The Boreal Summer Intraseasonal Oscillation Simulated in the NCEP Climate Forecast System: The Effect of Sea Surface Temperature

Abstract

Observational evidence has indicated the important role of the interaction of the atmosphere with the sea surface in the development and maintenance of the tropical intraseasonal oscillation (ISO). However, improvements in ISO simulations with fully coupled atmosphere–ocean general circulation models are limited and model dependent. This study further examines the effect of air–sea coupling and the basic-state sea surface temperature (SST) associated with the boreal summer intraseasonal oscillation (BSISO) in a 21-yr free run with the recently developed NCEP coupled Climate Forecast System (CFS) model. For this, the CFS run is compared with an Atmospheric Model Intercomparison Project–type long-term simulation forced by prescribed SST in the NCEP Global Forecast System (GFS) model and flux-corrected version of CFS (referred to as CFSA). The GFS run simulates significantly unorganized BSISO convection anomalies, which exhibit an erroneous standing oscillation. The CFS run with interactive air–sea coupling has limited improvements, including the generation of intraseasonal SST variation preceding the convection anomaly by ∼10 days. However, this simulation still does not show the observed continuous northward propagation over the Indian Ocean due to a cold model bias. The CFSA run removes the cold bias in the Indian Ocean and the simulation of the development and propagation of BSISO anomalies are significantly improved. Enhanced and suppressed convection anomalies exhibit the observed quadrupole-like configuration, and phase relationships between precipitation and surface dynamic and thermodynamic variables for the northward propagation are shown to be coherent and consistent with the observations. It is shown that the surface meridional moisture convergence is an important factor for the northward propagation of the BSISO. On the other hand, both the GFS and CFS runs do not realistically simulate an eastward-propagating equatorial mode. The CFSA run produces a more realistic eastward-propagation mode only over the Indian Ocean and Java Sea due to the improved mean state in SST, low-level winds, and vertical wind shear. Reasons for the failure of farther eastward propagation into the west Pacific in CFSA are discussed. This study reconfirms the significance of air–sea interactions. More importantly, however, the results suggest that in order for the influence of the coupled air–sea interaction to be properly communicated, the mean state SST in the coupled model should be reasonably simulated. This is because the basic-state SST itself acts to sustain BSISO convection and it makes the large-scale dynamical environment (i.e., easterly vertical wind shear or low-level westerly zonal wind) more favorable for the propagation of the moist Rossby–Kelvin wave packet.

Statistical‐dynamical prediction of the Madden–Julian oscillation using NCEP Climate Forecast System (CFS)

“…The previous dynamical extended-range forecast (DERF) run with GFS model demonstrated the existence of the predictability barrier over the Maritime Continent and western Pacific . This kind of the model deficiency was also found in a recent analysis on the CFS long-term simulation (Seo et al, 2007). This property in the operational coupled model is investigated in more detail in a separate paper (Seo et al, 2008).…”

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

“…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). The role of deficiencies in relation to background states in influencing the northward propagating BSISOs has also been suggested by many studies (Jiang and Li 2005;Ajayamohan and Goswami 2007;Seo et al 2007;Sabeerali et al 2013).…”

“…A new mass-flux cumulus parameterization scheme, which includes more sophisticated cloud processes and vertically variable entrainment, was introduced and the model shows improved simulation of deep convection and the associated vertical heating profile (Wu 2012). Given the important role of the model convective parameterization in the realism of ISO simulations (Seo et al 2007;Benedict et al 2013) (Wu et al 2008). Other major modification to the model physics includes a new convective parameterization scheme, a dry adiabatic adjustment scheme in which potential temperature is conserved, a modified scheme to calculate the sensible heat and moisture fluxes over the open ocean that takes into account the effect of ocean waves on the latent and sensible heat fluxes, and an empirical equation to compute the snow cover fraction (Wu and Wu 2004).…”

The Surface Diurnal Warm Layer in the Indian Ocean during CINDY/DYNAMO

“…These processes are core to the evolution of El Niño-Southern Oscillation (ENSO; Neelin et al 1998) on interannual time scales. On shorter, intraseasonal time scales, ocean-atmosphere interaction has a significant role in the development and eastward propagation of the Madden-Julian oscillation (MJO; Flatau et al 1997;Shinoda et al 1998;Woolnough et al 2000;Matthews 2004;Drushka et al 2012) and also in the northwardpropagating intraseasonal oscillations observed in the Indian Ocean during the boreal summer (BSISO; Fu and Wang 2004;Seo et al 2007).…”