Publications

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2019
Zhou, WY, Xie SP.  2019.  A conceptual spectral plume model for understanding tropical temperature profile and convective updraft velocities. Journal of the Atmospheric Sciences. 76:2801-2814.   10.1175/jas-d-18-0330.1   AbstractWebsite

The tropical tropospheric temperature is close to but typically cooler than that of the moist adiabat. The negative temperature deviation from the moist adiabat manifests a C-shape profile and is projected to increase and stretch upward under warming in both comprehensive climate models and idealized radiative-convective equilibrium (RCE) simulations. The increased temperature deviation corresponds to a larger convective available potential energy (CAPE) under warming. The extreme convective updraft velocity in RCE increases correspondingly but at a smaller fractional rate than that of CAPE. A conceptual model for the tropical temperature deviation and convective updraft velocities is formulated to understand these features. The model builds on the previous zero-buoyancy model but replaces the bulk zero-buoyancy plume by a spectrum of entraining plumes that have distinct entrainment rates and are positively buoyant until their levels of neutral buoyancy. Besides the negative temperature deviation and its increasing magnitude with warming, this allows the spectral plume model to further predict the C-shape profile as well as its upward stretch with warming. By representing extreme convective updrafts as weakly entraining plumes, the model is able to reproduce the smaller fractional increase in convective velocities with warming as compared to that of CAPE. The smaller fractional increase is mainly caused by the upward stretch in the temperature deviation profile with warming, which reduces the ratio between the integrated plume buoyancy and CAPE. The model thus provides a useful tool for understanding the tropical temperature profile and convective updraft velocities.

2017
Zhou, WY, Xie SP.  2017.  Intermodel spread around the Kuroshio-Oyashio Extension region in coupled GCMs caused by meridional variation of the westerly jet from atmospheric GCMs. Journal of Climate. 30:4589-4599.   10.1175/jcli-d-16-0831.1   AbstractWebsite

The Kuroshio-Oyashio Extension (KOE) is a region of energetic oceanic mesoscale eddies and vigorous air-sea interaction that can influence climate variability over the northwest Pacific and East Asia. General circulation models (GCMs) exhibit considerable differences in their simulated climatology around the KOE region. Specifically, there are substantial intermodel spreads in both sea surface temperature (SST) and the upper-level westerly jet. In this study, the cause for such large spreads is studied by analyzing 21 pairs of coupled and atmospheric GCMs from phase 5 of the Coupled Model Intercomparison Project (CMIP5). It is found that the intermodel spread of the climatological westerly jet among coupled GCMs is largely inherited from their atmospheric models rather than being due to their SST difference as previously thought. An anomalous equatorward shift in the simulated westerly jet can give rise to a cold SST bias around the KOE region as follows. The equatorward jet shift induces cyclonic surface wind anomalies over the North Pacific, which not only enhance the turbulent heat fluxes out of the ocean south of the KOE but also drive an anomalous cyclonic ocean circulation that brings colder (warmer) water into the north (south) of the KOE. The KOE region is consequently cooled due to both the atmospheric and oceanic effects. Such processes are demonstrated through idealized perturbation experiments using an ocean model. The results herein point to reducing atmospheric model errors in the westerly jet as the way forward to improve the coupled simulations around the KOE region.

2012
Tokinaga, H, Xie SP, Deser C, Kosaka Y, Okumura YM.  2012.  Slowdown of the Walker circulation driven by tropical Indo-Pacific warming. Nature. 491:439-+.   10.1038/nature11576   Abstract

Global mean sea surface temperature (SST) has risen steadily over the past century(1,2), but the overall pattern contains extensive and often uncertain spatial variations, with potentially important effects on regional precipitation(3,4). Observations suggest a slowdown of the zonal atmospheric overturning circulation above the tropical Pacific Ocean (the Walker circulation) over the twentieth century(1,5). Although this change has been attributed to a muted hydrological cycle forced by global warming(5,6), the effect of SST warming patterns has not been explored and quantified(1,7,8). Here we perform experiments using an atmospheric model, and find that SST warming patterns are the main cause of the weakened Walker circulation over the past six decades (1950-2009). The SST trend reconstructed from bucket-sampled SST and night-time marine surface air temperature features a reduced zonal gradient in the tropical Indo-Pacific Ocean, a change consistent with subsurface temperature observations(8). Model experiments with this trend pattern robustly simulate the observed changes, including the Walker circulation slowdown and the eastward shift of atmospheric convection from the Indonesian maritime continent to the central tropical Pacific. Our results cannot establish whether the observed changes are due to natural variability or anthropogenic global warming, but they do show that the observed slowdown in the Walker circulation is presumably driven by oceanic rather than atmospheric processes.