Monitoring heat wave precursors in Central Chile



Current SETI forecast, MJO condition, and observed temperatures in Central Chile



This is the last figure generated for the SETI forecast. Normally, this figure is updated once a day. If the figure is outdated, it might be due to data download issues (corrupt files) or calculation problems at the corresponding node.

The top panel shows SETI – the standardized extra-tropical index, representing mid-latitude circulation anomalies in the mid-troposphere over the SW Indian Ocean every 6h. SETI is regarded as a potential precursor of summer heat waves in Central Chile (Jacques-Coper et al., 2021). The vertical dashed line separates the GFS analysis for the previous 21 days (blue curve, to its left) from GEFS forecasts for the next 16 days (to its right). The GEFS-based SETI forecast comprises 20 ensemble members (orange curves) and the GEFS ensemble mean (red curve), and the operational GFS run (green curve). The operational run is performed at a higher spatial resolution and generally used in many online GFS charts and maps.


In the lower panel, colour circles indicate the observed phase and amplitude of the Wheeler and Hendon index of the Madden-Julian Oscillation (MJO; Wheeler & Hendon, 2004). MJO is the main mode of tropical intraseasonal variability and represents a further potential precursor of summer heat waves in Central Chile (Jacques-Coper et al., 2021).


Finally, red, blue, and green curves show maximum (Tx), mean (Tm), and minimum (Tn) temperature time series, respectively, as measured at the General Freire weather station near Curicó (34.97ºS, 71.22ºW, 225 m asl; managed by the Chilean National Weather Directorate, DMC).


For more information on how to interpret these figures, please refer to the “Instructions” tab.



Disclaimer: Please note that these products are for experimental use and do not represent official forecasts issued by any public or private institution.

Contact: Christian Segura (chseguraguzman@gmail.com), Martín Jacques Coper (martinjacques@udec.cl).

SETI analysis GFS

Here you can explore the past values of SETI.



Current SETI forecast (GFS)



Here you can explore the current forecast values of SETI. Use the time selector at the bottom to zoom in/out the time series.




SETI stands for standardized extra-tropical index and measures mid-latitude circulation anomalies in the mid-troposphere over the SW Indian Ocean every 6h. SETI is regarded as a potential precursor of summer heat waves in Central Chile ( Jacques-Coper et al., 2021 ).The time series corresponds to GFS analysis for the previous 21 days (blue curve, to its left) and GEFS forecasts for the next 16 days (to its right). The GEFS-based SETI forecast comprises 20 ensemble members (orange curves) and the GEFS ensemble mean (red curve), and the operational GFS run (green curve). The operational run is performed at a higher spatial resolution and generally used in many online GFS charts and maps.

For more information on how to interpret these figures, please refer to the “Instructions” tab.



Disclaimer: Please note that these products are for experimental use and do not represent official forecasts issued by any public or private institution.

Data sources:

- GEFS forecasts: geopotential height fields at 500 hPa (Z500) from the Global Forecast System (GFS) and the Global Ensemble Forecast System (GEFS) from NOAA / National Weather Service, USA, are accessed at https://nomads.ncep.noaa.gov/ .

Central Chile Temperature forecast (GFS)




South Central Chile Temperature forecast (GFS)






For more information on how to interpret these figures, please refer to the “Instructions” tab.



Disclaimer: Please note that these products are for experimental use and do not represent official forecasts issued by any public or private institution.

Data sources:

- GEFS forecasts: temperature fields at 2 meters above ground from the Global Forecast System (GFS) and the Global Ensemble Forecast System (GEFS) from NOAA / National Weather Service, USA, are accessed at https://nomads.ncep.noaa.gov/ .

Precursors of summer heat waves in central Chile


Between 16 and 14 days (i.e., approx. 2 weeks) before the heat wave starts in central Chile, we observe a dipole of anomalous atmospheric circulation over the SW Indian Ocean, southeast of Africa. To monitor this phenomenon, we have generated a numerical index called SETI (standardized extra-tropical index). High SETI values indicate the presence of a dipole like the one shown in Figure 1. SETI is a potential precursor of heat waves in central Chile through the propagation towards the East of an extra-tropical wave train (i.e., the alternation of positive and negative pressure centers in the mid-troposphere).


drawing

Figure 1. Dipole of anomalous mid-level atmospheric circulation over the Indian Ocean.

Some days later than in Figure 1, i.e. 13 to 10 days before the heat wave starts in central Chile, the extratropical wave train progresses eastward across the southern Indian Ocean. In the tropics, deep convection (rising air masses) occurs, promoting persistent precipitation. The location and intensity of the convection is modulated by the Madden-Julian oscillation (MJO). When convection occurs over the Maritime Continent, to the north of Australia, the MJO is active around phase 4. This phenomenon triggers a tropical wave train that moves to the southeast, on its way to South America. That is why we affirm that the MJO is another possible precursor of heat waves in central Chile.


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Figure 2. Deep convection in the tropics, over the Maritime Continent.

Then, between days 8 and 5 before the heat wave starts in central Chile, both wave trains (the extratropical and the tropical ones) converge and overlap, leading to a constructive wave superposition. Thus, the resulting centers of pressure are reinforced and continue to progress towards South America.


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Figure 3. The wave train is reinforced.

After a few days, the precipitation over the Maritime Continent has ceased and convection occurs over the South Pacific Convergence Zone. The reinforced wave train has arrived to south Chile via a high pressure center, which induces and strengthens the heat wave conditions over central Chile. This configuration tends to occur during MJO active phases 6 to 8.


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Figure 4. The heat wave starts in central Chile.


Standardized Extra-Tropical Index (SETI)


This monitoring platform is based on the results of Jacques-Coper et al. (2021). As discussed therein, the prominent anomalous dipole within the extra-tropical band in the SW Indian Ocean (Figure 1) that potentially precedes heat waves in central Chile is the motivation to compute the Extra-Tropical Index (ETI). To compute the ETI, we use 6-hourly GFS fields of geopotential height at 500 hPa (Z500; see “Data sources”). For each time, we enclose the areas corresponding to both action centres of said dipole as follows: 48º–56º S; 6º–22º E (western centre) and 54º–62º S; 58º–74º E (eastern centre), respectively. Then, we select the grid points within each center to extract the corresponding Z500 values, compute the mean of each center, and calculate ETI as their difference of means (i.e., eastern center mean minus western center mean).

For the standardized version of ETI (i.e., SETI), the mean Z500 time series from each centre of action (as defined for ETI) is standardized by subtracting its own mean and dividing by its standard deviation. Both statistics are computed for each, using a running window spanning for the last 40 days. Finally, the difference of both centres is standardized again in a similar manner, using its corresponding statistics for the last 40 days. Consequently, the 40-day running window retains variability in the synoptic-to-intraseasonal band, smoothing out longer periods such as the annual cycle.


Data and figure sources


SETI forecast, MJO condition, and observed temperatures in Central Chile


Model data: Z500 is accessed from the Global Forecast System (GFS) and the Global Ensemble Forecast System (GEFS) from NOAA / National Weather Service, USA, accessed at https://nomads.ncep.noaa.gov/.

MJO index monitoring: The observed Wheeler and Hendon MJO index is obtained from the Bureau of Meteorology, Australia, accessed at http://www.bom.gov.au/climate/mjo/graphics/rmm.74toRealtime.txt.

Temperature in central Chile: meteorological observations at the “General Freire” weather station near Curicó (34.97ºS, 71.22ºW, 225 m asl) are carried out by the Chilean National Weather Directorate (DMC) and accessed at XXX.

Figures from external repositories


SERVIMET archive: synoptic charts issued by the Chilean Navy Weather Service (SERVIMET) are accessed at http://web.directemar.cl/met/jturno/cartas/imagen.jpg

JMA archive: dynamic charts showing OLR and 200-hPa stream function and wave activity flux (anomaly) are issued by the Japan Meteorological Agency (JMA, Tokyo Climate Center, Climate Prediction Division), and are accessed at: https://ds.data.jma.go.jp/tcc/tcc/products/clisys/anim/anim_tp.html

MJO GFS-based forecast: figures from the US NOAA Climate Prediction Center, accessed at https://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/foregfs.shtml


Example of use


The figure below shows a particular situation (January 6, 2021) to exemplify how the information should be used. We observe on that day a SETI value slightly above 1. These conditions, although not very intense, show low spread (i.e., low dispersion of the ensemble members) and are forecast to persist for several days. Thus, the presence of a dipole over the SW Indian Ocean should be expected. This atmospheric configuration is related to a precursor of heat waves in central Chile through its connection with the aforementioned extratropical wave train. Additionally, we observe that the MJO index from the previous day corresponds to phase 3 and is slightly active (amplitude greater than 1). This fact suggests that a further precursor, namely MJO active phase 4, might occur during the subsequent days. With this knowledge, we can check the MJO forecasts available in the archives from external repositories. If a MJO transition into phase 6 is expected during the next ~1 week, this fact suggests that a tropical wave train might develop and constructively interact with the extre-tropical one, strengthening the circulation anomalies that promote persisting high temperatures in central Chile.


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Figure 5. SETI example.