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Nature of Synoptic Scale Systems – Southwest Monsoon

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Nature of Synoptic Scale Systems – Southwest Monsoon Nature of Synoptic scale systems – southwest monsoon Sunitha Devi. S ([email protected]) 17th December 2019, IITM, Pune Outline Introduction o Genesis of synoptic scale systems during monsoon o Why we should discuss them. Monsoon Depressions & higher intensity categories Low pressure areas & lower intense systems o Their characteristics as available in the Literature so far. Mid-tropospheric cyclonic circulations Off-shore troughs & vortices Mid-latitude westerly troughs Concluding remarks Introduction Environment for Synoptic Systems during the monsoon season (courtesy: Prof. P.V.Joseph) Area A: Moderate Vertical Wind Shear and so can support synoptic systems except severe cyclones Area B: Large Vertical Wind Shear due LLJ and TEJ and unable to have synoptic systems like Depressions and Cyclones Area C: Feeble Vertical Wind Shear & has an equatorial trough and high SST - produces intense tropical cyclones Area D: African Easterly Jet with core at 600 hPa (4 Kms) – Produces Easterly Waves Contribution to monsoon rainfall Fraction of monsoon precipitation attributed to (a) depressions, (b) low- pressure areas using an 800 km radius of influence. (Using k-means clustering analysis) [Hunt, K.M.R. & Fletcher, J.K. Clim Dyn (2019) 53: 1859. https://doi.org/10.1007/s00382-019-04744-x] Fraction of monsoon precipitation attributed to various types of low pressure systems Regions coloured by the LPA type responsible for the most precipitation. White stippling indicates where more than half the monsoon precipitation is attributed to LPAs; the white area indicates where no monsoon precipitation is attributed to LPAs Hunt, K.M.R. & Fletcher, J.K. Clim Dyn (2019) 53: 1859. https://doi.org/10.1007/s003 82-019-04744-x Monsoon Depressions Form in extension of monsoon trough into Bay of Bengal Sometimes Mid-Level genesis Hybrid systems (cold core-ish, need shear) Heavy/ very heavy rains often in the SW Quadrant Fig. from P.K. Das, © 1987 John Wiley & Sons Monsoon Depression: major characteristics •Synoptic-scale : about 2000 km of diameter • Location of initiation ‣ 80 % in the Bay of Bengal, ‣ 10% in Arabian Sea ‣ 10 % over land (Bangladesh) - some times from the remnant of Pacific typhoons - sometimes from the mid-level cyclonic circulations • MSLP ~ as low as 990 hPa • Lifespan : 3 to 5 days • Frequency : twice a month over Bay of Bengal • Move : westward or northwestward at 2 or 3 m/s along the monsoon trough, at least as far as Central India before dissipating • Monsoon Depression: A few more major characteristics •Closed circulation between surface-300 hPa max. intensity (wind, convergence) from 600 to 800 hPa • Temperature ‣ cold core between surface-600 hPa ‣ above, warm core between 500-200 hPa ‣ But in some of them, no cold core observed • Evolution : no risk of development of tropical cyclone because of the strong vertical shear (TEJ in upper troposphere and SW monsoon flow in low troposphere) • Origin hypothesis : baroclinic instability (eastward vertical tilt coupled with westward vertical shear ) + CISK (convergence linked to the surface trough) • Large diurnal variation in rainfall noticed Schematic horizontal cross section of a monsoon depression Daggupati & Sikka 1977: Deep convection and vertical velocity max. in the SW quadrant • Rain : in the SW quadrant between 100 mm to 300 mm per day Structure of monsoon depressions in ERA Hunt etal., 2016.On the Structure and Dynamics of Indian Monsoon Depressions [DOI: 10.1175/MWR-D-15-0138.1] Derived 3 dimensional composite structure of 106 monsoon depressions from 35 years of ERA interim re-analysis data. Confirmed many known features including the southwest monsoon rainfall maximum and other fields. In addition, attributed a significant asymmetry owing to the presence of the Himalayas, a bi-modal mid-tropospheric potential vorticity core, very large CAPE & very small CINE at the centre etc. Also analysed variations in the structure as a function of different background states like land-coast-Sea, diurnal, ENSO. Active – Break, IOD etc. Hunt etal., 2016 Hunt etal., 2016 Hunt etal., 2016 Hunt etal., 2016 Hunt etal., 2016 Hunt etal., 2016 2. Weaker low pressure systems Monsoon Lows - less intense than monsoon depressions form frequently during the monsoon season in the Bay of Bengal, the Arabian sea and over land in the monsoon trough region. According to the criteria followed by IMD, a low pressure area is called a ML if the wind speed within the associated cyclonic circulation is less than 17 knots. Also there are still weaker systems – cyclonic circulations in the lower & mid tropospheric levels Both contribute significantly to the monsoon rainfall. The total rainfall associated with a ML was found to be almost on par with that with a MD - Mooley and Shukla (1987) Trend in Monsoon Low pressure systems • Monsoon season [June–September] LPS show a statistically significant increasing trend after 1950 (Sikka 2006; Fig. a). • Such a trend is even more evident in the time series of total monsoon season LPS days (Fig. b). • This occurs despite a decrease in the depression and storm days (Fig. c), primarily after 1976 or so • Strong increase in the weaker Low pressure (Rajeevan et al. 2000; system days. (Fig.d) (Ajayamohan etal., 2009) Rao et al. 2004; Sikka 2006). 3. Mid tropospheric cyclonic circulations (MTCs) Wind flow and temperature around a Mid Tropospheric Cyclone Mid-tropospheric cyclonic circulation (MTC) Streamline and isotach at (left) 925 hPa, (right) 600 hPa Source : Atkinson, 1971, d’après Miller et Keshavamurthy, 1968 •Closed circulation between 700-300 hPa Maximum of intensity (wind, convergence) in mid-troposphere from 500 to 600 hPa At low and upper troposphere : absence or light signature in wind (manifested as a trough in the streamline) Main features of mid tropospheric cyclonic circulations • Location of initiation : Mid-tropospheric cyclone occur in NE of Arabian Sea, South Vietnam, South China Sea from • Period : from May to october • Synoptic scale : ~ 3000 km • Lifespan : from 3 to 7 days, even 10 days ! • Frequency : less than monsoon depression • Move : stationnary or westward • Temperature ‣ cold core between surface-600 hPa ‣ above between 500-200 hPa, hot core • Heavy rains : in western quadrant (location of ascending motion) up to 200 mm per day • Origin hypothesis ‣ for initiation, barotropic instability of the monsoon flow at 700 hP ‣ for growth, release of latent heat Monsoon Depression Vs MTC (Courtesy: Prof. P.V.Joseph) Off-shore Troughs and Vortices: . Wave troughs develop in the low level monsoon westerlies just off the west coast of peninsular India anywhere from north Kerala to south Gujarat when the monsoon is in the active phase. The development of the trough is seen to occur when there is a deepening of the north-south pressure gradient at the sea level and southerly / southwesterly/ weak easterly surface winds along parts of the coast instead of the normal westerly winds. 24 hour pressure changes and pressure departures from normal also indicate the presence of the off-shore trough. These troughs form more often near coastal Karnataka and slowly shift about 2 degrees of latitude per day northwards. Cyclonic vorticity may be present above 1 km also but in the form of shear. Small (meso-scale) vortices form in these off-shore troughs which cause heavy rainfall in small areas along the west coast of peninsular India away from the Western Ghats. 26-July-05 Phenomenal off shore convergence may be due to the combined effect of three met. factors. Movement of low from Bay of Bengal inland, which strengthens westerlies from the Arabian Sea Active off shore trough Previous weeks' low pressure remnants Western Disturbances, though not traditionally a monsoon system, could cause significant precipitation, occasionally Fig copyright by R. Stull, 2017: Practical Meteorology Copyright: S. Nandargi, IITM An example of western disturbance associated precipitation, J&K, September 2014 (year after Uttarakhand event) Cyclone & Depressions over north Indian Ocean during southwest monsoon -2018 CYCLONES & DEPRESSIONS OVER NORTH INDIAN OCEAN DURING SOUTHWEST MONSOON 2019 (i) Two Very Severe Cyclonic Storms, VAYU & HIKAA, both over the Arabian Sea (ii) One Deep Depression over the Bay of Bengal and a Depression over the Arabian Sea. AS more active. Cyclonic Storms during Monsoon season (1990 – 2018) Concluding remarks A decreasing trend in the monsoon depression frequency and a corresponding increase in the weaker category of monsoon lows has been established by several studies. At the same time, there exists a large intra seasonal and inter annual variability in the frequency of formation of synoptic systems (Southwest monsoon – 2019 witnessed the formation of 2 very severe cyclonic storms) Better representation of synoptic scale systems over the Indian Ocean & west Pacific in numerical models should enhance the skill of predicting the transition phases of Indian southwest monsoon. .
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