Trends and recent shifts in the ocean hydrography of the temperate north-east Atlantic Insights from...

Trends and recent shifts in the ocean hydrography of the temperate north-east Atlantic

Insights from routinely repeated sections around Spanish waters.

Spanish Institute of Oceanography

GAIC 201514-18 September, Galway, Ireland

Gonzalez-Pola, C., Velez-Belchi, P. Sanchez-Leal, R.F., Somavilla, R., Lavin, A. and Hernandez-Guerra, A.

Las Palmas University

NE Atlantic. Intergyre and EBC.

Iberian Peninsula and Canaries are located in the Eastern Boundary System of the North Atlantic. (Broad weak currents, upwelling and slope flows). West Iberia and BoB at the “intergyre region” (shadow area).

Tomczak and Godfrey, 1994.

Reid, 1994

Pollard et. al., 1996

Pingree, 1993.

Main outcome of this talk. A large shift in the hydrography of this region occurred about 10 years ago, likely having an impact on a broader scale.Below, spreading of the Mediterranean Water from the Gulf of Cadiz.

And below, a mixture of intermediate and deep water masses of polar origin.

NE Atlantic. Intergyre.Bay of Biscay, Western Iberian Margin and Canary Basin, all exceeding 5000 m.

Routinely sampling at:Bay of BiscayWest Galicia (Finisterre)Gulf of Cadiz.Canary Basin.

Starting in mid 90’s or early 00`s. Monthly programs focused on the shelf, yearly full-depth sampling, multidisciplinary programs.

ENACW

MW

LSW

ENADWLDW

Subsurface hydrography. Recent times. (I) Biscay

Radiales Project: Monthly sampling (hydrographical and biological in standard sections around Spanish waters).

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

• Santander Section. 7 stations, 1 in the shelf break, two over the deep ocean (2400 and 2800)

• Small ship, sampling limited to 1000m depth (1500 from 2006; full water column coverage from 2013 with a new vessel).

High frequency timeseries of central water properties since early 90`s.

67

S.Seb.


Winter 2005. Coldest in SW Europe (and NorthEast Temperate Atlantic) in many years

NCEP/NCAR Reanalysis Sea Level Pressure (mb) anomaly.Dec04-March05.

Anomalous atmospheric pattern.

Mixed layer depth reached

~350 dbar in 2005

(maximum previous record

since early 90`s ~240

dbar)

Accumulated light modal

water mixed with deeper

water to create a

denser variety.

Depth of Isopycnal Temp at Isopycnals

Modal waters become denser while warmer+ saltier (more difficult to get eroded by yearly winter mixing). Overall view, most heat accumulating at mid-depths

Subsurface hydrography. Recent times. (II) Western Iberia

The Deep Sections program is running since 2003, providing full-depth (>5500 m ) hydrography and biogeochemistry at Western Iberia.

2003-2010 semiannually, then annual.

Finisterre section. ~400 km. 20 Occupations so far.

Salinity. Summer 2014

Subsurface hydrography. Recent times. (II) Western Iberia

Large seasonality revealed, maximum at the MW levels. (Needed of seasonality removal when looking at interannual variability).0.

3ºC c

olde

r

0.5º

C w

arm

er

Variability for 2003-2014:

Same pattern as in Biscay for central waters ~+0.14ºC dec-1

Drop in the MW to LSW levels 2008-2011 (overall negative trend).

Negligible changes below 2000 m.

Overall Temp trend ~-0.015 ºC dec-1

Strong response to large-scale conditions (NAO)

Subsurface hydrography. Recent times. (III) Gulf of Cadiz

Area of intense mixing between overflow waters from the Mediterranean (MOW) and Central waters from the Atlantic (ENACW) to form what is known as Mediterranean Waters (MW).

Mooring line right at the passage since 2004 revealed seasonality due to the annual deep water formation cycle (warmer outflow in winter, Lafuente et.al. 2007).

Several multidisciplinary cruises since mid 90`s.

Warmer outflow in recent times (~0.3ºC respect to the 80`s) + warmer ENACW (presently stalled).

Very noisy signal within the last 20 years. (e.g.trend at 400 m 0.5±0.3 ºC dec-1 though)

Subsurface hydrography. Recent times. (IV) Canaries

Canary current system well embedded in the return path of the subptropical gyre.Deep sections have been run since late 90`s, with some interruptions.

Trends of ~0.20 ºC dec-1 again at the upper thermocline waters (200-600 m). No trends in the intermediate levels.Weak cooling at deep levels ~ - 0.01±0.01 ºC dec-1 Salinity trends: density compensating (heave). Large swings at intermediate levels

Summary of the observational recordSea Surface temperatures have raised notably during the last 40 years (+~0.6 ºC). Almost stalled within the last 10 years.

• Most detailed track of upper permanent thermocline since early 90`s (monthly).

• Warming at all levels peaking at 400-600 m, (up to 0.5ºC in 20 years).

• Shift in water mass properties after 2005, central waters of much warmer/saltier type.

• Annual track of full-depth since 2003.

• Central Waters (200-800) behaving as in Biscay.

• Lower thermocline waters (MW and LSW) subjected to strong variability (linked to large-scale atmospheric patterns). Passage of a cold+fresh anomaly in late 00`s

• No relevant changes in deep waters (2000-5500 m).

• Relatively well covered since late 90`s.

• Large warming of formed MW (>0.5ºC in 15 years).

• Warmer ENACW that mix to form MW (not surfacemost Atlantic Waters that enter the Western Mediterranean).

• Annual track of full-depth since late 90`s (with some gaps).

• Notable warming of upper permanent thermocline (near 0.4ºC in 20 yr), by heave.

• No changes below 600 m, even slight cooling at deep levels.

Shift in ENACW props Shift in circulation???

Central waters at Biscay and West Iberia shifted in 2005 to a much warmer/saltier (denser) type.

Such shift should have altered the zonal dynamic height.

Zonal dynamic height in three sections across the intergyre from WODB2001 (grey) and ARGO after 2005 (blue)

Shift in ENACW props. Shift in circulation???

Flow reversal hypothesis imply:

• Warmer/saltier ENACW type not incorporating to the subtropical gyre (anomaly never reached the Canaries)

• ENACW contributing to the North Atlantic current, favoring an enhancement of saltier waters export to the subpolar gyre and Nordic Seas (further consequence: saltier surface waters in deep convection regions enables heat transfer towards the deep).

• North Atlantic Current carrying WNACW into the Nordic Seas must rearrange.

?

Thanks!

IEO Ocean Hydrography Monitoring Programs

Galicia-Biscay RADIALES: L. Valdés, A. Bode.Galicia-Biscay Deep Sects. A. Lavín, C. González-PolaCadiz: Ricardo F. Sánchez.Canaries: Pedro Vélez, Alonso Hernández-Guerra

The Marine Strategy Framework Directive (UE directive) has arranged 5 marine regions (3 in the Atlantic).

Monitoring programs, including oceanic hydrography, have been run since 90`s or 00`s by the IEO at the three regions.

Other research projects and monitoring efforts available (CSIC, Universities, Puertos del Estado, Azti, SOCIB, PLOCAN etc)

Observed Changes. Subsurface hydrography. Recent times


Radiales Project: Monthly sampling (hydrographical and biological in standard sections around Spanish waters).

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

• Santander Section. 7 stations, 1 in the shelf break, two over the deep ocean (2400 and 2800)

• Small ship, sampling limited to 1000m depth (1500 from 2006; full water column coverage from 2013 with a new vessel).

High frequency timeseries of central water properties since early 90`s.

67

S.Seb.

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.

Quantifying water masses changes

1

1

2

2

Intermediate Levels. Changes at isopycnals and isobars. St7.M

WS

al M

inE

NA

CW

27.1-27.2 ➯ Significant and

progressive sin

king until 2005.

Then 27.1 outcropped surface

27.3 weak sinking

27.1//27.2 Thicker mode.

27.2// 27.3 Stable sin

ce 2005

➯

.

27.2// 27.3 2005

➯

shift triggered a 2-yr

isopycnal warming

that remains.


Intermediate Levels. Changes at isopycnals and isobars. St7.M

WS

al M

inE

NA

CW

Warming rates at all levels 0.004-0.026 ºC/yr.(~0.015ºC/yr on average. 0.32ºC in 21 years).

Salinity increase ~0.05 in 21 years.


Overall View, S temporal evolution

Quick shift (isopycnal)



Winter 2005. Coldest in SW Europe (and NorthEast Temperate Atlantic) in many years

NCEP/NCAR Reanalysis Sea Level Pressure (mb) anomaly.Dec04-March05.

Anomalous atmospheric pattern.

Mixed layer depth reached

~350 dbar in 2005

(maximum previous record

since early 90`s ~240

dbar)

Accumulated light modal

water mixed with deeper

water to create a

denser variety.

Subsurface hydrography. (II) Western IberiaThe Deep Sections program is running since 2003, providing full-depth (>5500 m ) hydrography and biogeochemistry at Western Iberia and Biscay.

2003-2010 semi-annually, then annual. Biscay series discontinued in 2011.

Finisterre section. ~400 km. 20 Occupations so far.

Salinity. Summer 2014

Subsurface hydrography. (II) Western Iberia

Large seasonality revealed, maximum at the MW levels. (Needed of seasonality removal when looking at interannual variability).0.

3ºC c

olde

r

0.5º

C w

arm

er

Variability for 2003-2014:

Same pattern as in Biscay for central waters ~+0.14ºC dec-1

Drop in the MW to LSW levels 2008-2011 (overall negative trend).

Negligible changes below 2000 m.

Overall Temp trend ~-0.015 ºC dec-1

Strong response to large-scale conditions (NAO)

Area of intense mixing between overflow waters from the Mediterranean (MOW) and Central waters from the Atlantic (ENACW) to form what is known as Mediterranean Waters (MW).

Subsurface hydrography. (III) Gulf of Cadiz

Several multidisciplinary cruises since mid 90`s.

Warmer outflow in recent times (~0.3ºC respect to the 80`s) + warmer ENACW (presently stalled).

Subsurface hydrography. (IV) Canaries

Canary current system well embedded in the return path of the subptropical gyre.Deep sections have been run since late 90`s, with some interruptions.

Trends of ~0.20 ºC dec-1 again at the upper thermocline waters (200-600 m). No trends in the intermediate levels.Weak cooling at deep levels ~- 0.01±0.01 ºC dec-1 Salinity trends: density compensating (heave). Large swings at intermediate levels

Trends and recent shifts in the ocean hydrography of the temperate north-east Atlantic Insights from...

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