Diagnosis, modeling, and predictability of large-scale weather and climate variations on time scales of days to millennia.
Variations of a PWC index and atmospheric convective mass flux (Mc) in NCAR/CAM4 model simulations in which only the ENSO-related and ENSO-unrelated portions of observed SST changes are prescribed.
How has global warming affected the tropical Pacific Walker Circulation?
The tropical Pacific Walker Circulation (PWC) is a planetary scale east-west overturning of the equatorial atmosphere with ascent over the western Pacific and descent over the eastern Pacific ocean. There’s great interest in how the PWC has been affected by 20th century global warming. As yet, however, there’s no clear observational consensus on whether it has weakened, strengthened, or stayed unchanged. On the other hand, a general slowdown of overturning atmospheric circulations has been argued to be physically necessary to explain the relatively weak response of global precipitation to global warming obtained in many climate models, but it’s not clear to what extent such an argument applies to regional overturning circulations such as the PWC. Still, many climate models do also simulate a 20th century weakening of the PWC. Is this correct?
We have recently published a detailed study (Sandeep et al., 2014) of 20th century PWC changes and their physical mechanisms using a combination of observational data sets and atmosphere-ocean coupled climate model simulations included in the latest Intergovernmental Panel on Climate Change report. The PWC weakens over the century in the coupled model simulations, but strengthens in the observational 20th Century Reanalysis data set and also in separate uncoupled atmospheric general circulation model (AGCM) simulations with prescribed observed changes in radiative forcings and sea surface temperatures (SSTs). We argue that the weakening in the coupled simulations is not a consequence of a reduced global convective mass flux expected from the simple argument outlined above, but is rather due to a weakening of the zonal equatorial Pacific SST gradient.
We provide further clarification through additional uncoupled AGCM simulations in which portions of the observed SST changes that are related or unrelated to the El Niño Southern Oscillation (ENSO) are prescribed separately as lower boundary conditions. Both sets of SST fields have a global warming trend, and both sets of simulations produce a weakening of the global convective mass flux (see figure). However, consistent with the strong role of the zonal SST gradient, the PWC strengthens in the simulations with the ENSO-unrelated SST forcing, which has a strengthening zonal SST gradient, despite the weakening of the global convective mass flux. Overall, our results suggest that the PWC strengthened during 20th century global warming, but also that this strengthening was partly masked by a weakening trend associated with ENSO-related PWC variability.
Sandeep, S, F Stordal, PD Sardeshmukh, and GP Compo. 2014. Pacific Walker Circulation variability in coupled and uncoupled models. Climate Dynam. 43(1-2):103- 117.