Atmospheric General Circulation Model Simulations Research Articles

The role of the Indian monsoon onset in the West African monsoon onset: observations and AGCM nudged simulations

In spring the inland penetration of the West African Monsoon (WAM) is weak and the associated rainband is located over the Guinean coast. Then within a few days deep convection weakens considerably and the rainband reappears about 20 days after over the Sahel, where it remains until late September signalling the summer rainy season. Over the period 1989–2008 a teleconnection induced by the Indian monsoon onset is shown to have a significant impact on the WAM onset, by performing composite analyses on both observational data sets and atmospheric general circulation model simulations ensembles where the model is nudged to observations over the Indian monsoon sector. The initiation of convective activity over the Indian subcontinent north of 15°N at the time of the Indian monsoon onset results in a westward propagating Rossby wave establishing over North Africa 7–15 days after. A back-trajectory analysis shows that during this period, dry air originating from the westerly subtropical jet entrance is driven to subside and move southward over West Africa inhibiting convection there. At the same time the low-level pressure field over West Africa reinforces the moisture transport inland. After the passage of the wave, the dry air intrusions weaken drastically. Hence 20 days after the Indian monsoon onset, convection is released over the Sahel where thermodynamic conditions are more favourable. This scenario is very similar in the observations and in the nudged simulations, meaning that the Indian monsoon onset is instrumental in the WAM onset and its predictability at intraseasonal scale.

Precipitation response to land subsurface hydrologic processes in atmospheric general circulation model simulations

Several studies have established that soil moisture increases after adding a groundwater component in land surface models, owing to the additional supply of subsurface water. However, the impact of groundwater on the spatial-temporal variability of precipitation has received little attention. This study explores how a groundwater representation in land surface models alters precipitation distributions through coupled groundwater-land-atmosphere simulations. Results indicate that the addition of groundwater yields a global increase in soil water content and evapotranspiration, a decrease in surface air temperature, and an increase in cloud cover fraction. These result in globally inhomogeneous changes in precipitation. In the boreal summer, tropical land regions show a positive anomaly in the Northern Hemisphere and a negative anomaly in the Southern Hemisphere. As a result, an asymmetric dipole is found over tropical land regions along the equator. Furthermore, in the transition climatic zone where the land and atmosphere are strongly coupled, precipitation also increases. Two main mechanisms are suggested for the two different regions with increased precipitation. The “rich-get-richer” mechanism is responsible for the positive precipitation anomalies over the tropical land regions, while a positive feedback of land-atmosphere interaction is the major contributor to increased precipitation over central North America. This study highlights the importance of land subsurface hydrologic processes in the climate system and has further implications for global water cycle dynamics.

The effect of zonally asymmetric ozone heating on the Northern Hemisphere winter polar stratosphere

[1] Previous modeling studies have found significant differences in winter extratropical stratospheric temperatures depending on the presence or absence of zonally asymmetric ozone heating (ZAOH), yet the physical mechanism causing these differences has not been fully explained. The present study describes the effect of ZAOH on the dynamics of the Northern Hemisphere extratropical stratosphere using an ensemble of free-running atmospheric general circulation model simulations over the 1 December - 31 March period. We find that the simulations including ZAOH produce a significantly warmer and weaker stratospheric polar vortex in mid-February due to more frequent major stratospheric sudden warmings compared to the simulations using only zonal mean ozone heating. This is due to regions of enhanced Eliassen-Palm flux convergence found in the region between 40°N–70°N latitude and 10–0.05 hPa. These results are consistent with changes in the propagation of planetary waves in the presence of ZAOH predicted by an ozone-modified refractive index.

An assessment and interpretation of the observed warming of West Antarctica in the austral spring

We synthesize variability and trends in multiple analyses of Antarctic near-surface temperature representing several independent source datasets and spatially complete reconstructions, and place these into the broader context of the behavior of other components of the climate system during the past 30–50 years. Along with an annual-mean trend during the past 50 years of about 0.1°C/decade averaged over Antarctica, there is a distinct seasonality to the trends, with insignificant change (and even some cooling) in austral summer and autumn in East Antarctica, contrasting with warming in austral winter and spring. Apart from the Peninsula, the seasonal warming is largest and most significant in West Antarctica in the austral spring since the late 1970s. Concurrent trends in sea ice are independent evidence of the observed warming over West Antarctic, with the decrease in sea ice area in the Amundsen and Bellingshausen Seas congruent with at least 50% of the inland warming of West Antarctica. Trends in near surface winds and geopotential heights over the high-latitude South Pacific are consistent with a role for atmospheric forcing of the sea ice and air temperature anomalies. Most of the circulation trend projects onto the two Pacific South American (PSA) modes of atmospheric circulation variability, while the Southern Annular Mode lacks a positive trend in spring that would otherwise cause a cooling tendency. The largest circulation trend is associated with the PSA-1 mode, a wave-train extending from the tropics to the high Southern latitudes. The PSA-1 mode is significantly correlated with SSTs in the southwestern tropical and subtropical Pacific. The increased SSTs in this region, together with the observed increase in rainfall, suggest that anomalous deep convection has strengthened or increased the occurrence of the Rossby wave-train associated with PSA-1. This hypothesis is supported by results from two ensembles of SST-forced atmospheric general circulation model simulations. Finally, the implications of the seasonality, timing, and spatial patterns of Antarctic temperature trends with respect to interpreting the relative roles of stratospheric ozone depletion, SSTs and increased atmospheric concentrations of greenhouse gasses are discussed.

FIRST STUDY OF POTENTIAL PREDICTABILITY OF RIVER DISCHARGES AND ITS DEPENDENCY ON HORIZONTAL RESOLUTION USING AGCM

Previous studies have used low-resolution models to investigate the potential predictability of river discharges. The present study conducts a preliminary investigation of this predictability based on a dynamical seasonal prediction method using an atmospheric general circulation model (AGCM) and studies its dependency on the AGCM’s horizontal resolution. I performed 60-km and 180-km mesh AGCM simulations forced with the observed sea-surface temperatures and sea-ice concentration and simulations performed by converting the 60-km mesh simulation results to 180-km mesh ones and vice-versa. The global geographical distributions of the potential predictability resemble those of previous studies; in addition, the river basin correction effect is reconfirmed and found in basins exceeding 104km2 in area. Comparisons of the potential predictability among the multiple simulations demonstrate that upscaling loses the detailed features of the potential predictability, while downscaling retains the details. Dominant sources of the difference in the potential predictability of river discharge between the 60 km and 180 km mesh AGCMsimulations are their atmospheric potential predictabilities.

The Roles of External Forcings and Internal Variabilities in the Northern Hemisphere Atmospheric Circulation Change from the 1960s to the 1990s

Abstract The Northern Hemisphere atmospheric circulation change from the 1960s to the 1990s shows a strong positive North Atlantic Oscillation (NAO) and a deepening of the Aleutian low. The issue regarding the contributions of external forcings and internal atmospheric variability to this circulation change has not been resolved satisfactorily. Previous studies have found the importance of tropical SST forcing. Here, this hypothesis is examined again using relatively large ensembles of atmospheric general circulation model simulations of the twentieth-century climate forced only by historically varying SST. The resulting ensemble-mean amplitude underestimates the observed change by at least 70%, although the spatial pattern is reproduced well qualitatively. Furthermore, AGCM experiments are performed to investigate other driving factors, such as the greenhouse gases, sea ice, the stratospheric ozone, as well as the contribution from atmospheric internal variability. The increase in ensemble-mean trend amplitude induced by these additional drivers was not enough to substantially improve the agreement with the observed trend. However, the full distribution of simulated trends reveals that the ensemble members at the upper tail are much closer to the observed amplitude. In the “best” ensemble, the 95th percentile of the simulated NAO trend amplitude remains at about 80% of the observed trend amplitude, with nearly equal contributions from external forcings and internal variability. The results also indicate that a complete set of driving factors and a correct simulation of stratospheric trends are important in bridging the gap between observed and modeled interdecadal variability in the North Atlantic winter circulation.

Multimodel Estimates of Atmospheric Response to Modes of SST Variability and Implications for Droughts

Abstract A set of idealized global model experiments was performed by several modeling centers as part of the Drought Working Group of the U.S. Climate Variability and Predictability component of the World Climate Research Programme (CLIVAR). The purpose of the experiments was to assess the role of the leading modes of sea surface temperature (SST) variability on the climate over the continents, with particular emphasis on the influence of SSTs on surface climate variability and droughts over the United States. An analysis based on several models gives more creditability to the results since it relies on the assessment of impacts that are robust across different models. Coordinated atmospheric general circulation model (AGCM) simulations forced with three modes of SST variability were analyzed. The results show that the SST-forced precipitation variability over the central United States is dominated by the SST mode with maximum loading in the central Pacific Ocean. The SST mode with loading in the Atlantic Ocean, and a mode that is dominated by trends in SSTs, lead to a smaller response. Based on the response to the idealized SSTs, the precipitation response for the twentieth century was also reconstructed. A comparison with the Atmospheric Model Intercomparison Project (AMIP) simulations forced with the observed SSTs illustrates that the reconstructed precipitation variability was similar to the one in the AMIP simulations, further supporting the conclusion that the SST modes identified in the present analysis play a dominant role in the precipitation variability over the United States. One notable exception is the Dust Bowl of the 1930s, and further analysis regarding this major climate extreme is discussed.

The crucial role of ocean–atmosphere coupling on the Indian monsoon anomalous response during dipole events

This paper examines an issue concerning the simulation of anomalously wet Indian summer monsoons like 1994 which co-occurred with strong positive Indian Ocean Dipole (IOD) conditions in the tropical Indian Ocean. Contrary to observations it has been noticed that standalone atmospheric general circulation models (AGCM) forced with observed SST boundary condition, consistently depicted a decrease of the summer monsoon rainfall during 1994 over the Indian region. Given the ocean–atmosphere coupling during IOD events, we have examined whether the failure of standalone AGCM simulations in capturing wet Indian monsoons like 1994 can be remedied by including a simple form of coupling that allows the monsoon circulation to dynamically interact with the IOD anomalies. With this view, we have performed a suite of simulations by coupling an AGCM to a slab-ocean model with spatially varying mixed-layer-depth (MLD) specified from observations for the 1994 IOD; as well as four other cases (1983, 1997, 2006, 2007). The specification of spatially varying MLD from observations allows us to constrain the model to observed IOD conditions. It is seen that the inclusion of coupling significantly improves the large-scale circulation response by strengthening the monsoon cross-equatorial flow; leading to precipitation enhancement over the subcontinent and rainfall decrease over south-eastern tropical Indian Ocean—in a manner broadly consistent with observations. A plausible physical mechanism is suggested to explain the monsoonal response in the coupled frame-work. These results warrant the need for improved monsoon simulations with fully coupled models to be able to better capture the observed monsoon interannual variability.

Permanent El Niño and the onset of Northern Hemisphere glaciations: Mechanism and comparison with other hypotheses

[1] The tropical Pacific transitioned from a “permanent El Nino” state, that is, without the current equatorial zonal gradient of sea surface temperatures, to one with an equatorial cold tongue in a process that started ∼4 Ma, during the early Pliocene. The latter stages of this transition, around ∼2.75 Ma, marked the onset of Northern Hemisphere glaciations. We explore the hypothesis that this onset of glaciation is causally linked to the development of the cold tongue guided by atmosphere general circulation model (AGCM) simulations forced by various hypothesized climate influences. A unique feature of our simulations is imposing permanent El Nino conditions not by imposing sea surface temperatures anomalies directly but rather through changing the implied ocean heat transport in the slab ocean component of the AGCM, done in such a way as to maintain global ocean heat balance. Similar to previous studies, we find increased near-surface temperature conditions occur during simulated permanent El Nino conditions at the locations of known ice sheet growth during glacial periods. We establish that this tropical-extratropical connection is mediated through atmospheric stationary waves, a well-known atmospheric pathway for today's El Nino influence on North America. The influence of the permanent El Nino condition is compared, through model simulations, to other mechanisms proposed to date to explain the onset of glaciations at ∼2.75 Ma: changes in atmospheric carbon dioxide (CO2) concentrations, closure of the Panama seaway, and changes in North Pacific seasonality. The exact comparison is done by calculating differences in the equilibrium line altitude at the known locations of ice sheet growth, an approach that permits the translation of climate anomalies into a measurement of glaciation trends in terms of glacier mass balance. We find that permanent El Nino and atmospheric CO2 are the leading influences in this comparison, leading us to conjecture that the two influences combined determined the long-term background conditions governing ice sheet growth. Within this long-term background evolution, however, we additionally find that the exact timing of Northern Hemisphere glaciations is likely to have come from favorable orbital conditions. Our results regarding permanent El Nino on Northern Hemisphere glaciations is, however, conditioned on a strong sensitivity of Northern Hemisphere climate changes to how the oceanic heat compensation to the permanent El Nino is achieved.

Climate forcings and climate sensitivities diagnosed from atmospheric global circulation models

Understanding the historical and future response of the global climate system to anthropogenic emissions of radiatively active atmospheric constituents has become a timely and compelling concern. At present, however, there are uncertainties in: the total radiative forcing associated with changes in the chemical composition of the atmosphere; the effective forcing applied to the climate system resulting from a (temporary) reduction via ocean-heat uptake; and the strength of the climate feedbacks that subsequently modify this forcing. Here a set of analyses derived from atmospheric general circulation model simulations are used to estimate the effective and total radiative forcing of the observed climate system due to anthropogenic emissions over the last 50 years of the twentieth century. They are also used to estimate the sensitivity of the observed climate system to these emissions, as well as the expected change in global surface temperatures once the climate system returns to radiative equilibrium. Results indicate that estimates of the effective radiative forcing and total radiative forcing associated with historical anthropogenic emissions differ across models. In addition estimates of the historical sensitivity of the climate to these emissions differ across models. However, results suggest that the variations in climate sensitivity and total climate forcing are not independent, and that the two vary inversely with respect to one another. As such, expected equilibrium temperature changes, which are given by the product of the total radiative forcing and the climate sensitivity, are relatively constant between models, particularly in comparison to results in which the total radiative forcing is assumed constant. Implications of these results for projected future climate forcings and subsequent responses are also discussed.

Role of tropical Pacific SSTs in global medieval hydroclimate: A modeling study

The role of tropical Pacific SSTs in driving global medieval hydroclimate is assessed. Using fossil coral records from Palmyra Atoll, tropical Pacific sea surface temperature (SST) boundary conditions are derived for the period 1320–1462 A.D. These boundary conditions consist of La Niña‐like mean state conditions in the tropical Pacific with inter‐annual and decadal variability about that altered state. The reconstructed SSTs in the tropical Pacific are used to force a 16 member ensemble of atmospheric general circulation model (AGCM) simulations, coupled to a one layer ocean model outside of the tropical Pacific. The AGCM simulations of medieval climate are compared with modern climate simulations for the period 1856–2005 A.D. and are shown to reproduce many aspects of medieval hydroclimate found in paleo‐proxy records for much of the Western Hemisphere, northern Eurasia, and the northern tropics. These results suggest that many features of global medieval hydroclimate changes can be explained by changes in tropical Pacific SSTs, though the potential role for other oceans is also discussed.

The Delayed Effect of Major El Niño Events on Indian Monsoon Rainfall

Abstract Previous studies have shown that boreal summer Indian monsoon rainfall is, on average, significantly above normal after major El Niño events. In this study, the underlying causes of this rainfall response are examined using both observational analysis and atmospheric general circulation model (AGCM) simulations. Moist static energy budgets for two strong El Niño events (1982/83 and 1997/98), estimated from monthly 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40), suggest that stronger low-level moisture transport and reduced moist stability associated with a warmer north Indian Ocean (NIO) can increase monsoon rainfall, despite a weakened monsoon circulation. The trade-off between a dynamically weaker monsoon and moist processes favoring enhanced monsoonal rainfall is broken during the late monsoon season (August–September) as the warm NIO enhances surface latent heat flux and the monsoon circulation relaxes back to the climatological mean. The monsoon circulation strength and the moist processes work together in the late season, which explains the observed tendency for monsoonal rainfall increases during the late monsoon season after strong winter El Niño conditions. Idealized AGCM experiments with a fixed-depth ocean mixed layer demonstrate that the remnant but weaker-than-peak warm SSTs in the eastern equatorial Pacific during spring and the early summer following winter El Niños substantially contribute to the NIO warming. The results suggest that local air–sea interactions in the tropical Indian Ocean after winter El Niño are strongly dependent on the details of El Niño’s decaying trend.

Low-Frequency Variability in a Turbulent Baroclinic Jet: Eddy–Mean Flow Interactions in a Two-Level Model

Abstract The origin of low-frequency variability in the midlatitude jet is investigated using a two-level baroclinic channel model. The model state fields are separated into slow and fast components using intermediate time- scale averaging. In the equation for the fast variables the nonlinear wave–wave interactions are parameterized as a stochastic excitation. The slowly varying ensemble mean eddy fluxes obtained from the resulting stochastic turbulence model are coupled with the slowly varying mean flow dynamics. This forms a coupled set of deterministic equations on the slow time scale that governs the dynamics of the eddy–mean flow interaction. The equilibria of this coupled system are found as a function of the excitation strength, which controls the level of turbulence. At low levels of turbulence the equilibrated flow with zonally symmetric mean forcing remains zonally symmetric, but as excitation increases it undergoes zonal symmetry-breaking bifurcations. Time-dependent flows arising from these bifurcations take the form of westward-propagating wavelike structures resembling blocking patterns. Features of these waves are characteristic of blocking in both observations and atmospheric general circulation model simulations including retrogression, eddy variance concentrated upstream of the waves, and eddy momentum flux forcing the waves.

Intraseasonal Variability in an Aquaplanet General Circulation Model

An aquaplanet atmospheric general circulation model simulation with a robust intraseasonal oscillation is analyzed. The SST boundary condition resembles the observed December‐April average with continents omitted, although with the meridional SST gradient reduced to be one‐quarter of that observed poleward of 10° latitude. Slow, regular eastward propagation at 5 m s−1 in winds and precipitation with amplitude greater than that in the observed MJO is clearly identified in unfiltered fields. Local precipitation rate is a strongly non‐linear and increasing function of column precipitable water, as in observations. The model intraseasonal oscillation resembles a moisture mode that is destabilized by wind‐evaporation feedback, and that propagates eastward through advection of anomalous humidity by the sum of perturbation winds and mean westerly flow. A series of sensitivity experiments are conducted to test hypothesized mechanisms. A mechanism denial experiment in which intraseasonal latent heat flux variability is removed largely eliminates intraseasonal wind and precipitation variability. Reducing the lower‐troposphere westerly flow in the warm pool by reducing the zonal SST gradient slows eastward propagation, supporting the importance of horizontal advection by the low‐level wind to eastward propagation. A zonally symmetric SST basic state produces weak and unrealistic intraseasonal variability between 30 and 90 day timescales, indicating the importance of mean low‐level westerly winds and hence a realistic phase relationship between precipitation and surface flux anomalies for producing realistic tropical intraseasonal variability.

Towards Direct Simulation of Future Tropical Cyclone Statistics in a High‐Resolution Global Atmospheric Model

We present a set of high‐resolution global atmospheric general circulation model (AGCM) simulations focusing on the model′s ability to represent tropical storms and their statistics. We find that the model produces storms of hurricane strength with realistic dynamical features. We also find that tropical storm statistics are reasonable, both globally and in the north Atlantic, when compared to recent observations. The sensitivity of simulated tropical storm statistics to increases in sea surface temperature (SST) is also investigated, revealing that a credible late 21st century SST increase produced increases in simulated tropical storm numbers and intensities in all ocean basins. While this paper supports previous high‐resolution model and theoretical findings that the frequency of very intense storms will increase in a warmer climate, it differs notably from previous medium and high‐resolution model studies that show a global reduction in total tropical storm frequency. However, we are quick to point out that this particular model finding remains speculative due to a lack of radiative forcing changes in our time‐slice experiments as well as a focus on the Northern hemisphere tropical storm seasons.

Influence of the Maritime Continent on the Boreal Summer Intraseasonal Oscillation

The effect of the Maritime Continent (MC) on the propagation characteristics of the boreal summer intraseasonal oscillation (ISO) over the Indo-western Pacific region were investigated by performing high-resolution (T159) atmospheric general circulation model (AGCM) simulations that remove and retain the MC. The most significant difference, as revealed by a finite domain wavenumber-frequency spectral analysis is the weakening of the northward propagation of ISO over the Asian monsoon region (65° -160°E) when the MC is removed; a less significant difference is the enhancement of the eastward propagation along the equator. The diagnosis of the vertical structure of the simulated ISO and the model mean flow indicates that the weakening of the northward propagation is primarily attributed to the reduction of the background easterly shear, low-level southerly and meridional humidity gradient, all of which contribute to the weakening of meridional asymmetries of vorticity and humidity fields with respect to the ISO convection center. The enhanced eastward propagation is possibly attributed to the strengthening of the mean convection over the MC in association with the increase of the local surface moisture and moist static energy.

Analysing and combining atmospheric general circulation model simulations forced by prescribed SST: northern extratropical response

The ECHAM 3.2 (T21), ECHAM 4 (T30) and LMD (version 6, grid-point resolution with 96 longitudes × 72 latitudes) atmospheric general circulation models were integrated through the period 1961 to 1993 forced with the same observed Sea Surface Temperatures (SSTs) as compiled at the Hadley Centre. Three runs were made for each model starting from different initial conditions. The mid-latitude circulation pattern which maximises the covariance between the simulation and the observations, i.e. the most skilful mode, and the one which maximises the covariance amongst the runs, i.e. the most reproducible mode, is calculated as the leading mode of a Singular Value Decomposition (SVD) analysis of observed and simulated Sea Level Pressure (SLP) and geopotential height at 500 hPa (Z500) seasonal anomalies. A common response amongst the different models, having different resolution and parametrization should be considered as a more robust atmospheric response to SST than the same response obtained with only one model. A robust skilful mode is found mainly in December-February (DJF), and in June-August (JJA). In DJF, this mode is close to the SST-forced pattern found by Straus and Shukla (2000) over the North Pacific and North America with a wavy out-of-phase between the NE Pacific and the SE US on the one hand and the NE North America on the other. This pattern evolves in a NAO-like pattern over the North Atlantic and Europe (SLP) and in a more N-S tripole on the Atlantic and European sector with an out-of-phase between the middle Europe on the one hand and the northern and southern parts on the other (Z500). There are almost no spatial shifts between either field around North America (just a slight eastward shift of the highest absolute heterogeneous correlations for SLP relative to the Z500 ones). The time evolution of the SST-forced mode is moderatly to strongly related to the ENSO/LNSO events but the spread amongst the ensemble of runs is not systematically related at all to the intensity of Nino3.4 SST anomalies. The leading reproducible mode in JJA is clearer and more skilful for SLP than for Z500 and also seems related to the SST time evolution of tropical Pacific. It is characterised by an out-of-phase between the whole North Pacific and a horseshoe shaped area from Eastern Siberia and Gulf of Mexico. The leading OM mode found in MAM and SON, are quite close to the DJF one (at least for the modelled anomalies), but they are less skilful than in DJF. The most skilful mode (i.e. SLP-Z500 mode in DJF and SLP mode in JJA) is almost similar to the most reproducible one during these particular seasons. In MAM and SON, the SST-forced pattern is very close to the wintertime one. The warm episodes in the central and eastern tropical Pacific are then associated with negative pressure anomalies at the sea level and also at 500 hPa over the whole North Pacifkc and from SE US Coast to Western Europe (from SE US Coast to Scandinavia for Z500) and positive pressure anomalies on Central Canada, north of 55°-60°N across the North Atlantic and also over Northern Siberia (in MAM). The variance forced by SST are lower in MAM and SON than in DJF and, as suggested above, the skill of this SST-forced mode is weak in MAM and almost close to zero in SON.

A review on aspects of climate simulation assessment

This paper reviews some aspects of evaluation of climate simulation, including the ITCZ, the surface air temperature (SAT), and the monsoon. A brief introduction of some recently proposed approaches in weather forecast verification is followed by a discussion on their possible application to evaluation of climate simulation. The authors suggest five strategies to extend the forecast verification methods to climate simulation evaluation regardless significant differences between the forecasts and climate simulations. It is argued that resolution, convection scheme, stratocumulus cloud cover, among other processes in the atmospheric general circulation model (AGCM) and the ocean-atmosphere feedback are the potential causes for the double ITCZ problem in coupled models and AGCM simulations, based on the system- and component-level evaluations as well as the downscaling strategies in some recent research. Evaluations of simulated SAT and monsoons suggest that both coupled models and AGCMs show good performance in representing the SAT evolution and its variability over the past century in terms of correlation and wavelet analysis but poor at reproducing rainfall, and in addition, the AGCM alone is not suitable for monsoon regions due to the lack of air-sea interactions.

A reconstructed dynamic Indian monsoon index extended back to 1880

The authors present a reconstruction of summer (June–July–August) mean dynamic Indian monsoon index (DIMI) back to 1880 based on a large number of historical surface observation data as well as information from the upper air data. The reconstruction shows a satisfying skill in terms of both the value of reduction of error and an evaluation against other independent monsoon indices. The skill of reconstruction increases over time with more predictor data (in particular upper-level data) becoming available. A comparison with the observed all Indian summer monsoon rainfall index (AIRI) shows a high consistence in both inter-decadal and inter-annual variability. The reconstruction shows stronger than normal monsoon during the 1880s, 1915–1925 (around 1920) and 1930–1945 (around 1940) as the AIRI. The El Nino/Southern Oscillation (ENSO)—monsoon relationship is reasonably captured in the reconstruction. Powers concentrating within quasi-biennial band stand out in the reconstruction as well as in the AIRI. A comparison of the reconstruction against an atmospheric general circulation model simulation with specified SST and external forcing agents spanning 1901–1999 indicates a slightly higher reproducibility of monsoon circulation than monsoon rainfall in terms of interannual variability. The relationship between the Asian continent warming and the ENSO–monsoon connection is also discussed by using the new dynamic index.

Parameterization of PBL Processes in an Atmospheric General Circulation Model: Description and Preliminary Assessment

Abstract This paper presents the basic features of a newly developed planetary boundary layer (PBL) parameterization, and the performance assessment of a version of the University of California, Los Angeles (UCLA), Atmospheric General Circulation Model (AGCM) to which the parameterization is incorporated. The UCLA AGCM traditionally uses a framework in which a sigma-type vertical coordinate for the PBL shares a coordinate surface with the free atmosphere at the PBL top. This framework facilitates an explicit representation of processes concentrated near the PBL top, which is crucially important especially for predicting PBL clouds. In the new framework, multiple layers are introduced between the PBL top and earth’s surface, allowing for predictions of the vertical profiles of potential temperature, total water mixing ratio, and horizontal winds within the PBL. The vertically integrated “bulk” turbulent kinetic energy (TKE) is also predicted for the PBL. The PBL-top mass entrainment is determined through an equation including the effects of TKE and the radiative and evaporative cooling processes concentrated near the PBL top. The surface fluxes are determined from an aerodynamic formula in which the velocity scale depends both on the square root of TKE and the grid-scale PBL velocity at the lowermost model layer. The turbulent fluxes within the PBL are determined through an approach that includes the effects of both large convective and small diffusive eddies. AGCM simulations with the new formulation of PBL are analyzed with a focus on the seasonal and diurnal variations. The simulated seasonal cycle of stratocumulus over the eastern oceans is realistic, as are the diurnal cycles of the PBL depth and precipitation over land. The simulated fluxes of latent heat, momentum, and shortwave radiation at the ocean surface and baroclinic activity in the middle latitudes show significant improvements over the previous versions of the AGCM based on the single-layer PBL.