Moisture Budget Research Articles

Regional Moisture Budget and Land‐Atmosphere Coupling Over the U.S. Southern Great Plains Inferred From the ARM Long‐Term Observations

AbstractTen‐year warm‐season (May–August) observations at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site are used to examine the large‐scale atmospheric moisture budget and the strength of land‐atmosphere coupling. Atmospheric moisture budget components, computed from the ARM variational analysis data, are compared between different wet/dry scenarios and convection regimes. Consistent with previous studies, large‐scale moisture flux convergence dominates the SGP regional moisture budget on the daily time scale, but surface evaporation does play a more prominent role on late‐afternoon deep convection days. Moreover, the relationship between moisture flux convergence and precipitation increases significantly from dry year to wet years. The land‐atmosphere coupling strength is quantified for local convective events within the “Local L‐A Coupling Process Chain” (Santanello et al., 2018, https://doi.org/10.1175/BAMS‐D‐17‐0001.1). The impact of land surface on the evolution of planetary boundary layer is highly dependent on the vegetation leaf area index. A significantly larger surface sensible heat flux is found over the SGP forest region in midsummer, which is accompanied by a much higher planetary boundary layer development and a large increase in shallow cumulus cloud fraction. The significant relationship between afternoon precipitation and surface turbulent flux only exists in the northeast part of the ARM SGP site, which is mainly covered by grassland/pasture.

Fast Biases in Monsoon Rainfall over Southern and Central India in the Met Office Unified Model

AbstractThe Met Office Unified Model (MetUM) is known to produce too little total rainfall on average over India during the summer monsoon period, when assessed for multiyear climate simulations. We investigate how quickly this dry bias appears by assessing the 5-day operational forecasts produced by the MetUM for six different years. It is found that the MetUM shows a drying tendency across the five days of the forecasts, for all of the six years (which correspond to two different model versions). We then calculate each term in the moisture budget, for a region covering southern and central India, where the dry bias is worst in both climate simulations and weather forecasts. By looking at how the terms vary with forecast lead time, we are able to identify biases in the weather forecasts that have been previously identified in climate simulations using the same model, and we attempt to quantify how these biases lead to a reduction in total rainfall. In particular, an anticyclonic bias develops to the east of India throughout the forecast, and it has a complex effect on the moisture available over the peninsula, and a reduction in the wind speed into the west of the region appears after about 3 days, indicative of upstream effects. In addition, we find a new bias that the air advected from the west is too dry from very early in the forecast, and this has an important effect on the rainfall.

CAM6 simulation of mean and extreme precipitation over Asia: sensitivity to upgraded physical parameterizations and higher horizontal resolution

Abstract. The Community Atmosphere Model version 6 (CAM6), released in 2018 as part of the Community Earth System Model version 2 (CESM2), is a major upgrade over the previous CAM5 that has been used in numerous global and regional climate studies. Since CESM2–CAM6 will participate in the upcoming Coupled Model Intercomparison Project phase 6 (CMIP6) and is likely to be adopted in many future studies, its simulation fidelity needs to be thoroughly examined. Here we evaluate the performance of a developmental version of the Community Atmosphere Model with parameterizations that will be used in version 6 (CAM6α), with a default 1∘ horizontal resolution (0.9∘×1.25∘, CAM6α-1∘) and a high-resolution configuration (approximately 0.25∘, CAM6α-0.25∘), against various observational and reanalysis datasets of precipitation over Asia. CAM6α performance is compared with CAM5 at default 1∘ horizontal resolution (CAM5-1∘) and a high-resolution configuration at 0.25∘ (CAM5-0.25∘). With the prognostic treatment of precipitation processes and the new microphysics module, CAM6α is able to better simulate climatological mean and extreme precipitation over Asia, better capture the heaviest precipitation events, better reproduce the diurnal cycle of precipitation rates over most of Asia, and better simulate the probability density distributions of daily precipitation over Tibet, Korea, Japan and northern China. Higher horizontal resolution in CAM6α improves the simulation of mean and extreme precipitation over northern China, but the performance degrades over the Maritime Continent. Moisture budget diagnosis suggests that the physical processes leading to model improvement are different over different regions. Both upgraded physical parameterizations and higher horizontal resolution affect the simulated precipitation response to the internal variability of the climate system (e.g., Asian monsoon variability, El Niño–Southern Oscillation – ENSO, Pacific Decadal Oscillation – PDO), but the effects vary across different regions. For example, higher horizontal resolution degrades the model performance in simulating precipitation variability over southern China associated with the East Asian summer monsoon. In contrast, precipitation variability associated with ENSO improves with upgraded physical parameterizations and higher horizontal resolution. CAM6α-0.25∘ and CAM6α-1∘ show an opposite response to the PDO over southern China. Basically, the response to increases in horizontal resolution is dependent on the CAM version.

Variations in ozone and greenhouse gases as drivers of Southern Hemisphere climate in a medium-complexity global climate model

The climate of the Southern Hemisphere (SH) is being affected not only by the global increase in the concentration of greenhouse gases (GHG) but also by the ongoing recovery of stratospheric ozone. The competing roles of both are usually studied by means of state-of-the-art global climate models (GCM) due to the complexity in the atmospheric processes and interactions involved. The main objective of this paper is to determine whether quantitatively similar results can be achieved using a simpler, medium-complexity GCM. This includes addressing the sensitivity of the medium-complexity model to changes in GHG and ozone concentrations as well as looking into the plausible mechanisms driving such patterns of variations in atmospheric circulation, precipitation and temperature extremes. Two sets of simulations are performed: one with fixed sea surface temperatures (SST) and another one making use of a mixed-layer ocean model coupled to the GCM. Results show that the GCM is highly sensitive to changes in the atmospheric composition, and that the largest differences are found when SST are allowed to vary accordingly, particularly in the case of temperature extremes. Specific analyses are also performed on changes in subtropical precipitation over the continental regions of the SH, with moisture budget decomposition being applied in order to provide explanations for the rainfall changes described in the manuscript.

Contrast of Evolution Characteristics of Boreal Summer and Winter Intraseasonal Oscillations over Tropical Indian Ocean

A most striking summer–winter difference of evolution of the intraseasonal oscillation (ISO) over the equatorial Indian Ocean is a quasi-stationary oscillation in boreal summer but eastward propagation in boreal winter. This feature is consistent with the observational fact that maximum ISO variance appears only in the eastern Indian Ocean in boreal summer while it appears across the entire basin in boreal winter. The cause of the distinctive propagation and initiation characteristics is investigated through the diagnosis of observational and reanalysis data for the period of 1982–2012. It is found that when the ISO convection appears over eastern Indian Ocean, a positive (negative) moisture tendency appears to the east of the convection in boreal winter (summer). It is the moisture tendency difference that is responsible for different propagation behavior in the summer and winter. A further diagnosis of the moisture budget indicates that the major difference lies in anomalous moisture advection by the mean flow. In addition, air–sea interaction also plays a role. While boreal winter ISO starts over western Indian Ocean, boreal summer ISO is initiated over central–eastern equatorial Indian Ocean, due to boundary layer moistening. The moisture increase is caused primarily by the horizontal advection of mean specific humidity by anomalous easterlies induced by preceding suppressed-phase ISO over eastern Indian Ocean. Besides, a delayed SST feedback also plays a role. The overall difference of ISO evolution between the summer and winter is regulated by the seasonal mean state including the mean SST and water vapor content.

Changes in Frequency of Large Precipitation Accumulations over Land in a Warming Climate from the CESM Large Ensemble: The Roles of Moisture, Circulation, and Duration

ABSTRACTProjected changes in the frequency of major precipitation accumulations (hundreds of millimeters), integrated over rainfall events, over land in the late twenty-first century are analyzed in the Community Earth System Model (CESM) Large Ensemble, based on the RCP8.5 scenario. Accumulation sizes are sorted by the local average recurrence interval (ARI), ranging from 0.1 to 100 years, for the current and projected late-twenty-first-century climates separately. For all ARIs, the frequency of exceedance of the given accumulation size increases in the future climate almost everywhere, especially for the largest accumulations, with the 100-yr accumulation becoming about 3 times more frequent, averaged over the global land area. The moisture budget allows the impacts of individual factors—moisture, circulation, and event duration—to be isolated. In the tropics, both moisture and circulation cause large future increases, enhancing the 100-yr accumulation by 23% and 13% (average over tropical land), and are individually responsible for making the current-climate 100-yr accumulation 2.7 times and 1.8 times more frequent, but effects of shorter durations slightly offset these effects. In the midlatitudes, large accumulations become about 5% longer in duration, but are predominantly controlled by enhanced moisture, with the 100-yr accumulation (land average) becoming 2.4 times more frequent, and 2.2 times more frequent due to moisture increases alone. In some monsoon-affected regions, the 100-yr accumulation becomes more than 5 times as frequent, where circulation changes are the most impactful factor. These projections indicate that changing duration of events is a relatively minor effect on changing accumulations, their future enhancement being dominated by enhanced intensity (the combination of moisture and circulation).

Diagnosing Moisture Sources for Flash Floods in the United States. Part II: Terrestrial and Oceanic Sources of Moisture

Abstract Backward trajectories were derived from North American Regional Reanalysis data for 19 253 flash flood reports published by the National Weather Service to determine the along-path contribution of the land surface to the moisture budget for flash flood events in the conterminous United States. The impact of land surface interactions was evaluated seasonally and for six regions: the West Coast, Arizona, the Front Range, Flash Flood Alley, the Missouri Valley, and the Appalachians. Parcels were released from locations that were impacted by flash floods and traced backward in time for 120 h. The boundary layer height was used to determine whether moisture increases occurred within the boundary layer or above it. Moisture increases occurring within the boundary layer were attributed to evapotranspiration from the land surface, and surface properties were recorded from an offline run of the Noah land surface model. In general, moisture increases attributed to the land surface were associated with anomalously high surface latent heat fluxes and anomalously low sensible heat fluxes (resulting in a positive anomaly of evaporative fraction) as well as positive anomalies in top-layer soil moisture. Over the ocean, uptakes were associated with positive anomalies in sea surface temperatures, the magnitude of which varies both regionally and seasonally. Major oceanic surface-based source regions of moisture for flash floods in the United States include the Gulf of Mexico and the Gulf of California, while boundary layer moisture increases in the southern plains are attributable in part to interactions between the land surface and the atmosphere.

Asymmetric effects of atmospheric circulation on the South China Sea summer monsoon onset

Observational analyses suggest a significant positive correlation between the year-to-year convection over the South China Sea (SCS)/western Pacific (WP) and the SCS summer monsoon (SCSSM) onset date. An investigation shows that there is an asymmetric relationship between the area-mean outgoing longwave radiation and the SCSSM onset date index. The analysis found that the influence of the intraseasonal scale circulation is the main cause of this asymmetric relationship. On the interannual scale, circulation and sea surface temperature (SST) anomalies are distributed symmetrically. During the convection active (inactive) years, the SST anomaly field indicates a La Ni n˜ a (El Ni n˜ o)-like pattern. The SCS-WP low-level westerly (easterly) anomaly is enhanced, leading to an increase (decrease) in moisture and ascending (descending) motion. The enhanced convection results in weakening (enhancement) of the subtropical high and provides a favorable condition for early (late) SCSSM establishment. On the intraseasonal time scale, the convection intensity is stronger during the active years than during the inactive years. The northwestward propagation is significant during the active years. This feature is not observed during inactive years. This difference is attributed to the asymmetric meridional distribution of the atmospheric convective instability and moisture disturbance during active years, not during inactive years. In active years, the atmosphere is potentially more (less) unstable and the moisture is to the north (south) of the intraseasonal oscillation (ISO) convective center, which is conducive to generating a favorable unstable environment for the development of new convection north of the ISO convection center, leading to northward convection propagation. The moistening in the SCS-WP is primarily attributed to the phase leading horizontal advection term. An ISO moisture budget analysis reveals that the largest positive contribution is the vertical moisture advection term.

Seasonal precipitation change in the Western North Pacific and East Asia under global warming in two high-resolution AGCMs

To estimate seasonal precipitation change over the western North Pacific and East Asia region (WNP-EA), we use high-resolution atmospheric Model (HiRAM) (50 km resolution) to conduct a series of simulations forced by Representative Concentration Pathways 8.5 (RCP8.5) scenario with the fifth phases of the Coupled Model Intercomparison project (CMIP5) ensemble sea surface temperature (SST) changes (RCP_Ens). The sensitivity of future projections to various SST forcing ( $$ {\text{SST}}_{\text{spa}}^{{\prime }} $$ ) is also assessed. We also assess the sensitivity associated with model dependence by comparing with the results of the Meteorological Research Institute-Atmospheric General Circulation Model (MRI-AGCM3.2S) projections. The major findings are: (1) weakened atmospheric circulation in all seasons in RCP_Ens experiment; (2) more precipitation over most of the northern East Asian continent and the northern WNP oceanic region in all seasons; (3) an anomalous anticyclonic circulation (AAC) together with decreased precipitation over the oceanic WNP-EA in the typhoon season; and (4) largest sensitivity to various $$ {\text{SST}}_{\text{spa}}^{{\prime }} $$ in spring comparing with other seasons. The aforementioned changes are similarly seen in both the HiRAM and MRI-AGCM3.2S, suggesting the reliability of our findings. The moisture budget indicates the dominance of dynamic contribution to the reduced precipitation. By contrast, the increased precipitation may be associated with various processes such as enhanced upward motion, increased water vapor or surface evaporation.

Mechanisms for an Amplified Precipitation Seasonal Cycle in the U.S. West Coast under Global Warming

Abstract The mean precipitation along the U.S. West Coast exhibits a pronounced seasonality change under warming. Here we explore the characteristics of the seasonality change and investigate the underlying mechanisms, with a focus on quantifying the roles of moisture (thermodynamic) versus circulation (dynamic). The multimodel simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5) show a simple “wet-get-wetter” response over Washington and Oregon but a sharpened seasonal cycle marked by a stronger and narrower wet season over California. Moisture budget analysis shows that changes in both regions are predominantly caused by changes in the mean moisture convergence. The thermodynamic effect due to the mass convergence of increased moisture dominates the wet-get-wetter response over Washington and Oregon. In contrast, mean zonal moisture advection due to seasonally dependent changes in land–sea moisture contrast originating from the nonlinear Clausius–Clapeyron relation dominates the sharpened wet season over California. More specifically, the stronger climatological land–sea thermal contrast in winter with warmer ocean than land results in more moisture increase over ocean than land under warming and hence wet advection to California. However, in fall and spring, the future change of land–sea thermal contrast with larger warming over land than ocean induces an opposite moisture gradient and hence dry advection to California. These results have important implications for projecting changes in the hydrological cycle of the U.S. West Coast.

Local and Remote Controls on Arctic Mixed‐Layer Evolution

AbstractIn this study Lagrangian large‐eddy simulation of cloudy mixed layers in evolving warm air masses in the Arctic is constrained by in situ observations from the recent PASCAL field campaign. A key novelty is that time dependence is maintained in the large‐scale forcings. An iterative procedure featuring large‐eddy simulation on microgrids is explored to calibrate the case setup, inspired by and making use of the typically long memory of Arctic air masses for upstream conditions. The simulated mixed‐phase clouds are part of a turbulent mixed layer that is weakly coupled to the surface and is occasionally capped by a shallow humidity layer. All eight simulated mixed layers exhibit a strong time evolution across a range of time scales, including diurnal but also synoptic fingerprints. A few cases experience rapid cloud collapse, coinciding with a rapid decrease in mixed‐layer depth. To gain insight, composite budget analyses are performed. In the mixed‐layer interior the heat and moisture budgets are dominated by turbulent transport, radiative cooling, and precipitation. However, near the thermal inversion the large‐scale vertical advection also contributes significantly, showing a distinct difference between subsidence and upsidence conditions. A bulk mass budget analysis reveals that entrainment deepening behaves almost time‐constantly, as long as clouds are present. In contrast, large‐scale subsidence fluctuates much more strongly and can both counteract and boost boundary‐layer deepening resulting from entrainment. Strong and sudden subsidence events following prolonged deepening periods are found to cause the cloud collapses, associated with a substantial reduction in the surface downward longwave radiative flux.

Estimating the role of upper Blue Nile basin moisture budget and recycling ratio in spatiotemporal precipitation distributions

Upper Blue Nile basin (UBNB) is the water tower of Ethiopia and downstream countries. It contributes significant moisture to the surrounding atmosphere. However, the contribution of the moisture from the basin to the precipitation in the UBNB is not well documented. Therefore, this paper is aimed to estimate the role of UBNB moisture budget and recycling ratio for spatiotemporal precipitation distributions. To this end, we used the European Center for Medium-range Weather Forecast (ECMWF) data from 1979 to 2017. The derived ECMWF results are correlated with in-situ observations with the correlation coefficient of 0.82. During summer season most of the UBNB moisture is converted to precipitation around the central parts of the study area, while in spring it contributes to the southern parts of the study area. Furthermore, the northwest part of the study area is affected by the basins moisture during the autumn season. The calculated recycling ratios for four seasons (summer, autumn, spring, and winter) are 9.70%, 16.33%, 19.01%, and 35.30% respectively with the annual average value of 20.11%. It is evident that the maximum amount of precipitation is captured from the local moisture during the winter season. Generally, UBNB moisture budget had a lesser contribution of precipitation over the study area. It rather contributed significant precipitation to the neighboring countries. Hence, further studies on moisture budget are required to explain this phenomenon in the context of Ethiopia.

Linking Atmospheric Rivers and Warm Conveyor Belt Airflows

Abstract Extreme precipitation associated with extratropical cyclones can lead to flooding if cyclones track over land. However, the dynamical mechanisms by which moist air is transported into cyclones is poorly understood. In this paper we analyze airflows within a climatology of cyclones in order to understand how cyclones redistribute moisture stored in the atmosphere. This analysis shows that within a cyclone’s warm sector the cyclone-relative airflow is rearwards relative to the cyclone propagation direction. This low-level airflow (termed the feeder airstream) slows down when it reaches the cold front, resulting in moisture flux convergence and the formation of a band of high moisture content. One branch of the feeder airstream turns toward the cyclone center, supplying moisture to the base of the warm conveyor belt where it ascends and precipitation forms. The other branch turns away from the cyclone center exporting moisture from the cyclone. As the cyclone travels, this export results in a filament of high moisture content marking the track of the cyclone (often used to identify atmospheric rivers). We find that both cyclone precipitation and water vapor transport increase when moisture in the feeder airstream increases, thus explaining the link between atmospheric rivers and the precipitation associated with warm conveyor belt ascent. Atmospheric moisture budgets calculated as cyclones pass over fixed domains relative to the cyclone tracks show that continuous evaporation of moisture in the precyclone environment moistens the feeder airstream. Evaporation behind the cold front acts to moisten the atmosphere in the wake of the cyclone passage, potentially preconditioning the environment for subsequent cyclone development.

Thermodynamic and Dynamic Responses to Deforestation in the Maritime Continent: A Modeling Study

Abstract Tropical deforestation can result in substantial changes in local surface energy and water budgets, and thus in atmospheric stability. These effects may in turn yield changes in precipitation. The Maritime Continent (MC) has undergone severe deforestation during the past few decades but it has received less attention than the deforestation in the Amazon and Congo rain forests. In this study, numerical deforestation experiments are conducted with global (i.e., Community Earth System Model) and regional climate models (i.e., Regional Climate Model version 4.6) to investigate precipitation responses to MC deforestation. The results show that the deforestation in the MC region leads to increases in both surface temperature and local precipitation. Atmospheric moisture budget analysis reveals that the enhanced precipitation is associated more with the dynamic component than with the thermodynamic component of the vertical moisture advection term. Further analyses on the vertical profile of moist static energy indicate that the atmospheric instability over the deforested areas is increased as a result of anomalous moistening at approximately 800–850 hPa and anomalous warming extending from the surface to 750 hPa. This instability favors ascending air motions, which enhance low-level moisture convergence. Moreover, the vertical motion increases associated with the MC deforestation are comparable to those generated by La Niña events. These findings offer not only mechanisms to explain the local climatic responses to MC deforestation but also insights into the possible reasons for disagreements among climate models in simulating the precipitation responses.

Enhanced Latent Heating over the Tibetan Plateau as a Key to the Enhanced East Asian Summer Monsoon Circulation under a Warming Climate

AbstractCoupled climate system models consistently show that the low-level southerly wind associated with the East Asian summer monsoon (EASM) is enhanced under anthropogenic greenhouse gas forcing, and the enhanced EASM was attributed to the enhanced land–sea thermal contrast by previous studies. Based on a comparison of the global warming scenarios with the present-day climate in an ensemble of 30 coupled models from phase 5 of the Coupled Model Intercomparison Project (CMIP5), we show evidence that changes in land–sea thermal contrast cannot explain the enhanced EASM circulation in terms of the seasonality. Indeed, the enhanced low-level southerly wind over East Asia is associated with a large-scale anomalous cyclone around the Tibetan Plateau (TP), and numerical simulation by the Linear Baroclinic Model suggests that the enhanced latent heating over the TP associated with enhanced precipitation is responsible for this low-level cyclone anomaly and the enhanced EASM circulation projected by the coupled models. Moisture budget analysis shows that enhanced hydrological recycling and enhanced vertical moisture advection due to increased specific humidity have the largest contribution to the increased precipitation over the TP, and more than half of the intermodel uncertainty in the projected change of EASM circulation is associated with the uncertainty in the changes of precipitation over the TP. Therefore, the TP plays an essential role in enhancing the EASM circulation under global warming through enhanced latent heating over the TP.

Effect of marginal topography around the Tibetan Plateau on the evolution of central Asian arid climate: Yunnan–Guizhou and Mongolian Plateaux as examples

Mountains are believed to have played an important role in the evolution of modern arid climate over central Asia. The main topography of the Tibetan Plateau (TP) suppresses the regional atmospheric rainfall by both the modulation of atmospheric circulation and blocking of water vapor transport from the ocean. In this study, the effect of Yunnan–Guizhou and Mongolian Plateaux (YGP and MP, respectively), two marginal topographies around the main TP, on the central Asian aridity are evaluated using general circulation model experiments. The results show that the precipitation over central Asia is significantly decreased by these two topographies. Compared to the whole TP-induced annual precipitation decrease of 0.45 mm/d, the contributions of the YGP and MP reach 0.14 mm/d and 0.08 mm/d, respectively. These two marginal mountains occupy approximately one half of the total change by the TP although they are much smaller in heights and sizes. The orographic forcing of the TP suppresses the precipitation significantly throughout the year while those of YGP and MP are mainly effective in boreal winter. A moisture budget analysis shows that all the mountains examined drive increases in subsidence and resulting decreases in humidity over central Asia, with smaller or opposing roles for changes in horizontal winds. These subsidence changes dominate the drying of Central Asia due to the TP and MP, and are largely driven by the influences of the topography on stationary waves. In contrast, the YGP dries Central Asia primarily through altering transient eddies. The forcing of YGP and MP originate from the mechanical blocking of the tropical easterly and mid-latitude westerly, respectively, which exert significant changes in atmospheric circulation. This implies that the effect of small-scale mountains on arid climate over central Asia might be underestimated and a considerable proportion of mechanical effect of the TP on the Asian aridity actually comes from its margins.

The coupling of deep convection with the resolved flow via the divergence of mass flux in the IFS

The resolution of the European Centre for Medium‐range Weather Forecast (ECMWF) integrated forecast system (IFS) is expected to reach 5 km in the coming decade. Assumptions in the parametrization of deep convection, such as that all of the compensating environmental flow occurs in the grid column, i.e. the convective and environmental mass fluxes cancel each other in term of mass transport, have to be challenged. In this paper, we further develop the original concept of separating the convective updraught from the subsiding branch of the overturning convective circulation and apply it to the global hydrostatic equations of the IFS. In practice, this constitutes a revised convection–dynamics coupling where the mass flux subsidence of the dynamical variables is not computed locally by the convection scheme, but instead is recomputed from the revised continuity equation and is effective through the semi‐Lagrangian advection of the dynamical core. Therefore horizontal divergence/convergence is also generated in the dynamics at the top/bottom of the convective columns, thus adding to the representation of deep convection a three‐dimensional character which is not present in traditional schemes. The proposed physics–dynamics coupling is intended to be applicable to any mass flux convection scheme and within any regional or global model. We first demonstrate the accuracy of the revised physics–dynamics coupling in terms of global temperature and moisture budgets. The potential impact of the coupling on the convective organization is demonstrated for an idealized squall line case at high horizontal resolution using the small planet testbed. Model reforecasts at 9 km and 5 km resolution confirm the viability of the method in terms of forecast skill and model climate. However, the model impacts are limited as the main factor that still determines the convective stabilization and organization is the current conceptual model of subgrid mass flux which, actually, remains unchanged.

Mechanism for asymmetric atmospheric responses in the western North Pacific to El Niño and La Niña

The cause of asymmetric atmospheric circulation responses over the tropical western North Pacific (WNP) to El Nino and La Nina was investigated through observational analyses and idealized modeling experiments. Firstly, column integrated moisture and moist static energy budget analyses were carried out to reveal the cause of asymmetric precipitation anomalies over the WNP. The result indicates that negative nonlinear moist enthalpy advection anomalies occur in both El Nino and La Nina, and they tend to induce a negative precipitation anomaly in the key WNP region and thus an anomalous anticyclone during both El Nino and La Nina winters. This, together with linear moist enthalpy advection, results in an asymmetric atmospheric circulation response. Secondly, the relative roles of the nonlinear moist enthalpy advection and the zonal shift of longitudinal location of anomalous heating over the central-eastern Pacific between El Nino and La Nina were investigated through an anomaly general circulation model. It is found that both the nonlinear advection and the zonally asymmetric heating contribute equally to the observed zonal shift of the anomalous WNP anticyclonic and cyclonic circulation centers between El Nino and La Nina.

Relative contributions of interdecadal and interannual SST variations to tropical precipitation decadal mean change in the late 1990s

A prominent precipitation decrease occurred over the equatorial central Pacific in the late 1990s, accompanied by precipitation increase around the Maritime Continent and over the equatorial America. Previous studies attributed the above change to La Nina-like decadal mean sea surface temperature (SST) cooling associated with a positive to negative phase switch of the Pacific Decadal Oscillation (PDO). Results of numerical experiments with an atmospheric general circulation model reveal that both the interdecadal and interannual components of SST variations contribute to the late 1990s’ precipitation reduction over the equatorial central Pacific in all the four seasons and the precipitation increase around the Maritime Continent in winter and summer. The accompanying precipitation increase over the Central America is mainly induced by the interdecadal components of SST variations. The contribution of interannual SST variations to the equatorial central Pacific precipitation decrease mostly stems from a larger rate of precipitation change with SST in positive than negative SST anomaly years, which leads to a residual decadal mean precipitation being larger during the period before than after the late 1990s. The moisture budget decomposition demonstrates that the dynamic effect associated with the vertical motion change dominates the tropical decadal mean precipitation changes in all the four seasons and the thermodynamic effect associated with the moisture change is small. This applies to the equatorial central Pacific, the Maritime Continent, and the Central America in both interdecadal and interannual SST forced simulations.

Influence of the Kuroshio Interannual Variability on the Summertime Precipitation over the East China Sea and Adjacent Area

AbstractMuch attention has been paid to the climatic impacts of changes in the Kuroshio Extension, instead of the Kuroshio in the East China Sea (ECS). This study, however, reveals the prominent influences of the lateral shift of the Kuroshio at interannual time scale in late spring [April–June (AMJ)] on the sea surface temperature (SST) and precipitation in summer around the ECS, based on high-resolution satellite observations and ERA-Interim. A persistent offshore displacement of the Kuroshio during AMJ can result in cold SST anomalies in the northern ECS and the Japan/East Sea until late summer, which correspondingly causes anomalous cooling of the lower troposphere. Consequently, the anomalous cold SST in the northern ECS acts as a key driver to robustly enhance the precipitation from the Yangtze River delta to Kyushu in early summer (May–August) and over the central ECS in late summer (July–September). In view of the moisture budget analysis, two different physical processes modulated by the lateral shift of the Kuroshio are identified to account for the distinct responses of precipitation in early and late summer, respectively. First, the anomalous cold SST in the northern ECS induced by the Kuroshio offshore shift is likely conducive to the earlier arrival of the mei-yu–baiu front at 30°–32°N and its subsequent slower northward movement, which may prolong the local rainy season, leading to the increased rain belt in early summer. Second, the persistent cold SST anomalies in late summer strengthen the near-surface baroclinicity and the associated strong atmospheric fronts embedded in the extratropical cyclones over the central ECS, which in turn enhances the local rainfall.