AOML
NOAA

Meridional Overturning Circulation

AMOC, Global Monsoons and Extreme Weather

Atlantic Meridional Overturning Circulation and Its Impact on Global Monsoons and Extreme Weather Events

Introduction

The Atlantic Meridional Overturning Circulation (AMOC) is characterized by northward flowing warm and salty water in the upper layer of the Atlantic Ocean, deep convection in the Nordic and Labrador Seas, and southward flowing colder water at depth. This circulation transports heat from the South Atlantic and tropical North Atlantic to the subpolar and polar North Atlantic, a region characterized by net cooling and freshening of the water at higher northern latitudes of the Atlantic in the Nordic and Labrador Seas, where the heat is released to the atmosphere with substantial impacts on climate. Funded by Climate Program Office (CPO) of NOAA, this project aims to advance our understanding of the mean pathways and variability of the AMOC and their link to global monsoons and extreme weather events. Three specific goals are to better understand and describe (1) the southward pathways of the upper and lower North Atlantic Deep Water, (2) interannual-to-decadal variability of the South Atlantic MOC (SAMOC), and (3) decadal modulations of global monsoons and US heat waves by SAMOC.

News and Highlights

05/23/2017:

2017 US AMOC science team meeting

PIs of the projects attended the 2017 US AMOC science team meeting, which was held in Santa Fe, NM during May 23-25, 2017. Sang-Ki Lee presented a poster "Southward pathways of the upper and lower North Atlantic Deep Water and their impact on the Atlantic meridional heat transport". Marlos Goes presented a poster entitled "The role of African dust on AMOC during Heinrich events". Hosmay Lopez had an oral presentation "Remote influence of Interdecadal Pacific Oscillation on the South Atlantic Meridional Overturning Circulation variability" and a poster presentation "A reconstructed South Atlantic Meridional Overturning Circulation time series since 1870". Shenfu Dong presented a poster "Altimetery-derived outh Atlantic Meridional Overturning Circulation between 20S and 35S since 1993".

05/03/2017:

Biweekly AMOC discussion meetings

PhOD has resumed the AMOC discussion meetings to discuss new papers and ideas concerning AMOC and related topics. The meeting is held biweekly at AOML (normally 2nd floor conference room). In the first meeting (May 31st), Alexandra Gronholz led the discussion of "Impact of slowdown of Atlantic overturning circulation on heat and freshwater transports" by Kelly et al. (2016).

04/19/2017:

Journal of Geophysical Research (JGR) Editors' Highlight

"Wind-driven ocean dynamics impact on the contrasting sea-ice trends around West Antarctica" by S.-K. Lee et al. (2017) was selected as Journal of Geophysical Research (JGR) Editors' Highlight: "Much of the work on the cause of Antarctic sea ice over recent decades has focused on atmospheric drivers but this paper focuses on the ocean's role. The authors analyse the trend of Antarctic sea ice over the past 35 years on the basis of satellite data and model simulations forced with atmospheric reanalysis products. Their findings suggest that ocean processes play a crucial role in determining the seasonality of sea ice trends. They also reveal that the sea-ice response is regional".

04/01/2017:

Welcome aboard Alexandra!

Dr. Alexandra Gronholz, a recent graduate from University of Bremen in Germany, joined our team as a postdoc in April, 2017. She will use the Modular Ocean Model version 6 to study the variability of the meridional overturning circulation in the South Atlantic and its link to the eddy processes. Her research will directly contribute to NOAA's long-term goal and objective, Climate Adaptation and Mitigation - Improved scientific understanding of the changing climate system and its impacts. Welcome aboard Alexandra!

Southward Pathways of the Upper and Lower North Atlantic Deep Water

PIs: Sang-Ki Lee, and Marlos Goes

The Atlantic Meridional Overturning Circulation (AMOC) transports the upper warm water northward and the deep cold water southward in the Atlantic, and is a key component of the global energy balance. In many of the climate models that participate the Coupled Model Inter-comparison Project Phase 5 (CMIP5), the amplitudes of the AMOC agree very well with or are even larger than the observed value of about 18 Sv at 26.5N; but they still show cold upper ocean temperature biases in the North Atlantic. This suggests that the AMOC strength may not be the only factor that determines the meridional ocean heat transport. A common symptom in these models is that the returning flow of the AMOC at depth is too shallow. A shallow returning flow would carry excessive heat southward; thus the net northward heat transport by the AMOC would be weaker than the observed. We will present quantitative analysis that supports the above hypothesis using available observations, CMIP5 model outputs, and ocean and sea-ice model experiments.

Interannual-to-Decadal Variability of the SAMOC

PIs: Shenfu Dong, and Sang-Ki Lee
Colloborators: Matthew Harrison (GFDL), Molly Baringer, and Gustavo Goni

Recent studies have suggested the possibility of the southern origin of the Atlantic MHT anomalies. These studies have used General Circulation Models (GCMs) to demonstrate covariability between the South Atlantic MOC (SAMOC) and the Southern Hemisphere westerlies at interannual to longer time scales. However, it has been pointed out that the sensitivity of the SAMOC to the changes in the Southern Hemisphere westerlies depends critically on the representation of mesoscale eddies in those models. The observation-based estimates of MHT in the South Atlantic have a wide range of values, which, to some extent, is due to the misrepresentation of the eddy heat transport. Therefore, understanding the variability of the MOC/MHT in the South Atlantic on various time scales and the role of eddies in the South Atlantic are essential ingredients toward achieving decadal predictability of the AMOC and its impact on climate. In our continued effort to diagnose the performance of climate models in the South Atlantic and to better understand the role of eddies, we will (1) diagnose and characterize interannual to decadal variability of the SAMOC in CMIP5 models, (2) explore the impact of resolving eddies in ocean GCMs on interannual to decadal variability of the simulated SAMOC, and (3) quantify the relationship of the SAMOC with the Southern Hemisphere westerlies and the Agulhas leakage.

Decadal Modulations of Global Monsoons and Extreme Weather Events by SAMOC

PIs: Hosmay Lopez, Shenfu Dong, and Sang-Ki Lee

There have been many efforts to understand the role of the Atlantic Meridional Overturning Circulation (AMOC) as a potential predictor of decadal climate variability, motivated partly by its inherent relationship with North Atlantic sea surface temperature. In contrast, there is currently limited knowledge about the underlying mechanisms that govern the South Atlantic Meridional Overturning Circulation (SAMOC) variability and how it might feedback into climate, partly due to the small number of direct observations in this ocean basin. Therefore, the majority of efforts to understand the dynamics of the AMOC and its climate impacts are focused on the North Atlantic. Nevertheless, the South Atlantic Ocean plays a key role in the global distribution of energy and is characterized by complex and unique ocean dynamic processes. For example, the Brazil-Malvinas Confluence and the Agulhas leakage play critical roles in the exchange of water masses. These potentially modify the long-term response of the SAMOC that could impact global atmospheric circulation, precipitation, and climate. For example, it is hypothesized that decadal variations of SAMOC could modulate the strength of global monsoons with 15 - 20 years of lead-time, suggesting that SAMOC is a potential predictor of global monsoon variability. We will present quantitative analysis that supports the above hypothesis.

Publications

Dong, S., S.L. Garzoli, and M.O. Baringer, 2011: The role of inter-ocean exchanges on decadal variations of the northward heat transport in the South Atlantic. J. Phys. Oceanogr., 41, 1498-1511.

Goes, M., I. Wainer, N. Signorelli, 2014: Investigation of the causes of historical changes in the sub-surface salinity minimum of the South Atlantic. J. Geophys. Res. Oceans, 119, 5654-5675, doi: 10.1002/2014JC009812.

Lee, S.-K., D. Volkov, H. Lopez, W. G. Cheon, A. L. Gordon, Y. Liu, and R. Wanninkhof, 2017: Wind-driven ocean dynamics impact on the contrasting sea-ice trends around West Antarctica. J. Geophys. Res., doi:10.1002/2016JC012416.

Lee, S.-K., W. Park, E. van Sebille, M. O. Baringer, C. Wang, D. B. Enfield, S. Yeager and B. P. Kirtman, 2011: What caused the significant increase in Atlantic ocean heat content since the mid-20th century? Geophys. Res. Lett., 38, L17607, doi:10.1029/2011GL048856. [highlighted in Oct. 6, 2011 issue of Nature as Community Choice]

Lee, S.-K. and C. Wang, 2010: Delayed advective oscillation of the Atlantic thermohaline circulation. J. Climate, 23, 1254-1261.

Liu, Y., S.-K. Lee, D. B. Enfield, B. A. Muhling, J. T. Lamkin, F. Muller-Karger and M. A. Roffer, 2015: Potential impact of climate change on the Intra-Americas Seas: Part-1. A dynamic downscaling of the CMIP5 model projections. J. Marine Syst., 148, 56-69, doi:10.1016/j.jmarsys.2015.01.007.

Lopez, H., G. Goni and S. Dong, 2016: A reconstructed South Atlantic Meridional Overturning Circulation time series since 1870. Geophys. Res. Lett., doi:10.1002/2017GL073227.

Lopez, H., S. Dong, S.-K. Lee and G. Goni, 2016: Decadal modulations of interhemispheric global atmospheric circulations and monsoons by the South Atlantic Meridional Overturning Circulation. J. Climate, 29, 1831-1851, doi:http://dx.doi.org/10.1175/JCLI-D-15-0491.1.

Lopez, H., S. Dong, S.-K. Lee and E. Campos, 2016: Remote influence of Interdecadal Pacific Oscillation on the South Atlantic Meridional Overturning Circulation variability. Geophys. Res. Lett., 43, 8250-8258, doi:10.1002/2016GL069067.

Wang, C., L. Zhang, S.-K. Lee, L. Wu and C. R. Mechoso, 2014: A global perspective on CMIP5 climate model biases. Nature Clim. Change, 4, 201-205, doi:10.1038/nclimate2118.