Climatological patterns of subseasonal eddy flux transfer based on the co‐spectral analysis over the Indian region and the derivation of an index of eddy transfer for operational tracking

Abstract

The heat and momentum flux transfer by transient eddies from the extratropical to the tropical (E2T) region causes a significant variation of weather and climate in the tropical region. Such transports also cause a regional departure from zonally symmetric large scale circulation patterns caused by the Hadley type transports. During boreal summer monsoon season, the zonally symmetric Hadley type circulation is typically considered in theoretical as well as modelling studies. The current study introduces a method based on the calculation of the E-vector components and co-spectra analysis to diagnose the zonally asymmetric climatological patterns of E2T transport and its subseasonal variability. The spectral analysis identifies the climatological as well as low and high frequency patterns of the transient eddy heat and momentum flux transfer over the Indian region and compares the result with traditional Eliassen-Palm (EP) Flux based stationary eddy patterns. The study identifies the zonal, seasonal, and vertical asymmetries in the transport of transient eddy heat and momentum flux. The study also clearly categorizes the seasonality and frequency dependence of northeast to the southwest tilt of transient eddies transporting momentum and heat flux. Based on the climatological patterns, the wavelet transform approach is used to study the subseasonal variability of the eddy transport indices. We define a set of indices, which identify the subseasonal temporal variation of the E2T transfer mechanism. These indices could be used to operationally track the E2T eddy flux transfer both in observation and in forecast models.

Eddy transport, wave–mean flow interaction, and Eddy forcing during the 2013 Uttarakhand extreme event in the reanalysis and S2S retrospective forecast data

“…On the synoptic scale, extratropical Rossby wave intrusion influenced the June 2013 heavy rainfall event over the Uttarakhand region through the southward extension of troughs and associated southward shift in the subtropical jet pattern. Such synergistic evolutions are documented earlier also (Raman and Rao, 1981; Kalshetti et al ., 2021). At a local scale, along with western Himalayan orography and moisture convergence associated with monsoon lows during this time, the upper‐level extratropical trough intrusion imparted a strong eddy forcing, which is evident through the existence of E‐vector divergence (Figure 10) and the conversion of mean kinetic energy to eddy kinetic energy (Figure 11).…”

“…Figure 3a shows the strengthening of the eddy asymmetry and EMF index (first and the second components of the E‐vector), which helps to understand the low‐frequency intrusion over the Indian region (Kalshetti et al ., 2021). To understand the synoptic situation, we first analyse the precipitation during June 2013.…”

“…However, in recent years, studies have shown that high-intensity rainfall events over the Himalayan region are sometimes associated with the coexistence of low-frequency monsoon intraseasonal oscillations and extratropical eddies (or Rossby waves or Western Disturbances). The eddy transport and fluctuations are evident through the associated heat and momentum transport towards or out of the Tropics during the summer monsoon, as shown in Kalshetti et al (2021). Disturbances originating in the extratropics can locally shift the jet stream equatorward and thus disturb the synoptic setup of the Indian summer monsoon.…”

“…The eddy transport and fluctuations are evident through the associated heat and momentum transport towards or out of the Tropics during the summer monsoon, as shown in Kalshetti et al . (2021). Disturbances originating in the extratropics can locally shift the jet stream equatorward and thus disturb the synoptic setup of the Indian summer monsoon.…”