[Eoas-seminar] Oceanography Dissertation Defense - Tachanat Bhatrasataponkul - Nov 5, 11:00 - 327OSB
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Fri Oct 26 16:10:03 EDT 2018
Name: Tachanat Bhatrasataponkul
Date: November 5, 2018
Time: 11:00 AM
Title: The Origin of the North Atlantic Cold Blob Revisited
The cold blob refers to an observationally unprecedented gyre-scale record-breaking cold of mean surface temperature over the subpolar North Atlantic. Its anomalous cold feature goes against the rising trend of global mean surface temperature under the context of a warming climate. Observations show that the Atlantic cold blob emerged since early 2014 and can penetrate deeper into the ocean interior beyond 500m depths. A sudden drop in upper ocean heat content is associated with an accumulative increase in freshwater content. Prior works pointed out that intense surface cooling during two consecutive winters was a primary driver. We hypothesize that surface forcing alone is insufficient for the cold blob to persist. Our analysis shows, for the first time, that variations in the net surface heat fluxes cannot explain the decline in upper ocean heat content during 2014-2017. Therefore, surface forcing fails to explain the origin of the cold blob.
To address alternative mechanisms, non-assimilative simulations based on a coupled ocean-sea ice model (GFDL MOM5/SIS1) with two different atmospheric forcings (MERRA2 and ERA-interim) are employed to investigate the transports of mass, heat, and freshwater within the cold blob area. Initial diagnosis verified that both model runs can reproduce the cold blob characteristics at similar magnitudes to Argo observation. Model results show a decreasing trend of heat transports at the southern boundary, implying that reduced poleward ocean heat transport likely accounts for the formation and persistence of the cold blob. Heat budget analysis reveals considerable changes in the residual heat fluxes and suggests that reduced warming for the 100m to 700m depths occurred since 2006 before turning into enhanced cooling during late 2013.
Increased freshwater transports are found at the western and southern boundaries. Salt budget analysis shows that the residual freshwater fluxes remain positive for the entire past decade and subsequently results in an accumulative surplus of freshwater content in this area. The model simulation with incorporating Greenland meltwater estimates reveals that approximate 200% amplification in freshwater storage during the present decade could imply the relative contribution of Greenland meltwater. In the long run, upper ocean cooling and freshening would lead to increased stratification and reduced mixing with deeper waters, therefore enhancing the likelihood that the subsurface cold blob persists.
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