Showing papers by "Rob Allan published in 2002"

Positive feedbacks between the Antarctic Circumpolar Wave and the global El Niño-Southern Oscillation Wave

Abstract: Atmospheric and oceanic teleconnections link the Antarctic Circumpolar Wave (ACW) in the Southern Ocean [White and Peterson, 1996] and the global El Nino Southern Oscillation (ENSO) wave (GEW) in the tropical Indo-Pacific Ocean [White and Cayan, 2000], both signals characterized by eastward phase propagation and 3- to 5-year-period variability. We extend the tropical standing mode of ENSO into the extratropics by regressing the Nino-3 sea surface temperature (SST) index against sea level pressure (SLP) anomalies over the globe, finding the Pacific-South America (PSA) pattern in SLP anomaly [Cai and Baines, 2001] straddling Drake Passage in the Southern Ocean. The amplitude of this PSA pattern is similar to1/3 that of the ACW in this domain and thus cannot be considered its principal driver. On the other hand, suppressing the tropical standing mode of ENSO in interannual ST (surface temperature) and SLP anomalies over the globe allows the GEW to be observed much more readily, whereupon its eastward phase propagation across the Warm Pool is found to remotely force the ACW in the eastern Pacific and western Atlantic sectors of the Southern Ocean through atmospheric teleconnections [Sardeshmukh and Hoskins, 1988] which propagate along with it. Subsequently, the ACW propagates this imposed GEW signal throughout the remainder of the Southern Ocean as a coupled wave in covarying ST and SLP anomalies, whereupon entering the Indian sector 1.5 to 2.5 years later it spawns a northern branch which takes another 1.5 to 2.5 years to propagate the ACW signal equatorward into the Warm Pool south of Indonesia. There it interferes constructively with the GEW. Thus the two forms of teleconnection, one fast and directed from the tropics to the high southern latitudes via the atmosphere and the other slow and directed from the high southern latitudes to the tropics via the ocean, complete a global circuit of 3- to 5-year duration that reinforces both the ACW and GEW and influences the tropical standing mode of ENSO.

A reconstruction of Madras (Chennai) mean sea-level pressure using instrumental records from the late 18th and early 19th centuries

Abstract: In and around the Indian Ocean basin, historical records documenting observations of climatic variables such as rainfall, surface-air and sea-surface temperature, and atmospheric pressure can be found in various publications by the European colonial powers of the time. However, it is only the most accessible of these records from the mid-to-late 1800s that have been transferred into electronic form and used in any contemporary climatic studies. A longer-term set of observations from the observatory at Madras (now Chennai), set up by the English East India Company, was found to have the potential to provide useful climatic data back to 1796. In this study, atmospheric pressure data recorded at the Madras Observatory, and at other climate stations in or near Madras, and in India, were recovered and examined for their quality and veracity. From the old observatory publications, pre-1841 records (that is prior to those documented in standard data compilations) were quality controlled, and the full record from 1796 was then reduced to mean sea level, standard gravity and 24 h means to produce a 205 year set of monthly observations. This monthly mean sea-level pressure (MSLP) record was correlated with monthly or seasonally stratified instrumental indices of the El Nino southern oscillation (ENSO) phenomenon and all-India rainfall, and a seasonally stratified palaeoclimatic reconstruction of the southern oscillation index. In general, the Madras MSLP record is shown to be robust over time, and thus a useful long-term measure of fluctuations in the ENSO phenomenon across the Indian Ocean basin and of the Indian summer monsoon. Copyright © 2002 Royal Meteorological Society.