Edited by: Emanuel Dutra, European Centre for Medium-Range Weather Forecasts, UK
Reviewed by: Olivia Martius, University of Bern, Switzerland; Eduardo Zorita, Hemlholtz-Zentrum Geesthacht, Germany
*Correspondence: Michael J. Pook, Commonwealth Scientific and Industrial Research Organisation Oceans and Atmosphere Flagship, GPO Box 1538, Hobart, TAS 7001, Australia e-mail:
This article was submitted to Atmospheric Science, a section of the journal Frontiers in Environmental Science.
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The rare occasions when Lake Eyre in central, southern Australia fills with water excite great interest and produce major ecological responses. The filling of other smaller lakes such as Lake Frome have less impact but can contribute important information about the current and past climates of these arid regions. Here, the dominant synoptic systems responsible for heavy rainfall over the catchments of Lake Eyre and Lake Frome since 1950 are identified and compared. Heavy rain events are defined as those where the mean catchment rainfall for 24 h reaches a prescribed threshold. There were 25 such daily events at Lake Eyre and 28 in the Lake Frome catchment. The combination of a monsoon trough at mean sea level and a geopotential trough in the mid-troposphere was found to be the synoptic system responsible for the majority of the heavy rain events affecting Lake Eyre and one in five of the events at Lake Frome. Complex fronts where subtropical interactions occurred with Southern Ocean fronts also contributed over 20% of the heavy rainfall events in the Frome catchment. Surface troughs without upper air support were found to be associated with 10% or fewer of events in each catchment, indicating that mean sea level pressure analyses alone do not adequately capture the complexity of the heavy rainfall events. At least 80% of the heavy rain events across both catchments occurred when the Southern Oscillation Index (SOI) was in its positive phase, and for Lake Frome, the SOI exceeded +10 on 60% of occasions, suggesting that the background atmospheric state in the Pacific Ocean was tilted toward La Niña. Hydrological modeling of the catchments suggests that the 12-month running mean of the soil moisture in a sub-surface layer provides a low frequency filter of the precipitation and matches measured lake levels relatively well.
The lakes of Australia's arid interior are ephemerally filled and are for most of the time salt-crusted playa surfaces. The filling of such large playa lakes (including Australia's largest—Lake Eyre) is therefore an important climatological phenomenon triggering large-scale ecological responses (e.g., Kingsford and Porter,
A number of attempts have been made to document the historical filling of Lake Eyre but little other work exists which documents when and how other large playa lakes (e.g., Torrens or Frome) have filled. Earlier work by the Lake Eyre Committee (
It is not known what synoptic and climatic conditions were responsible for ancient lake fillings. However, there are reliable synoptic data available for the period since about 1950 and analysis of this data set provides the opportunity to examine the modern circulation types associated with large rainfall events in these arid zone catchments. The results of this analysis may then offer some insights into the characteristics of the enhanced rainfall periods in the geological record. Hence, in this paper we investigate the types of synoptic systems responsible for a selection of recent historical heavy rainfall events over the catchments of Lake Eyre and Lake Frome using accepted methods of synoptic climatology (e.g., Pook et al.,
Lake-filling occurrences are not the result of isolated heavy precipitation events in the arid interior of Australia but rather follow extended periods of widespread rainfall over the catchments. While it is important to categorize the synoptic systems responsible for individual heavy rain events, it is also essential to place these synoptic events within the background atmospheric state. The intensity of the Australian monsoon has been shown to have a strong association with the filling of Lake Eyre (Allan,
Daily rainfall data for each catchment for the period January 1, 1900 to February 28, 2014 were obtained by spatial averaging of the Australian Bureau of Meteorology's high-quality gridded rainfall surfaces at 0.05 × 0.05° resolution, prepared for the Australian Water Availability Project (AWAP) (Jones et al.,
The synoptic analysis was primarily based on the National Centers for Environmental Prediction (NCEP)—National Center for Atmospheric Research (NCAR) climate reanalysis data set (Reanalysis 1) (Kalnay et al.,
The Reanalysis data set was supplemented by daily weather maps at 0000 UTC published in the Australian Bureau of Meteorology's “Monthly Weather Review” series (Simmonds and Richter,
Monthly means of the SOI were obtained from the Australian Bureau of Meteorology (
For each day on which the mean rainfall across the particular catchment network was greater than or equal to a threshold value, a particular synoptic system was identified as being responsible for the precipitation event. For the vast Lake Eyre catchment (see Figure
The primary classification of synoptic systems followed Pook et al. (
As this analysis extends over the austral summer, an extra category of “monsoon trough” has been added. The monsoon trough was identified by the analyst in cases where the shear line between the low-level westerlies in the north and the southeast trades in the south as inferred from the isobaric pattern was located over northern Australia (McBride,
For each event, the MSLP, 500 hPa, and the 1000-500 hPa thickness analyses were carefully evaluated. Hence, synoptic types have been specified according to a combination of MSLP and upper air criteria. As an example, Figure
Over the period from January 1950 to February 2014 there were 25 individual events for which the mean daily rainfall for the Lake Eyre catchment (see Figure
24-May-1955 | 23 | E/ly Tr/upper Tr |
25-May-1955 | 23 | E/ly Tr/upper Tr |
29-April-1968 | 21 | cutoff low |
05-March-1972 | 22 | monsoon Tr/upper Tr |
06-March-1972 | 21 | monsoon Tr/upper Tr |
07-February-1976 | 22 | monsoon Tr/upper Tr |
08-February-1976 | 24 | monsoon Tr/upper Tr |
09-February-1976 | 24 | monsoon Tr/upper Tr |
18-February-1977 | 21 | monsoon Tr/upper Tr |
19-February-1977 | 26 | monsoon Tr/upper Tr |
20-February-1977 | 23 | monsoon Tr/upper Tr |
10-July-1978 | 21 | E′ly Tr/upper Tr |
21-May-1981 | 21 | E′ly Tr/upper Tr |
12-January-1984 | 24 | monsoon Tr/cutoff low |
13-January-1984 | 30 | monsoon Tr/cutoff low |
31-March-1988 | 25 | E′ly Tr/upper Tr |
21-May-1990 | 22 | E′ly Tr/cutoff low |
22-May-1990 | 24 | cutoff low |
06-February-1991 | 22 | monsoon Tr |
28-February-1992 | 21 | monsoon Tr/upper Tr |
18-January-1995 | 29 | monsoon Tr/upper Tr |
19-January-1995 | 26 | cutoff low |
21-January-2007 | 22 | monsoon Tr/upper Tr |
28-February-2010 | 25 | monsoon Tr/upper Tr |
01-March-2010 | 26 | monsoon Tr/upper Tr |
The dominant system found to be responsible for the heavy rain events was the combination of a monsoon trough of low pressure on the MSLP surface and an upper trough as determined by the 500 hPa geopotential and 1000-500 hPa thickness analyses. An example of this synoptic type is shown in Figure
Troughs in the easterlies which were not assessed as satisfying the requirements of the monsoon trough but were associated with an upper trough on the 500 hPa surface were analyzed as the progenitors of a further 5 heavy rainfall events. One surface easterly trough was accompanied by a cutoff low at 500 hPa and 3 heavy rain events were associated with cutoff lows identifiable in both the upper air and at MSLP. Two of these events occurred in the austral autumn. Only one event was found to be caused by a surface trough without an accompanying trough in the middle and upper atmosphere. Additionally, only one heavy rain event occurred in the austral winter (10 July, 1978) and none in the austral spring, thus confirming the strong bias toward major rain events in the Lake Eyre catchment during summer and autumn.
For one event (21 January, 2007), an active monsoon trough was located over northern Australia while a cutoff low was centered just to the southwest of Adelaide and extended a cold trough at 500 hPa northwards over the Lake Eyre Basin. Although this location of the cutoff low would normally not be associated of itself with heavy rainfall over the majority of the Lake Eyre catchment, Figure
For the analysis period, there were 28 heavy rainfall events resulting in an average daily rainfall of 25 mm or more over the Lake Frome catchment (Table
02-February-1950 | 25 | E′ly Tr |
17-March-1950 | 47 | E′ly Tr/upper Tr |
24-May-1955 | 27 | E′ly Tr/cutoff low |
25-May-1955 | 36 | cutoff low |
17-January-1962 | 30 | complex front |
16-January-1968 | 25 | cutoff low |
29-December-1973 | 31 | monsoon Tr |
30-December-1973 | 26 | complex front |
29-January-1974 | 30 | monsoon Tr/upper Tr |
19-February-1974 | 27 | E′ly Tr/upper Tr |
23-October-1975 | 25 | cutoff low |
13-December-1975 | 32 | E′ly Tr/upper Tr |
08-February-1976 | 30 | monsoon Tr/upper Tr |
09-February-1976 | 49 | monsoon Tr/upper Tr |
10-February-1976 | 26 | monsoon Tr/upper Tr |
17-January-1979 | 25 | monsoon Tr/upper Tr |
22-February-1979 | 26 | E′ly Tr/cutoff low |
14-January-1984 | 44 | monsoon Tr/cutoff low |
26-January-1984 | 30 | complex front |
14-March-1989 | 49 | monsoonTr/upper Tr |
10-January-1990 | 26 | E′ly Tr/cutoff low |
16-March-1996 | 27 | complex front |
07-February-1997 | 31 | E′ly Tr |
12-February-2000 | 25 | complex front |
06-February-2011 | 40 | complex front |
29-February-2012 | 36 | E′ly Tr/upper Tr |
1-March-2012 | 29 | E′ly Tr/upper Tr |
15-February-2014 | 26 | E′ly Tr |
Monsoon troughs at MSLP combined with an upper trough were found to be responsible for about 20% of the 28 heavy rain events in the Lake Frome catchment. However, complex fronts were equally dominant causes of heavy rainfall events. These systems were characterized by strong interaction with a prefrontal trough and tropical moisture and often, a pre-existing subtropical cloud band, and are similar to the “interacting frontal types” of Wright (
The next most important category of synoptic systems is a trough in the easterly flow at MSLP accompanied by an upper trough. There were also three cases where cutoff lows were the dominant synoptic system associated with daily rain events and four events where cutoff lows were associated with a trough on the MSLP surface. Additionally, four events were associated with troughs at the surface that were not also accompanied by troughs in the middle and upper atmosphere.
Figure
Water levels in Lake Eyre have been recorded or estimated over many decades by the Lake Eyre Yacht Club (Bye et al.,
In order to better match the relatively slow lake-filling events with hydrological data, monthly precipitation data from AWAP are presented in Figure
In Figure
The Lake Frome record displays some similar characteristics to the much larger Lake Eyre catchment but the lake level peaks are considerably lower than those recorded for Lake Eyre. Figure
The strong tendency for the heavy rain events considered here to occur in the austral summer and autumn suggests that the intensity and persistence of the Australasian summer monsoon is a key factor in lake-filling episodes. In turn, the summer monsoon is closely influenced by the phase and intensity of the ENSO cycle. In order to investigate the association of ENSO with heavy rainfall in the lake catchments, the daily heavy rain events have been partitioned according to the mean value of the SOI for the month in which the event occurred.
Figure
A climatological analysis has been completed of the synoptic systems associated with heavy precipitation events in the Lake Eyre and Lake Frome catchments of central, southern Australia since 1950. The analysis reveals that tropical influences during the austral summer and autumn dominate significant daily precipitation occurrences in the Lake Eyre catchment. On the MSLP surface, these occurrences were characterized by convergence in a monsoon trough over northern Australia or, at the very least, a well-defined trough of low pressure in the easterly flow. However, the striking feature of the majority of these events was the presence of an accompanying trough or cutoff low in the middle troposphere (500 hPa) and a negative anomaly in 1000-500 hPa thickness relative to the long-term climatology. Further south, at Lake Frome, approximately one half of the events also had these characteristics but over 20% of events affecting that catchment were associated with Southern Ocean cold fronts which interacted with a prefrontal trough extending from the sub-tropics (complex fronts). Individual cutoff lows accounted for between 10 and 15% of heavy rain events in each catchment. Heavy rain events associated with surface troughs without upper air support were identified on only about 10% of occasions at Lake Eyre and 7% of occasions at Lake Frome. These findings indicate that synoptic analyses confined to mean sea level are inadequate for providing a complete understanding of these heavy rain events.
Although the climatology was based on heavy daily precipitation events, it is evident that one heavy fall of rain over a 24-h period is not capable of producing a lake-filling episode. These rare occurrences are associated with a persistence of rain events, usually in association with a well-developed Australasian summer monsoon which in turn, is closely influenced by the phase and intensity of the ENSO cycle. At least 80% of the heavy rain events examined in this study across both catchments occurred when the Southern Oscillation as measured by the SOI was in its positive phase. Strikingly, in the Lake Frome case, the SOI exceeded +10 on 60% of occasions, suggesting that the background atmospheric state in the Pacific Ocean and Australasia was tilted toward La Niña. At Lake Frome, all the events occurred outside winter, suggesting tropical interaction was a key component of the synoptic processes. Overall, it was a key finding of this study that the Lake Frome catchment derives significant rainfall from both tropical and subtropical synoptic types, and from connections between the higher latitudes and the tropics.
Hydrological modeling of the two catchments using high quality rainfall data as an input reveals that a 12-month running mean of the moisture content in the sub-surface soil layer coincides quite well with the historical lake levels. The degree of saturation of this sub-surface layer appears to act as a low pass filter on the catchment precipitation in a similar way to the finding of Ummenhofer et al. (
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Michael J. Pook, James S. Risbey and Peter R. Briggs acknowledge the contribution of the Commonwealth Scientific and Industrial Research Organisation Oceans and Atmosphere Flagship to this work. Caroline C. Ummenhofer was supported through the