Seasonal and Interannual Variability of of Mukalla, Yemen) and Ras Asir (northern corner of
the Somali Peninsula). The 900 km long GA spreads
eddy field and surface circulation in the
over an area of about 220 x 103 km2. The average depth
Gulf of Aden. is about 1800 m. Following the monsoonal cycle over
M. A. Al Saafani and S. S. C. Shenoi the northern Indian Ocean, the winds are westerly
Physical Oceanography Division, National Institute of during summer monsoon (June - September) and
Oceanography northeasterly during the winter monsoon (November -
Goa - 403 004, India. March). During the intermediate periods April - May
and October they are weak and unstable in their
From oceanographic perspective, the GA is
The circulation in the Gulf of Aden is inferred from important because it provides the passage for the Red
three different data sets: historical ship drifts, Sea Water (RAW), one of the most saline water mass in
hydrography, and satellite altimeter derived sea level the world oceans, to the Indian Ocean. Commercially
(Topex/Poseidon, Jason and ERS). The circulation in also, it is significant because it is the highway for
this semi-enclosed basin is marked with strong international trade since the start of recorded history ,
seasonality with reversals in the direction of flows twice nearly 3 million barrel of oil is transported through the
a year following the reversal in monsoonal winds. Gulf of Aden every day which increases the risk of
During the winter monsoon (November - February) tanker accident and oil pollution. In spite of the
there is an inflow from Arabian Sea; an extension of importance of the Gulf of Aden, very little information
Arabian Coastal Current (ACC). During southwest is available on Gulf of Aden's hydrography and general
monsoon (June - August) the flow is generally towards circulation. Most of the investigations in this area was
east especially along the northern coast of Gulf of Aden. aimed at understanding the exchanges between Red Sea
The geostrophic currents also show that the circulation and Gulf of Aden [2-3] or to study the spreading of Red
in the gulf is embedded with mesoscale eddies. These Sea outflow into the Gulf of Aden [4-10] and finally
westward propagating eddies appear to enter the Gulf of into the Arabian Sea 
Aden from the western Arabian Sea in winter. The During NE monsoon, winds drive the surface
relative contribution of mesoscale eddies to the water towards Bab-el-Mandab Strait, particularly along
circulation in the gulf were estimated using altimeter the Arabian coast. Along the Somali coast, there is a
derived Sea level anomaly (SLA) for the years 1993 to weak current in the opposite direction and a cyclonic
2003. The effect of these mesoscale eddies extend over eddy in the middle of the gulf . Cyclonic and anti-
the entire water column. The propagation speeds, of cyclonic eddies are also visible in satellite imagery as
these eddies, estimated using weekly spaced altimeter well as in hydrography .
derived SLA (2002 - 2003) is ~ 4.0 - 5.3 cm s-1. The sum During the SW monsoon, the surface water
of the speeds of second mode Rossby wave and the generally moves in the opposite direction. During this
mean current (4.8 cm s-1) matches with the propagation season upwelling is very intensive in three regions in the
speeds of eddies estimated using SLA. Hence, second Arabian Sea [14-16], one of these regions is the
mode baroclinic Rossby waves appear to be responsible southern coast of Arabia especially along the Omani
for the westward propagation of eddies in the Gulf of coast and the eastern part of the Yemeni coast along
Aden. The presence of these eddies in the temperature- Gulf of Aden. This seasonal upwelling is quite
salinity climatology confirms that they are not transient pronounced in SST imagery in the western Gulf of Aden
1. Introduction Though the presence of eddies in the Gulf of
The Gulf of Aden (GA), a narrow strip of water Aden were reported in earlier studies [5-6,12,17-18] it is
body that connects the Red Sea with the Indian Ocean not known whether they are formed locally in the gulf
extends east northeastward from the narrow Strait of or formed elsewhere and then propagated into the gulf.
Bab-el-Mandab to a line between Ras Baghashwa (east Reference  reports the propagation of anticyclonic
rings into the Gulf of Aden from Somali current through
Socotra passage. They estimated the propagation speed geostrophic currents show westward flow all over the
as 5 - 8 cm s-1. gulf with eddies embedded in it (Figures 2 and 3).
In the present study, the monthly mean Prominent is the anticyclonic eddy in the eastern part of
climatology of hydrographic data, ship drift and weekly gulf and the cyclonic eddy in the center. The weaker
altimeter data were used to describe the seasonal cycle anticyclonic eddy vanishes completely in March.
of circulation and eddy field in the Gulf of Aden. The During May, the flow inside the gulf is towards
satellite derived weekly Sea level anomaly (SLA) data east along both coasts and it is towards west in the
for 11 years are used to study the propagation of eddies. center. During this period three anticyclonic eddies are
The interannual variability in these eddies and their seen, one at the western part of the gulf, second at the
effects on the circulation are also analyzed. The center and the third near the mouth. Summer pattern is
remainder of this paper is organized as follows. In clearly seen in July when the Somali current is fully
section 2 we summarize the data sources and analysis developed in the east. During this month, the flow is
methods. In section 3 we present the results. In section 4 towards east along the Yemen coast while there is a
we discuss the results, and finally the summary is given weak westward flow along the Somali coast of gulf.
in section 5. The circulation pattern in September follows
the summer pattern with outflow along Yemeni side and
2. Data and Methodology: weak inflow along Somali side. Also the anticyclonic
A new hydrographic data set was assembled using eddy seen during July - August have become much
temperature-salinity profiles from the archives of (i) weaker. During November the flows reverses the
National Oceanographic Data Center (NODC) (ii) Japan direction. The winter pattern starts appearing with
Oceanographic Data Center (JODC), (iii) the CTD establishment of a cyclonic eddy at the center. Now
profiles collected during the Bab-el-Mandab Experiment there is an inflow along Yemeni side and outflow along
[3, 20], (iv) the CTD profiles of the Netherlands Indian Somali side.
Ocean Programme  and (v) the data collected during At the peak of winter, in December-January,
1984-85 by Marine Science and Resources Research when the winds are from the northeast, the surface
Center, Yemen . All profiles were subjected to currents are towards the west over most of the gulf with
quality checks following . After quality checks 4088 mesoscale eddies moderating the flow. This is more
unique profiles (west of 55o E) of temperature-salinity evident in altimeter data and hydrography derived
were retained for analysis. surface currents (Figure 2-3) rather than in ship drifts
The merged TOPEX/Poseidon and ERS-1/2 (Figure 1). Indications are also there to suggest
sea levels  for the period from 1993-2003 are used upwelling along the southern coast.
for the analysis of sea levels.
The effects of these eddies are felt at deeper depths also.
3. Result: Figure 4 shows the surface distribution of temperature
3.1 Seasonal Circulation: and vertical section of temperature along the axis of the
The seasonal cycle of surface circulation in the gulf and the SLA climatology during July. The location
Gulf of Aden is described based on three sources of data of anticyclonic eddy coincides with the deepening of
sets (i) the ship drifts, (ii) the geostrophic currents thermocline (up to ~ 600 m) in the section.
estimated using merged altimeter data, and (iii) the
geostrophic currents estimated using hydrographic data. 3.2 Propagation of eddies:
It may be noted that the ship drift is a combination of From monthly climatology of SLA (Figure 2) it
winds and currents while the other two represents only can see that eddies propagate westward in the Gulf of
the geostrophic component of surface current. Aden during winter. During summer, a strong
During January, the flows seen from the ship anticyclonic eddy is seen at the center of the Gulf of
drifts (Figure 1) are towards the west along the Yemen Aden (May – September).
and Somali coasts while it is towards east in the center. The propagation speed of these eddies can be
Altimeter as well as the hydrography derived calculated using the time-longitude plots of weekly SLA
(Figure 5). During 2002-03, the propagation speed in
the Gulf of Aden was found to be ~ 4.0-5.3 cm s-1.
Figure 3: Bimonthly distribution of sea surface
Figure 1: Bimonthly distribution of surface current
temperature and surface geostrophic current derived
derived from NOAA ship drift data.
from hydrographic data.
Figure 4: Distribution of SLHA for month of July
compared with SST distribution for the same month and
the vertical distribution of temperatue shows the
Figure 2: Bimonthly distribution of sea surface height
deepening of the thermocline in the region of the
and surface geostrophic current derived from SLHA
This speed is somewhat similar to the speeds of
second mode Rossby waves (3.5 cm s-1) estimated from
the Brunt-Väisäla frequency profiles derived from the
average density profile for the winter along 13 °N.
annual signal and interannual signal to the circulation in
the gulf were estimated by calculating the correlation
between the interannual signal, annual, and mesoscale
variation with the SLA (figure 6). In the absence of
interannual variability, the mesoscale eddies and the
annual variability caused by the monsoonal winds
appears to be the only contributors to the circulation in
the gulf. The contribution from the mesoscale eddies is
as important as the annual cycle.
Figure 5: Time/longitude plot oh SLHA for 2002/2003
used to calculate the westward propagation of the eddies
in the Gulf of Aden.
3.3 Mesoscale sea level variability:
Sea level anomaly (SLA) time series η(x,y,t) can be
considered as composed of three modes of variability;
(i) the interannual variability (ηi(x,y,t)), (ii) the annual
cycle (ηa(x,y,t)), and (iii) the high frequency variability Figure 6: Correlation between the time series of the SLA
(ηm(x,y,t)). Hence, the sea level anomaly can be with annual, interannual, and residual signals..
represented as 
The surface circulation in the Gulf of Aden was
η (x,y,t) = ηi (x,y,t) + ηa (x,y,t) + ηm (x,y,t)
described using different data sets for the first time. The
circulation in this semi enclosed basin is marked with
The annual cycle (1 year period) mainly reflects the
strong seasonality with reversal in the direction of flow
variations of sea level due to seasonal changes. All
twice a year following the reversal of the monsoonal
signals having period shorter than one year are
wind. During NE monsoon, northeasterly winds cause
considered here as high frequency signals or mesoscale
the westward surface currents. The presence of the
variability. Here we extract the the interannual
westward propagating eddies complicate the situation to
variability by filtering the data with a moving average
cause inflow along Yemen coast and outflow along
filter having a duration of 1 year (here 53 samples
Somali coast [12,17].
because of weekly data). Then the harmonic annual
cycle is approximated by a sine function given as
During SW monsoon the southwesterly winds causes
ηa (x,y,t) = A(x,y) Sin[2ωt + ϕ(x,y)]
eastward flows and an upwelling along the Yeman coast
where, ω, the frequency is one cycle per year.
of the gulf. Also during this season, an anticyclonic
Amplitude A and phase ϕ are estimated using a least
eddy forms in the gulf. The flow is mainly towards east
square method. Finally to estimate the high frequency
with some inflow from the Somali Current that flows
variability, the interannual variability and the annual
towards north along the Somali coast.
cycle are subtracted from the total SLA (η(x,y,t)).
Eddies in the Gulf of Aden influence not only
The relative contribution of mesoscale eddies,
the surface circulation (figure 4), but also the deeper
depths. At deeper depths, the eddies influence the Acknowledgment:
spreading of the Red Sea Water [5,6]. Reference 
reports that eddies first forms in the Somali Current and Hydrographic data from NODC and JODC were
then propagate into the gulf through Socotra passage. downloaded from
Our analysis shows that they propagate from the interior http://www.nodc.noaa.gov/OC5/WOD01/data2001.html,
Arabian Sea as a part of westward propagating annual http://www.jodc.go.ip/service.htm, the CTD profiles of
Rossby wave. The propagation speed of these eddies the Netherlands Indian Ocean Programme were
estimated from the time/longitude plots (4.0 – 5.3 cm s- downloaded from the NIOP web site. Authors would
1) of SLA matches well with the sum of second mode
like to acknowledge the help of J. Piechura for
Rossby wave (3.5 cm s-1), at these latitudes, and the providing the data collected during 1984-1985 by
speed of mean currents (1.3 cm s-1) in the gulf. Hence, Marine Science Resource and Research Center
we conclude that the second mode baroclinic Rossby (MSRRC), Yemen. Al Saafani thanks Sana'a
waves are responsible for the westward propagation of University, Government of Yemen for sponsoring the
eddies in the Gulf of Aden. Ph. D. research and Director, NIO for support and
computational facilities. The figures in this manuscript
The climatology of SLA created from 11 years weekly were prepared using FERRET and GMT softwares.
data (Figure 2) and the climatology of hydrography
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