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.
Ocean tracers such as heat, salt and carbon are perpetually carried by the global meridional overturning circulation (GMOC) and redistributed between hemispheres and across ocean basins from their source regions. The GMOC is therefore a crucial component of the global heat, salt and carbon balances. In a new article published in Geophysical Research Letter (Lee et al., 2019), Sang-Ki Lee and his team from NOAA/AOML, CIMAS/University of Miami and NCAR revisited our current understanding of the GMOC by using a global ocean model simulation with its temperature and salinity corrected toward observations. This study presents the derived GMOC, validates it against observations and summarizes in a schematic, which highlights several important but relatively under-explored aspects of the GMOC, including the pathway through which the heaviest water mass formed around the Antarctica is brought to the surface. The study further shows that some key aspects of the GMOC are poorly captured in a model run without the temperature and salinity corrections, suggesting that current climate models do not reproduce realistic paths of the GMOC and the associated global heat, salt and carbon balances.
Dr. Dongmin Kim is a recent graduate from Ulsan National Institute of Science and Technology (UNIST) of South Korea. He joined our team as a postdoc in August 1, 2018. Dongmin will utilize the Community Earth System Model version 2 (CESM2) to study the impact of the AMOC and its pathways on the global atmospheric dynamics. Welcome aboard Dongmin!
PIs of the projects attended the 2018 International AMOC science meeting, which was held in Miami, Fl during July 24-27, 2018. Hosmay Lopez had an oral presenation "The South Atlantic Meridional Overturning Circulation: A driver of climate variability and extremes". Marlos Goes had an oral presenation "Assessing uncertainties on the stability of the AMOC during Heinrich events using simulations from one Earth System model". Shenfu Dong presented a poster "SAMOC variations during the past 24 years and their role in ocean heat content changes".Sang-Ki Lee presented a poster "Pathways of the global meridional overturning circulation inferred from a data-constrained ocean & sea-ice model".
Climate projections for the twenty-first century suggest an increase in the occurrence of heat waves. However, the time at which externally forced signals of anthropogenic climate change (ACC) emerge against background natural variability (time of emergence (ToE)) has been challenging to quantify, which makes future heat-wave projections uncertain. In a new article published in Nature Climate Change (Lopez et al., 2018), Hosmay Lopez and his team combine observations and model simulations under present and future forcing to assess how internal variability and ACC modulate US heat waves. The team shows that ACC dominates heat-wave occurrence over the western United States and Great Lakes regions, with ToE that occurred as early as the 2020s and 2030s, respectively. In contrast, internal variability governs heat waves in the northern and southern Great Plains, where ToE occurs in the 2050s and 2070s; this later ToE is believed to be a result of a projected increase in circulation variability, namely the Great Plain low-level jet. Thus, greater mitigation and adaptation efforts are needed in the Great Lakes and western United States regions. This work was highlited in climate.gov.
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".
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).
"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".
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!
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.
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.
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.
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., R. Lumpkin, M. O. Baringer, C. S. Meinen, M. Goes, S. Dong, H. Lopez and S. G. Yeager, 2019: Global meridional overturning circulation inferred from a data-constrained ocean & sea-ice model. Geophys. Res. Lett., 45, https://doi.org/10.1029/2018GL080940.
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.
Lopez, H, R. West, S. Dong, G. Goni, B. Kirtman, S.-K. Lee, and R. Atlas, 2018: Early emergence of anthropogenically-forced heat waves in the western US and Great Lakes. Nature Clim. Change, https://doi.org/10.1038/s41558-018-0116-y.
Murphy, L. N., M. Goes, and A. Clement, 2017: Role of African dust in the Atlantic meridional overturning circulation during Heinrich events. Paleoceanogr., 32, 1291-1308, doi:10.1002/2017PA003150.
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.