More Like this

1. Transient Overturning Compensation between Atlantic and Indo-Pacific Basins

   https://doi.org/10.1175/JPO-D-20-0060.1  

   Sun, Shantong ; Thompson, Andrew F. ; Eisenman, Ian ( August 2020 , Journal of Physical Oceanography)

   null (Ed.)

   Abstract 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. Here we focus on the transient evolution of the global ocean overturning circulation in response to a perturbation to the NADW formation rate. 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 millennial) time scales 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 time scales 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 interbasin exchange in the response of the global ocean overturning circulation to a changing climate. 

   more » « less
2. Indo-Pacific Warming Induced by a Weakening of the Atlantic Meridional Overturning Circulation

   https://doi.org/10.1175/JCLI-D-21-0346.1  

   Sun, Shantong ; Thompson, Andrew F. ; Xie, Shang-Ping ; Long, Shang-Min ( January 2022 , Journal of Climate)

   Abstract 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, interbasin 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 time scales: a fast decadal surface warming and a slow centennial subsurface warming. We show that the transient interbasin 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 interbasin 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 interbasin 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 interbasin overturning circulation is a key component of the global ocean heat budget in a changing climate. 

   more » « less
3. Oceanic Pathways of an Active Pacific Meridional Overturning Circulation (PMOC)

   https://doi.org/10.1029/2020GL091935  

   Thomas, M. D. ; Fedorov, A. V. ; Burls, N. J. ; Liu, W. ( May 2021 , Geophysical Research Letters)

   In contrast to the modern‐day climate, North Pacific deep water formation and a Pacific meridional overturning circulation (PMOC) may have been active during past climate conditions, in particular during the Pliocene epoch (some 3–5 million years ago). Here, we use a climate model simulation with a robust PMOC cell to investigate the pathways of the North Pacific deep water from subduction to upwelling, as revealed by Lagrangian particle trajectories. We find that similar to the present‐day Atlantic Meridional Overturning Circulation (AMOC), most subducted North Pacific deep water upwells in the Southern Ocean. However, roughly 15% upwells in the tropical Indo‐Pacific Oceans instead—a key feature distinguishing the PMOC from the AMOC. The connection to the Indian Ocean is relatively fast, at about 250 years. The connection to the tropical Pacific is slower (∼800 years) as water first travels to the subtropical South Pacific then gradually upwells through the thermocline.

    

   more » « less
4. Hydroclimate Variability in the Equatorial Western Indian Ocean for the Last 250,000 Years

   https://doi.org/10.1029/2022PA004530  

   Windler, Grace ; Tierney, Jessica E. ; deMenocal, Peter B. ( January 2023 , Paleoceanography and Paleoclimatology)

   Indian Ocean sea surface temperatures impact precipitation across the basin through coupled ocean‐atmosphere responses to changes in climate. To understand the hydroclimate response over the western Indian Ocean and equatorial east Africa to different forcing mechanisms, we present four new proxy reconstructions from core VM19‐193 (2.98°N, 51.47°E) that span the last 250 ky. Sub‐surface water temperatures (Sub‐T; TEX₈₆) show strong precessional (23 ky) variability that is primarily influenced by maximum incoming solar radiation (insolation) during the Northern Hemisphere spring season, likely indicating that local insolation dominates the upper water column at this tropical location over time. Leaf waxes, on the other hand, reflect two different precipitation signals:δ¹³C_wax(in phase with boreal fall insolation) is likely reflecting vegetation changes in response to local rainfall over east Africa, whereasδD_precip(primarily driven by boreal summer insolation) represents changes in regional circulation associated with the summer monsoon. Glacial‐interglacial changes in ocean temperatures support glacial shelf exposure over the Maritime Continent in the eastern Indian Ocean and the subsequent weakening of the Indian Walker Circulation as a mechanism driving 100 ky climate variability across the tropical Indo‐Pacific. Additionally, the 100 ky spectral power inδD_precipsupports a basin‐wide weakening of summer monsoon circulation in response to glacial climates. Overall, the proxy records from VM19‐193 indicate that both precession and glacial‐interglacial cycles exert control over hydroclimate at this tropical location.

    

   more » « less
5. Freshwater Contributions to Decadal Variability of the Indonesian Throughflow

   https://doi.org/10.1029/2023GL103906  

   Wang, Shouyi ; Ummenhofer, Caroline C. ; Oppo, Delia W. ; Murty, Sujata A. ; Wagner, Patrick ; Böning, Claus W. ; Biastoch, Arne ( July 2023 , Geophysical Research Letters)

   The Makassar Strait, the main passageway of the Indonesian Throughflow (ITF), is an important component of Indo‐Pacific climate through its inter‐basin redistribution of heat and freshwater. Observational studies suggest that wind‐driven freshwater advection from the marginal seas into the Makassar Strait modulates the strait's surface transport. However, direct observations are too short (<15 years) to resolve variability on decadal timescales. Here we use a series of global ocean simulations to assess the advected freshwater contributions to ITF transport across a range of timescales. The simulated seasonal and interannual freshwater dynamics are consistent with previous studies. On decadal timescales, we find that wind‐driven advection of South China Sea (SCS) waters into the Makassar Strait modulates upper‐ocean ITF transport. Atmospheric circulation changes associated with Pacific decadal variability appear to drive this mechanism via Pacific lower‐latitude western boundary current interactions that affect the SCS circulation.

    

   more » « less