Sensitivity of the Indian Ocean circulation to surface wind stress

Transcription

1 Indian Journal of Marine Sciences Vol. 37(1), March 008, pp Sensitivity of the Indian Ocean circulation to surface wind stress A. C. Pandey* & Shailendra Rai K. Banerjee Centre of Atmospheric and Ocean Studies, University of Allahabad, Allahabad 11 00, India *[ ] There is a lot of debate on the responses of Antarctic Circumpolar Current (ACC) in relations to changes in Southern Hemisphere winds and how the momentum input by the surface wind stress can be transferred down to the ocean floor. An σ coordinate Ocean General Circulation model was used in the present study. The major circulations in the Southern as well as tropical Indian Ocean have been discussed. The sensitivity of model with respect to wind stress forcing has been performed by using the surface wind stress climatological data of Hellerman and dasilva for the Indian Ocean up to 60ºS. It has been found that the response of zonal wind stress over tropical Indian Ocean north of 5ºS was large. The response of change in surface wind stress was negligible in the Southern Indian Ocean. [Key words: Southern Indian Ocean, circulation in Indian Ocean, wind stress forcing] 1. Introduction Most of the modeling studies were concentrated on equatorial and tropical Indian Ocean and very little attention has been drawn on the Indian sector of Southern Ocean due to data scarcity, complex topography and mesoscale eddies 1 in this region. The upper circulation of Southern Indian Ocean (SIO) is limited by south equatorial current to the north, subtropical convergence to the south and restricted to the west by Agulhas Current system so it can be thought as the effect of anticyclonic wind driven gyre. The SIO links both the deep and upper circulation of Atlantic Ocean and Pacific Ocean 3. Currents of the ocean are divided as wind driven currents generated due to wind stress forcing and thermohaline currents due to density difference. It is generally thought that the wind stress is the major driving force for Antarctic Circumpolar Current (ACC) although some studies showed importance of thermohaline processes in driving ACC 4. There are number of studies stating responses of ACC in response to changes in the Southern Hemisphere winds 5-9 with various plausible explanations and hypothesis. However, the dynamics determining the transport of ACC with changing winds is still unclear. In the present study, attempt has been made to see the circulation response of SIO region by using wind stress data of two different sources. This will enable us to understand the sensitivity of wind stress forcing *Corresponding author Ph: Fax: +91 (53) in the Southern as well as tropical Indian Ocean. The model used in this work was Princeton Ocean Model (POM) which was developed initially for simulating coastal and regional simulations but now used world wide for basin scale simulations also 10, 11.. Model Settings and Data Princeton Ocean Model (POM) 1, 13 has been used in this study which is primitive equation, sigma coordinate, free surface. This Ocean General Circulation Model (OGCM) uses turbulent closure mixing scheme 14. Model bottom topography is shown in the Fig 1. Bottom topography of 5' resolution of terrain base is used and interpolated to model grid. The sigma coordinate has 16 vertical layers (σ = 0.0, , , , -0.14, -0.86, , -0.49, , , , , , , -0.99, -1.0) the resolution is fine for upper 8 layers and coarse for the lower 5 layers and σ = (z-η)/(h+ η) where η(x,y) and H(x,y) are the surface elevation and water depth, respectively. The model has a split time step, a two dimensional external time step of 15 sec and a three dimensional internal mode time step of 450 sec. The model grid extends from the deep ocean to 10 m depth on the coastal region to shallow region in the model simulation. The maximum grid between two adjacent grid points is ΔH/H < 0.. Model is initialized with the monthly climatology of Levitus 15,16 atlas for temperature and salinity. The Southampton Oceanography Centre (SOC) climatological data 17 is used for surface forcing of heat flux. The open lateral boundary conditions have been used in the current experiments. The north and

2 56 INDIAN J. MAR. SCI., VOL. 37, No. 1, MARCH 008 Fig. 1 Bottom topography of the model domain (in meter) (contour interval is 500 meters).

3 PANDEY & RAI: INDIAN OCEAN CIRCULATION 57 Fig. Vertical profile of (A) temperature and (B) salinity for experiment EXPT1 and (C) temperature and (D) salinity EXPT for 1, 5, 10 years of model run with respect to initial condition (IC). south open boundaries of the model are governed by the Sommerfeld radiation condition 18. Therefore, although the total transport on the open boundaries are set to zero, the internal velocities at each level are free to adjust geostrophically to the density field. A Smagorinsky-type 19 horizontal diffusion is used here and the diffusion coefficient was calculated using equation: A M u = 0.ΔxΔy x 1 v u + + x y 1/ v + y where, u and v are the horizontal velocity component and C is a coefficient taken here as 0.. Model is configured for the whole Indian Ocean extending in south up to Antarctic continent in SIO and some parts of Atlantic Ocean is also included to cover the Agulhas Retroflection region (0ºE to 150ºE and 70ºS to 30ºN). Two set of model experiments has been performed for sensitivity of surface wind stress in the Indian Ocean (SIO in particular). In the first experiment, the wind stress forcing of monthly climatological data of Comprehensive Ocean- Atmosphere Data Set (COADS) analyzed by da Silva 0 is used and named as EXPT1 whereas Hellerman & Rosenstein 1 Global Wind Stress Climatology in EXPT. In order to reduce the unrealistic temperature and salinity changes in the mixed layer we added a weak relaxation of temperature and salinity to the monthly climatological values with a relaxation time scale of 100 days. Fig. 3 Mean surface current for 10 year of model run (EXPT1) in the Indian Ocean simulated by POM with spatial resolution of 1º 1º for the seasons (A) January March and (B) June August (arrow length of 0.5 cm represents current speed of 0. m/sec).

4 58 INDIAN J. MAR. SCI., VOL. 37, No. 1, MARCH Results and Discussion The representation of temperature, salinity and model climate drift is an indicator of how long the model has been run in order to produce realistic results, in sense of Indian Ocean climate change. To evaluate the model climate drift during the simulation we have examined area averaged vertical profile of temperature and salinity. Figure shows the vertical profile of temperature and salinity for 1, 5, 10 years of model run with respect to initial condition for the both the experiments EXPT1 and EXPT. Model climate drift is negligible with respect to initial condition as shown in these figures. The changes in vertical structure reduce the vertical density gradient, resulting in a more diffuse model thermocline and halocline in comparison with the initial conditions. 3.1 Simulation of circulation in Indian Ocean Seasonal variation of surface current climatology produced by the model is discussed to show the ability of the model to capture all the major currents of Indian Ocean. Figures 3 and 4 showed the seasonal average of model produced circulations for the seasons January-March (JFM) and July-September (JAS) for summer and winter monsoon of tropical Indian Ocean from 10 years of model run for both the experiments. During the winter monsoon (JAS) the northward moving East African Coast Current (EACC) (roughly 45ºE, 5ºS) meets southward flowing Somali current (roughly 50ºE, 5ºN) in the band of ºS-5ºS and the two branches form the east ward flowing South Equatorial Counter Current (SECC) (roughly along 3ºS) as simulated by the model (Figs 3A, 4A). The west ward flowing Northeast monsoon current (NMC) (along 5ºN, 70ºE-85ºE) south of Sri Lanka and its movement toward West Fig. 4 Mean surface current for 10 year of model run (EXPT) in the Indian Ocean simulated by POM with spatial resolution of 1º 1º for the seasons (A) January March and (B) June August (arrow length of 0.5 cm represents current speed of 0. m/sec).

5 PANDEY & RAI: INDIAN OCEAN CIRCULATION 59 Indian Coast Current (WICC) 3 (along 71ºE, 8ºN- 0ºN) is clearly captured by the model (Fig 3A, 4A). During the summer monsoon the South Equatorial Current (SEC) (along 15ºS) and EACC meets with Somali current and is simulated by the model (Fig 4A,B). The southwest monsoon current (SMC) along 5ºN, 70ºE-85ºE south of Sri Lanka flows eastward in this season. In this season bifurcation of East Indian coastal current (EICC) (along 85ºE, 8ºN- 0ºN) in the Bay of Bengal was clearly seen which was supplied by SMC. In SIO region the SEC 4 which is driven by Southeast Trade winds was found to be present and it was the source of western boundary current west of Madagascar within the latitude band of 1ºS-5ºS. The splitting of SEC in northward moving EACC and southward moving Northeast Madagascar current (roughly 45ºE, 5ºS) was clearly evidenced by the model (Figs 3, 4). The major uninterrupted Antarctic Circumpolar Current (ACC) was well seen between 40ºS to 60ºS and its velocity was between 0 cm/sec to 40 cm/sec, which is in agreement with observational and modeling studies 5. The retroflection of Agulhas current system 6,7 i.e. separation of Agulhas current from the southern tip of South Africa was seen, in which, some part retroflected and meet the west wind drift current system and other part meet in the Benguella current system (roughly 10ºE, 5ºS), which was clearly seen in the model simulation (Figs 3, 4). The third important current in SIO region was the west wind drift current flowing westward along the coasts of Antarctica continent driven by west flowing wind stress, which was seen in the model simulation with a speed comparable to ACC. The Leeuwin current along west coast of Australia was also seen in the model (Figs 3, 4). Since the available observational data south of about 10ºS do not reveal much seasonal variability 8 there and the representation of circulation features in SIO showed identical circulation branches for all the seasons (Figs 3 and 4). Fig. 5 Seasonal average of difference of zonal component of wind stress forcing of EXPT and EXPT1 in the season (A) Jan Mar (B) Apr Jun (C) Jul Sep and (D) Oct Dec (N/m ).

6 60 INDIAN J. MAR. SCI., VOL. 37, No. 1, MARCH 008 Fig. 6 Seasonal average of difference of zonal component of ocean surface current of EXPT and EXPT1 in the season (A) Jan Mar (B) Apr Jun (C) Jul Sep and (D) Oct Dec. 3. Sensitivity of zonal wind stress on the circulation features of Indian Ocean To see the sensitivity of zonal wind stress over tropical as well as SIO, the difference of zonal wind stress (WU) for EXPT and EXPT1 has been calculated and averaged for the seasons January March (JFM), April June (AMJ), July September (JAS) and October December (OND) and the plots have been shown in Fig 5. Similarly, the model produced zonal surface current of EXPT1 has been subtracted from EXPT and averaged for JFM, AMJ, JAS and OND seasons and shown in Fig 6. In the JFM season, WU of Hellerman & Rosenstein 1 are stronger than that of dasilva 0 in the latitudinal bands of 50 S to 60 S and 30 S to 10 S (Fig 5A) in the range of N/m whereas the opposite is the case in the band of 30 S to 50 S. It is clear from Fig 6a that effect of surface circulation on zonal wind stress was negligible in the ACC region but it is prominent in equatorial region (10 S - 10 N) being a common feature for all the seasons. It was also clear from the Fig 6 that the effect of WU forcing on coastal regions was larger, even near Antarctic coasts. The difference of wind stress forcing of N/m in the region near Red Sea, around Srilanka and Indonesian throughflow (ITF) for AMJ season corresponds to a difference of m/s in zonal surface current (Fig 5B and 6B). The similar behavior was also seen for the SON season for the same regions (Figs 5C and Fig 6C). In the SIO region the ocean near Kerguelen Island, having shallow topography, showed response of zonal wind stress on the zonal circulation for all the seasons. 4. Conclusion The sigma coordinate ocean model POM has been configured for Indian Ocean up to Antarctica continent. Two experiments EXPT1 and EXPT have been performed by applying Hellerman and dasilva wind stress forcing keeping other variables same. The well defined circulation features have been captured by the model for both the experiments. Since the wind

7 PANDEY & RAI: INDIAN OCEAN CIRCULATION 61 stress forcing was different in both the experiments, the sensitivity with reference to zonal wind stress forcing has been checked. The effect of zonal wind stress in the ACC region is small whereas it was large in the equatorial and coastal regions. The reason of larger response of WU on zonal currents in equatorial region was due to the fact that the Ekman transport was larger in these regions as compared to mid and high latitudes if we consider forcing of wind stress only 9. The reason of larger response in coastal regions may be due to shallow bottom topography at these places. Acknowledgement We thank National Centre of Antarctic and Ocean Research (NCAOR), Goa and Ministry of Earth Sciences (Earlier known as Department of Ocean Development) for providing financial support. References 1 Kostianoy, A. G., Ginzburg, A. I., Lebedev, S. 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