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Liu, W, Lu J, Leung LR, Xie SP, Liu ZY, Zhu J.  2015.  The de-correlation of westerly winds and westerly-wind stress over the Southern Ocean during the Last Glacial Maximum. Climate Dynamics. 45:3157-3168.   10.1007/s00382-015-2530-4   AbstractWebsite

Motivated by indications from paleo-evidence, this paper investigates the changes of the Southern Westerly Winds (SWW) and westerly-wind stress between the Last Glacial Maximum (LGM) and pre-industrial in the PMIP3/CMIP5 simulations, highlighting the role of Antarctic sea ice in modulating the wind effect on ocean. Particularly, a de-correlation occurs between the changes in SWW and westerly-wind stress, caused primarily by an equatorward expansion of winter Antarctic sea ice that undermines the efficacy of wind in generating stress over the liquid ocean. Such de-correlation may reflect the LGM condition in reality, in view of the fact that the model which simulates this condition has most fidelity in simulating modern SWW and Antarctic sea ice. Therein two models stand out for their agreements with paleo-evidence regarding the change of SWW and the westerly-wind stress. They simulate strengthened and poleward-migrated LGM SWW in the atmosphere, consistent with the indications from dust records. Whilst in the ocean, they well capture an equatorward-shifted pattern of the observed oceanic front shift, with most pronounced equatorward-shifted westerly wind stress during the LGM.

Liu, JW, Zhang SP, Xie SP.  2013.  Two types of surface wind response to the East China Sea Kuroshio Front. Journal of Climate. 26:8616-8627.   10.1175/jcli-d-12-00092.1   AbstractWebsite

Effects of the sea surface temperature (SST) front along the East China Sea Kuroshio on sea surface winds at different time scales are investigated. In winter and spring, the climatological vector wind is strongest on the SST front while the scalar wind speed reaches a maximum on the warm flank of the front and is collocated with the maximum difference between sea surface temperature and surface air temperature (SST - SAT). The distinction is due to the change in relative importance of two physical processes of SST-wind interaction at different time scales. The SST front-induced sea surface level pressure (SLP) adjustment (SF-SLP) contributes to a strong vector wind above the front on long time scales, consistent with the collocation of baroclinicity in the marine boundary layer and corroborated by the similarity between the thermal wind and observed wind shear between 1000 and 850 hPa. In contrast, the SST modulation of synoptic winds is more evident on the warm flank of the SST front. Large thermal instability of the near-surface layer strengthens temporal synoptic wind perturbations by intensifying vertical mixing, resulting in a scalar wind maximum. The vertical mixing and SF-SLP mechanisms are both at work but manifest more clearly at the synoptic time scale and in the long-term mean, respectively. The cross-frontal variations are 1.5 m s(-1) in both the scalar and vector wind speeds, representing the vertical mixing and SF-SLP effects, respectively. The results illustrate the utility of high-frequency sampling by satellite scatterometers.

Liu, W, Xie S-P, Liu Z, Zhu J.  2017.  Overlooked possibility of a collapsed Atlantic Meridional Overturning Circulation in warming climate. Science Advances. 3   10.1126/sciadv.1601666   Abstract

Changes in the Atlantic Meridional Overturning Circulation (AMOC) are moderate in most climate model projections under increasing greenhouse gas forcing. This intermodel consensus may be an artifact of common model biases that favor a stable AMOC. Observationally based freshwater budget analyses suggest that the AMOC is in an unstable regime susceptible for large changes in response to perturbations. By correcting the model biases, we show that the AMOC collapses 300 years after the atmospheric CO2 concentration is abruptly doubled from the 1990 level. Compared to an uncorrected model, the AMOC collapse brings about large, markedly different climate responses: a prominent cooling over the northern North Atlantic and neighboring areas, sea ice increases over the Greenland-Iceland-Norwegian seas and to the south of Greenland, and a significant southward rain-belt migration over the tropical Atlantic. Our results highlight the need to develop dynamical metrics to constrain models and the importance of reducing model biases in long-term climate projection.

Liu, W, Xie SP.  2018.  An ocean view of the global surface warming hiatus. Oceanography. 31:72-79.   10.5670/oceanog.2018.217   AbstractWebsite

The rate of global mean surface temperature increase slowed during 1998-2012. We review oceanic changes during this global warming hiatus from different but related perspectives. In one perspective, we explore the physical mechanisms for sea surface temperature patterns and highlight the role of natural variability, particularly the Interdecadal Pacific Oscillation (IPO) and the Atlantic Multidecadal Oscillation (AMO) that both have chaotic/random phases. In the other perspective, we investigate how the hiatus relates to changes in energy fluxes at the top of the atmosphere and to the three-dimensional distribution of ocean heat content change on decadal timescales. We find that the recent surface warming hiatus is associated with a transition of the IPO from a positive to negative phase and with heat redistribution between the tropical Pacific and Indian Oceans. The AMO has shifted to a positive phase since the late 1990s, inducing a La Nina-type response over the tropical Pacific via a tropic-wide teleconnection, contributing to the global warming hiatus.

Liu, JW, Xie SP, Yang S, Zhang SP.  2016.  Low-cloud transitions across the Kuroshio Front in the East China Sea. Journal of Climate. 29:4429-4443.   10.1175/jcli-d-15-0589.1   AbstractWebsite

The East China Sea Kuroshio (ECSK) flows in the East Asian monsoon region where the background atmospheric circulation varies significantly with season. A sea surface temperature (SST) front associated with the ECSK becomes narrower and sharper from winter to spring. The present study investigates how low clouds respond to the ECSK front in different seasons by synthesizing spaceborne lidar and surface visual observations. The results reveal prominent cross-frontal transitions in low clouds, which exhibit distinct behavior between winter and spring. In winter, cloud responses are generally confined below 4 km by the strong background descending motion and feature a gradual cloud-top elevation from the cold to the warm flank of the front. The ice clouds on the cold flank of the ECSK front transform into liquid water clouds and rain on the warm flank. The springtime clouds, by contrast, are characterized by a sharp cross-frontal transition with deep clouds reaching up to 7 km over the ECSK. In both winter and spring, the low-cloud morphology exhibits a large transformation from the cold to the warm flank of the ECSK front, including increases in cloud-top height, a decline in smoothness of cloud top, and the transition from stratiform to convective clouds. All this along with the atmospheric soundings indicates that the decoupling of the marine atmospheric boundary layer (MABL) is more prevalent on the warm flank of the front. Thus, long-term observations reveal prominent cross-frontal low-cloud transitions in morphology associated with MABL decoupling that resemble a large-scale cloud-regime transition over the eastern subtropical Pacific.

Liu, L, Xie SP, Zheng XT, Li T, Du Y, Huang G, Yu WD.  2014.  Indian Ocean variability in the CMIP5 multi-model ensemble: the zonal dipole mode. Climate Dynamics. 43:1715-1730.   10.1007/s00382-013-2000-9   AbstractWebsite

The performance of 21 Coupled Model Intercomparison Project Phase 5 (CMIP5) models in the simulation of the Indian Ocean Dipole (IOD) mode is evaluated. Compared to CMIP3, CMIP5 models exhibit a similar spread in IOD intensity. A detailed diagnosis was carried out to understand whether CMIP5 models have shown improvement in their representation of the important dynamical and thermodynamical feedbacks in the tropical Indian Ocean. These include the Bjerknes dynamic air-sea feedback, which includes the equatorial zonal wind response to sea surface temperature (SST) anomaly, the thermocline response to equatorial zonal wind forcing, the ocean subsurface temperature response to the thermocline variations, and the thermodynamic air-sea coupling that includes the wind-evaporation-SST and cloud-radiation-SST feedback. Compared to CMIP3, the CMIP5 ensemble produces a more realistic positive wind-evaporation-SST feedback during the IOD developing phase, while the simulation of Bjerknes dynamic feedback is more unrealistic especially with regard to the wind response to SST forcing and the thermocline response to surface wind forcing. The overall CMIP5 performance in the IOD simulation does not show remarkable improvements compared to CMIP3. It is further noted that the El Nio-Southern Oscillation (ENSO) and IOD amplitudes are closely related, if a model generates a strong ENSO, it is likely that this model also simulates a strong IOD.

Liu, C, Xie SP, Li PL, Xu LX, Gao WD.  2017.  Climatology and decadal variations in multicore structure of the North Pacific subtropical mode water. Journal of Geophysical Research-Oceans. 122:7506-7520.   10.1002/2017jc013071   AbstractWebsite

The pycnostad of the North Pacific subtropical mode water (STMW) often displays multiple vertical minima in the potential vorticity profile. Argo profile data from 2004 to 2015 are used to investigate interannual to decadal variations of the multicore structure. Climatologically, about 24% pycostads of STMW have multicore structure, and most of them distribute in the region west of 150 degrees E. STMW cores are classified into three submodes-the cold, middle, and warm ones with potential temperatures of 16.0-17 degrees C, 17-18 degrees C, and 18-19.5 degrees C, respectively. The Kuroshio Extension (KE) varies between stable and unstable states. The unstable KE with large meanders increases the subsurface stratification and shoals the winter mixed layer east of 150 degrees E with warmer temperatures. There, the dominant STMW type varies from the cold single type in stable KE years (making up 72% of the profiles with STMW) to the middle single one (53%) in unstable years. The variation of the dominant STMW type in the region east of 150 degrees E subsequently affects the multicore structure of STMW west of 150 degrees E. In a broad region between 130 degrees E and 180 degrees E, profiles with STMW are fewer in unstable years but the proportion of multicore profiles increases among STMW profiles. This might be due to the split recirculation gyre with a chaotic KE.

Long, SM, Xie SP, Zheng XT, Liu QY.  2014.  Fast and slow responses to global warming: Sea surface temperature and precipitation patterns. Journal of Climate. 27:285-299.   10.1175/jcli-d-13-00297.1   AbstractWebsite

The time-dependent response of sea surface temperature (SST) to global warming and the associated atmospheric changes are investigated based on a 1% yr(-1) CO2 increase to the quadrupling experiment of the Geophysical Fluid Dynamics Laboratory Climate Model, version 2.1. The SST response consists of a fast component, for which the ocean mixed layer is in quasi equilibrium with the radiative forcing, and a slow component owing to the gradual warming of the deeper ocean in and beneath the thermocline. A diagnostic method is proposed to isolate spatial patterns of the fast and slow responses. The deep ocean warming retards the surface warming in the fast response but turns into a forcing for the slow response. As a result, the fast and slow responses are nearly opposite to each other in spatial pattern, especially over the subpolar North Atlantic/Southern Ocean regions of the deep-water/bottom-water formation, and in the interhemispheric SST gradient between the southern and northern subtropics. Wind-evaporation-SST feedback is an additional mechanism for the SST pattern formation in the tropics. Analyses of phase 5 of the Coupled Model Intercomparison Project (CMIP5) multimodel ensemble of global warming simulations confirm the validity of the diagnostic method that separates the fast and slow responses. Tropical annual rainfall change follows the SST warming pattern in both the fast and slow responses in CMIP5, increasing where the SST increase exceeds the tropical mean warming.

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.

Long, SM, Xie SP.  2015.  Intermodel variations in projected precipitation change over the North Atlantic: Sea surface temperature effect. Geophysical Research Letters. 42:4158-4165.   10.1002/2015gl063852   AbstractWebsite

Intermodel variations in future precipitation projection in the North Atlantic are studied using 23 state-of-art models from Phase 5 of the Coupled Model Intercomparison Project. Model uncertainty in annual mean rainfall change is locally enhanced along the Gulf Stream. The moisture budget analysis reveals that much of the model uncertainty in rainfall change can be traced back to the discrepancies in surface evaporation change and transient eddy effect among models. Results of the intermodel Singular Value Decomposition (SVD) analysis show that intermodel variations in local sea surface temperature (SST) pattern exert a strong control over the spread of rainfall projection among models through the modulation of evaporation change. The first three SVD modes explain more than 60% of the intermodel variance of rainfall projection and show distinct SST patterns with mode water-induced banded structures, reduced subpolar warming due to ocean dynamical cooling, and the Gulf Stream shift, respectively.

Luebbecke, JF, Boening CW, Keenlyside NS, Xie S-P.  2010.  On the connection between Benguela and equatorial Atlantic Ninos and the role of the South Atlantic Anticyclone. Journal of Geophysical Research-Oceans. 115   10.1029/2009jc005964   Abstract