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L AST SIXTY YEARS OF MIXED LAYER DEPTH VARIABILITY IN THE SOUTHERN B AY OF B ISCAY. D EEPENING OF WINTER MLD S CONCURRENT TO GENERALIZED UPPER WATER WARMING.

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Presentation on theme: "L AST SIXTY YEARS OF MIXED LAYER DEPTH VARIABILITY IN THE SOUTHERN B AY OF B ISCAY. D EEPENING OF WINTER MLD S CONCURRENT TO GENERALIZED UPPER WATER WARMING."— Presentation transcript:

1 L AST SIXTY YEARS OF MIXED LAYER DEPTH VARIABILITY IN THE SOUTHERN B AY OF B ISCAY. D EEPENING OF WINTER MLD S CONCURRENT TO GENERALIZED UPPER WATER WARMING TRENDS ? R. Somavilla, C. González-Pola, M. Ruiz-Villarreal and A. Lavín

2 LAST SIXTY YEARS OF MIXED LAYER DEPTH VARIABILITY IN THE SOUTHERN BAY OF BISCAY. DEEPENING OF WINTER MLDS CONCURRENT TO GENERALIZED UPPER WATER WARMING TRENDS? R. Somavilla, C. González-Pola, M. Ruiz-Villarreal and A. Lavín

3 LAST SIXTY YEARS OF MIXED LAYER DEPTH VARIABILITY IN THE SOUTHERN BAY OF BISCAY. DEEPENING OF WINTER MLDS CONCURRENT TO GENERALIZED UPPER WATER WARMING TRENDS? R. Somavilla, C. González-Pola, M. Ruiz-Villarreal and A. Lavín

4 Introduction MLD Atmosphere Ocean Interior Heat Storage Marine ecosystem and Global biogeochemical cycles Interannual √ Seasonal √ Daily √ … Long term variability?

5 In this work: Introduction MLD Atmosphere Ocean Interior Heat Storage Marine ecosystem and Global biogeochemical cycles Interannual √ Seasonal √ Daily √ … Long term variability? 1. Long term hydrographic time-series. 2.Upper ocean vertical structure climatology. 3. One dimensional water column model, GOTM model. Interannual, seasonal and decadal MLD variability in the southern Bay of Biscay.

6 Fact: Upper layers of the North Atlantic are warming. Long term warming trends Arbic & Owens (2001) 0.005ºC/yr [1920s to 1990s] Levitus et. at. (2005) 0.006ºC/yr [1955 to 2003] Potter & Lozier (2004) MW ºC/yr [1955 to 2003] Introduction Warming in the Eastern North Atlantic Recent strong warm anomalies Hollyday et. al (2008) Upp  T > 0.1ºC from 2000 Johnson & Gruber (2007) SPMW 0.7ºC Thierry et. al (2008) SPWW 1.4ºC González-Pola et al. ENACW and MW.0.30ºC

7 C. González Pola, A.Lavín, R.Somavilla, C.Rodriguez and E.Prieto W ATERS M ASSES V ARIABILITY FROM A M ONTHLY H YDROGRAPHICAL T IMESERIES AT THE B AY OF B ISCAY

8 Introduction Figure 1. Position of the VACLAN/COVACLAN projects sections (white dots); correntimeter moorings (black asterisk); the Santander standard section (black dots); and AGL buoy (red dot) in the Bay of Biscay and Eastern Atlantic margin Spanish Institute of Oceanography (IEO) Santander Observing System

9 This presentation will examine: 2. Simulation MLD variability using climatological profiles. I. Results Introduction Table 1. Resume of the forcings fields, data sets and time used for relaxation purposes and periods covered by the different simulations carried out. Atmospheric forcing fields Relaxation Period Q 0 and τQ0Q0 τTimetowards Simulation I.AI.BI.C1 month Hydrographic time-series II.A 6 months Hydrographic time-series II.B 6 months Upper ocean climatology III 6 months Upper ocean climatology Climatological profiles skills to reproduce MLD variability Reconstruction past evolution of MLD variability Prevailing factors governing MLD variability AIMS: 1. MLD, upper waters and intermediate water mass variability

10 This presentation will examine: 1. MLD, upper waters and intermediate water mass variability Results of the long-term run ( ). Constrains and reliability I. Results 4. Extreme winter mixing of 1963, 1965, II. Discussion III. Conclusions Introduction 5. Low-frequency variability in MLD and large-scale atmospheric patterns. 6. Winter MLD deepening trends and warming tendencies in the Bay of Biscay? 2. Simulation MLD variability using climatological profiles.

11 Results. 1. MLD, upper waters and intermediate water mass variability Fig. 3: (a) Temperature and (e) salinity upper layer temporal evolution from observations and from simulations I.A ((b) and (f)), I.B (c) and I.C (d). Black dots in (a) and (e) represent MLD estimation following the (Gonzalez-Pola et al., 2007) algorithm applied to IEOS6 and IEOS7 temperature proles. Black line in (b), (c), (d) and (f) indicates MLD estimated from GOTM model. Convection processes dominate winter MLD development Wind stress-driven turbulence controls summer MLD variability Extreme winter mixing of re-emergence Kantha & Clayson, 2002; Alexander et al., 2000 Reproduction of MLD seasonal cycle Winter ~ 200 m. Summer ~ 30 m. Prevailing factors governing MLD variability Convection + wind stress No wind stress No convection Santander standard section

12 Results. 1. MLD, upper waters and intermediate water mass variability: Extreme winter mixing 2005 Figure 5. (a) Sequence of temperature profiles, color code follows the legend with the October 2006 to December 2007 period changing gradually from yellow to red. (b, c, d, e, f, g and h) Time series of average θ, depth of isopycnal, salinity, potential vorticity, nutrients and chlorophyll at different pressure and isopycnal levels. c w cooling warming

13 Figure 3. Potential temperature anomaly (θ) within the mixed layer (100 dbar) in the Northeast Atlantic in spring 2005 from Argo floats. Re-emergence mechanism from Deser et al. (2003). Reference: Somavilla, R., C. González-Pola, C. Rodriguez, S. A. Josey, R. F. Sánchez, and A. Lavín (2009), Large changes in the hydrographic structure of the Bay of Biscay after the extreme mixing of winter 2005, J. Geophys. Res., 114, C01001, doi: /2008JC Results. 1. MLD, upper waters and intermediate water mass variability: Re-emergence mechanism 2006

14 East North Atlantic Central Water (ENACW).   ~ Pres ~ 350 dbar Lower bound of ENACW (Sal Min).   ~ Pres ~ 500 dbar Mediterranean Water (MW).   ~ Pres ~ 1000 dbar (core) Well sampled at the external station 7 (not conditioned by slope flows) Lavín et. al Results. 1. MLD, upper waters and intermediate water mass variability: Intermediate Water Masses. St7

15 It is possible to split changes at a fixed depth approximately in two main components (Levitus 1989, Bindoff & McDougall 1994, Arbic & Owens, 2001) : 1.Thermohaline properties variation at fixed isopycnal levels. Pure Warming//Freshening [air-sea fluxes variability] 2.Variations due to vertical displacement of isopycnal levels. Pure Heave [renewal rates, circulation changes] Approximate expression: Heating at isobaric levels “isobaric change” Heating at isopycnal levels “isopycnal change”. Modification of the thermohaline structure of the water masses Heating due to isopycnal level displacement “heave”. ‘Same water types’ but different proportions Results. 1. MLD, upper waters and intermediate water mass variability: Quantifying water masses changes

16 MW Sal Min ENACW ➯ Significant and progressive sinking until stable. Cooling pulse in 2009, back in // 27.3 ➯ Strong reduction (~7 dbar yr-1). This density level was getting depleted. Restoration in // 27.3 ➯ 2005 shift triggered a 2-yr isopycnal warming. Isopycnal cooling in 2009, back in 2010 Results. 1. MLD, upper waters and intermediate water mass variability: Changes at isopycnals and isobars

17 MW Sal Min ENACW Warming rates at all levels ºC/yr. (~0.020 ºC/yr on average. 0.30ºC in 15 years). Salinity increase ~0.06 in 15 years. Results. 1. MLD, upper waters and intermediate water mass variability: Changes at isopycnals and isobars

18 1992 to 2005 ENACW:  ↑ Heave+isop. 4:1 + Sal. Min.  ↑ Heave MW  ↑ Isop onwards ENACW:  ↕ Heave+isop.  ↓ →  ↑ Sal. Min.  ↑ (↔) Isop MW  ↑↔ Isop Results. 1. MLD, upper waters and intermediate water mass variability: Overall View,  S temporal evolution

19 Results. 2. Simulation MLD variability using climatological profiles. Climatological profiles skills to reproduce MLD variability Reproduction of MLD seasonal cycle Winter ~ 200 m. Summer ~ 30 m. Extreme winter mixing of re-emergence √ Effect of large scale lateral advection in thermocline water properties and stratification X Inclusion of shelf break advective anomalies Benefit for their use in studying the mixed layer along an oceanic large-scale region MLD HISTORICAL RECONSTRUCTION AND FUTURE SCENARIOS Climatological profiles

20 Discussion. 3. Results of the long-term run ( ). Constrains and reliability Extreme winter mixing of 1963, 1965, and 2005 Shallower MLD during the 70s and 80s First questions Reliability of atmospheric forcing???Reliability of atmospheric forcing √ Climatological profiles based on temp. profiles (1994 onwards) ???? Somavilla et al., 2009 Mean Winter Net Heat Loss 105 W·m -2 Mean Winter Net Heat Loss 90 W·m -2

21 Discussion. 3. Results of the long-term run ( ). Constrains and reliability. Reliability of ‘ climatological profiles + NOAA SST decadal warming ’

22 Discussion. 4. Extreme winter mixing of 1963, 1965 and , 1965, 2005 similar accumulated buoyancy flux at the end of the winter 1965 evenly distributed mixing episodes 1963 Extreme mixing episode mid January: 761 W/m 2 MLD from 70 to 150 meters 2005 Extreme mixing episodes end January and February 764 W/m 2 MLD from 150 to 330 meters MLD (black line) and net heat loss (blue solid line) during the winters of 1963, 1965 and Red dots represent MLD estimation following the Gonzalez-Pola et al. [2008] algorithm applied to IEOS6 temperature profiles. Somavilla et al., 2009

23 + NAO index years Subpolar gyre Anom. Q 0 >0 Deepening trend in MLD Colder SSTs Western (BoB) & Eastern lobes Anom. Q 0 <0 Shallowing trend in MLD Warmer SSTs Carton et al., 2008; Henson et al., 2009 Michaels et al., 1996; Paiva et al., 2002 Net heat loss anomaly in + and – NAO index years Discussion. 5. Low-frequency variability in MLD and large-scale atmospheric patterns.

24 Discussion. 5. Low-frequency variability in MLD and large-scale atmospheric patterns. - NAO index years Subpolar gyre Anom. Q 0 <0 Shallowing trend in MLD Warmer SSTs Western (BoB) & Eastern lobes Anom.Q 0 >0 Deepening trend in MLD Colder SSTs Carton et al., 2008; Henson et al., 2009 Michaels et al., 1996; Paiva et al., 2002

25 Discussion. 6. Winter MLD deepening trends and warming tendencies in the Bay of Biscay? SST Modelled ºC/decade NOAA SST recon ºC/decade 200 m Modelled ºC/decade Observations ºC/decade ‘ climatological profiles + NOAA SST decadal warming ’ 200 m. SST t Cte. Q 0 Shallower MLD Deeper MLD 200 m. SST t increasing. Q 0 Shallower MLD ++ Deeper MLD

26 Conclusions 1. As expected, at seasonal timescales winter mixed layer development is mostly conducted by convection processes while wind stress is responsible for mixing events during the spring-summer season. 2. Climatological profiles skills have enabled to use them for a first trial of reproduction of the last sixty years of MLD variability in the study area. Remarkable results have been obtained. An unexpected period of shallower MLDs seem to have occurred during the 1970s and 1980s which would have been concluded from mid1990s onwards by a deepening trend in MLD. 3. The reproduction of sea surface and 200 meters depth temperature time-series and the warming trend at both levels supports the counterintuitive outcome of shallower winter mixed layers concurrent to generalized upper water warming trends reported in several occasions for the area. 4. As found in other recent studies, long term trends in MLD in the southern Bay of Biscay seem to be related with decadal variability in North Atlantic Oscillation (NAO), being in phase and opposition with other cycles of deepening and shallowing trends in MLD found from subtropical-to-subpolar areas in the North Atlantic. 5. Favourable sequence of mixing events results in intense convection processes becoming determinant to explain interannual differences and extraordinary deepening of winter mixed layer as in years 2005, 1963 and 1965.

27 Many thanks for your attention Reference: Somavilla, R., C. González-Pola, M. Ruiz-Villareal and A. Lavín, Last sixty years of mixed layer depth variability in the southern Bay of Biscay. Deepening of winter MLDs concurrent to generalized upper water warming trends? Ocean Dynamics. DOI: /s

28 Discussion. 1. Extreme winter mixing of 1963, 1965 and 2005 Low-frequency variability pattern of atmospheric pressure identified as the Eastern Atlantic pattern (EATL). Rogers (1990) Negative state of the North Atlantic Oscillation (NAO) Atmospheric pressure anomaly during the winters 1963,1965 and Reference: Somavilla, R., C. González-Pola, C. Rodriguez, S. A. Josey, R. F. Sánchez, and A. Lavín (2009), Large changes in the hydrographic structure of the Bay of Biscay after the extreme mixing of winter 2005, J. Geophys. Res., 114, C01001, doi: /2008JC , 1965, 2005 similar accumulated buoyancy flux at the end of the winter

29 This presentation will examine: 2. Simulation MLD variability using climatological profiles. I. Results Introduction Table 1. Resume of the forcings fields, data sets and time used for relaxation purposes and periods covered by the different simulations carried out. Atmospheric forcing fields Relaxation Period Q 0 and τQ0Q0 τTimetowards Simulation I.AI.BI.C1 month Hydrographic time-series II.A 6 months Hydrographic time-series II.B 6 months Upper ocean climatology III 6 months Upper ocean climatology Climatological profiles skills to reproduce MLD variability Reconstruction past evolution of MLD variability Prevailing factors governing MLD variability AIMS: Effects of advection mantaining main thermocline 1. MLD, upper waters and intermediate water mass variability


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