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The response of low-latitude weather and climate to polar sea ice melt is not well understood. In this study, we run a suite of fully coupled simulations using the Community Earth System Model to investigate the effects of polar sea ice melt on Indian summer monsoon (ISM). In the coupled model simulation, the albedo of the sea ice is reduced in such a way that the increased absorption of the solar radiation would melt the sea ice. The effects of the Arctic and Antarctic sea ice melts on the ISM have been examined. The findings reveal that the combined melting of sea ice in the Arctic and Antarctic exerts a stronger influence on the ISM compared to the melting at a single pole. Specifically, the ISM exhibits a relatively weak sensitivity to the Arctic sea ice melt, while showing a stronger response to sea ice melt over the Antarctic and the combined melt over both poles. The response of summer monsoon to sea ice melt is characterized by a weakening of the circulation and a decrease in the continental precipitation, indicating an overall weakening of the monsoon system as a result of the polar sea ice melt. The intertropical convergence zone (ITCZ) and associated precipitation pattern shift equatorward in the sea ice melt experiments.

The latitude-altitude structure of the double intertropical convergence zone (DITCZ), its zonal variations, annual cycle and interannual variability over tropical oceans, which were least explored so far, are investigated using the long-term (2006–2018) joint analysis of spaceborne cloud radar and lidar observations that can profile all types of clouds. In contrast to satellite imager observations, this analysis can unambiguously discriminate the DITCZ core and the cirrus outflow emanating from them, which enables the characterization of DITCZ features, including their vertical structure, width and strength even when the equatorial region separating the DITCZ bands has overcast clouds or one of the bands is weak. The role of observed surface wind divergence (SWD) and sea surface temperature (SST) in the genesis and annual migration of the DITCZ bands as well as the ability of vertical wind obtained from reanalysis in capturing the observed DITCZ features are evaluated. The DITCZ occurs over the central Pacific throughout the year, while its occurrence is mainly limited to March-April over the eastern Pacific and the Atlantic and in December over the Indian Ocean. Annual variations of the occurrence, vertical extent, strength and latitudinal positions of the DITCZ bands are mainly driven by the combined effect of SWD and SST variations. The significant differential cloud radiative heating between the equator and the DITCZ bands, derived from the profiles of cloud water content, suggests that it would feedback in enhancing the middle and upper tropospheric wind convergence and downdraft over the equator and contribute to sustain the DITCZ.

The Indo-Pacific Warm Pool (IPWP) significantly influences the global hydrological cycle through its impact on atmospheric-oceanic circulation. However, gaining a comprehensive understanding of the hydrologic climate dynamics within the IPWP and its broader effects on the global climate have been hindered by spatial and temporal limitations in paleoclimate records on orbital timescales. In this study, we reconstructed precipitation records (approximated from δ18Osw-ivc) over the past 450 kyr, based on planktonic foraminiferal Mg/Ca and δ18O data obtained from International Ocean Discovery Program Site U1486 in the western tropical Pacific. The δ18Osw-ivc record revealed a generally consistent pattern with precession variations over the past 450 kyr, closely corresponding to changes in boreal summer insolation at the equator. The δ18Osw-ivc record displayed an anti-phased relationship with Chinese speleothem δ18O records on the precession band, with lower precipitation in the western tropical Pacific and higher precipitation in the East Asia summer monsoon region during periods of high Northern Hemisphere summer insolation. This anti-phased correlation primarily resulted from the north-south migration of the Intertropical Convergence Zone (ITCZ), influenced by the interhemispheric insolation contrast. By considering additional δ18Osw-ivc records from various locations within the IPWP region, we identified synchronous precipitation changes within the IPWP on the precession band. The synchronization of precipitation on both margins of the ITCZ’s seasonal range and differences between central and marginal regions of the ITCZ within the IPWP revealed the expansion and contraction of the ITCZ on precession band.

Previous studies on the relationship between polar motion and water mass change have mainly concentrated on the excitation of polar motion via global terrestrial water storage changes (TWSC). In view of the uneven distribution of global terrestrial water storage, the relationship between regional water mass change and polar motion needs to be further explored owing to the lack of documented results. In addition, given the uncertainty in the estimation of TWSC, it is required to develop appropriate indices to describe water mass change from different perspectives. The Amazon River basin (referred to Amazon hereafter), containing the world’s largest river, located at around the 90°W longitude, is selected as the study area. Water vapor flux, precipitation, runoff and TWSC are selected as the indices of water mass changes to reveal the relationship between polar motion and water mass change in this giant basin. The Mann-Kendall (M-K) method, the accumulated anomaly analysis method and the curvature method are used to identify the abrupt change points; the least squares method is used to estimate the trends, and the Fast Fourier Transform (FFT) and the Ensemble Empirical Mode Decomposition (EEMD) are used to perform a periodic analysis, for all the above indices. It is shown that, of all the indices from 1948 to 2011, water vapor flux is the most closely related index to polar motion. In detail, precipitation and water vapor flux contain beat periods of polar motion; water vapor flux, precipitation and polar motion have a common M-K test abrupt change point (occurring in ca. 1968) at the 0.05 significance level; water vapor flux has a similar accumulated anomaly curve with that of polar motion; and water vapor flux is more highly correlated with polar motion than most other indexes. It is found, just like global TWSC, the χ 2 component of the excitation via water vapor flux and water storage change in the Amazon follows that of observed polar motion; χ 1 does not follow. However, the pattern in the Amazon that the χ 2 component of the excitation by water follows that of observed polar motion is at a more significant level than in global. Finally, the new index termed Location of Vapor-based InterTropical Convergence Zone (LVITCZ) we proposed to describe the annual mean latitudinal location of water mass change shows a more close and visual relationship between water mass change and polar motion than other chosen indices do.

This study presents the global climate model IPSL‐CM6A‐LR developed at Institut Pierre‐Simon Laplace (IPSL) to study natural climate variability and climate response to natural and anthropogenic forcings as part of the sixth phase of the Coupled Model Intercomparison Project (CMIP6). This article describes the different model components, their coupling, and the simulated climate in comparison to previous model versions. We focus here on the representation of the physical climate along with the main characteristics of the global carbon cycle. The model's climatology, as assessed from a range of metrics (related in particular to radiation, temperature, precipitation, and wind), is strongly improved in comparison to previous model versions. Although they are reduced, a number of known biases and shortcomings (e.g., double Intertropical Convergence Zone [ITCZ], frequency of midlatitude wintertime blockings, and El Niño–Southern Oscillation [ENSO] dynamics) persist. The equilibrium climate sensitivity and transient climate response have both increased from the previous climate model IPSL‐CM5A‐LR used in CMIP5. A large ensemble of more than 30 members for the historical period (1850–2018) and a smaller ensemble for a range of emissions scenarios (until 2100 and 2300) are also presented and discussed. Plain Language Summary Climate models are unique tools to investigate the characteristics and behavior of the climate system. While climate models and their components are developed gradually over the years, the sixth phase of the Coupled Model Intercomparison Project (CMIP6) has been the opportunity for the Institut Pierre‐Simon Laplace to develop, test, and evaluate a new configuration of its climate model called IPSL‐CM6A‐LR. The characteristics and emerging properties of this new model are presented in this study. The model climatology, as assessed from a range of metrics, is strongly improved, although a number of biases common to many models do persist. The equilibrium climate sensitivity and transient climate response have both increased from the previous climate model IPSL‐CM5A‐LR used in CMIP5. Key Points The IPSL‐CM6A‐LR model climatology is much improved over the previous version, although some systematic biases and shortcomings persist A long preindustrial control and a large number of historical and scenario simulations have been performed as part of CMIP6 The effective climate sensitivity of the IPSL model increases from 4.1 to 4.8 K between IPSL‐CM5A‐LR and IPSL‐CM6A‐LR

The intertropical convergence zone, where global rainfall is greatest, is a narrow belt of clouds usually centred about six degrees north of the Equator; this Review links its migrations on various timescales to the atmospheric energy balance. Factors behind ITCZ movement The Intertropical Convergence Zone (ITCZ), a vast area of high precipitation encircling much of the tropics, is not stable. It moves north and south on seasonal to millennial time scales. This Review presents a unified conceptual framework that explains ITCZ migrations in terms of the atmospheric energy balance, based on historical and palaeoclimate records. The authors propose that ITCZ location is governed by energy flux into and transport across the tropics. For example, as at present, greater atmospheric energy transport from north to south will place the ITCZ in the Northern Hemisphere. Cold periods, which may reverse the energy transport, will lead to southerly ITCZ movement. The ITCZ position is sensitive to slight shifts in the atmospheric energy balance however, because the factors controlling it are small differences between large terms. Rainfall on Earth is most intense in the intertropical convergence zone (ITCZ), a narrow belt of clouds centred on average around six degrees north of the Equator. The mean position of the ITCZ north of the Equator arises primarily because the Atlantic Ocean transports energy northward across the Equator, rendering the Northern Hemisphere warmer than the Southern Hemisphere. On seasonal and longer timescales, the ITCZ migrates, typically towards a warming hemisphere but with exceptions, such as during El Niño events. An emerging framework links the ITCZ to the atmospheric energy balance and may account for ITCZ variations on timescales from years to geological epochs.

Although it is well established that transpiration contributes much of the water for rainfall over Amazonia, it remains unclear whether transpiration helps to drive or merely responds to the seasonal cycle of rainfall. Here, we use multiple independent satellite datasets to show that rainforest transpiration enables an increase of shallow convection that moistens and destabilizes the atmosphere during the initial stages of the dry-to-wet season transition. This shallow convection moisture pump (SCMP) preconditions the atmosphere at the regional scale for a rapid increase in rain-bearing deep convection, which in turn drives moisture convergence and wet season onset 2–3 mo before the arrival of the Intertropical Convergence Zone (ITCZ). Aerosols produced by late dry season biomass burning may alter the efficiency of the SCMP. Our results highlight the mechanisms by which interactions among land surface processes, atmospheric convection, and biomass burning may alter the timing of wet season onset and provide a mechanistic framework for understanding how deforestation extends the dry season and enhances regional vulnerability to drought.

Understanding observed changes to the global water cycle is key to predicting future climate changes and their impacts. While many datasets document crucial variables such as precipitation, ocean salinity, runoff, and humidity, most are uncertain for determining long-term changes. In situ networks provide long time series over land, but are sparse in many regions, particularly the tropics. Satellite and reanalysis datasets provide global coverage, but their long-term stability is lacking. However, comparisons of changes among related variables can give insights into the robustness of observed changes. For example, ocean salinity, interpreted with an understanding of ocean processes, can help cross-validate precipitation. Observational evidence for human influences on the water cycle is emerging, but uncertainties resulting from internal variability and observational errors are too large to determine whether the observed and simulated changes are consistent. Improvements to the in situ and satellite observing networks that monitor the changing water cycle are required, yet continued data coverage is threatened by funding reductions. Uncertainty both in the role of anthropogenic aerosols and because of the large climate variability presently limits confidence in attribution of observed changes.

The Paleocene—Eocene Thermal Maximum (PETM; 56 Ma) is one of our best geological analogs for understanding climate dynamics in a “greenhouse” world. However, proxy data representing the event are only available from select marine and terrestrial sedimentary sequences that are unevenly distributed across Earth’s surface, limiting our view of the spatial patterns of climate change. Here, we use paleoclimate data assimilation (DA) to combine climate model and proxy information and create a spatially complete reconstruction of the PETM and the climate state that precedes it (“PETM-DA”). Our data-constrained results support strong polar amplification, which in the absence of an extensive cryosphere, is related to temperature feedbacks and loss of seasonal snow on land. The response of the hydrological cycle to PETM warming consists of a narrowing of the Intertropical Convergence Zone, off-equatorial drying, and an intensification of seasonal monsoons and winter storm tracks. Many of these features are also seen in simulations of future climate change under increasing anthropogenic emissions. Since the PETM-DA yields a spatially complete estimate of surface air temperature, it yields a rigorous estimate of global mean temperature change (5.6 ◦C; 5.4 ◦C to 5.9 ◦C, 95% CI) that can be used to calculate equilibrium climate sensitivity (ECS). We find that PETM ECS was 6.5 ◦C (5.7 ◦C to 7.4 ◦C, 95% CI), which is much higher than the present-day range. This supports the view that climate sensitivity increases substantially when greenhouse gas concentrations are high.

Compared to the eastern Pacific and eastern Atlantic, tropical cyclogenesis associated with breakdown of the intertropical convergence zone (ITCZ) was less documented over the western North Pacific (WNP). A typical case in boreal summer 2006 over the WNP, in which tropical cyclogenesis is induced by ITCZ breakdown in association with synoptic‐scale wave train (SWT), is examined through observation analysis and numerical simulations. Observational analysis displays that a northwest–southeast‐oriented SWT developed and propagated northwestward several days prior to ITCZ breakdown. Furthermore, the comparisons of simulation results reveal the role of the SWT in ITCZ breakdown. On one hand, the SWT within the ITCZ region is conducive to break down the ITCZ by enhancing the potential vorticity (PV), strengthening the meridional PV gradient and producing an evident sign reversal of PV gradient. On the other hand, the anomalous anticyclonic circulation related to the SWT north of ITCZ can reduce the meridional scale of PV band and thus increase the meridional PV gradient to accelerate ITCZ breakdown.