Atmospheric General Circulation Model Simulations Research Articles

Oceanic Influence and Lapse Rate Changes Dominate the Recent Amplified Saharan Warming

AbstractThe surface air temperature (SAT) over the Sahara Desert has increased at a much faster rate than average tropical land in recent decades. This study examines the relative roles of anthropogenic greenhouse gas (GHG) forcing and sea surface temperature (SST) change in the observed Saharan temperature increase during boreal warm season from 1979 to 2020 using atmospheric general circulation model simulations. It is found that the SST forcing dominates the observed Saharan warming. Further analysis shows that the warming ocean forces the Saharan SAT increase by moving more energy to the Sahara Desert, while the water vapor feedback plays a secondary role. The reason for the stronger Saharan warming than the average tropical land given the same SST forcing is also explored. We found that the largest contributor to the warming contrast is the lapse‐rate feedback, which is attributable to the difference in the vertical warming profile.

Understanding the Forcing Mechanisms of the 1931 Summer Flood along the Yangtze River, the World’s Deadliest Flood on Record

Abstract The 1931 Yangtze River flood in eastern China, which had an associated death toll of over 2 million, is regarded as one of the world’s deadliest natural disasters on record. However, due to the lack of meteorological data before the 1950s, the causes of this event are rarely investigated. Here, we combine multiple lines of evidence from recently available historical observations, reanalysis datasets, and atmospheric general circulation model simulations driven by historical sea surface temperatures (SSTs) that cover the early twentieth century to reveal the physical mechanisms underlying this flood. We find that the flooding in 1931 along the Yangtze River valley was dominated by July rainfall. Although the rainfall totals in July 1931 were not the largest in history (ranking second over the past century), the totals exceeded those for many other pluvial years between 1951 and 2010 in terms of its persistence, which was associated with a steady western Pacific subtropical high (WPSH). The flooding resulted from the combined effects of tropical El Niño–related SST forcing and extratropical wave activities over the Eurasian continent. On the one hand, warm SST anomalies in the tropical Indian Ocean following an El Niño event led to the southwestward extension of the WPSH. On the other hand, the southward shift of the westerly jet due to extratropical wave activities prevented the normal northward movement of the WPSH typical in July. Additionally, the impact that the preceding springtime precipitation and soil moisture had on the summer flood of 1931 is also discussed.

The Influences of the Multi‐Scale Sea Surface Temperature Anomalies in the North Pacific on the Jet Stream in Winter

AbstractUsing Climate Forecast System Reanalysis (CFSR) data and numerical simulations, the impacts of the multi‐scale sea surface temperature (SST) anomalies in the North Pacific on the boreal winter atmospheric circulations are investigated. The basin‐scale SST anomaly as the Pacific Decadal Oscillation (PDO) pattern, a narrow meridional band of frontal‐scale smoothed SST anomaly in the subtropical front zone (STFZ) and the spatial dispersed eddy‐scale SST anomalies within the STFZ are the three types of forcings. The results of statistical methods find that all three oceanic forcings may correspond to the winter North Pacific jet changing with the similar pattern. Furthermore, several atmospheric general circulation model simulations are used to reveal the differences and detail processes of the three forcings. The basin‐scale cold PDO‐pattern SST anomaly first causes negative turbulent heat flux anomalies, atmospheric cooling, and wind deceleration in the lower atmosphere. Subsequently, the cooling temperature with an amplified southern lower temperature gradient and baroclinity brings a lagging middle warming because of the enhanced atmospheric eddy heat transport. The poleward and upward development of baroclinic fluctuations eventually causes the acceleration of the upper jet. The smoothed frontal‐ and eddy‐scales SST anomalies in the STFZ cause comparable anomalous jet as the basin‐scale by changing the upward baroclinic energy and Eliassen‐Palm fluxes. The forcing effects of multi‐scales SST anomalies coexist simultaneously in the mid‐latitude North Pacific, which can cause similar anomalous upper atmospheric circulations. This is probably why it is tricky to define the certain oceanic forcing to the specific observed atmospheric circulation variation.

Weakening of Indian Summer Monsoon Synoptic Activity in Response to Polar Sea Ice Melt Induced by Albedo Reduction in a Climate Model

AbstractThe effect of polar sea ice melt on low latitude climate is little known. To understand the response of the Indian summer monsoon (ISM) synoptic activity to the sea ice melt, we have run a suite of coupled and uncoupled climate model simulations. In one set of simulations, the albedo of sea ice is reduced so that it would melt due to increased absorption of solar radiation. The coupled model simulation with a reduced sea ice albedo resulted in an almost complete melting of the sea ice in summer in both hemispheres. A high‐resolution (50 km) atmospheric general circulation model (AGCM) is forced with the climatological annual cycles of sea surface temperature (SST) and sea ice concentrations (SIC) from the coupled model outputs to better resolve synoptic scale variability. In the high‐resolution AGCM simulations forced with SST and SIC from the sea ice melt experiments, the ISM circulation weakened substantially, and the monsoon low‐pressure systems (LPS) activity experienced an overall decline of 23%, with a widespread weakening in the south and a moderate strengthening over the north, in response to a decline of 78% (24%) in SIC over the Arctic (Antarctic) in the June–September season. The changes in the LPS activity in response to polar sea ice melt are found to be mostly driven by the changes in low‐level absolute vorticity and vertical shear over the Bay of Bengal.

European precipitation response to Indian ocean dipole events

This study examines the effect of the Indian Ocean Dipole (IOD) on the precipitation variability in the European region. The observed summer precipitation response to IOD-related sea-surface temperature anomalies is reproduced using Atmospheric General Circulation Model (AGCM) ensemble experiments and analyzed using observations and re-analyses patterns. Precipitation associated with the strong negative independent IOD events (while El-Nino/Southern oscillation-related effect excluded) may increase up to 30–40 mm/month according to observations/re-analyses data and up to 10–12 mm/month according to numerical ensemble AGCM simulation experiments over the Central European area and decrease at the eastern part of the Russian Plain and the Ural Mountains.

Wintertime Weakening of Low-Cloud Impacts on the Subtropical High in the South Indian Ocean

AbstractTo elucidate the unique seasonality in the coupled system of the subtropical Mascarene high and low-level clouds, the present study compares wintertime cloud radiative impacts on the high with their summertime counterpart through coupled and atmospheric general circulation model simulations. A comparison of a fully coupled control simulation with another simulation in which the radiative effects of low-level clouds are artificially switched off demonstrates that the low-cloud effect on the formation of the Mascarene high is much weaker in winter. Background climatology plays an important role in this seasonality of the Mascarene high reinforcement. Relative to summer, the suppression of deep convection due to low-level clouds that acts to reinforce the high is much weaker in winter. This arises from 1) seasonally lower sea surface temperature in concert with the smaller sea surface temperature reduction due to the deeper ocean mixed layer and the weaker cloud radiative effect under weaker insolation and 2) seasonally stronger subtropical subsidence associated with the Hadley circulation in winter. As verified through atmospheric dynamical model experiments, enhanced cloud-top radiative cooling by low-level clouds acts to reinforce the wintertime Mascarene high in comparable magnitude as in summer. The present study reveals that the self-sustaining feedback with low-level clouds alone is insufficient for replenishing the full strength of the wintertime Mascarene high. This implies that another internal feedback pathway and/or external driver must be operative in maintaining the wintertime high.

Asymmetric impacts of El Niño and La Niña on the Pacific–North American teleconnection pattern: the role of subtropical jet stream

The asymmetric impacts of El Niño and La Niña on the Pacific–North American teleconnection pattern in boreal winter have important implications for the surface air temperature and precipitation anomalies in North America. Previous studies have shown that the varying tropical convective heating contributes to the zonal shift of the teleconnection pattern during different El Niño/Southern Oscillation phases. In this study, using reanalysis, atmospheric general circulation model (AGCM) simulations, and a linear baroclinic model, we further present that the discrepancy of the subtropical jet stream (STJ) during El Niño and La Niña also contributes to the asymmetry. The atmospheric anomalies readily extract kinetic energy and effectively develop at the exit of the STJ. During El Niño (La Niña) years, as the central-eastern tropical Pacific warms up (cools down), the meridional temperature gradient in central subtropical Pacific increases (decreases), leading to the eastward (westward) shift of the STJ. The movement of the STJ leads to the shift of the location where disturbance develops most efficiently, ultimately contributing to the asymmetry of the teleconnection pattern.

Response of the Upper‐Level Monsoon Anticyclones and Ozone to Abrupt CO2 Changes

AbstractThe summer monsoon anticyclones are the dominant climatological features of the Northern Hemispheric (NH) summertime circulation in the upper troposphere and lower stratosphere (UTLS). However, the response of these anticyclones to the increased levels of remains highly uncertain, as does the impact on the distribution of UTLS ozone and other tracers. This study examines the response of the NH summertime monsoon anticyclones and UTLS ozone to the abrupt increase in forcing using output from a suite of coupled ocean–atmosphere general circulation model simulations. These models show an equatorward shift of the Asian summer monsoon anticyclone, a weakening of the North American summer monsoon anticyclone, and a stronger westerly flow penetrating deep into the tropics above the Pacific Ocean and North America. We use additional idealized experiments from atmosphere‐only general circulation models with prescribed SSTs and sea ice concentration to isolate the direct atmospheric radiative effects from the indirect effect of SST warming on the UTLS monsoon anticyclones. Comparison between atmosphere‐only and coupled ocean–atmosphere experiments shows that SST warming is the principal mechanism producing UTLS monsoonal circulation changes. The experiments result in a significant reduction up to 40%–50% of the UTLS ozone in the northern tropics, which could have an impact on radiative balance near the surface.

Monthly Modulations of ENSO Teleconnections: Implications for Potential Predictability in North America

AbstractUsing a high-resolution atmospheric general circulation model simulation of unprecedented ensemble size, we examine potential predictability of monthly anomalies under El Niño Southern Oscillation (ENSO) forcing and back-ground internal variability. This study reveals the pronounced month-to-month evolution of both the ENSO forcing signal and internal variability. Internal variance in upper-level geopotential height decreases (∼ 10%) over the North Pacific during El Niño as the westerly jet extends eastward, allowing forced signals to account for a greater fraction of the total variability, and leading to increased potential predictability. We identify February and March of El Niño years as the most predictable months using a signal-to-noise analysis. In contrast, December, a month typically included in teleconnection studies, shows little-to-no potential predictability. We show that the seasonal evolution of SST forcing and variability leads to significant signal-to-noise relationships that can be directly linked to both upper-level and surface variable predictability for a given month. The stark changes in forced response, internal variability, and thus signal-to-noise across an ENSO season indicate that subseasonal fields should be used to diagnose potential predictability over North America associated with ENSO teleconnections. Using surface air temperature and precipitation as examples, this study provides motivation to pursue ‘windows of forecast opportunity’, in which statistical skill can be developed, tested, and leveraged to determine times and regions in which this skill may be elevated.

Asymmetric impact of the boreal spring Pacific Meridional Mode on the following winter El Niño‐Southern Oscillation

AbstractA number of previous studies have indicated that boreal spring Pacific Meridional Mode (SPMM) can exert notable impacts on the occurrence of El Niño‐Southern Oscillation (ENSO) event during the subsequent winter. Here, we reveal that the impact of the SPMM on the following winter ENSO is asymmetric. During positive SPMM (+SPMM) years, strong westerly wind anomalies are induced over the tropical western Pacific via wind‐evaporation‐SST (WES) feedback. These westerly wind anomalies lead to pronounced sea surface temperature warming in the tropical central and eastern Pacific, which further develop to an El Niño‐like pattern via positive atmosphere–ocean interaction. However, easterly wind anomalies over the tropical western Pacific during negative SPMM (−SPMM) years are much weaker. The weak easterly wind anomalies cannot lead to a clear La Niña‐like pattern in the following winter. We suggest that the asymmetric strength of the westerly and easterly wind anomalies over the tropical western Pacific is due to the asymmetric intensity of the WES feedback over the tropical north Pacific, with stronger intensity in the +SPMM years. The asymmetric trade wind charging process also partly contributes to the asymmetric SPMM–ENSO relation. Atmospheric general circulation model simulations confirm that the intensity of the +SPMM‐generated westerly wind anomalies over the tropical western Pacific is stronger than that of the easterly wind anomalies related to the −SPMM. The present study indicates that it is necessary to consider the phases of the SPMM when predicting winter ENSO events based on the preceding SPMM.

Impact of Atlantic multidecadal oscillation on interannual relationship between ENSO and East Asian early summer monsoon

AbstractImpact of the Atlantic multidecadal oscillation (AMO) on the interannual relationship between the El Niño‐Southern Oscillation (ENSO) and East Asian early summer monsoon (EAESM) is investigated using observations and community atmosphere model (CAM) outputs. We find that the interannual ENSO–EAESM relationship displays a prominent decadal variation during 1951–2018, which is consistent with the AMO phase change, that is, a high ENSO–EAESM correlation appears during the negative AMO phase, while no significant correlation is found during the positive AMO phase. Further analyses reveal that the AMO‐related sea surface temperature anomalies (SSTAs) in the western North Pacific, rather than in the North Atlantic, modulate the interannual relationship between the El Niño and EAESM. These SSTAs can prolong the existence of the anomalous western North Pacific anticyclone (WNPAC) from the ENSO mature winter to the subsequent early summer, thus affecting the ENSO–EAESM relationship for the El Niño events. In contrast, the AMO modulates the interannual relationship between the La Niña and EAESM through the North Atlantic and North Pacific. The North Atlantic warming‐induced anomalous anticyclone weakens the background cyclone over the western North Pacific during the positive AMO phase, and the mid‐latitude air–sea interaction in the North Pacific also influence the anomalous cyclone. The observed AMO's impact on the interannual relationship between the ENSO and EAESM is confirmed by atmosphere general circulation model simulations using the CAM.

Performance of CMIP5 atmospheric general circulation model simulations over the Asian summer monsoon region

Performance of twenty-eight state of art atmosphere general circulation models (AGCMs) in simulating regional characteristic features of summer monsoon rainfall and circulation over the south Asian and the western-north Pacific regions are examined. AGCMs depict good representation of climatological spatial distribution of monsoon rainfall over both the regions. Interestingly, the year-to-year variability of the south Asian (Indian) monsoon rainfall is well simulated by the models compared to the circulation features. Whereas, for the western-north Pacific region, the AGCMs depict poor representation of monsoon rainfall variability, but circulation features are simulated with better skill. Thus suggesting that, models with good performance of monsoon rainfall variability need not necessarily have better representation of monsoon circulation features. Further analysis indicate that most models with good skill in simulating monsoon variability have better representation of monsoon-SST teleconnections with the Indo-Pacific SSTs. For the south Asian region, better simulating models have good representation of monsoon-ENSO teleconnection whereas most models have poor representation of monsoon-IOD relationship, suggesting requirement for realistic representation of these teleconnections in the models. For the western-north Pacific region, most models have unrealistic relationship of monsoon rainfall with SSTs during preceding winter and concurrent monsoon season, indicating that the poor (better) skill in simulating rainfall (circulation) probably arise from the unrealistic (realistic) representation of monsoon-SST teleconnections. Thus, the importance of realistic representation of monsoon-SST teleconnections in the climate models have been brought out for better simulation of monsoon variability over the south Asian and the western-north Pacific regions. • Performance of twenty-eight state of art atmosphere general circulation models in representing the summer monsoon rainfall and circulation over the south Asia and the western-north Pacific are examined. • Study brings out the importance of realistic representation of monsoon-SST teleconnections in the climate models for better simulation of the monsoon over the south Asia and the western-north Pacific. • The models which depict higher skill in simulating the interannual variability of monsoon have a better representation of monsoon-SST teleconnection over most of the Indo-Pacific region.

Using the fast impact of anthropogenic aerosols on regional land temperature to constrain aerosol forcing.

Anthropogenic aerosols have been postulated to have a cooling effect on climate, but its magnitude remains uncertain. Using atmospheric general circulation model simulations, we separate the land temperature response into a fast response to radiative forcings and a slow response to changing oceanic conditions and find that the former accounts for about one fifth of the observed warming of the Northern Hemisphere land during summer and autumn since the 1960s. While small, this fast response can be constrained by observations. Spatially varying aerosol effects can be detected on the regional scale, specifically warming over Europe and cooling over Asia. These results provide empirical evidence for the important role of aerosols in setting regional land temperature trends and point to an emergent constraint that suggests strong global aerosol forcing and high transient climate response.

Assessing Global and Local Radiative Feedbacks Based on AGCM Simulations for 1980–2014/2017

AbstractWe examine radiative feedbacks based on short‐term climate variability by analyzing atmospheric general circulation model (AGCM) simulations, including Atmospheric Model Intercomparison Project within CMIP phase 6 (AMIP6) with known effective radiative forcing (ERF) for 1980–2014 and one with zero ERF for 1980–2017. We first verify the Kernel‐Gregory feedback calculation by showing that both clear‐sky radiative fluxes and all‐sky radiative feedbacks from the kernel method agree with model simulations. We find that global‐mean net feedback for 1980–2017/2014 is −2 W m−2 K−1, about twice the feedback estimated for long‐term warming (4 × CO2) experiments. This difference is mainly caused by a near‐zero global‐mean net cloud feedback for 1980–2017/2014. We show that the lapse rate feedback for 1980–2017/2014 is the largest contributor to the amplified temperature change over the three poles (Arctic, Antarctic, and Tibetan Plateau), followed by surface albedo feedback and Planck feedback deviation from its global mean. Except for a higher surface albedo feedback in Antarctic, other feedbacks are similar between Arctic and Antarctic.

A tropospheric pathway of the stratospheric quasi-biennial oscillation (QBO) impact on the boreal winter polar vortex

Abstract. The quasi-biennial oscillation (QBO) is quasi-periodic oscillation of the tropical zonal wind in the stratosphere. When the tropical lower stratospheric wind is easterly (westerly), the winter Northern Hemisphere (NH) stratospheric polar vortex tends to be weak (strong). This relation is known as the Holton–Tan relationship. Several mechanisms for this relationship have been proposed, especially linking the tropics with high latitudes through stratospheric pathway. Although QBO impacts on the troposphere have been extensively discussed, a tropospheric pathway of the Holton–Tan relationship has not been explored previously. Here, we propose a tropospheric pathway of the QBO impact, which may partly account for the Holton–Tan relationship in early winter, especially in the November–December period. The study is based on analyses of observational data and results from a simple linear model and atmospheric general circulation model (AGCM) simulations. The mechanism is summarized as follows: the easterly phase of the QBO is accompanied with colder temperature in the tropical tropopause layer, which enhances convective activity over the tropical western Pacific and suppresses it over the Indian Ocean, thus enhancing the Walker circulation. This convection anomaly generates a Rossby wave train, propagating into the midlatitude troposphere, which constructively interferences with the climatological stationary waves, especially in wavenumber 1, resulting in enhanced upward propagation of the planetary wave and a weakened polar vortex.

Global Monsoon Responses to Decadal Sea Surface Temperature Variations during the Twentieth Century: Evidence from AGCM Simulations

AbstractMultidecadal 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.

The sensitivity of climatological SST to slab ocean model thickness

A pronounced tropical cooling (> 4 °C) and high-latitude warming in the annual mean sea surface temperature (SST) climatology is found in a numerical experiment conducted with a coupled model consisting of an atmospheric general circulation model (AGCM) coupled to a slab ocean model (SOM) in which the time-independent SOM thickness is reduced by a factor of two. The results suggest that biases in the ocean mixed layer depth could be contributing to SST biases in coupled atmosphere–ocean general circulation models. These changes in annual mean SST are noteworthy since in simple climate models the SOM thickness controls the amplitude and phase of the SST annual cycle, but it plays no role in determining the annual mean SST. Results from the numerical experiment indicate that halving of the SOM thickness not only changes annual cycle amplitude but results in asymmetrical changes in the annual cycle that rectify onto annual mean SST. The changed SOM thickness is found to primarily affect the surface net solar flux and latent heat flux components. However, due to the SOM equilibrium energy budget constraint, the annual mean net surface heat flux does not change, and the solar and latent heat fluxes changes compensate for each other. There is no such constraint on the annual cycle of heat fluxes, where the solar and latent components act to reinforce each other over much of the ocean, with the notable exception of the tropic Pacific warm pool region. We investigate the influence of the amplitude of the SST annual cycle on the annual mean SST through the net surface heat flux and its components using a two-step approach. First, two prescribed SST AGCM simulations are carried out, both with the annual mean SST of the full-thickness SOM coupled model simulation: one forced with the SST annual cycle of the full-thickness SOM coupled simulation, and the other forced with the SST annual cycle of the half-thickness SOM coupled simulation. Second, the impacts of these changes in the annual cycle on the annual mean SST are assessed in a full-thickness AGCM-SOM coupled simulation forced by the net, annual-mean, surface heat flux changes produced by the SST annual cycle changes in the first step. It is found that these net surface heat flux changes can largely reproduce the annual mean SST sensitivity to SOM thickness found in the coupled SOM thickness halving experiment. The effects on the annual mean heat flux components and annual mean SST from changes in the SOM thickness in an idealized limiting case are also considered.

Connecting Direct Effects of CO2 Radiative Forcing to Ocean Heat Uptake and Circulation

AbstractThe ocean's response to direct atmospheric effects of increased carbon dioxide's (CO2) radiative forcing is examined. These direct effects are defined as the climate changes that result from forcing on a fast time scale of about a year, independent of the slower surface warming that the forcing also provokes. To evaluate how these direct effects impact ocean heat uptake and circulation, output of atmospheric general circulation model (GCM) simulations are used to force an ocean GCM with comprehensive boundary conditions. Perturbation simulations with the prescribed response to a quadrupling of atmospheric CO2 include altered surface winds, freshwater fluxes, downwelling shortwave radiation, and downwelling longwave cloud radiative effect. The perturbation simulations show that the intensification and poleward shift of surface winds, particularly in the Southern Ocean, strengthen the shallow overturning circulation in the tropical Pacific and deep overturning in the Atlantic. This, in turn, has a cooling effect on the global ocean at shallow depths. A two‐layer energy balance model, designed to capture transient global mean climate change, is adapted to account for the altered ocean heat uptake from direct effects. The direct change in global mean ocean heat uptake is a decrease of about 0.3 W/m2 for quadrupling of CO2, offsetting about 5% of the surface longwave forcing.

Summer Temperature over the Tibetan Plateau Modulated by Atlantic Multidecadal Variability

Abstract Rapid warming has led to an aggregated environmental degradation over the Tibetan Plateau (TP) in the last few decades, including accelerated glacier retreat, early snowmelt, permafrost degradation, and forest fire occurrence. Attribution of this warming in recent decades has mainly been focused on anthropogenic forcing. Yet, linkages to the Atlantic multidecadal variability (AMV), an essential part of the climate system causing decadal to centennial fluctuations of temperature, remains poorly understood for the TP, especially at long time scales. Using well-replicated tree-ring width records, we reconstructed 358 years of summer minimum temperature (MinT) of the whole TP. This reconstruction matches the recent warming signal recorded since the 1980s, and captures 63% of the variance in 1950–2005 instrumental records. A teleconnection from the North Atlantic to the TP is further identified based in observations and simulations with an atmospheric general circulation model (AGCM). We propose that half of the multidecadal variability of TP summer MinT can be explained by the AMV over the past three and a half centuries. Both observations and AGCM simulations indicate that the AMV warm phase induces a zonal dipole response in sea level pressure across the Atlantic–Eurasia region, with anomalously high surface pressure and corresponding downward atmospheric motion over the TP. We propose that the descending motion during warm AMV phases causes negative rainfall and positive temperature anomalies over the TP. Our findings highlight that the AMV plays a role in the multidecadal temperature variability over the TP.

An examination of the Northern Hemisphere mid-latitude storm track interannual variability simulated by climate models\u2014sensitivity to model resolution and coupling

The model fidelity in simulating the Northern Hemisphere storm track interannual variability and the connections of this variability to the low frequency atmospheric variations and oceanic variations are examined based on the atmospheric European Centre for Medium-Range Weather Forecasts (ECMWF) model and coupled NCAR Community Climate System Model (CCSM) systems at different horizontal resolutions. The atmospheric general circulation model (AGCM) runs are forced by observed sea surface temperatures (SST) with varying atmospheric resolutions, while the coupled general circulation model (CGCM) runs have a fixed atmospheric resolution but varying oceanic resolutions. The phases, between the North Pacific (NP) and North Atlantic (NA) sectors, of the simulated hemisphere-scale Empirical Orthogonal Function (EOF) modes of the storm track fluctuations change with the model resolution, suggesting the storm track variability in NP and NA basins are largely independent. The models can qualitatively reproduce the basin-scale EOFs of both NP and NA storm track variability. These EOFs are not sensitive to either atmospheric or oceanic model horizontal resolutions, but their magnitudes from the CGCM runs are substantially underestimated. The storm track variations over NP basin are hybrid of internal atmospheric variations and external forcing from the underlying conditions, but the fluctuations over the NA basin are merely atmospheric internal variability. The NP storm track variability from SST forcing accounts for 4.4% of the total variance in observations, while it only has less than 2% of the total in all AGCM simulations. The external forcing to the storm track variations is more realistically reproduced in the higher atmospheric resolution runs. The air–sea coupling makes the SST feedbacks to the atmospheric internal variability, absent in the atmospheric ECMWF model hindcasts, emerge in the coupled CCSM simulations.