Collaborative Research: The Atlantic Meridional Overturning Circulation and Internal Climate Variability

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The Atlantic meridional overturning circulation (AMOC) is a global ocean circulation in which colder and denser surface water sinks in the subpolar North Atlantic and flows southward at depth, reaching Antarctica and circulating through the world oceans until it eventually resurfaces and returns north from the high southern latitudes. The AMOC transports a substantial amount of heat from the Southern Hemisphere and the tropics into the North Atlantic, and variations of the AMOC are thought to be implicated in long-term variations in North Atlantic sea surface temperature (SST). The SST variations have impacts on the frequency and intensity of Atlantic hurricanes, prolonged drought in the Sahel, north-south shifts of the intertropical convergence zone (ITCZ), and rainfall changes over much of the globe including the US. This project seeks to understand the basic dynamics which drive AMOC variability as well as its effects on SST and surface climate, focusing in particular on the extent to which the slow variations of the AMOC are driven by "weather noise", the short-term and somewhat impulsive forcing of the ocean by the passage of surface weather systems. Despite its impulsive nature forcing by weather systems can induce a low-frequency ocean response due to the reddening effect of upper ocean thermal inertia. The PIs have developed a method to quantify the effect of weather noise in climate model simulations in which an ensemble of atmospheric models are coupled to a single ocean model. The surface fluxes through which the atmosphere affects the ocean are averaged over all the ensemble models before applying them to the ocean, so the ocean only feels the effects of the ensemble mean fluxes. Each atmospheric model produces its own weather systems which are unrelated to the weather generated in the other models, thus the weather noise averages out in the ensemble mean and does not affect the ocean. Here the PIs employ this strategy, which they refer to as an Interactive Ensemble (IE), using the Community Earth System Model (CESM). The IE-CESM is an updated version of the IE system they developed under previous funding (AGS-1137902/1137911), and the ensemble includes the land surface and sea ice component models as well as the atmosphere model.The PIs assess the role of weather noise in generating AMOC variability through comparisons between IE-CESM simulations and control runs using the standard CESM (in which weather noise is not removed). Additional ocean-only simulations are used to assess the role of internal ocean variability in AMOC variability. Additional simulations use a hybrid IE (HyIE) configuration, in which atmospheric forcing from the IE is used in some regions but forcing from a single atmospheric model is used in others. Weather noise is thus applied only over specified regions, and the local and remote responses of the regionally confined noise forcing can be evaluated. One hypothesis to be tested is that the AMOC responds primarily to weather noise forcing over the Labrador sea where most of the deep water formation occurs. Alternatively, remote noise forcing could affect the AMOC, which is trapped along the western boundary, through the generation of westward-propagating oceanic Rossby waves. The response of the surface climate to AMOC variability is also considered, with particular attention to the north-south shifts of the intertropical convergence zone (ITCZ) that are the expected consequence of changes in cross-equatorial transport accompanying AMOC fluctuations.As noted above AMOC variability is a matter of practical as well as scientific interest, given its association with surface climate effects such as drought and hurricane activity. In addition, the PIs are working with the CESM developers to make the IE framework available to the broader research community. The project also supports two graduate students, thereby providing for the future work force in this research area.