Relationships between subseasonal‐to‐seasonal predictability and spatial scales in tropical rainfall

Abstract

Subseasonal to seasonal (S2S) tropical rainfall predictability is assessed both from an analysis of the spatial scales of observed rainfall variability data, as well as from an S2S model reforecast skill. Observed spatial scales are quantified from gridded observed daily rainfall data, in terms of the size (area) of daily contiguous wet grid‐points (referred to as ‘wet patches’), as well as from the spatial autocorrelations of 7–91‐day running averages of rainfall. Model S2S reforecast skill is measured using the anomaly correlation coefficient between observed and simulated weekly and monthly rainfall from an 11‐member ensemble of European Centre for Medium‐Range Weather Forecasts (ECMWF) reforecasts (1998–2017). Both measures of S2S predictability are found to be systematically lower over land than sea, usually peaking at the start or end of the rainy season and decreasing during the core. Small spatial scales and low skill over equatorial/northern tropical Africa and western Amazonia coincide with small daily rainfall patch size and strong synoptic‐scale (≤7 days) variability there. Over most of South and SE Asia, daily wet patches are larger and strongly modulated by intraseasonal oscillations, boosting S2S rainfall predictability, while this is offset by large daily mean rainfall intensities that increase the noise. In consequence, S2S rainfall skill here generally remains low. Several land areas (as around Maritime Continent from the Philippines to Northern Australia, Eastern and Southern Africa, Eastern South America) exhibit larger spatial scales and skill, especially where the relative amplitude of SST‐forced interannual variations is strong. Most of the Maritime Continent illustrates such behaviour, but even here, the time‐averaged spatial scales and skill drop during the core of the rainy season.

Storm types in Bangladesh: duration, intensity and area of intra‐daily wet events

“…In that context, we could view the duration as a pivotal characteristic: it is positively and largely related to both total amount (Figure 4) and area (Figure 7) of WEs. In consequence, total amount is also positively related to area, as already observed from daily rainfall across the whole tropical zone (Moron and Robertson, 2021). On the contrary, mean intensity is rather independent from duration, total amount as well as area of WEs.…”

“…7) of wet events. In consequence, total amount is also positively related to area, as already observed from daily rainfall across the whole tropical zone (Moron and Robertson 2021). On the contrary, mean intensity is rather independent from duration, total amount as well as area of wet events.…”

“…It may be especially critical here for ST#1 and also ST#2. But, it still suggests that duration/area/total amount and intensity are two different (and here independent) characteristics of any tropical WE (Moron and Robertson, 2021). It remains to understand what are the main drivers of both properties and their space–time variations across the year, and also at interannual time scale.…”

“…Lastly, this approach gives some clues about the predictability issue since the potential predictability of seasonal amounts is considered to be moderate at best (Chowdhury and Ward, 2007; Hossain et al ., 2019; Kelley et al ., 2020; Acharya et al ., 2021). A poor predictability at interannual time scale may be related to various sources including intense and very localized rainy events (Stephenson et al ., 1999) and the lack or weakness of space–time organization of instantaneous rainfall by predictable regional to large scales (Moron and Robertson, 2021). Even a predictable signal at intra‐seasonal time scale (BSISO and QBW here) is not necessarily beneficial to the interannual time scale if it leads to a near‐zero residual at the end of the rainy season.…”

Surface‐driven amplification of Madden–Julian oscillation circulation anomalies across East Africa and its influence on the Turkana jet

“…Given that most of East Africa is a semi-arid or arid environment, where surface turbulent fluxes are sensitive to soil moisture variations (Gallego-Elvira et al, 2016;Soares et al, 2019), we expect surface turbulent fluxes to be strongly modulated by the MJO, and in turn possibly feed back onto the atmosphere at similar scales to the original precipitation forcing. Though intraseasonal rainfall variability is smaller than daily to weekly fluctuations across the majority of the Tropics (Moron and Robertson, 2021), previous studies have shown the importance of tropical intraseasonal land-atmosphere feedbacks (Saha et al, 2012;Unnikrishnan et al, 2017;Chug and Dominguez, 2019;Talib et al, 2022). Within the Tropics, most studies have focused on the impact of intraseasonal precipitation variability on land-atmosphere coupling across India (Sikka and Gadgil, 1980;Srinivasan et al, 1993;Ferranti et al, 1999;Saha et al, 2012;Unnikrishnan et al, 2017).…”