Lewis J. Gramer1,2,*, Jun Zhang1,2,
Ghassan Alaka2, Andrew Hazelton1,2,
Sundararaman Gopalakrishnan2
1 - Cooperative Institute for Marine and Atmospheric
Studies, Miami, FL 33149.
2 - NOAA Atlantic Oceanographic and Meteorological
Lab, Miami, FL 33149.
* - Corresponding author: Dr. Lew Gramer, 8930
Caribbean Blvd., Cutler Bay, FL 33157. Email:lew.gramer@noaa.gov.
Telephone: +1-305-772-7933. ORCID:https://orcid.org/0000-0003-4772-1991
Key Points
- Hurricanes approaching land tend to force downwelling over the ocean
shelf;
- Coastal downwelling sustains warmer sea-surface temperatures (SSTs)
over the shelf;
- Sustained SSTs and air-sea contrasts over the shelf increase enthalpy
(heat and moisture) fluxes contributing directly to hurricane
intensification near landfall, particularly for larger or
slower-moving hurricanes.
Abstract
This study demonstrates a link between coastal downwelling and tropical
cyclone (TC) intensification. We show coastal downwelling increases
air-sea enthalpy (heat, moisture) fluxes ahead of TCs approaching land,
creating conditions conducive to intensification even in the presence of
typically inhibiting factors like strong vertical wind shear. The study
uses a coupled TC model (HWRF-B) and buoy observations to demonstrate
that coastal downwelling developed as three TCs in 2020 approached land.
Results show that downwelling maintained warmer sea-surface temperatures
over the ocean shelf, enhancing air-sea temperature/humidity contrasts.
We found that in such cases the resulting air-sea enthalpy fluxes can
replenish the boundary layer even when cool, dry air intrudes, as in
sheared storms and storms approaching land. Warm, moist air advects into
the inner core, enhancing convective development, thus providing energy
for TC intensification. These results indicate coastal downwelling
should be considered in forecasting TC intensity change before landfall.
Plain Language Summary
We examined forecasts for three hurricanes in 2020 that intensified near
landfall. Using a coupled air-sea hurricane model, we found that
tropical storm-force winds blowing parallel to the coast forced water
near the ocean surface to move toward shore. Winds appear to often blow
parallel to the coast when tropical cyclones (hurricanes) are near land.
The model showed that this onshore transport caused the water level to
rise near the coastal boundary - a process called coastal Ekman
convergence. This convergence forced water downward along the sloping
seafloor and back away from shore, forming a circular exchange of water
between the shelf and the open ocean; this exchange is called coastal
downwelling. We demonstrate that incipient coastal downwelling brought
warmer surface water over the shelf, heating and adding moisture to the
air and thus providing more energy to these tropical cyclones. We
further show that the additional energy provided by coastal downwelling
can contribute to intensification of larger or slower-moving tropical
cyclones before landfall. We suggest it is important to validate the
modeling of coastal downwelling in future forecast models, in order to
reliably forecast tropical cyclone intensity near landfall.
Index Terms: 3372, 4504, 4534, 4217, 4219
Keywords: landfalling hurricanes, coastal downwelling, shelf
oceanography, rapid intensification, tropical cyclones, air-sea enthalpy
flux
1. Introduction
In 2009 the National Oceanic and Atmospheric Administration (NOAA)
established the 10-year Hurricane Forecast Improvement Program (HFIP;
Gopalakrishnan et al. 2021) to improve the accuracy and reliability of
hurricane forecasts and to extend forecast lengths (lead times) with
increased certainty, especially before landfall. Predicting intensity
changes in tropical cyclones (TCs), particularly rapid intensification
(RI; Kaplan et al. 2015), is a complex, multiscale problem. This
forecast guidance challenge assumes special significance when TCs
intensify just before landfall. This paper introduces a previously
unexplored mechanism by which some Atlantic TCs intensify prior to
landfall, despite conditions generally deemed unfavorable for
intensification.
Many studies have shown that high sea surface temperatures (SSTs)
provide the necessary energy for TCs by increasing air-sea enthalpy
(heat and moisture) fluxes, resulting in more sustained eyewall
convection, a warmer core, a stronger pressure gradient between the
center and the environment, and stronger winds in the eyewall region
(Emanuel et al. 2004; Jaimes et al. 2015; Zhang et al. 2020a; Zhang et
al. 2020b). Nevertheless, stronger near-surface winds can provide
negative feedback on TC intensification in two ways, frictional
dissipation, which is always present (Wang and Xu 2010; see Eq. S1) and
SST cooling, which is dependent on air-sea interaction and ocean
dynamics. SST cooling refers to the wake of cool surface water that TCs
over the open ocean can create (e.g., Bender and Ginis 2000). Such wakes
result from upwelling due to divergence of near-surface ocean currents
and most importantly to rapid turbulent entrainment of cooler water into
the oceanic mixed layer. The resultant SST cooling can significantly
weaken TCs (Walker et al. 2014; Guo et al. 2020), particularly
slower-moving (e.g., translation speed <2 m/s) and larger ones
(radius of maximum winds ~50 km; Halliwell et al. 2015).
Environmental vertical wind shear (VWS) also weakens TCs through several
processes (Wong and Chan 2004; Wang et al. 2015): vortex center
misalignment (Kaplan et al. 2015), mid-tropospheric dry-air intrusion in
the TC inner core (Bhalachandran et al. 2019; Tang and Emanuel 2010),
upper-tropospheric divergent fluxes of entropy and potential energy
(Riemer et al. 2010), and downdrafts carrying cool, dry air into the
boundary layer (e.g., Bhalachandran et al., 2019; Riemer et al. 2010;
Tang and Emanuel 2012). Forecasters typically expect TCs that are
experiencing significant VWS to weaken, particularly TCs also impacted
by ocean cooling.
When approaching landfall, however, TCs do sometimes intensify despite
ocean mixing, the presence of VWS, and the negative effects of land
interaction on the TC. In this study, we establish how three landfalling
storms over the Caribbean Sea and the Gulf of Mexico intensified before
landfall, and we specifically highlight the importance to that
intensification of oceanographic processes over the coastal shelf. The
three cases considered were landfalling TCs Sally, Hanna, and Eta from
the 2020 season. The study used the ocean-coupled Basin-scale Hurricane
Weather Research and Forecast (HWRF-B) system (Alaka et al. 2020) and
observations collected from NOAA buoys to test three hypotheses:
- TCs approaching land tend to force coastal downwelling over the ocean
shelf.
- Coastal downwelling sustains warmer SSTs over the shelf.
- Sustained SSTs and resulting air-sea contrasts over the shelf enhance
enthalpy fluxes that contribute directly to TC intensification near
landfall, particularly for larger or slower-moving TCs.
2. Methods
The model in these case studies was the quasi-operational Basin-scale
HWRF (HWRF-B; Zhang et al. 2016; Alaka et al. 2017, 2019, 2020)
developed under HFIP. This model uses a fixed outer domain at 13.5 km
horizontal resolution, with two TC-following nested domains at 4.5 and
1.5 km resolution, respectively. Up to five sets of nested domains can
be deployed for a given initialization time to produce high-resolution
forecasts for multiple TCs in the North Atlantic and eastern North
Pacific. HWRF-B is coupled to MPIPOM-TC, an instance of the Princeton
Ocean Model for TCs (Yablonsky et al. 2015) initialized with a two-day
spinup from the Real-Time Ocean Forecasting System (RTOFS; Mehra and
Rivin 2010).
Three TC case studies from the 2020 North Atlantic hurricane season were
evaluated: Sally, Hanna, and Eta. Each TC experienced intensification
while approaching land and interacted with the ocean shelf for a period
of one day or more. To elucidate processes leading to intensification,
model sea-surface fields were analyzed at forecast hours before and
immediately after intensification. Supporting Information Table S1
details forecast initialization times for each case and forecast hours
chosen for detailed analysis: the forecast hour immediately following
intensification and a forecast hour prior to intensification that was
near the peak of both air-sea flux and storm kinetic energy (see below).
We evaluated model currents and sea temperatures using
quality-controlled buoy observations from the NOAA National Data Buoy
Center (NDBC 2009; Winant et al. 1994).
We examined the relationship of air-sea fluxes to total storm kinetic
energy and frictional dissipation using a model energy budget following
Trenberth (1997) and Kato et al. (2016). See Supporting Information
Equation S1 for details.
3. Results
Figure 1 shows the effect of downwelling-favorable winds over the shelf
(i.e., TC surface winds >17.5 ms-1blowing parallel to the coast with the coastline on their right) on
near-surface ocean currents and sea-surface elevation. (We note that
downwelling-favorable winds may be common when a TC is offshore of
land.) These winds force surface Ekman transport toward the lateral
boundary of the coast. This transport results in surface current
convergence over time, causing a “bulge” in sea surface elevation over
the shelf, in contrast with the sea-surface depression produced in the
TC’s wake. The bulge over the shelf is the precondition for coastal
downwelling.
The right column of Figure 1 shows that after intensification, sea
surface elevation for Sally (Fig. 1b) and Hanna (Fig. 1d) continued to
increase dramatically. Approaching landfall, however, we note that storm
surge also contributed to these elevation increases. For Eta, wind speed
increase before landfall was small. However, surface Ekman convergence
continued to increase through landfall (Fig. 1e,f), consistent with the
other cases.
Figure 2 contrasts TC coastal downwelling effects on sea temperature and
ocean currents with the effects of the TC wake in the open ocean:
Coastal downwelling maintained warmer SSTs. In Sally (Fig. 2a), in
particular, the shelf effect was apparent along the entire west Florida
shelf following the storm’s passage. In each storm’s wake, by contrast,
spatial variations in surface wind velocity within the storm caused
Ekman divergence and upwelling while TC winds drove energetic
mixed-layer deepening (see below), leading to SST cooling.
The middle column of Fig. 2 shows side views (vertical profiles) of
coastal downwelling effects on temperature and ocean currents.
Downwelling circulation advected warm water ashore and then downward,
depressing ocean isotherms near the shelf. The black contour shows
depression of the isotherm for 26 °C, a critical temperature threshold
for TC intensification (Cione and Uhlhorn 2003; Cione 2015). Apparent in
currents further from the shelf is an upward return flow, a
characteristic of the cross-shelf circulation cell of coastal
downwelling. For the two Gulf of Mexico storms (top and middle panels),
the model shows low-salinity water derived from RTOFS initialization on
the shelf (not shown). The resulting shelf stratification is consistent
with intensified shoreward surface transport and the quick development
of the downwelling front (Csanady 2002; Austin and Lentz 2002).
The right column of Figure 2 shows profiles along a line from the inner
core into each storm’s wake. In contrast with the downwelling seen in
the middle column, these profiles show the effects of upwelling and
mixed-layer deepening, particularly on the 26 °C isotherm.
Figure S1 in the Supporting Information compares surface model
temperatures with near-surface buoy observations from the Gulf of Mexico
during Sally’s passage. Sea temperatures from a deep-ocean buoy (Fig.
S1b) showed a rapid decline of >1.5 °C
d-1 during TC passage consistent with a developing
cold wake, while multiple buoys on the shelf (Fig. S1c shows one
example) recorded markedly slower declines of 0.2-0.4 °C
d-1, consistent with sustained SST from downwelling
together with slower direct air-sea cooling.
Figure 3 shows all three storms had positive total (latent+sensible)
air-sea enthalpy flux in the shelf region potentially aiding
intensification despite moderate-to-strong deep-layer (i.e., 200-850
hPa) VWS and surface wind field asymmetries. The enthalpy fluxes prior
to intensification (left panels) are more prominent over the shelf than
the deeper ocean. Peak heat fluxes per grid-point for Sally, Hanna, and
Eta were 1128, 1223, and 1570 Wm-2, respectively, with
all peaks occurring over the shelf. Enhanced fluxes over the shelf
illustrate coastal downwelling’s impact on air-sea temperature and
humidity contrasts before intensification.
The asymmetries in these enhanced shelf fluxes led to initial
asymmetries in surface winds. However, after intensification (right
panels), these asymmetric fluxes counterintuitively led to a more
symmetrized storm. While areas of substantial shelf enthalpy flux in all
panels correspond with areas of large air-sea humidity and temperature
contrast, they do not always correspond with areas of strongest winds
(not shown). This supports the interpretation that shelf-based energy
fluxes resulted from higher SSTs over the shelf rather than simple wind
asymmetries.
Figure 4 shows that the boundary layer is replenished by the shelf
air-sea enthalpy fluxes seen in Fig. 3, even when cool, dry air
intrudes, as in strongly sheared storms like Sally and Eta. The figure
shows cool, dry air with low equivalent potential temperature
(θe) being carried by downdrafts into the boundary layer
of each storm. Enthalpy fluxes in the boundary layer over the shelf
replenish heat and moisture, increasing θe between the
left and right panels. During intensification, the
higher-θe air is transported along streamlines of
surface air circulation toward the inner core of each storm. The
transport of high-entropy air into the inner core enhances deep
convection, resulting in symmetrization and intensification just before
landfall.
Figures 3 and 4 show TC axisymmetrization and contraction, leading to
intensification (Chen et al. 2018; Wadler et al. 2021). Fluxes from the
shelf region together with boundary layer inflow enhance boundary layer
entropy. When advected into the inner core, this leads to eyewall
convection. This drives diabatic heating, which is favorable for
symmetric intensification of the TC vortex (Chen et al. 2018),
especially when it is located inside the radius of maximum winds (e.g.
Vigh and Schubert 2009; Chen and Gopalakrishnan, 2015). All three storms
were relatively slow-moving (Table S1), increasing the effect of the
cold wake on fluxes over deeper water, while VWS enhanced the initial
asymmetries; for both of these reasons, the subsequent intensification
of each storm was remarkable.
In Figure 5, for each case we evaluate the simplified energy budget (Eq.
S1). Change in the total kinetic energy and in maximum 10 m winds for
all three storms is positively correlated with the difference between
the total enthalpy fluxes and frictional dissipation. Note that enthalpy
fluxes over the shelf region constituted an important fraction of the
total flux (30-90%) leading up to and during intensification. The
increase in kinetic energy lagged increases in surface flux by 6-12 h,
supporting the argument that surface enthalpy fluxes are important for
TC intensity change.
Significant increases in total enthalpy flux preceded intensification of
15 ms-1 in 36 h for Sally (Fig. 5a) and rapid
intensification (Kaplan and DeMaria 2003) for Hanna (Fig. 5b), and the
shelf effect contributed a majority of the total flux in both cases.
Although Eta had intensified largely over the open ocean, it did
encounter insular and continental shelf water starting in hour 72. And
just before its first landfall during the time when it was approaching
its maximum forecast intensity, 30-40% of its total enthalpy flux was
derived from shelf waters (Fig. 5c). These shelf fluxes occurred in both
the inner and outer core of Eta (see Fig. 3e). For all three cases, high
enthalpy fluxes supported boundary-layer recovery of downdraft-induced
low entropy air upshear before entering the updrafts in the downshear
quadrant, countering the negative effect of ventilation on TC
intensification.
4. Discussion and
Conclusions
This paper investigated three hurricane case studies using the coupled
HWRF-B / MPIPOM-TC modeling system. We found that these TCs forced
coastal downwelling. Coastal downwelling develops when surface winds
blow with the land to their right in the Northern Hemisphere (Sreenivas
and Gnanaseelan 2014; Kuo et al. 2014; Shen et al. 2017; Zhang et al.
2019). We further found that coastal downwelling associated with each TC
sustained SSTs over the shelf, consistent with other studies focused on
non-TC conditions (e.g., Choboter et al. 2011). Shelf circulation during
typhoons in the western north Pacific has also been observed to sustain
SSTs (Kuo et al. 2014, Zhang et al. 2019, Zhang et al. 2021).
Here, however, we established a direct link between SST sustenance,
enhanced enthalpy fluxes, and intensification for these three Atlantic
TCs. Despite dry air intrusion due to persistent shear, enhanced air-sea
enthalpy fluxes resulting from warmer SSTs over the shelf caused all
three TCs to intensify before landfall. Surface wind field forecasts and
observations (not shown) indicated that warmer, moister air in the
boundary layer was carried into each TC’s inner core over a period of
6-12 h. In each of the three cases, an increase in total kinetic energy
and intensification of surface winds both followed within another 6-12
h.
A simplified energy budget showed that a peak in residual of air-sea
enthalpy flux minus frictional dissipation was correlated with a peak in
TC intensity, similar to results for peak intensity found by Wang and Xu
(2010). The lag of 12-18 h between peak air-sea enthalpy flux and peak
TC intensity was consistent with idealized modeling studies (Halliwell
et al. 2015). Our results support the conclusion that surface enthalpy
fluxes are important for TC intensity change, constituting an energy
source that is sufficient to compensate for the energy loss due to
frictional dissipation (Emanuel 1986). Similarly, we find agreement with
Zhang et al. (2017) that the importance of surface enthalpy fluxes for
TC intensification hinges on the role they play in boundary-layer
recovery. What is novel here is the importance of air-sea enthalpy
fluxes due to TC-driven coastal downwelling in particular.
Future work will examine the relationship between shelf fluxes and storm
structure (wind field sizes), as this may also constitute an important
component of TC response to the shelf effect. Mechanisms for
boundary-layer recovery within the TC core must also be explored in more
detail in future, potentially using idealized studies. Finally, a recent
study suggests that insolation differences related to cloud cover may be
important for TC intensification near landfall (Lok et al. 2021): the
energy budget discussed here will be extended to consider insolation as
well as terms for potential and internal energy advection.
This study demonstrated that the coupled HWRF-B/MPIPOM-TC system
appropriately modeled important coastal and shelf ocean dynamics related
to TC intensification, namely the shelf effect due to coastal
downwelling. We further showed that this oceanographic process can
influence the boundary-layer recovery and atmospheric convection in
intensifying TCs. These case studies represent three TCs from just one
basin and hurricane season (2020 North Atlantic) that were impacted by
coastal downwelling. Other cases from the 2017-2020 North Atlantic and
eastern North Pacific hurricane seasons are being analyzed to confirm
the relative prevalence of this shelf effect in causing landfalling TCs
to intensify.
Future model-observational studies should analyze other oceanographic
mechanisms that may contribute to the shelf effect as well, including
coastal-trapped and continental shelf waves (Shen et al. 2021) and
barrier-layer intensification (Balaguru et al. 2020, Rudzin et al.
2020). Implications of the shelf effect for TCs in a changing climate
will also certainly bear further examination. Finally, future work must
evaluate how the shelf effect can impact coastal communities and marine
ecosystems, e.g., from enhanced bottom temperatures and cross-shore
transport; such impacts may be significant (see, e.g., Dzwonkowski et
al. 2020).
Above all, this study establishes the important role of coastal
downwelling in increasing storm intensity before landfall. We recommend
that modeling of the shelf effect be validated in future upgrades of
operational coupled TC forecast models, such as those now under
development by HFIP (Gall et al. 2013), in order to reliably forecast
the intensity of landfalling TCs.
Acknowledgments
The authors wish to thank the Hurricane Forecast Improvement Program
(HFIP) as well as funding from the Hurricane Supplemental (HSUP), NOAA
Grant NA19OAR0220187. Internal review comments from F Marks, R Rogers,
and R Lumpkin all improved the work. LJG is particularly thankful to
lively recent discussions with HS Kim, and earlier discussions with SJ
Monismith. Finally, suggestions from E Becker, L Albritton, K Nielsen,
and N Deyo of the University of Miami all improved the clarity and
concision of the manuscript.
Data
Graphical products associated with TC forecasts described in this text
can be found online at:
https://storm.aoml.noaa.gov/
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