Sensitivity of Australian Rainfall to Inter-El Niño Variations
TL;DR: In this paper, the authors suggest that Australian rainfall is sensitive to the zonal distribution of SST anomalies during El Nino and, in particular, the greatest sensitivity is to the SST variations on the eastern edge of the Pacific warm pool rather than in the eastern Pacific where EL Nino variations are typically largest.
Abstract: Australia typically experiences drought during El Nino, especially across the eastern two-thirds of the continent during austral spring (September–November). There have, however, been some interesting departures from this paradigm. For instance, the near-record-strength El Nino of 1997 was associated with near-normal rainfall. In contrast, eastern Australia experienced near-record drought during the modest El Nino of 2002. This stark contrast raises the issue of how the magnitude of the drought is related to the character and magnitude of El Nino, for instance as measured by the broadscale sea surface temperature (SST) anomaly in the equatorial eastern Pacific. Internal (unpredictable) atmospheric noise is one plausible explanation for this contrasting behavior during these El Nino events. Here, the authors suggest that Australian rainfall is sensitive to the zonal distribution of SST anomalies during El Nino and, in particular, the greatest sensitivity is to the SST variations on the eastern edge of the Pacific warm pool rather than in the eastern Pacific where El Nino variations are typically largest. Positive SST anomalies maximized near the date line in 2002, but in 1997 maximum anomalies were shifted well into the eastern Pacific, where their influence on Australian rainfall appears to be less. These findings provide a plausible physical basis for the view that forecasting the strength of El Nino is not sufficient to accurately predict rainfall variations across Australia during El Nino.
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01 Apr 2012
1,699 citations
TL;DR: In this paper, the authors use a suite of climate models incorporating anthropogenic changes in greenhouse gases to show that the occurrence ratio of the new type of El Nino is projected to increase by up to a factor of five by the late twenty-first century.
Abstract: In recent decades, a distinctly different type of El Nino has emerged. 'Normal' El Ninos are climate events associated with anomalous warming in the eastern Pacific, and climatic consequences elsewhere varying from floods to drought. In the alternative version, sometimes called El Nino Modoki (modoki meaning 'similar but different' in Japanese), the warm pool is shifted westward and is flanked to the east and west by cooler water. Sang-Wook Yeh and colleagues use a suite of climate models incorporating anthropogenic changes in greenhouse gases to show that the occurrence ratio of the new type of El Nino is projected to increase by up to a factor of five by the late twenty-first century. A distinctly different type of El Nino event, causing global climate effects dramatically different from those caused by the canonical El Nino, was observed in the late twentieth century. Using data from projected global warming scenarios it is now demonstrated that this new type of El Nino event is likely to become progressively more common in the future as a result of anthropogenic climate change. El Nino events, characterized by anomalous warming in the eastern equatorial Pacific Ocean, have global climatic teleconnections and are the most dominant feature of cyclic climate variability on subdecadal timescales. Understanding changes in the frequency or characteristics of El Nino events in a changing climate is therefore of broad scientific and socioeconomic interest. Recent studies1,2,3,4,5 show that the canonical El Nino has become less frequent and that a different kind of El Nino has become more common during the late twentieth century, in which warm sea surface temperatures (SSTs) in the central Pacific are flanked on the east and west by cooler SSTs. This type of El Nino, termed the central Pacific El Nino (CP-El Nino; also termed the dateline El Nino2, El Nino Modoki3 or warm pool El Nino5), differs from the canonical eastern Pacific El Nino (EP-El Nino) in both the location of maximum SST anomalies and tropical–midlatitude teleconnections. Here we show changes in the ratio of CP-El Nino to EP-El Nino under projected global warming scenarios from the Coupled Model Intercomparison Project phase 3 multi-model data set6. Using calculations based on historical El Nino indices, we find that projections of anthropogenic climate change are associated with an increased frequency of the CP-El Nino compared to the EP-El Nino. When restricted to the six climate models with the best representation of the twentieth-century ratio of CP-El Nino to EP-El Nino, the occurrence ratio of CP-El Nino/EP-El Nino is projected to increase as much as five times under global warming. The change is related to a flattening of the thermocline in the equatorial Pacific.
1,248 citations
01 Jan 2013
958 citations
Earth System Research Laboratory1, Geophysical Fluid Dynamics Laboratory2, Georgia Institute of Technology3, University of California, Irvine4, University of Arizona5, Texas A&M University6, University of Reading7, Woods Hole Oceanographic Institution8, University of Miami9, California Institute of Technology10, Hanyang University11
TL;DR: The authors surveys the current state of knowledge of ENSO diversity, identifies key gaps in understanding, and outlines some promising future research directions, as well as identifying key gaps and promising future directions.
Abstract: El Nino–Southern Oscillation (ENSO) is a naturally occurring mode of tropical Pacific variability, with global impacts on society and natural ecosystems. While it has long been known that El Nino events display a diverse range of amplitudes, triggers, spatial patterns, and life cycles, the realization that ENSO’s impacts can be highly sensitive to this event-to-event diversity is driving a renewed interest in the subject. This paper surveys our current state of knowledge of ENSO diversity, identifies key gaps in understanding, and outlines some promising future research directions.
761 citations
Cites background from "Sensitivity of Australian Rainfall ..."
...2006) and Australia (Wang and Hendon 2007), while the very strong 1997/98 El Niño had very little effect on precipitation in both regions....
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...El Niño events have also been associated with reduced precipitation over northern, central, and peninsular India (Rasmusson and Carpenter 1982; Shukla and Paolino 1983; Ropelewski and Halpert 1987), as well as over northern and eastern Australia (Wang and Hendon 2007; Cai and Cowan 2009; Taschetto and England 2009)....
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TL;DR: This article identified and documented a suite of large-scale drivers of rainfall variability in the Australian region, including El Nino-Southern Oscillation (ENSO), Indian Ocean dipole (IOD), Madden-Julian oscillation and atmospheric blocking.
Abstract: This work identifies and documents a suite of large-scale drivers of rainfall variability in the Australian region. The key driver in terms of broad influence and impact on rainfall is the El Nino–Southern Oscillation (ENSO). ENSO is related to rainfall over much of the continent at different times, particularly in the north and east, with the regions of influence shifting with the seasons. The Indian Ocean dipole (IOD) is particularly important in the June–October period, which spans much of the wet season in the southwest and southeast where IOD has an influence. ENSO interacts with the IOD in this period such that their separate regions of influence cover the entire continent. Atmospheric blocking also becomes most important during this period and has an influence on rainfall across the southern half of the continent. The Madden–Julian oscillation can influence rainfall in different parts of the continent in different seasons, but its impact is strongest on the monsoonal rains in the north. Th...
699 citations
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TL;DR: The NCEP-DOE Atmospheric Model Intercomparison Project (AMIP-II) reanalysis is a follow-on project to the "50-year" (1948-present) N CEP-NCAR Reanalysis Project.
Abstract: The NCEP–DOE Atmospheric Model Intercomparison Project (AMIP-II) reanalysis is a follow-on project to the “50-year” (1948–present) NCEP–NCAR Reanalysis Project. NCEP–DOE AMIP-II re-analysis covers the “20-year” satellite period of 1979 to the present and uses an updated forecast model, updated data assimilation system, improved diagnostic outputs, and fixes for the known processing problems of the NCEP–NCAR reanalysis. Only minor differences are found in the primary analysis variables such as free atmospheric geopotential height and winds in the Northern Hemisphere extratropics, while significant improvements upon NCEP–NCAR reanalysis are made in land surface parameters and land–ocean fluxes. This analysis can be used as a supplement to the NCEP–NCAR reanalysis especially where the original analysis has problems. The differences between the two analyses also provide a measure of uncertainty in current analyses.
5,177 citations
Additional excerpts
...2 dataset (Kanamitsu et al. 2002)....
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TL;DR: In this article, a simple analytic model is constructed to elucidate some basic features of the response of the tropical atmosphere to diabatic heating, showing that there is considerable east-west asymmetry which can be illustrated by solutions for heating concentrated in an area of finite extent.
Abstract: A simple analytic model is constructed to elucidate some basic features of the response of the tropical atmosphere to diabatic heating. In particular, there is considerable east-west asymmetry which can be illustrated by solutions for heating concentrated in an area of finite extent. This is of more than academic interest because heating in practice tends to be concentrated in specific areas. For instance, a model with heating symmetric about the equator at Indonesian longitudes produces low-level easterly flow over the Pacific through propagation of Kelvin waves into the region. It also produces low-level westerly inflow over the Indian Ocean (but in a smaller region) because planetary waves propagate there. In the heating region itself the low-level flow is away from the equator as required by the vorticity equation. The return flow toward the equator is farther west because of planetary wave propagation, and so cyclonic flow is obtained around lows which form on the western margins of the heating zone. Another model solution with the heating displaced north of the equator provides a flow similar to the monsoon circulation of July and a simple model solution can also be found for heating concentrated along an inter-tropical convergence line.
3,799 citations
TL;DR: In this article, the amplitude and phase of the Arm harmonic fitted to the 24-month composite values are plotted in the form of a vector for each station, which reveals both the regions of spatially coherent ENSO-related precipitation and the phase of this signal in relation to the evolution of the composite episode.
Abstract: We investigate the “typical” global and large-scale regional precipitation patterns that are associated with the El Nino/Southern Oscillation (ENSO). Monthly precipitation time series from over 1700 stations are analyzed using an empirical method designed to identify regions of the globe that have precipitation variations associated with ENSO. Monthly mean ranked precipitation composites are computed over idealized 2-year ENSO episodes for all stations that include data for at least five ENSOs. The amplitude and phase of the Arm harmonic fitted to the 24-month composite values are plotted in the form of a vector for each station. When plotted on a global map, these vectors reveal both the regions of spatially coherent ENSO-related precipitation and the phase of this signal in relation to the evolution of the composite episode. Time cries of precipitation for the coherent regions identified in the harmonic vector map are examined to determine the magnitudes of the ENSO-related precipitation and th...
3,608 citations
TL;DR: The new NOAA operational global sea surface temperature (SST) analysis is described in this paper, which uses 7 days of in situ (ship and buoy) and satellite SST.
Abstract: The new NOAA operational global sea surface temperature (SST) analysis is described. The analyses use 7 days of in situ (ship and buoy) and satellite SST. These analyses are produced weekly and daily using optimum interpolation (OI) on a 1° grid. The OI technique requires the specification of data and analysis error statistics. These statistics are derived and show that the SST rms data errors from ships are almost twice as large as the data errors from buoys or satellites. In addition, the average e-folding spatial error scales have been found to be 850 km in the zonal direction and 615 km in the meridional direction. The analysis also includes a preliminary step that corrects any satellite biases relative to the in situ data using Poisson's equation. The importance of this correction is demonstrated using recent data following the 1991 eruptions of Mt. Pinatubo. The OI analysis has been computed using the in situ and bias-corrected satellite data for the period 1985 to present.
2,766 citations
TL;DR: In this article, the evolution of sea surface temperature (SST) anomalies, surface wind fields, and precipitation anomaly patterns during major warm episodes in the eastern and central tropical Pacific are described in terms of composite SST and wind fields (30°N−30°S) for six warm episodes since 1949, and time series and cross-spectral analyses of mean monthly data along six shipping lanes which cross the equator between the South American coast and 170°W.
Abstract: Surface marine observations, satellite data, and station observations of surface pressure and precipitation are used to describe the evolution of sea surface temperature (SST) anomalies, surface wind fields, and precipitation anomaly patterns during major warm episodes in the eastern and central tropical Pacific. The sequence of events is described in terms of composite SST and wind fields (30°N–30°S) for six warm episodes since 1949, and time series and cross-spectral analyses of mean monthly data along six shipping lanes which cross the equator between the South American coast and 170°W. During the months preceding a warm episode, the equatorial easterlies are stronger than normal west of the dateline. This and other coherent and strongly developed anomaly patterns over the western equatorial Pacific and South Pacific are associated with a South Pacific Convergence Zone (SPCZ) located southwest of its normal position. During October-November prior to El Nino, the equatorial easterly anomalies i...
2,647 citations