Mesoscale variability in the Arabian Sea from HYCOM model results and observations: impact on the Persian Gulf Water path

Abstract

Abstract. The Arabian Sea and Sea of Oman circulation and water masses, subject to monsoon forcing, reveal a strong seasonal variability and intense mesoscale features. We describe and analyze this variability and these features, using both meteorological data (from ECMWF reanalyses), in situ observations (from the ARGO float program and the GDEM -Generalized Digital Environmental mode -climatology), satellite altimetry (from AVISO) and a regional simulation with a primitive equation model (HYCOM -the Hybrid Coordinate Ocean Model). The model and observations display comparable variability, and the model is then used to analyze the three-dimensional structure of eddies and water masses with higher temporal and spatial resolutions than the available observations. The mesoscale features are highly seasonal, with the formation of coastal currents, destabilizing into eddies, or the radiation of Rossby waves from the Indian coast. The mesoscale eddies have a deep dynamical influence and strongly drive the water masses at depth. In particular, in the Sea of Oman, the Persian Gulf Water presents several offshore ejection sites and a complex recirculation, depending on the mesoscale eddies. The associated mechanisms range from coastal ejection via dipoles, alongshore pulses due to a cyclonic eddy, to the formation of lee eddies downstream of Ra's Al Hamra. This water mass is also captured inside the eddies via several mechanisms, keeping high thermohaline characteristics in the Arabian Sea. The variations of the outflow characteristics near the Strait of Hormuz are compared with variations downstream.

Mesoscale eddies and submesoscale structures of Persian Gulf Water off the Omani coast in Spring 2011

“…5). This more dilute PGW is well marked in the blue profile; it a priori originated from the outflow during a previous season, and must have recirculated at the western end of the Sea of Oman, as observed in a numerical simulation (L'Hégaret et al, 2015). The coastal outflow of PGW appears on the brown and red profiles (upper panels of Fig.…”

“…During other seasons, the path of the PGW in the Gulf of Oman is less regular, as shown by observations and by numerical modeling; also, PGW can exit in the form of short pulses (see Banse, 1997;Senjyu et al, 1998;Bower et al, 2000;Prasad et al, 2001;Thoppil and Hogan, 2009;Wang et al, 2012;Wang et al, 2013). Recently, ARGO floats (see Carton et al, 2012;L'Hégaret et al, 2013) and HYCOM numerical simulations (see L'Hégaret et al, 2015) confirmed that during other seasons, PGW can be expelled from the coast into the Sea of Oman. These ejections were related to the presence of mesoscale eddies in the Sea of Oman, and especially to the presence of a dipole in spring; different offshore ejection mechanisms were identified.…”

“…In 2011, the wind stress curl off Ra's Al Hadd changed sign from March to May. During this period, the wind work leads to a deepening of the surface eddies (see L'Hégaret et al, 2015 andVic et al, 2014). In Fig.…”

“…These biases are due to thermal inertia of the sensors, and mostly occur between the lowering and rising of the device, which result in a delay between the conductivity and temperature measurements (see Barth et al, 1996). These errors are corrected by applying a couple of coefficients, a first one correcting the amplitude of the signals between the rising and the lowering of the SeaSoar, and a second coefficient correcting the time delay (see Lueck andPicklo, 1990 andMensah et al, 2008). Once corrected, the SeaSoar measurements 2 CTD for conductivity, temperature, depth 3 ADCP for acoustic Doppler current profiler 4 XBT and XCTD for expendable bathythermograph and CTD 5 VM-ADCP for vessel-mounted acoustic Doppler current profiler are validated against CTD station or XCTD cast data (such stations or casts were achieved along the SeaSoar transects).…”

“…Al Saafani et al (2007) identified eddies in the Gulf of Aden generated by Rossby waves emitted from the Indian coast or amplified in the interior of the basin. This mechanism is also present in the northern Arabian Sea, with Rossby waves being forced by wind and by coastal Kelvin waves (see L'Hégaret et al, 2015).…”