Mixed Rossby-Gravity waves and their association with the intrusion of extratropical disturbances

Abstract

Mixed Rossby-gravity (MRG) waves are tropical synoptic-scale disturbances which exhibit mixed properties of low frequency Rossby waves and high frequency gravity waves. A comprehensive understanding on the major factors that aid the growth and maintenance of the MRG waves is crucial for improving the weather and subseasonal forecast skills in the tropics. Based on the location of its origin, the physical processes that cause the occurrence of MRG waves are generally classified into two groups, in situ and external forcing mechanisms. Extratropical disturbances are considered as the major external forcing that impacts the occurrence of MRG waves. Despite many modeling and observational studies that suggest extratropical disturbances as a crucial element in the genesis of MRG waves, its contribution in the growth and maintenance of the MRG waves are not known. In the current study, as a first step to address this important issue, we primarily focus on quantifying the association between the intrusion of extratropical disturbances and the occurrence of the MRG waves. Since both the intrusion of extratropical disturbances and MRG waves exhibit episodic character, identification of both the disturbances as events is critical for quantifying their possible association. The episodes of the intruding extratropical disturbances are identified based on potential vorticity (PV) on 350 K isentropic surface. We exploit the PV conservation principle to identify the air of extratropical origin in the tropics. We have identified 3584 extratropical disturbances that intruded into tropics for the period 1979-2022. A major fraction of such events is observed in the central-east Pacific and Atlantic Ocean domains when the westerly ducts are observed. While the intruding extratropical disturbances predominantly propagate eastward, 25% of the intruding extratropical disturbances exhibit westward propagation and such disturbances are observed mainly during summer season. Because the intrusion of extratropical disturbances is mainly confined to the upper troposphere, MRG wave events at 200 hPa are identified using equatorial meridional wind as the key variable by following the wave fitting method. We have identified 2390 MRG events for the period 1979-2019 and 37% of them are associated with the intrusion of extratropical disturbances (such MRG events are labeled as intrusion MRG events). It is noteworthy that nearly 70% of the intrusion MRG events occurred in the winter hemisphere where the westerly wind ducts are well developed. Over the central–east Pacific during boreal winter, it is observed that the intrusion MRG wave events have a bigger amplitude and have a larger meridional extent compared to non-intrusion MRG wave events. They also exhibit a similar spatial scale as the extratropical disturbances implying that resonant interactions may be a primary mechanism for the genesis of MRG wave events. In addition to the intrinsic westward propagating MRG waves, eastward propagating MRG wave-like disturbances are observed in the Western Hemisphere. Theoretical studies showed that the large-scale zonal winds can modulate the MRG waves by modifying its restoring force and causing a shift in its intrinsic frequencies. MRG waves that undergo such modulation by the large-scale winds are named as Doppler shifted MRG waves. Detailed analyses show that the eastward propagating MRG-like disturbances are indeed the Doppler shifted eastward propagating MRG (E-MRG) waves. We have identified 730 E-MRG wave events using the Empirical Mode Decomposition (EMD) method. As expected, 75% of the E-MRG wave events occurred when the large-scale winds were westerly. It is intriguing to note the occurrence of E-MRG wave events when the large-scale winds were easterly. Further analysis revealed that the westerly phase of the Kelvin waves is instrumental in setting up the conducive environment for the MRG wave event to undergo the Doppler shifting. The westerly phase of the Kelvin waves explained more than 75% of the E-MRG wave events that occurred when the large-scale winds were easterly.