How can the Indian monsoon affect heatwaves in Europe?
It is easy to assume that our weather is influenced predominantly by local processes. In the extra-tropical regions (between 30° and 60° latitude North or South of the equator) such as Europe, the weather is mostly dictated by a succession of cyclones, bringing rain, and anti-cyclones, bringing drier and sunnier conditions. The tropical region (between 30° South and 30° North of the equator) including most of India, has a different kind of weather where organised thunderstorms (convection) bring rain outside of periods of sunshine. These differences created a fundamental separation between the two regions, which was reflected in early stages of NWP (Numerical Weather Prediction). Early models had an artificial boundary at or above the equator, justified by the difference between regions but also due to the limited computing power and data available at the time. However, as the field progressed, this boundary was questioned, and it was proven that connection pathways between the extra-tropics and the tropics existed. More importantly, if we wanted to improve the forecast in the extra-tropical regions outside of the 5-7 days range, we needed to include the tropics into our models (1).
The CAFE EU-funded project focuses on improving the prediction of extreme weather in the sub-seasonal range (2 weeks to 2 months). At this time range, including long lasting and far reaching processes is paramount to understand the drivers of meteorological phenomena and improve their prediction. My research within the CAFE project focuses on the prediction of heatwaves over Europe. In this short blog, I will be exploring the potential impact of the tropics on European heatwaves.
Tropical to extra-tropical pathways: Teleconnections
As stated earlier, pathways or connections have been found between tropical weather and extra-tropical weather (2). These pathways are called teleconnections. In the tropics, precipitation occur mostly through large and organised convection (thunderstorm activity) driven by the intense heating of the sun. Extreme convection or anomalous convection can create strong disturbances in the atmosphere. These disturbances can then cause global scale atmospheric waves called Rossby Waves. These waves coming from the tropic can propagate north and south into extra-tropical regions. When interacting with the atmospheric circulation at higher latitudes, they can induce extreme weather conditions, such as cold or warm spells and dry or wet extremes.
Tropical convection can be influenced by meteorological patterns that operate at different time scales. The most well known is ENSO (El Niño Southern Oscillation) which is characterised by two phases affecting weather on the seasonal scale: El Niño and La Niña. During El Niño, warmer than average SST (sea surface temperature) reaches the eastern Pacific along the equator while during La Niña colder SST is experienced in the same region. The warmer ocean is accompanied by changes in the local atmospheric conditions resulting in anomalous convection affecting weather across the globe. For example, during El Niño boreal winters (winter in the northern Hemisphere), the southern USA experience a wetter winter season. This is shown in figure 1 which represents the more recurrent impacts of El Niño globally.
Figure 1: Global impact of El Nino conditions on precipitation.
The MJO (Madden-Julian Oscillation) is an eastward propagation of regions of increased and suppressed convection at a shorter, subseasonal scale. This pattern of convection can affect dominant modes of weather in the Euro-Atlantic region such as the NAO (North Atlantic Oscillation).
As seen above, current research focuses on boreal winter. Summer teleconnections and by extension heatwaves have been less studied. Recent studies have shown that teleconnections do also exist during summer. Some studies actually highlighted that the tropics influenced some heatwaves, such as the notorious 2003 and 2010 heatwaves over Europe (3, 4). Because of these promising findings, my research on heatwaves includes looking at the effect of convection in the tropics on the occurrence of heatwaves in Europe.
Heatwaves and tropical convection
To start with, it is important to define heatwaves. Heatwaves are periods of continuous extreme warm temperatures in summer. The most extreme heatwaves can impact our society deeply by affecting our crop production and our health, which is why it is crucial to improve their predictions. They are due to continuous clear-sky conditions in the summer. These conditions occur when a persistent anti-cyclone, also called blocking, suppresses any possible convection or precipitation. Our research looks at an extended European region stretching from the Atlantic to the Ural region of Russia and from northern Africa to the top of Scandinavia. During our research, we found that European heatwaves could be partitioned into 5 different types depending on the geographical location of the heatwave, with one of them being located over western Russia (Fig 2).
Figure 2: Average temperature anomaly at 2-meters (colouring) and geopotential height anomaly (contouring: continuous black lines represent positive anomalies – anti-cyclonic conditions – and dashed lines negative anomalies – cyclonic conditions) during western Russian events.
The persistent anti-cyclones mentioned earlier, can be triggered and/or maintained by Rossby Waves (5). Which can themselves be triggered by strong tropical convection, as described earlier. In summer, the main mode of tropical convection variability is known as the BSISO (Boreal Summer Intra-Seasonal Oscillation). The BSISO is the summer version of the MJO and is regarded as a potential source of predictability on the subseasonal range. The BSISO explains the onset and decay but also the active and break phases of the monsoon over Eastern Asia. We use two indices to monitor the different phases of BSISO before heatwaves. The 2-dimensional phase space diagram in Figure 3, shows how both indices (EOF1 and EOF2) vary 14 days before the onset of heatwaves over western Russia (each line representing one event). We observe that a significant number of events propagate from the lower left to lower right quadrant of the plot. This propagation corresponds to the onset of the monsoon over India. At the same time, by analysing the atmospheric circulation for these different events, we observe how Rossby Waves propagate towards the region of the heatwave. This shows how increased convection in the tropics could be used as precursor for the forecast of heatwaves over Europe.
Figure 3: 2-dimensional diagram following the evolution of a subset of Russian events 14 days before their onset using 2 indices (constructed using EOF: Empirical Orthogonal Functions). Each line corresponds to one event.
In the last decades research moved from a local approach to a global approach, as the relevance of teleconnections became evident. Progress in computing power, allowed for more complex NWP models which include the entire globe and more of the complex physics driving the dynamics of our weather, such as teleconnections. This allows us to better understand the interactions between remote regions such as the tropics and the extra-tropics. Further on, correct simulations of the teleconnections improve our predictions especially at a longer time-scale. In this blog post, we showed their relevance for heatwaves, an extreme event with important impact on our society, whose frequency is likely to increase in a changing climate. Teleconnections can be used as an added precursor to generate earlier warnings and increase our confidence in the forecast of heatwaves
In the coming months, our research will be using forecast data to determine the relevance of these findings in improving the forecast of heatwaves. Stay tuned for more results.
(1) Baumhefner, D. P. (1971). On the Effects of an Imposed Southern Boundary on Numerical Weather Prediction in the Northern Hemisphere, Journal of Atmospheric Sciences, 28(1), 42-54. https://journals.ametsoc.org/view/journals/atsc/28/1/1520-0469_1971_028_0042_oteoai_2_0_co_2.xml
(2) O’Reilley, C.H., Woollings, T.,Zanna, L., & Weisheimer, A. (2018). The Impact of Tropical Precipitation on Summertime Euro-Atlantic Circulation via a Circumglobal Wave Train, Journal of Climate, 31(16), 6481-6504. https://journals.ametsoc.org/view/journals/clim/31/16/jcli-d-17-0451.1.xml
(3) Cassou, C., Terray, L., Phillips, A.S., 2005. Tropical Atlantic influence on European heatwaves. J. Clim. 18 (15), 2805–2811.
(4) Lau, W. K. M., & Kim, K. (2012). The 2010 Pakistan Flood and Russian Heat Wave: Teleconnection of Hydrometeorological Extremes, Journal of Hydrometeorology, 13(1), 392-403. https://journals.ametsoc.org/view/journals/hydr/13/1/jhm-d-11-016_1.xml
(5) Masato, G., Hoskins, B.J. and Woollings, T.J. (2012), Wave-breaking characteristics of midlatitude blocking. Q.J.R. Meteorol. Soc., 138: 1285-1296. https://doi.org/10.1002/qj.990