The role of air–sea coupling in the simulation of the Madden–Julian oscillation in the Hadley Centre model

Abstract

The role of air-sea coupling in the simulation of the Madden-Julian oscillation (MJO) is explored using two configurations of the Hadley Centre atmospheric model (AGCM), GA3.0, which differ only in F, a parameter controlling convective entrainment and detrainment. Increasing F considerably improves deficient MJO-like variability in the Indian and Pacific Oceans, but variability in and propagation through the Maritime Continent remains weak.By coupling GA3.0 in the tropical Indo-Pacific to a boundary-layer ocean model, Kprofile parametrization (KPP), and employing climatological temperature corrections, well-resolved air-sea interactions are simulated with limited alterations to the mean state. At default F, when GA3.0 has a poor MJO, coupling produces a stronger MJO with some eastward propagation, although both aspects remain deficient. These results agree with previous sensitivity studies using AGCMs with poor variability. At higher F, coupling does not affect MJO amplitude but enhances propagation through the Maritime Continent, resulting in an MJO that resembles observations. A sensitivity experiment with coupling only in the Indian Ocean reverses these improvements, suggesting that coupling in the Maritime Continent and West Pacific is critical for propagation. We hypothesize that, for AGCMs with a poor MJO, coupling provides a 'crutch' to augment MJO-like activity artificially through high-frequency SST anomalies.In related experiments, we employ the KPP framework to analyze the impact of air-sea interactions in the fully coupled GA3.0, which at default F shows a similar MJO to the uncoupled GA3.0. This is due to compensating effects: an improvement from coupling and a degradation from mean-state errors. Future studies on the role of coupling should separate these effects carefully.

MetUM-GOML1: a near-globally coupled atmosphere–ocean-mixed-layer model

“…Vitart and Molteni, 2009). The impact of air-sea interactions on the eastward propagation of the MJO here within the nearglobally coupled MetUM-GOML is consistent with a similar MetUM mixed-layer ocean coupled simulation with coupling only in the Indo-Pacific (Klingaman and Woolnough, 2014).…”

“…This is important because the mean state is known to affect modes of variability (e.g. the MJO; Inness et al, 2003;Ray et al, 2011) and the perceived impact of coupling on that variability (Klingaman and Woolnough, 2014). Within this framework the role of air-sea interactions can be studied in a coupled model with the "right" basic state, thus, limiting the possibility that changes in the variability are a consequence of changes to the mean state.…”

“…At the coupling boundary a linear interpolation blends the coupled and reference SSTs and sea ice to remove any discontinuities. A regionally coupled configuration of this framework, with coupling in the tropical Indo-Pacific, is described in Klingaman and Woolnough (2014).…”

“…This is a further diagnostic recommended by the CLIVAR MJO Task Force (Kim et al, 2009), which has previously been applied to MJO-filtered OLR to investigate the role of local air-sea interactions in the MetUM GA3.0 (Klingaman and Woolnough, 2014).…”

Propagation of the Madden–Julian Oscillation and scale interaction with the diurnal cycle in a high-resolution GCM

“…4.2. These diagnostics share some similarities with the MJO activity diagrams of Klingaman and Woolnough (2014a;Fig. 1) and Klingaman and Woolnough (2014b;Fig.…”