Upper Ocean Response to the Tropical Cyclone Ockhi and its Impact on Primary Production in Arabian Sea
S Saranya Ganesh1, S Abhilash2*, A K Sahai1, Athira U1, S Josepha P1, P Vijaykumar2, Max Millan3 and B Chakrapani2
1Indian Institute of Tropical Meteorology, India
2Department of Atmospheric Sciences, Cochin University of Science and Technology, India
3School of Global studies, University of Sussex, UK
Submission: July 08, 2019; Published: July 29, 2019
*Correspondence author: S Abhilash, Department of Atmospheric Sciences, Cochin University of Science and Technology, Cochin, India
How to cite this article:S Saranya Ganesh, S Abhilash, A K Sahai, Athira U, S Josepha P, et al. Upper Ocean Response to the Tropical Cyclone Ockhi and
its Impact on Primary Production in Arabian Sea. Oceanogr Fish Open Access J. 2019; 10(3): 555786.
This study investigates the upper ocean response and primary productivity over Arabian Sea due to tropical cyclone ‘Ockhi’, a rare tropical storm system that formed over southwest Bay of Bengal and moved westward into the Arabian Sea while rapidly intensifying into a severe storm. Multi-satellite data products comprising Sea Surface Temperature (SST), turbulent fluxes, surface wind, and Ekman pumping derived from European Reanalysis – 5 (ERA5) daily surface wind data is used to examine the evolution and life cycle of the storm. MODIS derived Chlorophyll-a (Chlor-a), daily averaged Photosynthetically Available Radiation (PAR) and Particulate Organic Carbon (POC) are utilized for detailed analysis of the impact of the storm. Evolution of SST starting from the week prior to Ockhi shows that Southeast Arabian Sea and South-west Bay of Bengal regions had high values ranging from 28 to more than 30ºC, temperatures favorable enough to fuel the intensification of Ockhi. As the storm system moved north-westward, a clear reduction in SST is observed over the west-central Arabian Sea, affirming the upper ocean cooling associated with strong upwelling in the storm centre. A ten-fold increase in Chlor-a concentrations from 0.5 to 5 mg/m3 was observed after the passage of Ockhi, accompanied by a high wind-stress curl. The increase in PAR is primarily driven by reduction in clouds post-cyclone and a corresponding increase in POC concentrations is evident over the cold pool along the trail of storm. The maximum concentration of bloom is observed over central Arabian Sea where Ockhi remained stationary as it recurved under the influence of strong sub-tropical westerlies. The present study concludes that slow progression of Ockhi and its re-curving path contributed to enhanced upwelling through Ekman pumping and uplifting of thermocline which resulted in the reduction in SST and contributed to increase in ocean primary production.
The two major seasons leading to the development of intense cyclonic storms over the North Indian Ocean (NIO) basin are during the pre-monsoon (Apr-May-June) and post-monsoon (October-November-December) periods. In general, cyclogenesis over Arabian Sea occurs either in situ over Southeast Arabian Sea and over the Lakshadweep regions due to upper air circulations resulting from active monsoon troughs or pre-monsoon surges or in rare cases from remnants of cyclones from the Bay of Bengal that move across the south peninsular India because most cyclones that form over Bay of Bengal weaken after landfall (IMD, 2016). In addition, Arabian Sea is relatively colder than Bay of Bengal and hence inhibits the formation and intensification of storms. However, studies have suggested that the warming of NIO in recent decades may lead to increase in the frequency of extremely severe storms over the region [1,2]. Cyclone Ockhi was named an “unusual cyclone with a rare track”
by the media and scientific community alike as it was the first
storm after 1925 to severely affect the southwestern coast of
India that traversed over open sea for more than 2500 km before landfall (IMD, 2018). Its unprecedented intensification was rapid and near the coastlines resulting in massive destruction of over 10,000 homes along the coast and after its passage, almost 660 unaware fishermen who had ventured into the sea from Kerala, Tamil Nadu and Lakshadweep islands were reported missing. As per IMD records, the only other recorded cyclonic storm to follow a similar track affecting Sri Lanka and southern peninsular India was the one that formed in December 1912.
It has been recognized since the 1930s that lower tropospheric westward traveling disturbances provide the “seedling” circulations for a large proportion of tropical cyclones [3-5]. These are known as easterly waves, which are generally embedded in the trade wind flow in the lower levels of troposphere and are usually observed to occur through October to April with a periodicity of about 3 to 4 days having wavelengths of 2000 to 2500 km. Cyclone Ockhi also owes its
genesis to such an easterly wave which moved over the Comorin
Sea and southeast BoB on 28 November, 2017. By early hours
of 29 November, the low pressure developed into a depression,
passed over Sri Lanka and intensified into a deep depression
moving in a west-northwestward track moving towards
the south coast of India. The storm then underwent rapid
intensification and developed from deep depression into a Very
Severe Cyclonic Storm (VSCS: maximum sustained wind speed
greater than 64 knots) by December 1, 2017 and slowly moved
over the Lakshadweep islands into southeast Arabian Sea.
Ockhi maintained its intensity till early December 3, as it moved
towards northwest. Between December 3 and 4, under the
influence of strong upper level westerlies, the storm recurved
from east-central Arabian Sea and underwent rapid weakening
into a depression, gradually dissipating into a low-pressure area.
It made landfall over the South Gujarat coast on 6th of December.
Track of Ockhi from 29 November to 6 December is given in Figure 1.
The upper ocean responses associated with tropical cyclones
includes reduction in SST along the storm track, changes in
thermocline, surface mixed layer depth and associated upwelling
. These responses are more pronounced in the cases of slowly
moving cyclones. Satellite data has been used extensively to
study the ocean responses, ocean-atmosphere interactions and
ocean primary productivity due to the passage of tropical storms
as in situ observations are difficult during the storms [7-12]. Lin
et al.  used datasets from the Tropical Rainfall Measurement
Mission (TRMM) satelite, SST and NASA QuickSCAT ocean surface
wind vectors and showed that the strength of surface wind over
the cold SST patches weakens relative to that over surrounding
warm SST and this wind speed anomalies continue to exist
until the low SST patches disappear. This study highlights the
upper ocean response and associated variations, observed over
Arabian Sea due to VSCS Ockhi using multi-satellite data.
The NIO basin including both Arabian Sea and Bay of Bengal
covering 0 – 25°N and 50 – 100°E is the area considered for
the present study. The details of VSCS Ockhi is obtained from
storm reports of IMD and Joint Typhoon Warning Centre. ERA-5
daily averaged data sets of u and v components of wind at 10m
height with a resolution of 1x1 degree is used to calculate Ekman
pumping due to storm. Infrared Brightness Temperature (BT)
during the storm period is obtained from Geostationary satellite
datasets of National Oceanic and Atmospheric Administration
(NOAA) Climate Prediction Centre . Daily SST data (4 km
resolution) is obtained from Advanced Very High-Resolution
Radiometer (AVHRR-SST) from NOAA. Turbulent fluxes including
latent and sensible heat fluxes along with surface wind stress and
wind vector data are obtained from Scatterometer Satellite-1
(Scat-SAT1) through Meteorological and Oceanic Satellite Data
Archival Centre of Indian Space Research Organization .
Scanning Scatterometer is an active microwave device designed
to determine ocean surface level wind vectors through estimation
of radar backscatter. The Ku-band pencil beam scatterometer is
operating at 13.515 GHz providing a ground resolution cell of
size 25×25 km.
Chlorophyll-a (Chlor-a), daily averaged Photosynthetically
Available Radiation (PAR) and Particulate Organic Carbon (POC)
derived from Moderate Resolution Imaging Spectroradiometer
(MODIS) [16,17] are used to analyze the variation in ocean
primary production before, during and after the passage of
Ockhi. Algorithm for MODIS Chlor-a concentration/POC uses
an empirical relationship derived from in situ measurements
of Chlor-a/POC and blue-to-green band ratios of in situ
remote sensing reflectances (Rrs) returning the near-surface
concentrations in mg m-3. The PAR is defined as the quantum
energy flux from the Sun in the 400-700 nm range. Algorithm
for MODIS daily mean PAR estimates daily average PAR at the
ocean surface in units of einstein/m2day1. For ocean colour
applications, PAR is a common input used in modelling marine
primary productivity. Implementation of this algorithm is
contingent on the availability of observed top-of-atmosphere
radiances in the visible spectral regime that do not saturate over
Brightness Temperature in infrared spectra of geostationary
satellite from 27 November to 6 December is shown in Figure 2.
Under favorable conditions, the low pressure concentrated into
a depression by 29 November and moved westward crossing
Sri Lanka. The storm emerged in the Comorin Sea, intensifying
into a deep depression in the early hours of 30 November and
moved north-westwards strengthening into a cyclonic storm on
the same day moving near the South Kerala and Kanyakumari
coasts. The BT near the eyewall during this time is less than
200K. Ockhi intensified further as it crossed Lakshadweep Islands on December 1 and became a VSCS over southeast
Arabian Sea. It reached peak intensity of more than 150 km/h on
December 2 with an estimated central pressure of less than 980
hPa. Ockhi maintained its intensity till early hours of December
3, thereafter, came under the influence of strong subtropical
westerly ridge that was present to the north of 14°N and started
its re-curvature towards north-east and gradually weakened.
The storm made landfall near the south coast of Gujarat as a
well-marked low pressure in the early morning of 6th December
Previous studies show that upper ocean cooling is influenced
by storm intensity, cyclone translational speed, initial mixed layer
depth, Ekman pumping velocity and pre-existing circulation
patterns. Montgomery et al.  suggested that decreasing
translational speed along with high storm intensity can lead to
larger upper ocean response. According to IMD, Ockhi had a 12-
hour average translational speed of 15 kmh-1 which satisfies the
slow-moving criteria till its mature stage. The storm attained
maximum intensity on December 2 and maintained its strength
till December 3 before recurving and weakening. SST, surface
wind stress and heat fluxes are compared here to understand
the upper ocean response to Ockhi. Thus, synergic use of these
multi-satellite data sets enables us to understand the physical
and dynamical responses of upper ocean during and after the
passage of Ockhi
Figure 3 shows the AVHRR-SST superimposed with surface
wind vectors from ScatSAT-1 over NIO from November 29 to
December 5; corresponding Scat-SAT 1 analysed wind stress
curl (shaded) and stress vectors are presented in Figure 4. Warm
SSTs ranging from 29 to more than 30°C over South-East Arabian
Sea and in some areas over equatorial Indian Ocean provided
favorable conditions for storm development. On 29th November,
a strong surface circulation with weak wind stress is evident
over Sri Lanka and neighborhood which moved westward over
South coast of India by November 30. It may also be noted
that there is no significant SST variation in this region after the
storm passage. But as the storm progressed north-westwards,
a reduction in SST by approximately 1° is observed over areas
left behind by the storm. Figure 5 shows the Ekman transport
during the evolution of Ockhi from 28 November to 6 December
(left to right) . Strong upwelling is observed along the track
of Ockhi throughout its lifetime. As Ockhi intensified and crossed
Lakshadweep region, wind stress curl was having its maximum
The positive Ekman pumping near the storm centre also
became more prominent both in intensity and surface area.
During December 3 to 4, as the storm came under the influence
of subtropical westerlies, upwelling was maximum in the same
region and thus associated cooling being more prominent. On
December 5, an anomalous cold pool with SST ranging from 26.5
to less than 25°C is observed near the region where Ockhi had
maximum intensity and underwent re-curvature . The region
of cold SST anomaly is called cold patch, which lies between 10-
15°N and 67 – 75°E. Thus, by comparing SST, surface wind, wind
stress curl and Ekman pumping, one can clearly see that, surface
wind is maximum on December 2, which in turn increases the
wind stress curl to more than 4 N/m3 on December 3 leading to
strong upwelling . The effect of this wind stress on SST by
anomalous cooling over the region with a spatial scale of 100 to
400 km is observed to begin one day after intensification of the
storm to VSCS and wind stress curl is found to decrease after
December 4, resulting in reduced upwelling along the storm
track. Lin et al.  reported similar SST evolution and its link
to wind stress curl over the Western Pacific during the passage
The latent (Figure 6) and sensible heat fluxes (Figure 7)
during the life cycle of Ockhi is presented in supplementary file.
The latent heat flux values in the order of 250 to 300 Wm-2 before
the genesis of the cyclone increased to 300 to more than 350 Wm-2
during rapid intensification. Sensible heat flux values initially
ranged between 15 to 30 Wm-2 and later increased over the
entire South Arabian sea as Ockhi propagated northwestward.
The highest values are seen over the storm area
The changes in surface chlor-a, daily averaged PAR and POC
concentrations before, during and after the passage of Ockhi are
shown in Figure 4. Due to spatial discontinuity in the satellite
swath, 8-day average values of the data are used for analysis.
8-day composite values during November 18–26 is taken for
analysing the ocean state before storm, 8-day composite during
November 27 to December 4 represents the data during the
active period of the storm and composite during December 5 to
December 12 is considered to study the impact after the storm
passage. The area between 10-20°N and 65-75°E covers the path
traversed by Ockhi. The area impacted by cyclone is marked using
red ellipse in Figure 4. Low concentration of Chlor-a is found
over the area before the passage of the cyclone accompanied
by high values of PAR and low POC concentrations . During
the active period of the cyclone, Chlor-a was completely absent
over the area under the influence of the storm. The primary
productivity might have also been influenced by low values of
PAR, mostly due to overcast sky associated with Ockhi. Due to
the reduction in Chlor-a and PAR, POC was also absent during
the period when storm was active.
Upper ocean dynamical and thermodynamically response
to cyclone passage is more pronounced after the passage of the
cyclone. As evident from, the wind stress curl started increasing
from 1 December to 4 December. In response to these high
values of wind stress curl and hence storm induced upwelling,
sea surface over the regions through Ockhi passed started to
cool after December 4. This delayed response is quite common,
however the wind speed dependency and slow propagation of
Ockhi also contributed to the large values of wind stress curl
during this period as previously reported by Lin et al., 2003
over Western Pacific. After December 4, chlor-a concentration
increases to more than 5 mgm-3 along the storm track where
extreme cooling was observed from AVHRR-SST analyses. As
the System moved further north, PAR levels also increased
to 45-49 einsteinm-2day-1 over equatorial Indian Ocean along
with decrease in cloud cover. Similarly, POC concentrations
also increased during the period from the range of 10-100 to
190-370 mgm-3 near this cold patch. The latent (Figure 8) and
sensible heat fluxes during the life cycle of Ockhi is presented
in supplementary file. The latent heat flux is in the order of 250
to 300 Wm-2 before the genesis of the cyclone, which increased
to 300 to more than 350 Wm-2 as the storm underwent rapid
intensification. During genesis, sensible heat flux values range
between 15 to 30 Wm-2 and later increased over the entire South
Arabian Sea as Ockhi propagated northwestward. The highest
values are seen over the storm area.
The upper ocean response over the Arabian Sea due to
the passage of VSCS Ockhi is examined using visible, IR and
microwave channels of multiple satellites. Large area of
cooling developed in the east-central Arabian Sea in response
to high wind stress curl and stronger upwelling due to Ekman
transport over regions where the storm had maximum intensity
and minimum translational speed. The increase in upwelling
is associated with enhanced Ekman pumping and uplifting
of thermocline. Increase in wind-stress curl associated with
high surface wind clearly led to the reduction in SST followed
by increase in PAR, resulting in high concentrations of Chlor-a
and associated enhancement of plankton population along the
track of Ockhi. It is evident that wind stress induced upwelling
and cold patch of SST along with increase in PAR indeed lead to higher Chlor-a and POC concentrations in the region of maximum
storm intensity and minimum translational speed. A delay of
1-2 days is observed between the maximum wind-stress curl
and maximum Chlor-a and POC concentrations. Increased cloud
cover might have contributed to the reduction in PAR values
during the active periods of the storm and hence to the delay
in attaining maximum primary productivity as evident from
peak Chlor_a and POC concentrations. The study underlines the
potential of combined use of remotely sensed data from multiple
sources for carrying out impact assessment of intense weather
events occurring over oceanic regions
The Multi-Satellite datasets used are listed in the references
and plotting interfaces (GrADs, NCL, Ferret etc) are gratefully
acknowledged. Data from the ERA5 analysis were generated
using the Copernicus Climate Change Service and are publicly
available on the ECMWF website. SGS is thankful to SPPU for PhD
admission and IITM for research fellowship. SA acknowledge the
facilities provided under SAC/EPSA
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