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This study reports on the analysis of the results from a 20 km grid spacing, Regional Coupled ocean–atmosphere Model (RCM) integration over the Western Pacific Warm Pool (WP2). The RCM was integrated over a 20‐year period (1986–2005) using reanalysis boundary conditions for the atmosphere and the ocean. This is a first‐of‐a‐kind study with an RCM at 20 km over the WP2. The RCM simulation shows reasonable fidelity of the mean state and of the Madden–Julian Oscillation (MJO). We utilize this successful integration of the RCM to understand a well‐known observed feature of MJOs in the WP2 to be of the strongest amplitude during the December–March period of the year. Our analysis of the model integration reveals that the recharge of moist static energy (MSE) prior to peak MJO convection and its discharge during and after the convection explains the MJO in the simulation. The recharge/discharge of the MSE is shown to be largely dictated by horizontal advection, which is stemmed to a small extent by column‐integrated radiative heating and surface evaporation. This balance of forces in the evolution of the MSE anomalies and their corresponding variations with sea‐surface temperature (SST) anomalies at MJO time‐scales in the WP2 is strongest in the December–March period in the RCM simulation. [ABSTRACT FROM AUTHOR]

This study examines the simulation of the heatwave events over Florida from the integration of two regional climate models (RCMs) at comparatively high resolution (at 10 km grid spacing) for two decades of the current climate (1986–2005) followed by their projection in the mid‐21st century (2041–2060). The two RCMs are coupled ocean–atmosphere (CRSM) and atmosphere only (URSM), which are forced at the lateral boundaries by the corresponding simulation of the Community Climate System Model v4 (CCSM4) for both the atmosphere and the ocean. The analysis reveals a reasonable simulation of the frequency, intensity and characteristic kinematic and thermodynamic features of the heatwave events over Florida from both the RCMs with the differences between URSM and CRSM being insignificant. The future projections (2041–2060) are presented only for URSM since the projected differences from CRSM were insignificant. We found the projected change of higher frequency ranging between 20 to over 50% increase relative to the late 20th‐century frequency of heatwave events. Similarly, the projections suggest a broadening of the season of their occurrence by over 2 months. These projected changes are associated with a projected increase in the mean surface temperature, a slight increase of surface humidity but with a mean reduction of surface relative humidity, and a westward dislocation of the North Atlantic Subtropical High. [ABSTRACT FROM AUTHOR]

We evaluate the impact of air‐sea coupling in simulating the characteristics of the Indian summer monsoon (ISM) and its intraseasonal oscillations (ISOs) by using a coupled ocean‐atmosphere regional climate model (CRSM) and an atmosphere‐only regional climate model (URSM). These 20‐km resolution regional climate models (RCMs) were driven by the boundary conditions from Community Climate System Model version 4 (CCSM4) global model. The mean ISM rainfall simulated by URSM and CRSM is comparable with observations and shows an improvement over the CCSM4. The systematic error in the spatial distribution of the mean June–September low‐level and upper‐level winds, sea surface temperature, and latent heat flux shows that CRSM performs better than URSM, suggesting the benefits of air‐sea coupling in the RCM simulations of the mean ISM. Both the CRSM and the URSM perform reasonably and comparably in capturing the spatiotemporal evolution of 10–20‐day high‐frequency and 20–70‐day low‐frequency ISOs of the ISM rainfall. These RCMs simulated the observed features of the ISO‐filtered rainfall anomalies that propagate at a faster rate across the Arabian Sea compared to that over the Bay of Bengal. Furthermore, the difference in the track density of the monsoon low‐pressure systems between the wet and the dry phases of ISOs shows insignificant differences between CRSM and URSM but is found to be superior to the CCSM4 simulation. This study suggests a relative insensitivity of air‐sea coupling in the RCM for simulating ISO and LPS features of ISM, which is likely abetted by the realistic ISO signals in the parent CCSM4. Plain Language Summary: We generated two different high‐resolution regional climate simulations: an atmosphere‐only regional climate model (RCM) and a coupled ocean‐atmosphere RCM forced with coarse‐resolution global fields. By using these high‐resolution regional climate simulations, we investigate how air‐sea coupling affects the observed characteristics of the Indian summer monsoon (ISM) and its intraseasonal oscillations (ISOs). The results of the 20‐year regional climate simulation from each of the RCMs reveal that they reasonably capture the observed mean climatology and ISOs of the ISM. The benefit of air‐sea coupling in simulating the mean ISM is clear from the comparisons between the coupled and uncoupled RCM simulations. Nonetheless, the differences in the simulated characteristics of the ISO and the monsoon low‐pressure systems between the RCMs were trivial. On the other hand, both the RCMs considerably outperform the global model in simulating the monsoon low‐pressure systems and their association with the ISOs of the ISM. This study concludes that ISO simulation in the RCM could be insensitive to air‐sea coupling because of realistic ISO signals in the driving global climate model. Key Points: Air‐sea coupling in dynamically downscaled simulation improves the mean Indian summer monsoon relative to the parent general circulation models (GCM) simulationCoupled ocean‐atmosphere and atmosphere‐only regional climate models (RCMs) simulate intraseasonal oscillations (ISOs) of the monsoon with only subtle differencesRCMs add value to downscaling by simulating realistic intraseasonal variability of the monsoon low‐pressure systems absent in the parent GCM [ABSTRACT FROM AUTHOR]

In this study, we present the results of a regional model (regional spectral model‐regional ocean model [(RSM‐ROMS]) simulation of the South Asian Summer Monsoon (SASM). The RSM‐ROMS integration is carried out at 20 km grid spacing over a period of 25 years (1986–2010). The simulation is forced by global atmospheric and oceanic reanalysis. The RSM‐ROMS simulation shows a realistic alignment of the simulated rainfall along the orographic features of the domain. Furthermore, the RSM‐ROMS simulates the observed feature of convection over continental SASM region being more vigorous with dominance of mixed warm and cold phase hydrometeors in contrast to the dominance of the warm rain process in the neighboring tropical oceans. Similarly, the upper ocean features of contrasting mixed layer and thermocline depths between the northern and equatorial Indian Ocean are also simulated in the RSM‐ROMS. Intra‐Seasonal Oscillation (ISO) of the SASM at 10–20 and 20–70 days are also simulated in the RSM‐ROMS with many of its features verifying with observations. For example, the 20–70 days ISO are of higher amplitude and its meridional propagation is slower in Bay of Bengal compared to that over Arabian Sea. Additionally, RSM‐ROMS shows 12.3 Monsoon Low Pressure Systems (LPSs) per season that is comparable to 14.6 per season from observations. Furthermore, the observed intraseasonal contrasts of LPS between the wet and dry spells of ISO is also reproduced in the RSM‐ROMS. Plain Language Summary: The South Asian Summer Monsoon (SASM) climate represents a complex mix of variations across many spatio‐temporal scales over a region with unique topographic and bathymetric features. As a result, the simulation of the SASM climate offers a stiff challenge to numerical climate models. In this study, we evaluate the added value of a regional coupled ocean‐atmosphere model in downscaling the SASM climate from a 2.5° × 2.5° global atmospheric reanalysis and 0.5° × 0.5° global ocean reanalysis to 20 km grid spacing. The regional model demonstrates significant skill in capturing the observed features of the intra‐seasonal oscillations of the SASM besides displaying reasonable fidelity of simulating and improving the mean climate of the SASM. Furthermore, at 20 km grid spacing, the regional model simulates the seasonal and intraseasonal activity of the monsoon low pressure systems that is comparable to observations. Therefore, the regional model at 20 km grid spacing of this study clearly demonstrates its added value to its driving global reanalysis in the simulation of the SASM climate, and its intra‐seasonal variations. The regional model simulation also demonstrates its fidelity from the verification of the seasonal and intra‐seasonal variability of the monsoon low pressure systems of the SASM. Key Points: The regional climate model displays a verifiable intraseasonal variability of precipitationThe regional climate model simulates the observed unique feature of intraseasonal variations of the monsoon low pressure systemsThe regional climate model shows the observed contrast of convection with warm rain process dominating over ocean compared to that over land [ABSTRACT FROM AUTHOR]

In this study, projected changes in mean northeast monsoon (NEM) rainfall and associated extreme rainfall and temperature events, over peninsular India (PI) and its six subdivisions, are quantified. High‐resolution dynamically downscaled simulations of the Weather Research and Forecasting (WRF) regional climate model driven by the boundary conditions from the Community Climate System Model version 4 (CCSM4) model (WRF‐CCSM4) are compared with statistically downscaled simulations of NASA Earth Exchange Global Daily Downscaled Projections (NEX‐GDDP). Over PI, these downscaled simulations show low bias in mean NEM rainfall (≤ − 0.44 mm·day−1) and high pattern correlation coefficient (≥0.75), giving confidence in their future projections. Under future warming over PI, both downscaled simulations project future significant enhancement in NEM rainfall with WRF‐CCSM4 projecting 1.98 mm·day−1 (83.78% change with respect to the present‐day mean) whereas the multimodel ensemble (MME) of eight NEX‐GDDP models project 0.67 ± 0.58 mm·day−1 (19.78%) by the midddle of the century and 1.42 ± 0.97 mm·day−1 (42.76%) by the end of the century. Analysis of extreme rainfall events shows that WRF‐CCSM4 projects future enhancement (reduction) in extreme rainfall (R95p) days over 91.4% (8.6%) of grid‐points over PI. In future, coastal areas of Karnataka and Andhra Pradesh will likely experience increased extreme rainfall occurrence by more than 25 days and 15–20 days respectively. Projected future enhancement in the mean and extreme NEM rainfall is attributed to the increased precipitable water under a warming climate. Future projection of extreme temperature indices shows an increase in minimum and maximum temperatures over PI during the NEM season. Over PI, future winter nights and days are found to be warmer than those in the present day and the temperature change in future winter nights is found to be larger than that in winter days. This climate change information would help decision‐makers in evaluating existing policies and devising revised policies to reduce risk due to climate change. [ABSTRACT FROM AUTHOR]

Abstract: An efficient, reliable very high resolution dynamical downscaling model, a regional climate model (Weather Research and Forecasting‐Advanced Research Weather Research and Forecasting) one‐way nested into skillful general circulation model (National Center for Atmospheric Research‐Community Climate System Model version 4), is configured and implemented for ecologically sensitive, densely populated west coast of India encompassing Western Ghats (WG) having complex, meridionally oriented orography and wide biodiversity. This model with 3 km resolution resolves orographic features enabling realistic simulation of physical and dynamical characteristics of present‐day Indian summer monsoon (ISM) and extreme events, particularly recent trends in ISM rainfall over WG as observed. Marked skill of this model provides confidence in its future climate projection at regional scale. Future ISM rainfall projection shows significant increase (reduction) over 50.7% (5.8%) of Indian grid points. Significant reduction (10–20% of mean) over WG is due to upper‐tropospheric warming effect that stabilizes the atmosphere. Projected changes in extreme events show overall increase in warm days and warm nights over India with maximum increase over South India. Projected changes show widespread increase in wet days over most of India and reduction over WG. Projection of consecutive dry days implies wetter future for most parts of India but strengthened drought conditions for WG. Wind extreme projection shows strengthened (weakened) low (high) winds probability over WG and increase (decrease) in very high (low) winds over central India. This study establishes the importance of (i) employing sufficiently high‐resolution model, (ii) using bias‐corrected boundary data, and (iii) configuring model for realistic present‐day climate over complex topographic coastlines such as the west coast, in order to obtain useful climate change information for adaptation measures. [ABSTRACT FROM AUTHOR]