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The Indonesian Throughflow's Effect on Global Climate Determined from the COLA Coupled Climate System

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  • 1 Department of Meteorology, University of Maryland at College Park, College Park, Maryland
  • | 2 Center for Ocean–Land–Atmosphere Studies, Calverton, Maryland
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Abstract

Blocking the Indonesian archipelago by a land bridge, and so preventing flow from the western equatorial Pacific Ocean into the eastern Indian Ocean, causes a global readjustment in the coupled ocean–atmosphere's climate. Notable features found in 10-yr-averaged fields when compared with the control climate are an ENSO-like signal over the equatorial Pacific, with warmer SSTs in the eastern basin, westerly anomalies in wind stress, and increased precipitation along the equator. The band of increased precipitation is flanked by bands of decreased rainfall. Over the Indian Ocean, blocking of the throughflow results in signatures similar to those associated with the Indian Dipole Mode. In the eastern basin, there are cooler SSTs and heat content anomalies. The southeast trades are increased with an associated increase in latent heating. The precipitation belt is shifted northwestward to give decreased rainfall over southern Indonesia and increased rainfall to the north; there is a slight increase in rainfall over eastern Africa, and SSTs are warmer in the western half of the basin. The Atlantic, midlatitudes, and polar regions are affected via atmospheric teleconnections.

Net surface heat flux differences in regions of significant SST difference in the equatorial Pacific and Indian Oceans are about 2 times as large as found previously in ocean-only simulations, indicating a positive feedback. Experiments with the atmospheric GCM component forced by the Indian Ocean SST anomalies generated by blocking the throughflow reproduce the changes in surface heat flux and winds found in the coupled model simulations over the southern Indian Ocean. In the equatorial Pacific, positive feedback is provided by the Bjerknes mechanism. In the Indian Ocean, the positive feedback loop comprises a change in oceanic heat flux divergence, which changes SST, and in turn net surface heat flux and surface winds, which further change the oceanic circulation and heat flux divergence; the primary effect is on meridional advection. The feedback is sufficiently strong that the anticyclonic–cyclonic pair of Sverdrup gyres generated in the tropical southern Indian Ocean with lateral viscous effects alter the path of the depth-integrated throughflow transport from zonal across the Indian Ocean, then southward along the east African coast, to southwesterly. The Island Rule indicates another positive feedback in the coupled system, albeit weak, as the modification to the wind stresses (attributed to the zonal ones in the equatorial Pacific), which results from blocking the throughflow, acts to decrease further the throughflow. Interannual variability in the total throughflow transport is not well predicted by the Island Rule. Its seasonal cycle appears driven by the cycle in the equatorial Pacific, as both lead by two months that found in stand-alone ocean GCMs.

Additional affiliation: Earth System Science Interdisciplinary Center, University of Maryland at College Park, College Park, Maryland

Corresponding author address: Dr. Roxana C. Wajsowicz, Department of Meteorology, University of Maryland at College Park, 3433 Computer and Space Science Building, College Park, MD 20742-2425.Email: roxana@atmos.umd.edu

Abstract

Blocking the Indonesian archipelago by a land bridge, and so preventing flow from the western equatorial Pacific Ocean into the eastern Indian Ocean, causes a global readjustment in the coupled ocean–atmosphere's climate. Notable features found in 10-yr-averaged fields when compared with the control climate are an ENSO-like signal over the equatorial Pacific, with warmer SSTs in the eastern basin, westerly anomalies in wind stress, and increased precipitation along the equator. The band of increased precipitation is flanked by bands of decreased rainfall. Over the Indian Ocean, blocking of the throughflow results in signatures similar to those associated with the Indian Dipole Mode. In the eastern basin, there are cooler SSTs and heat content anomalies. The southeast trades are increased with an associated increase in latent heating. The precipitation belt is shifted northwestward to give decreased rainfall over southern Indonesia and increased rainfall to the north; there is a slight increase in rainfall over eastern Africa, and SSTs are warmer in the western half of the basin. The Atlantic, midlatitudes, and polar regions are affected via atmospheric teleconnections.

Net surface heat flux differences in regions of significant SST difference in the equatorial Pacific and Indian Oceans are about 2 times as large as found previously in ocean-only simulations, indicating a positive feedback. Experiments with the atmospheric GCM component forced by the Indian Ocean SST anomalies generated by blocking the throughflow reproduce the changes in surface heat flux and winds found in the coupled model simulations over the southern Indian Ocean. In the equatorial Pacific, positive feedback is provided by the Bjerknes mechanism. In the Indian Ocean, the positive feedback loop comprises a change in oceanic heat flux divergence, which changes SST, and in turn net surface heat flux and surface winds, which further change the oceanic circulation and heat flux divergence; the primary effect is on meridional advection. The feedback is sufficiently strong that the anticyclonic–cyclonic pair of Sverdrup gyres generated in the tropical southern Indian Ocean with lateral viscous effects alter the path of the depth-integrated throughflow transport from zonal across the Indian Ocean, then southward along the east African coast, to southwesterly. The Island Rule indicates another positive feedback in the coupled system, albeit weak, as the modification to the wind stresses (attributed to the zonal ones in the equatorial Pacific), which results from blocking the throughflow, acts to decrease further the throughflow. Interannual variability in the total throughflow transport is not well predicted by the Island Rule. Its seasonal cycle appears driven by the cycle in the equatorial Pacific, as both lead by two months that found in stand-alone ocean GCMs.

Additional affiliation: Earth System Science Interdisciplinary Center, University of Maryland at College Park, College Park, Maryland

Corresponding author address: Dr. Roxana C. Wajsowicz, Department of Meteorology, University of Maryland at College Park, 3433 Computer and Space Science Building, College Park, MD 20742-2425.Email: roxana@atmos.umd.edu

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