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Short period variations of the aerosol mass concentrations over Bay of Bengal: Association with quasi-periodic variations in the Marine Atmospheric Boundary Layer parameters and fluxes
Journal of Atmospheric and Solar-Terrestrial Physics 77 (2012) 78–84
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Short period variations of the aerosol mass concentrations over Bay of Bengal: Association with quasi-periodic variations in the Marine Atmospheric Boundary Layer parameters and fluxes S. Naseema Beegum 1, K. Krishna Moorthy n, D. Bala Subrahamanyam, N.V.P. Kiran Kumar, S. Suresh Babu, M. Mohan Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum 695022, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 July 2011 Received in revised form 8 November 2011 Accepted 24 November 2011 Available online 6 December 2011
Analysis of the time-series of daily mean total aerosol mass concentrations (MT), measured within the Marine Atmospheric Boundary Layer (MABL) over the Bay of Bengal (BoB) revealed the presence of short-period modulations with periods of 4–6 days, 8–10 days as well as quasi 16-day (14–20 days). These were found to be distinctively associated with similar oscillations in the concurrently measured MABL parameters such as air temperature (AT), pressure (P), relative humidity (RH), wind speed (WS), sea surface temperature (SST) and derived parameters such as Momentum Flux (MF), Latent Heat Flux (LHF) and Sensible Heat Flux (SHF). Examination of the phase relations revealed that the 4–6 days and quasi 16-day periodicities in AT, P, RH and SST maintained a nearly in-phase (very small phase difference o 7 201) variation between themselves and also with similar periodicities in the aerosol mass concentration MT. On the other hand, the periodicities in WS, SHF, LHF and MF, though were nearly in-phase among themselves, exhibited an out-of-phase (phase difference 180 7201) relation with that of MT. Interestingly, the 8–10 day periodicities revealed a different phase relationship; the variables AT, P, SST, WS and MT were in-phase and these variables were out-of-phase with similar periodicities in RH and the fluxes. It was also observed that all the three waves represented westward propagating Rossby waves. An easterly phase in the wind was found to result in advection of particles from the East Asia to BoB, as evident from the out-of-phase relationship between the periodicities of 4–6 days and quasi 16-day in MT and zonal wind. The meridional component, that was stronger than its zonal amplitude in the 8–10 day periodicity, resulted in enhanced advection of particles from the southern part of Bay of Bengal in comparison with that from the Eastern region, leading to an in-phase relationship between MT and zonal wind. & 2011 Elsevier Ltd. All rights reserved.
Keywords: W_ICARB MABL aerosols Bay of Bengal aerosols Rossby waves
1. Introduction Tropical atmosphere is the seat of strong wave activities of various time scales, which can be categorized according to their scale sizes, triggering and restoring mechanisms as gravity waves, tides and planetary waves, each having its own distinct physical and geometrical properties (Holton, 1972). Among these waves, those having periodicities greater than one day and the horizontal dimensions comparable to the radius of the Earth are called the planetary waves, the most important ones at the tropics being the eastward propagating circulation anomalies having periodicities of 30–60 days known as the Madden Julian Oscillations (MJO, n
Corresponding author. Tel.: þ91 471 2562404; fax: þ91 471 2706535. E-mail address: [email protected] (K.K. Moorthy). 1 Current affiliation: National Physical Laboratory, Dr. KS Krishnan Marg, New Delhi 110012, India. 1364-6826/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2011.11.012
Madden and Julian, 1971) and the westward-propagating periodicities with shorter time periods than MJO, in both zonal and meridional components, are called the (Rossby waves) Planetary waves (Rossby, 1939; Blinova, 1943). These oscillations appear most clearly over the Indian and western Pacific Oceans (Sperber, 2003) and involve many meteorological variables, including the zonal wind, surface pressure, temperature and humidity. These waves, being excited by several means, including interactions of the mean flow with mountains, convective events and instabilities of various kinds, have been found to produce their signatures in the concentrations of atmospheric trace species including aerosols (Beegum et al., 2009). It is reported in the literature that the wave activities are more vigorous over vast oceans compared to the continental regions as the large-scale air–sea interactions play an important role in the variability of both ocean and atmosphere on monthly/seasonal time scales. Latent heat release in the tropics drives planetary
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Fig. 1. Cruise track followed during W_ICARB. The positions of the ship at 6:30 UT of all the days of measurements are marked by the circles in the track. The monthly mean prevailing wind vectors of January 2009 at 850 hPa are also superposed on the same figure.
scale atmospheric circulations. Resulting changes in cloud cover would affect both the atmospheric and oceanic boundary layers through a variety of feedbacks, such as precipitation, wind speed and sea surface temperature (e.g., Webster et al., 1996). Nakazawa (1988) noted that convection occurred in association with the active intra-seasonal oscillations (ISO) and was organized into large systems called super-cloud clusters. These super-cloud clusters are composed of several individual cloud clusters that tend to propagate westward, while the envelope of super-cloud clusters propagates eastward with the ISO. Several investigators have reported the presence of two dominant intra-seasonal modes with periods of 30–60 days and 10–20 days in atmospheric parameters over the tropical Indian Ocean (Krishnamurti and Ardunay, 1980; Yasunari, 1981; Krishnamurti et al., 1988; Fedulina et al., 2004; Beegum et al., 2009). This article focuses on the characterization of air–sea interaction processes over various time scales by examining the short period fluctuations in the time-series data of concurrently measured surface aerosol mass concentrations as well as Marine Atmospheric Boundary Layer (MABL) parameters (the surface wind speed (WS), sea surface temperature (SST), air temperature (AT), relative humidity (RH) and the derived parameters Sensible Heat Flux, (SHF), Momentum Flux (MF) and Latent Heat Flux (LHF)) over Bay of Bengal during the winter season of 2009. Even though the wave activity over this equatorial oceanic region is rather pronounced during winter season, such studies are virtually non-existent. Surface layer meteorological measurements, in conjunction with the aerosol mass concentration measurements, collected onboard the oceanographic research vessel (ORV) Sagar Kanya during its cruise #SK254 dedicated to the Winter Integrated Campaign for Aerosols gases and Radiation Budget (W_ICARB) field experiment formed the database of the present study. During this campaign, the entire BoB, the region between 76.61 and 97.51E and 3.51 and 211N has been explored extensively in a span of 34 days (from 27th December 2008 to 30th January 2009), along the cruise track shown in Fig. 1
2. Campaign, instruments and data During the campaign, the ORV sailed off the port of Chennai on 27th December 2008 and after the field expedition called at Kochi on 30th January 2009, following the track denoted by the solid line in Fig. 1. During this period, continuous measurements of instantaneous size segregated mass concentration of composite (total) aerosols were made using a 10-stage Quartz Crystal Microbalance (QCM)
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cascade Impactor (model PC-2 California instruments, USA) having 50% lower size cut off at each of its 10 size bins at 425 mm, 12.5, 6.4, 3.2, 1.6, 0.8, 0.4, 0.2, 0.1, and 0.05 mm for size bins from 1 to 10; the sum of the individual-bin mass concentrations yielding the total mass concentration, MT, of ambient aerosols. The QCM sampled the ambient air through the community inlet at a constant flow rate of 0.24 l/min with a sampling time of 300 s. Measurements were repeated at regular intervals of 30 min, round the clock. Measurements were restricted to the periods of RH o78% at the deck level, in view of the affinity of the quartz crystal to changes in RH for higher RH (Pillai and Moorthy, 2001; Nair et al., 2008). But there were no data loss on application of this restriction, as there were only one or two points in the hourly data and in the present study we have taken the daily averages, omitting those values. In general, the uncertainties in the measured MT values were in the range of 10–20% at very low mass concentrations (r10 mg m 3) and the error reduces to o10% for the high mass concentrations (430 mg m 3). More details of the instrumentation, analysis and error budget are given elsewhere (Pillai and Moorthy, 2001; Nair et al., 2009). Concurrent surface layer meteorological measurements were carried out using an Automatic Weather Station (AWS) fitted with several meteorological sensors on a 7-m long retractable boom fixed to the bow of the ship. Overall, there were five slow response sensors: cup anemometer for wind speed, wind vane (wind direction), platinum resistance thermometer (air temperature), capacitance-based humicap (relative humidity) and barometer (atmospheric pressure), all connected to a data logger placed near the retractable boom. The entire AWS assembly was powered with a solar panel. Data from these sensors were recorded every 10 min. The orientation and alignment of the sensors on the boom were adjusted such that these measurements were least affected by heat contamination and flow distortion, respectively, due to the exhaust and the bulk nature of ship. Beside these, manual measurements of SST were made at hourly intervals with a bucket thermometer. The navigation parameters of the ship such as its velocity, header angle from true North and exact geographical location in the terms of longitude and latitude were continuously recorded at every second using a GPS (Global Positioning System). More details of the instrumentation and measurement techniques are given elsewhere (Subrahamanyam et al., 2011).
2.1. Data processing and correction for ship’s motion As the measurements of meteorological parameters were made onboard the ship, these would produce errors in the wind components due to the motion of the ship. For correcting the wind speeds for flow distortion, we calculated the zonal and meridional components of apparent winds and ship’s speed. As the apparent wind is resultant of the true wind and ship’s velocity, it is resolved trigonometrically by solving the two zonal and meridional components. In this way, wind speed measurements were corrected for the movement of the ship. However, the retractable boom was kept sufficiently away from the ship’s stack ( 50 m away from the stack) in order to avoid the contamination by the local flow distortion over the bulk body. In the case of ship-borne platform, accurate measurements of vertical winds become difficult because of the pitch and roll motion of the ship, and therefore estimation of the air–sea interaction parameters largely depend on the bulk aerodynamic algorithms (Godfrey et al., 1991; Subrahamanyam and Radhika, 2002, 2003; Bhat et al., 2003; Ramana et al., 2004; Alappattu et al., 2008). Hence in the present study, the surface layer turbulent fluxes of momentum, heat and moisture have been estimated through an iterative scheme based on the bulk aerodynamic algorithm (Smith, 1988; Subrahamanyam and Radhika, 2002, 2003; Subrahamanyam et al., 2011).
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3. Prevailing meteorology The prevailing meteorology during the W_ICARB comprised predominantly of calm synoptic conditions with moderate winds and clear/partially clear skies. No major weather systems or cyclonic depressions were encountered in the study area during the cruise period, except perhaps the thunderstorm that occurred on the night of 28th January 2009 in the northern Indian Ocean. The monthly mean prevailing synoptic winds of January 2009 at 850 hPa (NCEP, National Centre for Environmental Prediction) are superposed on the cruise track in Fig. 1. An anti-cyclonic circulation, centered at 201N over the Indian landmass resulted in calm/ weak northerlies/north-easterlies to prevail over the northern BoB. Moderate easterlies/north-easterlies prevailed over the mid and southern BoB (below 121N), with higher wind speeds in eastern parts. This synoptic wind pattern revealed the prevalence of continental air mass over the entire BoB during the campaign period; however they originated from distinct geographical regions. Examination of the spatial pattern of wind vectors over BoB revealed that the pattern remained almost similar during the entire cruise period as shown in Fig. 2, where the average wind patterns for the first (January 1–15) and second (January 16–30) halves of the campaign period are shown (in the left and middle panels) along with the difference between the two (in the right panel). Even though the magnitude of winds decreased in the second half of the campaign, the wind direction did not show any significant change. The extreme right panel of Fig. 2 reveals that the difference in absolute magnitude between the two halves remained very low for most of the oceanic regions, except in the eastern BoB (east of Andaman–Nicobar islands). This evidences that the synoptic conditions remained nearly stable during the campaign period.
observed during the first half and a subsequent decrease thereafter in the second half is attributed mostly to the source proximity of the northern as well as eastern continental locations. Similar latitudinal gradients are observed in both the column and surface aerosol parameters over BoB during the same period (Nair et al., 2008, 2009; Moorthy et al., 2010). With a view to examining the short period modulations in MT, the time-series data is detrended (Priestley, 1988) by fitting two different slopes for the first and second halves (Fig. 3 (top panel)). The resulting timeseries of MT is shown in the bottom panel of Fig. 3. Since the wind pattern was consistent throughout the cruise period, the modulations observed in the variables cannot be attributed to the changes in the prevailing winds, but might be associated with the modulations in the other atmospheric variables. Careful examination of the time-series of surface layer meteorological parameters and the air–sea fluxes revealed similar fluctuations modulated by a weak trend (not shown in figure) in all the parameters except in WS. These trends, associated with the latitudinal trends in the parameters for the study region (in winter season, as we move from tropics to near extra-tropics), were removed by detrending and the resulting time-series of these parameters shown in Fig. 4 reveal prominent fluctuations. 4.2. The wavelet analysis With a view to delineate the dominant periodicities in all these parameters, the detrended time-series data was subjected to wavelet analysis using a Morlet wavelet (Torrence and Compo, 1998). Morlet is a complex wavelet, which can be decomposed into real and imaginary parts (Torrence and Compo, 1998). The mother wavelet of the morlet is a plane wave modulated by a Gaussian of the form, CðxÞ ¼ C expððx2 =2ÞÞexpðioxÞ, where o is
4. Results and discussion 4.1. Variations in surface aerosol mass concentrations and ABL parameters The time-series of the daily average values of the total aerosol mass concentrations (MT) for the entire cruise period is shown in Fig. 3 (top panel). The figure revealed an increasing trend in the first half of the cruise and a decreasing trend in the second half, superposed with several short period modulations. During the first half, the ship was moving northwards from Chennai to the northern most region ( 221N), thereafter in the eastern BoB (oceanic region in the east of BoB), in close proximity to the east Asian region. In the second half, it was traversing from Port Blair ( 121N) to the equatorial region ( 21N) through the mid ocean as evident from the cruise track (Fig. 1). Hence the increase in MT
Fig. 3. Time series of the daily mean total mass concentrations (MT). The top panel shows the variations of the raw data. The dotted lines show the corresponding least square linear fits for the two slopes. The bottom panel shows the time series of the detrended data, where the trends have been removed and the fluctuations become more prominent.
Fig. 2. Spatial pattern of wind vectors at 850 hpa averaged over the first half (January 1–15) of the cruise (left panel), second half (January 16–30) of the cruise (middle panel) and the difference between the two (right panel).
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Fig. 4. Time series of the surface measured MABL parameters (detrended) of air temperature (AT, 1C), relative humidity (RH, %), pressure (P, hPa), wind speed (WS, m s 1), sea surface temperature (SST, 1C) and derived parameters such as Sensible Heat Flux (SHF, W m 2), Latent Heat Flux (LHF, W m 2) and Momentum Flux (MF, N m 2).
the angular frequency.The wavelet analysis was performed in the time-series of the daily mean data of MT and surface meteorological parameters and the resulting spectra are shown in Fig. 5. The spectrum of MT (in the top left panel) revealed the presence of two strong periodicities of 4–6 days and 8–10 days. Another mode of quasi-16 day is also present in significant amplitude with periods in the range of 14–20 days. In addition, signatures of a weak, but significant 2–3 periodicity are also observed. At this juncture it may also be kept in mind that 2–3 day periodicities would be partly contaminated by north–south movement of the ship with an average periodicity of 3 days (Fig. 1) and hence the periodicity is not discussed further in the manuscript. Similar analysis of the wavelet spectra of the surfacelayer meteorological parameters such as AT, WS, RH, P, SST and estimated values of air–sea interface fluxes also revealed the periodicities of 2–3 days, 4–6 days and 8–10 days and quasi-16 day. As the fluxes of LHF, SHF and MF depend on wind speed and relative humidity, it is logical to expect these fluctuations to be similar to those in the surface parameters. Based on the observations over tropical oceanic regions several investigators have reported the presence of various oscillations in meteorological parameters of different time scales. Among these the most important is the quasi-16 day mode (Chen and Chen, 1993; Chatterjee and Goswami, 2004). Chatterjee and Goswami (2004) have studied the atmospheric quasi biweekly mode based on the OLR and NCEP reanalysis wind vectors at lower atmosphere and have suggested that it is a convectively coupled first meridional mode equatorial Rossby wave. Sengupta et al. (2004) have reported the presence of 10–20 day periodicity in the meridional ocean currents in the eastern Indian Ocean and suggested that the observed biweekly variability is due to equatorially trapped mixed Rossby-gravity waves generated by sub-seasonal variability of winds. Parekh et al. (2004) have reported that sea surface temperature measured by the Tropical Rainfall Measuring Mission Microwave Imager (TMI) over the Bay of Bengal during summer monsoon was found to exhibit a lowperiod intra-seasonal mode of 8–16 days, in addition to the 30–60
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days mode. Janicot and Sultan (2001) have also observed a quasiperiodic signal of about 15 days in the rainfall and wind fields at 925 hPa over West Africa based on long database of 23 years. The presence of the quasi 16-day fluctuations in zonal winds over the eastern tropical Atlantic and Northwest Africa have been also noted by Viltard et al. (1997). Intensive study of synoptic-scale disturbances over central and West Africa and the tropical Atlantic has revealed the existence of two types of westward propagating wave-like fluctuations with periodicity of 3 to 5 days (westward speed of 9–10 m s 1) in meridional wind (Carlson, 1969; Viltard et al., 1997) and a longer periodicity of 6–9 days (westward speed of 11 m s 1) (Yanai and Murakami, 1970) in zonal wind. Hareesh Kumar et al. (2001) found prominent peaks at around 10.6 days in the meridional component of ocean surface wind and air temperature as well as SST. Hence the periodicities in the meteorological parameters can cause either advection or dispersion and produce subsequent signatures on aerosol or other trace species concentrations (Beegum et al., 2009). In order to examine the strength of these oscillations and direction of propogation, we have estimated the amplitudes and phase angles of each of the periodicities for all parameters and these are listed in Table 1. As the periodicities observed are in 95% confidence level (significant at 5%), the estimated phase angles are also significant. The phase angles of the periodicities of 4–6 days and quasi 16-day revealed a nearly in-phase relationship between AT, P, RH and SST; the difference in the phase angles being o 7201). Similarly the variations of WS, LHF, SHF and MF are observed to be in-phase among themselves and out-of-phase with the variations in MT. Interestingly, even though similar phase relationships between different parameters are observed for both the waves, the phase difference between the corresponding variables for these two waves were approximately 1801 (within 7201), indicating that these waves were varying nearly out-of-phase spatially. But, the phase estimation of the 8–10 day periodicity revealed certain different characteristics in which AT, P, SST, WS and MT are nearly in-phase and these variables are nearly out-ofphase with the RH, SHF, LHF and MF. This observation is significant owing to the fact that the observed periodicities in MT showed a nearly in-phase association with WS in contrary to the near-out-of-phase variation in the other two waves. Examination of the phase propagation helps to ascertain the type of the wave observed in the meteorological parameters (such as wind field), which result in corresponding modulations in the mass concentrations. As such, we deduced the zonal and meridional phase propagation of these waves by analyzing the NCEP wind vectors (U for zonal propagation and V for meridional propagation) at 850 hPa as a function of longitude and latitude, respectively, for U and V. The results are shown in Fig. 6. As the zero phase position of the waves move westward in U for all the three periodicities (Fig. 6(a)–(c)), these are the westward propagating planetary waves (Rossby waves). These waves also have meridional components, which are found to propagate northwards (Fig. 6(d)–(f)). At this juncture it is interesting to examine the zonal and meridional amplitude of these periodicities. As such, the timeseries of zonal and meridional component of wind at the mid oceanic region of BoB (centered around 101N, and 851E as a representative) at 850 hPa (NCEP) from January 1 to 31 of 2009 has been subjected to wavelet analysis and the resulting spectra are shown in the Fig. 7. From the figure, it is clear that the 8–10 day periodicity is stronger (higher amplitude) in the meridional component as compared to the zonal wind, whereas the 4–6 days and quasi-16 day periodicities are stronger in the zonal component. The phase propagation of the meridional component of the 8–10 day periodicity revealed that the wave has a strong
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Fig. 5. Wavelet spectra of time series of (detrended) of air temperature (AT, 1C), relative humidity (RH, %), pressure (P, hPa), wind speed (WS, m s 1), sea surface temperature (SST, 1C) and derived parameters such as Sensible Heat Flux (SHF, W m 2), Latent Heat Flux (LHF, W m 2) and Momentum Flux (MF, N m 2).
Table 1 The amplitudes and phases of the observed periodicities in MT as well as in observed MABL parameters. Periodicity (days)
4–6 8–10 14–20
Amplitude of the parameters
Phase angles of the parameters (deg.)
AT (1C)
P (hPa)
RH (%)
WS (m s 1)
SST (1C)
SHF (W m 2)
LHF (W m 2)
MF (W m 2)
MT (mg m 3)
AT
0.34 0.34 0.15
1.2 0.68 0.94
4.35 1.97 2.1
0.87 0.66 0.46
0.51 0.48 0.23
2.96 2.92 1.27
40.4 25.8 14.8
0.02 0.017 0.01
0.40 0.58 0.94
32 33 30 226 25 231 201 227 20 26 17 205 24 7 224 219 192 10 186 180 214 6 198 17 6 7 186
meridional component with a phase speed of 5 m s 1, whereas for the other two waves the meridional phase propagation is weak (o1 m s 1) and hence the meridional component of these waves are found to be nearly stationary. Shie et al. (2006) have reported a positive correlation between air temperature and SST as well as with RH over tropical oceans. It is reported that the SST and the fluxes of momentum and heat (in the MABL) exhibit periodicities associated with intra-seasonal oscillations of convection over the western Pacific and Indian Oceans (Shinoda et al., 1998). It is also reported that the intraseasonally varying wind speed and SST are anti-correlated (outof-phase), because SST acts to reduce the amplitude of the latent and sensible heat fluxes (Shinoda et al., 1998). It is reported in the
P
RH
WS SST SHF LHF MF
MT
literature that the air–sea interaction provides the energy source for large-scale disturbances such as intra-seasonal oscillations. Such large-scale disturbances will not develop spontaneously due to convection alone, but the variations in surface heat fluxes, induced by the surface wind variations, associated with the disturbance itself and is termed wind-induced surface heat exchange (WISHE, e.g. Emanuel, 1987; Neelin et al., 1987). As the wind speed is the main determinant of the fluxes, the zonally propagating waves observed in the fluxes are the aftermath of the periodicities in the zonal wind. Here the observed periodicities in MT are nearly out-of-phase with that of the westward propagating quasi-16 day and 4–6 day periodicities present in the zonal wind. This is in agreement with earlier observations that the Bay of
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Fig. 6. Phase propagation of the quasi 16-day, 4–6 days and 8–10 days periodicities in the zonal (a– c) and meridional winds (d–f).
phase relationship between MT and the fluxes for the periodicities. Since the 8–10 periodicity is having a strong meridional component (in wind), the resulting advection component is southeasterly, with strong contribution from the (more clean) southern part of Bay of Bengal, and this leads to the reduced amplitude of this periodicity in MT for the periodicity. Hence an in-phase relationship between the two is observed. Even though, the periodicities of 4–6 days and quasi-16 day have same direction of propagation, it is found that they have a phase difference of 1801 spatially. The studies on the periodicities of aerosol concentrations are important from the point of view of air quality as well as to reduce the uncertainties in the satellite retrieval of aerosol products. They are also useful in understanding the changes in aerosol forcing due to natural planetary scale oscillations in winds.
5. Conclusions
Fig. 7. Wavelet spectra of the zonal and meridional components of winds at 850 hPa (NCEP) from January 1 to 31 of 2009 at the mid oceanic region of Bay of Bengal BoB (centered around 101N, and 851E).
Bengal is highly influenced by the advected aerosols from East Asia (Moorthy et al., 2003, 2010; Moorthy and Babu, 2006). This further corroborates our earlier observations of the near-out-of-
Continuous and collocated measurements (made onboard the oceanic research vessel over the entire Bay of Bengal (BoB) during the period of W_ICARB) of total aerosol mass concentrations (MT) as well as the surface meteorological parameters (AT, P, RH, WS, SST) and derived fluxes (MF, LHF, SHF), revealed distinct temporal variations. The main results are summarized below 1. Analysis of the daily mean time-series of MT as well as all the surface meteorological parameters revealed short period modulations with periods of 4–6 days, 8–10 days as well as quasi
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16-day with significant amplitudes and the periodicities are found to be associated with westward propagating planetary (Rossby) waves. 2. The phase estimation of the periodicities of 4–6 days and quasi 16-day) revealed an in-phase relationship between the parameters AT, P, RH and SST. Similarly the variations of WS, LHF, SHF and MF showed in-phase variations and these variables were found to have out-of-phase relationship with MT for both the periodicities. 3. Phase estimation of 8–10 day periodicity revealed a different phase relationship; while the variables AT, P, SST, WS and MT were nearly in-phase, these were nearly out-of-phase with the variations in RH and fluxes. Even though, all the waves have westward phase propagation, the 4–6 days and quasi 16-day waves exhibited an out-of-phase relationship. 4. The periodicities in the meteorological parameters can cause either advection or dispersion and produce subsequent signatures on aerosol concentrations. As the easterly component of the quasi 16-day and 4–6 day periodicities are stronger, these would favor advection from the eastern continental locations and hence would produce an out-of-phase relationship between WS and MT. In contrast to these waves, due to the strong meridional component of the 8–10 periodicity, the resulting advection component is south-easterly with strong contribution from the southern part of Bay of Bengal and this tends to reduce the observed amplitude of MT and hence an inphase relationship between MT and SST.
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Short period variations of the aerosol mass concentrations over Bay of Bengal: Association with quasi-periodic variations in the Marine Atmospheric Boundary Layer parameters and fluxes S. Naseema Beegum 1, K. Krishna Moorthy n, D. Bala Subrahamanyam, N.V.P. Kiran Kumar, S. Suresh Babu, M. Mohan Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum 695022, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 July 2011 Received in revised form 8 November 2011 Accepted 24 November 2011 Available online 6 December 2011
Analysis of the time-series of daily mean total aerosol mass concentrations (MT), measured within the Marine Atmospheric Boundary Layer (MABL) over the Bay of Bengal (BoB) revealed the presence of short-period modulations with periods of 4–6 days, 8–10 days as well as quasi 16-day (14–20 days). These were found to be distinctively associated with similar oscillations in the concurrently measured MABL parameters such as air temperature (AT), pressure (P), relative humidity (RH), wind speed (WS), sea surface temperature (SST) and derived parameters such as Momentum Flux (MF), Latent Heat Flux (LHF) and Sensible Heat Flux (SHF). Examination of the phase relations revealed that the 4–6 days and quasi 16-day periodicities in AT, P, RH and SST maintained a nearly in-phase (very small phase difference o 7 201) variation between themselves and also with similar periodicities in the aerosol mass concentration MT. On the other hand, the periodicities in WS, SHF, LHF and MF, though were nearly in-phase among themselves, exhibited an out-of-phase (phase difference 180 7201) relation with that of MT. Interestingly, the 8–10 day periodicities revealed a different phase relationship; the variables AT, P, SST, WS and MT were in-phase and these variables were out-of-phase with similar periodicities in RH and the fluxes. It was also observed that all the three waves represented westward propagating Rossby waves. An easterly phase in the wind was found to result in advection of particles from the East Asia to BoB, as evident from the out-of-phase relationship between the periodicities of 4–6 days and quasi 16-day in MT and zonal wind. The meridional component, that was stronger than its zonal amplitude in the 8–10 day periodicity, resulted in enhanced advection of particles from the southern part of Bay of Bengal in comparison with that from the Eastern region, leading to an in-phase relationship between MT and zonal wind. & 2011 Elsevier Ltd. All rights reserved.
Keywords: W_ICARB MABL aerosols Bay of Bengal aerosols Rossby waves
1. Introduction Tropical atmosphere is the seat of strong wave activities of various time scales, which can be categorized according to their scale sizes, triggering and restoring mechanisms as gravity waves, tides and planetary waves, each having its own distinct physical and geometrical properties (Holton, 1972). Among these waves, those having periodicities greater than one day and the horizontal dimensions comparable to the radius of the Earth are called the planetary waves, the most important ones at the tropics being the eastward propagating circulation anomalies having periodicities of 30–60 days known as the Madden Julian Oscillations (MJO, n
Corresponding author. Tel.: þ91 471 2562404; fax: þ91 471 2706535. E-mail address: [email protected] (K.K. Moorthy). 1 Current affiliation: National Physical Laboratory, Dr. KS Krishnan Marg, New Delhi 110012, India. 1364-6826/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2011.11.012
Madden and Julian, 1971) and the westward-propagating periodicities with shorter time periods than MJO, in both zonal and meridional components, are called the (Rossby waves) Planetary waves (Rossby, 1939; Blinova, 1943). These oscillations appear most clearly over the Indian and western Pacific Oceans (Sperber, 2003) and involve many meteorological variables, including the zonal wind, surface pressure, temperature and humidity. These waves, being excited by several means, including interactions of the mean flow with mountains, convective events and instabilities of various kinds, have been found to produce their signatures in the concentrations of atmospheric trace species including aerosols (Beegum et al., 2009). It is reported in the literature that the wave activities are more vigorous over vast oceans compared to the continental regions as the large-scale air–sea interactions play an important role in the variability of both ocean and atmosphere on monthly/seasonal time scales. Latent heat release in the tropics drives planetary
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Fig. 1. Cruise track followed during W_ICARB. The positions of the ship at 6:30 UT of all the days of measurements are marked by the circles in the track. The monthly mean prevailing wind vectors of January 2009 at 850 hPa are also superposed on the same figure.
scale atmospheric circulations. Resulting changes in cloud cover would affect both the atmospheric and oceanic boundary layers through a variety of feedbacks, such as precipitation, wind speed and sea surface temperature (e.g., Webster et al., 1996). Nakazawa (1988) noted that convection occurred in association with the active intra-seasonal oscillations (ISO) and was organized into large systems called super-cloud clusters. These super-cloud clusters are composed of several individual cloud clusters that tend to propagate westward, while the envelope of super-cloud clusters propagates eastward with the ISO. Several investigators have reported the presence of two dominant intra-seasonal modes with periods of 30–60 days and 10–20 days in atmospheric parameters over the tropical Indian Ocean (Krishnamurti and Ardunay, 1980; Yasunari, 1981; Krishnamurti et al., 1988; Fedulina et al., 2004; Beegum et al., 2009). This article focuses on the characterization of air–sea interaction processes over various time scales by examining the short period fluctuations in the time-series data of concurrently measured surface aerosol mass concentrations as well as Marine Atmospheric Boundary Layer (MABL) parameters (the surface wind speed (WS), sea surface temperature (SST), air temperature (AT), relative humidity (RH) and the derived parameters Sensible Heat Flux, (SHF), Momentum Flux (MF) and Latent Heat Flux (LHF)) over Bay of Bengal during the winter season of 2009. Even though the wave activity over this equatorial oceanic region is rather pronounced during winter season, such studies are virtually non-existent. Surface layer meteorological measurements, in conjunction with the aerosol mass concentration measurements, collected onboard the oceanographic research vessel (ORV) Sagar Kanya during its cruise #SK254 dedicated to the Winter Integrated Campaign for Aerosols gases and Radiation Budget (W_ICARB) field experiment formed the database of the present study. During this campaign, the entire BoB, the region between 76.61 and 97.51E and 3.51 and 211N has been explored extensively in a span of 34 days (from 27th December 2008 to 30th January 2009), along the cruise track shown in Fig. 1
2. Campaign, instruments and data During the campaign, the ORV sailed off the port of Chennai on 27th December 2008 and after the field expedition called at Kochi on 30th January 2009, following the track denoted by the solid line in Fig. 1. During this period, continuous measurements of instantaneous size segregated mass concentration of composite (total) aerosols were made using a 10-stage Quartz Crystal Microbalance (QCM)
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cascade Impactor (model PC-2 California instruments, USA) having 50% lower size cut off at each of its 10 size bins at 425 mm, 12.5, 6.4, 3.2, 1.6, 0.8, 0.4, 0.2, 0.1, and 0.05 mm for size bins from 1 to 10; the sum of the individual-bin mass concentrations yielding the total mass concentration, MT, of ambient aerosols. The QCM sampled the ambient air through the community inlet at a constant flow rate of 0.24 l/min with a sampling time of 300 s. Measurements were repeated at regular intervals of 30 min, round the clock. Measurements were restricted to the periods of RH o78% at the deck level, in view of the affinity of the quartz crystal to changes in RH for higher RH (Pillai and Moorthy, 2001; Nair et al., 2008). But there were no data loss on application of this restriction, as there were only one or two points in the hourly data and in the present study we have taken the daily averages, omitting those values. In general, the uncertainties in the measured MT values were in the range of 10–20% at very low mass concentrations (r10 mg m 3) and the error reduces to o10% for the high mass concentrations (430 mg m 3). More details of the instrumentation, analysis and error budget are given elsewhere (Pillai and Moorthy, 2001; Nair et al., 2009). Concurrent surface layer meteorological measurements were carried out using an Automatic Weather Station (AWS) fitted with several meteorological sensors on a 7-m long retractable boom fixed to the bow of the ship. Overall, there were five slow response sensors: cup anemometer for wind speed, wind vane (wind direction), platinum resistance thermometer (air temperature), capacitance-based humicap (relative humidity) and barometer (atmospheric pressure), all connected to a data logger placed near the retractable boom. The entire AWS assembly was powered with a solar panel. Data from these sensors were recorded every 10 min. The orientation and alignment of the sensors on the boom were adjusted such that these measurements were least affected by heat contamination and flow distortion, respectively, due to the exhaust and the bulk nature of ship. Beside these, manual measurements of SST were made at hourly intervals with a bucket thermometer. The navigation parameters of the ship such as its velocity, header angle from true North and exact geographical location in the terms of longitude and latitude were continuously recorded at every second using a GPS (Global Positioning System). More details of the instrumentation and measurement techniques are given elsewhere (Subrahamanyam et al., 2011).
2.1. Data processing and correction for ship’s motion As the measurements of meteorological parameters were made onboard the ship, these would produce errors in the wind components due to the motion of the ship. For correcting the wind speeds for flow distortion, we calculated the zonal and meridional components of apparent winds and ship’s speed. As the apparent wind is resultant of the true wind and ship’s velocity, it is resolved trigonometrically by solving the two zonal and meridional components. In this way, wind speed measurements were corrected for the movement of the ship. However, the retractable boom was kept sufficiently away from the ship’s stack ( 50 m away from the stack) in order to avoid the contamination by the local flow distortion over the bulk body. In the case of ship-borne platform, accurate measurements of vertical winds become difficult because of the pitch and roll motion of the ship, and therefore estimation of the air–sea interaction parameters largely depend on the bulk aerodynamic algorithms (Godfrey et al., 1991; Subrahamanyam and Radhika, 2002, 2003; Bhat et al., 2003; Ramana et al., 2004; Alappattu et al., 2008). Hence in the present study, the surface layer turbulent fluxes of momentum, heat and moisture have been estimated through an iterative scheme based on the bulk aerodynamic algorithm (Smith, 1988; Subrahamanyam and Radhika, 2002, 2003; Subrahamanyam et al., 2011).
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3. Prevailing meteorology The prevailing meteorology during the W_ICARB comprised predominantly of calm synoptic conditions with moderate winds and clear/partially clear skies. No major weather systems or cyclonic depressions were encountered in the study area during the cruise period, except perhaps the thunderstorm that occurred on the night of 28th January 2009 in the northern Indian Ocean. The monthly mean prevailing synoptic winds of January 2009 at 850 hPa (NCEP, National Centre for Environmental Prediction) are superposed on the cruise track in Fig. 1. An anti-cyclonic circulation, centered at 201N over the Indian landmass resulted in calm/ weak northerlies/north-easterlies to prevail over the northern BoB. Moderate easterlies/north-easterlies prevailed over the mid and southern BoB (below 121N), with higher wind speeds in eastern parts. This synoptic wind pattern revealed the prevalence of continental air mass over the entire BoB during the campaign period; however they originated from distinct geographical regions. Examination of the spatial pattern of wind vectors over BoB revealed that the pattern remained almost similar during the entire cruise period as shown in Fig. 2, where the average wind patterns for the first (January 1–15) and second (January 16–30) halves of the campaign period are shown (in the left and middle panels) along with the difference between the two (in the right panel). Even though the magnitude of winds decreased in the second half of the campaign, the wind direction did not show any significant change. The extreme right panel of Fig. 2 reveals that the difference in absolute magnitude between the two halves remained very low for most of the oceanic regions, except in the eastern BoB (east of Andaman–Nicobar islands). This evidences that the synoptic conditions remained nearly stable during the campaign period.
observed during the first half and a subsequent decrease thereafter in the second half is attributed mostly to the source proximity of the northern as well as eastern continental locations. Similar latitudinal gradients are observed in both the column and surface aerosol parameters over BoB during the same period (Nair et al., 2008, 2009; Moorthy et al., 2010). With a view to examining the short period modulations in MT, the time-series data is detrended (Priestley, 1988) by fitting two different slopes for the first and second halves (Fig. 3 (top panel)). The resulting timeseries of MT is shown in the bottom panel of Fig. 3. Since the wind pattern was consistent throughout the cruise period, the modulations observed in the variables cannot be attributed to the changes in the prevailing winds, but might be associated with the modulations in the other atmospheric variables. Careful examination of the time-series of surface layer meteorological parameters and the air–sea fluxes revealed similar fluctuations modulated by a weak trend (not shown in figure) in all the parameters except in WS. These trends, associated with the latitudinal trends in the parameters for the study region (in winter season, as we move from tropics to near extra-tropics), were removed by detrending and the resulting time-series of these parameters shown in Fig. 4 reveal prominent fluctuations. 4.2. The wavelet analysis With a view to delineate the dominant periodicities in all these parameters, the detrended time-series data was subjected to wavelet analysis using a Morlet wavelet (Torrence and Compo, 1998). Morlet is a complex wavelet, which can be decomposed into real and imaginary parts (Torrence and Compo, 1998). The mother wavelet of the morlet is a plane wave modulated by a Gaussian of the form, CðxÞ ¼ C expððx2 =2ÞÞexpðioxÞ, where o is
4. Results and discussion 4.1. Variations in surface aerosol mass concentrations and ABL parameters The time-series of the daily average values of the total aerosol mass concentrations (MT) for the entire cruise period is shown in Fig. 3 (top panel). The figure revealed an increasing trend in the first half of the cruise and a decreasing trend in the second half, superposed with several short period modulations. During the first half, the ship was moving northwards from Chennai to the northern most region ( 221N), thereafter in the eastern BoB (oceanic region in the east of BoB), in close proximity to the east Asian region. In the second half, it was traversing from Port Blair ( 121N) to the equatorial region ( 21N) through the mid ocean as evident from the cruise track (Fig. 1). Hence the increase in MT
Fig. 3. Time series of the daily mean total mass concentrations (MT). The top panel shows the variations of the raw data. The dotted lines show the corresponding least square linear fits for the two slopes. The bottom panel shows the time series of the detrended data, where the trends have been removed and the fluctuations become more prominent.
Fig. 2. Spatial pattern of wind vectors at 850 hpa averaged over the first half (January 1–15) of the cruise (left panel), second half (January 16–30) of the cruise (middle panel) and the difference between the two (right panel).
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Fig. 4. Time series of the surface measured MABL parameters (detrended) of air temperature (AT, 1C), relative humidity (RH, %), pressure (P, hPa), wind speed (WS, m s 1), sea surface temperature (SST, 1C) and derived parameters such as Sensible Heat Flux (SHF, W m 2), Latent Heat Flux (LHF, W m 2) and Momentum Flux (MF, N m 2).
the angular frequency.The wavelet analysis was performed in the time-series of the daily mean data of MT and surface meteorological parameters and the resulting spectra are shown in Fig. 5. The spectrum of MT (in the top left panel) revealed the presence of two strong periodicities of 4–6 days and 8–10 days. Another mode of quasi-16 day is also present in significant amplitude with periods in the range of 14–20 days. In addition, signatures of a weak, but significant 2–3 periodicity are also observed. At this juncture it may also be kept in mind that 2–3 day periodicities would be partly contaminated by north–south movement of the ship with an average periodicity of 3 days (Fig. 1) and hence the periodicity is not discussed further in the manuscript. Similar analysis of the wavelet spectra of the surfacelayer meteorological parameters such as AT, WS, RH, P, SST and estimated values of air–sea interface fluxes also revealed the periodicities of 2–3 days, 4–6 days and 8–10 days and quasi-16 day. As the fluxes of LHF, SHF and MF depend on wind speed and relative humidity, it is logical to expect these fluctuations to be similar to those in the surface parameters. Based on the observations over tropical oceanic regions several investigators have reported the presence of various oscillations in meteorological parameters of different time scales. Among these the most important is the quasi-16 day mode (Chen and Chen, 1993; Chatterjee and Goswami, 2004). Chatterjee and Goswami (2004) have studied the atmospheric quasi biweekly mode based on the OLR and NCEP reanalysis wind vectors at lower atmosphere and have suggested that it is a convectively coupled first meridional mode equatorial Rossby wave. Sengupta et al. (2004) have reported the presence of 10–20 day periodicity in the meridional ocean currents in the eastern Indian Ocean and suggested that the observed biweekly variability is due to equatorially trapped mixed Rossby-gravity waves generated by sub-seasonal variability of winds. Parekh et al. (2004) have reported that sea surface temperature measured by the Tropical Rainfall Measuring Mission Microwave Imager (TMI) over the Bay of Bengal during summer monsoon was found to exhibit a lowperiod intra-seasonal mode of 8–16 days, in addition to the 30–60
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days mode. Janicot and Sultan (2001) have also observed a quasiperiodic signal of about 15 days in the rainfall and wind fields at 925 hPa over West Africa based on long database of 23 years. The presence of the quasi 16-day fluctuations in zonal winds over the eastern tropical Atlantic and Northwest Africa have been also noted by Viltard et al. (1997). Intensive study of synoptic-scale disturbances over central and West Africa and the tropical Atlantic has revealed the existence of two types of westward propagating wave-like fluctuations with periodicity of 3 to 5 days (westward speed of 9–10 m s 1) in meridional wind (Carlson, 1969; Viltard et al., 1997) and a longer periodicity of 6–9 days (westward speed of 11 m s 1) (Yanai and Murakami, 1970) in zonal wind. Hareesh Kumar et al. (2001) found prominent peaks at around 10.6 days in the meridional component of ocean surface wind and air temperature as well as SST. Hence the periodicities in the meteorological parameters can cause either advection or dispersion and produce subsequent signatures on aerosol or other trace species concentrations (Beegum et al., 2009). In order to examine the strength of these oscillations and direction of propogation, we have estimated the amplitudes and phase angles of each of the periodicities for all parameters and these are listed in Table 1. As the periodicities observed are in 95% confidence level (significant at 5%), the estimated phase angles are also significant. The phase angles of the periodicities of 4–6 days and quasi 16-day revealed a nearly in-phase relationship between AT, P, RH and SST; the difference in the phase angles being o 7201). Similarly the variations of WS, LHF, SHF and MF are observed to be in-phase among themselves and out-of-phase with the variations in MT. Interestingly, even though similar phase relationships between different parameters are observed for both the waves, the phase difference between the corresponding variables for these two waves were approximately 1801 (within 7201), indicating that these waves were varying nearly out-of-phase spatially. But, the phase estimation of the 8–10 day periodicity revealed certain different characteristics in which AT, P, SST, WS and MT are nearly in-phase and these variables are nearly out-ofphase with the RH, SHF, LHF and MF. This observation is significant owing to the fact that the observed periodicities in MT showed a nearly in-phase association with WS in contrary to the near-out-of-phase variation in the other two waves. Examination of the phase propagation helps to ascertain the type of the wave observed in the meteorological parameters (such as wind field), which result in corresponding modulations in the mass concentrations. As such, we deduced the zonal and meridional phase propagation of these waves by analyzing the NCEP wind vectors (U for zonal propagation and V for meridional propagation) at 850 hPa as a function of longitude and latitude, respectively, for U and V. The results are shown in Fig. 6. As the zero phase position of the waves move westward in U for all the three periodicities (Fig. 6(a)–(c)), these are the westward propagating planetary waves (Rossby waves). These waves also have meridional components, which are found to propagate northwards (Fig. 6(d)–(f)). At this juncture it is interesting to examine the zonal and meridional amplitude of these periodicities. As such, the timeseries of zonal and meridional component of wind at the mid oceanic region of BoB (centered around 101N, and 851E as a representative) at 850 hPa (NCEP) from January 1 to 31 of 2009 has been subjected to wavelet analysis and the resulting spectra are shown in the Fig. 7. From the figure, it is clear that the 8–10 day periodicity is stronger (higher amplitude) in the meridional component as compared to the zonal wind, whereas the 4–6 days and quasi-16 day periodicities are stronger in the zonal component. The phase propagation of the meridional component of the 8–10 day periodicity revealed that the wave has a strong
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Fig. 5. Wavelet spectra of time series of (detrended) of air temperature (AT, 1C), relative humidity (RH, %), pressure (P, hPa), wind speed (WS, m s 1), sea surface temperature (SST, 1C) and derived parameters such as Sensible Heat Flux (SHF, W m 2), Latent Heat Flux (LHF, W m 2) and Momentum Flux (MF, N m 2).
Table 1 The amplitudes and phases of the observed periodicities in MT as well as in observed MABL parameters. Periodicity (days)
4–6 8–10 14–20
Amplitude of the parameters
Phase angles of the parameters (deg.)
AT (1C)
P (hPa)
RH (%)
WS (m s 1)
SST (1C)
SHF (W m 2)
LHF (W m 2)
MF (W m 2)
MT (mg m 3)
AT
0.34 0.34 0.15
1.2 0.68 0.94
4.35 1.97 2.1
0.87 0.66 0.46
0.51 0.48 0.23
2.96 2.92 1.27
40.4 25.8 14.8
0.02 0.017 0.01
0.40 0.58 0.94
32 33 30 226 25 231 201 227 20 26 17 205 24 7 224 219 192 10 186 180 214 6 198 17 6 7 186
meridional component with a phase speed of 5 m s 1, whereas for the other two waves the meridional phase propagation is weak (o1 m s 1) and hence the meridional component of these waves are found to be nearly stationary. Shie et al. (2006) have reported a positive correlation between air temperature and SST as well as with RH over tropical oceans. It is reported that the SST and the fluxes of momentum and heat (in the MABL) exhibit periodicities associated with intra-seasonal oscillations of convection over the western Pacific and Indian Oceans (Shinoda et al., 1998). It is also reported that the intraseasonally varying wind speed and SST are anti-correlated (outof-phase), because SST acts to reduce the amplitude of the latent and sensible heat fluxes (Shinoda et al., 1998). It is reported in the
P
RH
WS SST SHF LHF MF
MT
literature that the air–sea interaction provides the energy source for large-scale disturbances such as intra-seasonal oscillations. Such large-scale disturbances will not develop spontaneously due to convection alone, but the variations in surface heat fluxes, induced by the surface wind variations, associated with the disturbance itself and is termed wind-induced surface heat exchange (WISHE, e.g. Emanuel, 1987; Neelin et al., 1987). As the wind speed is the main determinant of the fluxes, the zonally propagating waves observed in the fluxes are the aftermath of the periodicities in the zonal wind. Here the observed periodicities in MT are nearly out-of-phase with that of the westward propagating quasi-16 day and 4–6 day periodicities present in the zonal wind. This is in agreement with earlier observations that the Bay of
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83
Fig. 6. Phase propagation of the quasi 16-day, 4–6 days and 8–10 days periodicities in the zonal (a– c) and meridional winds (d–f).
phase relationship between MT and the fluxes for the periodicities. Since the 8–10 periodicity is having a strong meridional component (in wind), the resulting advection component is southeasterly, with strong contribution from the (more clean) southern part of Bay of Bengal, and this leads to the reduced amplitude of this periodicity in MT for the periodicity. Hence an in-phase relationship between the two is observed. Even though, the periodicities of 4–6 days and quasi-16 day have same direction of propagation, it is found that they have a phase difference of 1801 spatially. The studies on the periodicities of aerosol concentrations are important from the point of view of air quality as well as to reduce the uncertainties in the satellite retrieval of aerosol products. They are also useful in understanding the changes in aerosol forcing due to natural planetary scale oscillations in winds.
5. Conclusions
Fig. 7. Wavelet spectra of the zonal and meridional components of winds at 850 hPa (NCEP) from January 1 to 31 of 2009 at the mid oceanic region of Bay of Bengal BoB (centered around 101N, and 851E).
Bengal is highly influenced by the advected aerosols from East Asia (Moorthy et al., 2003, 2010; Moorthy and Babu, 2006). This further corroborates our earlier observations of the near-out-of-
Continuous and collocated measurements (made onboard the oceanic research vessel over the entire Bay of Bengal (BoB) during the period of W_ICARB) of total aerosol mass concentrations (MT) as well as the surface meteorological parameters (AT, P, RH, WS, SST) and derived fluxes (MF, LHF, SHF), revealed distinct temporal variations. The main results are summarized below 1. Analysis of the daily mean time-series of MT as well as all the surface meteorological parameters revealed short period modulations with periods of 4–6 days, 8–10 days as well as quasi
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16-day with significant amplitudes and the periodicities are found to be associated with westward propagating planetary (Rossby) waves. 2. The phase estimation of the periodicities of 4–6 days and quasi 16-day) revealed an in-phase relationship between the parameters AT, P, RH and SST. Similarly the variations of WS, LHF, SHF and MF showed in-phase variations and these variables were found to have out-of-phase relationship with MT for both the periodicities. 3. Phase estimation of 8–10 day periodicity revealed a different phase relationship; while the variables AT, P, SST, WS and MT were nearly in-phase, these were nearly out-of-phase with the variations in RH and fluxes. Even though, all the waves have westward phase propagation, the 4–6 days and quasi 16-day waves exhibited an out-of-phase relationship. 4. The periodicities in the meteorological parameters can cause either advection or dispersion and produce subsequent signatures on aerosol concentrations. As the easterly component of the quasi 16-day and 4–6 day periodicities are stronger, these would favor advection from the eastern continental locations and hence would produce an out-of-phase relationship between WS and MT. In contrast to these waves, due to the strong meridional component of the 8–10 periodicity, the resulting advection component is south-easterly with strong contribution from the southern part of Bay of Bengal and this tends to reduce the observed amplitude of MT and hence an inphase relationship between MT and SST.
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