Abstract
Model simulations suggest that Madden–Julian oscillation (MJO) activity changes under the anthropogenic climate change background. However, satellite observations, which provide information on MJO convection activity, are not available before the 1970s, hindering research on the long-term historical variability of MJO. This study aims at extending the data length of MJO indices that include both MJO circulation and convection features, such as the widely used Real-Multivariate MJO (RMM) Index, to the pre-satellite era. This paper introduces a new MJO Index construction method, in which the outgoing longwave radiation (OLR) input is derived from upper-level geopotential, and names it as the Geopotential-Based Multivariate MJO (GMM) Index. The GMM Index is derived from 1902 to 2008 based on the 20th century reanlaysis product, by assuming that the relationship between OLR and geopotential does not change over time, and is compared with the filtered version of the RMM (FMM) Index during 1981–2008 and historical observed precipitation records in the 20th century. The GMM Index is shown to (1) have the same climatological properties as the FMM Index, (2) be statistically highly correlated to the FMM Index, and (3) be able to indicate MJO activities and MJO’s convection features in the pre-satellite era. The overall bivariate correlation between the FMM and GMM indices based on ERA-20C is 0.964. Evaluation results confirm the validity of the proposed MJO Index construction method, which could capture MJO convection activity in the pre-satellite era and can be applied to all MJO indices that require OLR inputs. This study provides an alternative way that overcomes the difficulty of historical MJO studies and will be beneficial to our understanding of the long-term change of MJO.
Highlights
The Madden-Julian Oscillation (MJO; Madden and Julian 1971, 1972), named after Madden and Julian, is an important tropical atmospheric system often treated as a bridge linking weather and climate (Zhang 2013)
Are 0.946 and 0.954, respectively. These indicate that the univariate EOF results of the derived outgoing longwave radiation (OLR) anomalies show consistent MJO convection features with that of observed OLR
The BCORR of the univariate OLR index based on ECMWF Re-analysis Interim (ERA-I) 150-hPa ∇x z ′ of all MJO phases on inactive, active, very active and extremely active MJO days are 0.682, 0.815, 0.847 and 0.887, respectively; and, the BCORR of the univariate OLR index based on ERA-20C 150-hPa ∇x z ′ of MJO
Summary
The Madden-Julian Oscillation (MJO; Madden and Julian 1971, 1972), named after Madden and Julian, is an important tropical atmospheric system often treated as a bridge linking weather and climate (Zhang 2013). The MJO, despite being named as an oscillation, acts more like an eastward-moving atmospheric pulse with convection clusters 5000–10,000 km spatial scale (Nakazawa 1988; Lau et al.2012) and notable zonal wind anomalies along the equator. Lower-tropospheric equatorial westerly and easterly anomalies are observed to the west and east of the MJO deep convection center, respectively. The low-level westerly and easterly anomalies are, respectively, the Rossby and Kelvin wave responses to the heat source over the deep convection center (Gill 1980; Wang and Rui 1990; Hsu and Li 2012; Wang and Chen 2017). The upper-level zonal wind shows similar Rossby-Kelvin wave responses. Given the dominant planetary-scale circulation and convection features of the MJO, MJO signals are usually extracted from outgoing longwave radiation (OLR) and tropospheric zonal wind data
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
