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Abrupt Change in Deep Water Formation in the Greenland Sea: Results from

Hydrographic and Tracer Time Series

SEARCH Open Science mtg., October 03

P. Schlosser, J. Karstensen, D. Wallace, J. Bullister, and J. Blindheim

CU/L-DEO, IfM Kiel, NOAA PMEL, IMR Bergen

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Outline

Background

Early hydrography tracer observations

Tracer and hydrographic data from the 1990s

Evolution of deep waters

Evolution of intermediate waters

Connection to outflow waters

Summary

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Greenland Sea in the context of Atlantic THC

Levitus salinity map

AMAP

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Greenland Sea in the context of Nordic seas

Red: warm, salty Atlantic Water; Blue: cold, fresh Arctic outflowBlack: deep circulation

Inflow (Sv): NA: ca. 8 Sv; Bering Strait: ca. 1 SvOutflow (Sv): Near surface: ca. 3 Sv; Overflows: ca. 6 Sv

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Mean surface conditions

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Sections across the central

Greenland Sea (75N; 6/99)

1. Cyclonic circulation2. Doming of isopycnals3. Polar/Arctic front4. Relatively homogeneous

deep water

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Evolution of tracer concentrations in surface waters

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Early Hydrography/tracer time series

Boenisch et al., 1997

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2000 m to bottom2000 m to bottom 200 to 2000 m

Evolution of T/S and tracer properties

Boenisch et al., 1997

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Surveys during the 1990s

Cruises: Johan Hjort; IMR Norway, Johan Blindheim

10 cruises1991 to 2000

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Hydrography/tracer time series during the 1990s

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Evolution of deep water T/S properties

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Evolution of deep water tracer properties

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Background stratification versus features in convective cells. Conv. cells are characterized by temp. max. at the bottom of the feature.

Imprints from deep convection

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Evolution of tracer inventories

Tracer inventories indicate formation of intermediate waters in 1994, 1999, and possibly 1992.

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Evolution of T and S anomalies

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During convection:

Re-distribution of tracersNo net mass transport

After convection:

Mass transport by eddies andmeridional circulation

Greenland Sea

Sea ice may be important

Convection and tracer profiles

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Properties on overflow water isopycnals

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Heat fluxes required to restore 1970’s conditions

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Summary

Change in deep water formation rates1. Abrupt change in DWF rate occurred around 19802. Reduction in DWF rate: ca. 80%; more or less constant

since 19803. Greenland Sea gyre still on trend away from properties

known from instrumental records. ‘Recovery’ needs significant forcing.

Deep waters1. Quasi-linear trends in T (10 mK yr-1) and S (1 ppm yr-1)2. Advection of water from Arctic Ocean3. Little influence of atmosphere (isolated from atm. Forcing

on short time scales)

Intermediate waters1. Variability in formation2. Warmer and fresher (sea ice plays little role)3. Events: 94/95 and 99/00 (possibly 92/93)4. Transport: 0.1 to 0.2 Sv (0.5 during conv. events)

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Hydrography/tracer time series during the 1990s

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Background stratification versus features in convective cells. Conv. cells are characterized by temp. max. and its depth.

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GS variability

Local Regional

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GS variability

Local Regional

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Factors controlling GSDW

Arctic outflow (quasi constant)Deep convection (sporadic)Connection to surface temperature?