Forcing of anthropogenic aerosols on temperature trends of the sub-thermocline southern Indian Ocean

Tim Cowan,Ariaan Purich,Leon Rotstayn,Matthew H England,Wenju Cai

doi:10.1038/srep02245

Tim Cowan, Ariaan Purich + Show 3 more

Open Access

https://doi.org/10.1038/srep02245

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Abstract

In the late twentieth century, the sub-thermocline waters of the southern tropical and subtropical Indian Ocean experienced a sharp cooling. This cooling has been previously attributed to an anthropogenic aerosol-induced strengthening of the global ocean conveyor, which transfers heat from the subtropical gyre latitudes toward the North Atlantic. From the mid-1990s the sub-thermocline southern Indian Ocean experienced a rapid temperature trend reversal. Here we show, using climate models from phase 5 of the Coupled Model Intercomparison Project, that the late twentieth century sub-thermocline cooling of the southern Indian Ocean was primarily driven by increasing anthropogenic aerosols and greenhouse gases. The models simulate a slow-down in the sub-thermocline cooling followed by a rapid warming towards the mid twenty-first century. The simulated evolution of the Indian Ocean temperature trend is linked with the peak in aerosols and their subsequent decline in the twenty-first century, reinforcing the hypothesis that aerosols influence ocean circulation trends.

Highlights

In the late twentieth century, the sub-thermocline waters of the southern tropical and subtropical Indian Ocean experienced a sharp cooling
While Indian Ocean Thermal Archive (IOTA) is included in the trend average in (b), the original source data was unavailable for significance testing, IOTA is excluded in the significance test
The one inconsistency is the lack of an extratropical warming in Simple Ocean Data Assimilation (SODA)-Parallel Ocean Program (POP), which may be due to the scarcity of observations in the regions south of 35uS13

Summary

Introduction

In the late twentieth century, the sub-thermocline waters of the southern tropical and subtropical Indian Ocean experienced a sharp cooling. While some modelling studies suggest the Pacific had very little impact on the subsurface IO prior to the 1990s9,17, future surface warming in the tropical and subtropical Pacific may lead to wind-induced redistributions of upper ocean water and a shoaling thermocline in the southern IO18. Anthropogenic forcings, such as GHGs have already been attributed as the major cause of historic ocean warming[19]. While IOTA is included in the trend average in (b), the original source data was unavailable for significance testing (only trend data were provided), IOTA is excluded in the significance test

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Conclusion