The long‐lived monsoon depressions of 2006 and their linkage with the Indian Ocean Dipole

Abstract

A highlight of the 2006 boreal summer monsoon season was the enhanced activity of long-lived monsoon depressions and low-pressure systems (LPS) over the Indian region. Another important phenomenon during this period was the evolution of a positive Indian Ocean Dipole (PIOD) event. Although previous studies have investigated the impact of PIOD on the large-scale monsoon response, their influence on monsoon LPS activity is not well understood. Based on detailed diagnostic analyses of monsoon LPS during 2006, as well as those associated with other PIOD events during 1958-2007, the present work addresses two specific issues concerning the roles of (a) PIOD-induced large-scale circulation changes and (b) internal feedbacks between latent heating and dynamics, in sustaining the monsoon LPS activity. The results show that PIOD conditions generally favour increased propensity of long-lived (>5 days) LPS with long westward tracks extending into northwest India. The average contribution of long-lived monsoon LPS to the total is found to be approximately 12% higher during PIOD episodes as compared to non-PIOD. The PIOD events showed two important large-scale elements conducive for enhancement of LPS activity: (a) strengthening of cross-equatorial moisture transport from south-eastern tropical Indian Ocean into the Bay of Bengal and (b) enrichment of barotropic instability of monsoon flow. Estimates of latent-heating profiles from TRMM-satellite products during the 2006 LPS showed heating in the mesoscale updrafts above 600 hPa with maximum approximately 400 hPa; while cooling prevailed in lower levels. Stratiform precipitation covered approximately 70-85% of rain area during the prolonged LPS; and the large-scale monsoon Hadley-type circulation was found to be intensified with strong mid-level inflows entering the stratiform rain region. The overall findings suggest that the PIOD-induced background circulation together with internal feedbacks between mesoscale convective systems and large-scale circulation can effectively enhance the longevity of monsoon LPS. These results should serve as important inputs for numerical weather forecasting of extreme rainfall events associated with the regional monsoon phenomenon.

Influence of the Atlantic zonal mode on monsoon depressions in the Bay of Bengal during boreal summer

“…Krishnan et al [] have shown that the presence of strong and persistent cross‐equatorial moisture transport into the BoB is favorable for the formation of monsoon depressions. The seasonal composite of midtropospheric (600 hPa) relative humidity in the BoB shows a slight increase (~5%) during the cold phase of AZM relative to its warm phase (Figures g–i).…”

“…The National Centres for Environmental Prediction product for wind, humidity, and air temperature data (1979–2012) [ Kanamitsu et al , ] are used to elucidate the differences in large‐scale atmospheric conditions between the cold and warm phases of AZM in the BoB. Following Krishnan et al [], the integrated moisture transport between 1000 and 600 hPa is estimated by using the expression, , where g is the acceleration due to gravity; q is the specific humidity; and V is the horizontal wind vector on pressure levels. Since the AZM is phase locked to the seasonal cycle (June–August), we focus here only on the June–August period.…”

“…As reported by earlier studies, the monsoon depressions (wind speed between 17 knots and 33 knots; 1 knot = 0.5 m/s = 1.85 km/h), which form during the boreal summer monsoon, contribute significantly to seasonal monsoon rainfall [e.g., Ramage , ; Goswami et al , ; Krishnamurthy and Ajayamohan , ; Ajayamohan et al , ; Krishnan et al , ]. Monsoon depressions generally form over the northern part of the BoB (some form over land also) and propagate westward or northwestward with a typical life span of 3 to 6 days and bring a substantial amount of rainfall to the core monsoon region along their tracks.…”

Structure, characteristics, and simulation of monsoon low‐pressure systems in CFSv2 coupled model

“…They argued that the presence of positive vorticity at low levels during active ISO phase coupled with enhanced meridional wind shear increases the probability of LPS genesis along the quasi-stationary monsoon trough. Also, some evidence exists regarding the impact of slowly varying modes of variability, viz., the El-Niño and Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD), on monsoon LPS [Singh et al, 2002;Hunt et al, 2016;Krishnan et al, 2011;Ajayamohan and Rao, 2008].…”

Sensitivity of Indian summer monsoon simulation to physical parameterization schemes in the WRF model

“…The intra seasonal oscillation of precipitation affects the seasonal mean precipitation (Goswami & Ajayamohan 2001) and hence the interannual variability of the boreal summer precipitation over India. The interannual variability in precipitation over India during the boreal summer season is affected by various climate modes such as the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD; Behera et al 1999, Saji et al 1999, Krishnan & Swapna 2009, Krishnan et al 2011.…”