Fast and slow Kelvin waves in the Madden-Julian Oscillation of a GCM

The structure of the Madden-Julian Oscillation (MJO) in an 1800-day integration of the Hadley Centre Unified Model was analysed, and interpreted within a Kelvin wave framework. The model was forced with constant equinoctial (March) boundary conditions so that a ``clean'' MJO signal could be separated from the effects of the seasonal cycle and forced interannual variability. The simulated MJO was fairly realistic in terms of its large-scale spatial structure and propagation characteristics, although its period of 30 days (corresponding to an average phase speed of 15 \mps) was shorter than that observed. The signal in deep convection was less coherent than in observations, and appeared to move eastward as a sequence of discrete convective anomalies, rather than by a smooth eastward propagation. Both ``fast'' and ``slow'' equatorial Kelvin waves appeared to play an important role in the eastward propagation of the simulated MJO. Enhanced convection over the Indian Ocean was associated with a ``fast'' equatorial Kelvin wave that propagated eastward at 55 m s-1 over the Pacific. On reaching the west coast of South America, a component of this Kelvin wave propagated northward and southward as a trapped wave along the mountain ranges of Central America and the Andes, in agreement with observations. The anomalous surface easterlies over the tropical eastern Pacific associated with this fast Kelvin wave enhanced the climatological mean easterlies and led to positive convective anomalies over the eastern Pacific consistent with the WISHE mechanism. However, WISHE was not able to account for the eastward development of the convective anomalies over the Indian Ocean/western Pacific region. By splitting the equatorial divergence anomalies of the simulated MJO into their du/dx and dv/dy components, the role of Kelvin wave dynamics in the ``slow'' (15 m s-1) average eastward propagation of the simulated MJO was examined. Although the two components were of comparable magnitude, the \dudx\ component exhibited a pronounced eastward propagation which tended to be disrupted by the \dvdy\ component, thus supporting the paradigm of an underlying, but strongly modified, Kelvin wave mechanism

Observations of the diurnal cycle of outgoing longwave radiation from the Geostationary Earth Radiation Budget instrument

The Geostationary Earth Radiation Budget instrument on Meteosat-8, located over Africa, provides unprecedented temporal sampling (~17 minutes) of the broadband emitted thermal and reflected solar radiances. We analyse the diurnal cycle of the outgoing longwave radiation (OLR) fluxes derived from the thermal radiances for July 2006. Principal component (PC) analysis separates the signals of the surface temperature response to solar heating and of the development of convective clouds. The first two PCs explain most of the OLR variations: PC1 (surface heating) explains 82.3% of the total variance and PC2 (cloud development) explains 12.8% of the variance. Convection is initiated preferentially over mountainous regions and the cloud then advects downstream in the ambient flow. Diurnal variations are much weaker over the oceans, but a coherent signal over the Gulf of Guinea suggests that the cloudiness is modulated by the diurnally varying contrast between the Gulf and the adjacent land mass

Modelling monsoons: understanding and predicting current and future behaviour

The global monsoon system is so varied and complex that understanding and predicting its diverse behaviour remains a challenge that will occupy modellers for many years to come. Despite the difficult task ahead, an improved monsoon modelling capability has been realized through the inclusion of more detailed physics of the climate system and higher resolution in our numerical models. Perhaps the most crucial improvement to date has been the development of coupled ocean-atmosphere models. From subseasonal to interdecadal time scales, only through the inclusion of air-sea interaction can the proper phasing and teleconnections of convection be attained with respect to sea surface temperature variations. Even then, the response to slow variations in remote forcings (e.g., El Niño—Southern Oscillation) does not result in a robust solution, as there are a host of competing modes of variability that must be represented, including those that appear to be chaotic. Understanding the links between monsoons and land surface processes is not as mature as that explored regarding air-sea interactions. A land surface forcing signal appears to dominate the onset of wet season rainfall over the North American monsoon region, though the relative role of ocean versus land forcing remains a topic of investigation in all the monsoon systems. Also, improved forecasts have been made during periods in which additional sounding observations are available for data assimilation. Thus, there is untapped predictability that can only be attained through the development of a more comprehensive observing system for all monsoon regions. Additionally, improved parameterizations - for example, of convection, cloud, radiation, and boundary layer schemes as well as land surface processes - are essential to realize the full potential of monsoon predictability. A more comprehensive assessment is needed of the impact of black carbon aerosols, which may modulate that of other anthropogenic greenhouse gases. Dynamical considerations require ever increased horizontal resolution (probably to 0.5 degree or higher) in order to resolve many monsoon features including, but not limited to, the Mei-Yu/Baiu sudden onset and withdrawal, low-level jet orientation and variability, and orographic forced rainfall. Under anthropogenic climate change many competing factors complicate making robust projections of monsoon changes. Absent aerosol effects, increased land-sea temperature contrast suggests strengthened monsoon circulation due to climate change. However, increased aerosol emissions will reflect more solar radiation back to space, which may temper or even reduce the strength of monsoon circulations compared to the present day. Precipitation may behave independently from the circulation under warming conditions in which an increased atmospheric moisture loading, based purely on thermodynamic considerations, could result in increased monsoon rainfall under climate change. The challenge to improve model parameterizations and include more complex processes and feedbacks pushes computing resources to their limit, thus requiring continuous upgrades of computational infrastructure to ensure progress in understanding and predicting current and future behaviour of monsoons

Uncertainty in weather and climate prediction

Following Lorenz's seminal work on chaos theory in the 1960s, probabilistic approaches to prediction have come to dominate the science of weather and climate forecasting. This paper gives a perspective on Lorenz's work and how it has influenced the ways in which we seek to represent uncertainty in forecasts on all lead times from hours to decades. It looks at how model uncertainty has been represented in probabilistic prediction systems and considers the challenges posed by a changing climate. Finally, the paper considers how the uncertainty in projections of climate change can be addressed to deliver more reliable and confident assessments that support decision-making on adaptation and mitigation

Observed Changes in the Lifetime and Amplitude of the Madden–Julian Oscillation Associated with Interannual ENSO Sea Surface Temperature Anomalies

The Madden-Julian Oscillation (MJO) is analysed using the reanalysis zonal wind and satellite outgoing longwave radiation-based indices of Wheeler and Hendon for the 1974-2005 period. The average life time of MJO events varies with season, being 36 days for events whose central date occurs in December, and 48 days for events in September. The life time of the MJO in the equinoctial seasons (March-May and October-December) is also dependent on the state of the El Nino-Southern Oscillation (ENSO). During October-December it is only 32 days under El Nino conditions, increasing to 48 days under La Nina conditions, with similar values in northern spring. This difference is due to faster eastward propagation of the MJO convective anomalies through the Maritime Continent and western Pacific during El Nino, consistent with theoretical arguments concerning equatorial wave speeds. The analysis is extended back to 1950 by using an alternative definition of the MJO based on just the zonal wind component of the Wheeler and Hendon indices. A rupture in the amplitude of the MJO is found in 1975, at the same time as the well known rupture in the ENSO time series, that has been associated with the Pacific Decadal Oscillation. The mean amplitude of the MJO is 16% larger in the post-rupture period (1976-2005) compared to the pre-rupture period (1950-1975). Before the 1975 rupture, the amplitude of the MJO is a maximum (minimum) under El Nino (La Nina) conditions during northern winter, and a minimum (maximum) under El Nino (La Nina) conditions during northern summer. After the rupture, this relationship disappears. When the MJO-ENSO relationship is analysed using all year round data, or a shorter data set, as in some previous studies, no relationship is found

Atmospheric response to observed intraseasonal tropical sea surface temperature anomalies

The major tropical convective and circulation features of the intraseasonal or Madden-Julian Oscillation (MJO) are simulated as a passive response to observed MJO sea surface temperature (SST) anomalies in an atmospheric general circulation model (AGCM), strengthening the case for ocean-atmosphere interactions being central to MJO dynamics. However, the magnitude of the surface fluxes diagnosed from the MJO cycle in the AGCM, that would feed back onto the ocean in a coupled system, are much weaker than in observations. The phasing of the convective-dynamical model response to the MJO SST anomalies and the associated surface flux anomalies is too fast compared to observations of the (potentially) coupled system, and would act to damp the SST anomalies

Evaluation of the Met Office global forecast model using Geostationary Earth Radiation Budget (GERB) data

Simulations of the top-of-atmosphere radiative-energy budget from the Met Office global numerical weather-prediction model are evaluated using new data from the Geostationary Earth Radiation Budget (GERB) instrument on board the Meteosat-8 satellite. Systematic discrepancies between the model simulations and GERB measurements greater than 20 Wm-2 in outgoing long-wave radiation (OLR) and greater than 60 Wm-2 in reflected short-wave radiation (RSR) are identified over the period April-September 2006 using 12 UTC data. Convective cloud over equatorial Africa is spatially less organized and less reflective than in the GERB data. This bias depends strongly on convective-cloud cover, which is highly sensitive to changes in the model convective parametrization. Underestimates in model OLR over the Gulf of Guinea coincide with unrealistic southerly cloud outflow from convective centres to the north. Large overestimates in model RSR over the subtropical ocean, greater than 50 Wm-2 at 12 UTC, are explained by unrealistic radiative properties of low-level cloud relating to overestimation of cloud liquid water compared with independent satellite measurements. The results of this analysis contribute to the development and improvement of parametrizations in the global forecast model