Research Summary
Below are some quick summaries of my research.
Transient inter-basin overturning AMOC depth and deep ocean stratification Antarctic sea ice
Transient inter-basin overturning circulation and its role in climate change
Transient overturning compensation between Atlantic and Indo-Pacific basins
Climate models consistently project (i) a decline in the formation of North Atlantic Deep Water (NADW) and (ii) a strengthening of the Southern Hemisphere westerly winds in response to anthropogenic greenhouse gas forcing. These two processes suggest potentially conflicting tendencies of the Atlantic Meridional Overturning Circulation (AMOC): a weakening AMOC due to changes in the North Atlantic but a strengthening AMOC due to changes in the Southern Ocean.
We propose that the adjustment of the Indo-Pacific overturning circulation is a critical component in mediating AMOC changes. Using a hierarchy of ocean and climate models, we show that the Indo-Pacific overturning circulation provides the first response to AMOC changes through wave processes, whereas the Southern Ocean overturning circulation responds on longer (centennial to millenial) timescales that are determined by eddy diffusion processes. Changes in the Indo-Pacific overturning circulation compensate AMOC changes, which allows the Southern Ocean overturning circulation to evolve independently of the AMOC, at least over timescales up to many decades. In a warming climate, the Indo-Pacific develops an overturning circulation anomaly associated with the weakening AMOC that is characterized by a northward transport close to the surface and a southward transport in the deep ocean, which could effectively redistribute heat between the basins. Our results highlight the importance of inter-basin exchange in the response of the global ocean overturning circulation to a changing climate.
Centennial changes in the ITF connected to AMOC [a schematic]
The great ocean conveyor belt is a classical logo for the global ocean overturning circulation. In this paradigm, the North Atlantic Deep Water (NADW) enters the Pacific, comes back to the surface through diapycnal mixing, and returns to the Atlantic Ocean after entering the Indian Ocean as part of the Indonesian Throughflow (ITF). However, later observational studies suggested that deep waters mainly return to the surface via along-isopycnal pathways in the Southern Ocean, with ITF playing only a small role in closing the global overturning circulation.
Here we argue that, although the conveyor belt is not an accurate representation of the mean-state global ocean overturning circulation, it is a key component of the overturning’s transient response to surface forcing perturbations. In response to an AMOC slowdown, the Indian Ocean develops a northward surface transport anomaly that converges mass and modifies sea surface height in the Indian Ocean, which weakens the ITF. Through this transient version of the conveyor belt circulation, changes in the high-latitude North Atlantic (e.g., Arctic sea ice melt) can affect the climate in the low-latitude Indo-Pacific Ocean. An intriguing corollary is the potential to use the ITF to monitor or interpret long-term trends in the AMOC.
Ref: Sun and Thompson (GRL, 2020) and Sun et al. (JPO, 2020)
Indo-Pacific warming induced by a weakening of the AMOC: An inter-basin seesaw [a schematic]
The reorganization of the Atlantic Meridional Overturning Circulation (AMOC) is often associated with changes in Earth’s climate. These AMOC changes are communicated to the Indo-Pacific basins via wave processes and induce an overturning circulation anomaly that opposes the Atlantic changes on decadal to centennial time scales. We examine the role of this transient, inter-basin overturning response, driven by an AMOC weakening, both in an ocean-only model with idealized geometry and in a coupled CO$_2$ quadrupling experiment, in which the ocean warms on two distinct timescales: a fast decadal surface warming and a slow centennial subsurface warming. We show that the transient inter-basin overturning produces a zonal heat redistribution between the Atlantic and Indo-Pacific basins. Following a weakened AMOC, an anomalous northward heat transport emerges in the Indo-Pacific, which substantially compensates for the Atlantic southward heat transport anomaly. This zonal heat redistribution manifests as a thermal inter-basin seesaw between the high-latitude North Atlantic and the subsurface Indo-Pacific and helps to explain why Antarctic temperature records generally show more gradual changes than the Northern Hemisphere during the last glacial period. In the coupled CO$_2$ quadrupling experiment, we find that the inter-basin heat transport due to a weakened AMOC contributes substantially to the slow centennial subsurface warming in the Indo-Pacific, accounting for more than half of the heat content increase and sea level rise. Thus, our results suggest that the transient inter-basin overturning circulation is a key component of the global ocean heat budget in a changing climate.
Ocean overturning circulation in steady state
Processes that set the depth of AMOC: Southern Ocean vs North Atlantic
Paleoclimate proxy data suggest that the AMOC was shallower at the Last Glacial Maximum than its preindustrial depth. Previous studies emphasized the Southern Ocean surface buoyancy forcing in setting the AMOC depth. Using a combination of model simulations and conceptual theories, we show that dipyacnal processes, ignored in previous studies, could diminish the influece of Southern Ocean surface buoyancy forcing on the AMOC depth. A new schematic based on surface buoyancy distributions in both the Southern Ocean and the North Atlantic is proposed.
Ref: Sun et al. (2018), Sun and Liu (2017), Sun et al. (JClim, 2020)
Glacial-interglacial changes in deep ocean stratification set by Southern Ocean surface buoyancy forcing
Previous studies have suggested that the global ocean density stratification below ∼3000 m is approximately set by its direct connection to the Southern Ocean surface density, which in turn is constrained by the atmosphere. Here the role of Southern Ocean surface forcing in glacial-interglacial stratification changes is investigated using a comprehensive climate model and an idealized conceptual model. Southern Ocean surface forcing is found to control the global deep ocean stratification up to ∼2000 m, which is much shallower than previously thought and contrary to the expectation that the North Atlantic surface forcing should strongly influence the ocean at intermediate depths. We show that this is due to the approximately fixed surface freshwater fluxes, rather than a fixed surface density distribution in the Southern Ocean as was previously assumed. These results suggest that Southern Ocean surface freshwater forcing controls glacial-interglacial stratification changes in much of the deep ocean.
Ref: Sun et al. (2016)
Sea ice
Observed Antarctic sea ice expansion reproduced in a climate model after correcting biases in sea ice drift velocity
The Antarctic sea ice area expanded significantly during 1979-2015. This is at odds with state-of-the-art climate models, which typically simulate a receding Antarctic sea ice cover in response to increasing greenhouse forcing. Here we investigate the hypothesis that this discrepancy between models and observations occurs due to simulation biases in the sea ice drift velocity. As a control we use the Community Earth System Model (CESM) Large Ensemble (“LENS”), which has 40 realizations of past and future climate change that all undergo Antarctic sea ice retreat during recent decades. We modify CESM to replace the simulated sea ice velocity field with a satellite-derived estimate of the observed sea ice motion, and we simulate 3 realizations of recent climate change (“ObsVi”). We find that the Antarctic sea ice expands in all 3 of these realizations, with the simulated spatial structure of the expansion bearing resemblance to observations. The results suggest that the reason CESM has failed to capture the observed Antarctic sea ice expansion is due to simulation biases in the sea ice drift velocity, implying that an improved representation of sea ice motion is crucial for more accurate sea ice projections.