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The Indian summer monsoon rainfall (ISMR) during June to September contributes most of the annual rainfall over India and plays an important role in Indian agriculture and thus the economy. It exhibits high spatio-temporal variabilities forced from both internal and external factors, which are important for better understanding and prediction of ISMR. Since the internal factors, mainly in the form of intraseasonal oscillations set a limit to the predictability, the major focus is given to the external forcing factors including the coupled air–sea interactions, sea surface temperature variations, snow cover, etc. This paper mainly aims to review the results of recent research analysis on ISMR variability and the major climate factors that determine the variability. Focus is given on the contributions from the coupled ocean–atmosphere processes in the Indian and Pacific Oceans to the ISMR variability [(primarily the El Niño Southern Oscillation (ENSO)] and Indian Ocean Dipole (IOD). Several studies were carried out in recent decades to explore the ISMR variabilities and their influences from tropical oceans. The studies, which focused the impact of ENSO and IOD on the ISMR variability have been considered in exploring their relationships and observed changes in recent decades. In the backdrop of varying relationship of ISMR with ENSO and IOD in the regional scale, it is important to study further the regional teleconnection of ISMR variabilities with oceanic factors, especially from the Indian and Pacific Ocean basin.

In this work, the roles of El Niño–Southern Oscillation (ENSO) in the variability and predictability of the Pacific–North American (PNA) pattern and precipitation in North America in winter are examined. It is noted that statistically about 29% of the variance of PNA is linearly linked to ENSO, while the remaining 71% of the variance of PNA might be explained by other processes, including atmospheric internal dynamics and sea surface temperature variations in the North Pacific. The ENSO impact is mainly meridional from the tropics to the mid–high latitudes, while a major fraction of the non-ENSO variability associated with PNA is confined in the zonal direction from the North Pacific to the North American continent. Such interferential connection on PNA as well as on North American climate variability may reflect a competition between local internal dynamical processes (unpredictable fraction) and remote forcing (predictable fraction). Model responses to observed sea surface temperature and model forecasts confirm that the remote forcing is mainly associated with ENSO and it is the major source of predictability of PNA and winter precipitation in North America.

This study investigates the impact of ship emissions on clouds over a shipping corridor in the southeastern Atlantic, employing geostationary-based observations, which have not been previously used in studies of this kind. Based on CLAAS-3, the 20-year (2004-2023) CLoud property dAtA set using SEVIRI (the geostationary Spinning Enhanced Visible and InfraRed Imager), the diurnal, seasonal and long-term corridor effects on clouds are examined. Results show a significant impact of ship emissions on cloud microphysics, consistent with the Twomey effect: an increase in cloud droplet number concentration (N.sub.d) and a decrease in effective radius (r.sub.e). Additionally, cloud liquid water path (W) decreases, though changes in cloud fraction are more subtle. No clear impact on cloud optical thickness is found, implying an overall minor radiative effect of the ship emissions, although methodological limitations to detect changes in the corridor cannot be excluded. Seasonal and diurnal variations of the impact are evident, influenced by regional conditions and by the cloud thinning during the day, respectively. The long-term analysis reveals a weakening of the shipping corridor effect on N.sub.d and r.sub.e, presumably following the International Maritime Organization's 2020 stricter regulations on sulfur emissions, and broader regional changes in W and cloud fraction, associated with sea surface temperature variations. Focusing on a climatically important cloud regime, and including novel aspects, namely the diurnal and full seasonal cycle analyses, this study highlights the advantages and potential of geostationary satellite-based cloud observations for studying aerosol-cloud interactions.

Soil temperature regulates biogeochemical processes and is a key environmental factor affecting salt marsh ecosystems. Previous studies on soil temperature and heat transport in intertidal marshes predominantly focused on short‐term changes, leaving seasonal variations unclear. This study conducted a yearlong field and modeling investigation to examine temporal and spatial temperature variations in a creek‐marsh section under estuarine and meteorological influences. The results showed that inundating tidal water propagated the seasonal water temperature signal to marsh soils, especially at low elevations, thereby modulating air temperature‐induced soil temperature variations and distributions. The response of soil temperature to air and tidal water temperatures exhibited a damped and delayed pattern. In contrast, tide‐induced porewater circulations near the creek facilitated the temperature responses. A regression model incorporating a Gamma distribution function was developed to quantify the delayed and cumulative thermal effects of tidal water and air on shallow soil temperatures. The model coefficients varied along the creek‐marsh section, capturing the seasonal regulation of periodic tidal inundation on soil temperature in the low‐elevation marsh and near the creek. Sensitivity analyses indicated that relative sea level rise lowered yearly‐averaged temperatures for the marsh platform by enhancing latent heat export. Surface water warming increased the marsh temperatures at lower elevations. This study demonstrates that the creek‐marsh topography, warming, and relative sea level rise jointly affect soil temperature dynamics, advancing our understanding of temperature‐dependent biogeochemical processes in marsh ecosystems. Key Points Seasonal temperature distributions along a creek‐marsh section were identified Shallow soil temperature dynamics were predominated by seasonal tidal water and air temperatures Relative sea level rise decreased the yearly‐averaged temperature in the high‐elevation marsh

Low-frequency changes in the tropical Indian Ocean surface temperature have previously been investigated in the context of the Indian Ocean basin-wide (IOBM) and dipole (IOD) modes. The IOBM and IOD are the leading eigenmodes estimated from a traditional anomaly of SST. This approach ignores the possibility of multiple seasonal cycles (SCs) having different geographic patterns and interannually modulating amplitudes. The analyses presented here are anchored on the four sets of multivariate seasonal cycles independently extracted from the monthly observations of sea surface temperature (SST), surface wind, and surface pressure variations. We show that the secular warming, encapsulated by the monotonic variations of the first SC of SST (SST–SC1), differs from the previous linear trend patterns and has the most significant variance in the Indian Ocean Warm Pool (IOWP). Hence, these warming tendencies quantify the monotonic expansion rates of IOWP. The most significant interannual responses of Indian Ocean SST to remote forces (such as El Niño and La Niña) are also captured by SST–SC1. Unlike the traditional IOBM but similar to SST–SC1’s secular warming, these remotely forced interannual signals also have considerable variances in IOWP. The interannual variations in SST’s third seasonal cycle (i.e., SST–SC3) inherit SST–SC3’s dipole pattern but diverge from classical IOD in many aspects and are predominantly controlled by local processes. However, they are insufficient to account for the total interannual signals on their own. The collective interannual variations of four seasonal cycles—with significant variances off Africa’s eastern shores—demonstrate basin-wide unipolar patterns. Hence, SST interannual signals in the north-western Indian Ocean and the constantly growing warming in the IOWP influence climate and weather over countries surrounding the Indian Ocean. Thus, this study offers a simple way to separate three types of climate signals: secular, internal, and remotely induced climate fluctuations.

Multidecadal variations in the global land monsoon were observed during the twentieth century, with an overall increasing trend from 1901 to 1955 that was followed by a decreasing trend up to 1990, but the mechanisms governing the above changes remain inconclusive. Based on the outputs of two atmospheric general circulation models (AGCMs) forced by historical sea surface temperature (SST) covering the twentieth century, supplemented with AGCM simulations forced by idealized SST anomalies representing different conditions of the North Atlantic and tropical Pacific, evidence shows that the observed changes can be partly reproduced, particularly over the Northern Hemisphere summer monsoon (NHSM) domain, demonstrating the modulation of decadal SST changes on the long-term variations in monsoon precipitation. Moisture budget analysis is performed to understand the interdecadal changes in monsoon precipitation, and the dynamic term associated with atmospheric circulation changes is found to be prominent, while the contribution of the thermodynamic term associated with humidity changes can lead to coincident wetting over the NHSM domain. The increase (decrease) in NHSM land precipitation during 1901–55 (1956–90) is associated with the strengthening (weakening) of NHSM circulation and Walker circulation. The multidecadal scale changes in atmospheric circulation are driven by SST anomalies over the North Atlantic and the Pacific. A warmer North Atlantic together with a colder eastern tropical Pacific and a warmer western subtropical Pacific can lead to a strengthened meridional gradient in mid-to-upper-tropospheric thickness and strengthened trade winds, which transport more water vapor into monsoon regions, leading to an increase in monsoon precipitation.

Variations in snow depth have complex effects on soil microclimate. Snow insulates soil and thus regulates, along with air temperature, the nature, and extent of soil freezing. There is great uncertainty about the main drivers of soil freezing, which have important effects on ecosystem carbon and nitrogen cycling processes and might change as climate warms and snowfall decreases as part of climate change. Here, we utilitze sites from a variety of elevations and aspects within the northern hardwood forest at the Hubbard Brook Experimental Forest (New Hampshire, USA) to investigate relationships between seasonal snowpack, soil freezing, and soil microclimate across this gradient using 8 years of bi-weekly snowpack and soil frost-depth measurements, and continuous soil climate monitoring. We utilize a time-integrated snowpack descriptor and find that snowpacks with lower seasonal snow water equivalents result in more soil temperature variation and deeper soil frost but have no effect on variation in soil moisture. Seasonal snow water equivalent of the snowpack influences the date of rapid soil warming in the spring, which in turn influences both summer soil moisture and an index of annual cumulative soil heat. These results show that snowpack dynamics, which are highly sensitive to changes in climate, have wide-ranging effects on soil microclimate year-round and thus could have important implications for ecosystem carbon and nitrogen cycling processes.

In this paper, we present a semi-coupled theory to compute the temperature variation due to the tide-generating force. The tidal volume strain is first derived in a pure elastic homogeneous sphere, in terms of the classic Love’s solution. Then the temperature variation is obtained by solving the inhomogeneous heat conduction equation by considering both the isothermal and adiabatic conditions on the surface. The results show that the magnitude of the tidal temperature variation can be more than 1 mK, which is detectable by the current precision thermometer.

To understand inter-annual to decadal summer monsoon precipitation variations in Myanmar, we developed new tree-ring chronologies of Tectona grandis (teak) from three sampling sites in north-central Myanmar where the climatic proxy is sparse. A regional chronology (spanning 1700–2016) derived from three site chronologies showed a strongly positive precipitation sensitivity during the summer monsoon (R = 0.71), indicating that slow tree growth was detected in years of deficient precipitation. We reconstructed monsoon precipitation (May–October) for the period 1770–2016, with robust calibration-verification statistics. Our reconstruction revealed 22 (16) extremely dry (wet) years over the past 247 years. Several dry and wet episodes recorded in our reconstruction are consistent with other precipitation proxies from tropical Asia, such as the East Indian drought in 1790–1796 and the Victorian Holocaust drought in 1888–1890. The 2.0–4.0-year high-frequency periodicities revealed from spectral peaks and dominant regions of high spatial correlations indicated the summer precipitation in Myanmar is linked with broader-scale ocean-atmospheric circulations, mainly associated with the El Niño-Southern Oscillation (ENSO) activities due to sea surface temperature variations in the tropical Pacific Ocean. Coherent relationships of our reconstructed series with ENSO-related climate indices further support the dynamics of monsoon precipitation variability in Myanmar is inter-linked with global climate systems. Our reconstruction inferred from teak tree rings may be useful to provide valuable insight into the impacts of extreme weather events associated with monsoon hydroclimate in Myanmar.

Through the latter half of the twentieth century, meridional shifts in tropical precipitation have been associated with severe droughts. Although linked to a variety of causes, the origin of these shifts remains elusive. Here, it is shown that they are unlikely to arise from internal variability of the climate system alone, as simulated by coupled ocean–atmosphere climate models. Similar to previous work, the authors find that anthropogenic and volcanic aerosols are the dominant drivers of simulated twentieth-century tropical precipitation shifts. Models that include the cloud-albedo and lifetime aerosol indirect effects yield significantly larger shifts than models that lack aerosol indirect effects and also reproduce most of the southward tropical precipitation shift in the Pacific. However, all models significantly underestimate the magnitude of the observed shifts in the Atlantic sector, unless driven by observed SSTs. Mechanistically, tropical precipitation shifts are driven by interhemispheric sea surface temperature variations, which are associated with hemispherically asymmetric changes in low-latitude surface pressure, winds, and low clouds, as well as the strength, location, and cross-equatorial energy transport of the Hadley cells. Models with a larger hemispheric aerosol radiative forcing gradient yield larger hemispheric temperature contrasts and, in turn, larger meridional precipitation shifts. The authors conclude that aerosols are likely the dominant driver of the observed southward tropical precipitation shift in the Pacific. Aerosols are also significant drivers of the Atlantic shifts, although one cannot rule out a contribution from natural variability to account for the magnitude of the observed shifts.