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Key Factors Modulating the Threat of the Arabian Sea’s Tropical Cyclones to the Gulf Countries

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Key Factors Modulating the Threat of the Arabian Sea’s Tropical Cyclones to the Gulf Countries

Abstract and Figures

Tropical cyclones (TC) are one of the biggest natural hazards with significant threat to life and property due to storm surge, flooding and extreme winds. Combined, these hazards substantially increase the potential for loss of life and damage especially in populated landfall-locations such as the countries on the Arabian Gulf and Sea of Oman. Hence, it is important to identify the factors that modulate the trajectory of tropical cyclones in order to better predict their lifetime and dynamics. Since 1900, only two TCs moved into the Sea of Oman and made landfall on the southeastern coast of the Arabian Peninsula (AP): TC Shaheen in 2021 and Gonu in 2007. In this study, the mechanisms behind the exceptional trajectories of these two TCs are investigated. Both TCs developed during the active phase of the Madden-Julian Oscillation (MJO), and benefited from above-average sea surface temperatures (SSTs), ocean heat content and reduced vertical wind shear. Their paths over the Indian Ocean were controlled by large-scale forcings, whereas regional-scale processes, such as the local SST gradients and the Arabian Heat Low (AHL) defined their trajectories near landfall location over the AP. An exceptionally deep AHL was present during these events. The AHL is found to interact two-way with TCs: its associated circulation drags the TCs inland while the TCs, through the advection of cooler marine-air inland, cause the collapse of the AHL. It is recommended to account for the AHL as fluctuations in its position and strength can determine where future TCs make landfall.
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1. Introduction
The Arabian Sea is a body of water located in the northwestern Indian Ocean. It extends to about 25°N and from
the shores of Somalia on the western side, to the Arabian Peninsula (AP), Iran and Pakistan on the northwestern
and northern sides, and western India on the eastern side (Qasim, 1982). The atmospheric circulation here is
dominated by the Asian monsoon system, with prevailing low-level southwesterly winds in the summer season
and northeasterly winds in the winter months (Schott etal., 2009). Tropical cyclone (hereafter TC) formation
is unfavorable in July and August due to the (a) presence of substantial vertical wind shear, when the strong
low-level southwesterly winds co-occur with upper-level easterlies associated with the Asian summer monsoon
anticyclone (Basha etal., 2020), and (b) copious amounts of precipitation that lower the sea surface tempera-
tures (SSTs) through local air-sea interactions (Fu etal.,2002). Hence, TCs typically form in the pre-monsoon
(May and June) or post-monsoon (October and November) months, with an average of one storm per season
(Al-Maskari,2012; Gray,1968). Cooler SSTs, prevailing anticyclonic relative vorticity, and higher vertical wind
shear arising from the presence of the subtropical jet at upper-levels prevent storms from developing in the
winter months (Evan & Camargo,2011). The main trigger of TC formation in the northern Indian Ocean is the
Madden-Julian Oscillation (MJO; Madden & Julian,1994), which is the dominant mode of tropical intraseasonal
(30–90days) variability. The MJO is characterized by regions of enhanced/suppressed convection that propagate
eastwards across the tropics, with a poleward component in the boreal summer season (Kawamura etal.,1996).
Krishnamohan etal.(2012) reported that roughly 82% of the TCs which formed in the area from 1979 to 2008
occurred during active phase of the MJO. There is also considerable interannual variability, with the higher SSTs
and a more favorable dynamic and thermodynamic forcing in Central Pacific El Nino and Southern Oscillation
(ENSO; Ashok & Yamagata,2009) as opposed to eastern Pacific El Nino Southern Oscillation (ENSO) events,
leading to an increased likelihood of TC formation in the Arabian Sea (Sumesh & Ramesh Kumar,2013). The
recent increase in SSTs in the basin in conjunction with the Pacific Decadal Oscillation (Mantua & Hare,2002)
also contributes to more active TC seasons (Rajeevan etal.,2013).
While TCs are regularly seen in the Arabian Sea, it is rare for them to move into the Sea of Oman, a small body
of water that borders Oman, the United Arab Emirates (UAE), Iran and western Pakistan. Only two such occur-
rences have been reported since 1900 (Mahmoud,2021): TC Gonu in 2007 which, as of 2021, is the strongest TC
Abstract Tropical cyclones (TC) are one of the biggest natural hazards with significant threat to life and
property due to storm surge, flooding and extreme winds. Combined, these hazards substantially increase the
potential for loss of life and damage especially in populated landfall-locations such as the countries on the
Arabian Gulf and Sea of Oman. Hence, it is important to identify the factors that modulate the trajectory of TC
in order to better predict their lifetime and dynamics. Since 1900, only two TCs moved into the Sea of Oman
and made landfall on the southeastern coast of the Arabian Peninsula (AP): TC Shaheen in 2021 and Gonu in
2007. In this study, the mechanisms behind the exceptional trajectories of these two TCs are investigated. Both
TCs developed during the active phase of the Madden-Julian Oscillation, and benefited from above-average
sea surface temperatures (SSTs), ocean heat content and reduced vertical wind shear. Their paths over the
Indian Ocean were controlled by large-scale forcings, whereas regional-scale processes, such as the local SST
gradients and the Arabian Heat Low (AHL) defined their trajectories near landfall location over the AP.An
exceptionally deep AHL was present during these events. The AHL is found to interact two-way with TCs: its
associated circulation drags the TCs inland while the TCs, through the advection of cooler marine-air inland,
cause the collapse of the AHL. It is recommended to account for the AHL as fluctuations in its position and
strength can determine where future TCs make landfall.
FRANCIS ET AL.
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Attribution-NonCommercial-NoDerivs
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Key Factors Modulating the Threat of the Arabian Sea's
Tropical Cyclones to the Gulf Countries
Diana Francis1 , Ricardo Fonseca1 , and Narendra Nelli1
1Environmental and Geophysical Sciences (ENGEOS) Lab, Khalifa University, Abu Dhabi, United Arab Emirates
Key Points:
Cyclones Shaheen and Gonu
developed in the active phase of the
Madden-Julian Oscillation and in a
La Nina state in the northern Indian
Ocean
Positive surface and subsurface ocean
temperature anomalies fueled the
storms and played important role in
their paths
An abnormally deep Arabian Heat
Low over the Arabian Peninsula
dragged the cyclones inland
and modulated their exceptional
trajectories
Supporting Information:
Supporting Information may be found in
the online version of this article.
Correspondence to:
D. Francis,
diana.francis@ku.ac.ae
Citation:
Francis, D., Fonseca, R., & Nelli, N.
(2022). Key factors modulating the
threat of the Arabian Sea's tropical
cyclones to the Gulf countries. Journal
of Geophysical Research: Atmospheres,
127, e2022JD036528. https://doi.
org/10.1029/2022JD036528
Received 20 JAN 2022
Accepted 27 MAY 2022
Author Contributions:
Conceptualization: Diana Francis
Data curation: Ricardo Fonseca,
Narendra Nelli
Formal analysis: Ricardo Fonseca
Investigation: Diana Francis, Ricardo
Fonseca
Supervision: Diana Francis
Validation: Diana Francis, Narendra
Nelli
Visualization: Ricardo Fonseca,
Narendra Nelli
10.1029/2022JD036528
RESEARCH ARTICLE
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on record in the Arabian Sea (e.g., Dibajnia etal.,2010; Najar & Salvekar,2010), and TC Shaheen in October
2021 (Terry etal.,2022). Before 1900, there were two storms in the 1890s that propagated into the Sea of Oman:
a destructive TC in early June 1890 whose trajectory resembles that of TC Shaheen, which dropped 286mm
of rain in Muscat in a 24-hr period and led to death of at least 757 people (Membery,2002), and a storm in
early June 1898 that dissipated in the Sea of Oman (Knapp etal.,2010). Deshpande etal.(2010) noted that the
track of TC Gonu was influenced by the presence of a ridge to its west over the AP and the interaction with a
mid-latitude trough over Iran. The mechanisms responsible for such features, however, have not been discussed.
Alimohammadi etal.(2021) emphasized the role of the SSTs in the track of TC Gonu, not just its magnitude but
the spatial pattern as well, and stressed the important role of the observed SST gradient on its trajectory. TC Gonu
also had substantial impacts in the ocean state, with warmer and high saline water flowing in from the Arabian
Gulf to the Sea of Oman (Wang etal.,2012). Besides a warm ocean surface, higher-than-average temperatures in
the water column also promote the intensification of TCs and play a vital role in the TCs' intensity. This is because
colder subsurface waters will eventually be upwelled by the storm's circulation and impact the surface fluxes and
therefore the TC's strength (Morey etal.,2006; Pasquero and Emanuel,2008). In fact, some authors argue that the
ocean heat content (OHC), defined as the integral of the product of the seawater's density to its temperature from
the ocean surface to a given depth, may be even more important than the SSTs (e.g., Hallam etal.,2021; Wada &
Usui,2007). Although it has not been recorded so far, TCs can also make it into the Arabian Gulf, where the SSTs
in the summer can exceed 33°C in western UAE and eastern Qatar, and where an overall warming trend at a rate
of up to 0.08Kyr
−1 has been observed (Nesterov etal.,2021). Idealized simulations suggest that a TC developing
within the Arabian Gulf or moving from the Sea of Oman/Arabian Sea into the Gulf could pick up winds in excess
of 400 kph, potentially making it the strongest storm ever recorded on Earth (Lin & Emanuel,2016). Together
with a storm surge of more than 7m, such a system would likely have devastating effects on coastal cities around
the Gulf such as Abu Dhabi and Dubai. What is more, anthropogenic forcing may increase the likelihood of
extremely severe TCs in the Arabian Sea in the post-monsoon period, when TC Shaheen developed (Murakami
etal., 2017). This stresses the need to better understand the large-scale atmospheric circulation that favors the
movement of a TC into the Sea of Oman and potentially into the Gulf, and the forcings responsible for them. This
need is becoming more urgent as TCs may occur more frequently in a warming climate including in the northern
Indian Ocean (e.g., Knutson etal., 2020), as a result of the higher SSTs and moisture holding capacity of the
atmosphere (IPCC,2021).
The main objective of this paper is to identify the key factors, from larger to local scales, modulating the trajec-
tories of TCs in the Arabian Sea. It is structured as follows. In Section2, a summary of the datasets employed
and the methodology followed is provided. In Section3, an overview of TCs Shaheen and Gonu at their different
stages of evolution is given using both satellite-derived and reanalysis data. The large-scale and regional-scale
atmospheric conditions that explain their trajectory are discussed in Section4. The main findings of the study
are outlined in Section5.
2. Methodology and Datasets
In this study, a total of three satellite-derived and two reanalysis datasets are employed. In order to visualize
the spatial structure and evolution of the TCs, visible satellite images from the Moderate Resolution Imaging
Spectroradiometer (MODIS; Kaufman etal.,1997,2002) instrument onboard the National Aeronautic and Space
Administration's (NASA's) Terra and Aqua satellites are extracted from NASA's WorldView website (https://
worldview.earthdata.nasa.gov/). Insight into the vertical extent of the clouds and potential dust lifting triggered
by the storms is gained through the inspection of false-color red-green-blue (RGB; Banks etal.,2019; Francis
etal.,2019; Francis, Fonseca, Nelli, etal.,2022) satellite images from the Spinning Enhanced Visible and Infra-
red Imager (SEVIRI; Schmetz etal.,2002) instrument onboard the Meteosat Second Generation spacecraft.
RGB images are available every 15min at a 0.05° ×0.05° (∼5.6 km) resolution over the domain 60°S–60°N
and 60°W–60°E. High-resolution SSTs from the Group of High-Resolution Sea Surface Temperature (GHRSST;
Nesterov etal.,2021) data set, at a 0.01°×0.01° (∼1.1km) spatial resolution available on a daily basis from 2002
to present, are considered to explore the SSTs during the formation and development of the TCs. An evaluation of
the satellite-derived and in situ measured SSTs over the Arabian Gulf and Indian Ocean shows a good agreement,
with root mean standard deviations of less than 1K (Jangid etal.,2017; Nesterov etal.,2021), making the data
set suitable to be used in this study.
Writing – original draft: Ricardo
Fonseca
Writing – review & editing: Diana
Francis
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The reanalysis datasets employed are the European Center for Medium Range Weather Forecasting's (ECMWF)
ERA-5 (Hersbach etal.,2020) for the atmospheric fields, and the Ocean and Sea Ice Reanalysis 5 (ORAS5; Zuo
etal.,2019) for the OHC. ERA-5 data, available on an hourly basis and at a spatial resolution of 0.25°×0.25°
(∼27km) from 1950 to present, is selected as it is one of the best performing reanalysis in the Middle East and
South Asia. Fonseca etal. (2021) noted the good agreement between the air temperature predicted by ERA-5
and that observed at the location of individual weather stations in the AP in the summer season. Mahto and
Mishra(2019) and Arshad etal.(2021) found that ERA-5 outperforms other commonly used reanalyzes over
India and Pakistan, respectively. The Ocean and Sea Ice Reanalysis System 5 (ORAS5) data set, also available at
0.25°×0.25° but on a monthly basis from 1958 to present, is chosen as the predicted OHC is in general agree-
ment with that measured in situ in the northern Indian Ocean (Albert & Bhaskaran,2020), the target region in
this study.
An important meteorological feature in the AP is the Arabian Heat Low (AHL). The AHL is a thermal low that
develops mostly in the summer months as a result of the strong heating of the surface by solar radiation. It is
diagnosed following Fonseca etal.(2021). In particular, it is defined as the top 10% of the 700–925hPa thickness
(hereafter low-level atmospheric thickness, LLAT) in the domain 10°–35°N and 40°–60°E. In the computation of
the LLAT, water bodies and regions for which the 925hPa pressure level is below the surface, for which vertical
extrapolation is performed in the ERA-5 data, are excluded. This metric works best at night just before sunrise,
while during daytime the minimum in sea-level pressure more accurately depicts the AHL. The intertropical
discontinuity (ITD), the boundary between the hotter and drier desert air and the cooler and more moist Arabian
Sea air, is also considered given its role in convection initiation. It is defined using the 15°C dewpoint temperature
surface over the domain 13°–38°N and 43°–70°E, following Fonseca etal.(2021).
3. Overview of Tropical Cyclones Shaheen and Gonu
TC Shaheen, a name which means falcon in Arabic, developed in the eastern Arabian Sea from a cyclonic
circulation that moved off India in late September 2021 (Figure1a). This last system was named Gulab, and
formed in the Bay of Bengal on 24 September 2021 in association with the active phase of the MJO in that region
(FigureS1a). For a tropical disturbance to survive for such a long stretch over land (Gonu crossed India from east
to west in the final days of September 2021) is remarkable but not unprecedented. When the land surface is very
moist, as was the case over India after a wetter than average September according to the Indian Meteorological
Department (IMD,2021a), and the low-level atmosphere resembles a tropical one with a reduced temperature
variation, a tropical system can behave as if it were over water, maintaining its strength or even intensifying
over land. This phenomenon is dubbed as “brown ocean effect” (Andersen & Shepherd,2017), and has been
observed worldwide such as over Australia (Yoo etal.,2020) and North America (Nair etal.,2019). When the
vortex associated with the remnants of Gulab tracked into the northern Arabian Sea on 30 September, the system
was slow to develop. However, once it moved westwards into the northwestern Arabian Sea into an area of
above average SSTs (Figure2a) and subsurface ocean temperatures (Figure2b) as well as reduced vertical wind
shear (200–850hPa), generally below ±8ms
−1 (Figure2c) which is considered weak to moderate shear in the
region (Uddin etal.,2021), it intensified rapidly acquiring a well-developed eye (Figure1a). During this time,
the central pressure dropped to 984hPa and the maximum sustained surface wind reached about 31ms
−1, with
the storm maintaining its strength until making landfall in Oman (Figure1d). The false color RGB images given
in Figures1b and1c show the eye and surrounding deep convective clouds clearly, and that the TC-associated
circulation did not lead to major dust lifting in the region. Once over the Sea of Oman, the storm turned south-
westwards, dissipating as it moved inland (Figure1a). TC Shaheen wreaked havoc in the country, with torrential
rainfall, 60kt (∼31ms
−1) winds and 12-m waves, leaving at least 11 people dead and leading to losses of more
than 80 million USD (Al Shaibany,2021; Terry etal.,2022). As discussed in IMD(2021b), both the trajectory
and intensity of TC Shaheen were well simulated by IMD's operational model, with an error in the landfall site
of 5.5km 60hr before the storm made landfall, and a discrepancy in the wind speed of 2.4km (∼1.2ms
−1) for
a forecast lead time of 72hr, both roughly an order of magnitude smaller than the 2016–2020 average errors.
The cooler SSTs south of the track of TC Shaheen (Figure2a) generate a meridional SST gradient, with the storm
moving westwards toward an area of a positive SST gradient and where the highest SSTs are found, a behavior
that is in line with that observed for other storms in the region (e.g., Srinivas etal.,2016). Figure2b shows the
OHC anomalies in the top 300m of the ocean for October 2021 from the ORAS5 data set. Positive anomalies of
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Figure 1.
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about 1–2×109Jm
−2 are seen over most of the northern Arabian Sea, the vast majority more than one standard
deviation away from the mean, with absolute values roughly twice than those observed over the Bay of Bengal
in the period 2007–2016 (Albert & Bhaskaran,2020). The borderline La Nina that was taking place at the time
(FigureS1b) may explain the higher OHCs, as the two have been found to be correlated in the northern Indian
Ocean (Albert & Bhaskaran,2020). Hence, both the surface and subsurface ocean temperatures are favorable
for TC development and intensification. The vertical wind shear remained weak along the TC path (Figures2c
and 2d) which is crucial as strong shear disrupts the storm's circulation and can lead to its ultimate demise
(Gray,1968). To the north and south of the track, however, the shear was much larger due to the presence of the
subtropical jet and monsoon flow, respectively. As seen in the anomaly plot (Figure2d) the subtropical jet was
displaced to the north of its climatological position, which explains the reduced shear in the Sea of Oman and
northern Arabian Sea. The borderline La Nina (FigureS1b) also likely contributed to lower vertical wind shears
in the region (Hoarau etal.,2012).
TC Gonu developed in the Arabian Sea in June 2007, Figure3a, and to date is the strongest storm ever recorded
in the region (Deshpande etal., 2021), with maximum sustained surface winds reaching 65m s
−1 on 04 June
(Figure1d). As TC Shaheen, and in general for TCs that develop in the northern Indian Ocean, it coincided with
the passage of the active phase of the MJO, but this time over the western northern Indian Ocean (FigureS1a). The
Figure 1. (a) Visible satellite images from the Moderate Imaging Spectroradiometer (MODIS) instrument onboard the National Aeronautic and Space Administration's
(NASA's) Terra and Aqua satellites over the Arabian Sea in October 2021. Credit: NASA Worldview. (b–c) False-color Red-Green-Blue (RGB) satellite images derived
from the measurements taken by the Spinning Enhanced Visible and Infrared Imager (SEVIRI) instrument onboard NASA's Terra and Aqua satellites over southern
Arabian Peninsula (10°–40°N and 30°–65°E) on (b) 02 October 2021 at 03 UTC and (c) 03 October 2021 at 06 UTC. The magenta to pink shading denotes dust; white
regions are sandy areas; thick high-level clouds are shaded in orange or brown; thin high-level clouds are given in dark brown to black; dry land is shaded in pale blue
during daytime and pale green at night. (d) Estimated central pressure (blue; hPa) and maximum sustained surface wind (red; ms
−1) from 00 UTC on 30 September
2021 to 06 UTC on 04 October 2021 taken from IMD(2021b).
Figure 2. (a) The Group of High-Resolution Sea Surface Temperature (GHRSST) anomalies (K) on 30 September 2021 with respect to the 23 September–06 October
2002–2020 climatology. (b) Ocean and Sea Ice Reanalysis System 5 (ORAS5) ocean heat content anomalies (10
9Jm
−2), integrated from the surface down to 300m
depth, for October 2021 with respect to October 1979–2020 climatology. The stipple in panels (a) and (b) denotes anomalies that are at least one standard deviation
away from the mean. (c) Shows the 200–850hPa vertical wind shear (ms
−1) from ERA-5 for the domain 20°–120°E and 0°–50°N on 30 September at 00 UTC, with
the anomalies with respect to the 1979–2020 climatology given in panel (d). In all panels, the 3-hourly Tropical cyclones (TC) Shaheen's track from 00 UTC on 30
September to 06 UTC on 04 October 2021, as given by IMD(2021b), is plotted as a solid black line.
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Figure 3. As Figure1 but for Tropical cyclones (TC) Gonu in early June 2007. The red-green-blue (RGB) images in (b) and
(c) are on 06 June at 18 UTC and 07 June 2007 at 09 UTC, respectively, while the time-series in (d) is plotted from 18 UTC
on 01 June to 03 UTC on 07 June and is taken from Tyagi etal.(2011).
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timing and genesis location of TC Gonu suggests that the monsoon vortex, a low-pressure system which devel-
ops off the western coast of India around Goa prior to the summer monsoon onset (Krishnamurti etal.,1981),
played a role in its formation, as noted by Harikumar et al. (2014). The monsoon vortex, whose frequency
has decreased in recent decades, is known to promote the development of TCs (Deepa & Oh,2014). Aided by
mostly above-average SSTs (Figure4a), climatologically average values of OHC (Figure4b) with absolute values
comparable to those seen in October 2021, and reduced vertical wind shear (Figure4c), it intensified quickly
making landfall in the eastern tip of Oman on 06 June (Figure3a). Afterward, the storm moved into the Sea of
Oman on 07 June. Here, the cooler surface (Figure4a) and subsurface (Figure4b) ocean temperatures, coupled
with a wind shear exceeding 20ms
−1 (Figure4c), as a result of a southward displacement of the subtropical jet
(Figure4d), weakened the storm which dissipated over southern Iran. The wind shear threshold below which TCs
can develop and persist being 8–10ms
−1 (Panda etal.,2015; Patricola etal.,2016).
Gonu was the first TC to hit Iran since 1898, with the winds higher than 111kph (∼31ms
−1), torrential rains
in excess of 70mm, and 5.8 m waves causing widespread damage with losses of ∼216 million USD and 23
deaths (Panahi etal.,2010). As opposed to TC Shaheen, there was widespread dust lifting in the region around
the time of TC Gonu, as seen both in the visible images (brown shading; Figure2a) and RGB snapshots (pink
and magenta shading; Figures2b and2c). This may be attributed to the fact that it occurred before the summer
monsoon season, when the soils in the land regions around the Arabian Sea are generally very dry. The contrast
between the cooler SSTs to the east and the warmer SSTs to the west over the northern Arabian Sea (Figure4a)
helped to shape TC Gonu's path. As noted by Tyagi etal.(2011), the performance of operational dynamical and
statistical models for TC Gonu was rather poor, with landfall position errors of up to 930km for a forecast lead
time of 72hr, with the models predicting landfall up to 18hr earlier than observed. The higher performance for
TC Shaheen reflects the major improvements made to numerical models in recent years, including the assimila-
tion of additional surface and satellite data available in the region.
Figure 4. As Figure2 but for Tropical cyclones (TC) Gonu in early June 2007. The Group of High-Resolution Sea Surface Temperature (GHRSST) and ERA-5 fields
are given on 031 June 2007 while the Ocean and Sea Ice Reanalysis System 5 (ORAS5) anomalies are for June 2007. The 3-hourly track of TC Gonu from 1800 UTC
on 013 June to 039 UTC on 07 June 2007, as given by Tyagi etal.(2011), is plotted as a solid black line.
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4. Atmospheric Forcing
4.1. Large-Scale Environment
The trajectory of a TC is highly influenced by the large-scale atmospheric environment such as by the monsoon
circulation, intraseasonal variability associated with for example, the MJO, position and strength of the subtropi-
cal anticyclone and the presence of tropical upper-tropospheric troughs (Camargo etal.,2007; Chen etal.,2009;
Shan & Yu,2020). Other factors at play are the TC's own circulation (Li & Wang,1992), SSTs (Wang etal.,2021)
and the genesis location (Harr & Elsberry,1991). Modes of variability such as ENSO also impact the TC genesis
and track through a modulation of the SSTs and atmospheric circulation (Camargo etal.,2007; Xie etal.,2009).
The large-scale environment in early October 2021 is summarized in Figures5a–5d, which show the upper- and
lower-level streamfunction (velocity potential) and rotational (divergent) wind anomalies from ERA-5 reanaly-
sis data. In the period 01–06 October 2021 there was anomalous convection over southern and southeast Asia,
as evidenced by the upper-level divergence and low-level convergence in the region (Figures5c and 5d). In
line with theoretical arguments (Jin & Hoskins,1995), there is a quadrupole in the streamfunction anomalies
(Figures5a and5b): at upper-levels a pair of anticyclones to the west of the heating anomaly, over western and
central Indian Ocean, and a pair of cyclones to the east of it, over western and central Pacific, and the opposite
phase at lower-levels. The anticyclone that extends from India to Iraq and northern Saudi Arabia will block TC
Shaheen from moving northwards into Asia, with the associated easterly to northeasterly flow directing it to the
AP.At lower-levels, the anomalous low pressure over southeastern AP is consistent with a deeper thermal low
as discussed in Section4.2, with the associated cyclonic circulation dragging the TC inland into Oman. The
enhanced convection over the Maritime Continent and suppressed convective activity over the East Pacific and
Atlantic Ocean are likely linked with the borderline La Nina (FigureS1b) reinforced by the MJO for which the
active phase was over the Maritime Continent at the time (FigureS1a). The wavetrain in the Northern Hemi-
sphere at 200hPa resembles the composite for La Nina events and the Phase 3 of the MJO presented in Moon
etal.(2011), stressing the role of both forcings on the atmospheric state in early October 2021.
Figure 5. Streamfunction (shading; m
2s
−1) and rotational wind (arrows; ms
−1) anomalies at (a) 200hPa and (b) 850hPa and velocity potential (shading; m
2s
−1) and
divergent wind (arrows; ms
−1) anomalies at (c) 200hPa and (d) 850hPa from ERA-5 with respect to 1979–2020 for 01–06 October 2021.
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Figure6 is as Figure5 but for early June 2007, when TC Gonu developed over the Indian Ocean. Despite there
being a La Nina at the time (FigureS1b), the MJO being in phases 8 and 1 (FigureS1a) led to more active
convection over Africa and eastern Indian Ocean and suppressed convection over the western and central Pacific
(Figures6c and 6d). The response to this anomalous heating comprises a low-level anticyclone over southern
Asia to the east of the forcing, seen in the 850hPa streamfunction anomalies (Figure6b). Together with a trough
over Iran arising from an equatorward displaced Mediterranean storm track, TC Gonu moved northwestwards
into Iran, where the interaction with land and increased vertical shear in excess of 20ms
−1 (Figure4c) from a
southward displaced and stronger subtropical jet (Figure6a) led to the demise of the storm. The wavetrain over
the Northern Hemisphere has roughly the opposite phase to that in early October 2021 when TC Shaheen devel-
oped over the Arabian Sea, consistent with a shift in the MJO convective region (Moon etal.,2011).
A comparison of Figures5 and6 reveals that, over the northern Indian Ocean, the large-scale anomalies in early
June 2007 and October 2021 are roughly the opposite, in line with the shift in the position of the heating anoma-
lies. However, in both cases there is a reduction of the climatological vertical wind shear: the low-level easterlies
in June 2007 weaken the monsoon flow in the Arabian Sea, while the upper-level easterlies in October 2021
reduce the westerly shear associated with the subtropical jet. The lower pressures at 850hPa over the northern
Arabian Sea in October 2021 and to the southwest of India in June 2007 also promoted the development of the
two TCs. While the combination of the MJO phase and ENSO state in early June 2007 and October 2021 is not
unique, it is also not a preferred one (Dasgupta etal.,2021). Together with an anomalously placed AHL discussed
in Section4.2, the large-scale circulation in both periods helped to steer TCs Shaheen and Gonu into the Sea of
Oman.
4.2. Regional-Scale Environment
Figure7 shows ERA-5 and GHRSST fields from 02–04 October 2021 when TC Shaheen was over the Sea of
Oman.
Figure 6. As Figure5 but for 01–09 June 2007.
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Figure 7.
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By October, when the lower solar zenith angle and the longer nights allow for stronger surface radiative cooling
and the SSTs in the Arabian Gulf and Sea of Oman are also dropping rapidly, the AHL and the ITD are of much
reduced dynamical interest. This can be seen in the middle panels, which show the October climatological state:
the AHL is restricted to a very small region around central and southeastern Saudi Arabia, while the ITD (purple
line) is largely in its wintertime position along the southern AP's coastline. However, in early October 2021 both
features were still in their summertime mode: the ITD extended into southern parts of Iran while the AHL covered
a wide area from Iraq and southwestern Iran to southern Oman. This is consistent with the warmer-than-aver-
age surface and air temperatures in the region (not shown). As TC Shaheen moved into the Sea of Oman, there
was a minimum in sea-level pressure over southern parts of Oman which corresponds to the AHL (Figure7b).
The associated counter-clockwise circulation, together with the easterly to northeasterly winds at higher levels,
helped to drag the system inland. As it moved into central parts of Oman, TC Shaheen merged with the AHL
(Figure7c) while the cooler and more moist air advected by the storm from the Arabian Sea triggered its collapse.
Together with a ridge building over Iraq and northern Saudi Arabia and associated cold air advection into the
region (Figures7b and7c), the AHL weakened considerably, becoming closer to its climatological position for
the month of October.
The interaction between TC Shaheen and the AHL led to a strong cyclonic circulation over southern Oman which
disrupted the southwesterly flow in the Arabian Sea. At the same time, the more moist air it brought inland,
coupled with the higher-than-average SSTs over the Strait of Hormuz and eastern Arabian Gulf, allowed the ITD
to shift northwards into southern Iran, where convection developed on 06 October (not shown). This stresses the
remote effect a tropical system can have on the precipitation in the region through changes in the atmospheric
circulation. It also highlights the need to closely monitor the AHL/ITD position and strength as they can modulate
Figure 7. Sea-level pressure (shading; hPa), Intertropical Discontinuity (ITD; purple line) and 10-m horizontal wind vectors (arrows; ms
−1) (top) and sea surface
temperatures (SSTs) (shading; K) and 200hPa horizontal winds (arrows; ms
−1) (bottom) at 12 UTC on (a) 02, (b) 03 and (c) 04 October 2021. The leftmost
panel shows the fields for 2021, the middle panel gives the October climatology and the rightmost panel presents the difference between the two (the intertropical
discontinuity (ITD) is not plotted in the latter). The SSTs are extracted from the Group of High-Resolution Sea Surface Temperature (GHRSST) data set and are daily,
with the 01–06 October 2002–2020 climatology used. All other fields are from ERA-5, and the climatology employed is that of October 1979–2020.
Figure 7. (Continued)
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the path of tropical systems besides their role in summertime convection (Fonseca etal.,2021; Francis, Temimi,
etal.,2021) and dust activity (Francis, Chaboureau, etal.,2021). A comparison of the SSTs before the TC entered
the Sea of Oman (Figure7a) and when it was inland (Figure7c) highlight its cooling effect on the ocean surface:
TCs draw energy from surface evaporation leading to upwelling of colder deeper ocean water and hence to a drop
of the surface temperatures by a couple of degrees (e.g., D’Asaro,2003).
Figure8 is as Figure7 but for early June 2007 when TC Gonu affected mostly Oman and Iran. The AHL also
extended eastwards and southeastwards compared to its climatological position on 05–06 June as in early October
2021 (Figures8a and8b). In this case, the associated cyclonic circulation reinforced the monsoon winds over the
Arabian Sea and helped to propel the system northwestwards into the Sea of Oman. Once it moved into the Sea
of Oman, the AHL played a limited role in its trajectory, which was essentially controlled by the presence of a
trough over Iran and a ridge over India (Figures6a and6b). The reason for the reduced influence of the AHL on
TC Gonu's path on 07–08 June is the two-way interaction between the systems: the cyclonic circulation of TC
Gonu advected the cooler and more moist air from the Sea of Oman inland which led to a retreat of the AHL. In
fact, by 07 June 2007 at 12 UTC the sea-level pressure was close to its climatological state over most of Oman
(Figure8c). As in early October 2021, and buoyed by the warmer-than-average SSTs over the Gulf, the ITD
shifted northward into parts of UAE and southern Iran, beyond its climatological position for the month of June,
further enhancing the rainfall in the latter. A comparison with the regional-scale environment in October 2021
when TC Shaheen developed reveals that the AHL played a key role in modulating the TC trajectory with the
interaction between the TC and AHL leading to a weakening and retreat of the latter and a northward shift of the
ITD which impacted the local meteorological conditions. The cooling of the SSTs in the Sea of Oman by roughly
2–3K after the storm moved through is also seen in this case.
As noted above, the cyclonic circulation associated with the AHL helped to drag TC Shaheen inland into Oman
in October 2021 and direct TC Gonu to the Sea of Oman in June 2007. A TC that skirted the Sea of Oman was
TC Phet in June 2010. TC Phet developed over the central Arabian Sea on 31 May in a region where the SSTs
were around 30°C and the wind shear was below 10ms
−1 (Panda etal.,2015). It formed when the MJO was in
Phase 1 (FigureS1a) and in a La Nina state (FigureS1b), with a ridge over western India directing the cyclone
northwestwards, as is the case for TC Gonu. However, and as opposed to TC Gonu, instead of moving to the Sea
of Oman, TC Phet turned northeastwards into the northern Arabian Sea and Pakistan (Haggag & Badry,2012). In
order to investigate whether a weaker and/or retreated AHL may explain the different trajectories. Figure9 shows
the anomalies in the ERA-5 and GHRSST fields for 06 June 2007 and 04 June 2010 at 03 UTC, when TCs Gonu
and Phet were over northeastern Oman. A clear contrast in the spatial extent of the AHL can be seen: while on
06 June 2007 the thermal low extended into parts of central Oman and neighboring UAE and Saudi Arabia, on
04 June 2010 it was restricted to central and northern Saudi Arabia and Iraq. A similar conclusion is reached by
inspecting the sea-level pressure field which, over the eastern AP, was generally higher than the June climatology
(except around the TC) in 2010 but lower in 2007 (not shown). The results of Figure9 further stress the impor-
tance of closely monitoring the position and strength of the AHL, given its role in the trajectories of TCs that
approach the eastern AP.Besides the AHL, differences in the large-scale circulation (e.g., the upper-level winds
were southeasterly in the Sea of Oman on 06 June 2007 but weaker and more variable in direction on 04 June
2010, as seen in Figure9) and in the spatial pattern of SSTs (a more meridional gradient prevailed in 2007 while
a more zonal one dominated in 2010) are also behind the divergent paths taken by the two systems.
5. Discussion and Conclusions
In this study, the processes from local to larger scales responsible for the propagation of tropical cyclones (TC) in
the Arabian Sea toward the Sea of Oman and the Arabian Gulf are investigated, using satellite-derived and reanal-
ysis data. The case of TC Shaheen in October 2021 and Gonu in June 2007, the only two TCs to date since 1900
to move into the Sea of Oman, were analyzed to identify the key factors modulating their exceptional trajectories.
This is particularly pertinent as the expected increase in anthropogenic forcing and SSTs in the Arabian Sea with
climate change may lead to more of such storms (Knutson etal.,2020; Murakami etal.,2017). In addition, it is
possible that TCs will move into the Arabian Gulf with disastrous consequences for coastal cities such as Abu
Dhabi and Dubai (Lin & Emanuel,2016). As a result, it is crucial to understand what mechanisms modulate their
tracks so as to better predict them in the future.
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Both TC Shaheen and Gonu coincided with the passage of the active phase of the MJO over their genesis region.
In both early October 2021 and early June 2007, the SSTs and the OHC were above-average in the regions where
the TCs developed, with a vertical wind shear generally within ±8ms
−1 and therefore below the commonly used
Figure 8. As Figure7 but for 05–07 June 2007. The climatology used for Group of High-Resolution Sea Surface
Temperature (GHRSST) is 01–09 June 2002–2020 and for ERA-5 is the June 1979–2020 average.
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8–10ms
−1 threshold for TC development (Patricola etal.,2016). The combination of these factors aided in the
intensification of the storms. The two systems developed in a La Nina environment, when the vertical wind shear
is typically reduced in the region.
On the larger-scales, the atmospheric response to the MJO and ENSO heating anomalies played an important
role in the TC tracks. An anomalous ridge extending from northern India to Egypt prevented TC Shaheen from
moving into Asia, while a high pressure over India and a low pressure extending into Iran in early June 2007
directed TC Gonu northwestwards into southern Iran. The weakening of TC Gonu over the Sea of Oman was
mostly related to locally cooler surface and subsurface ocean temperatures and a higher vertical wind shear in
excess of 20ms
−1 associated with a southward shift in the position of the subtropical jet. On the other hand, in
early October 2021 the SSTs and OHC were above average in the Sea of Oman and the subtropical jet was shifted
northwards with respect to its climatological position, which allowed TC Shaheen to rapidly intensify over the
region. In addition, the west-east trajectory of TC Shaheen is a response to the meridional SST gradient in the
Arabian Sea, whereas in early June 2007 the SST gradient was in the zonal direction, with the path of TC Gonu
exhibiting a strong meridional component.
The regional-scale circulations over the AP were also of note, in particular the AHL. A stronger and southeast-
ward displaced AHL in early October 2021 forced TC Shaheen to move inland into Oman, while in the case of
TC Gonu in early June 2007 the AHL helped to shape the storm's track mostly while it was over the Arabian
Sea. There is a two-way destructive interaction between the TC and AHL: while the AHL circulation affects the
path of a TC, the TC's circulation, in particular through the advection of cooler and more moist marine air into
the AP, leads to its collapse and retreat. The latter took place after the system had moved inland in the case of
TC Shaheen but while it was still over the water in the case of TC Gonu. The role of the AHL on the trajectories
of TCs that move into the eastern AP is further stressed by comparing the paths of TC Gonu in early June 2007
with that of TC Phet in early June 2010. Having developed in similar MJO and ENSO conditions as TC Gonu and
at the same time of the year, after hitting northeastern Oman, TC Phet tracked northeastwards into the northern
Arabian Sea and Pakistan instead of moving into the Sea of Oman like TC Gonu. In the case of TC Phet, the AHL
was confined to central parts of Saudi Arabia, and did not interact with the storm's circulation. Besides the AHL,
Figure 8. (Continued)
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Figure 9. Low-Level (700–925hPa) Atmospheric Thickness (LLAT; shading; m), Arabian Heat Low (AHL; stipple) and 850hPa horizontal winds (arrows; ms
−1)
(top) and sea surface temperatures (SSTs) (shading; K) and 200hPa horizontal winds (arrows; ms
−1) (bottom) at 03 UTC on (a) 06 June 2007 and (b) 04 June 2010.
The leftmost panel shows the fields for 2007/2010, the middle panel gives the June climatology and the rightmost panel presents the difference between the two (the
Arabian Heat Low is not plotted in the latter). The climatology used for Group of High-Resolution Sea Surface Temperature (GHRSST) is 01–09 June 2002–2020 for
(a) and 01–07 June 2002–2020 for (b), and for ERA-5 is the June 1979–2020 average for (a and b).
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TCs also impact the position of the ITD, which can lead to precipitation in regions far away from those directly
affected by the TC such as southern Iran on 06 October 2021 after TC Shaheen had dissipated over Oman.
The results discussed in this work highlight the role of regional-scale circulations, in particular the AHL and the
subtropical jet, on the trajectory of TCs that approach the AP, driving them into the Sea of Oman and potentially
into the Arabian Gulf. This stresses the need to closely monitor and accurately account for these patterns in
weather and climate models used to predict TC trajectories and landfall projections. An extension of this work
would be to investigate the interaction between TC and dust in the region. This will be left for future work.
Conflict of Interest
The authors declare no conflicts of interest relevant to this study.
Data Availability Statement
All the data used to generate the figures in this study has been uploaded to Francis, Fonseca, and Nelli(2022)
(https://zenodo.org/record/6562813). All data sources used are freely available from the following websites: (a)
ERA-5 reanalysis data can be downloaded from the Copernicus' Climate Change Service website (Hersbach
etal.,2018a,b; Hersbach etal.,2019a,b); (b) the Group of High-Resolution Sea Surface Temperature (GHRSST)
daily sea surface temperature data is available on the National Aeronautic and Space Administration's website
(JPL MUR MEaSUREs Project, 2015); (c) false color satellite images generated from the measurements
collected by the Spinning Enhanced Visible and Infrared Imager (SEVIRI) instrument are available on the Euro-
pean Organisation for the Exploitation of Meteorological Satellites' website (EUMETSAT,2017); (d) the Ocean
and Sea Ice Reanalysis System 5 (ORAS5) monthly data is available on the Copernicus' Climate Change Service
Website (Zuo etal.,2018); (e) Moderate Resolution Imaging Spectroradiometer (MODIS) satellite images can
be downloaded from the National Aeronautic and Space Administration's Worldview application (Boller,2022).
All figures are generated using the Interactive Data Language (IDL; Bowman & Fowler,2015) software version
8.8.1.
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who made several insightful comments
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