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Impacts of interruption of the Agulhas leakage on the tropical Atlantic in coupled ocean–atmosphere simulations

Impacts of interruption of the Agulhas leakage on the tropical Atlantic in coupled ocean–atmosphere simulations
Impacts of interruption of the Agulhas leakage on the tropical Atlantic in coupled ocean–atmosphere simulations
In this paper we use a coupled ocean-atmosphere model to investigate the impact of the interruption of Agulhas leakage of Indian ocean water on the Tropical Atlantic, a region where strong coupled ocean atmosphere interactions occur. The effect of a shut down of leakage of Indian ocean water is isolated from the effect of a collapse of the MOC. In our experiments, the ocean model is forced with boundary conditions in the southeastern corner of the domain that correspond to no interocean exchange of Indian ocean water into the Atlantic. The southern boundary condition is taken from the Levitus data and ensures a MOC in the Atlantic. Within this configuration, instead of warm and salty Indian ocean water temperature (cold) and salinity (fresh) anomalies of southern ocean origin propagate into the South Atlantic and eventually reach the equatorial region, mainly in the thermocline. This set up mimics the closure of the “warm water path” in favor of the “cold water path”. As part of the atmospheric response, there is a northward shift of the Intertropical Convergence Zone (ITCZ). The changes in Trade Winds lead to reduced Ekman pumping in the equatorial region. This leads to a freshening and warming of the surface waters along the equator. Especially in the Cold Tongue region, the cold and fresh subsurface anomalies do not reach the surface due to the reduced upwelling. The anomaly signals are transported by the Equatorial undercurrent and spread away from the Equator within the thermocline. Part of the anomaly eventually reaches the Tropical North Atlantic, where it affects the Guinea Dome. Surprisingly, the main effect at the surface is small on the equator and relatively large at the Guinea Dome. In the atmosphere, the northward shift of the ITCZ is associated with a band of negative precipitation anomalies and higher salinities over the Tropical South Atlantic. An important implication of these results is that the modified water characteristics due to a shut down of the Agulhas leakage remain largely unaffected when crossing the equatorial Atlantic and therefore can affect the deepwater formation in the North Atlantic. This supports the hypothesis that the Agulhas leakage is an important source region for climate change and decadal variability of the Atlantic.
0930-7575
989-1003
Haarsma, Reindert J.
52de705c-7df6-48c6-a891-99c8a01f4b09
Campos, Edmo J.D.
feececd5-54ab-48ab-9b97-c53cf121f8bc
Drijfhout, Sybren
a5c76079-179b-490c-93fe-fc0391aacf13
Hazeleger, Wilco
21d5b030-1127-424f-b22b-0a470dcb5b32
Severijns, Camiel
89ffbb61-7552-4b88-933d-b963d39ad1b0
Haarsma, Reindert J.
52de705c-7df6-48c6-a891-99c8a01f4b09
Campos, Edmo J.D.
feececd5-54ab-48ab-9b97-c53cf121f8bc
Drijfhout, Sybren
a5c76079-179b-490c-93fe-fc0391aacf13
Hazeleger, Wilco
21d5b030-1127-424f-b22b-0a470dcb5b32
Severijns, Camiel
89ffbb61-7552-4b88-933d-b963d39ad1b0

Haarsma, Reindert J., Campos, Edmo J.D., Drijfhout, Sybren, Hazeleger, Wilco and Severijns, Camiel (2011) Impacts of interruption of the Agulhas leakage on the tropical Atlantic in coupled ocean–atmosphere simulations. Climate Dynamics, 36 (5-6), 989-1003. (doi:10.1007/s00382-009-0692-7).

Record type: Article

Abstract

In this paper we use a coupled ocean-atmosphere model to investigate the impact of the interruption of Agulhas leakage of Indian ocean water on the Tropical Atlantic, a region where strong coupled ocean atmosphere interactions occur. The effect of a shut down of leakage of Indian ocean water is isolated from the effect of a collapse of the MOC. In our experiments, the ocean model is forced with boundary conditions in the southeastern corner of the domain that correspond to no interocean exchange of Indian ocean water into the Atlantic. The southern boundary condition is taken from the Levitus data and ensures a MOC in the Atlantic. Within this configuration, instead of warm and salty Indian ocean water temperature (cold) and salinity (fresh) anomalies of southern ocean origin propagate into the South Atlantic and eventually reach the equatorial region, mainly in the thermocline. This set up mimics the closure of the “warm water path” in favor of the “cold water path”. As part of the atmospheric response, there is a northward shift of the Intertropical Convergence Zone (ITCZ). The changes in Trade Winds lead to reduced Ekman pumping in the equatorial region. This leads to a freshening and warming of the surface waters along the equator. Especially in the Cold Tongue region, the cold and fresh subsurface anomalies do not reach the surface due to the reduced upwelling. The anomaly signals are transported by the Equatorial undercurrent and spread away from the Equator within the thermocline. Part of the anomaly eventually reaches the Tropical North Atlantic, where it affects the Guinea Dome. Surprisingly, the main effect at the surface is small on the equator and relatively large at the Guinea Dome. In the atmosphere, the northward shift of the ITCZ is associated with a band of negative precipitation anomalies and higher salinities over the Tropical South Atlantic. An important implication of these results is that the modified water characteristics due to a shut down of the Agulhas leakage remain largely unaffected when crossing the equatorial Atlantic and therefore can affect the deepwater formation in the North Atlantic. This supports the hypothesis that the Agulhas leakage is an important source region for climate change and decadal variability of the Atlantic.

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Published date: 2011
Organisations: Physical Oceanography

Identifiers

Local EPrints ID: 348348
URI: http://eprints.soton.ac.uk/id/eprint/348348
ISSN: 0930-7575
PURE UUID: 32648a01-2565-4506-a145-ba8f4ee0fc1f
ORCID for Sybren Drijfhout: ORCID iD orcid.org/0000-0001-5325-7350

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Date deposited: 12 Feb 2013 12:18
Last modified: 15 Mar 2024 03:44

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Contributors

Author: Reindert J. Haarsma
Author: Edmo J.D. Campos
Author: Wilco Hazeleger
Author: Camiel Severijns

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