The Combined QBO and ENSO Influence on Tropical Cyclone Activity over the North Atlantic Ocean

The Quasi-Biennal Oscillation (QBO) and the El Niño-Southern Oscillation (ENSO) largely modulate the zonal wind in the tropics. Previous studies showed that QBO phases produce changes in deep convection through an increase/decrease in the tropopause height over the tropics and subtropics. This study investigates the combined effects of QBO and ENSO on tropical cyclone activity by modulating tropopause height. We found that tropopause height increases over the Gulf of Mexico, the Caribbean region, and the Western North Atlantic Ocean during La Niña + QBOW, allowing deeper tropical convection to develop over those regions. As a consequence, TC activity over those regions is not only increased in number but also enhanced in intensity. Conversely, during El Niño + QBOE, most deep tropical convection is inhibited over those same regions due to the decrease in tropopause height over the subtropics. We conclude that QBO effects on TCs and deep convection should be studied in combination with ENSO. Additional comparative studies using long record data at high vertical resolution are needed to fully understand to what extent QBO interacts with ENSO in the lower tropical stratosphere and upper tropical troposphere.

Tropical Convection in the Caribbean and Surrounding Region during a Regional, Warming Sea-Surface Temperature Period, 1982–2020

Warming sea-surface temperatures (SSTs) have implications for the climate-sensitive Caribbean region, including potential impacts on precipitation. SSTs have been shown to influence deep convection and rainfall, thus understanding the impacts of warming SSTs is important for predicting regional hydrometeorological conditions. This study investigates the long-term annual and seasonal trends in convection using the Galvez-Davison Index (GDI) for tropical convection from 1982–2020. The GDI is used to describe the type and potential for precipitation events characterized by sub-indices that represent heat and moisture availability, cool/warm mid-levels at 500 hPa, and subsidence inversion, which drive the regional Late, Early, and Dry Rainfall Seasons, respectively. Results show that regional SSTs are warming annually and per season, while regionally averaged GDI values are decreasing annually and for the Dry Season. Spatial analyses show the GDI demonstrates higher, statistically significant correlations with precipitation across the region than with sea-surface temperatures, annually and per season. Moreover, the GDI climatology results show that regional convection exhibits a bimodal pattern resembling the characteristic bimodal precipitation pattern experienced in many parts of the Caribbean and surrounding region. However, the drivers of these conditions need further investigation as SSTs continue to rise while the region experiences a drying trend.

Causal maps versus correlation maps: visual analysis of tropical-extratropical atmospheric teleconnections using causal discovery

<p>In climatology, correlation maps are often used to study the relationships between one 1D time series and a (spatiotemporal) 2D or even 3D field. However, correlation measures do not necessarily capture causal relationships and similarities in correlation maps obtained from different indices may appear if the set of indices contains correlated variables. Causal discovery tools such as the Peter and Clark – Momentary conditional independence (PCMCI) algorithm can help in disentangling spurious from causal links in both linear and nonlinear frameworks. In the linear case considered in the present work, PCMCI extends standard correlation analysis by removing the confounding effects of autocorrelation, indirect links and common drivers. Combining PCMCI and Causal Effect Networks on a 2D field helps identifying, and subsequently discarding the spurious correlations and thereby allows to retain only the causal links. The resulting visualization technique is referred to as a “causal map”.</p><p>In this presentation, we illustrate the application of causal maps in combination with maximum covariance analysis to assess how tropical convection interacts with mid-latitude circulation during boreal summer at different intraseasonal timescales. The obtained causal maps reveal the dominant patterns of interaction and highlight specific mid-latitude regions that are most strongly connected to tropical convection. In general, the identified causal teleconnection patterns are only mildly affected by ENSO variability and the tropical-mid-latitude linkages remain similar under different types of ENSO phases. Still, La Niña strengthens the South Asian monsoon generating a stronger response in the mid-latitudes, while during El Niño periods, the western North Pacific summer monsoon pattern is reinforced. Our study paves the way for a process-based validation of boreal summer teleconnections in (sub-)seasonal forecast models and climate models and therefore provides important clues towards improved sub-seasonal and climate projections.</p><p> </p><p>Reference: G. Di Capua, J. Runge, R.V. Donner, B. van den Hurk, A.G. Turner, R. Vellore, R. Krishnan, D. Coumou: Dominant patterns of interaction between the tropics and mid-latitudes in boreal summer: Causal relationships and the role of time-scales. Weather and Climate Dynamics, 1, 519-539 (2020)</p>