Interdecadal Pacific Oscillation Research Articles

A seesaw in the South Pacific western and eastern subtropical mode waters

Abstract The South Pacific Western Subtropical Mode Water (WSTMW) and Eastern Subtropical Mode Water (ESTMW) are important components of the subtropical meridional overturning cells. By synthesizing observations, reanalysis products, and eddy-resolving ocean model results, we find an interannual volume seesaw in the WSTMW and ESTMW during 1980-2020. Further analysis of the data and model outputs indicates that this volume seesaw is closely related to the El Niño–Southern Oscillation (ENSO) through its modulation of the Ekman pumping velocity, buoyancy fluxes, and mixed layer depth over the formation regions of the two water masses. This negative correlation between the WSTMW and ESTMW modulates the depth of the permanent pycnocline and directly contributes to the east-west contrast between the two water masses in their core layer properties, especially, during negative phase of the Interdecadal Pacific Oscillation (IPO) since 2000.

Climate Extremes in the New Zealand Region: Mechanisms, Impacts and Attribution

ABSTRACTAs global surface temperatures have increased with human‐induced climate change, notable compound climate extremes in the New Zealand (NZ) region associated with atmospheric heatwaves (AHWs) and marine heatwaves (MHWs) have occurred in the past 6 years. Natural modes of variability that also played a key role regionally include the Interdecadal Pacific Oscillation (IPO), El Niño/Southern Oscillation (ENSO) and changes in the location and strength of the westerlies as seen in the Southern Annular Mode (SAM). Along with mean warming of 0.8°C since 1900, a negative phase of the IPO, La Niña phase of ENSO and a strongly positive SAM contributed to five compound warm extremes in the extended austral summer seasons (NDJFM) of 1934/35, 2017/18, 2018/19, 2021/22 and 2022/23. These are the most intense coupled ocean/atmosphere (MHWs/AHWs) heatwaves on record with average temperature anomalies over land and sea +0.8°C to 1.1°C above 1991–2020 averages. The number of days above 25°C and above the 90th percentile of maximum temperature has increased, while the number of nights below 0°C and below the 10th percentile has decreased. Coastal waters around NZ recently experienced their longest MHW in the satellite era (1982‐present) of 289 days through 2023. The estimated recurrence interval reduces from 1 in 300‐years for the AHW event during the 1930s climate to a 1 in 25‐year event for the most recent decade. Consequences include major loss of ice of almost one‐third volume from Southern Alps glaciers from 2017 to 2021 with rapid melt of seasonal snow in all four cases. Above‐average temperatures in the December/January grape flowering period resulted in advances in veraison (the onset of ripening); and higher‐than‐average grape yields in 2022 and 2023 vintages. Marine impacts include widespread sea‐sponge bleaching around northern and southern NZ.

A climate change signal in the tropical Pacific emerges from decadal variability.

The eastern tropical Pacific has defied the global warming trend. There has been a debate about whether this observed trend is forced or natural (i.e., the Interdecadal Pacific Oscillation; IPO) and this study shows that there are two patterns, one that oscillates along with the IPO, and one that is emerging since the mid-1950s, herein called the Pacific Climate Change (PCC) pattern. Here we show these have distinctive and distinguishable atmosphere-ocean signatures. While the IPO features a meridionally broad wedge-shaped SST pattern, the PCC pattern is marked by a narrow equatorial cooling band. These different SST patterns are related to distinct wind-driven ocean dynamical processes. We further show that the recent trends during the satellite era are a combination of IPO and PCC. Our findings set a path to distinguish climate change signals from internal variability through the underlying dynamics of each.

Uncovering Interdecadal Pacific Oscillation’s Dominance in Shaping Low-Frequency Sea Level Variability in the South China Sea

The low-frequency sea level variability in the South China Sea (SCS) is examined using high-resolution regional ocean model simulations that span the last six decades. The analysis reveals interdecadal oscillations with a periodicity of 12–13 years as the dominant mode of sea level variability in the SCS. The fluctuations in the Luzon Strait transport (LST) are identified as primary drivers of interannual to interdecadal sea level variability, rather than atmospheric forcing within the SCS. Fourier spectrum analysis is employed to investigate the association between SCS sea level variability and the Interdecadal Pacific Oscillation (IPO), using principal components of SCS sea surface height anomalies, wind stress curl, wind stress components, net short wave flux, as well as the LST and various climate indices. The variations in the SCS sea level are driven by the IPO, which modifies the LST and ocean heat content, impacting the steric sea level.

Pacific Interannual and Multidecadal Variability Recorded in δ18O of South American Summer Monsoon Precipitation

AbstractThe South American summer monsoon (SASM) generates important hydroclimatic impacts in (sub‐)tropical South America and isotopic tracers recorded in paleoclimatic archives allow for assessing its long‐term response to Pacific variability prior to modern observations. Stable oxygen isotopes in precipitation integrate hydroclimatic changes during the SASM mature phase from December to February (DJF) in response to the Interdecadal Pacific Oscillation (IPO) and El Niño—Southern Oscillation (ENSO), respectively. Here, results from the isotope‐enabled Community Atmosphere Model v.5 are compared with highly resolved and precisely dated isotopic records from speleothems, tree rings, lake and ice cores during the industrial era (1880–2000 CE) and validated against observations from the International Atomic Energy Agency (IAEA) network. Pacific sea surface temperatures (SSTs) are coupled to the isotopic composition of SASM precipitation through perturbations in the Walker circulation associated with low‐ (IPO) and high‐frequency (ENSO) variability, impacting convective activity over tropical South America and the tropical Atlantic. Changes in convection over this monsoon entrance region ultimately control the downstream oxygen isotopic composition of precipitation recorded in paleoclimate archives. Overall, model results, paleoclimate records and IAEA data agree on the isotopic response to Pacific SST forcing. These results highlight the potential for long isotopic paleoclimate records to reconstruct Pacific climate variability on both high‐ and low‐frequency timescales. Furthermore, the isolation of the IPO signal in a diverse set of isotopic archives invites the reinterpretation of other paleoclimate proxies for identifying this historically overlooked forcing.

Coral Sr/Ca-SST reconstruction from Fiji extending to ~1370 CE reveals insights into the Interdecadal Pacific Oscillation.

The southwestern tropical Pacific is a key center for the Interdecadal Pacific Oscillation (IPO), which regulates global climate. This study introduces a groundbreaking 627-year coral Sr/Ca sea surface temperature reconstruction from Fiji, representing the IPO's southwestern pole. Merging this record with other Fiji and central tropical Pacific records, we reconstruct the SST gradient between the southwestern and central Pacific (SWCP), providing a reliable proxy for IPO variability from 1370 to 1997. This reconstruction reveals distinct centennial-scale temperature trends and insights into Pacific-wide climate impacts and teleconnections. Notably, the 20th century conditions, marked by simultaneous basin-scale warming and weak tropical Pacific zonal-meridional gradients, deviate from trends observed during the past six centuries. Combined with model simulations, our findings reveal that a weak SWCP gradient most markedly affects IPO-related rainfall patterns in the equatorial Pacific. Persistent synchronous western and central Pacific warming rates could lead to further drying climate across the Coral Sea region, adversely affecting Pacific Island nations.

Large-Scale Climate Modes Drive Low-Frequency Regional Arctic Sea Ice Variability

Abstract Summer Arctic sea ice is declining rapidly but with superimposed variability on multiple time scales that introduces large uncertainties in projections of future sea ice loss. To better understand what drives at least part of this variability, we show how a simple linear model can link dominant modes of climate variability to low-frequency regional Arctic sea ice concentration (SIC) anomalies. Focusing on September, we find skillful projections from global climate models (GCMs) from phase 6 of the Coupled Model Intercomparison Project (CMIP6) at lead times of 4–20 years, with up to 60% of observed low-frequency variability explained at a 5-yr lead time. The dominant driver of low-frequency SIC variability is the interdecadal Pacific oscillation (IPO) which is positively correlated with SIC anomalies in all regions up to a lead time of 15 years but with large uncertainty between GCMs and internal variability realization. The Niño-3.4 index and Atlantic multidecadal oscillation have better agreement between GCMs of being positively and negatively related, respectively, with low-frequency SIC anomalies for at least 10-yr lead times. The large variations between GCMs and between members within large ensembles indicate the diverse simulation of teleconnections between the tropics and Arctic sea ice and the dependence on the initial climate state. Further, the influence of the Niño-3.4 index was found to be sensitive to the background climate. Our results suggest that, based on the 2022 phases of dominant climate variability modes, enhanced loss of sea ice area across the Arctic is likely during the next decade. Significance Statement The purpose of this study is to better understand the drivers of low-frequency variability of Arctic sea ice. Teasing out the complicated relationships within the climate system takes a large number of examples. Here, we use 42 of the latest generation of global climate models to construct a simple linear model based on dominant named climate features to predict regional low-frequency sea ice anomalies at a lead time of 2–20 years. In 2022, these modes of variability happen to be in the phases most conducive to low Arctic sea ice concentration anomalies. Given the context of the longer-term trend of sea ice loss due to global warming, our results suggest accelerated Arctic sea ice loss in the next decade.

Performance of CMIP5 and CMIP6 models in reproducing the Interdecadal Pacific Oscillation and its global impacts

AbstractThis study assessed the capability of the historical simulations of phase 5 and 6 of the Coupled Model Intercomparison Project (CMIP5/6) in reproducing the temporal and spatial characteristics of the Interdecadal Pacific Oscillation (IPO) and its impact on global surface air temperature (SAT), surface equivalent potential temperature (Thetae_sfc) and precipitation. The IPO index time series simulated by CMIP5/6 models deviated from observations and struggled to capture the phase evolution characteristics of the IPO. Nevertheless, CMIP5/6 models successfully captured the horseshoe‐shaped sea surface temperature anomaly in the Pacific. Additionally, the CMIP5/6 models were able to simulate the IPO's 10–30‐year period. Notably, the simulated IPO index exhibited a statistically significant upward trend, which was absent in observations. Additionally, the IPO‐related global land SAT, Thetae_sfc and precipitation simulated by CMIP5/6 models performed differently in boreal winter and boreal summer. Furthermore, the IPO‐related global land SAT performed better in CMIP5/6 models during boreal winter than that in boreal summer. In CMIP6 models, it improved during both boreal winter and summer compared to CMIP5 models. In terms of the IPO‐related global land Thetae_sfc, CMIP5/6 models also performed better during boreal winter than in boreal summer. However, CMIP5 models outperformed CMIP6 models during the boreal summer. In terms of the IPO‐related global land precipitation, CMIP5/6 models performed better during boreal summer compared to boreal winter. Moreover, the IPO‐related global land precipitation in CMIP6 models improved significantly in boreal winter, but almost the same in boreal summer, compared to CMIP5 models. Further studies showed that the enhancements in simulating IPO's spatial pattern did not correspond to improvements in the model's ability to simulate IPO's global teleconnections.

Distinct Modulations of Northwest Pacific Tropical Cyclone Precipitation by Atlantic Multidecadal Oscillation and Interdecadal Pacific Oscillation

AbstractThe interdecadal variability of tropical cyclone precipitation (TCP) over the western North Pacific (WNP) has not been thoroughly explored in previous studies. Here, we show that the TCP variations are modulated by both the Atlantic Multidecadal Oscillation (AMO) and Interdecadal Pacific Oscillation (IPO) as evidenced by reanalysis data and model experiments. A clustering analysis of tropical cyclone tracks shows that the AMO dominates a dipole pattern of TCP anomalies in the South China Sea and along the coastal eastern China. Meanwhile, the IPO dominates TCP over the southeastern WNP. Further analyses show that the AMO, particularly its extratropical component, affects TCP over the WNP by triggering an eastward‐propagating Rossby‐wave train, resulting in a pair of anomalous gyres over the WNP. Contrastly, the IPO modulates TCP by stimulating tropical circulation anomalies via the tropical pathway. These findings shed light on improving near‐term TCP forecast and its regional influence on East Asia.

Unraveling the Role of the Interdecadal Pacific Oscillation in Recent Tropical Expansion via Large‐Ensemble Simulations

AbstractObservational evidence has shown that the Earth's tropics have widened since 1980. However, climate models underestimate the observed tropical expansion rate, with a large spread among individual models. The proposal of internal variability to account for model–observation differences is hindered by the limited availability of sufficient realizations from models in the Coupled Model Intercomparison Project (CMIP), restricting the accuracy of quantitative contribution estimation. The emergence of a single model initial‐condition large ensemble provides a new opportunity to quantify the role of internal variability. Here, using large‐ensemble simulations from two individual models complemented with CMIP Phase 6 (CMIP6) simulations, we show evidence that the recent widening of the tropics is mainly caused by internal variability related to the Interdecadal Pacific Oscillation (IPO). The positive‐to‐negative phase transition of the IPO from 1980 to 2014 reduced the meridional tropospheric temperature gradient, resulting in poleward shifts in tropical edges. After adjusting the IPO trends simulated by individual realizations to ensure consistency with the observations, the IPO phase transition can account for at least 73% (66%) of the observed tropical expansion rate in the Northern Hemisphere based on the metric of the meridional stream function (surface zonal wind). The IPO is also essential for shaping tropical expansion‐related precipitation changes. Our results underscore the significance of considering internal variability when explaining model–observation differences and understanding intermodel uncertainty.

Decadal Thermal Variability of the Upper Southern Ocean: Zonal Asymmetry

Abstract As the major sink of anthropogenic heat, the Southern Ocean has shown quasi-symmetric, deep-reaching warming since the mid-twentieth century. In comparison, the shorter-term heat storage pattern of the Southern Ocean is more complex and has notable impacts on regional climate and marine ecosystems. By analyzing observational datasets and climate model simulations, this study reveals that the Southern Ocean exhibits prominent decadal (>8 years) variability extending to ∼700-m depth and is characterized by out-of-phase changes in the Pacific and Atlantic–Indian Ocean sectors. Changes in the Pacific sector are larger in magnitude than those in the Atlantic–Indian Ocean sectors and dominate the total heat storage of the Southern Ocean on decadal time scales. Instead of heat uptake through surface heat fluxes, these asymmetric variations arise primarily from wind-driven heat redistribution. Pacemaker and preindustrial simulations of the Community Earth System Model version 1 (CESM1) suggest that these variations in Southern Ocean winds arise primarily from natural variability of the tropical Pacific, as represented by the interdecadal Pacific oscillation (IPO). Through atmospheric teleconnection, the positive phase of the IPO gives rise to higher-than-normal sea level pressure and anticyclonic wind anomalies in the 50°–70°S band of the Pacific sector. These winds lead to warming of 0–700 m by driving the convergence of warm water. The opposite processes, involving cyclonic winds and upper-layer divergence, occur in the Atlantic–Indian Ocean sector. These findings aid our understanding of the time-varying heat storage of the Southern Ocean and provide useful implications on initialized decadal climate prediction.

Following the Rain: Climates of Opinion

This article foregrounds La Niña’s role in establishing climate optimism in the southeastern states of Australia in the 1860s and 1870s. It examines the perceptions of climate as influenced by the cycles of the El Niño Southern Oscillation (ENSO), underscored by the decadal cycles of the Interdecadal Pacific Oscillation (IPO) and the influence in some areas of the intensity of the subtropical ridge. It finds an abundance of very wet La Niña years coincided with a key period of Australian immigration and the expansion of pastoralism and agriculture. The article concludes that this period of productivity and expansion substantially contributed to a false sense of optimism. It suggests nineteenth-century ideas about climate can illuminate contemporary cultural understandings.

Competing impacts of tropical Pacific and Atlantic on Southern Ocean inter-decadal variability

The observed Southern Ocean sea surface temperature (SST) has experienced prominent inter-decadal variability nearly in phase with the Inter-decadal Pacific Oscillation (IPO), but less associated with the Atlantic Multidecadal Variability (AMV), challenging the prevailing view of Pacific-Atlantic synergistic effects. Yet, the mechanisms of distinct trans-hemispheric connections to the Southern Ocean remain indecisive. Here, by individually constraining the observed cold-polarity and warm-polarity IPO and AMV SSTs in a climate model, we show that the IPO is influential in initiating a basin-wide Southern Ocean response, with the AMV secondary. A tropical Pacific-wide cooling triggers a basin-scale Southern Ocean cold episode through a strong Rossby wave response to the north-to-south cross-equatorial weakened Hadley circulation. By contrast, due to the competing role of tropical Pacific cooling, an Atlantic warming partly cools the Southern Ocean via a weak Rossby wave response to the south-to-north cross-equatorial enhanced Hadley circulation. Conversely, tropical Pacific warming leads to a warm Southern Ocean episode. Our findings highlight that properly accounting for the tropical Pacific SST variability may provide a potential for skillful prediction of Southern Ocean climate change and more reliable estimates of climate sensitivity, currently overestimated by the misrepresented Southern Ocean warming.

Quantifying Contributions of External Forcing and Internal Variability to Arctic Warming During 1900–2021

AbstractArctic warming has significant environmental and social impacts. Arctic long‐term warming trend is modulated by decadal‐to‐multidecadal variations. Improved understanding of how different external forcings and internal variability affect Arctic surface air temperature (SAT) is crucial for explaining and predicting Arctic climate changes. We analyze multiple observational data sets and large ensembles of climate model simulations to quantify the contributions of specific external forcings and various modes of internal variability to Arctic SAT changes during 1900–2021. We find that the long‐term trend and total variance in Arctic‐mean SAT since 1900 are largely forced responses, including warming due to greenhouse gases and natural forcings and cooling due to anthropogenic aerosols. In contrast, internal variability dominates the early 20th century Arctic warming and mid‐20th century Arctic cooling. Internal variability also explains ∼40% of the recent Arctic warming from 1979 to 2021. Unforced changes in Arctic SAT are largely attributed to two leading modes. The first is pan‐Arctic warming with stronger loading over the Eurasian sector, accounting for 70% of the unforced variance and closely related to the positive phase of the unforced Atlantic Multidecadal Oscillation (AMO). The second mode exhibits relatively weak warming averaged over the entire Arctic with warming over the North American‐Pacific sector and cooling over the Atlantic sector, explaining 10% of the unforced variance and likely caused by the positive phase of the unforced Interdecadal Pacific Oscillation (IPO). The AMO‐related changes dominate the unforced Arctic warming since 1979, while the IPO‐related changes contribute to the decadal SAT changes over the North American‐Pacific Arctic.

Sources of low-frequency variability in observed Antarctic sea ice

Abstract. Antarctic sea ice has exhibited significant variability over the satellite record, including a period of prolonged and gradual expansion, as well as a period of sudden decline. A number of mechanisms have been proposed to explain this variability, but how each mechanism manifests spatially and temporally remains poorly understood. Here, we use a statistical method called low-frequency component analysis to analyze the spatiotemporal structure of observed Antarctic sea ice concentration variability. The identified patterns reveal distinct modes of low-frequency sea ice variability. The leading mode, which accounts for the large-scale, gradual expansion of sea ice, is associated with the Interdecadal Pacific Oscillation and resembles the observed sea surface temperature trend pattern that climate models have trouble reproducing. The second mode is associated with the central Pacific El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode and accounts for most of the sea ice variability in the Ross Sea. The third mode is associated with the eastern Pacific ENSO and Amundsen Sea Low and accounts for most of the pan-Antarctic sea ice variability and almost all of the sea ice variability in the Weddell Sea. The third mode is also related to periods of abrupt Antarctic sea ice decline that are associated with a weakening of the circumpolar westerlies, which favors surface warming through a shoaling of the ocean mixed layer and decreased northward Ekman heat transport. Broadly, these results suggest that climate model biases in long-term Antarctic sea ice and large-scale sea surface temperature trends are related to each other and that eastern Pacific ENSO variability is a key ingredient for abrupt Antarctic sea ice changes.

Impacts of internal variability on winter temperature fluctuations over the Tibetan Plateau

The climate comprises an externally forced component and internal variability within the climate system. This variability arises from interactions among climate subsystems, significantly impacting climate change, particularly over the Tibetan Plateau (TP). Therefore, understanding the mechanisms governing TP temperature responses to internal variability is crucial. In this study, winter temperature components over the TP in response to internal variability and external forcing are distinguished using reconstructed monthly temperature data and large ensemble simulations from 11 CMIP6 coupled models spanning 1901 to 2014. Analysis reveals two cold periods (1901–1931, 1956–1999) and two warm periods (1932–1955, 2000–2014) within the internal-variability component from 1901 to 2014. These periods are significantly associated with the Atlantic Multi-decadal Oscillation (AMO) and Interdecadal Pacific Oscillation (IPO), explaining 38.9% of the winter temperature's internal-variability component over the TP. Manipulating AMO+ (AMO-) without IPO influence or IPO- (IPO+) without AMO impact intensifies the winter warm (cold) anomaly across the entire TP. This suggests that AMO and IPO contribute to the temperature's internal-variability component over the TP. Specifically, in winter, both AMO+ (AMO-) and IPO- (IPO+) induce a “− + −+” (+ − +−) Rossby wave train over Eurasia, influencing the East Asian trough, East Asian jet stream, Ural Blocking high, and the Siberian high, resulting in a warm (cold) anomaly over the TP. Moreover, our findings indicate that AMO and IPO collaboratively determine the cold or warm anomaly and its intensity over the TP in winter.

Salinity effect-induced ENSO amplitude modulation in association with the interdecadal Pacific Oscillation

Salinity effect-induced ENSO amplitude modulation in association with the interdecadal Pacific Oscillation

Impact of the North Pacific Meridional Mode on the Tropical Pacific Modulated by the Interdecadal Pacific Oscillation

Abstract The North Pacific meridional mode (NPMM) peaking in boreal spring influences El Niño–Southern Oscillation (ENSO) properties in the ensuing winter. Whether the precursory impact of NPMM on the spatial diversity of ENSO has decadal variation remains unknown. Using long-term reanalysis datasets, we find that the interdecadal Pacific oscillation (IPO) significantly modulates the NPMM forcing on two types of ENSO. During the positive IPO (+IPO) phase, a strengthened background Aleutian low and southward-shifted storm track, in comparison to the negative IPO (−IPO) phase, produce stronger basin-scale negative geopotential height tendency anomalies over the North Pacific through synoptic-scale eddy–mean flow interaction. Such strong background negative tendency facilitates an Aleutian low–like pressure monopole rather than a North Pacific Oscillation (NPO)-like pressure dipole in boreal spring, leading to a weak NPMM that cannot effectively promote development of either a central Pacific (CP) or an eastern Pacific (EP) ENSO. By contrast, the NPO-like dipole enhances in boreal spring during −IPO, corresponding to stronger and more frequently occurring NPMM events that induce a robust CP-ENSO-like response in boreal winter. Moreover, the −IPO-related tropical Pacific mean states and the associated positive feedbacks cause a strong decrease in mixed layer temperature variance in the equatorial eastern Pacific, but a slight increase in the central Pacific, thus further contributing to the enhanced correlation between NPMM and CP-ENSO. Therefore, −IPO has played a role in the stronger impact of NPMM on CP-ENSO since the 1990s, and the modulation effects of IPO should be considered in understanding the extratropical–tropical climatic connection and ENSO spatial diversity.

A hybrid statistical downscaling prediction model with timescale decomposition for summer precipitation over Hainan Island, China

AbstractBy observing well‐known sea surface temperature (SST) indices and outputs from Climate Forecast System version 2 (CFSv2) hindcasts and predictions, in this study, we developed a hybrid statistical downscaling prediction model (HSDPM) based on a timescale decomposition approach, which effectively improved the forecasting ability of summer (June–August) precipitation over Hainan Island (HNSP) in China; this precipitation presented multiple obvious timescale variabilities. We found that the interannual variability of the HNSP is closely related to the observed North Atlantic tripole SST anomalies in the preceding winter and the summer sea level pressure over the South China Sea by CFSv2 predicted in March. On the other hand, the interdecadal variability in the HNSP is linked to the prewinter Pacific Interdecadal Oscillation (PDO) according to the observations, the CFSv2 predictions of the summer surface air temperature over the tropical central‐east Pacific. On this basis, both the interannual and interdecadal components of the HNSP are effectively predicted using the corresponding predictors via multiple regression; thus, the HSDPM is ultimately established by combining the above two components. Compared with the original CFSv2 model, the HSDPM model achieves a considerable improvement in performance in predicting the HNSP. Specifically, the temporal correlation coefficient, ratio of root‐mean‐square error and anomaly sign consistency rate between the observed and HSDPM‐predicted HNSPs are 0.70, 17.4% and 80.5%, respectively, which are significantly greater than the above three forecasting index values of 0.23, 38.3% and 48.8%, respectively, obtained from the original CFSv2 predictions. The application of the HSDPM may be beneficial for drought and flood prevention and mitigation in Hainan Island.

An underlying factor of increasing early winter precipitation in the Hokuriku region of Japan in recent decades

AbstractUsing a reanalysis dataset and large‐ensemble simulation results, this study examines a possible factor of increasing trend in early winter precipitation in recent decades in the Hokuriku region of Japan. Monthly precipitation in December has a significant increasing trend after the early 1990s, which is different from those in January and February. The increasing precipitation in December is related to that in the sea surface upward latent heat flux due to intensified winter monsoon circulation and warming sea surface temperatures (SSTs) over the Sea of Japan. December averaged SSTs show a trend pattern in recent decades that is similar to the negative phase of the interdecadal Pacific oscillation (IPO), accompanied by positive trends from the eastern Indian Ocean to the western tropical Pacific. The enhanced trend of convection over the Bay of Bengal is seen; suggesting a combined effect of climatologically high SSTs and IPO‐related warmed SSTs over the region. Trends in recent decades of an upper‐level wavy pattern from South Asia to near Japan along the subtropical jet associated with enhanced convection near the Bay of Bengal and the related pressure drop from Japan to the north are seen, which contribute to intensified winter monsoon circulation.