A Possible Volcanic Signal in NCEP/NCAR Reanalysis Data

Yung-An Lee,Li-Jen Fang

doi:10.3319/tao.2000.11.4.895(a)

Abstract

In this study we used the multi-channel singular spectrum analysis (MSSA) to extract climate signals from National Centers for Environmental Prediction / National Center for Atmospheric Research (NCEP/NCAR) reanalysis monthly mean temperature data between 1950 and 1998. The signal associated with three major volcanic eruptions during this period was clearly shown in the sixth principal component (PC) of the MSSA. The CD was clearly shown in the sixth principal component (PC) of the MSSA. The time evolution of this PC revealed both interannual and interdecadal scale variations suggesting that major volcanic eruptions not only have an effect on short-term climate variability but may also be associated with interdecadal variability in the atmospheric temperature field. The associated anomaly patterns of this PC showed general cooling in the lower atmosphere and warming in the upper atmosphere. Furthermore its time lag maps show anomaly patterns in the equatorial Pacific similar to those of EI Nino-Southern Oscillation (ENSO) warm events. These results suggest that a major volcanic eruption might enhance the strength of an ENSO warm event. Nevertheless, since these eruptions occurred rarely and since each one of them roughly coincided with an ENSO warm event, we are unable to assert that there is a relationship from observed data alone. Despite the irregular occurrence of major volcanic eruptions in history, this study demonstrated that MSSA has the ability to extract temporally irregular but meaningful signal from observed data. Hence MSSA is a very valuable tool for climate study.

Highlights

Major explosive volcanic eruptions inject massive amounts of dust and gases into the*1Cnsotrirteustepoonf dAitnmgoasupthheorricadPdhryessicss: D, Nr.aYtiounnga-l ACnenLterea,lUniversity, Institute ofChung-Li, Taiwan Atmospheric Physics, National CentralTAO, Vol 11, No 4, December 2000 atmosphere
-1 eo s the tropical Pacific suggest that the sixth principal component (PC) of the multi-channel singular spectrum analysis (MSSA) may be the volcanic signal
The use of a 30-month window length prevents us from ruling out the possibility that the sixth mode may be contaminated by interdecadal El Nino-SouthernOscillation (ENSO)-like variability the influence of major volcanic eruptions on a decadal scale is still an open question

Summary

INTRODUCTION

Major explosive volcanic eruptions inject massive amounts of dust and gases into the. Mass and Portman (1989) used the superposed epoch method of analysis (Composite method) to examine annual and monthly variations in four factors after major volcanic eruptions during the past century; these were surface air temperature, sea surface temperature, surface pressure, and precipitation They used various data (such as the Volcanic Explosive Index and the Dust Veil Index) to determine if an eruption was a major one, but the main selection criterion was whether the eruption created a significant stratospheric dust veil. Roback and Mao (1992, 1995) extended Mass and Portman's analysis to regional and seasonal pat terns with a global surface temperature dataset They used regression coefficients derived from the Southern Oscillation index (SOI) to remove the ENSO signal from .surface tempera ture anomalies at each grid point. In section four, we summarize the major findings and discuss some of the implications

DATA AND ANALYSIS MET HOD

3.RESULTS

SUMMARY AND DISCUSSION