Matthew England - Recent projects

Professor Matthew England
Climate Change Research Centre (CCRC)
School of Mathematics
The University of New South Wales
Sydney NSW 2052 Australia

M.England

unsw.edu.au

RECENT PROJECTS

Oceanic predictors of drought cycles for Australia and other Indian and Pacific Ocean rim nations

Collaborators: Caroline Ummenhofer, Alex Sen Gupta, Agus Santoso, Mike Pook, Andrea Taschetto and Khalia Hill

Project Aims:

There has been a great deal of concern about the decline in rainfall over southwest Western Australia and along Australia's east coast in the past few decades. There are also other Indian Ocean rim nations suffering a recent decline in rainfall, notably sub-Saharan Africa. To exacerbate the problem, annual rainfall variations for some of these region are very large, with changes of up to 70% from one year to the next. The goal of this project has been to examine the mechanisms of rainfall variability for all regions of Australia, with a view to improving predictive skill for years/seasons of anomalously high and low rainfall. Regions of interest include southwest Western Australia (SWWA), the east coast extending from Queensland down to Victoria, Tasmania, and tropical Australia. Of these regions, SWWA, the east coast and parts of Tasmania have suffered recent rainfall decline, whereas tropical Australia (particularly to the northwest) has seen an increase (Fig. 1).

Figure 1. Trend in annual total precipitation (mm per decade) over Australia during 1950-2005 (Source: Bureau of Meteorology).

1. Southwest Western Australia

Over SWWA there are periods of unusually high and unusually low rainfall with no clear link to well-documented climate modes such as the El Niño / Southern Oscillation (ENSO). The aim of this part of the project was to examine what determines rainfall extremes over southwest Western Australia combining observations, reanalysis data, and a long-term natural integration of the global coupled climate system.

The area of interest is highlighted below in Fig. 2a. While excellent progress has been made in our understanding of what has caused the region’s decline in rainfall since 1970, relatively little is known about what controls the large variations in seasonal and annual rainfall rates. We were interested in finding out what aspect of the climate system was forcing years of unusually low rainfall. We were also keen to investigate the possible causes of high rainfall seasons. This information is vital for the management of Western Australia’s freshwater supply and for the region’s agricultural sector.

Figure 2a. Annual mean precipitation (mm per yr) over Australia (Source: Bureau of Meteorology); the study area over southwest Western Australia (SWWA) is indicated.

Background:

The Australian continent experiences harsh extremes in climate. Rainfall over southwest Western Australia has shown a decline during winter over the past century, linked to greenhouse gas increases and possibly also to stratospheric ozone depletion over Antarctica. While much attention has been paid to this decrease in yearly rainfall, little is known about the very large year-to-year changes in SWWA rainfall (refer to Fig. 2b). Over these shorter time-scales, the region’s interannual rainfall variability remains poorly understood, as traditional predictors for Australian climate, such as the Southern Oscillation Index, appear to resolve little of the region’s variability. We set out to explore how southwest Western Australian rainfall might be controlled by the adjacent Indian and Southern Oceans, while also examining possible modulated effects from the tropical Pacific. The project involved an investigation of interannual rainfall extremes over SWWA using observations, reanalysis data, and a fully coupled climate model.

Figure 2b. Time-series of the observed annual rainfall anomaly (mm per yr) for SWWA during 1970-2003; dashed lines indicate ± one standard deviation and years with rainfall exceeding these criteria are marked as extremes (filled circles). Indian Ocean Dipole (IOD) years are also indicated.

Results:

In the paper England et al. [2006] we showed a distinctive pattern of Indian Ocean sea surface temperature (SST) anomalies that characterised the southwest Australian rainfall variations, involving a dipole structure with unusually warm and cool waters adjacent to Western Australia (see Figs 3 and 4 below). Extreme events in rainfall were found to be part of a large-scale phenomenon spanning the Indian Ocean basin, extending south to 50ºS.

The characteristic pattern of Indian Ocean sea surface temperature anomalies during extreme rainfall years turned out to be remarkably consistent between the reanalysis fields and the coupled climate model, but different from previous definitions of SST dipoles in the region. In particular, the dipole exhibits peak amplitudes in the eastern Indian Ocean adjacent to the west coast of Australia. During dry years, anomalously cool water appears in the tropical/subtropical eastern Indian Ocean, adjacent to a region of unusually warm water in the subtropics off SWWA. This dipole of anomalous SST seesaws in sign between dry and wet years, and appears to occur in phase with a large-scale reorganization of winds over the tropical/subtropical Indian Ocean (Figs. 3, 4). The wind field alters SST via anomalous ‘Ekman’ transport in the tropical Indian Ocean and via anomalous air-sea heat fluxes in the subtropics. The winds also change the large-scale advection of moisture onto the SWWA coast. At the basin scale, the anomalous wind field can be interpreted as a large-scale acceleration (deceleration) of the Indian Ocean climatological mean anticyclone during dry (wet) years. In addition, dry (wet) years see a strengthening (weakening) and coinciding southward (northward) shift of the subpolar westerlies, which results in a similar southward (northward) shift of the rain-bearing fronts associated with the subpolar front.

Figure 3. Composite maps of (a,b) rainfall and (c,d) SST anomaly overlaid on vectors of wind stress anomaly during dry and wet years in the climate model. Anomalies are only shown where they exceed a 90% significance level as estimated by a two-tailed t-test. The corresponding composite maps from the observed climatology data sets (SST shown in Fig. 4 at right) bear a striking similarity to the climate model results.

Figure 4. Schematic diagram showing the connection between Indian Ocean climate variability and (a) dry, (b) wet years over southwest Western Australia. Sea surface temperature anomalies are shown as actual observed composite fields (color shaded in °C). Wind anomalies are shown as bold arrows, pressure anomalies by H (high) and L (low), and rainfall anomalies by sun/cloud symbols.

An analysis of the seasonal evolution of the climate extremes revealed a progressive amplification of anomalies in SST and atmospheric circulation toward a wintertime maximum, coinciding with the season of highest SWWA rainfall. The anomalies in SST can appear as early as the summertime months, however. This has important implications for predictability of SWWA rainfall extremes. This is an important aspect of the work as oceanic precursors to climate extremes are at the heart of improving lead forecast times of Australian rainfall.

Predictability:

During the past 12 months the predictability of the Indian Ocean - SWWA rainfall link has been investigated [Ummenhofer et al., 2007a]. This has involved exploring ensemble sets of atmospheric model experiments wherein the ocean SST is perturbed by the anomalies characterising wet and dry years (see Fig. 5a). Figure 5b shows the frequency distribution of large-scale rainfall spatially averaged across SWWA summed for the months Jan-Dec, May-Sep, and June-August for both the DRY (left) and WET (right) cases. Apparent in this analysis is the fact that the respective ensemble sets are significantly drier/wetter over SWWA in the perturbed SST experiments as compared to the CONTROL case. This can be seen by comparing the histograms of the control ensemble set with the anomaly experiments. In each case there is a skewing of the histogram to the dry or wet end of the spectrum, in line with the imposed SST anomaly fields. This demonstrates that the Indian Ocean plays a role in controlling SWWA rainfall patterns - particularly seasons of anomalously high and low rainfall. Previous studies had assumed that Indian Ocean SST anomalies were simply symptomatic of large-scale atmospheric forcing.

(a) SST perturbation anomalies used in the AGCM predictability experiments (averaged for MJJAS)

(b) Rainfall event histograms binned by 20 mm/month ranges.

Frequency distribution of large-scale rainfall spatially averaged across SWWA summed for the months of May-September for the Control, DRY, and WET cases.

Outcomes:

A key outcome of this study is a new opportunity to develop predictive indices – akin to the Southern Oscillation Index – for southwest Western Australia rainfall. An example is shown below in Figure 6, where sea surface temperature anomalies are shown for April 2005. The labels P1 and P2 in Fig. 6a indicate the location of the SST dipole documented in our SWWA study. The analyses undertaken were completed in January 2005, so that 2005 provided the first test of the robustness of our findings. The pattern of Indian Ocean temperatures in autumn 2005 (Fig. 6a) developed into the characteristic ‘wet’ year precursors seen in Figs. 3 and 4 above. We had to wait several months to find out whether this SST anomaly would lead to increased SWWA rainfall, as our climate analyses would suggest. As it turned out, the 2005 winter saw record high rainfall over many regions of southwest WA (see Fig. 6b below). For details of this record rainfall see a press release from the Australian Bureau of Meteorology. If the web-link to the BoM site is down, see a copy of this press release by clicking here.

Figure 6a. Satellite measurements of sea surface temperature anomalies in April 2005 (blue = cool anomalies, red = warm anomalies).

Figure 6b. Rainfall decile ranges for Australia during April – June 2005 (Source: Australian Bureau of Meteorology). Early winter rainfall over southwest Western Australia reached record high levels in 2005 after several years of average and below average rainfall.

In the above predictability experiments it is clear that the Indian Ocean SST has the capacity to force years of high and low rainfall, yet we still need to quantify the prediction lead times involved. This will involve running AGCM experiments with SST anomalies persisting into various stages of the year, then quantifying the resulting SWWA rainfall anomalies during the winter season. It is hoped that forecast lead times of the order of 2-3 months will have statistically significant predictive skill. This would be a major advance for seasonal/interannual forecasting over SWWA as previously the region was thought to be insensitive to Indian Ocean thermal variability. As the oceans develop and retain heat anomalies on much longer time-scales than the atmosphere, this has positive implications for the region's agricultural and water sectors as they will have the opportunity to better plan for anomalous seasons.

Conclusions:

Our discovery of an oceanic precursor to periods of unusually high and unusually low rainfall will ultimately lead to improved predictability of southwest Western Australia freshwater supply. Ensemble atmospheric and coupled climate model experiments, as shown in Figure 5 above, have demonstrated that the above SST index significantly improves skill at forecasting SWWA rainfall.

2. New Zealand

The techniques and methods employed in the SWWA study have now also been applied to studies of New Zealand rainfall extremes and trends [Ummenhofer and England, 2007; Ummenhofer et al., 2007b]. New Zealand rainfall is characterised by high regional variations with particularly high rainfall along the west coast of the South Island (Fig. 7). The interannual extremes in New Zealand rainfall and their modulation by modes of Southern Hemisphere climate variability were examined in observations and a coupled climate model. North Island extreme dry (wet) years are found to be characterized by locally increased (reduced) sea level pressure (SLP), cold (warm) sea surface temperature (SST) anomalies in the southern Tasman Sea and to the north of the island, and coinciding reduced (enhanced) evaporation upstream of the mean southwesterly airflow. During extreme dry (wet) years in South Island precipitation, an enhanced (reduced) meridional SLP gradient occurs, with circumpolar strengthened (weakened) subpolar westerlies and an easterly (westerly) anomaly in zonal wind in the subtropics. As a result, via Ekman transport, anomalously cold (warm) SST appears under the subpolar westerlies, while anomalies of the opposite sign occur further north.

Figure 7. Annual mean precipitation map for New Zealand for the period 1960-2004 (in mm per year).

The phase and magnitude of the resulting SST and evaporation anomalies cannot account for the rainfall extremes over the South Island, suggesting a purely atmospheric mode of variability as the driving factor, in this case the Southern Annular Mode (SAM). New Zealand rainfall variability is predominantly modulated by two Southern Hemisphere climate modes, namely the El Niño -Southern Oscillation (ENSO) and the SAM, with a latitudinal gradation in influence of the respective phenomena, and a notable interaction with orographic features. While this heterogeneity is apparent both latitudinally and as a result of orographic effects, climate modes can force local rainfall anomalies with considerable variations across both islands. North Island precipitation is for the most part regulated by both local air-sea heat fluxes and circulation changes associated with the tropical ENSO mode. In contrast for the South Island, the influence of the large-scale general atmospheric circulation dominates, especially via the strength and position of the subpolar westerlies, which are modulated by the extratropical SAM.

3. Tasmania

This project has also assessed the magnitude and mechanisms of interanuual rainfall variability over Tasmania [Hill et al., 2007]. In particular, interannual rainfall variability over Tasmania is examined using observations, reanalysis data and an unforced 1000-yr coupled climate model integration. Tasmania's rainfall is dominated by an east-west gradient of mean rainfall and variability. Over the past 100 years the rainfall over the island has seen a signicant increasing trend in the southwest and a decreasing trend elsewhere. The empirical orthogonal functions (EOFs) of rainfall over Tasmania show a leading mode (explaining 63% of total variance) of coherent island-wide in phase anomalies with dominant periods of 2 and 6 years. The second and third EOF's together account for 25% of total variation, characterised by out of phase east-west anomalies, and a trend in time. Composites of the reanalysis data indicate the influence of the SAM on dry years over Tasmania, while an ENSO anomaly pattern is apparent in the equatorial Pacific Ocean during wet years. Composites of rainfall during each ENSO phase suggest a notable influence of this climate mode on Tasmanian rainfall, although there is a decreasing impact toward the south of the island (Fig. 8a,b). In contrast, the Southern Annular Mode (SAM) acts to keep the west wet in its negative phase, while the east remains drier (Fig. 8c,d). During the positive phase of the SAM, the majority of the island is dry except for the southwest region, where a notable increase in rainfall is present. This region of wetter conditions coincides with the area of positive rainfall trends during the past century, and is likely due to a modest shift in the mean latitude of extratopical cyclones over the Southern Hemisphere since 1950.

Figure 8. Composites of anomalous rainfall (mm per year) over Tasmania during anomalous phases of ENSO (a) La-Niño and (b) El--Niño. Composites of anomalous rainfall over Tasmania during the anomalous (+/- 1 standard deviation) phases of the Southern Annular Mode; (c) positive and (d) negative.

4. Tropical Australia

In this part of the study we investigated the interseasonal and inter-event variations in the impact of El Niño on Australian rainfall using available observations from the post-satellite era. Of particular interest is the difference in impact between classical El Niño events wherein peak sea surface temperature (SST) anomalies appear in the eastern Pacific, and the recently termed El Niño `Modoki' events that are characterized by peak SST anomalies in the central Pacific.

A clear interseasonal and inter-event difference is apparent, with the classical El Niño signature peaking in austral spring, and the Modoki signature peaking in austral summer. The maximum rainfall response for Modoki events lags the peak SST anomalies by one season, compared to only one month for traditional El Niños. Most interestingly, the Modoki and non-Modoki El Niño events exhibit a marked difference in rainfall impact over Australia: while classical El Niños are associated with a significant reduction in rainfall over northeastern and southeastern Australia, the Modoki events appear to drive a large-scale decrease in rainfall over northwestern and northern Australia. Furthermore, we find that Australian rainfall variations during March-April-May are more sensitive to the Modoki SST anomaly pattern than the traditional El Niño anomalies to the east. (Figs. 9, 10).

Figure 9. Leading mode of SVD analysis between SST in the tropical Pacific (upper left panel) during SON and Australian rainfall (upper right panel) lagged by 1 month (1979 to 2003). Lower panel: Time series of the SVD expansion coecient. Blue line: associated with the SST mode (degrees Celsius). Green line: associated with the rainfall mode (mm/month). Black line: NINO3 Index scaled by 0.3. The correlation coefficient between the SST and rainfall time series is 0.52 and between NINO3 and SST is 0.98.

Figure 10. Leading mode of SVD analysis between SST in the tropical Pacific (upper left panel) during DJF and Australian rainfall (upper right panel) lagged by 1 season (i.e. MAM). Lower panel: Time series of the SVD expansion coecientcient. Blue line: associated with the SST mode (degrees Celsius). Green line: associated with the rainfall mode (mm/month). Black line: El Niño Modoki Index (EMI) scaled by 0.5. The correlation ccoefficient between the SST and rainfall time series is 0.44 and between the EMI and SST is 0.64.

5. East Africa

The most recent featured aspect of this project has been the discovery of a link between the SST mode used to force SWWA rainfall anomalies, and precipitation over East Africa. In particular, we have identified links between extreme wet conditions over East Africa and Indian Ocean sea surface temperatures (SST) during the "short rain" season in October-November.

During periods of enhanced East African rainfall, Indian Ocean SST anomalies reminiscent of a tropical Indian Ocean Dipole (IOD) event are observed. Ensemble simulations with an atmospheric general circulation model (AGCM) are used to understand the relative effect of local and large-scale Indian Ocean SST anomalies on above-normal East African precipitation. The importance of the various tropical and subtropical IOD SST poles, both individually and in combination, is quantified (refer to regions in Fig. 11).

Figure 11. Average Oct-Nov SST anomaly (in degrees C) used to force the atmospheric model to explore rainfall anomalies over the Indian Ocean rim nations (in particular Africa for this sub-project). The perturbation is applied for the entire Indian Ocean, as well as for the separate peak SST anomalies seen in the western, eastern, and southern parts of the Indian Ocean.

In the simulations, enhanced East African "short rains" are predominantly driven by the local warm SST anomalies in the western equatorial Indian Ocean, while the eastern cold pole of the tropical IOD is of lesser importance (Fig. 12). The changed East African rainfall distribution can be explained by a reorganization of the atmospheric circulation induced by the SST anomalies. A reduction in sea level pressure over the western half of the Indian Ocean and converging wind anomalies over East Africa lead to moisture convergence and increased convective activity over the region. The pattern of large-scale circulation changes over the tropical Indian Ocean and adjacent land masses is consistent with an anomalous strengthening of the Walker cell. The seasonal cycle of various indices related to the SST and the atmospheric circulation in the equatorial Indian Ocean are examined to assess their potential usefulness for seasonal forecasting. This SST pattern (Fig. 11) is likely the basis of a useful Indian Ocean Index for predicting SWWA and east African rainfall.

Figure 12. Frequency distribution of total precipitation spatially averaged across East Africa: cumulative rainfall amount (in mm) summed for the months October-November for atmospheric model experiments forced by: (a) the Indian Ocean SST anomaly of Fig. 11, (b) eastern and southern anomalies [PeI+sI], (c) eastern and western anomalies [PeI+wI], (d) eastern anomalies only [PeI], (e) southern anomalies only [PsI], and (f) western anomalies only [PwI]. The shaded gray rainfall distribution represents the Control unperturbed simulations (normalized to the number of ensemble members in the perturbed cases), while the perturbed cases are indicated with black outlines. The following significance levels hold, as determined by a Mann-Whitney test: with the exception of (e) all significant at 99% level.

References:

England, M.H., C.C. Ummenhofer, and A. Santoso, 2006: Interannual rainfall extremes over southwest Western Australia linked to Indian Ocean climate variability. J. Climate, 19, 1948-1969. Reprint.

England, M.H., C.C. Ummenhofer, and A. Santoso, 2006: Indian Ocean variability linked to interannual rainfall extremes over southwest Western Australia, CLIVAR Exchanges, 11(4), p. 28. Reprint.

[138] England, M. H., A. Santoso, S. J. Phipps, and C. C. Ummenhofer, 2009: Role of the Indonesian Throughflow in controlling regional mean climate and rainfall variability, J. Climate, submitted.

[121] Hill, K.J., A. Santoso and M.H. England, 2009: Interannual Tasmanian rainfall variability associated with large-scale climate modes, J. Climate, in press. Preprint.

Luffman, J.J., A.S. Taschetto, and M. H. England, 2008: Global and regional climate response to late 20th Century warming over the Indian Ocean, J. Climate, submitted.

[124] Taschetto, A.S. and M. H. England, 2009: El Nino Modoki impacts on Australian rainfall, J. Climate, in press.

[107] Taschetto, A. S. and M. H. England, 2009: An analysis of late twentieth century trends in Australian rainfall, Int. J. Climatol., 29, 791-807. Reprint.

[133] Taschetto, A.S., C.C. Ummenhofer, A. Sen Gupta, and M. H. England, 2009: The effect of anomalous warming in the central Pacific on the Australian monsoon, Geophys. Res. Lett., in press.

[132] Ummenhofer, C.C., A. Sen Gupta, A.S. Taschetto, and M. H. England, 2009: Modulation of Australian precipitation by meridional gradients in East Indian Ocean Sea Surface Temperature, J. Climate, in press.

[123] Ummenhofer, C.C., A. Sen Gupta, M.H. England, and C.J.C. Reason, 2009: Contributions of Indian Ocean sea surface temperatures to enhanced East African rainfall, J. Climate, 22, 993-1013. Reprint.

[117] Ummenhofer, C.C., M. H. England, P.C. McIntosh, G.A. Meyers, M.J. Pook, J.S. Risbey, A. Sen Gupta, A.S. Taschetto, 2009: What causes southeast Australia's worst droughts?, Geophys. Res. Lett., 36, L04706, doi:10.1029/2008GL036801. Reprint.

[116] Ummenhofer, C.C., A. Sen Gupta and M.H. England, 2009: Causes of late Twentieth Century trends in New Zealand precipitation, J. Climate, 22, 3-19. Reprint.

[108] Ummenhofer, C.C., A. Sen Gupta, M.J. Pook, and M.H. England, 2008: Anomalous rainfall over southwest Western Australia forced by Indian Ocean sea surface temperatures, J. Climate, 21, 5113-5134. Reprint.

[99] Ummenhofer C.C., A. Sen Gupta, M.J. Pook, and M. H. England, 2007: Seasonal rainfall anomalies over Western Australia forced by Indian Ocean SST - Scope for improved forecasting. CLIVAR Exchanges, 12(4), 30-32. Reprint.

[97] Ummenhofer, C.C. and M.H. England, 2007: Interannual extremes in New Zealand precipitation linked to modes of Southern Hemisphere climate variability, J. Climate, 20, 5418-5440. Preprint.

Did the Indian Ocean SST patterns work in 2006?

By mid-winter in the Southern Hemisphere in 2006 the southwest Western Australian region had witnessed record low levels of rainfall for the January - June period (see Fig. 13a below). This very dry period for SWWA has coincided with an SST anomaly pattern (Fig. 13b) that is reminiscent of the canonical "dry year" pattern described by England et al. [2006], as shown in the schematic diagram of Fig 4a above. Thus, since the time that our analyses were accepted for publication in the Journal of Climate at the beginning of 2005, both the 2005 and 2006 rainfall seasons over SWWA have shown extreme patterns that are consistent with the scenarios detailed in this Journal of Climate paper.

Figure 13a. Rainfall ranges for Western Australia during January - June 2006 (Source: Australian Bureau of Meteorology). Early winter rainfall over southwest Western Australia has been at record low levels in 2006.

Figure 13b. Satellite measurements of sea surface temperature anomalies in April 2006 (blue = cool anomalies, red = warm anomalies).

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