Royal Society Meeting on Abrupt Climate Change: Evidence, Mechanisms and Implications, 4-5 February 2003. Session 2:- Modern observations and processes

Royal Society Meeting on Abrupt Climate Change: Evidence, Mechanisms and Implications, 4-5 February 2003.

Session 2:- Modern observations and processes

Recent Changes in the North Atlantic Bob Dickson, CEFAS

with Ruth Curry, WHOI and Igor Yashayaev, BIO



1. What have been the changes in forcing?2. What ocean changes are we expecting?3. What changes do we find?4. What observing strategy to adopt?



1. What Changes in the Forcing?

When we plot air temperature as a function of latitude and time, two things are clear: 1) the World is warmer. Including 2002, all ten of the warmest years since records began in 1861 have occurred since 1990; Jones and Moberg, 2003. 2) in the last two decades the distribution of warming has become global. Courtesy Tom Delworth, GFDL

….and our instrumental and proxy records suggest that the NAO in the 1990s may have been at a 600 year extreme positive state.

Phil Jones CRU, in press



2. What changes of global scale and importance are we expecting to observe in the ocean?

2i. A slowdown of the MOC, and 2ii. An acceleration of the water cycle

Most (but not all) coupled climate models anticipate a slow down of the Atlantic Meridional Overturning Circulation (MOC)

under greenhouse gas forcing as a result of freshening and warming of subpolar seas

IPCC, 2001

The Water Cycle Will Accelerate With Global Warming

• A warmer atmosphere will carry more water vapor, because of the exponential increase of vapor pressure with temperature.

• An enhanced water cycle will change the distribution of salinity in the upper ocean.

• A program for monitoring salinity changes is needed.

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Vapor Pressure of Water as Function of Temperature

Temperature, C

Ray Schmitt,WHOI pers comm



3. What changes do we observe?

We can’t measure change in the MOC directly

But we can measure a range of its known or supposed associates e.g.:3a) an increase in the freshwater fluxes from the Arctic that are supposed to slow it down.3b) slowing or density-change in the overflows that “drive” it.3c) changes in the trans-ocean gradients of steric height that will reflect a change in overturning rate.



3a. Changes in Freshwater Flux from the Arctic.

Schematic of the northern loop of the ocean’s “Great Conveyor”. McCartney et al 1996

The elements of an ASOF freshwater flux array is funded and is partly in place. Few results yet….....but we do have one proxy measure of f’w flux from the AR7W Line

Osterhus/Vinje

Dickson/Meincke

Falkner

MellingPrinsenberg

Lee

Yashayaev

Meincke SFB 512

The offshore density gradient in the 0-150m layer from Labrador Shelf to the Central Lab Sea is our only (proxy) measure of the changing f’w flux to the NW Atlantic. This gradient has progressively steepened with the NAO over the past 4 decades (equivalent to a 20% incr. In S’going transport) largely thro’ freshening of shelf & upper-Slope waters.

Data from AR7W, 0-150m.

Density gradient

Density



3b. Changes in the Overflowsi) transportii) temperature (see paper)iii) salinity

The main indication we have of a change in overflow transport is proxy evidence, and applies to the eastern overflow. There, the depth of the t=28.0 isopycnal in the upstream ‘reservoir’ of the Norwegian Sea controls the pressure head that drives overflow through the Faroe-Bank Channel (Hansen et al 2001).

Increased precipitation (+14 cm/ winter)

Weak, shallow Greenland Sea convection

Decreased local sea-ice formation

Over the past 4 decades, for a variety of reasons associated with the amplifying NAO the fresh water accession to the Nordic Seas has increased steadily

A.O. ice melt?

Increased ice & freshwater flux through Fram Strait

The 50-year record at OWS M in the Norwegian Sea is our best benchmark of this broadscale change, showing a long term freshening reaching to depths of >1 km.

Salinity at OWS M, Norwegian Sea, 1950-97, Courtesy Svein Østerhus, UiB

Depth of the t=28.0 isopycnal at OWS M

As a result, the t = 28.0 isopycnal has itself steadily deepened, by 70m in 50 years, (Hansen et al 2001).

Hansen et al (2001) use this finding to suggest that the coldest, deepest part of the eastern overflow may have decreased by 20% since 1950.

30-day mean speed in bottom 100m at Denmark Strait site UK1, 1986-2002 (inc TTO-array July 1990 to June 1991)

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There is no such evidence of decreasing flow speeds from the western overflow through Denmark Strait. There, the 187 monthly means of speed collected from the overflow core since 1986 (z 2000m) show little sign of seasonal variability…..

Neither has there been any obvious change in the transport of the overflow core.



3biii. Changes in overflow salinity

The broadscale freshening of the Nordic Seas over the past 4 decades has reached depths of >1 km, so is accessible to the two main overflows which cross the Greenland-Scotland Ridge.

Salinity at OWS M, Norwegian Sea, 1950-97, Courtesy Svein Østerhus, UiB

Tapping-off this layer, the two dense overflows that renew and ventilate the deep ocean have also freshened over the past 4 decades.

Dickson et al, Nature

2002.

…so that if we construct salinity time series at intervals along the spreading pathways of both overflows from

their sills to the deep Labrador Sea…

… we find that the entire system of overflow and entrainment that ventilates the deep Atlantic has undergone a remarkably rapid and remarkably steady freshening over the past four decades.

A change in the ocean-climate of sub-arctic seas has thus been transferred to the deep and abyssal ocean at the headwaters of the “Great Conveyor”

Dickson et al 2002

NAO-

NAO+

The resulting full-depth change in the Labrador Sea salinity is believed to be the largest change in the instrumented oceanographic record. [By 1992, equivalent to adding an extra 6 m of fresh water at the sea surface].

Reported with the usual objectivity!

Fresh water gain by layers

2 Principal Accumulated Range* Resident Fresh Water

36.884 - 36.966 Labrador Sea Water 2.52 m

36.968 - 37.060 Northeast Atlantic Deep 0.63 m

37.062 - 37.182 Denmark Strait Overflow 0.31 m

Total: ~3.5 m

…..Or more generally over the NW Atlantic, equivalent to mixing in an extra 3.5 m of freshwater, unevenly distributed over the watercolumn.

The result has been a dramatic shift in the -S relation for waters of the NW Atlantic. Igor Yashayaev, unpublished

2ii. A change in the Atlantic hydrologic cycle?

The expected acceleration of the water cycle has not yet found in terrestrial pan evaporation data (Roderick & Farquhar 2002), but then……the Oceans contain 96% of the Earth’s water, experience 86% of planetary evaporation and receive 78% of planetary precipitation…...

Numbers from Ray Schmitt, WHOI

As we follow the deep freshening south through the western Atlantic to the W Line

….we encounter something else

Curry, Dickson and Yashayaev, in press

This is Salinity Maximum Water (SMW), formed in the subtropical gyre at the Atlantic E-P maximum, where density layers in the range 0= 25.50-26.50 are ventilated.

Curry, Dickson & Yashayaev, in press

Over the same 40-year period that salinities of high latitude water masses have freshened, salinities at the E-P maximum have been increasing …. a shift in the entire Atlantic hydrological cycle?

Curry, Dickson and Yashayaev, in press

Calculated for surface ton =26.50 (waters outcropping S of

30N in the E-P max) or 27.0 (S of 40N)

This change in salinity is not confined to the NAO or even the Atlantic. We compare changes late ‘50s-early ‘60s to 1990s from 30S to 60N through the W Basins, through main centres of SSS, E-P and steric height.

FRESHER• NEADW & DSOW• LSW & Surface Labrador Sea• AAIW & UCDW & Vent.Therm

SALTIER

• Surface Tropics / Subtropics• MOW & UNADW (S)

Curry, Dickson & Yashayaev, in press

ATLANTIC WATER MASSES 1990-99 minus 1956-64

30S EQ 60N

The same symmetrical pattern of change, freshening intermediate waters from high N & S latitudes and more-saline upper ocean at low lats has been shown for the Pacific and Indian oceans by Wong et al 1999, 2001 and Bindoff & McDougall 2000.

“It is therefore tantalizing to speculate that the observed symmetrical freshening of NPIW and AAIW/SAMW represents a trend in the climate system that could be induced by anthropogenic sources. A natural extension to this work would be to investigate whether the freshening of AAIW/SAMW is circumpolar in extent”

Wong, Bindoff and Church, 2001

3c. A change in the trans-ocean steric gradient?

THC Overturning vs Atlantic Meridonal Steric Gradient in HADCM 3(Thorpe et al 2001)

We assume that change in the MOC will be associated with some measurable change in the trans-ocean density gradient, e.g. HadCM3 suggests a close correlation between Atlantic overturning rate and the S-N gradient of steric height from 30S - 60N through the W Atlantic.

Atlantic Water Mass Variability Atlantic Water Mass Variability and the Meridional Density Gradient and the Meridional Density Gradient

Over the 40-year observational record, the meridional steric height gradient has varied only by about 20 cm, largely driven by changes at its northern end.

a) T contribution

b) S contribution

The change in steric height in the Labrador Sea, 1958 to 2002, been dominated by the effect of cooling rather than freshening a net lowering of steric height.

Courtesy of Igor Yashayaev, BIO, Canada

c) Steric Height change

THC Overturning vs Atlantic Meridonal Steric Gradient in HADCM 3(Thorpe et al 2001)

Mapping the observed steric gradient changes (20 cm) onto the Thorpe et al results, we find that the Atlantic MOC strength has varied little over the past 40 years.

1995

1970

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20 cm

…. But also that a 50% reduction from the present MOC strength could result from just a 40 cm drop in the gradient below the 1995 gradient.

Change in the zonal trans-ocean density gradient along 25N, ---the Marotzke, Bryden and Cunningham proposal in NERC-RAPID

Since mass transport between 2 points depends only on the pressure difference between them, their moored array along 26.5N will measure the overturning rate by continuous observation of density at the W‘ern & E’ern boundaries.

A 1st look at changing gradients of steric height along 24N suggest two problems. High-amplitude variability in the west and small variability elsewhere cf the W Atlantic line.

So in summary, the observed changes are:

4. Slowdown of the Atlantic MOC ?

3. Change in Atlantic Hydrologic cycle

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Vapor Pressure of Water as Function of Temperature

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Zonal ?2. Change in steric gradient

Meridional

1. Influence of Northern Seas on the density field of the N. Atlantic

Change in Arctic Atlantic f’w flux ? Change in overflow T Change in overflow S Change in overflow Transport

?

As accompaniments to the extreme climatic forcing of recent decades, the following obs. seem relevant:• 40 year increase in the s’going f’w flux circuiting around the Lab Sea margins.• 50 year decrease in deep, dense overflow from FSC• 40 year freshening of both GIN-Sea & its overflows• 40 year increase in E-P & upper ocean salinity in the subtropics• some global evidence of the expected multi-decadal increase in the water cycle.Though no evidence yet of any sustained change in the MOC, the above explains & justifies the current $120M+, 4-5 year focus on the oceans role in climate (EC, UK, Norway, USA, Canada).

4. What observing strategy?

The first task is to measure all of the Arctic-Subarctic Ocean Fluxes that connect the Arctic to the N Atlantic. ASOF is already proceeding to implementation (http://asof.npolar.no)

plusArrays for monitoring zonal and meridional gradients in steric height

plusBoundary arrays across DWBC





3bii. Changes in overflow temperature

30-day mean temperature in bottom 100m at Denmark Strait site UK1, 1986 - 2002 (inc TTO-array 7/90 to 6/91)

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The current meter thermistor data from 2000m in the overflow core do reveal a dramatic decadal variation in mean temperatures. These values are not smoothed. Each 30-day mean is simply the average of 720 hourly

values.

…and the temperature and salinity of the overflow core at 2000m off Angmagssalik clearly determine the density of the DSOW-derived layer of the abyssal Labrador Sea a further one year later.

Potential Temperature

NAO-

NAO+

Documents

Royal Society Meeting on Abrupt Climate Change: Evidence, Mechanisms and Implications, 4-5 February 2003. Session 2:- Modern observations and processes