Last sixty years of mixed layer depth variability in the southern Bay of Biscay

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

[email protected]

mailto:[email protected]



DEEPENING OF WINTER MLDS CONCURRENT TO GENERALIZED UPPER WATER WARMING TRENDS?


[email protected]




DEEPENING OF WINTER MLDS CONCURRENT TO GENERALIZED UPPER

WATER WARMING TRENDS?


[email protected]



Introduction

MLD

Atmosphere

Ocean Interior Heat Storage

Marine ecosystem and

Global biogeochemical cycles

Interannual √

Seasonal √

Daily √

…

Long term

variability?

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.

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 0.010 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 2000Johnson & Gruber (2007) SPMW 0.7ºC 1993-2003Thierry et. al (2008) SPWW 1.4ºC 1996-2003González-Pola et al. ENACW and MW .0.30ºC 1994-2010

[email protected]

C. González Pola, A.Lavín, R.Somavilla, C.Rodriguez and E.Prieto

WATERS MASSES VARIABILITY FROM A MONTHLY HYDROGRAPHICAL TIMESERIES AT THE BAY

OF BISCAY


Introduction

http://www.seriestemporales-ieo.net/

http://www.vaclan.es/

http://www.boya_agl.ieo.es/

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

This presentation will examine:

2. Simulation MLD variability 1995-2008 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

Q0 and τ Q0 τ Time towards

Simulation

I.A I.B I.C 1 month Hydrographic time-series 1995-2008

II.A 6 months

Hydrographic time-series 1995-2008

II.B 6 months

Upper ocean climatology 1995-2008

III 6 months


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 1994-2010.



3. Results of the long-term run (19482008). Constrains and reliability

I. Results

4. Extreme winter mixing of 1963, 1965, 2005.

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?


Results.


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 2005.

2006 re-emergence

Kantha & Clayson, 2002; Alexander et al., 2000

Reproduction of MLD seasonal cycle

W

inter ~ 2

00 m.

S

ummer ~ 3

0 m.

Prevailing factors governing MLD

variability

Convection + wind stress

No wind stress

No convection

Santander standard section

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

Figure 3. Potential temperature anomaly (θ) within themixed 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:10.1029/2008JC004974.

Results.

1. MLD, upper waters and intermediate water mass variability: Re-emergence mechanism 2006

East North Atlantic Central Water (ENACW). ~27.1-2 Pres ~ 350 dbarLower bound of ENACW (Sal Min). ~27.2-3 Pres ~ 500 dbarMediterranean Water (MW). ~27.3-27.6 Pres ~ 1000 dbar (core)Well sampled at the external station 7 (not conditioned by slope flows)

Lavín et. al. 2006

Results.

1. MLD, upper waters and intermediate water mass variability: Intermediate Water Masses. St7

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.

1

1

2

2

Results.

1. MLD, upper waters and intermediate water mass variability: Quantifying water masses changes

MW

Sal M

inEN

ACW

27.1-27.2 ➯ Significant and

progressive sinking until 2005. 27.3

stable.

Cooling pulse in 2009, back in 2010

27.2// 27.3 ➯ Strong reduction (~7 dbar

yr-1). This density level was getting

depleted. Restoration in 2005.

27.2// 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

MW

Sal M

inEN

ACW

Warming rates at all levels 0.010-0.030 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

1992 to 2005ENACW: ↑ Heave+isop. 4:1

+

Sal. Min. ↑ Heave

MW ↑ Isop.

2005 onwardsENACW: ↕Heave+isop.

↓ → ↑ Sal. Min. ↑ (↔) Isop

MW ↑↔ Isop

Results.1. MLD, upper waters and intermediate water mass variability: Overall View, S temporal evolution

Results.

2. Simulation MLD variability 1995-2008 using climatological profiles. Climatological

profiles skills to reproduce MLD

variability

Reproduction of MLD seasonal cycle

W

inter ~ 2

00 m.

S

ummer ~ 3

0 m.

Extreme winter mixing of 2005.

2006 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

MLDHISTORICAL

RECONSTRUCTION AND FUTURE SCENARIOS

Climatological profiles

Discussion.3. Results of the long-term run (19482008). Constrains and reliability

Extreme winter mixing of 1963, 1965, and 2005Shallower 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

Discussion.3. Results of the long-term run (19482008). Constrains and reliability.

Reliability of ‘climatological

profiles + NOAA SST decadal warming’

Discussion.4. Extreme winter mixing of 1963, 1965 and 2005

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

0

50

100

150

200

250

300

350

ML

D(m

)

0

150

300ML

D (

m)

0

300

600

Net

hea

t los

s (W

·m-2

)

0

150

300ML

D (

m)

0

300

600

Net

hea

t los

s (W

·m-2

)

Nov Dec Jan Feb Mar Apr

0

150

300

Time (months)

ML

D (

m)

0

300

600

Net

hea

t los

s (W

·m-2

)

1963, 1965, 2005 similar accumulated buoyancy flux at the end of the winter

1965evenly distributed mixing

episodes

1963Extreme mixing episode

mid January: 761 W/m2

MLD from 70 to 150 meters

2005Extreme mixing episodes end January and February

764 W/m2

MLD from 150 to 330 metersMLD (black line) and net heat loss (blue solid line) during the winters of 1963, 1965 and 2005. Red dots represent MLD estimation following the Gonzalez-Pola

et al. [2008] algorithm applied to IEOS6 temperature profiles.

Somavilla et al., 2009

+ NAO index years

Subpolar gyreAnom. Q0 >0

Deepening trend in MLDColder SSTs

Western (BoB) & Eastern lobesAnom. Q0<0

Shallowing trend in MLDWarmer 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.

Discussion.5. Low-frequency variability in MLD and large-scale atmospheric patterns.

- NAO index years

Subpolar gyreAnom. Q0<0

Shallowing trend in MLDWarmer SSTs

Western (BoB) & Eastern lobesAnom.Q0 >0

Deepening trend in MLDColder SSTs

Carton et al., 2008; Henson et al., 2009 Michaels et al., 1996; Paiva et al., 2002

Discussion.6. Winter MLD deepening trends and warming tendencies in the Bay of Biscay?

SST 1948-2008Modelled -0.019 ºC/decade

NOAA SST recon. +0.026 ºC/decade

200 m. 1980-2008 Modelled 0.224 ºC/decade

Observations 0.263 ºC/decade

‘climatological profiles + NOAA SST

decadal warming’

200 m.

SST

t

Cte. Q0ShallowerMLD

DeeperMLD

200 m.

SST

t

increasing. Q0ShallowerMLD

++ DeeperMLD

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.

Many thanks for your attention

Reference: Somavilla, R., C. González-Pola, M. Ruiz-Villareal and A. Lavín, 2011. 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: 10.1007/s10236-011-0407-6

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)

200519651963

Negative state of the North Atlantic Oscillation (NAO)

Atmospheric pressure anomaly during the winters 1963,1965 and 2005.

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:10.1029/2008JC004974.

1963, 1965, 2005 similar accumulated buoyancy flux

at the end of the winter



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

Q0 and τ Q0 τ Time towards

Simulation

I.A I.B I.C 1 month Hydrographic time-series 1995-2008

II.A 6 months

Hydrographic time-series 1995-2008

II.B 6 months


III 6 months


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


Documents

Last sixty years of mixed layer depth variability in the southern Bay of Biscay