h (because of an aircraft maintenance issue). Surface wind speeds at AXBT positions were interpolated temporally and spatially from these estimates. Corresponding surface wind directions were approximated by assuming they were tangential to Dorian’s center to calculate wind direction shifts over time. Calculating shifts in wind direction required assuming a radially independent inflow angle, the effects of which should be small (Zhang and Uhlhorn 2012) relative to the bearing shifts associated with TC passage.

Surface wind stress magnitude τ was calculated from the interpolated SFMR wind speeds USFMR assuming a surface air density ρa of 1.13 kg m−3 (the approximate density of saturated air at 30°C and 1000 hPa):
τ=ρaCD(USFMR)2.
The dimensionless drag coefficient CD parameterization used in this study was presented in Eq. (9) of Hwang (2018), where a0 = 8.5 × 10−4, a1 = 9.48 × 10−4, a2 = 0.419, and Uref = 65 m s−1:
CD=a0+a1(U102a22Uref)exp[(U10/Uref)22a22].
This parameterization was selected as it was developed from microwave radiometer observations in TCs. For the purposes of this study, U10 and USFMR are approximated to be equal to one another. Integrated surface wind stresses for 9-km OISST grid points with a closest point of approach of less than 100 km from Dorian were calculated using surface stresses interpolated at 15-min intervals over the period when each grid point was within 250 km of Dorian’s center.

3. Results

a. Ocean observations

Two sets of AXBT transects were examined to better understand the upper-ocean temperature structure in the vicinity of Hurricane Dorian between 31 August and 3 September 2019. In total, 32 of the 72 AXBTs deployed during this period composed five ocean temperature transects. Three poststorm transects were collected across the TC wake as Dorian approached the Bahamas, and two prestorm transects were collected across the Gulf Stream between the Bahamas and the Florida coast (Figs. 1a,c). Although the cold wake transects were still in Dorian’s wind field, the three transects were collected 125, 150, and 169 km from the TC center, and SFMR wind measurements were at most 41, 38, and 48 kt during the transect deployments. Because they were collected behind Dorian’s hurricane-force wind field and after the majority of the surface wind stress was imparted on the ocean surface, they are referred to as poststorm transects in this study. Conversely, the Gulf Stream transects were collected ahead of Dorian’s center, outside the hurricane-force wind field, and are therefore referred to as prestorm transects in this study.

The three wake transects were collected at nearly the same storm-relative position, across the TC track between 150 and 200 km behind NHC best-track (Avila et al. 2020) TC center (Fig. 1c) and about 12 h apart. The first transect includes data from four AXBTs deployed in 24 min beginning at 2349 UTC 31 August 2019; the second includes data from five AXBTs deployed over 33 min starting at 1351 UTC 1 September 2019, and the third includes data from eight AXBTs deployed in 30 min starting at 2343 UTC 1 September 2019. Dorian crossed each transect location about 11 h prior to deployment of the first AXBT (TC crossing times denoted by vertical lines in Fig. 1b) while increasing in intensity from 130 to 150 kt and slowing in translation speed from 4.8 to 4.4 m s−1 (Fig. 1b). Surface wind measurements from aircraft center fix passes in closest temporal proximity to Dorian’s passage over the transect locations reflect the intensification of this major hurricane (Figs. 2d–f).

Fig. 2.
Fig. 2.

The AXBT temperature transects collected across Dorian’s wake beginning at (a) 2349 UTC 31 Aug 2019 at 73.5°W, (b) 1351 UTC 1 Sep 2019 at 75.2°W, and (c) 2343 UTC 1 Sep 2019 at 76.0°W. Dashed black lines mark AXBT profile locations, solid magenta lines indicate the TC track, which crossed the transects at 26.0°, 26.4°, and 26.4°N, respectively; and white contours denote 20°, 24°, and 26°C isotherms. Radial SFMR surface wind profiles from the aircraft pass through the TC center in closest proximity to each transect at (d) 1632 UTC 31 Aug 2019, (e) 0210 UTC 1 Sep 2019, and (f) 1132 UTC 1 Sep 2019. Each SFMR profile was collected within 3 h (and 31 km) of the TC crossing the transect. Negative and positive distances denote left and right of track, respectively. Also shown are AXBT transects collected across the Gulf Stream beginning at (g) 2209 UTC 1 Sep 2019 and (h) 1059 UTC 3 Sep 2019. Gray shading shows bathymetry along each transect. Transect distances are relative to their intercept point (between dashed blue and magenta lines in Fig. 1a).

Citation: Monthly Weather Review 151, 6; 10.1175/MWR-D-22-0271.1

All three poststorm transects exhibit a cold wake structure, including a cooler, deeper mixed layer right of track and an SST minimum near or just right of the storm center (Figs. 2a–c). At the ocean surface, minimum temperatures observed along each transect were 27.0°, 27.6°, and 23.3°C and cross-track SST variability spanned ranges of 2.1°, 1.7°, and 6.0°C, respectively. Below the surface, cross-track variability extended to at least the 20°C isotherm and 300-m depth in every case. Notably, along the third transect the SST observed at the southernmost AXBT was 29.3°C and the SST observed just south of Dorian’s track was 23.3°C, a gradient of 6°C over less than 70 km. Moreover, SSTs in the third transect remained 2.5°C cooler than those at either edge of the transect up to 70 km right of track. The subsurface variability was also particularly noteworthy along the third transect, in which the 26°C isotherm depth varied by 107 m (Fig. 2c; 0 m along track to 107 m right of track).

The analysis of these transects is presented as precisely as the observations allow; however, it is noted that the magnitude and center of these cold wakes could differ from those identified here if the actual SST minimum occurred between AXBT observation locations (particularly for the first two transects, which had lower spatial resolution near the TC track). Additionally, slight variations in storm-relative positions of each transect along with slight reductions in TC translation speed may mean that the upper-ocean response captured in each transect reflects a slightly different phase of poststorm inertial oscillations (inertial periods of 27.3 and 25.5 h correspond to latitudes 26° and 28°N, respectively); therefore, any comparison between transects should allow for this possibility. Last, the assumed AXBT fall rate used to calculate depth includes a ±5% error (Sessions et al. 1975) that varies with each profile due to current differences between drop positions. That said, it is the presence and general structure of the wake that is of primary importance here.

One relevant assumption is that the observed cold wakes are TC induced, rather than a function of pre-TC ocean variability. Three factors lend confidence to this assumption. First, prestorm SST variability across each wake transect is slight: 0.5°, 0.4°, and 0.2°C, respectively (Fig. 1a). Next, prestorm surface currents near the transects are relatively weak (Fig. 1a; averaging 0.05 m s−1 and never exceeding 0.2 m s−1). Third, due to the first two factors it is likely that the prestorm subsurface horizontal temperature gradient across each transect is minimal. A second relevant assumption is that the AXBT data are valid. Confidence in these measurements is high, as each observation was heavily scrutinized following objective quality control procedures that were applied to all profiles (as described in Densmore et al. 2021). The 23.3°C SST observation in the third wake transect was further scrutinized as AXBT profiles collected in strong TC cold wakes can be difficult to distinguish from late starts resulting from equipment malfunction (Densmore et al. 2021). Based on AXBT signal timing and strength, it was determined that this observation is valid and merits inclusion in this analysis.

In addition to these three wake transects collected at different geographic locations but nearly the same storm-relative position, two nearly collocated transects were collected across the Gulf Stream ahead of Dorian’s center over 36 h apart (and therefore at different storm-relative positions; see Fig. 1c). The first transect includes data from nine AXBTs deployed between 2209 and 2233 UTC 1 September 2019 when Dorian was over 200 km away, and the second includes data from six AXBTs deployed between 1059 and 1122 UTC 3 September 2019, less than 100 km ahead (northwest) of Dorian.

In the first pre-TC Gulf Stream transect (Fig. 2g), SSTs are uniformly warm (29°–30.5°C) and subsurface isotherms gradually deepen offshore (e.g., the 26°C isotherm first reaches a maximum depth near 79.4°W, 7 km east of the transect intercept point). Thirty-seven hours later, changes are evident across the second Gulf Stream transect (Fig. 2h). At the surface, temperatures decreased by up to 2°–3°C as Dorian approached. Below the surface, the 24° and 26°C isotherms deepened in the western half of the transect and shallowed in the eastern half. Variations in observation spacing and transect alignment in the dynamically complex region of the Gulf Stream and continental shelf preclude precise temperature intercomparisons; however, mixed layer cooling spans the length of the transects, which presents an unusual situation, that is, SST cooling well ahead of TC passage.

Two TC-related factors are likely responsible for this unusual occurrence. First, after passing the wake transect locations at greater than 4 m s−1, Dorian moved at less than 3 m s−1 throughout the Gulf Stream AXBT deployment period, including about 24 h in which the storm slowed below 1 m s−1. This slowing translation speed suggests that the primary mechanism driving the ocean response had likely shifted from shear-induced mixing at the storm wake (poststorm) transect locations to upwelling at the Gulf Stream (prestorm) transects. The dynamical reasoning behind this shift is that a slow-moving storm extends the duration of forcing at the sea surface and slows the rotation of the local wind at each point, which enables Ekman suction to develop in the upper ocean beneath a surface wind stress field with positive relative vorticity and contribute to surface cooling. Both the longer forcing and slower rotation of the local wind are exacerbated by Dorian’s 90° right turn (between approximately 1200 UTC 2 September and 0600 UTC 3 September), from a westward to a northward heading (Figs. 1b and 3a), which maintained northeasterly winds and positive relative vorticity over the Gulf Stream transect location even as the storm-relative position of the Gulf Stream transects shifted from the right front to the left front quadrant (Fig. 1c). Together, these two TC factors (translation speed less than 3 m s−1 and 90° right turn) resulted in sustained winds up to tropical storm force from a near-constant direction for over 36 h at the Gulf Stream transect locations, which facilitated mixed-layer cooling, suggesting an upwelling response that was likely modulated by strong Gulf Stream currents and complex shelf dynamics.

Fig. 3.
Fig. 3.

(a) Hurricane Dorian translation speed and heading. (b) Estimated surface winds (vectors, with north up) at the 13 AXBT pair locations between the deployment times for each pair. Individual (gray) and mean (red) differences between AXBT pair profiles where wind direction shifts Δθ of (c) greater than (conducive for mixing) and (d) less than (conducive for upwelling) 75° were identified. Also shown are example initial (dashed) and final (solid) AXBT pair profiles indicative of (e) mixing and (f) upwelling. Warming and cooling are denoted by red and blue shading, respectively.

Citation: Monthly Weather Review 151, 6; 10.1175/MWR-D-22-0271.1

b. Dynamical response

The effects of Dorian’s reduced translation speed and 90° right turn are evident in the surface wind speeds and directions (Figs. 3a,b) over the AXBT pair locations (Fig. 4a). The wind direction over AXBT pairs 1–3 shifted as a result of TC passage prior to the storm’s right turn (Figs. 3a,b). At pair 1, the second AXBT profile was collected shortly after this wind shift and before Dorian turned right. However, wind speeds over AXBT pairs 2 and 3 remained elevated and wind directions remained nearly constant as Dorian slowed and turned before the second AXBT in each pair was collected. This extended forcing period reduced the likelihood that near-inertial oscillations had begun at those locations and, along with the reduced surface wind rotation rate, meant that upwelling may have been a contributing factor to changes in the upper-ocean temperature. Temperature profile pairs at all three locations, however, illustrate a mixing-dominated response (e.g., Fig. 3e), with surface cooling and subsurface warming present in every case (Fig. 3c), indicating that the shear-driven mixing due to the wind shift during TC passage had greater impact than any potential subsequent upwelling (Jaimes and Shay 2009). At the 10 other locations, winds remained nearly constant in direction as Dorian slowed and turned. Profiles in 8 of those 10 AXBT pairs exhibited an upwelling response (e.g., Fig. 3f), and in 7 pairs cooling occurred at all depths (Fig. 3d). A large increase in subsurface temperatures was observed in the westernmost pair (pair 4), likely due to high spatial and temporal variability induced by the Gulf Stream (Figs. 1a and 4a). The subsurface warming and mixed layer cooling observed at pair 10 (right of track) indicated a characteristic mixing response, despite only 10° of clockwise surface wind rotation over the location in the 15.3 h between the two AXBT observations. This is likely due to the 73° of clockwise surface wind rotation noted at a nearly adjacent location (pair 3) in the 30.1 h preceding the launch of the first AXBT in pair 10 (Fig. 3b; cf. wind barbs for AXBT pairs 3 and 10).

Fig. 4.
Fig. 4.

(a) Maximum OISST-measured cooling for 2-day periods with start dates spanning from 31 Aug to 5 Sep 2019. Numbers indicate the location of AXBT pairs collected at times identified in the legend, and gray dots denote grid points where maximum cooling was observed between 2 and 4 Sep. Also shown is integrated wind stress vs maximum 2-day OISST cooling, separated for maximum cooling attained during (c) 2– 4 Sep and (b) all other periods. Green, blue, and red markers indicate OISST grid points along (within 10 km), left, and right of track at TC passage, respectively. Number of grid points within each subset (left of track, along track, and right of track for 2–4 Sep and all other periods) and corresponding r2 values for each subset are provided in the legend.

Citation: Monthly Weather Review 151, 6; 10.1175/MWR-D-22-0271.1

Surface cooling was present at all 13 AXBT pair locations, ranging from 0.25° to 3°C and averaging 1.6°C (Figs. 3c,d). To verify that these observed SST changes are due to the upper-ocean response to Dorian and not sharp prestorm temperature gradients, OISSTs were interpolated for each of the 26 AXBT positions 2 days prior to deployment, and differences were evaluated for each AXBT pair. The greatest prestorm difference between AXBT pair OISSTs was 0.1°C (not shown), more than an order of magnitude less than the mean SST change detected in the AXBT pairs and the 2-day cooling measured by satellite along most of Dorian’s track. As in the wake transect analysis, because preexisting variability detected by satellite is so small relative to the variability due to the storm, it is presumed that these AXBT observations primarily captured ocean temperature changes associated with the response to Dorian.

The full horizontal structure of Dorian’s cold wake becomes more apparent when examining maximum 2-day SST changes captured by OISSTs (Fig. 4a). While these data do not necessarily capture the full spatial or temporal patterns in the ocean response to Dorian (Wentz et al. 2000), strong surface cooling is evident, often ranging from 1.5° to 3.0°C. Of particular note is the location where the cooling takes place relative to the TC track. As Hurricane Dorian made landfall in the Abaco Islands (26.5°N, 77°W) at 1640 UTC 1 September 2019 (Avila et al. 2020), the storm had begun to slow (to near 3 m s−1) but had not yet begun to turn north (TC heading was about 270°). East of that landfall location, greater SST cooling is present right of the TC track (Fig. 4a), which is consistent with the right-of-track SST minima observed in the AXBT wake transects (Figs. 2a–c). However, after the turn, SST cooling is more evenly distributed across the TC track (e.g., from 27° to 29°N), including the area of the pre-TC Gulf Stream transects (Fig. 2), and especially in the area of maximum 2-day OISST cooling (3.5°C) just north of Grand Bahama Island (Fig. 4a). This reduction in the cross-track cooling gradient corroborates the shift in the dominant upper-ocean response mechanism from shear-driven mixing to upwelling.

Stratifying these maximum 2-day OISST cooling values at locations within 100 km of Dorian by total integrated surface wind stress yields additional details. First, across all time periods (Figs. 4b,c), cooling generally increases with integrated wind stress. Next, separating the 2-day period most associated with uniform cross-track cooling (2–4 September, denoted by gray dots in Fig. 4a) from all (five) other start dates substantiates that the reduction in right-of-track cooling bias depicted geographically across all dates in Fig. 4a is temporally consistent with Dorian’s slow translation speed and 90° right turn. For all periods except 2–4 September (Fig. 4b), integrated stress values are low (most less than 4 × 105 Pa s), and greater cooling is consistently present right of the TC track. This cross-track disparity is greatly diminished, however, during the 2–4 September maximum cooling period (Fig. 4c), which is consistent with the shift from a mixing- to an upwelling-dominant response identified in the wake transects and AXBT pairs.

The increased right-of-track cooling bias in mixing dominant environments is substantiated by comparing r2 values for locations that are left and right of track with maximum surface cooling over 2–4 September and all other periods. These r2 values are an order of magnitude greater (0.21 vs 0.02) right of track than left of track for all 48-h cooling periods except 2–4 September but are only twice as large (0.57 vs 0.33) over 2–4 September. In addition, r2 values are larger for the 2–4 September (upwelling dominant) cooling period, regardless of storm-relative position (left or right of track). This is likely because wind rotation rate affects the upper-ocean response in mixing-dominant regimes and likely confounds the relationship between OISST-indicated cooling and integrated wind stress in Fig. 4b. In an upwelling-dominant environment where wind rotation rates are smaller, the response magnitude is more closely related to the net surface wind stress, corresponding to increased correlations in Fig. 4c.

Since the integrated wind stress calculation includes both wind speed and duration, which are maximized over points close to the TC track during a slow right turn, elevated integrated wind stress values during the 2–4 September period make intuitive sense. In fact, 90% of all points with integrated wind stress values in excess of 4 × 105 Pa s exhibit maximum 2-day OISST cooling between 2 and 4 September. However, during this period Dorian not only slows and turns, but also weakens in intensity by about 60 kt. The associated significant reduction notwithstanding, the wind stress imparted to the ocean at these locations was elevated and coincident with mutual negative feedback between Dorian’s intensity and upper-ocean temperatures.

One aspect unaccounted for by the wind stress calculation is the effect of wind direction rotation rate on the upper ocean—for example, the persistent forcing from a uniform direction that results from a turning TC as evidenced during Dorian. Separating the relative impacts of slow translation speed and changing TC heading is beyond the scope of this paper but may be critical to identifying the dominant upper-ocean response mechanism in situations in which TC heading and translation speed changes vary independently.

4. Conclusions

From 31 August to 3 September 2019, 72 AXBT temperature–depth profiles were successfully collected on seven weather reconnaissance missions into Hurricane Dorian. In total, 32 of the 72 AXBTs composed two sets of transects at post- and pre-TC locations (collected before and after Dorian slowed and turned, respectively), which revealed TC-induced mixed layer cooling that was likely driven by different mechanisms.

The three post-TC transects captured a cold wake signature that extended to at least 300 m and included SST differences of up to 6°C over 70 km. In each transect, the mixed layer was coldest and deepest right of track, consistent with Dorian’s passage over the transect locations as a major hurricane with a translation speed greater than 4 m s−1. The two pre-TC (Gulf Stream) transects, collected while Dorian moved less than 3 m s−1 and turned right 90°, revealed an unusual TC characteristic: mixed layer cooling that extended over 100 km ahead of the storm.

The 90° right turn in TC heading and reduction in TC translation speed below 3 m s−1 for over 36 h reduced the rotation rate of local surface winds, which altered the stress imparted to the ocean surface and affected the subsurface currents sufficiently to shift the dominant upper-ocean cooling mechanism from shear-driven mixing to upwelling. This shift, detected in the transects and corroborated in 12 of the 13 AXBT pair profiles, nearly eliminated the right-of-track bias in sea surface cooling, as evident in both the maximum 2-day OISST cooling and the relationship of that cooling to integrated wind stress.

While integrated stress incorporates the effects of wind speed and duration, it does not account for changes in wind direction resulting from a change in TC heading. The rotation of wind direction affects its alignment with upper-ocean currents, which is (along with wind speed and forcing duration) a primary factor separating these two response mechanisms. Similarly, while the general 4 m s−1 threshold for TC upper-ocean response mechanisms accounts for the effects of TC translation speed on mixed layer cooling mechanism, no similar threshold exists for TC heading change or a combination of TC heading change and translation speed. We hypothesize that both the magnitude and direction components of the wind stress vector are critical to the evolution of the upper-ocean currents and both TC heading change and translation speed are primary factors in differentiating shear-driven mixing and upwelling as the dominant upper-ocean response to TC passage. Effects of a TC heading change on the upper-ocean response are particularly worth noting given that TCs rarely travel in a straight line from formation to decay, and major heading changes often correspond to very slow translation speeds (Hall and Kossin 2019). Other noteworthy landfalling storms from the past decade in the tropical Atlantic that exhibited a ≥60° heading change as major hurricanes include Hurricanes Joaquin (2015), Matthew (2016), and Zeta (2020). Tropical cyclone slowing and stalling events have increased in frequency and are expected to continue to do so (Kossin 2018), underscoring the importance of observing and accurately modeling TC–cold wake interactions (Schade and Emanuel 1999; Chen et al. 2017).

This study highlights the importance of both TC heading changes and slow translation speeds to the upper-ocean response mechanism. We detected these features with AXBTs, but higher-precision in situ measurements are needed to quantify these relationships. Specifically, frequent and persistent measurements of upper-ocean current speed and direction, along with concurrent temperature, salinity, and pressure observations, are needed to accurately and objectively link wind speed and direction to current speed and direction and subsequently to changes in upper-ocean structure and mixed layer cooling during TC passage.

Acknowledgments.

The authors are grateful for the support of the U.S. Air Force Reserve Command 53rd Weather Reconnaissance Squadron, who supported the AXBT deployments used in this study. In addition, the authors thank Jordan Sun, Grace Rovira-Melendez, Hunter McAlister, Julia Von Fecht, Matthew Kuhn, Jeffrey Kerling, and Kyle Rushing for their roles in AXBT data acquisition and quality control. This work was sponsored by the Office of Naval Research under Grant N000141812819 and the U.S. Navy’s Civilian Institution Office with the MIT–WHOI Joint Program. Argo data were collected and made freely available by the International Argo Program and the national programs that contribute to it (https://argo.ucsd.edu; https://www.ocean-ops.org). The Argo Program is part of the Global Ocean Observing System. NDBC buoy data were collected and made freely available by NOAA/NDBC.

Data availability statement.

The archived quality-controlled AXBT data (Sanabia 2020) are available online (https://accession.nodc.noaa.gov/0209221). Daily 9-km OISSTs from Remote Sensing Systems are accessible online (http://data.remss.com/SST/daily/mw_ir/v05.0/netcdf/2019/), OSCAR currents can be found online (https://podaac.jpl.nasa.gov/dataset/OSCAR_L4_OC_third-deg), and SFMR data are available online (https://www.aoml.noaa.gov/hrd/Storm_pages/dorian2019/sfmr.html). Argo data are available for download (https://doi.org/10.17882/42182), and NOAA NDBC historical buoy observations are accessible online (https://www.ndbc.noaa.gov/).

APPENDIX

OISST Data Validation

To validate the use of OISSTs to estimate sea surface cooling in Hurricane Dorian’s wake, 4093 observations from three NOAA National Data Buoy Center (NDBC) buoys (National Data Buoy Center 1971; identifiers 41010, 41016, and 41017) between 25 August and 4 September 2019, 2 profiles from Argo float 4902110 collected on 25 August and 4 September 2019 (Argo 2023), and 70 of the 72 AXBT profiles used in this study were compared with collocated OISSTs (Fig. A1a). Corresponding OISSTs for each observation were interpolated spatially and temporally, and all observations collocated with valid OISST data (sufficiently far from land and free of heavy cloud cover) were included.

Fig. A1.
Fig. A1.

(a) Argo, AXBT, and NDBC profile locations incorporated in the OISST validation overlaid with Hurricane Dorian’s track. Track colors denote Dorian’s intensity following the Saffir–Simpson scale. (b) Observed SSTs vs interpolated OISSTs, colored by observation source. (c) Observed SSTs vs interpolated OISSTs, only for observations collected within 2 h of OISST valid times (1200 UTC daily) and at least 150 km from Hurricane Dorian’s center.

Citation: Monthly Weather Review 151, 6; 10.1175/MWR-D-22-0271.1

Two major modes of deviation between observations and corresponding OISSTs are apparent: NDBC and AXBT observations in the range of 29°–31°C up to 2°C warmer than corresponding OISSTs, and AXBT SSTs up to 6°C cooler than corresponding OISSTs (Fig. A1b). The observations warmer than corresponding OISSTs are likely due to diurnal heating. Because OISSTs are effective daily at 1200 UTC (0800 EST, local time), they would not capture diurnal heating. Observations cooler than corresponding OISSTs are likely due to cooling associated with Dorian’s passage unresolved by OISSTs, because either the observation (and corresponding cooling) occurred farther away from the 1200 UTC valid times for the OISSTs or the observation was taken within the storm environment where heavy cloud cover and precipitation limited the accuracy of the OISST sensing.

To verify that the discrepancies between observations and corresponding OISSTs are due to diurnal heating or proximity to Hurricane Dorian (neither of which are assumed to be a factor in this study because cooling is calculated using OISSTs valid at the same time of day sufficiently far ahead of and behind Dorian to be free of heavy cloud cover), the validation was constrained to observations collected within 2 h of the OISST valid time (1200 UTC) each day at least 150 km from Hurricane Dorian’s center. Only 634 NDBC observations and 6 AXBT profiles met those criteria, and those data are presented in Fig. A1c. Observed SSTs meeting those constraints agree closely with corresponding OISSTs, with the exception of one AXBT profile located in a region of high spatial variability in the Gulf Stream. Given the close agreement between OISSTs and observed SSTs, it is assumed that the OISSTs are sufficiently accurate to quantify cooling associated with Hurricane Dorian’s passage in this study.

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    • Search Google Scholar
    • Export Citation
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  • Fig. 1.

    (a) Hurricane Dorian track and intensity, pre-TC 9-km Optimum Interpolation SSTs [OISSTs (shaded) for 30 Aug 2019; Wentz et al. 2000], ⅓° OSCAR currents (arrows, for the 27 Aug 2019 pentad; ESR 2009), and AXBT deployment locations colored by flight. Gulf Stream currents are capped at 1 m s−1 for readability (marked with red vectors; mean velocity in the included segment of the Gulf Stream is 1.2 m s−1). Black solid and dotted lines indicate best-track hurricane- and tropical storm–force wind swaths, respectively. Colored lines denote locations of wake (solid blue, red, and green; poststorm) and Gulf Stream (dashed blue and magenta; prestorm) transects. (b) Dorian intensity, translation speed, and heading for the period in (a). Vertical lines denote the time Dorian that crossed each wake transect, colored as in (a), and the horizontal line marks a 4 m s−1 translation speed. (c) Storm-relative AXBT transect positions, colored as in (a) and (b). The arrow indicates storm direction.

  • Fig. 2.

    The AXBT temperature transects collected across Dorian’s wake beginning at (a) 2349 UTC 31 Aug 2019 at 73.5°W, (b) 1351 UTC 1 Sep 2019 at 75.2°W, and (c) 2343 UTC 1 Sep 2019 at 76.0°W. Dashed black lines mark AXBT profile locations, solid magenta lines indicate the TC track, which crossed the transects at 26.0°, 26.4°, and 26.4°N, respectively; and white contours denote 20°, 24°, and 26°C isotherms. Radial SFMR surface wind profiles from the aircraft pass through the TC center in closest proximity to each transect at (d) 1632 UTC 31 Aug 2019, (e) 0210 UTC 1 Sep 2019, and (f) 1132 UTC 1 Sep 2019. Each SFMR profile was collected within 3 h (and 31 km) of the TC crossing the transect. Negative and positive distances denote left and right of track, respectively. Also shown are AXBT transects collected across the Gulf Stream beginning at (g) 2209 UTC 1 Sep 2019 and (h) 1059 UTC 3 Sep 2019. Gray shading shows bathymetry along each transect. Transect distances are relative to their intercept point (between dashed blue and magenta lines in Fig. 1a).

  • Fig. 3.

    (a) Hurricane Dorian translation speed and heading. (b) Estimated surface winds (vectors, with north up) at the 13 AXBT pair locations between the deployment times for each pair. Individual (gray) and mean (red) differences between AXBT pair profiles where wind direction shifts Δθ of (c) greater than (conducive for mixing) and (d) less than (conducive for upwelling) 75° were identified. Also shown are example initial (dashed) and final (solid) AXBT pair profiles indicative of (e) mixing and (f) upwelling. Warming and cooling are denoted by red and blue shading, respectively.

  • Fig. 4.

    (a) Maximum OISST-measured cooling for 2-day periods with start dates spanning from 31 Aug to 5 Sep 2019. Numbers indicate the location of AXBT pairs collected at times identified in the legend, and gray dots denote grid points where maximum cooling was observed between 2 and 4 Sep. Also shown is integrated wind stress vs maximum 2-day OISST cooling, separated for maximum cooling attained during (c) 2– 4 Sep and (b) all other periods. Green, blue, and red markers indicate OISST grid points along (within 10 km), left, and right of track at TC passage, respectively. Number of grid points within each subset (left of track, along track, and right of track for 2–4 Sep and all other periods) and corresponding r2 values for each subset are provided in the legend.

  • Fig. A1.

    (a) Argo, AXBT, and NDBC profile locations incorporated in the OISST validation overlaid with Hurricane Dorian’s track. Track colors denote Dorian’s intensity following the Saffir–Simpson scale. (b) Observed SSTs vs interpolated OISSTs, colored by observation source. (c) Observed SSTs vs interpolated OISSTs, only for observations collected within 2 h of OISST valid times (1200 UTC daily) and at least 150 km from Hurricane Dorian’s center.

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