Intraseasonal Variability in Coupled GCMs: The Roles of Ocean Feedbacks and Model Physics

Abstract

The interaction of ocean coupling and model physics in the simulation of the intraseasonal oscillation (ISO) is explored with three general circulation models: the Community Atmospheric Model, versions 3 and 4 (CAM3 and CAM4), and the superparameterized CAM3 (SPCAM3). Each is integrated coupled to an ocean model, and as an atmosphere-only model using sea surface temperatures (SSTs) from the coupled SPCAM3, which simulates a realistic ISO. For each model, the ISO is best simulated with coupling. For each SST boundary condition, the ISO is best simulated in SPCAM3.Near-surface vertical gradients of specific humidity, Dq (temperature, DT), explain ;20% (50%) of tropical Indian Ocean latent (sensible) heat flux variance, and somewhat less of west Pacific variance. In turn, local SST anomalies explain ;5% (25%) of Dq (DT) variance in coupled simulations, and less in uncoupled simulations. Ergo, latent and sensible heat fluxes are strongly controlled by wind speed fluctuations, which are largest in the coupled simulations, and represent a remote response to coupling. The moisture budget reveals that wind variability in coupled simulations increases east-of-convection midtropospheric moistening via horizontal moisture advection, which influences the direction and duration of ISO propagation.These results motivate a new conceptual model for the role of ocean feedbacks on the ISO. Indian Ocean surface fluxes help developing convection attain a magnitude capable of inducing the circulation anomalies necessary for downstream moistening and propagation. The ''processing'' of surface fluxes by model physics strongly influences the moistening details, leading to model-dependent responses to coupling.

Analysis of MJO Wind-Flux Feedbacks in the Indian Ocean Using RAMA Buoy Observations

“…This work builds from previous observational and modeling work (i.e., Maloney andEsbensen 2005 and2007;Back and Bretherton 2005;Araligidad and Maloney 2008;Maloney 2009;DeMott et al 2014) that has suggested the importance of wind-induced fluxes for destabilizing the MJO, and supports a recent body of work referred to as moisture mode theory (e.g., Fuchs 2007, 2009;Sugiyama 2009a, b;Sobel et al 2010;Sobel and Maloney 2012).…”

Three different downstream fates of the boreal-summer MJOs on their passages over the Maritime Continent

“…14a, c). Although weak negative moisture anomalies still present in the troposphere, column-integrated moisture tendencies are all positive over the MC and western Pacific, which apparently favor Indian Ocean MJO convection moving over the MC and entering western Pacific (e.g., DeMott et al 2014;Feng et al 2015). On the other hand, western Pacific troposphere for type-II MJO is still very dry with negative column-integrated moisture tendency (Fig.…”

“…14d-f) are positive over the MC and western Pacific in all three cases likely due to the evaporation of cloud liquid water (Yanai et al 1973;DeMott et al 2014) and the detrainments of cloud water from shallow cumulus and congestus (Johnson et al 1999). Around 100°E and 120°E (Fig.…”

“…14j-l). Moisture tendencies (a-c) with statistically significant levels above the 5 and 25% are drawn in thick and thin contours DeMott et al (2014) and Feng et al (2015) have suggested that positive column-integrated moisture tendency ahead of Indian Ocean MJO convection is the key factor to ensure MJO propagation over the MC. The above result, however, suggests that the positive moisture tendency alone is not a sufficient condition.…”

An evaluation of boreal summer intra-seasonal oscillation simulated by BCC_AGCM2.2

“…Although the exact reason is not clear, it is generally believed that inadequate representation and description of moisture processes and the associated scale interactions are likely the major culprit (Fu et al 2006;Sooraj and Seo 2013;Demott et al 2014). Some recent studies have suggested that interactions between different cloud regimes, circulation, and diabatic heating in the boundary layer may play a critical role for the evolution of the BSISO (Chattopadhyay et al 2009;Jiang et al 2011;Abhik et al 2013).…”