Multi-satellite altimetry and GOCE geoid based surface and subsurface currents in the Mediterranean Sea

ADT (𝜼𝜼𝒂𝒂𝒂𝒂𝒂𝒂)on sea surface can be calculated from the following equation and filtered which is called Pointwise approach: 𝜼𝜼𝒂𝒂𝒂𝒂𝒂𝒂 =𝟐𝟐𝟐𝟐[𝑺𝑺𝑺𝑺𝑺𝑺 − 𝑵𝑵 ] (1) Relative dynamic topography (𝜼𝜼𝒓𝒓𝒓𝒓𝒓𝒓 )at certain depth can be computed by the following equation: [Cadden et al., 2009] 𝜼𝜼𝒓𝒓𝒓𝒓𝒓𝒓 =∫ 𝑷𝑷𝑹𝑹 𝟎𝟎𝒅𝒅𝒅𝒅 𝝆𝝆(2) 𝒅𝒅means pressure,𝑷𝑷𝑹𝑹is reference pressure at certain depth under sea surface；𝝆𝝆is density which can be calculated from sea water temperature and salinity data. Absolute geostrophic current velocities at different layers can be derived by the combination of the use of 𝜼𝜼𝒂𝒂𝒂𝒂𝒂𝒂 and 𝜼𝜼𝒓𝒓𝒓𝒓𝒓𝒓 [Wunsch and Gaposchkin,1980;Cadden et al., 2009] 𝒖𝒖𝑹𝑹=−[𝒈𝒈 𝒇𝒇 𝝏𝝏𝜼𝜼𝒂𝒂𝒂𝒂𝒂𝒂 𝝏𝝏𝒚𝒚 +𝒈𝒈 𝒇𝒇 𝝏𝝏𝜼𝜼𝒓𝒓𝒓𝒓𝒓𝒓 𝝏𝝏𝒚𝒚 ](3) 𝒗𝒗𝑹𝑹= [𝒈𝒈 𝒇𝒇 𝝏𝝏𝜼𝜼𝒂𝒂𝒂𝒂𝒂𝒂 𝝏𝝏𝒙𝒙 −𝒈𝒈 𝒇𝒇 𝝏𝝏𝜼𝜼𝒓𝒓𝒓𝒓𝒓𝒓 𝝏𝝏𝒙𝒙 ](4) 𝒈𝒈is gravitational acceleration, 𝒇𝒇is Coriolis parameter, 𝒖𝒖,and 𝒗𝒗means east-west, and north-south component of sea surface geostrophic velocities, respectively.𝒖𝒖𝑹𝑹,and 𝒗𝒗𝑹𝑹 means geostrophic velocities at certain depth. Methodology The 5th International GOCE User Workshop, UNESCO, Paris, France, 25-28 November, 2014 Data Along-track TOPEX, JASON-1, JASON-2, ERS-2 and ENVISAT Sea Surface Height (SSH) data provided by Radar Altimetry Database System (RADS) were used to generate along-track Absolute Dynamic Topography (ADT) by using geoid calculated from GOCE-TIM5(up to 280 degree) [Pail et al., 2011]and ITG-GRACE2010S(up to 180 degree) [Mayer-Gürr et al., 2010]gravity field models as reference datum.Afterwards, the along-track ADT were used to calculate the geostrophic currents after gridding.The quality of the results is evaluated by comparing with in-situ surface current velocities collaborated, processed and distributed by the Global Drifter Program (GDP) [V2.07,Lumpkin and Johnson, 2013]. Monthly T/S profile data (GLOBAL_REP_PHYS_001_013), derived from Argo profiling floats, XBT, CTD and moorings measurements, are provided by MyOcean on a 0.25oregular grid covering latitude 82oSto 90oNand 0o~360oEwith 33 layers of 0 ~ 5500 mdepths during the time span of 1993 to 2011.With T/S profiles, we are able to derive Relative Dynamic Topography (RDT) and then to calculate absolute geostrophic currents at different layers when combining with ADT.In the study, data covering 1996~2011 are used for analysis. Multi-Satellite Altimetry and GOCE Geoid Based Surface and Subsurface Currents in the Mediterranean Sea Juan Jose Benjamin1,Chi-Hung Chang2, Chung-Yen Kuo2, C.K. Shum3, Yuchan Yi3 1Department of Geotechnical Engineering and Geosciences, Technical University of Catalonia, [email protected] 2Department of Geomatics, National Cheng Kung University, [email protected], [email protected] 3Division of Geodetic Science, School of Earth Sciences, The Ohio State University, ck[email protected]du, [email protected] The Mediterranean Sea (MS) is asemi-enclosed "true" ocean bordering European nations from the Western Europe to the near-, middle-East and Africa.MS’s tremendous resources could be influenced by global climate change.MS is connected with the North Atlantic Ocean through the Strait of Gibraltar, where an upper layer of Atlantic Water flows eastward into MS and alower layer of Mediterranean water outflows westward.The salty outflow water from MS may alter water patterns in the Atlantic Ocean and influence the meridional overturning circulation, which are important for heat transports from lower to higher latitudes.Therefore, accurately continuous monitoring of the current velocities and the circulation patterns could help us to understand the mechanism of the circulation and transport.In the study, we focus on the use of contemporary multi-mission satellite altimetry data and objectively analyzed MBT/XBT/Argo hydrographic data to calculate surface and subsurface geostrophic current velocities by referencing to contemporary accurate geoid models, include the GOCE and/or GRACE geoid models.The estimated current velocities are validated with insitu observations to evaluate the performance of different geoid models used for the calculation of geostrophic currents. Abstract Introduction Since Mediterranean Sea (MS) is characterized by avery active Thermohaline Circulation (THC) strongly resembling the global conveyor belt, it is very suitable to be used as ascaled-down model for studying the mechanism and variation of global overturning circulation and predicting how ocean responds to climate changes, and has been highlighted as one of the most vulnerable regions [IPCC, 2007]. Multiple climate-change-induced transients may lead to the variations and the shift of conveyor belt and therefore affect the characteristic intermediate and deep water masses of the whole basin [Roether et al., 1996;Klein et al., 1999]. Recent observational studies demonstrated that THC in MS represents avery high spatiotemporal variability driven by anthropogenic environmental changes [Bethoux and Gentili,1999]. Due to the importance of MS, the study will focus on the use of contemporary multi-satellite altimetry data combining with latest GOCE and GRACE gravity field models (GOCET-TIM5 and ITG-GRACE2010S) and in-situ T/S profile to calculate surface and subsurface geostrophic current velocities to analyze their performance and also the temporal variation of currents at different layers. HWHM (km)RMS (cm/s) GOCE-TIM5 GRACE2010S 40 30.23 72.47 50 17.35 56.95 58 12.39 43.93 80 8.66 24.01 90 8.65 18.49 100 8.87 14.82 111 9.20 12.10 125 9.61 10.04 150 10.12 8.95 166 10.35 8.96 Table 1. RMS values between altimetryderived geostrophic current velocity and in-situ data from GDP.Spatial resolution of GOCE-TIM5and GRACE2010Sis 71 km and 111 km,respectively. Results (a) (b) Fig. 1 Locations with the largest 50%number of drifter-days per square degree (a) and current velocity is larger than the average (b). To analyze the performance of GOCE and GRACE gravity field models on the determination of geostrophic currents, estimated mean surface geostrophic currents (MSG) were compared to in-situ data from GDP.In the MS, locations with the largest 50%number of drifter-days per square degree (over 43.53 days) in-situ data from GDP were chosen (See Fig.1-a).Among these selected locations, we further chose the locations with current velocity larger than the average (> 8.65 cm/s)for the comparison (See Fig. 1-b). Fig. 2 RMS differences of altimetryderived geostrophic current velocities and in-situ data from GDP as afunction of HWHM. Along-track ADT based on geoid undulations derived from GOCE-TIM5and ITGGRACE2010Sgravity field models were both smoothed using aGaussian filter with different half-weights at half-maximum (HWHM).Root Mean Square (RMS) of current velocities at the locations shown in Fig. 1-b is plotted as a function of HWHM (See Fig.2), while the values are listed in Table 1. Fig.2 shows that ADT based on GOCE-TIM5has smaller RMS and faster convergence of RMS than ITGGRACE2010Sdoes.Clearly, applying wider HWHM is necessary for ITG-GRACE2010S. Fig. 4 shows the current velocities and corresponding directions in the southern MS.Sections marked in red in the south of (1) Sicilia Island and (2) Ionian Basin showing aeastward and southward pattern with speed at around 10 cm/s, respectively, are picked for time-depth transect plot in Fig. 5 and 6. Table 1indicates that RMS converges to 8.65 cm/s with the HWHM of 90 km using GOCE-TIM5, while ITGGRACE2010Stakes the HWHM of 150 km for giving the convergence to 8.95 cm/s.Fig. 3 shows the mean current velocity in the MS.With the narrower filter HWHM, results from GOCE-TIM5reserve more detail signals than ITG-GRACE2010S. Following analysis were based on GOCE-TIM5. (a) (b) (c) (d) Fig. 3 Geostrophic current velocity based on (a) ITG-GRACE2010S and (b) GOCE-TIM5with filter HWHM equals 150 km and 90 km, respectively.(c) In-situ current speed from GDP and (d) number of drifter-day over MS. Conclusions •GOCE-TIM5with narrower filter HWHM than ITG-GRACE2010Sto reach asmaller RMS compared with in-situ data from GDP and it also preserves more detail current patterns. •Geostrophic current velocities and the patterns at different layers were resolved by combining altimetry, gravity field model, and T/S profile data.In the section in the south of Sicilia Island, currents in the upper 150 mshows asoutheastward to northeastward varying pattern.Meridional component of subsurface geostrophic current in the section in the south of Ionian Basin also shows an inter-annual cycle. In Fig. 5 and 6, upper and lower panels indicate the current velocity in zonal and meridional direction, respectively.Fig. 5 shows the transect in the south of Sicilia Island, while Fig. 6 shows the transect in the south of Ionian Basin.Xaxis indicates time, while yaxis indicates depth in meter. Currents in the south of Sicilia Island are mainly eastward but in meridional direction it shows the variation in direction.During 1998- 2002 and 2009-2011 it shows asouthward pattern, while in other periods it is northward.Below the depth of 150 m,the subsurface current is mainly northwestward (See Fig. 5). In the south of Ionian Basin, zonal geostrophic current is mainly westward, while meridional current shows avariant pattern.Especially, during 1999,2002,2008,and 2010,meridional component shows a northward pattern, which indicates that subsurface current here shows awhether southwestward or northwestward pattern. Fig. 5 Time-depth current velocities transect in the south of Sicilia Island. Fig. 4 Current velocities with directions and selected sections for time-depth plots in the southern MS. Fig. 6 Time-depth current velocities transect in the south of Ionian Basin.