Home > Actualités > Atmospheric heat waves in New Caledonia – Research by Enora Cariou
CLIPSSA

Enora CARIOU’s final year internship

National School of Meteorology

February – August 2023

Supervisors: Alexandre Peltier (Météo-France), Christophe Menkes (IRD)

Enora Cariou completed a 6-month internship, which included three presentations (at the start, mid-term and end of the internship) to the CLIPSSA supervisory team and the community of researchers from the IRD, IFREMER, Météo-France, etc.

She defended her end-of-internship thesis on 24 August 2023 in front of the Ecole Nationale de Météorologie, IRD and Météo-France teams.

Main objective of the study

The aim of this study was to take stock of the heat waves that have affected New Caledonia over the last 40 years. The research is based on the Excess Heat Factor (EHF) index, which characterises the intensity of heat waves and is generated from in-situ temperature data from eight Météo-France stations and ERA5 Land reanalyses.

Context, issues and specific objectives

10 years ago, Météo-France and the New Caledonian government set up a heatwave monitoring and forecasting tool to guard against any health risks associated with hyperthermia. The population of New Caledonia has a number of co-morbidities – diabetes, severe hypertension, heart failure, obesity – which make it particularly vulnerable to heatstroke that lasts through the night. The hot weather monitoring and forecasting tool developed by Météo-France has enabled the government to take measures to limit the risk of hyperthermia during the hot season. Although the health authorities have expressed their satisfaction with the quality of the forecasts, scientific knowledge of heat waves has not improved. The aim of the course was therefore to answer the following questions :

  • What are the characteristics of heat waves in New Caledonia (duration, intensity, number)?
  • What are the trends?
  • What atmospheric mechanisms are involved?
  • Were the most significant episodes localised or synoptic in scale?

The work plan focused on the following points:

  • Temporal characteristics of localized heat waves in New Caledonia : definition of the Excess Heat Factor (EHF) index to describe heat waves (data, methods and behaviour of the index).
  • Spatial and temporal characteristics of heat waves on a regional scale (swSPCZ): climatologies, trends, teleconnections with ENSO and weather patterns.
  • Study of the most intense episodes: application of archetypal analysis to EHF data.

Main results

Studies of annual and seasonal climatologies and long-term trends have revealed an average of 4 to 5 heat waves across the region, lasting an average of 4.5 to 5 days. There are more heatwaves on the south-east coast of Grande Terre and on the Loyalty Islands, but they are more intense on the relief and the west coast. The cool season (April to October) also sees more intense and longer episodes. There has been a significant increase in the number of heat waves over most of the country, but the signal is less clear for average intensities and durations. The study of the impact of different atmospheric phenomena on the number of heat waves across the three types of weather (Tropical Weather, Unstable Trade Winds and Southern Disturbance) favoured the number of heat wave days. In addition, La Nina also appears to significantly favour the number of heatwaves. Finally, the test carried out on the Madden-Julian Oscillation revealed two phases with a significant but opposite effect on the occurrence of heatwaves.

The study of teleconnections and the analysis of the most intense events, using the Archetypal Analysis method, demonstrate a significant influence of ENSO on the number and intensity of heat waves. Weather patterns in the region also influence the occurrence of heat waves. Finally, the Archetypal analysis reveals geographical patterns during the most intense heat waves. In order to improve this analysis, statistical tests should be set up to confirm these findings. In New Caledonia, the intense heat wave in the summer of 2015-2016 caused massive coral bleaching, which was not expected. In fact, during a Niño, temperatures are normally cooler. Here, we are seeing a teleconnection with El Niño, which creates intense heat waves. This link has yet to be proven.

It would also be interesting to apply the analysis over the whole year and not just the warm season (not done due to lack of time). In fact, by running the algorithm several times over the warm season alone to deduce the optimum number of archetypes, the calculation time was around 48 hours. However, it is possible to reduce the size of the dataset by applying a principal component analysis beforehand.

Perspectives 

To continue this work, statistical tests will have to be carried out to verify the impact of El Niño on the intensity of heat waves. In addition, the ERA5 Land re-analyses show non-negligible biases in relation to observations, particularly in terms of temperature values and the number of heat waves detected. Further work is required using other models. As part of CLIPSSA’s high-resolution numerical simulations over New Caledonia, it would therefore be interesting, in future work, to calculate the HFE on these data. Here, we have applied the archetypal analysis only to the warm season. Applying it over the whole year would make it possible to compare patterns in different seasons.

Finally, coral reefs are threatened by bleaching, particularly as a result of marine heat waves. We could therefore consider developing a line of research linking atmospheric and marine heat waves.

Read her thesis, presentation and article here!

A poster summarising his work was presented by Alexandre Peltier at the 30th Conference of the Australian Meteorological and Oceanographic Society (AMOS) – 3 to 9 February 2024 in Canberra.