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2015
Xie, SP, Deser C, Vecchi GA, Collins M, Delworth TL, Hall A, Hawkins E, Johnson NC, Cassou C, Giannini A, Watanabe M.  2015.  Towards predictive understanding of regional climate change. Nature Climate Change. 5:921-930.   10.1038/nclimate2689   AbstractWebsite

Regional information on climate change is urgently needed but often deemed unreliable. To achieve credible regional climate projections, it is essential to understand underlying physical processes, reduce model biases and evaluate their impact on projections, and adequately account for internal variability. In the tropics, where atmospheric internal variability is small compared with the forced change, advancing our understanding of the coupling between long-term changes in upper-ocean temperature and the atmospheric circulation will help most to narrow the uncertainty. In the extratropics, relatively large internal variability introduces substantial uncertainty, while exacerbating risks associated with extreme events. Large ensemble simulations are essential to estimate the probabilistic distribution of climate change on regional scales. Regional models inherit atmospheric circulation uncertainty from global models and do not automatically solve the problem of regional climate change. We conclude that the current priority is to understand and reduce uncertainties on scales greater than 100 km to aid assessments at finer scales.

Mei, W, Lien CC, Lin II, Xie SP.  2015.  Tropical cyclone-induced ocean response: A comparative study of the South China Sea and tropical Northwest Pacific*(,+). Journal of Climate. 28:5952-5968.   10.1175/jcli-d-14-00651.1   AbstractWebsite

The thermocline shoals in the South China Sea (SCS) relative to the tropical northwest Pacific Ocean (NWP), as required by geostrophic balance with the Kuroshio. The present study examines the effect of this difference in ocean state on the response of sea surface temperature (SST) and chlorophyll concentration to tropical cyclones (TCs), using both satellite-derived measurements and three-dimensional numerical simulations. In both regions, TC-produced SST cooling strongly depends on TC characteristics (including intensity as measured by the maximum surface wind speed, translation speed, and size). When subject to identical TC forcing, the SST cooling in the SCS is more than 1.5 times that in the NWP, which may partially explain weaker TC intensity on average observed in the SCS. Both a shallower mixed layer and stronger subsurface thermal stratification in the SCS contribute to this regional difference in SST cooling. The mixed layer effect dominates when TCs are weak, fast-moving, and/or small; and for strong and slow-moving TCs or strong and large TCs, both factors are equally important. In both regions, TCs tend to elevate surface chlorophyll concentration. For identical TC forcing, the surface chlorophyll increase in the SCS is around 10 times that in the NWP, a difference much stronger than that in SST cooling. This large regional difference in the surface chlorophyll response is at least partially due to a shallower nutricline and stronger vertical nutrient gradient in the SCS. The effect of regional difference in upper-ocean density stratification on the surface nutrient response is negligible. The total annual primary production increase associated with the TC passage estimated using the vertically generalized production model in the SCS is nearly 3 times that in the NWP (i.e., 6.4 +/- 0.4 x 10(12) versus 2.2 +/- 0.2 x 10(12) g C), despite the weaker TC activity in the SCS.

Xu, Y, Xie SP.  2015.  Ocean mediation of tropospheric response to reflecting and absorbing aerosols. Atmospheric Chemistry and Physics. 15:5827-5833.   10.5194/acp-15-5827-2015   AbstractWebsite

Radiative forcing by reflecting (e.g., sulfate, SO4) and absorbing (e.g., black carbon, BC) aerosols is distinct: the former cools the planet by reducing solar radiation at the top of the atmosphere and the surface, without largely affecting the atmospheric column, while the latter heats the atmosphere directly. Despite the fundamental difference in forcing, here we show that the structure of the tropospheric response is remarkably similar between the two types of aerosols, featuring a deep vertical structure of temperature change (of opposite sign) at the Northern Hemisphere (NH) mid-latitudes. The deep temperature structure is anchored by the slow response of the ocean, as a large meridional sea surface temperature (SST) gradient drives an anomalous interhemispheric Hadley circulation in the tropics and induces atmospheric eddy adjustments at the NH mid-latitudes. The tropospheric warming in response to projected future decline in reflecting aerosols poses additional threats to the stability of mountain glaciers in the NH. Additionally, robust tropospheric response is unique to aerosol forcing and absent in the CO2 response, which can be exploited for climate change attribution.

Liu, W, Lu J, Xie SP.  2015.  Understanding the Indian Ocean response to double CO2 forcing in a coupled model. Ocean Dynamics. 65:1037-1046.   10.1007/s10236-015-0854-6   AbstractWebsite

This study investigates the roles of multiple ocean-atmospheric feedbacks in the oceanic response to increased carbon dioxide by applying an overriding technique to a coupled climate model. The annual-mean sea surface temperature (SST) response in the Indian Ocean exhibits a zonal-dipolar warming pattern, with a reduced warming in the eastern and enhanced warming in the western tropical Indian Ocean (TIO), reminiscent of the Indian Ocean Dipole (IOD) pattern. The development of the dipole pattern exhibits a pronounced seasonal evolution. The overriding experiments show that the wind-evaporation-sea surface temperature (WES) feedback accounts for most of the enhanced warming in the western and central TIO during May-July with reduced southerly monsoonal wind and contributes partially to the reduced warming in the eastern TIO during June-September. The Bjerknes feedback explains most of the reduced warming in the eastern TIO during August-October, accompanied by a reduction of precipitation, easterly wind anomalies, and a thermocline shoaling along the equator. Both feedbacks facilitate the formation of the dipolar warming pattern in the TIO. The residual from the Bjerknes and WES feedbacks is attributable to the "static" response to increasing CO2. While the static SST response also contributes to the seasonal SST variations, the static precipitation response is relatively uniform in the TIO, appearing as a general increase of precipitation along the equatorial Indian Ocean during June-September.

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.

Li, G, Xie SP, Du Y.  2015.  Climate model errors over the South Indian Ocean thermocline dome and their effect on the basin mode of interannual variability. Journal of Climate. 28:3093-3098.   10.1175/jcli-d-14-00810.1   AbstractWebsite

An open-ocean thermocline dome south of the equator is a striking feature of the Indian Ocean (IO) as a result of equatorial westerly winds. Over the thermocline dome, the El Nino-forced Rossby waves help sustain the IO basin (IOB) mode and offer climate predictability for the IO and surrounding countries. This study shows that a common equatorial easterly wind bias, by forcing a westward-propagating downwelling Rossby wave in the southern IO, induces too deep a thermocline dome over the southwestern IO (SWIO) in state-of-the-art climate models. Such a deep SWIO thermocline weakens the influence of subsurface variability on sea surface temperature (SST), reducing the IOB amplitude and possibly limiting the models' skill of regional climate prediction. To the extent that the equatorial easterly wind bias originates from errors of the South Asian summer monsoon, improving the monsoon simulation can lead to substantial improvements in simulating and predicting interannual variability in the IO.

Dai, A, Fyfe JC, Xie S-P, Dai X.  2015.  Decadal modulation of global surface temperature by internal climate variability. Nature Clim. Change. advance online publication: Nature Publishing Group   10.1038/nclimate2605   Abstract

Despite a steady increase in atmospheric greenhouse gases (GHGs), global-mean surface temperature (T) has shown no discernible warming since about 2000, in sharp contrast to model simulations, which on average project strong warming1, 2, 3. The recent slowdown in observed surface warming has been attributed to decadal cooling in the tropical Pacific1, 4, 5, intensifying trade winds5, changes in El Niño activity6, 7, increasing volcanic activity8, 9, 10 and decreasing solar irradiance7. Earlier periods of arrested warming have been observed but received much less attention than the recent period, and their causes are poorly understood. Here we analyse observed and model-simulated global T fields to quantify the contributions of internal climate variability (ICV) to decadal changes in global-mean T since 1920. We show that the Interdecadal Pacific Oscillation (IPO) has been associated with large T anomalies over both ocean and land. Combined with another leading mode of ICV, the IPO explains most of the difference between observed and model-simulated rates of decadal change in global-mean T since 1920, and particularly over the so-called ‘hiatus’ period since about 2000. We conclude that ICV, mainly through the IPO, was largely responsible for the recent slowdown, as well as for earlier slowdowns and accelerations in global-mean T since 1920, with preferred spatial patterns different from those associated with GHG-induced warming or aerosol-induced cooling. Recent history suggests that the IPO could reverse course and lead to accelerated global warming in the coming decades.

Li, G, Xie SP, Du Y.  2015.  Monsoon-induced biases of climate models over the tropical Indian Ocean. Journal of Climate. 28:3058-3072.   10.1175/jcli-d-14-00740.1   AbstractWebsite

Long-standing biases of climate models limit the skills of climate prediction and projection. Overlooked are tropical Indian Ocean (IO) errors. Based on the phase 5 of the Coupled Model Intercomparison Project (CMIP5) multimodel ensemble, the present study identifies a common error pattern in climate models that resembles the IO dipole (IOD) mode of interannual variability in nature, with a strong equatorial easterly wind bias during boreal autumn accompanied by physically consistent biases in precipitation, sea surface temperature (SST), and subsurface ocean temperature. The analyses show that such IOD-like biases can be traced back to errors in the South Asian summer monsoon. A southwest summer monsoon that is too weak over the Arabian Sea generates a warm SST bias over the western equatorial IO. In boreal autumn, Bjerknes feedback helps amplify the error into an IOD-like bias pattern in wind, precipitation, SST, and subsurface ocean temperature. Such mean state biases result in an interannual IOD variability that is too strong. Most models project an IOD-like future change for the boreal autumn mean state in the global warming scenario, which would result in more frequent occurrences of extreme positive IOD events in the future with important consequences to Indonesia and East Africa. The Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) characterizes this future IOD-like projection in the mean state as robust based on consistency among models, but the authors' results cast doubts on this conclusion since models with larger IOD amplitude biases tend to produce stronger IOD-like projected changes in the future.

Chikamoto, Y, Timmermann A, Luo JJ, Mochizuki T, Kimoto M, Watanabe M, Ishii M, Xie SP, Jin FF.  2015.  Skilful multi-year predictions of tropical trans-basin climate variability. Nature Communications. 6   10.1038/ncomms7869   AbstractWebsite

Tropical Pacific sea surface temperature anomalies influence the atmospheric circulation, impacting climate far beyond the tropics. The predictability of the corresponding atmospheric signals is typically limited to less than 1 year lead time. Here we present observational and modelling evidence for multi-year predictability of coherent trans-basin climate variations that are characterized by a zonal seesaw in tropical sea surface temperature and sea-level pressure between the Pacific and the other two ocean basins. State-of-the-art climate model forecasts initialized from a realistic ocean state show that the low-frequency trans-basin climate variability, which explains part of the El Nino Southern Oscillation flavours, can be predicted up to 3 years ahead, thus exceeding the predictive skill of current tropical climate forecasts for natural variability. This low-frequency variability emerges from the synchronization of ocean anomalies in all basins via global reorganizations of the atmospheric Walker Circulation.

Chow, CH, Liu QY, Xie SP.  2015.  Effects of Kuroshio Intrusions on the atmosphere northeast of Taiwan Island. Geophysical Research Letters. 42:1465-1470.   10.1002/2014gl062796   AbstractWebsite

The Kuroshio loses bathymetric support off northeast Taiwan Island, causing large variability in its path. The resultant covariability of sea surface temperature (SST) and the lower atmosphere is investigated using satellite observations. In winter and spring off northeast Taiwan Island, the intrusions of warm Kuroshio water onto the continental shelf cause a large increase in local SST, intensify the northeasterly monsoonal winds, and lead to the increases in water vapor and rainfall. Key to this air-sea interaction is the existence of anomalous heat advection by the Kuroshio intrusions. The Kuroshio intrusions are partly due to westward propagating ocean eddies east of Taiwan Island with a lead time of 3weeks, hinting at the possibility of improved weather prediction near northeast Taiwan Island by considering ocean variability east of Taiwan Island.

Kilpatrick, TJ, Xie S-P.  2015.  ASCAT observations of downdrafts from mesoscale convective systems. Geophysical Research Letters.   10.1002/2015GL063025   AbstractWebsite

Downdrafts of air cooled by evaporating raindrops are an essential component of mesoscale convective systems (MCSs). Here we use surface wind observations from the Advanced SCATterometer (ASCAT) to identify MCS downdrafts over the western equatorial Pacific Ocean as regions of horizontal wind divergence exceeding 10-4 s-1. More than 1300 downdrafts are identified over the observation period (2009–2014). The downdraft signal in the surface winds is validated with satellite measurements of brightness temperature and rainfall rate, and surface buoy measurements of air temperature; composite analysis with these measurements indicates ASCAT detects downdrafts that lag the peak convection by 8–12 h. While ASCAT resolves mesoscale downdrafts in regions of light rain, a composite against buoy air temperature indicates that ASCAT fails to resolve the stronger convective-scale downdrafts associated with heavy rainfall at squall fronts. Nevertheless, the global observations by the satellite scatterometer open a new avenue for studying MCSs.

Diao, Y, Xie SP, Luo DH.  2015.  Asymmetry of winter European surface air temperature extremes and the North Atlantic Oscillation. Journal of Climate. 28:517-530.   10.1175/jcli-d-13-00642.1   AbstractWebsite

Interannual variations of winter warm and cold extremes in Europe are investigated. It is found that the variations are closely connected to the phase of the North Atlantic Oscillation (NAO). The leading EOF of the winter cold (warm) surface air temperature (SAT) extreme frequency shows an enhanced occurrence over western (eastern) Europe. The SAT probability distribution function of the cold extreme winter exhibits both a decrease of the mean SAT and a marked increase in SAT variance, whereas it shows only a shift of the mean SAT to the warmer side for extreme warm winters. This study reveals an asymmetry in location between the cold and warm extremes, caused by the NAO modulations of blocking events and other submonthly variations. Winters with frequent cold extremes are mainly accompanied by the eastern Atlantic blocking. The circulation causes not only marked local cooling but also increased SAT gradient, resulting in both enhanced SAT variance and increased occurrence of cold extremes. By contrast, winters with frequent warm extremes are associated with the northeast-southwest tilted positive NAO pattern. The warm advection by the submonthly perturbations is responsible for the development of warm extremes. The reduced SAT gradient due to enhanced warm advection weakens SAT variance over northern Europe. Thus, the cold extremes are larger in terms of deviations from the monthly mean than the warm extremes.

Feng, M, Hendon HH, Xie SP, Marshall AG, Schiller A, Kosaka Y, Caputi N, Pearce A.  2015.  Decadal increase in Ningaloo Nino since the late 1990s. Geophysical Research Letters. 42:104-112.   10.1002/2014gl062509   AbstractWebsite

Ningaloo Nino refers to the episodic occurrence of anomalously warm ocean conditions along the subtropical coast of Western Australia (WA). Ningaloo Nino typically develops in austral spring, peaks in summer, and decays in autumn, and it often occurs in conjunction with La Nina conditions in the Pacific which promote poleward transport of warm tropical waters by the Leeuwin Current. Since the late 1990s, there has been a marked increase in the occurrence of Ningaloo Nino, which is likely related to the recent swing to the negative phase of the Interdecadal Pacific Oscillation (IPO) and enhanced El Nino-Southern Oscillation variance since 1970s. The swing to the negative IPO sustains positive heat content anomalies and initiates more frequent cyclonic wind anomalies off the WA coast so favoring enhanced poleward heat transport by the Leeuwin Current. The anthropogenically forced global warming has made it easier for natural variability to drive extreme ocean temperatures in the region.

Liu, JW, Xie SP, Zhang SP.  2015.  Effects of the Hawaiian Islands on the vertical structure of low-level clouds from CALIPSO lidar. Journal of Geophysical Research-Atmospheres. 120:215-228.   10.1002/2014jd022410   AbstractWebsite

The steady northeast trade winds impinge on the Hawaiian Islands, producing prominent island wakes of multispatial scales from tens to thousands of kilometers. Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) reveal rich three-dimensional structures of low-level clouds that are induced by the islands, distinct from the background environment. The cloud frequency peaks between 1.5 and 2.0km in cloud top elevation over the windward slopes of the islands of Kauai and Oahu due to orographic lifting and daytime island heating. In the nighttime near-island wake of Kauai, CALIPSO captures a striking cloud hole below 1.6km as the cold advection from the island suppresses low-level clouds. The cyclonic eddy of the mechanical wake behind the island of Hawaii favors the formation of low-level clouds (below 2.5km), and the anticyclonic eddy suppresses the low-level cloud formation, indicative of the dynamical effect on the vertical structure of low-level clouds. In the long Hawaiian wake due to air-sea interaction, low-level clouds form over both the warmer and colder waters, but the cloud tops are 400-600m higher over the warm than the cold waters. In addition, the day-night differences and the sensitivity of low-level clouds to the background trade wind inversion height are also studied. Key Points

Qu, X, Huang G, Hu KM, Xie SP, Du Y, Zheng XT, Liu L.  2015.  Equatorward shift of the South Asian high in response to anthropogenic forcing. Theoretical and Applied Climatology. 119:113-122.   10.1007/s00704-014-1095-1   AbstractWebsite

The South Asian high (SAH) is a huge anticyclone in the upper troposphere. It influences the climate and the distribution of trace constituents and pollutants. The present study documents the change in the SAH and precipitation under global warming, as well as the possible link between the changes, based on 17 Coupled Model Intercomparison Project Phase 5 (CMIP5) model simulations. The CMIP5 historical simulation reproduces reasonably the tropospheric circulation (including the SAH), precipitation, and moisture. Under global warming, more than 75 % of the CMIP5 models project a southward shift of the SAH. The southward shift is more significant in the models with stronger anticyclonic circulation in the south part of the climatological SAH. The precipitation response displays a contrasting feature: negative over the southeastern equatorial Indian Ocean (IO) and positive over the tropical northern IO, the Bay of Bengal, and the equatorial western Pacific. The results of a linear baroclinic model (LBM) show that the regional rainfall changes over the Bay of Bengal and the equatorial western Pacific have a main contribution to the southward shift of the SAH. In addition, the precipitation and the surface wind responses over the Indo-Pacific region are well coupled. On one hand, the surface wind anomaly affects the rainfall response through altering the SST and moisture. On the other hand, the condensational heating released by regional rainfall changes sustains the surface wind response.

Mei, W, Xie SP, Zhao M, Wang YQ.  2015.  Forced and internal vriability of tropical cyclone track density in the Western North Pacific. Journal of Climate. 28:143-167.   10.1175/jcli-d-14-00164.1   AbstractWebsite

Forced interannual-to-decadal variability of annual tropical cyclone (TC) track density in the western North Pacific between 1979 and 2008 is studied using TC tracks from observations and simulations by a 25-km-resolution version of the GFDL High-Resolution Atmospheric Model (HiRAM) that is forced by observed sea surface temperatures (SSTs). Two modes dominate the decadal variability: a nearly basinwide mode, and a dipole mode between the subtropics and lower latitudes. The former mode links to variations in TC number and is forced by SST variations over the off-equatorial tropical central North Pacific, whereas the latter might be associated with the Atlantic multidecadal oscillation. The interannual variability is also controlled by two modes: a basinwide mode driven by SST anomalies of opposite signs located in the tropical central Pacific and eastern Indian Ocean, and a southeast-northwest dipole mode connected to the conventional eastern Pacific ENSO. The seasonal evolution of the ENSO effect on TC activity is further explored via a joint empirical orthogonal function analysis using TC track density of consecutive seasons, and the analysis reveals that two types of ENSO are at work. Internal variability in TC track density is then examined using ensemble simulations from both HiRAM and a regional atmospheric model. It exhibits prominent spatial and seasonal patterns, and it is particularly strong in the South China Sea and along the coast of East Asia. This makes an accurate prediction and projection of TC landfall extremely challenging in these regions. In contrast, basin-integrated metrics (e.g., total TC counts and TC days) are more predictable.

Brown, PT, Li W, Xie S-P.  2015.  Regions of significant influence on unforced global mean surface air temperature variability in climate models. Journal of Geophysical Research: Atmospheres.   10.1002/2014JD022576   Abstract

We document the geographic regions where local variability is most associated with unforced global mean surface air temperature (GMT) variability in Coupled Model Intercomparison Project Phase 5 coupled global climate models (GCMs) at both the subdecadal and interdecadal timescales. For this purpose, Regions of Significant Influence on GMT are defined as locations that have a statistically significant correlation between local surface air temperature (SAT) and GMT (with a regression slope greater than 1), and where local SAT variation leads GMT variation in time. In both GCMs and observations, subdecadal timescale GMT variability is most associated with SAT variation over the eastern equatorial Pacific. At the interdecadal timescale, GMT variability is also linked with SAT variation over the Pacific in many GCMs, but the particular spatial patterns are GCM dependent, and several GCMs indicate a primary association between GMT and SAT over the Southern Ocean. We find that it is difficult to validate GCM behavior at the interdecadal timescale because the pattern derived from observations is highly depended on the method used to remove the forced variability from the record. The magnitude of observed GMT variability is near the ensemble median at the subdecadal timescale but well above the median at the interdecadal timescale. GCMs with a stronger subdecadal relationship between GMT and SAT over the Pacific tend to have more variable subdecadal GMT while GCMs with a stronger interdecadal relationship between GMT and SAT over parts of the Southern Ocean tend to have more variable GMT.

2014
Zhou, ZQ, Xie SP, Zheng XT, Liu QY, Wang H.  2014.  Global warming-induced changes in El Nino teleconnections over the North Pacific and North America. Journal of Climate. 27:9050-9064.   10.1175/jcli-d-14-00254.1   AbstractWebsite

El Nino-Southern Oscillation (ENSO) induces climate anomalies around the globe. Atmospheric general circulation model simulations are used to investigate how ENSO-induced teleconnection patterns during boreal winter might change in response to global warming in the Pacific-North American sector. As models disagree on changes in the amplitude and spatial pattern of ENSO in response to global warming, for simplicity the same sea surface temperature (SST) pattern of ENSO is prescribed before and after the climate warming. In a warmer climate, precipitation anomalies intensify and move eastward over the equatorial Pacific during El Nino because the enhanced mean SST warming reduces the barrier to deep convection in the eastern basin. Associated with the eastward shift of tropical convective anomalies, the ENSO-forced Pacific-North American (PNA) teleconnection pattern moves eastward and intensifies under the climate warming. By contrast, the PNA mode of atmospheric internal variability remains largely unchanged in pattern, suggesting the importance of tropical convection in shifting atmospheric teleconnections. As the ENSO-induced PNA pattern shifts eastward, rainfall anomalies are expected to intensify on the west coast of North America, and the El Nino-induced surface warming to expand eastward and occupy all of northern North America. The spatial pattern of the mean SST warming affects changes in ENSO teleconnections. The teleconnection changes are larger with patterned mean warming than in an idealized case where the spatially uniform warming is prescribed in the mean state. The results herein suggest that the eastward-shifted PNA pattern is a robust change to be expected in the future, independent of the uncertainty in changes of ENSO itself.

Richter, I, Behera SK, Doi T, Taguchi B, Masumoto Y, Xie SP.  2014.  What controls equatorial Atlantic winds in boreal spring? Climate Dynamics. 43:3091-3104.   10.1007/s00382-014-2170-0   AbstractWebsite

The factors controlling equatorial Atlantic winds in boreal spring are examined using both observations and general circulation model (GCM) simulations from the coupled model intercomparison phase 5. The results show that the prevailing surface easterlies flow against the attendant pressure gradient and must therefore be maintained by other terms in the momentum budget. An important contribution comes from meridional advection of zonal momentum but the dominant contribution is the vertical transport of zonal momentum from the free troposphere to the surface. This implies that surface winds are strongly influenced by conditions in the free troposphere, chiefly pressure gradients and, to a lesser extent, meridional advection. Both factors are linked to the patterns of deep convection. Applying these findings to GCM errors indicates, that, consistent with the results of previous studies, the persistent westerly surface wind bias found in most GCMs is due mostly to precipitation errors, in particular excessive precipitation south of the equator over the ocean and deficient precipitation over equatorial South America. Free tropospheric influences also dominate the interannual variability of surface winds in boreal spring. GCM experiments with prescribed climatological sea-surface temperatures (SSTs) indicate that the free tropospheric influences are mostly associated with internal atmospheric variability. Since the surface wind anomalies in boreal spring are crucial to the development of warm SST events (Atlantic Ninos), the results imply that interannual variability in the region may rely far less on coupled air-sea feedbacks than is the case in the tropical 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.

Hu, KM, Huang G, Zheng XT, Xie SP, Qu X, Du Y, Liu L.  2014.  Interdecadal variations in ENSO influences on Northwest Pacific-East Asian early summertime climate simulated in CMIP5 models. Journal of Climate. 27:5982-5998.   10.1175/jcli-d-13-00268.1   AbstractWebsite

The present study investigates interdecadal modulations of the El Nino-Southern Oscillation (ENSO) influence on the climate of the northwest Pacific (NWP) and East Asia (EA) in early boreal summer following a winter ENSO event, based on 19 simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5). In the historical run, 8 out of 19 models capture a realistic relationship between ENSO and NWP early summer climate-an anomalous anticyclone develops over the NWP following a winter El Nino event- and the interdecadal modulations of this correlation. During periods when the association between ENSO and NWP early summer climate is strong, ENSO variance and ENSO-induced anomalies of summer sea surface temperature (SST) and tropospheric temperature over the tropical Indian Ocean (TIO) all strengthen relative to periods when the association is weak. In future projections with representative concentration pathways 4.5 and 8.5, the response of TIO SST, tropospheric temperature, and NWP anomalous anticyclone to ENSO all strengthen regardless of ENSO amplitude change. In a warmer climate, low-level specific humidity response to interannual SST variability strengthens following the Clausius-Clapeyron equation. The resultant intensification of tropospheric temperature response to interannual TIO warming is suggested as the mechanism for the strengthened ENSO effect on NWP-EA summer climate.

Xu, LX, Xie SP, McClean JL, Liu QY, Sasaki H.  2014.  Mesoscale eddy effects on the subduction of North Pacific mode waters. Journal of Geophysical Research-Oceans. 119:4867-4886.   10.1002/2014jc009861   AbstractWebsite

Mesoscale eddy effects on the subduction of North Pacific mode waters are investigated by comparing observations and ocean general circulation models where eddies are either parameterized or resolved. The eddy-resolving models produce results closer to observations than the noneddy-resolving model. There are large discrepancies in subduction patterns between eddy-resolving and noneddy-resolving models. In the noneddy-resolving model, subduction on a given isopycnal is limited to the cross point between the mixed layer depth (MLD) front and the outcrop line whereas in eddy-resolving models and observations, subduction takes place in a broader, zonally elongated band within the deep mixed layer region. Mesoscale eddies significantly enhance the total subduction rate, helping create remarkable peaks in the volume histogram that correspond to North Pacific subtropical mode water (STMW) and central mode water (CMW). Eddy-enhanced subduction preferentially occurs south of the winter mean outcrop. With an anticyclonic eddy to the west and a cyclonic eddy to the east, the outcrop line meanders south, and the thermocline/MLD shoals eastward. As eddies propagate westward, the MLD shoals, shielding the water of low potential vorticity from the atmosphere. The southward eddy flow then carries the subducted water mass into the thermocline. The eddy subduction processes revealed here have important implications for designing field observations and improving models.

Maloney, ED, Jiang XA, Xie SP, Benedict JJ.  2014.  Process-oriented diagnosis of East Pacific warm pool intraseasonal variability. Journal of Climate. 27:6305-6324.   10.1175/jcli-d-14-00053.1   AbstractWebsite

June-October east Pacific warm pool intraseasonal variability (ISV) is assessed in eight atmospheric general circulation simulations. Complex empirical orthogonal function analysis is used to document the leading mode of 30-90-day precipitation variability in the models and Tropical Rainfall Measuring Mission observations. The models exhibit a large spread in amplitude of the leading mode about the observed amplitude. Little relationship is demonstrated between the amplitude of the leading mode and the ability of models to simulate observed north-northeastward propagation. Several process-oriented diagnostics are explored that attempt to distinguish why some models produce superior ISV. A diagnostic based on the difference in 500-850-hPa averaged relative humidity between the top 5% and the bottom 10% of precipitation events exhibits a significant correlation with leading mode amplitude. Diagnostics based on the vertically integrated moist entropy budget also demonstrate success at discriminating models with strong and weak variability. In particular, the vertical component of gross moist stability exhibits a correlation with amplitude of -0.9, suggesting that models in which convection and associated divergent circulations are less efficient at discharging moisture from the column are better able to sustain strong ISV. Several other diagnostics are tested that show no significant relationship with leading mode amplitude, including the warm pool mean surface zonal wind, the strength of surface flux feedbacks, and 500-850-hPa averaged relative humidity for the top 1% of rainfall events. Vertical zonal wind shear and 850-hPa zonal wind do not appear to be good predictors of model success at simulating the observed northward propagation pattern.

Kang, SM, Xie SP.  2014.  Dependence of climate response on meridional structure of external thermal forcing. Journal of Climate. 27:5593-5600.   10.1175/jcli-d-13-00622.1   AbstractWebsite

This study shows that the magnitude of global surface warming greatly depends on the meridional distribution of surface thermal forcing. An atmospheric model coupled to an aquaplanet slab mixed layer ocean is perturbed by prescribing heating to the ocean mixed layer. The heating is distributed uniformly globally or confined to narrow tropical or polar bands, and the amplitude is adjusted to ensure that the global mean remains the same for all cases. Since the tropical temperature is close to a moist adiabat, the prescribed heating leads to a maximized warming near the tropopause, whereas the polar warming is trapped near the surface because of strong atmospheric stability. Hence, the surface warming is more effectively damped by radiation in the tropics than in the polar region. As a result, the global surface temperature increase is weak (strong) when the given amount of heating is confined to the tropical (polar) band. The degree of this contrast is shown to depend on water vapor- and cloud-radiative feedbacks that alter the effective strength of prescribed thermal forcing.

Mei, W, Xie SP, Zhao M.  2014.  Variability of tropical cyclone track density in the North Atlantic: Observations and high-resolution simulations. Journal of Climate. 27:4797-4814.   10.1175/jcli-d-13-00587.1   AbstractWebsite

Interannual-decadal variability of tropical cyclone (TC) track density over the North Atlantic (NA) between 1979 and 2008 is studied using observations and simulations with a 25-km-resolution version of the High Resolution Atmospheric Model (HiRAM) forced by observed sea surface temperatures (SSTs). The variability on decadal and interannual time scales is examined separately. On both time scales, a basinwide mode dominates, with the time series being related to variations in seasonal TC counts. On decadal time scales, this mode relates to SST contrasts between the tropical NA and the tropical northeast Pacific as well as the tropical South Atlantic, whereas on interannual time scales it is controlled by SSTs over the central eastern equatorial Pacific and those over the tropical NA. The temporal evolution of the spatial distribution of track density is further investigated by normalizing the track density with seasonal TC counts. On decadal time scales, two modes emerge: one is an oscillation between track density over the U.S. East Coast and midlatitude ocean and that over the Gulf of Mexico and the Caribbean Sea and the other oscillates between low and middle latitudes. They might be driven by the preceding winter North Atlantic Oscillation and concurrent Atlantic meridional mode, respectively. On interannual time scales, two similar modes are present in observations but are not well separated in HiRAM simulations. Finally, the internal variability and predictability of TC track density are explored and discussed using HiRAM ensemble simulations. The results suggest that basinwide total TC counts/days are much more predictable than local TC occurrence, posing a serious challenge to the prediction and projection of regional TC threats, especially the U.S. landfall hurricanes.