A local positive feedback of the tropical Pacific ocean-atmosphere system on interdecadal timescales

SINCE THE 1990S, THE CLIMATIC VARIABILITY ON INTERDE- CADAL TIME SCALES BECAME THE FOCUS OF THE INTERNATIONAL CLIMATOLOGY RESEARCH MISSIONS[1―3]. ON TIME SCALES OF A DECADE OR MORE, THE OCEAN CIRCULATION PREDOMINATEDHEAT BALANCE AND HYDROLOGICAL CYCLE, S…     

大西洋热盐环流减弱对热带太平洋气候平均态及年际变率的影响

利用一个完全耦合的海气模式, 通过对比分析两组试验中海表温度、盐度、风应力等气候态变化特征以及ENSO强度和频率的变化, 研究热带太平洋气候平均态及年际变率对热盐环流减弱的响应。在北大西洋高纬地区注入1 Sv淡水后, 大西洋经向翻转流(AMOC)减弱约90%, 这直接导致向北的经向热量输送减少, 使北大西洋有明显降温, 南大西洋略有升温。这些变化会经过大气和海洋的远程传播以及局地海气反馈作用, 影响热带太平洋气候平均态: 赤道东西太平洋的SST都略有增温, 但纬向温度梯度和纬向风应力并没有太大变化, 赤道太平洋温跃层的深度和倾斜度也基本保持不变。相应地, ENSO强度和频率也没有明显变化。由此得出结论: 热盐环流减弱会引起全球气候平均态的变化, 但对热带太平洋的年际变率没有太大影响。     

The tropical Pacific-Indian Ocean temperature anomaly mode and its effect

Temperature anomaly in the Indian Ocean is closely related to that in the Pacific Ocean because of the Walker circulation and the Indonesian throughflow. So only the El Ni?o/Southern Oscillation (ENSO) in the Pacific cannot entirely explain the influence of sea surface temperature anomaly (SSTA) on climate variation. The tropical Pacific-Indian Ocean temperature anomaly mode (PIM) is presented based on the comprehensive research on the pattern and feature of SSTA in both Indian Ocean and Pacific Ocean. The features of PIM and ENSO mode and their influences on the climate in China and the rainfall in India are further compared. For proving the observation results, numerical experiments of the global atmospheric general circulation model are conducted. The results of observation and sensitivity experiments show that presenting PIM and studying its influence are very important for short-range climate prediction.     

Weak response of the Atlantic thermohaline circulation to an increase of atmospheric carbon dioxide in IAP／LASG Climate System Model

Response of the Atlantic thermohaline circulation (THC) to global warming is examined by using the climate system model developed at IAP/LASG. The evidence indicates that the gradually warming climate associated with the increased atmospheric carbon dioxide leads to a warmer and fresher sea surface water at the high latitudes of the North Atlantic Ocean, which prevents the down-welling of the surface water. The succedent reduction of the pole-toequator meridional potential density gradient finally results in the decrease of the THC in intensity. When the atmospheric carbon dioxide is doubled, the maximum value of the Atlantic THC decreases approximately by 8%. The associated poleward oceanic heat transport also becomes weaker. This kind of THC weakening centralizes mainly in the northern part of the North Atlantic basin, indicating briefly a local scale adjustment rather than a loop oscillation with the whole Atlantic “conveyor belt” decelerating.     

Magnitude and timing of temperature change in the Indo-Pacific warm pool during deglaciation

Ocean-atmosphere interactions in the tropical Pacific region have a strong influence on global heat and water vapour transport and thus constitute an important component of the climate system. Changes in sea surface temperatures and convection in the tropical Indo-Pacific region are thought to be responsible for the interannual to decadal climate variability observed in extra-tropical regions, but the role of the tropics in climate changes on millennial and orbital timescales is less clear. Here we analyse oxygen isotopes and Mg/Ca ratios of foraminiferal shells from the Makassar strait in the heart of the Indo-Pacific warm pool, to obtain synchronous estimates of sea surface temperatures and ice volume. We find that sea surface temperatures increased by 3.5-4.0 degrees C during the last two glacial-interglacial transitions, synchronous with the global increase in atmospheric CO2 and Antarctic warming, but the temperature increase occurred 2,000-3,000 years before the Northern Hemisphere ice sheets melted. Our observations suggest that the tropical Pacific region plays an important role in driving glacial-interglacial cycles, possibly through a system similar to how El Ni?o/Southern Oscillation regulates the poleward flux of heat and water vapour.     

Advancing decadal-scale climate prediction in the North Atlantic sector

The climate of the North Atlantic region exhibits fluctuations on decadal timescales that have large societal consequences. Prominent examples include hurricane activity in the Atlantic, and surface-temperature and rainfall variations over North America, Europe and northern Africa. Although these multidecadal variations are potentially predictable if the current state of the ocean is known, the lack of subsurface ocean observations that constrain this state has been a limiting factor for realizing the full skill potential of such predictions. Here we apply a simple approach-that uses only sea surface temperature (SST) observations-to partly overcome this difficulty and perform retrospective decadal predictions with a climate model. Skill is improved significantly relative to predictions made with incomplete knowledge of the ocean state, particularly in the North Atlantic and tropical Pacific oceans. Thus these results point towards the possibility of routine decadal climate predictions. Using this method, and by considering both internal natural climate variations and projected future anthropogenic forcing, we make the following forecast: over the next decade, the current Atlantic meridional overturning circulation will weaken to its long-term mean; moreover, North Atlantic SST and European and North American surface temperatures will cool slightly, whereas tropical Pacific SST will remain almost unchanged. Our results suggest that global surface temperature may not increase over the next decade, as natural climate variations in the North Atlantic and tropical Pacific temporarily offset the projected anthropogenic warming.     

Impact of the closure of Indonesian seaway on climate: A numerical modeling study

Using a global OGCM and its relevant coupled ocean-atmosphere GCM with the contemporary, 6 MaBP and 14 MaBP oceanic topography, respectively, a series of numerical experiments are implemented in order to investigate the effect of the north shift of Australian continent on the tropical oceanic circulation, especially the formation of the western Pacific warm pool. The numerical experiments of the individual OGCM forced by the modern atmospheric circulation indicate that the closure of Indonesian passage results in warming in the tropical Pacific Ocean and cooling in the tropical Indian Ocean; furthermore, it also results in change in source of the Indonesian Through Flow (ITF) water, e.g. ITF mainly originates from the south Pacific at 14 MaBP, but it mainly originates from the north Pacific now. The coupled model shows similar results as the individual OGCM qualitatively.     

Holocene Southern Ocean surface temperature variability west of the Antarctic Peninsula

The disintegration of ice shelves, reduced sea-ice and glacier extent, and shifting ecological zones observed around Antarctica highlight the impact of recent atmospheric and oceanic warming on the cryosphere. Observations and models suggest that oceanic and atmospheric temperature variations at Antarctica's margins affect global cryosphere stability, ocean circulation, sea levels and carbon cycling. In particular, recent climate changes on the Antarctic Peninsula have been dramatic, yet the Holocene climate variability of this region is largely unknown, limiting our ability to evaluate ongoing changes within the context of historical variability and underlying forcing mechanisms. Here we show that surface ocean temperatures at the continental margin of the western Antarctic Peninsula cooled by 3-4 °C over the past 12,000 years, tracking the Holocene decline of local (65° S) spring insolation. Our results, based on TEX(86) sea surface temperature (SST) proxy evidence from a marine sediment core, indicate the importance of regional summer duration as a driver of Antarctic seasonal sea-ice fluctuations. On millennial timescales, abrupt SST fluctuations of 2-4 °C coincide with globally recognized climate variability. Similarities between our SSTs, Southern Hemisphere westerly wind reconstructions and El Ni?o/Southern Oscillation variability indicate that present climate teleconnections between the tropical Pacific Ocean and the western Antarctic Peninsula strengthened late in the Holocene epoch. We conclude that during the Holocene, Southern Ocean temperatures at the western Antarctic Peninsula margin were tied to changes in the position of the westerlies, which have a critical role in global carbon cycling.     

Remobilization of southern African desert dune systems by twenty-first century global warming

Although desert dunes cover 5 per cent of the global land surface and 30 per cent of Africa, the potential impacts of twenty-first century global warming on desert dune systems are not well understood. The inactive Sahel and southern African dune systems, which developed in multiple arid phases since the last interglacial period, are used today by pastoral and agricultural systems that could be disrupted if climate change alters twenty-first century dune dynamics. Empirical data and model simulations have established that the interplay between dune surface erodibility (determined by vegetation cover and moisture availability) and atmospheric erosivity (determined by wind energy) is critical for dunefield dynamics. This relationship between erodibility and erosivity is susceptible to climate-change impacts. Here we use simulations with three global climate models and a range of emission scenarios to assess the potential future activity of three Kalahari dunefields. We determine monthly values of dune activity by modifying and improving an established dune mobility index so that it can account for global climate model data outputs. We find that, regardless of the emission scenario used, significantly enhanced dune activity is simulated in the southern dunefield by 2039, and in the eastern and northern dunefields by 2069. By 2099 all dunefields are highly dynamic, from northern South Africa to Angola and Zambia. Our results suggest that dunefields are likely to be reactivated (the sand will become significantly exposed and move) as a consequence of twenty-first century climate warming.     

Influence of mean climate change on climate variability from a 155-year tropical Pacific coral record

Today, the El Ni?o/Southern Oscillation (ENSO) system is the primary driver of interannual variability in global climate, but its long-term behaviour is poorly understood. Instrumental observations reveal a shift in 1976 towards warmer and wetter conditions in the tropical Pacific, with widespread climatic and ecological consequences. This shift, unique over the past century, has prompted debate over the influence of increasing atmospheric concentrations of greenhouse gases on ENSO variability. Here we present a 155-year ENSO reconstruction from a central tropical Pacific coral that provides new evidence for long-term changes in the regional mean climate and its variability. A gradual transition in the early twentieth century and the abrupt change in 1976, both towards warmer and wetter conditions, co-occur with changes in variability. In the mid-late nineteenth century, cooler and drier background conditions coincided with prominent decadal variability; in the early twentieth century, shorter-period (approximately 2.9 years) variability intensified. After 1920, variability weakens and becomes focused at interannual timescales; with the shift in 1976, variability with a period of about 4 years becomes prominent. Our results suggest that variability in the tropical Pacific is linked to the region's mean climate, and that changes in both have occurred during periods of natural as well as anthropogenic climate forcing.     

Increasing destructiveness of tropical cyclones over the past 30 years

Theory and modelling predict that hurricane intensity should increase with increasing global mean temperatures, but work on the detection of trends in hurricane activity has focused mostly on their frequency and shows no trend. Here I define an index of the potential destructiveness of hurricanes based on the total dissipation of power, integrated over the lifetime of the cyclone, and show that this index has increased markedly since the mid-1970s. This trend is due to both longer storm lifetimes and greater storm intensities. I find that the record of net hurricane power dissipation is highly correlated with tropical sea surface temperature, reflecting well-documented climate signals, including multi-decadal oscillations in the North Atlantic and North Pacific, and global warming. My results suggest that future warming may lead to an upward trend in tropical cyclone destructive potential, and--taking into account an increasing coastal population--a substantial increase in hurricane-related losses in the twenty-first century.     

Eastern Pacific cooling and Atlantic overturning circulation during the last deglaciation

Surface ocean conditions in the equatorial Pacific Ocean could hold the clue to whether millennial-scale global climate change during glacial times was initiated through tropical ocean-atmosphere feedbacks or by changes in the Atlantic thermohaline circulation. North Atlantic cold periods during Heinrich events and millennial-scale cold events (stadials) have been linked with climatic changes in the tropical Atlantic Ocean and South America, as well as the Indian and East Asian monsoon systems, but not with tropical Pacific sea surface temperatures. Here we present a high-resolution record of sea surface temperatures in the eastern tropical Pacific derived from alkenone unsaturation measurements. Our data show a temperature drop of approximately 1 degrees C, synchronous (within dating uncertainties) with the shutdown of the Atlantic meridional overturning circulation during Heinrich event 1, and a smaller temperature drop of approximately 0.5 degrees C synchronous with the smaller reduction in the overturning circulation during the Younger Dryas event. Both cold events coincide with maxima in surface ocean productivity as inferred from 230Th-normalized carbon burial fluxes, suggesting increased upwelling at the time. From the concurrence of equatorial Pacific cooling with the two North Atlantic cold periods during deglaciation, we conclude that these millennial-scale climate changes were probably driven by a reorganization of the oceans' thermohaline circulation, although possibly amplified by tropical ocean-atmosphere interaction as suggested before.     

Features of tropical cyclone landfalls over East Asia corresponding to three types of Pacific warming decaying phase

This study examines the tropical cyclone （TC） landfall activities over the East Asia in three types of decaying phase of warm sea surface temperature anomalies （SSTAs） over the equatorial central-eastern Pacific： eastern Pacific warming decaying to La Nifia, eastern Pacific warming decaying to a neutral E1 Nifio-Southern Oscilla- tion phase, and a central Pacific warming decaying year. Results show that, for the type of eastern Pacific wanning decaying to La Nifia, more TCs make landfall over Hainan Island and Beibu Gulf, whereas fewer TCs reach eastern China coast. In particular, the number of landfalling TCs remarkably decreases in the decaying phase of eastern Pacific E1 Nifio to a neutral year. During the decaying phase of central Pacific E1 Nifio events, more TCs tend to make landfall over southern China, Indochina Peninsula and the Philippines. The anomalies of atmospheric circu- lation and environmental conditions induced by the SSTAs over the tropical Pacific in the different decaying types are responsible for the evident variation in features of TC landfall.     

Attribution of observed surface humidity changes to human influence

Water vapour is the most important contributor to the natural greenhouse effect, and the amount of water vapour in the atmosphere is expected to increase under conditions of greenhouse-gas-induced warming, leading to a significant feedback on anthropogenic climate change. Theoretical and modelling studies predict that relative humidity will remain approximately constant at the global scale as the climate warms, leading to an increase in specific humidity. Although significant increases in surface specific humidity have been identified in several regions, and on the global scale in non-homogenized data, it has not been shown whether these changes are due to natural or human influences on climate. Here we use a new quality-controlled and homogenized gridded observational data set of surface humidity, with output from a coupled climate model, to identify and explore the causes of changes in surface specific humidity over the late twentieth century. We identify a significant global-scale increase in surface specific humidity that is attributable mainly to human influence. Specific humidity is found to have increased in response to rising temperatures, with relative humidity remaining approximately constant. These changes may have important implications, because atmospheric humidity is a key variable in determining the geographical distribution and maximum intensity of precipitation, the potential maximum intensity of tropical cyclones, and human heat stress, and has important effects on the biosphere and surface hydrology.     

Millennial-scale trends in west Pacific warm pool hydrology since the Last Glacial Maximum

Models and palaeoclimate data suggest that the tropical Pacific climate system plays a key part in the mechanisms underlying orbital-scale and abrupt climate change. Atmospheric convection over the western tropical Pacific is a major source of heat and moisture to extratropical regions, and may therefore influence the global climate response to a variety of forcing factors. The response of tropical Pacific convection to changes in global climate boundary conditions, abrupt climate changes and radiative forcing remains uncertain, however. Here we present three absolutely dated oxygen isotope records from stalagmites in northern Borneo that reflect changes in west Pacific warm pool hydrology over the past 27,000 years. Our results suggest that convection over the western tropical Pacific weakened 18,000-20,000 years ago, as tropical Pacific and Antarctic temperatures began to rise during the early stages of deglaciation. Convective activity, as inferred from oxygen isotopes, reached a minimum during Heinrich event 1 (ref. 10), when the Atlantic meridional overturning circulation was weak, pointing to feedbacks between the strength of the overturning circulation and tropical Pacific hydrology. There is no evidence of the Younger Dryas event in the stalagmite records, however, suggesting that different mechanisms operated during these two abrupt deglacial climate events. During the Holocene epoch, convective activity appears to track changes in spring and autumn insolation, highlighting the sensitivity of tropical Pacific convection to external radiative forcing. Together, these findings demonstrate that the tropical Pacific hydrological cycle is sensitive to high-latitude climate processes in both hemispheres, as well as to external radiative forcing, and that it may have a central role in abrupt climate change events.     

Effect of remote sea surface temperature change on tropical cyclone potential intensity

The response of tropical cyclone activity to global warming is widely debated. It is often assumed that warmer sea surface temperatures provide a more favourable environment for the development and intensification of tropical cyclones, but cyclone genesis and intensity are also affected by the vertical thermodynamic properties of the atmosphere. Here we use climate models and observational reconstructions to explore the relationship between changes in sea surface temperature and tropical cyclone 'potential intensity'--a measure that provides an upper bound on cyclone intensity and can also reflect the likelihood of cyclone development. We find that changes in local sea surface temperature are inadequate for characterizing even the sign of changes in potential intensity, but that long-term changes in potential intensity are closely related to the regional structure of warming; regions that warm more than the tropical average are characterized by increased potential intensity, and vice versa. We use this relationship to reconstruct changes in potential intensity over the twentieth century from observational reconstructions of sea surface temperature. We find that, even though tropical Atlantic sea surface temperatures are currently at a historical high, Atlantic potential intensity probably peaked in the 1930s and 1950s, and recent values are near the historical average. Our results indicate that--per unit local sea surface temperature change--the response of tropical cyclone activity to natural climate variations, which tend to involve localized changes in sea surface temperature, may be larger than the response to the more uniform patterns of greenhouse-gas-induced warming.     

Recent Northern Hemisphere tropical expansion primarily driven by black carbon and tropospheric ozone

Observational analyses have shown the width of the tropical belt increasing in recent decades as the world has warmed. This expansion is important because it is associated with shifts in large-scale atmospheric circulation and major climate zones. Although recent studies have attributed tropical expansion in the Southern Hemisphere to ozone depletion, the drivers of Northern Hemisphere expansion are not well known and the expansion has not so far been reproduced by climate models. Here we use a climate model with detailed aerosol physics to show that increases in heterogeneous warming agents--including black carbon aerosols and tropospheric ozone--are noticeably better than greenhouse gases at driving expansion, and can account for the observed summertime maximum in tropical expansion. Mechanistically, atmospheric heating from black carbon and tropospheric ozone has occurred at the mid-latitudes, generating a poleward shift of the tropospheric jet, thereby relocating the main division between tropical and temperate air masses. Although we still underestimate tropical expansion, the true aerosol forcing is poorly known and could also be underestimated. Thus, although the insensitivity of models needs further investigation, black carbon and tropospheric ozone, both of which are strongly influenced by human activities, are the most likely causes of observed Northern Hemisphere tropical expansion.     

Increasing risk of Amazonian drought due to decreasing aerosol pollution

The Amazon rainforest plays a crucial role in the climate system, helping to drive atmospheric circulations in the tropics by absorbing energy and recycling about half of the rainfall that falls on it. This region (Amazonia) is also estimated to contain about one-tenth of the total carbon stored in land ecosystems, and to account for one-tenth of global, net primary productivity. The resilience of the forest to the combined pressures of deforestation and global warming is therefore of great concern, especially as some general circulation models (GCMs) predict a severe drying of Amazonia in the twenty-first century. Here we analyse these climate projections with reference to the 2005 drought in western Amazonia, which was associated with unusually warm North Atlantic sea surface temperatures (SSTs). We show that reduction of dry-season (July-October) rainfall in western Amazonia correlates well with an index of the north-south SST gradient across the equatorial Atlantic (the 'Atlantic N-S gradient'). Our climate model is unusual among current GCMs in that it is able to reproduce this relationship and also the observed twentieth-century multidecadal variability in the Atlantic N-S gradient, provided that the effects of aerosols are included in the model. Simulations for the twenty-first century using the same model show a strong tendency for the SST conditions associated with the 2005 drought to become much more common, owing to continuing reductions in reflective aerosol pollution in the Northern Hemisphere.     

Numerical simulation of the sensitivity of the Pacific subtropical-tropical meridional cell to global warming

Sensitivity of the Pacific subtropical-tropical meridional cell to global warming is examined by using a global ocean-atmosphere coupled model developed at LASG/IAP. Results indicate that associated with the increasing of atmospheric carbon dioxide, the most prominent signals of global warming locate at high latitudes, and the change of middle and low latitudes, in particular the surface wind, is relatively weak, which leads to a weak response of the Pacific subtropical-tropical meridional cell. At the time of atmospheric carbon dioxide doubling, the change of the meridional cell strength is smaller than the amplitude of natural variability.     

Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model

The continued increase in the atmospheric concentration of carbon dioxide due to anthropogenic emissions is predicted to lead to significant changes in climate. About half of the current emissions are being absorbed by the ocean and by land ecosystems, but this absorption is sensitive to climate as well as to atmospheric carbon dioxide concentrations, creating a feedback loop. General circulation models have generally excluded the feedback between climate and the biosphere, using static vegetation distributions and CO2 concentrations from simple carbon-cycle models that do not include climate change. Here we present results from a fully coupled, three-dimensional carbon-climate model, indicating that carbon-cycle feedbacks could significantly accelerate climate change over the twenty-first century. We find that under a 'business as usual' scenario, the terrestrial biosphere acts as an overall carbon sink until about 2050, but turns into a source thereafter. By 2100, the ocean uptake rate of 5 Gt C yr(-1) is balanced by the terrestrial carbon source, and atmospheric CO2 concentrations are 250 p.p.m.v. higher in our fully coupled simulation than in uncoupled carbon models, resulting in a global-mean warming of 5.5 K, as compared to 4 K without the carbon-cycle feedback.