Publications

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2019
Mei, W, Kamae Y, Xie SP, Yoshida K.  2019.  Variability and predictability of North Atlantic hurricane frequency in a large ensemble of high-resolution atmospheric simulations. Journal of Climate. 32:3153-3167.   10.1175/jcli-d-18-0554.1   AbstractWebsite

Variability of North Atlantic annual hurricane frequency during 1951-2010 is studied using a 100-member ensemble of climate simulations by a 60-km atmospheric general circulation model that is forced by observed sea surface temperatures (SSTs). The ensemble mean results well capture the interannual-to-decadal variability of hurricane frequency in best track data since 1970, and suggest that the current best track data might underestimate hurricane frequency prior to 1966 when satellite measurements were unavailable. A genesis potential index (GPI) averaged over the main development region (MDR) accounts for more than 80% of the SST-forced variations in hurricane frequency, with potential intensity and vertical wind shear being the dominant factors. In line with previous studies, the difference between MDR SST and tropical mean SST is a useful predictor; a 1 degrees C increase in this SST difference produces 7.05 +/- 1.39 more hurricanes. The hurricane frequency also exhibits strong internal variability that is systematically larger in the model than observations. The seasonal-mean environment is highly correlated among ensemble members and contributes to less than 10% of the ensemble spread in hurricane frequency. The strong internal variability is suggested to originate from weather to intraseasonal variability and nonlinearity. In practice, a 20-member ensemble is sufficient to capture the SST-forced variability.

2018
Zhou, WY, Xie SP.  2018.  A hierarchy of idealized monsoons in an intermediate GCM. Journal of Climate. 31:9021-9036.   10.1175/jcli-d-18-0084.1   AbstractWebsite

A hierarchy of idealized monsoons with increased degrees of complexity is built using an intermediate model with simplified physics and idealized land-sea geometry. This monsoon hierarchy helps formulate a basic understanding about the distribution of the surface equivalent potential temperature (e), which proves to provide a general guide on the monsoon rainfall. The zonally uniform monsoon in the simplest aquaplanet simulations is explained by a linearized model of the meridional distribution of (e), which is driven by the seasonally varying solar insolation and damped by both the monsoon overturning circulation and the local negative feedback. The heat capacities of the surface and the atmosphere give rise to an intrinsic time scale that causes the monsoon migration to lag behind the sun and reduces the monsoon extent and intensity. Monsoons with a zonally confined continent can be understood based on the zonally uniform monsoon by considering the ocean influence on the land through the westerly jet advection, which reduces the monsoon extent and induces zonal asymmetry. Monsoon responses to more realistic factors such as land geometry, albedo, and ocean heat flux are consistently predicted by their impacts on the surface (e) distribution. The soil moisture effect, however, does not fully fit into the surface (e) argument and provides additional control on monsoon rainfall by inducing regional circulation and rainfall patterns.

Shi, JR, Xie SP, Talley LD.  2018.  Evolving relative importance of the Southern Ocean and North Atlantic in anthropogenic ocean heat uptake. Journal of Climate. 31:7459-7479.   10.1175/jcli-d-18-0170.1   AbstractWebsite

Ocean uptake of anthropogenic heat over the past 15 years has mostly occurred in the Southern Ocean, based on Argo float observations. This agrees with historical simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), where the Southern Ocean (south of 30 degrees S) accounts for 72% +/- 28% of global heat uptake, while the contribution from the North Atlantic north of 30 degrees N is only 6%. Aerosols preferentially cool the Northern Hemisphere, and the effect on surface heat flux over the subpolar North Atlantic opposes the greenhouse gas (GHG) effect in nearly equal magnitude. This heat uptake compensation is associated with weakening (strengthening) of the Atlantic meridional overturning circulation (AMOC) in response to GHG (aerosol) radiative forcing. Aerosols are projected to decline in the near future, reinforcing the greenhouse effect on the North Atlantic heat uptake. As a result, the Southern Ocean, which will continue to take up anthropogenic heat largely through the mean upwelling of water from depth, will be joined by increased relative contribution from the North Atlantic because of substantial AMOC slowdown in the twenty-first century. In the RCP8.5 scenario, the percentage contribution to global uptake is projected to decrease to 48% +/- 8% in the Southern Ocean and increase to 26% +/- 6% in the northern North Atlantic. Despite the large uncertainty in the magnitude of projected aerosol forcing, our results suggest that anthropogenic aerosols, given their geographic distributions and temporal trajectories, strongly influence the high-latitude ocean heat uptake and interhemispheric asymmetry through AMOC change.

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.

2016
Lintner, BR, Langenbrunner B, Neelin JD, Anderson BT, Niznik MJ, Li G, Xie SP.  2016.  Characterizing CMIP5 model spread in simulated rainfall in the Pacific Intertropical Convergence and South Pacific Convergence Zones. Journal of Geophysical Research-Atmospheres. 121:11590-11607.   10.1002/2016jd025284   AbstractWebsite

Current-generation climate models exhibit various errors or biases in both the spatial distribution and intensity of precipitation relative to observations. In this study, empirical orthogonal function analysis is applied to the space-model index domain of precipitation over the Pacific from Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations to explore systematic spread of simulated precipitation characteristics across the ensemble. Two significant modes of spread, generically termed principal uncertainty patterns (PUPs), are identified in the December-January-February precipitation climatology: the leading PUP is associated with the meridional width of deep convection, while the second is associated with tradeoffs in precipitation intensity along the South Pacific Convergence Zone, the Intertropical Convergence Zone (ITCZ), and the spurious Southern Hemisphere ITCZ. An important factor distinguishing PUPs from the analogy to time series analysis is that the modes can reflect either true systematic intermodel variance patterns or internal variability. In order to establish that the PUPS reflect the former, three complementary tests are performed by using preindustrial control simulations: a bootstrap significance test for reproducibility of the intermodel spatial patterns, a check for robustness over very long climatological averages, and a test on the loadings of these patterns relative to interdecadal sampling. Composite analysis based on these PUPs demonstrates physically plausible relationships to CMIP5 ensemble spread in simulated sea surface temperatures (SSTs), circulation, and moisture. Further analysis of atmosphere-only, prescribed SST simulations demonstrates decreased spread in the spatial distribution of precipitation, while substantial spread in intensity remains. Key Points Systematic spread in CMIP5 simulation of Pacific region rainfall is investigated by using empirical mode reduction techniques Two significant modes of model spread are identified for the DJF rainfall climatology These modes are interpreted in terms of spread in simulated patterns of SST and circulation

Long, SM, Xie SP.  2016.  Uncertainty in tropical rainfall projections: Atmospheric circulation effect and the ocean coupling. Journal of Climate. 29:2671-2687.   10.1175/jcli-d-15-0601.1   AbstractWebsite

Uncertainty in tropical rainfall projections under increasing radiative forcing is studied by using 26 models from phase 5 of the Coupled Model Intercomparison Project. Intermodel spread in projected rainfall change generally increases with interactive sea surface temperature (SST) warming in coupled models compared to atmospheric models with a common pattern of prescribed SST increase. Moisture budget analyses reveal that much of the model uncertainty in tropical rainfall projections originates from intermodel discrepancies in the dynamical contribution due to atmospheric circulation change. Intermodel singular value decomposition (SVD) analyses further show a tight coupling between the intermodel variations in SST warming pattern and circulation change in the tropics. In the zonal mean, the first SVD mode features an anomalous interhemispheric Hadley circulation, while the second mode displays an SST peak near the equator. The asymmetric mode is accompanied by a coupled pattern of wind-evaporation-SST feedback in the tropics and is further tied to interhemispheric asymmetric change in extratropical shortwave radiative flux at the top of the atmosphere. Intermodel variability in the tropical circulation change exerts a strong control on the spread in tropical cloud cover change and cloud radiative effects among models. The results indicate that understanding the coupling between the anthropogenic changes in SST pattern and atmospheric circulation holds the key to reducing uncertainties in projections of future changes in tropical rainfall and clouds.

Xie, SP, Kosaka Y, Okumura YM.  2016.  Distinct energy budgets for anthropogenic and natural changes during global warming hiatus. Nature Geoscience. 9:29-+.   10.1038/ngeo2581   AbstractWebsite

The Earth's energy budget for the past four decades can now be closed(1), and it supports anthropogenic greenhouse forcing as the cause for climate warming. However, closure depends on invoking an unrealistically large increase in aerosol cooling(2) during the so-called global warming hiatus since the late 1990s (refs 3,4) that was due partly to tropical Pacific Ocean cooling(5-7). The difficulty with this closure lies in the assumption that the same climate feedback applies to both anthropogenic warming and natural cooling. Here we analyse climate model simulations with and without anthropogenic increases in greenhouse gas concentrations, and show that top-of-the-atmosphere radiation and global mean surface temperature are much less tightly coupled for natural decadal variability than for the greenhouse-gas-induced response, implying distinct climate feedback between anthropogenic warming and natural variability. In addition, we identify a phase difference between top-of-the-atmosphere radiation and global mean surface temperature such that ocean heat uptake tends to slow down during the surface warming hiatus. This result deviates from existing energy theory but we find that it is broadly consistent with observations. Our study highlights the importance of developing metrics that distinguish anthropogenic change from natural variations to attribute climate variability and to estimate climate sensitivity from observations.

2015
Zhou, ZQ, Xie SP.  2015.  Effects of climatological model biases on the projection of tropical climate change. Journal of Climate. 28:9909-9917.   10.1175/jcli-d-15-0243.1   AbstractWebsite

Climate models suffer from long-standing biases, including the double intertropical convergence zone (ITCZ) problem and the excessive westward extension of the equatorial Pacific cold tongue. An atmospheric general circulation model is used to investigate how model biases in the mean state affect the projection of tropical climate change. The model is forced with a pattern of sea surface temperature (SST) increase derived from a coupled simulation of global warming but uses an SST climatology derived from either observations or a coupled historical simulation. The comparison of the experiments reveals that the climatological biases have important impacts on projected changes in the tropics. Specifically, during February-April when the climatological ITCZ displaces spuriously into the Southern Hemisphere, the model overestimates (underestimates) the projected rainfall increase in the warmer climate south (north) of the equator over the eastern Pacific. Furthermore, the global warming-induced Walker circulation slowdown is biased weak in the projection using coupled model climatology, suggesting that the projection of the reduced equatorial Pacific trade winds may also be underestimated. This is related to the bias that the climatological Walker circulation is too weak in the model, which is in turn due to a too-weak mean SST gradient in the zonal direction. The results highlight the importance of improving the climatological simulation for more reliable projections of regional climate change.

Yang, Y, Xie SP, Wu LX, Kosaka Y, Lau NC, Vecchi GA.  2015.  Seasonality and predictability of the Indian Ocean Dipole Mode: ENSO forcing and internal variability. Journal of Climate. 28:8021-8036.   10.1175/jcli-d-15-0078.1   AbstractWebsite

This study evaluates the relative contributions to the Indian Ocean dipole (IOD) mode of interannual variability from the El Nino-Southern Oscillation (ENSO) forcing and ocean-atmosphere feedbacks internal to the Indian Ocean. The ENSO forcing and internal variability is extracted by conducting a 10-member coupled simulation for 1950-2012 where sea surface temperature (SST) is restored to the observed anomalies over the tropical Pacific but interactive with the atmosphere over the rest of the World Ocean. In these experiments, the ensemble mean is due to ENSO forcing and the intermember difference arises from internal variability of the climate system independent of ENSO. These elements contribute one-third and two-thirds of the total IOD variance, respectively. Both types of IOD variability develop into an east-west dipole pattern because of Bjerknes feedback and peak in September-November. The ENSO forced and internal IOD modes differ in several important ways. The forced IOD mode develops in August with a broad meridional pattern and eventually evolves into the Indian Ocean basin mode, while the internal IOD mode grows earlier in June, is more confined to the equator, and decays rapidly after October. The internal IOD mode is more skewed than the ENSO forced response. The destructive interference of ENSO forcing and internal variability can explain early terminating IOD events, referred to as IOD-like perturbations that fail to grow during boreal summer. The results have implications for predictability. Internal variability, as represented by preseason sea surface height anomalies off Sumatra, contributes to predictability considerably. Including this indicator of internal variability, together with ENSO, improves the predictability of IOD.

2013
Du, Y, Xie SP, Yang YL, Zheng XT, Liu L, Huang G.  2013.  Indian Ocean Variability in the CMIP5 Multimodel Ensemble: The Basin Mode. Journal of Climate. 26:7240-7266.   10.1175/jcli-d-12-00678.1   AbstractWebsite

This study evaluates the simulation of the Indian Ocean Basin (IOB) mode and relevant physical processes in models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). Historical runs from 20 CMIP5 models are available for the analysis. They reproduce the IOB mode and its close relationship to El Nino-Southern Oscillation (ENSO). Half of the models capture key IOB processes: a downwelling oceanic Rossby wave in the southern tropical Indian Ocean (TIO) precedes the IOB development in boreal fall and triggers an antisymmetric wind anomaly pattern across the equator in the following spring. The anomalous wind pattern induces a second warming in the north Indian Ocean (NIO) through summer and sustains anticyclonic wind anomalies in the northwest Pacific by radiating a warm tropospheric Kelvin wave. The second warming in the NIO is indicative of ocean-atmosphere interaction in the interior TIO. More than half of the models display a double peak in NIO warming, as observed following El Nino, while the rest show only one winter peak. The intermodel diversity in the characteristics of the IOB mode seems related to the thermocline adjustment in the south TIO to ENSO-induced wind variations. Almost all the models show multidecadal variations in IOB variance, possibly modulated by ENSO.