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Climate models emphasize the need to investigate inter-hemispheric climatic interactions. However, these models often underestimate the inter-hemispheric differences in climate change. With the wide application of reanalysis data since 1948, we identified a dipole pattern between the geopotential heights (GPHs) in Northeast Asia and Antarctica on the interdecadal scale in boreal summer. This Northeast Asia/Antarctica (NAA) dipole pattern is not conspicuous on the interannual scale, probably in that the interannual inter-hemispheric climate interaction is masked by strong interannual signals in the tropics associated with the El Niño-Southern Oscillation (ENSO). Unfortunately, the instrumental records are not sufficiently long-lasting to detect the interdecadal variability of the NAA. We thus reconstructed GPHs since 1565, making using the proxy records mostly from tree rings in Northeast Asia and ice cores from Antarctica. The strength of the NAA is time-varying and it is most conspicuous in the eighteenth century and after the late twentieth century. The strength of the NAA matches well with the variations of the solar radiation and tends to increase in along with its enhancement. In boreal summer, enhanced heating associated with high solar radiation in the Northern Hemisphere drives more air masses from the South to the North. This inter-hemispheric interaction is particularly strong in East Asia as a result of the Asian summer monsoon. Northeast Asia and Antarctica appear to be the key regions responsible for inter-hemispheric interactions on the interdecadal scale in boreal summer since they are respectively located at the front and the end of this inter-hemispheric trajectory.

The Tibetan Plateau (TP) is called the “third pole” and the “Asian water tower”, and climate change over the TP is evident in recent decades. However, the elevation dependency warming (EDW, larger temperature increases with higher elevation) over the TP under global warming of 1.5 °C and 2 °C is not well understood. In this study, future changes in the monthly mean, maximum, and minimum temperature over the TP derived from 21 global climate models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) are investigated using a midrange/high emission scenario (RCP4.5/8.5) in which the global surface temperature has risen by 1.5 °C and 2 °C relative to the pre-industrial period. The multi-model ensemble mean of 21 CMIP5 models indicates that the TP has rapidly warmed to a larger degree than the global mean and the whole China. Overall, the mean temperature over the TP under RCP4.5/8.5 scenarios under global warming of 1.5 °C and 2 °C will increase by 2.11/2.10 °C and 2.89/2.77 °C, respectively, particularly in the western TP. The midrange emission scenario RCP4.5 shows larger temperature changes under global warming of 1.5 °C and 2 °C than the high emission scenario RCP8.5. Furthermore, a robust EDW over the TP is found to intensify under global warming of 1.5 °C and 2 °C, which is probably contributed by the snow/ice-albedo feedback in the elevation range between 3.5 and 4 km over the TP. The EDW over the TP raises more robust under global warming of 2 °C than 1.5 °C. This study suggests that the TP is being influenced by global warming approximately 10 years earlier than the global scale under global warming of 1.5 °C and 2 °C, and the EDW under global warming of 1.5 °C and 2 °C will have potentially serious consequences for the third pole environment.

As Arctic sea ice declines in response to climate change, a shift from thick multiyear ice to a thinner ice cover is occurring. With this transition, ice thicknesses approach a threshold below which ice no longer insulates the atmosphere from oceanic surface fluxes. While this is well known, there are no estimates of the magnitude of this threshold, nor of the proportion of sea ice area that is below this threshold as ice thins. We determine this threshold by simulating the atmospheric response to varying thicknesses, ranging from 0.0 to 2.0 m and determine that threshold to be 0.40–0.50 m. The resulting “effective” ice area is 4–14% lower than reported total ice area, as 0.39–0.97 × 106 km2 of the total ice area falls below the threshold throughout the twentieth century, including during notable ice minima. The atmosphere above large non-insulating ice-covered regions is susceptible to more than 2 °C of warming despite ice presence. Observed mean Arctic Ocean ice thickness is projected to fall below this threshold as early as the mid-2020s. Studies on ocean–atmosphere interactions in relation to sea ice area should focus on this insulating sea ice area, where ice is at least 0.40–0.50 m thick, and treat ice regions below 0.40–0.50 m thickness with caution.

In order to test the sensitivity of regional climate to regional-scale atmosphere-land cover feedbacks, we have employed a regional climate model asynchronously coupled to an equilibrium vegetation model, focusing on the western United States as a case study. CO2-induced atmosphere-land cover feedbacks resulted in statistically significant seasonal temperature changes of up to 3.5°C, with land cover change accounting for up to 60% of the total seasonal response to elevated atmospheric CO2 levels. In many areas, such as the Great Basin, albedo acted as the primary control on changes in surface temperature. Along the central coast of California, soil moisture effects magnified the temperature response in JJA and SON, with negative surface soil moisture anomalies accompanied by negative evaporation anomalies, decreasing latent heat flux and further increasing surface temperature. Additionally, negative temperature anomalies were calculated at high elevation in California and Oregon in DJF, MAM and SON, indicating that future warming of these sensitive areas could be mitigated by changes in vegetation distribution and an associated muting of winter snow-temperature feedbacks. Precipitation anomalies were almost universally not statistically significant, and very little change in mean seasonal atmospheric circulation occurred in response to atmosphere-land cover feedbacks. Further, the mean regional temperature sensitivity to regional-scale land cover feedbacks did not exceed the large-scale sensitivity calculated elsewhere, indicating that spatial heterogeneity does not introduce non-linearities in the response of regional climate to CO2-induced atmosphere-land cover feedbacks.

El Niño/Southern Oscillation (ENSO) is the predominant interannual variability of the global climate system. How might ENSO change in a warmer world? The dominant two Combined Empirical Orthogonal Functions (CEOF) of the equatorial ocean temperature and zonal and vertical motion identify two modes that shown a transition in the eastern Pacific from a warming eastward/downward motion to a cooling westward/upward flow. These results also suggest consistent changes to the west and at depths down to 300 m. These dominate CEOFs provide a compact tool for assessing Coupled Model Intercomparison Project Phase 5 ocean model output for both the recent historical period and for the latter part of the twenty first century. Most of the analyzed models replicate well the spatial patterns of the dominant observational CEOF modes, but nearly always underestimate the magnitudes. Comparing model output for the twentieth and twenty first centuries there is very little change between the spatial patterns of the ENSO modes of the two periods. This lack of response to climate change is shown to be partly related to competing influences of climatic changes in the mean ocean circulation.

This study investigates the interdecadal enhancement of the South China Sea–Western North Pacific monsoon trough (MT) and its relationship with tropical cyclone (TC) genesis in the mid-2000s. Analyses reveal pronounced intensification of the MT, increased synoptic-scale wave activity, and more TC genesis over the South China Sea –Philippine Sea after the mid-2000s. Due to the delayed South China Sea summer monsoon (SCSSM) withdrawal, in the three-dimensional circulation structure, the low-level (850-hPa) MT endures for an extended period, accompanied by a typical cyclonic circulation and more potent convergence and upward motion over the southern South China Sea–Philippine Sea. This creates a beneficial background for TC genesis. Meanwhile, alterations in the intensity and position of the mid-level (500-hPa) Western North Pacific subtropical high and the upper-level (200-hPa) South Asian high impact the vertical circulation structures, thus influencing the overall environmental conditions. Increased moisture, cyclonic anomalies, and enhanced low-level (upper-level) convergence (divergence) act constructively for TC genesis. Intensified barotropic and baroclinic eddy kinetic energy conversions strengthen synoptic-scale systems like synoptic-scale waves, also facilitating TC formation. Anomalous sea surface temperature (SST) warming over the Philippine Sea excites a cyclonic anomaly through an equatorial Rossby wave response due to convective heating, maintaining the MT structure. Model simulations also demonstrate that the warming of SST in September has a positive effect on maintaining the MT structure. In summary, warmer Philippine Sea SST leads to delayed SCSSM withdrawal associated with the persistent MT accompanied by advantageous environmental conditions and more active synoptic-scale waves, leading to the interdecadal increase in TC genesis over the South China Sea–Philippine Sea during the SCSSM withdrawal phase since the mid-2000s.

The El Niño–Southern Oscillation (ENSO) is one of the dominant climate forcings affecting the interannual climate variability in many regions worldwide. Using the ERA-Interim monthly data for the period 1980–2016, for the first time the impact of the eastern Pacific (EP) and central Pacific (CP) El Niños on the climate of Iran is investigated. Results indicate that the ENSO cycle contributes to the interannual climate variability over Iran. Indeed, about 26% (23%) of the variance in annual precipitation over Iran is explained by annual SST changes in the Niño 3.4 region (annual changes of the Southern Oscillation Index). In spite of the seasonality of the ENSO signal and its interevent variability, annually all regions of Iran are anomalously wet during the EP El Niño, and dry during La Niña events. The CP El Niño events result in anomalously wet conditions over northwestern, northern, northeastern and western Iran, but dry conditions over central, eastern, southwestern, southern and southeastern Iran. However, the impact of the CP El Niño on the annual precipitation of Iran is not statistically significant, in contrast to the statistically significant impact of both La Niña and the EP El Niño. An equatorward displacement of the subtropical jet stream over Southwest Asia is found during the EP El Niño, while strengthening of the jet is found during La Niña, and they are statistically significant at the $$95\%$$ level. These changes in the position and intensity of the subtropical jet stream alter the position of the troughs and ridges of the Rossby waves and their speed, which contribute to the interannual climate variability over Iran. Equatorward displacement of the jet during the EP El Niño leads to the equatorward displacement of the Mediterranean storm track, while the intensified jet during La Niña weakens the quasi-stationary mid-tropospheric planetary waves.

Heat and cold waves may have considerable human and economic impacts in Europe. Recent events, like the heat waves observed in France in 2003 and Russia in 2010, illustrated the major consequences to be expected. Reliable Early Warning Systems for extreme temperatures would, therefore, be of high value for decision makers. However, they require a clear definition and robust forecasts of these events. This study analyzes the predictability of heat and cold waves over Europe, defined as at least three consecutive days of $${\text {T}}_{\text {min}}$$ and $${\text {T}}_{\text {max}}$$ above the quantile Q90 (under Q10), using the extended ensemble system of ECMWF. The results show significant predictability for events within a 2-week lead time, but with a strong decrease of the predictability during the first week of forecasts (from 80 to 40% of observed events correctly forecasted). The scores show a higher predictive skill for the cold waves (in winter) than for the heat waves (in summer). The uncertainties and the sensitivities of the predictability are discussed on the basis of tests conducted with different spatial and temporal resolutions. Results demonstrate the negligible effect of the temporal resolution (very few errors due to bad timing of the forecasts), and a better predictability of large-scale events. The onset and the end of the waves are slightly less predictable with an average of about 35% (30%) of observed heat (cold) waves onsets or ends correctly forecasted with a 5-day lead time. Finally, the forecasted intensities show a correlation of about 0.65 with those observed, revealing the challenge to predict this important characteristic.

The Global Soil Wetness Project (GSWP) is an international initiative aimed at producing global data sets of soil wetness and energy and water fluxes by driving land surface models with state-of-the-art 1° by 1° atmospheric forcing and land surface parameters. It also provides a unique opportunity to develop and test land surface parameterizations at the global scale, using multi-year off-line simulations that are not affected by the systematic errors found in atmospheric models. Nevertheless, the accuracy and reliability of the 10−year GSWP-2 atmospheric forcing remain questionable. A first comparison using the high-resolution Rhône-AGGregation (Rhône-AGG) database reveals that the baseline GSWP-2 precipitation forcing is drastically overestimated over the Rhône river basin. Hydrological simulations driven with each dataset and using the ISBA land surface model and the MODCOU river routing model are also compared. The simulated river discharges are validated against a dense network of river gauges and are generally less realistic when using the GSWP-2 instead of the Rhône-AGG precipitation forcing. Secondly, the GSWP-2 precipitation forcing is compared with three alternative data sets (GPCP-2, CRU-2, CMAP) at the global scale. Moreover, the results of a global sensitivity study to the precipitation forcing conducted with six land surface models are shown. The TRIP river routing model is used to convert daily runoff from all models into river discharges, which are compared at 80 gauging stations distributed over the globe. In agreement with the regional evaluation, the results reveal that the baseline GSWP-2 precipitation forcing is generally overestimated over the mid and high latitudes, which implies systematic errors in the simulated discharges. This study reveals that the empirical wind corrections applied to the GSWP-2 precipitation forcing are exaggerated, whereas the GPCP satellite adjustments seem to be useful for simulating realistic annual mean river discharges over the East Siberian river basins.

This study examines uncertainties of projected spring surface air temperature (SAT) and precipitation trends during 2006–2060 at regional scales over the mid-high latitudes of Eurasia due to internal variability based on 40 ensemble members projections of CCSM3. The 40 ensemble members are initiated at a slightly different atmospheric conditions but with the same external forcing. Thus, the differences of the projected spring SAT and precipitation trends among the 40 ensemble members are attributed to the internal variability. Results suggest that superposition of internal variability and external forcing leads to a large spread of projected spring SAT and precipitation trends over Eurasia. In comparison, the projected spring precipitation trend has a larger uncertainty than the spring SAT trend. In particular, the signal-to-noise ratios of spring SAT (precipitation) trends are larger than two (one) over most regions of the mid-high latitudes of Eurasia. The internal atmospheric circulation variability is an important source of the uncertainties of the projected spring SAT and precipitation trends. The first mode of the internal atmospheric circulation variability resembles the Arctic Oscillation pattern. The second mode displays feature similar to the North Atlantic Oscillation and anomalous Siberian High patterns. A dynamical adjustment technique is employed to reduce internal atmospheric circulation generated variability in spring SAT and precipitation trends. Result indicates that projected trends of the dynamically adjusted spring SAT and precipitation over the next 55 years over Eurasia are similar across the 40 ensemble members both in the spatial structure and amplitude.