A document by Matt Luongo. Click on the document to view its contents.

Global warming will soon reach the Paris Agreement targets of 1.5C°/2°C temperature increase. Under a business-as-usual scenario, the time to reach these targets varies widely among climate models. Using Coupled Model Intercomparison Project phase 5 and 6, we show that a 2°C near-future global warming rate is determined by Southern Ocean (SO) state closely tied with a low-level cloud (LLC) amount feedback strength during the reference (1861-1900) period; climate models with cold SO tend to accompany more low-level cloudiness and Antarctic sea ice concentration due to a strong LLC amount feedback. Consequently, initially cold SO models tend to simulate a fast near-future warming rate by absorbing more downward shortwave radiation compared to initially warm SO models because more LLC disappears due to a strong LLC amount feedback during the 2°C rise. Our results demonstrate that climate models that correctly simulate initial SO state can improve near-future projections with reduced uncertainties.

Previous studies have found that Northern Hemisphere aerosol-like cooling induces a La Nina-like quasi-equilibrium response in the tropical Indo-Pacific. Here, we explore a coupled atmosphere-ocean feedback pathway by which this response is communicated. We override ocean surface wind stress in a comprehensive climate model to decompose the total ocean-atmosphere response to forced extratropical cooling into the response of surface buoyancy forcing alone and surface momentum forcing alone. In the subtropics, the buoyancy-forced response dominates: the positive low cloud feedback amplifies sea surface temperature (SST) anomalies which are then communicated to the tropics via wind-driven evaporative cooling. In the deep tropics, the momentum-driven Bjerknes feedback creates zonally asymmetric SST patterns in the Indian and Pacific basins. Although subtropical cloud feedbacks are model-dependent, our results suggest this feedback pathway is robust across a suite of models such that models with a stronger subtropical low cloud response exhibit a stronger La Nina response.

Equatorial Pacific decadal variability (EPDV) modulates global climate. Although EPDV is suggested to be generated by both air-sea thermodynamically coupled slab ocean models (SOM) and fully coupled dynamic ocean models (DOM), the reason of EPDV simulated by the two distinct hierarchies of models remains unclear. This ambiguity arises from a gap in the dynamical framework between SOM and DOM. To fill the gap, we conducted a novel experiment (Clim-tau) that retains only the effects of thermodynamic coupling and mean ocean current on EPDV (without anomalous ocean current). We showed that in Clim-tau, thermodynamic-driven EPDV as in SOM is largely damped by equatorial Pacific mean upwelling; whereas involving anomalous ocean current as in DOM, the damped EPDV will be further amplified. Finally, we discussed the role of ocean dynamics in the observed EPDV. Our study highlights that SOM may misinterpret the physical mechanisms in the regions where ocean dynamics is important.