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The North Pacific Oscillation (NPO) serves as a pivotal atmospheric circulation pattern within the North Pacific. It has a considerable influence on the climate of East Asia and North America in the North Pacific Basin, and can also affect the climate of more distant regions via the atmospheric teleconnections. Our study has revealed a significant eastward shift of the southern lobe of the NPO from 178°E to 155°W during the boreal winter (November–December–January–February–March, NDJFM) after 1993/1994 in observations. The eastward shift of the NPO after 1993/1994 leads to stronger subtropical northeasterly trade winds during the boreal winter over the eastern North Pacific. As the strongest climatic factor influencing the strength of the wind-evaporation-sea temperature (WES) feedback mechanism, the enhanced subtropical northeast trade winds after 1993/1994 stimulate a more efficient WES feedback mechanism over the eastern North Pacific. Furthermore, the intensification of the WES feedback mechanism after 1993/1994 subsequently enhances the influence of the NPO on the tropical Pacific sea surface temperature (SST).

The strengthening of the Pacific trade winds in recent decades has been unmatched in the observational record stretching back to the early twentieth century. This wind strengthening has been connected with numerous climate-related phenomena, including accelerated sea-level rise in the western Pacific, alterations to Indo-Pacific ocean currents, increased ocean heat uptake, and a slow-down in the rate of global-mean surface warming. Here we show that models in the Coupled Model Intercomparison Project phase 5 underestimate the observed range of decadal trends in the Pacific trade winds, despite capturing the range in decadal sea surface temperature (SST) variability. Analysis of observational data suggests that tropical Atlantic SST contributes considerably to the Pacific trade wind trends, whereas the Atlantic feedback in coupled models is muted. Atmosphere-only simulations forced by observed SST are capable of recovering the time-variation and the magnitude of the trade wind trends. Hence, we explore whether it is the biases in the mean or in the anomalous SST patterns that are responsible for the under-representation in fully coupled models. Over interannual time-scales, we find that model biases in the patterns of Atlantic SST anomalies are the strongest source of error in the precipitation and atmospheric circulation response. In contrast, on decadal time-scales, the magnitude of the model biases in Atlantic mean SST are directly linked with the trade wind variability response.

This study simultaneously examines wind speed trends at the land–ocean interface, and below–above the trade-wind inversion layer in the Canary Islands and the surrounding Eastern North Atlantic Ocean: a key region for quantifying the variability of trade-winds and its response to large-scale atmospheric circulation changes. Two homogenized data sources are used: (1) observed wind speed from nine land-based stations (1981–2014), including one mountain weather station (Izaña) located above the trade-wind inversion layer; and (2) simulated wind speed from two atmospheric hindcasts over ocean (i.e., SeaWind I at 30 km for 1948–2014; and SeaWind II at 15 km for 1989–2014). The results revealed a widespread significant negative trend of trade-winds over ocean for 1948–2014, whereas no significant trends were detected for 1989–2014. For this recent period wind speed over land and ocean displayed the same multi-decadal variability and a distinct seasonal trend pattern with a strengthening (late spring and summer; significant in May and August) and weakening (winter–spring–autumn; significant in April and September) of trade-winds. Above the inversion layer at Izaña, we found a predominance of significant positive trends, indicating a decoupled variability and opposite wind speed trends when compared to those reported in boundary layer. The analysis of the Trade Wind Index (TWI), the North Atlantic Oscillation Index (NAOI) and the Eastern Atlantic Index (EAI) demonstrated significant correlations with the wind speed variability, revealing that the correlation patterns of the three indices showed a spatio-temporal complementarity in shaping wind speed trends across the Eastern North Atlantic.

The intertropical convergence zone (ITCZ) is a region encircling the globe near the equator where the surface trade winds of both the hemispheres meet. It is characterized by deep convective clouds and accounts for more than 30% of global precipitation. Conventionally, parameters such as low outgoing longwave radiation (OLR), high precipitation and strong surface wind convergence (SWC) are used to identify and characterize the ITCZ. These parameters suffer from some inherent issues while identifying ITCZ. A new method based on the identification of deep convective cloud cores (DCCCs), derived from brightness temperature (T B ) data of the water vapour absorption channels of Sondeur Atmosphérique du Profil d’Humidité Intertropicale par Radiométrie (SAPHIR), aboard Megha-Tropiques (MT) satellite is proposed in this work. The present method identifies the DCCCs and performs better over (a) anvils and cirrus dominated areas where OLR based methods face difficulties, (b) coastal and continental areas where satellite scatterometer data are unavailable to estimate surface convergence and (c) windward side of orographic regions where rainfall based methods fail. The global mean position of ITCZ is north of the equator, with maximum northward migration up to 24 ∘ N during June–August over the Asian summer monsoon region and maximum southward migration up to 20 ∘ S during December–February over the Indian and the Pacific Oceans. The ITCZ has a broad structure over the West Pacific Ocean (WPO) and Indian Ocean (IO), extending from 10 ∘ N to 10 ∘ S latitudes, and has a narrow-band structure over the Central Pacific Ocean (CPO), East Pacific Ocean (EPO), and Atlantic Ocean (AO). Over AO and WPO, the ITCZ has reduced strength in the occurrence frequency of DCCC (OFD) considerably during El-Ni n ~ o periods compared to normal years. The trend in DCCC over the tropics is insignificant during the study period of 2011–2018. Low orbital inclination satellites similar to Megha-Tropiques, with the primary objective of repeated measurements of water vapour and energy budget parameters over the tropics, are of high demand to the research community for data continuity.

A conceptual model connecting seasonal loss of Arctic sea ice to midlatitude extreme weather events is applied to the 21st-century intensification of Central Pacific trade winds, emergence of Central Pacific El Nino events, and weakening of the North Pacific Aleutian Low Circulation. According to the model, Arctic Ocean warming following the summer sea-ice melt drives vertical convection that perturbs the upper troposphere. Static stability calculations show that upward convection occurs in annual 40- to 45-d episodes over the seasonally ice-free areas of the Beaufort-to-Kara Sea arc. The episodes generate planetary waves and higher-frequency wave trains that transport momentum and heat southward in the upper troposphere. Regression of upper tropospheric circulation data on September sea-ice area indicates that convection episodes produce wave-mediated teleconnections between the maximum ice-loss region north of the Siberian Arctic coast and the Intertropical Convergence Zone (ITCZ). These teleconnections generate oppositely directed trade-wind anomalies in the Central and Eastern Pacific during boreal winter. The interaction of upper troposphere waves with the ITCZ air–sea column may also trigger Central Pacific El Nino events. Finally, waves reflected northward from the ITCZ air column and/or generated by triggered El Nino events may be responsible for the late winter weakening of the Aleutian Low Circulation in recent years.

The Victoria mode (VM), as a basin-scale sea surface temperature (SST) pattern over the North Pacific, is suggested to facilitate subsequent development of El Niño–Southern Oscillation (ENSO) through both the seasonal footprinting mechanism (SFM) and the trade wind charging (TWC) mechanism. The present study aims at investigating the distinct roles and relative contributions to ENSO of the SFM and the TWC mechanism associated with the VM using atmospheric and oceanic reanalysis data as well as modeling simulations. Our results reveal that the positive SST anomalies (SSTAs) over the subtropical northeast Pacific (SNP) related to the VM effectively trigger the initiation of ENSO via the SFM, which emphasizes an air–sea surface thermodynamic-coupling process. In contrast, the negative SSTAs over the western North Pacific (WNP) associated with the VM primarily induce ENSO via a thermocline–SST feedback process, known as the TWC mechanism. Further analysis indicates that the SFM related to the VM may play a relatively independent role in affecting ENSO and is more closely linked to ENSO than is the TWC mechanism related to the VM, which is shown to be reasonably reproduced by the Community Earth System Model. Additionally, the SFM associated with the positive (negative) SNP SSTAs may induce fewer El Niño events (more La Niña events) than the TWC mechanism related to the positive (negative) WNP SSTAs. Our findings suggest that the SFM and the TWC mechanism associated with the VM both contribute to enhanced predictive skill for ENSO.

The El Niño Southern Oscillation (ENSO) is one of the most prominent modes of coupled variability with sizable impacts on global climate and weather patterns, which makes the ability to predict the occurrence and development of ENSO events of fundamental importance. In order to achieve accurate and timely predictions, a well-established strategy is to understand and monitor known ENSO precursors. In this paper, we focus on North Pacific Oscillation (NPO)-related precursors, namely the trade wind charging and the Northern Pacific meridional mode (TWC/NPMM). In particular, we assess whether the TWC/NPMM mode and its relationship with ENSO is reconstructed across the CMIP6 protocol-driven ensemble High Resolution Model Intercomparison Project, which was developed to systematically test the impact of increased horizontal resolution. Here, we see that the TWC/NPMM is a consistent precursor of ENSO across the ensemble, notwithstanding some spatial variations in the reconstruction. Furthermore, previous analyses on observationally-based data show that the TWC/NPMM-ENSO relationship is robust, albeit not stationary, and its variations can influence the characteristic variability of ENSO itself. In particular, during those years when the TWC/NPMM-ENSO coupling is weak, ENSO oscillates regularly with constant periodicity; whereas, when the coupling is strong, ENSO shows a more stochastic behavior. A selected subset of better-performing HighResMIP models are able to reproduce the non-stationarity of the TWC/NPMM-ENSO coupling and to recreate how these variations are reflected in the characteristics of ENSO variability, similar to what was recorded in the observational analysis. In parallel with these analyses, we also assess their sensitivity to horizontal resolution and find that there is no consistent impact of resolution on the results described above.

This paper examines the “too bright” issue pertaining to non‐planetary boundary layer (PBL) clouds over the South Pacific trade‐wind region and its potential link to the falling ice radiative effects (FIREs). We run sensitivity experiments with CESM2‐CAM6 (CESM2) global climate model with FIREs on (SON) and off (NOS). The model exhibits more in‐cloud liquid water content (CLWC) and droplet above the PBL in NOS, leading to larger shortwave (SW) reflectivity at the top of the atmosphere than in SON over the trade wind regions. CMIP6 models are divided into three subsets: separately calculates the radiative effects of cloud ice and falling ice (SON2), combined (SON1) and without falling ice (NOS). SON2 models exhibit improved CLWC and SW reflectivity similar to CESM2‐SON, while NOS and SON1 models are akin to CESM2‐NOS owing to weaker surface wind stress and warmer ocean surface, caused by the lack of FIREs over the convective zones. Plain Language Summary This paper examines why some clouds simulated by global climate models (GCMs) tend to reflect too much sunlight back into space (i.e., appear too bright) in the South Pacific trade‐wind region. It explores whether this is linked to the effects of falling ice (FIREs) over the convective zones of the Pacific Intertropical Convergence Zone (ITCZ). Using the CESM2‐CAM6 GCM, we conducted experiments with FIREs turned on (CESM2‐SON) and off (CESM2‐NOS), primarily occurring in the ITCZ. The results showed that when FIREs are turned off, the model predicts more liquid water and cloud droplets above the planetary boundary layer, causing these clouds to reflect more sunlight over the trade‐wind regions. Additionally, we analyzed different climate models from the Coupled Model Intercomparison Project (CMIP6), categorizing them into three groups: those that separately account for the radiative effects of cloud ice and falling ice (SON2), as in CESM2‐SON; those that combine these effects (SON1); and those without falling ice (NOS). The SON2 models showed a better vertical spatial distribution of liquid water content and sunlight reflectivity over the trade‐wind region, while the NOS and SON1 models performed similarly to CESM2‐NOS. Key Points Cloud properties in CESM2 with falling ice radiative effects (FIREs) (SON) are compared to without FIREs (NOS) over the trade‐wind region CESM2 NOS exhibits more liquid water content above the planetary boundary layer (PBL) with more SW reflectivity at top of the atmosphere (TOA) over the trade‐wind region compared to CESM2 SON Similar improvements are found in CESM2‐SON‐type subset of CMIP6 models, linked to stronger surface wind stress and cooler ocean surface

The boreal winter North Pacific Oscillation (NPO) has a significant impact on the tropical Pacific large scale air-sea interaction in the following spring and summer. But it is unclear whether the boreal winter NPO is associated with the El Niño development or decaying during following summer. This study confirms the relationship between the winter NPO-like sea level pressure anomalies and following summertime El Niño and reveals an interdecadal change in this relationship. It tends to promote the development of sea surface temperature anomaly in central equatorial Pacific in the following summer when the subtropical lobe of the NPO located west of its average position. But after a significant eastward shift of NPO’s subtropical lobe, it is more conducive to the development of sea surface temperature anomaly in eastern equatorial Pacific. The eastward shift of the NPO’s subtropical lobe altered both the seasonal foortprint mechanism and the trade wind charging mechanism associated with the NPO, thus profoundly influenced the summertime El Niño. The findings in the present study have implications for a better understanding of the relationship between the boreal winter NPO and the El Niño in developing summer.

We investigate whether and how spatial organization affects the pathway to precipitation in large‐domain hectometer simulations of the North Atlantic trades. We decompose the development of surface precipitation (P) in warm shallow trade cumulus into a formation phase, where cloud condensate is converted into rain, and a sedimentation phase, where rain falls toward the ground while some of it evaporates. With strengthened organization, rain forms in weaker updrafts from smaller cloud droplets so that cloud condensate is less efficiently converted into rain. At the same time, organization creates a locally moister environment and modulates the microphysical conversion processes that determine the raindrops' size. This reduces evaporation and more of the formed rain reaches the ground. Organization thus affects how the two phases contribute to P, but only weakly affects the total precipitation efficiency. We conclude that the pathway to precipitation differs with organization and suggest that organization buffers rain development. Plain Language Summary Clouds in the trade‐wind region organize into a variety of spatial patterns. We investigate how this spatial organization influences rain development in simulations of trade‐wind convection. We divide the formation of surface precipitation into two phases. In the first phase, rain forms from the collision of cloud droplets or the collection of cloud droplets by raindrops. In the second phase, rain falls toward the ground while some of the rain evaporates. Our study shows that as organization strengthens, rain forms less efficiently, but a larger fraction of that rain reaches the ground as evaporation is reduced. Thus, organization in the simulations affects the way surface rain is generated. Our analyses suggest that it does so by modulating the cloud vertical motion in which rain forms, the local moisture environment through which rain falls and the microphysical conversion processes. Key Points The development of surface precipitation in simulated trade‐wind convection is decomposed into a formation and sedimentation phase As organization strengthens, less cloud condensate is converted into rain, but more rain reaches the ground as evaporation is suppressed Organization affects rain formation by modulating the local moisture environment, cloud vertical motion and microphysical properties