Dr Muhammad Adnan Abid

Abstract:

During early boreal winter, the extra-tropical atmospheric circulation is influenced by Rossby waves propagating from the Indian Ocean towards the North Atlantic-European (NAE) regions, resulting in a North Atlantic Oscillation (NAO)-like pattern. The mechanisms driving these teleconnections are not well understood and are crucial for improving model skills. This study investigates these mechanisms using the ERA5 dataset and tests the predictive capabilities of the ECMWF-SEAS5 hindcast, exploring potential reasons for a weak model response. Linear regression methods are employed to examine the extra-tropical links with the Indian Ocean dipole (IOD), both in isolation and in combination with the El Niño-Southern Oscillation (ENSO). Our findings demonstrate a connection between October IOD sea surface temperature anomalies and December Indian Ocean precipitation patterns. Furthermore, a correlation between the October IOD and December NAO time series suggests a link between the IOD and NAE circulation. The early winter European response to a positive IOD is characterized by a north-south precipitation dipole and a large positive surface air temperature anomaly. Positive feedback from transient eddy forcing reinforces the wavenumber-3-like propagation across extra-tropical regions, with ENSO playing a minor role compared to the IOD. This phenomenon is particularly evident in regions such as the North Pacific and North Atlantic, where wave energy propagation is intensified. Although SEAS5 replicates the NAO response, its magnitude is significantly weaker. The model struggles to simulate the delayed rainfall dipole response to the IOD accurately and shows structural discrepancies compared to reanalysis data.

Abstract:

Early winter (October-December; OND) precipitation is critical for Pakistan’s water resources, agriculture and socio-economic stability, yet the large-scale drivers and physical mechanism driving its interannual variability remains poorly understood. Using in-situ observations and ERA5 reanalysis for 1981–2023, we have investigated the dominant mode of early winter precipitation variability over Pakistan and its associated atmospheric teleconnections. The leading Empirical Orthogonal Function (EOF1) explains approximately 60% of the total variance, with the maximum interannual variability centered over northern Pakistan, including the Upper Indus Basin. A Pakistan Precipitation Index (PRI) is developed over this core precipitation region. Regression analysis demonstrates that positive PRI anomalies are associated with anomalous upper-level divergence over central-eastern equatorial Pacific and western Indian Ocean, compensated by convergence over Maritime Continent. A quasi-stationary Rossby wave train excites from tropical pacific and propagates eastward along the subtropical westerly jet into Eurasia producing an anomalous upper-level trough over southwest Asia including Pakistan. This favors enhanced ascent and cyclonic circulation anomalies along with moisture transport from western Indian Ocean and Bay of Bengal, resulting in above normal precipitation over Pakistan. The PRI is significantly correlated (r = 0.5) with the warm central-eastern equatorial Sea Surface Temperature (SST) anomalies (i.e., Niño3.4 index), indicating the El Niño-Southern Oscillation (ENSO) as a dominant mode of early winter precipitation variability. During El Niño, the warm SST anomalies in central-eastern equatorial Pacific enhance upper-level divergence, excite stationary Rossby wave and shift the subtropical westerly jet southward. At same time, the enhanced moisture transport and wave activity from the western Indian Ocean, favors convergence over Pakistan. In contrast, La Niña produces the opposite response. This shows ENSO along with the Indian Ocean strongly influences Pakistan precipitation variability in early winter. These findings provide valuable basis for seasonal prediction and support water resource management, agriculture and climate resilient planning across Pakistan.