The forgotten role of wave dynamics in modulating the low cloud response to warm pool warming

Abstract

The Pattern Effect describes the dependence of top-of-atmosphere radiation anomalies on changes in the pattern of sea surface temperatures. The emerging consensus in the field explains the impact of Pacific warm pool temperature on radiation using Convective Quasi-Equilibrium Weak Temperature Gradient (QE-WTG) theory: warm pool warming leads to increase in free-tropospheric temperatures across the tropics, a strengthening of inversion, increased cloud cover in the East Pacific low cloud decks, and negative radiative anomalies. Here we call on overlooked past results and new simulations from the Energy Exascale Earth System model to show that Rossby waves dominate the low-cloud response over the subtropical East Pacific low cloud decks, leading to decrease cloud cover in the low cloud decks. While the global radiative response is negative and consistent with QE-WTG, it is dominated by the response of the deep tropics, rather than the subtropical low cloud decks.

Figures (2)

• Figure 1: Bony Decomposition: A-C: Climatology of EIS, LCC, and $\omega_{500}$ in the E3SMv3 control simulation. Ellipses indicate equatorial warming patches applied in three Green's Function simulations. D-F: Tropical Pacific response of LCC to three patches, centered at 100$^\circ$W (blue circles), 140$^\circ$W (orange triangles), and 180$^\circ$W (green squares). The responses are time- and spatially-averaged the Pacific basin between 35$^\circ$S and 35$^\circ$N, and binned by the climatological values of EIS, LCC, and $\omega_{500}$. Bin edges for the Bony plots (D-L) are identical to the contour edges in climatology plots (A-C). Black vertical lines in Bony plots correspond to the black contour in the climatology. G-I: Same as D-F, but for the tropical response of net TOA radiation to the three patches. J-L: Same as D-F, but where TOA is weighted by the frequency of each EIS, LCC, and $\omega_{500}$ bin (red line, computed using Kernel Density estimation). The area under each curve is thus equal to the average change over the Tropical Pacific.
• Figure 2: Greens Function Responses: Responses of TOA radiation, LCC, EIS, Z500, and quasi-geostrophic stream function anomaly $\psi'$plumb_three-dimensional_1985 to three equatorial warming patches centered at 100$^\circ$W (left column), 140$^\circ$W (middle column), and 180$^\circ$W (right column). The corresponding plumb_three-dimensional_1985 wave activity flux is indicated by the vector arrows. The locations of the warming patches are indicated by the black ellipses. Top plots show annual average (ANN) anomalies for TOA, LCC, and EIS, and Z500. Due to seasonal differences in wave propagation, lower plots show boreal winter (DJF) and boreal summer (JJA) anomalies for Z500, $\psi'$ and wave activity flux.