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Li, G, Xie SP.  2014.  Tropical Biases in CMIP5 Multimodel Ensemble: The Excessive Equatorial Pacific Cold Tongue and Double ITCZ Problems. Journal of Climate. 27:1765-1780.   10.1175/jcli-d-13-00337.1   AbstractWebsite

Errors of coupled general circulation models (CGCMs) limit their utility for climate prediction and projection. Origins of and feedback for tropical biases are investigated in the historical climate simulations of 18 CGCMs from phase 5 of the Coupled Model Intercomparison Project (CMIP5), together with the available Atmospheric Model Intercomparison Project (AMIP) simulations. Based on an intermodel empirical orthogonal function (EOF) analysis of tropical Pacific precipitation, the excessive equatorial Pacific cold tongue and double intertropical convergence zone (ITCZ) stand out as the most prominent errors of the current generation of CGCMs. The comparison of CMIP-AMIP pairs enables us to identify whether a given type of errors originates from atmospheric models. The equatorial Pacific cold tongue bias is associated with deficient precipitation and surface easterly wind biases in the western half of the basin in CGCMs, but these errors are absent in atmosphere-only models, indicating that the errors arise from the interaction with the ocean via Bjerknes feedback. For the double ITCZ problem, excessive precipitation south of the equator correlates well with excessive downward solar radiation in the Southern Hemisphere (SH) midlatitudes, an error traced back to atmospheric model simulations of cloud during austral spring and summer. This extratropical forcing of the ITCZ displacements is mediated by tropical ocean-atmosphere interaction and is consistent with recent studies of ocean-atmospheric energy transport balance.

Li, JX, Wang GH, Xie SP, Zhang R, Sun ZY.  2012.  A winter warm pool southwest of Hainan Island due to the orographic wind wake. Journal of Geophysical Research-Oceans. 117   10.1029/2012jc008189   Abstract

A winter warm pool off the southwest coast of Hainan Island is uncovered from high resolution satellite measurements and field observations. The warm pool is characterized by warm temperature relative to the surroundings. It forms in October, intensifies from November to next January, and decays in February. Our results show that the wind wake in the northeast winter monsoon due to the orographic blockage by mountains of Hainan Island plays an important role in generating the warm pool by reducing surface latent heat flux. The core temperature of the warm pool is correlated with the El Nino and Southern Oscillation.

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.

Li, G, Xie SP, He C, Chen ZS.  2017.  Western Pacific emergent constraint lowers projected increase in Indian summer monsoon rainfall. Nature Climate Change. 7:708-+.   10.1038/nclimate3387   AbstractWebsite

The agrarian-based socioeconomic livelihood of densely populated South Asian countries is vulnerable to modest changes in Indian summer monsoon (ISM) rainfall(1-3). How the ISM rainfall will evolve is a question of broad scientific and socioeconomic importance(3-9). In response to increased greenhouse gas (GHG) forcing, climate models commonly project an increase in ISM rainfall(4-9). This wetter ISM projection, however, does not consider large model errors in both the mean state and ocean warming pattern(9-11). Here we identify a relationship between biases in simulated present climate and future ISM projections in a multi-model ensemble: models with excessive present-day precipitation over the tropical western Pacific tend to project a larger increase in ISM rainfall under GHG forcing because of too strong a negative cloud-radiation feedback on sea surface temperature. The excessive negative feedback suppresses the local ocean surface warming, strengthening ISM rainfall projections via atmospheric circulation. We calibrate the ISM rainfall projections using this 'present-future relationship' and observed western Pacific precipitation. The correction reduces by about 50% of the projected rainfall increase over the broad ISM region. Our study identifies an improved simulation of western Pacific convection as a priority for reliable ISM projections.

Li, J, Xie S-P, Cook ER, Huang G, D'Arrigo R, Liu F, Ma J, Zheng X-T.  2011.  Interdecadal modulation of El Nino amplitude during the past millennium. Nature Climate Change. 1:114-118.   10.1038/nclimate1086   Abstract
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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.

Li, G, Xie SP.  2012.  Origins of tropical-wide SST biases in CMIP multi-model ensembles. Geophysical Research Letters. 39   10.1029/2012gl053777   AbstractWebsite

Long-standing simulation errors limit the utility of climate models. Overlooked are tropical-wide errors, with sea surface temperature (SST) biasing high or low across all the tropical ocean basins. Our analysis based on Coupled Model Intercomparison Project (CMIP) multi-model ensembles shows that such SST biases can be classified into two types: one with a broad meridional structure and of the same sign across all basins that is highly correlated with the tropical mean; and one with large inter-model variability in the cold tongues of the equatorial Pacific and Atlantic. The first type can be traced back to biases in atmospheric simulations of cloud cover, with cloudy models biasing low in tropical-wide SST. The second type originates from the diversity among models in representing the thermocline depth; models with a deep thermocline feature a warm cold tongue on the equator. Implications for inter-model variability in precipitation climatology and SST threshold for convection are discussed. Citation: Li, G., and S.-P. Xie (2012), Origins of tropical-wide SST biases in CMIP multi-model ensembles, Geophys. Res. Lett., 39, L22703, doi: 10.1029/2012GL053777.

Li, XC, Xie SP, Gille ST, Yoo C.  2016.  Atlantic-induced pan-tropical climate change over the past three decades. Nature Climate Change. 6:275-+.   10.1038/nclimate2840   AbstractWebsite

During the past three decades, tropical sea surface temperature (SST) has shown dipole-like trends, with warming over the tropical Atlantic and Indo-western Pacific but cooling over the eastern Pacific. Competing hypotheses relate this cooling, identified as a driver of the global warming hiatus(1,2), to the warming trends in either the Atlantic(3,4) or Indian Ocean(5). However, the mechanisms, the relative importance and the interactions between these teleconnections remain unclear. Using a state-of-the-art climate model, we show that the Atlantic plays a key role in initiating the tropical-wide teleconnection, and the Atlantic-induced anomalies contribute similar to 55-75% of the tropical SST and circulation changes during the satellite era. The Atlantic warming drives easterly wind anomalies over the Indo-western Pacific as Kelvin waves and westerly anomalies over the eastern Pacific as Rossby waves. The wind changes induce an Indo-western Pacific warming through the wind-evaporation-SST effect(6,7), and this warming intensifies the La Nina-type response in the tropical Pacific by enhancing the easterly trade winds and through the Bjerknes ocean dynamical processes(8). The teleconnection develops into a tropical-wide SST dipole pattern. This mechanism, supported by observations and a hierarchy of climate models, reveals that the tropical ocean basins are more tightly connected than previously thought.

Li, JB, Xie SP, Cook ER.  2014.  El Nino phases embedded in Asian and North American drought reconstructions. Quaternary Science Reviews. 85:20-34.   10.1016/j.quascirev.2013.11.014   AbstractWebsite

The amplitude of El Nil-ID-Southern Oscillation (ENSO) varies substantially at each phase of its evolution, affecting the timing and patterns of atmospheric teleconnections around the globe. Instrumental records are too short to capture the full behavior of ENSO variability. Here we use the well-validated Monsoon Asia Drought Atlas (MADA) and North America Drought Atlas (NADA) for the past 700 years, and show that tree-ring records from different regions represent tropical sea surface temperature (SST) conditions at various phases of ENSO. Three modes of tree-ring based summer drought variability are found to be correlated with ENSO: summer droughts over the Maritime Continent and Southwest North America (NA), and a dipole mode between Central and South Asia. A lagged correlation analysis is performed to determine the time when precipitation and temperature anomaly imprints on summer droughts as recorded in tree-rings. Drought anomalies in the Maritime Continent and Southwest NA represent ENSO at the developing and peak phases respectively, while those over Central/South Asia are associated with tropical-wide SST anomalies (including the Indian Ocean) at the decay phase of ENSO. Thus proxy records from different regions can provide valuable information on long-term behavior of ENSO at different phases. (C) 2013 Elsevier Ltd. All rights reserved.

Li, G, Xie SP, Du Y, Luo YY.  2016.  Effects of excessive equatorial cold tongue bias on the projections of tropical Pacific climate change. Part I: the warming pattern in CMIP5 multi-model ensemble. Climate Dynamics. 47:3817-3831.   10.1007/s00382-016-3043-5   AbstractWebsite

The excessive cold tongue error in the equatorial Pacific has persisted in several generations of climate models. Based on the historical simulations and Representative Concentration Pathway (RCP) 8.5 experiments in the Coupled Model Intercomparison Project phase 5 (CMIP5) multi-model ensemble (MME), this study finds that models with an excessive westward extension of cold tongue and insufficient equatorial western Pacific precipitation tend to project a weaker east-minus-west gradient of sea surface temperature (SST) warming along the equatorial Pacific under increased greenhouse gas (GHG) forcing. This La Nia-like error of tropical Pacific SST warming is consistent with our understanding of negative SST-convective feedback over the western Pacific warm pool. Based on this relationship between the present simulations and future projections, the present study applies an "observational constraint" of equatorial western Pacific precipitation to calibrate the projections of tropical Pacific climate change. After the corrections, CMIP5 models robustly project an El Nio-like warming pattern, with a MME mean increase by a factor of 2.3 in east-minus-west gradient of equatorial Pacific SST warming and reduced inter-model uncertainty. Corrections in projected changes in tropical precipitation and atmospheric circulation are physically consistent. This study suggests that a realistic cold tongue simulation would lead to a more reliable tropical Pacific climate projection.

Li, JB, Xie SP, Cook ER, Morales MS, Christie DA, Johnson NC, Chen FH, D'Arrigo R, Fowler AM, Gou XH, Fang KY.  2013.  El Nino modulations over the past seven centuries. Nature Climate Change. 3:822-826.   10.1038/nclimate1936   AbstractWebsite

Predicting how the El Nino/Southern Oscillation (ENSO) will change with global warming is of enormous importance to society(1-4). ENSO exhibits considerable natural variability at interdecadal-centennial timescales(5). Instrumental records are too short to determine whether ENSO has changed(6) and existing reconstructions are often developed without adequate tropical records. Here we present a seven-century-long ENSO reconstruction based on 2,222 tree-ring chronologies from both the tropics and mid-latitudes in both hemispheres. The inclusion of tropical records enables us to achieve unprecedented accuracy, as attested by high correlations with equatorial Pacific corals(7,8) and coherent modulation of global teleconnections that are consistent with an independent Northern Hemisphere temperature reconstruction(9). Our data indicate that ENSO activity in the late twentieth century was anomalously high over the past seven centuries, suggestive of a response to continuing global warming. Climate models disagree on the ENSO response to global warming(3,4), suggesting that many models underestimate the sensitivity to radiative perturbations. Illustrating the radiative effect, our reconstruction reveals a robust ENSO response to large tropical eruptions, with anomalous cooling in the east-central tropical Pacific in the year of eruption, followed by anomalous warming one year after. Our observations provide crucial constraints for improving climate models and their future projections.

Li, JB, Xie SP, Cook ER, Chen FH, Shi JF, Zhang DD, Fang KY, Gou XH, Li T, Peng JF, Shi SY, Zhao YS.  2019.  Deciphering human contributions to Yellow River flow reductions and downstream drying using centuries-long tree ring records. Geophysical Research Letters. 46:898-905.   10.1029/2018gl081090   AbstractWebsite

The Yellow River flow has decreased substantially in recent decades, and the river often dried up in the lower reach and failed to reach the sea. Climate change and human disruption have been suggested as major causes of the flow reduction, but quantification of their relative contribution is challenging due to limited instrumental records and disturbance by dams. Here we use a basin-wide tree ring network to reconstruct the Yellow River flow for the past 1,200 years and show that the flow exhibits marked amplitude variations that are closely coupled to the hydrological mean state swings at multidecadal to centennial timescales. Recent flow should have increased to the highest level of the past 1,200 years if there were no human disruption. However, human activities have caused a loss of nearly half of natural flow since the late 1960s and are the main culprit for recent downstream flow reduction. Plain Language Summary Recent Yellow River flow reductions have had major impacts on China's economy and water policy. The short and heavily human-modified gauge records are unable to reveal natural flow variability now and in the past. Here we use tree rings to reconstruct long-term Yellow River flow, which enables an assessment of natural flow variability and the detection of human contributions to recent flow reductions. Our 1,200-year reconstruction reveals that under natural conditions the Yellow River flow should have increased markedly since the early twentieth century. However, the observed flow decreased since the late 1960s and such a decrease must be predominately caused by human interventions instead of climate change.

Li, G, Xie SP, Du Y.  2016.  A robust but spurious pattern of climate change in model projections over the tropical Indian Ocean. Journal of Climate. 29:5589-5608.   10.1175/jcli-d-15-0565.1   AbstractWebsite

Climate models consistently project reduced surface warming over the eastern equatorial Indian Ocean (IO) under increased greenhouse gas (GHG) forcing. This IO dipole (IOD)-like warming pattern, regarded as robust based on consistency among models by the new Intergovernmental Panel on Climate Change (IPCC) report, results in a large increase in the frequency of extreme positive IOD (pIOD) events, elevating the risk of climate and weather disasters in the future over IO rim countries. These projections, however, do not consider large model biases in both the mean state and interannual IOD variance. In particular, a "present-future relationship" is identified between the historical simulations and representative concentration pathway (RCP) 8.5 experiments from phase 5 of the Coupled Model Intercomparison Project (CMIP5) multimodel ensemble: models with an excessive IOD amplitude bias tend to project a strong IOD-like warming pattern in themean and a large increase in extreme pIOD occurrences under increased GHG forcing. This relationship links the present simulation errors to future climate projections, and is also consistent with our understanding of Bjerknes ocean-atmosphere feedback. This study calibrates regional climate projections by using this present-future relationship and observed IOD amplitude. The results show that the projected IOD-like pattern of mean changes and frequency increase of extreme pIOD events are largely artifacts of model errors and unlikely to emerge in the future. These results illustrate that a robust projection may still be biased and it is important to consider the model bias effect.

Lin, L, Xu YY, Wang ZL, Diao CR, Dong WJ, Xie SP.  2018.  Changes in extreme rainfall over India and China attributed to regional aerosol-cloud interaction during the late 20th century rapid industrialization. Geophysical Research Letters. 45:7857-7865.   10.1029/2018gl078308   AbstractWebsite

Both mean and extreme rainfall decreased over India and Northern China during 1979-2005 at a rate of 0.2%/decade. The aerosol dampening effects on rainfall has also been suggested as a main driver of mean rainfall shift in India and China. Conflicting views, however, exist on whether aerosols enhance or suppress hazardous extreme heavy rainfall. Using Coupled Model Intercomparison Project phase 5 (CMIP5) multimodel ensemble, here we show that only a subset of models realistically reproduces the late-20th-century trend of extreme rainfall for the three major regions in Asia: drying in India and Northern China and wetting in Southern China, all consistent with mean rainfall change. As a common feature, this subset of models includes an explicit treatment of the complex physical processes of aerosol-cloud interaction (i.e., both cloud-albedo and cloud-lifetime effects), while simulation performance deteriorates in models that include only aerosol direct effect or cloud-albedo effect. The enhanced aerosol pollution during this rapid industrialization era is the leading cause of the spatially heterogeneous extreme rainfall change by dimming surface solar radiation, cooling adjacent ocean water, and weakening moisture transport into the continental region, while GHG warming or natural variability alone cannot explain the observed changes. Our results indicate that the projected intensification of regional extreme rainfall during the early-to-mid 21st-century, in response to the anticipated aerosol reduction, may be underestimated in global climate models without detailed treatment of complex aerosol-cloud interaction. Plain Language Summary Over Asia, a robust pattern of drying-wetting-drying trend over three most populated regions (India, South China, and North China, respectively) have been observed in the past few decades. Yet the cause of the 30-year trend is rather unclear, with conflicting arguments on the importance of natural variability, the greenhouse gas, land cover, and aerosols. Most of the previous studies, however, fail to provide a holistic explanation for all three major regions simultaneously. The aerosol-cloud interaction-induced oceanic cooling, as we show here, provides a critical piece in reproducing the past trend. Only a fraction of climate models with complex treatment of aerosol-cloud interaction capture the observed pattern; thus, unconstrained model data set provides biased outlook of extreme rainfall in this region.

Lin, XP, Xie SP, Chen XP, Xu LL.  2006.  A well-mixed warm water column in the central Bohai Sea in summer: Effects of tidal and surface wave mixing. Journal of Geophysical Research-Oceans. 111   10.1029/2006jc003504   Abstract
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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

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

Liu, WT, Xie XS, Polito PS, Xie SP, Hashizume H.  2000.  Atmospheric manifestation of tropical instability wave observed by QuikSCAT and tropical rain measuring mission. Geophysical Research Letters. 27:2545-2548. Abstract
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Liu, W, Xie SP, Lu J.  2016.  Tracking ocean heat uptake during the surface warming hiatus. Nature Communications. 7   10.1038/ncomms10926   AbstractWebsite

Ocean heat uptake is observed to penetrate deep into the Atlantic and Southern Oceans during the recent hiatus of global warming. Here we show that the deep heat penetration in these two basins is not unique to the hiatus but is characteristic of anthropogenic warming and merely reflects the depth of the mean meridional overturning circulation in the basin. We find, however, that heat redistribution in the upper 350m between the Pacific and Indian Oceans is closely tied to the surface warming hiatus. The Indian Ocean shows an anomalous warming below 50m during hiatus events due to an enhanced heat transport by the Indonesian throughflow in response to the intensified trade winds in the equatorial Pacific. Thus, the Pacific and Indian Oceans are the key regions to track ocean heat uptake during the surface warming hiatus.

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.

Liu, W, Lu J, Xie SP, Fedorov A.  2018.  Southern Ocean heat uptake, redistribution, and storage in a warming climate: The role of meridional overturning circulation. Journal of Climate. 31:4727-4743.   10.1175/jcli-d-17-0761.1   AbstractWebsite

Climate models show that most of the anthropogenic heat resulting from increased atmospheric CO2 enters the Southern Ocean near 60 degrees S and is stored around 45 degrees S. This heat is transported to the ocean interior by the meridional overturning circulation (MOC) with wind changes playing an important role in the process. To isolate and quantify the latter effect, we apply an overriding technique to a climate model and decompose the total ocean response to CO2 increase into two major components: one due to wind changes and the other due to direct CO2 effect. We find that the poleward-intensified zonal surface winds tend to shift and strengthen the ocean Deacon cell and hence the residual MOC, leading to anomalous divergence of ocean meridional heat transport around 60 degrees S coupled to a surface heat flux increase. In contrast, at 45 degrees S we see anomalous convergence of ocean heat transport and heat loss at the surface. As a result, the wind-induced ocean heat storage (OHS) peaks at 46 degrees S at a rate of 0.07 ZJ yr(-1) (degrees lat)(-1) (1 ZJ = 10(21) J), contributing 20% to the total OHS maximum. The direct CO2 effect, on the other hand, very slightly alters the residual MOC but primarily warms the ocean. It induces a small but nonnegligible change in eddy heat transport and causes OHS to peak at 42 degrees S at a rate of 0.30 ZJ yr(-1) (degrees lat)(-1), accounting for 80% of the OHS maximum. We also find that the eddy-induced MOC weakens, primarily caused by a buoyancy flux change as a result of the direct CO2 effect, and does not compensate the intensified Deacon cell.

Liu, JW, Xie SP, Norris JR, Zhang SP.  2014.  Low-level cloud response to the Gulf Stream front in winter using CALIPSO. Journal of Climate. 27:4421-4432.   10.1175/jcli-d-13-00469.1   AbstractWebsite

A sharp sea surface temperature front develops between the warm water of the Gulf Stream and cold continental shelf water in boreal winter. This front has a substantial impact on the marine boundary layer. The present study analyzes and synthesizes satellite observations and reanalysis data to examine how the sea surface temperature front influences the three-dimensional structure of low-level clouds. The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite captures a sharp low-level cloud transition across the Gulf Stream front, a structure frequently observed under the northerly condition. Low-level cloud top (<4 km) increases by about 500 m from the cold to the warm flank of the front. The sea surface temperature front induces a secondary low-level circulation through sea level pressure adjustment with ascending motion over the warm water and descending motion over cold water. The secondary circulation further contributes to the cross-frontal transition of low-level clouds. Composite analysis shows that surface meridional advection over the front plays an important role in the development of the marine atmospheric boundary layer and low-level clouds. Under cold northerly advection over the Gulf Stream front, strong near-surface instability leads to a well-mixed boundary layer over the Gulf Stream, causing southward deepening of low-level clouds across the sea surface temperature front. Moreover, the front affects the freezing level by transferring heat to the atmosphere and therefore influences the cross-frontal variation of the cloud phase.

Liu, QY, Xie SP, Li LJ, Maximenko NA.  2005.  Ocean thermal advective effect on the annual range of sea surface temperature. Geophysical Research Letters. 32   10.1029/2005gl024493   Abstract
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