Mean-state biases and interannual variability affect perceived sensitivities of the Madden-Julian oscillation to air-sea coupling

Atmosphere–ocean feedbacks often improve the Madden–Julian oscillation (MJO) in climate models, but these improvements are balanced by mean-state biases that can degrade the MJO through changing the basic state on which the MJO operates. The Super-Parameterized Community Atmospheric Model (SPCAM3) produces perhaps the best representation of the MJO among contemporary models, which improves further in a coupled configuration (SPCCSM3) despite considerable mean-state biases in tropical sea-surface temperatures and rainfall. We implement an atmosphere–ocean-mixed-layer configuration of SPCAM3 (SPCAM3-KPP) and use a flux-correction technique to isolate the effects of coupling and mean-state biases on the MJO. When constrained to the observed ocean mean state, air–sea coupling does not substantially alter the MJO in SPCAM3, in contrast to previous studies. When constrained to the SPCCSM ocean mean state, SPCAM3-KPP fails to produce an MJO, in stark contrast to the strong MJO in SPCCSM3. Further KPP simulations demonstrate that the MJO in SPCCSM3 arises from an overly strong sensitivity to El Niño–Southern Oscillation (ENSO) events. Our results show that simulated inter-annual variability and coupled-model mean-state biases affect the perceived response of the MJO to coupling. This is particularly concerning in the context of internal variability in coupled models, as many MJO sensitivity studies in coupled models use relatively short (20–50 year) simulations that undersample interannual–decadal variability. Diagnosing the effects of coupling on the MJO requires simulations that carefully control for mean-state biases and interannual variability

Diagnosing ocean feedbacks to the BSISO: SST-modulated surface fluxes and the moist static energy budget

The oceanic feedback to the atmospheric boreal summer intraseasonal oscillation (BSISO) is examined by diagnosing the sea surface temperature (SST) modification of surface fluxes and the moist static energy (MSE) on intraseasonal scales. SST variability affects intraseasonal surface latent heat (LH) and sensible heat (SH) fluxes, through its influence on air-sea moisture and temperature gradients (delta-q and delta-T). According to bulk formula decomposition, LH is mainly determined by wind-driven flux perturbations, while SH is more sensitive to thermodynamic flux perturbations. SST fluctuations tend to increase the thermodynamic flux perturbations over active BSISO regions, but this is largely offset by the wind-driven flux perturbations. Enhanced surface fluxes induced by intraseasonal SST anomalies are located ahead (north) of the convective center over both the Indian Ocean and western Pacific, favoring BSISO northward propagation. Analysis of budgets of column-integrated MSE () and its time rate of change (d/dt) show that SST-modulated surface fluxes can influence the development and propagation of the BSISO, respectively. LH and SH variability induced by intraseasonal SSTs maintain 1-2% of /day over the equatorial western Indian Ocean, Arabian Sea and Bay of Bengal, but damp about 1% of /day over the western North Pacific. The contribution of intraseasonal SST variability to d/dt can reach 12-20% over active BSISO regions. These results suggest that SST variability is conducive, but perhaps not essential, for the propagation of convection during the BSISO life cycle

The convection connection: how ocean feedbacks affect tropical mean moisture and MJO propagation

The response of the Madden-Julian oscillation (MJO) to ocean feedbacks is studied with coupled and uncoupled simulations of four general circulation models (GCMs). Monthly mean SST from each coupled model is prescribed to its respective uncoupled simulation, to ensure identical SST mean state and low-frequency variability between simulation pairs. Consistent with previous studies, coupling improves each model's ability to propagation MJO convection beyond the Maritime Continent. Analysis of the MJO moist static energy budget reveals that improved MJO eastward propagation in all four coupled models arises from enhanced meridional advection of column water vapor (CWV). Despite the identical mean state SST in each coupled and uncoupled simulation pair, coupling increases mean-state CWV near the Equator, sharpening equatorial moisture gradients and enhancing meridional moisture advection and MJO propagation. CWV composites during MJO and non-MJO periods demonstrate that the MJO itself does not cause enhanced moisture gradients. Instead, analysis of low-level subgrid-scale moistening conditioned by rainfall rate (R) and SST anomaly reveals that coupling enhances low-level convective moistening for R > 5 mm/day; this enhancement is most prominent near the Equator. The low-level moistening process varies among the four models, which we interpret in terms of their ocean model configurations, cumulus parameterizations, and the sensitivity of convection to column relative humidity

Diagnosing ocean feedbacks to the MJO: SST-modulated surface fluxes and the moist static energy budget

The composite effect of intraseasonal sea surface temperature (SST) variability on the Madden-Julian Oscillation (MJO) is studied in the context of the column integrated moist static energy (MSE) budget using data from the European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-I). SST fluctuations influence the Delta-q and Delta-T parts of the bulk surface latent and sensible heat flux calculations, respectively, each of which influence column MSE. Reynolds decomposition of latent and sensible heat fluxes (LH and SH) reveal that the thermodynamic perturbations modestly offset the equatorial wind-driven perturbations and MSE, but strongly offset the subtropical wind-driven perturbations and MSE. Column moistening east of MJO convection is opposed by wind-driven perturbations and supported by thermodynamic perturbations. Impacts of intraseasonal SST fluctuations are analyzed by recomputing surface flux component terms using 61-day running-mean SST. Differences between "full SST" and "smoothed SST" projections onto MSE and its tendency yield the "SST effect" on the MJO MSE budget. Particularly in the Indian Ocean, intraseasonal SST fluctuations maintain equatorial MSE anomalies at a rate of 1%-2% per day, and damp subtropical MSE anomalies at a similar rate. Vertical advection exports 10%-20% of MSE per day, implying that the SST modulation of surface fluxes offsets roughly 10% of equatorial MSE export and amplifies by 10% the subtropical MSE export by vertical advection. SST fluctuations support MJO propagation by encouraging on-equator convection and the circulation anomalies that drive MJO propagation, and by contributing up to 10% of MSE tendencies across the Warm Pool

Boreal summer intraseasonal oscillation in a superparameterized general circulation model: effects of air–sea coupling and ocean mean state

The effect of air-sea coupling on the simulated boreal summer intraseasonal oscillation (BSISO) is examined using atmosphere--ocean-mixed-layer coupled (SPCAM3-KPP) and uncoupled configurations of the superparameterized (SP) Community Atmospheric Model, version 3 (SPCAM3). The coupled configuration is constrained to either observed ocean mean state or the mean state from the SP coupled configuration with a dynamic ocean (SPCCSM), to understand the effect of mean-state biases on the BSISO. All configurations overestimate summer mean subtropical rainfall and its intraseasonal variance. All configurations simulate realistic BSISO northward propagation over the Indian Ocean and western Pacific, in common with other SP configurations. Prescribing the 31-day smoothed sea-surface temperature (SST) from the SPCAM3-KPP simulation in SPCAm3 worsens the overestimated BSISO variance. In both coupled models, the phase relationship between intraseasonal rainfall and SST is well captured. This suggests that air-sea coupling improves the amplitude of the simulated BSISO and contributes to the propagation of convection. Constraining SPCAM3-KPP to the SPCCSM3 mean state also reduces the overestimated BSISO variability, but weakens BSISO propagation. Using the SPCCSM3 mean state also introduces a one-month delay to the BSISO seasonal cycle compared to SPCAM3-KPP with the observed ocean mean state, which matches well with observations. Based on a Taylor diagram, both air-sea coupling and SPCCSM3 mean state SST biases generally lead to higher simulated BSISO fidelity, largely due to their abilities to suppress the overestimated subtropical BSISO variance

Rain‐induced stratification of the equatorial Indian Ocean and its potential feedback to the atmosphere

Abstract: Surface freshening through precipitation can act to stably stratify the upper ocean, forming a rain layer (RL). RLs inhibit subsurface vertical mixing, isolating deeper ocean layers from the atmosphere. This process has been studied using observations and idealized simulations. The present ocean modeling study builds upon this body of work by incorporating spatially resolved and realistic atmospheric forcing. Fine‐scale observations of the upper ocean collected during the Dynamics of the Madden‐Julian Oscillation field campaign are used to verify the General Ocean Turbulence Model (GOTM). Spatiotemporal characteristics of equatorial Indian Ocean RLs are then investigated by forcing a 2D array of GOTM columns with realistic and well‐resolved output from an existing regional atmospheric simulation. RL influence on the ocean‐atmosphere system is evaluated through analysis of RL‐induced modification to surface fluxes and sea surface temperature (SST). This analysis demonstrates that RLs cool the ocean surface on time scales longer than the associated precipitation event. A second simulation with identical atmospheric forcing to that in the first, but with rainfall set to zero, is performed to investigate the role of rain temperature and salinity stratification in maintaining cold SST anomalies within RLs. Approximately one third, or 0.1°C, of the SST reduction within RLs can be attributed to rain effects, while the remainder is attributed to changes in atmospheric temperature and humidity. The prolonged RL‐induced SST anomalies enhance SST gradients that have been shown to favor the initiation of atmospheric convection. These findings encourage continued research of RL feedbacks to the atmosphere

Atmospheric convection and air-sea interactions over the tropical oceans: scientific progress, challenges, and opportunities

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Hagos, S., Foltz, G. R., Zhang, C., Thompson, E., Seo, H., Chen, S., Capotondi, A., Reed, K. A., DeMott, C., & Protat, A. Atmospheric convection and air-sea interactions over the tropical oceans: scientific progress, challenges, and opportunities. Bulletin of the American Meteorological Society, 101(3), (2020): E253-E258, doi:10.1175/BAMS-D-19-0261.1.Over the past 30 years, the scientific community has made considerable progress in understanding and predicting tropical convection and air–sea interactions, thanks to sustained investments in extensive in situ and remote sensing observations, targeted field experiments, advances in numerical modeling, and vastly improved computational resources and observing technologies. Those investments would not have been fruitful as isolated advancements without the collaborative effort of the atmospheric convection and air–sea interaction research communities. In this spirit, a U.S.- and International CLIVAR–sponsored workshop on “Atmospheric convection and air–sea interactions over the tropical oceans” was held in the spring of 2019 in Boulder, Colorado. The 90 participants were observational and modeling experts from the atmospheric convection and air–sea interactions communities with varying degrees of experience, from early-career researchers and students to senior scientists. The presentations and discussions covered processes over the broad range of spatiotemporal scales (Fig. 1).The workshop was sponsored by the United States and International CLIVAR. Funding was provided by the U.S. Department of Energy, Office of Naval Research, NOAA, NSF, and the World Climate Research Programme. We thank Mike Patterson, Jennie Zhu, and Jeff Becker from the U.S. CLIVAR Project Office for coordinating the workshop

Vertical structure and modulation of TOGA COARE convection: a radar perspective

April 1996.Also issued as author's dissertation (Ph.D.) -- Colorado State University, 1996.Includes bibliographical references.Tropical convection is an important component of the general circulation due to its role in driving large scale circulations which redistribute energy received at the equator to higher latitudes. The behavior of these large scale circulations is sensitive to the vertical distribution of diabatic heating produced by mesoscale precipitation system, which are driven by precipitation formation processes. Convection that occurs in the western Pacific warm pool plays a particularly important role in driving large scale circulations such as the Walker circulation and the inter-annual El Nino-Southern Oscillation (ENSO), which is influenced by the 30-60 day intraseasonal oscillation (ISO). Budget studies of diabatic heating profiles reveal two basic modes of heating: one with positive heating throughout the depth of the troposphere associated with convective precipitation, and another with positive heating in the upper troposphere and negative heating (cooling) in the lower troposphere associated with stratiform precipitation. Because stratiform heating profile shapes vary little from region to region and case to case, variations in total (convective plus mesoscale) heating profile shapes are thought to be linked to variations in the shape of convective heating profiles (Houze, 1989). However, long-term observations of convective vertical structures in the tropics-particularly the western Pacific warm pool--are either non-existent or limited to a few locations. As a component of the recently completed Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean-Atmosphere Response Experiment (COARE), the MIT 5-cm Doppler radar, mounted aboard the NOAA Research Vessel John V. Vickers, was deployed to a fixed location within the warm pool region for three approximately 30-day periods and continuously monitored the three-dimensional structure of precipitating systems. In this study, radar data were partitioned into convective and stratiform components in order to address two main goals: 1) to determine the characteristic distributions of convective vertical structure during COARE and their variability over time, and 2) to relate observed variations of convective vertical structure to larger scale environmental variables. Distributions of convective feature heights and 30 dBZ contour heights (an indicator of convective vigor) reveal that convective activity is modulated by the phase of the ISO as well as by intrusions of low-level dry subtropical air. However, at least some deep convection was nearly always present. The most intense convection--as observed by convective feature reflectivity profiles--occurred in environments with the greatest thermal buoyancy (the vertical distribution of CAPE) experienced by a parcel lifted from the mixed layer. However, these periods were also characterized by strong inhibitors to convective development which limited intense convective activity to just a few days. Convective heating profiles-computed from a combination of budget-derived total beating, radar-derived rainfall characteristics, and ideal and computed radiative heating profiles-varied in a manner consistent with the variations in vertical reflectivity and thermal buoyancy profiles. Namely, less vertically intense convection was associated with a lower altitude convective heating maximum than was observed for those days with convective reflectivity profiles indicative of more vigorous convection. Possibilities for future research include a rigorous comparison of convective characteristics between the COARE and GATE regions, modeling studies of the influence of low-level dry air on convective activity and its relation to tropospheric drying and testing a refinement of passive microwave rainfall retrieval algorithmsSponsored by the National Aeronautics and Space Administration under Graduate Student Fellowship NGT-30099-0 and grant NAG5-2692