A statistical analysis of the eddies detected by the automated tracking procedure from January 1993 to February 2017 in the Bay of Bengal is presented in this section. As a result, 1 089 eddies, which include 583 cyclones (36 065 independent eddy) and 506 anticyclones (29 547 independent eddy), with lifetimes ≥ 30 d are detected in the eddy tracking procedure, and a regional census statistic is performed on 1089 eddies.
The propagation directions and movement characteristics of eddies in the Bay of Bengal can be displayed by analyzing the eddy trajectories. Most mesoscale eddies (approximately 90% cyclones and 93% anticyclones) propagate westward in the Bay of Bengal, which consistent with previous studies (Chen et al, 2012; Cui et al., 2016; Lin et al., 2019). Conversely, less eddies have net eastward displacement, and they have much smaller lifetimes and much shorter propagation distances than the westward eddies. These eastward eddies mostly propagate eastward with the action of some strong currents and the effects of strong monsoons, especially the eddies restricted to near EICC can be expected from advection of the eddies by the seasonal northeast currents (Cui et al., 2016; Kumar and Chakraborty, 2011). Short-lived eastward eddies are closely associated with strong current variations and complicated dynamic processes because of air-sea interactions (Chen et al., 2012).
The analysis of these trajectories with lifetimes≥30 d in the Bay of Bengal shows that 53% cyclones and 55% anticyclones propagate southward (equatorward), with eddy lifetime increasing to 60, 90, 120 d, the ratios turn into 63% and 60%, 69% and 64%, 78% and 70% (Table 1), respectively. With the increasing lifetime, the difference in the number of northward eddies and southward eddies becomes more and more obvious: for eddies with lifetimes≥90 d, the number of southward eddies is double the number of northward eddies (67%:33%); for eddies with lifetimes≥120 d, the number of south-north eddies differs by three times (75%:25%). The result reveals that there is a clear southward (equatorward) preference for eddies with long lifetimes in the Bay of Bengal, especially cyclones.
Lifetime≥30 d Lifetime≥60 d Lifetime≥90 d Lifetime≥120 d Southward 586 (CE: 307; AE: 279) 281(CE: 147; AE: 134) 136(CE: 78; AE: 58) 77 (CE: 46; AE: 31) Northward 503(CE: 276; AE: 227) 176 (CE: 88; AE: 88) 68 (CE: 35; AE: 33) 26(CE: 13; AE: 13) Percentage/% 54:46(CE: 53:47; AE: 55:45) 61:39 (CE: 63:37; AE: 60:40) 67:33 (CE: 69:31; AE: 64:36) 75:25 (CE: 78:22; AE: 70:30) Note: Bold numbers represent all eddies; CE and AE mean cyclonic and anticyclonic eddies, respectively.
Table 1. The number of southward and northward eddies with different lifetimes, and their percentage (southward/northward)
The eddies in different areas of the Bay of Bengal show different north-southward preferences (Fig. 1). In terms of the northward and southward eddies with lifetimes≥60 d, the northward eddies are mainly distributed in the southern bay, while the southward eddies are mainly located in the middle and northern bay. It can be clearly seen that at the dividing line of 12°N, the eddies to the south of the line mostly propagate northwestward, and the eddies to the north of the line mostly propagate southwestward. The meridional eddy movement pattern was like that found in Chen et al. (2012) and Lin et al. (2019). Most of the northward eddies originate mainly outside the bay and in the southern bay (south of 12°N), and then propagate into the western bay along the southwest monsoon current; there are also some eddies formed in the southwestern bay that will enter the western bay from advection of the northward EICC (Fig. 1a). The eddies generated in the northern and middle bay (north of 12°N) mostly propagate southwest due to topographical constraints, and in the southern bay, few eddies propagate south to the area outside the bay (Fig. 1b). Cheng et al. (2018) suggested that when equatorial wind-driven downwelling (upwelling) Kelvin waves propagate to the tip of the Irrawaddy Delta off Myanmar, eddies in the central bay are generated there with periods of 30–120 d that subsequently propagate southwestward. The eddies with different propagation preferences in the northern and southern bay are very important for water transport in the Bay of Bengal. For example, eddies to the south of 12°N mostly propagate northward and disappear in the background field in the western bay, and they will transport the Arabian Sea High-Salinity Water (ASHSW, Fig. 4) near the equator into the western Bay of Bengal. These high-temperature and high-salt waters will continue to be transported to the northern bay along with the northward EICC and even affect the temperature and salt structure in the whole bay. In contrast, eddies to the north of 12°N mostly propagate southward and will carry low-temperature, low-salt, and low-density waters from river runoff and net precipitation to the middle and southern bay, which will affect the temperature and salinity and circulation structure of the entire Bay of Bengal.
Figure 1. The northward (a) and southward (b) propagation trajectories of the cyclonic (blue lines) and anticyclonic (red lines) eddies with lifetimes≥60 d.
The averaged kinetic properties of eddies in different lifetimes show that the eddies with longer lifetime have the stronger kinetic properties in the Bay of Bengal. That is to say, the stronger eddies can maintain a more stable and uniform rotational structure to ensure that they can last a longer lifetime in the ocean. The changes of eddy kinetic properties during the evolution of eddies with different lifetimes are shown in Figs 2a-c. Eddy properties are generally low in the eddy generation and disappearance stages, and eddy properties are generally high in the middle maturation stage of the eddy lifetime. The kinetic properties of all eddies exhibit symmetrical and parabolic changes with the eddy evolution in the complete lifetime. Moreover, the eddy properties in different lifetimes are basically same in the eddy generation and disappearance stages. On average, no matter eddies with what longer lifetime in the Bay of Bengal, eddies generally have an amplitude of 4 cm, a radius of 75 km, and an EKE of 1×104 cm2/s2 when they appear or disappear in the ocean. The eddy properties in different lifetimes have the largest difference in the middle maturation stage of the eddy lifetime. The max amplitude of eddies with lifetimes of 30–60 d generally does not exceed 8 cm, while the max amplitude of eddies with lifetimes ≥ 150 d can reach 16 cm in the middle lifetime stage. Generally speaking, in the stable and mature stage of eddy evolution, the eddies with longer lifetime have the stronger kinetic properties; while in the eddy generation and disappearance stages, the eddies with different lifetimes have the similar kinetic properties.
Figure 2. Evolution of eddy kinetic properties with the lifetime. a–c. The changes of eddy kinetic properties during the evolution of eddies with different lifetimes. The eddies with different lifetimes are divided into five categories: [3060) d, [60, 90) d, [90, 120) d, [120, 150) d, and langer than 150 d (the numbers of cyclonic and anticyclonnic eddies are: 348 and 284, 122 and 131, 54 and 47, 20 and 22, 39 and 22, respectively). Here cyclonic and anticyclonic eddies are considered together, and the eddy lifetime has been normalized. d–f. The normalized relative properties of all cyclonic (blue line) and anticyclonic (red line) eddies with lifetime≥30 d evolve with the normalized lifetime.
The relative changes of eddy kinetic properties during the lifetime evolution are shown in Figs 2d-f. From the figure, it can be clearly seen that the eddy properties increase significantly in the former one-fifth stage of the lifetime, in which the eddy amplitude increase by more than 1 times, the EKE increase by more than 4 times, and the eddy radius also increase significantly. The middle 2/5 to 4/5 lifetime is the stable and mature stage of eddies. At this stage, eddy properties are relatively stable, and the eddy energy density is very concentrated; the eddy amplitude and radius are both at 90% of the maximum value. The last 1/5 stage of the lifetime is the dying period of eddies, which is exactly the opposite of the first 1/5 lifetime stage. During this period, the eddy kinetic properties decrease rapidly, and finally the eddy disappears in the ocean.
The automated eddy identification provides the estimates of the eddy amplitude, radius scale and EKE as defined in Section 2.2. These kinetic properties of eddies with lifetimes ≥30 d over a 24-year period are analyzed here. There are some differences between the average properties of cyclones and anticyclones in the Bay of Bengal (Table 2). Specifically, the average amplitude and radius of cyclonic eddies are 9.7 cm and 116 km, and that of anticyclonic eddies are 9.0 cm and 122 km. The average amplitude intensity of anticyclonic eddies is smaller than that of cyclones, but the spatial scale is larger than that of cyclones. Although the radius of cyclonic eddies is small, their average eddy kinetic energy EKE (4.7×104 cm2/s2) is significantly larger than that of anticyclones (3.78×104 cm2/s2), indicating that the ocean kinetic energy carried by cyclonic eddies is more concentrated (higher eddy energy density). Of course, the magnitude of eddy energy is more intuitively reflected in the rotational speed of eddies. In terms of the maximum rotational speed Umax, cyclonic eddies (41.5 cm/s) are significantly larger than anticyclonic eddies (36.4 cm/s), indicating that the cyclonic eddies rotate faster than the anticyclones in the Bay of Bengal. This outcome is likely related to the intrusion of the offshore monsoon current caused by the summer southwest monsoon in the southwestern bay. During this period, many intense cyclonic eddies are generated in the eastern region of Sri Lanka, and the cyclones generally rotate very fast with the intrusion of the monsoon current (Nuncio and Kumar, 2012; Chen et al., 2018). In general, cyclonic eddies are stronger than anticyclones in terms of eddy amplitude, EKE, and maximum rotational speed, but the spatial scale of cyclones is smaller than that of anticyclones, which means that cyclones have a higher eddy energy density than anticyclones.
AM/cm R/km EKE/(cm2·s–2) Umax/(cm·s–1) Cyclones 9.70 116 4.70×104 41.5 Anticyclones 9.00 122 3.78×104 36.4
Table 2. Average kinetic properties of eddies with lifetimes≥30 d in the Bay of Bengal
The numbers of eddy occurrences for cyclonic and anticyclonic eddies are shown in Figs 3a and b. Although the Bay of Bengal region is not large enough for longer lifetime eddies due to the topography constraints, there are also abundant mesoscale eddies in the bay similar to the world’s oceans (Chelton et al., 2011b). Especially, eddies are concentrated in the northwestern bay, where the seasonal western boundary current EICC exists. An important reason for the distribution is the baroclinic instability of the EICC induced by a middle topography mutation of the western bay and by the local monsoon conversion (Babu et al., 1991; Hacker et al., 1998; Cui et al., 2016; Chen et al., 2012, 2018). From the polarity geographical distribution (Fig. 3c), it can be seen that the preferences of cyclones or anticyclones are indistinctive (values of P are near 0) in most regions of the Bay of Bengal, e.g., in the northern, central and southern bay where there are similar numbers of cyclones and anticyclones. However, there are still slight preferences in some local regions. Separately, the western bay and the southern region outside the bay tend to the occurrence of cyclonic eddies, while in the eastern bay (specifically, the region of 12°–16°N, 88°–94°E) there is a slight preference for the anticyclonic eddy occurrences. It is worth noting that near the southeastern region outside the bay (the northwest of Sumatra), the eddy polarity P shows significant negative values, indicating that the number of cyclones is significantly greater than that of anticyclones. This outcome occurs because a large number of cyclonic eddies often occur near the eastern region of Sumatra Island (5°N, 93°E), and they gradually move northwest into the Bay of Bengal or disappear into the southern bay mouth (Fig. 1a). Chen et al. (2017) show that in addition to local wind and current instability, parts of eddy energy in the Bay of Bengal originates from the equator. Equatorial-origin wave signals significantly enhance the EKE levels in the northwest of Sumatra, in the form of reflected Rossby waves and coastal Kelvin waves, respectively (Chen et al., 2018).
Figure 3. Census statistics for numbers (a and b) of cyclonic (CEs) and anticyclonic (AEs) eddies and their polarity distribution (c) for each 1°×1° region (smoothed using a 3°×3° window), the maps of the mean amplitude (d and e), radius (g and h), and EKE (j and k) of CEs and AEs, and their variations with latitude (f, i, and l). The red and blue lines in f, i and l represent the mean zonal properties of CEs and AEs, respectively; and the black lines represent the mean zonal properties and standard deviations of all eddies.
The maps of the mean amplitude (Figs 3 d and e), radius (Figs 3 g and h), and EKE (Figs 3 j and k) of cyclonic and anticyclonic eddies are shown in Fig. 3, respectively, and their variations with latitude are also shown in Figs 3f, i, and l panels. In terms of eddy amplitude, approximately two thirds of all detected eddies have amplitudes<10 cm, and 15% have amplitudes>15 cm in the Bay of Bengal, which consistent with Cui et al. (2016) and Lin et al. (2019). From the maps of mean eddy amplitude (Figs 3d and e), eddies with large amplitudes occur only in the relatively confined regions of highly unstable currents, such as the EICC and eastern Sri Lanka where eddies have amplitudes>12 cm (especially, near the EICC amplitudes>15 cm); over the rest of the bay, the mean eddy amplitudes are generally less than 10 cm. The large-amplitude eddies most likely form as meanders that pinch off the western boundary currents, or from baroclinic instability in the regions of strong and unstable currents. Chen et al. (2012) suggested that when the baroclinic instability is stronger, more energy from the mean is converted to the eddy energy, which makes eddies stronger and more stable but fewer in number. Moreover, the EICC is a very obvious seasonally changing current, and during the southwest monsoon in summer and the northeast monsoon in winter, the flow direction is completely opposite. Therefore, during the turn of the current, more large-amplitude eddies are more likely to be generated due to the instability of the EICC (Cheng et al., 2013; Chen et al., 2018). In eastern Sri Lanka, due to the intrusion of the southwest monsoon current and the effect of negative wind stress curl, there are often eddy-train structures in which strong cyclones and anticyclones alternately move northwest into the western bay in summer and autumn (Cui et al., 2016; Kumar and Chakraborty, 2011; Vinayachandran et al., 1999). Therefore, the eddy amplitudes are also high in eastern Sri Lanka. Generally, compared with eddy amplitudes in the regions of strong western boundary currents (e.g., the Gulf Stream and Kuroshio extensions, Cui et al., 2017; Fu, 2009; Chelton et al., 2011b), the mean eddy amplitude in the Bay of Bengal is lower, and the number of large-amplitude eddies is fewer. This outcome occurs mainly because the Bay of Bengal has only a seasonal western boundary current EICC that changes with the monsoon, and its velocity and intensity are not as strong as other western boundary currents in the world’s oceans (Cheng et al., 2013).
The eddy radii are mainly scattered in 50–150 km for approximately 80% eddies in Bay of Bengal. This latitudinal dependence of eddy scale is evident from the geographical distributions of the mean radius of cyclonic and anticyclonic eddies (Fig. 3g and h), which are characterized as an essentially monotonic increase from approximately 60 km at 20° latitude to approximately 160 km in the near-equatorial regions (Fig. 3i). This latitudinal dependence of eddy scale is closely related to the Rossby radius of deformation (Chen et al., 2012; Lin et al., 2019). The eddy radii in the central of the bay are greater than that in the nearshore area. In terms of eddy polarity, the radii of anticyclonic eddies are larger than those of cyclones at most latitudes, especially in the western and central bay which similar to the result of Lin et al. (2019); while radii of cyclonic and anticyclonic eddies are not much different in the low-latitude regions (Fig. 3i). It shows that the eddy scale difference between the cyclones and anticyclones is limited to the interior of the bay.
The geographical distributions of the mean EKE (Figs 3j and k) are consistent with the eddy scale. Although the mean scale of anticyclonic eddies is larger than that of cyclones, the latter carries more energy than the former, which is consistent with their amplitudes. Clear high EKE distributions are shown in eastern Sri Lanka, especially for cyclonic eddies (Fig. 3j). The high-intensity eddies here are closely related to the southwest monsoon current invading the Bay of Bengal in summer. Generally, due to the southwest monsoon prevailing over the entire North Indian Ocean in summer, the low-latitude equatorial region produces a strong and fast-flowing southwest monsoon current, which enters the Bay of Bengal at a very fast flow rate. Under the influence of the southwest monsoon and the barrier of the Sri Lankan Islands, a significant negative wind stress vorticity occurs in the eastern seas, and a cyclonic eddy often appears here (Dandapat and Chakraborty, 2016). Then, the eddy often interacts with the southwest monsoon current, which makes the eddy rotational speed and eddy intensity significantly stronger (Patnaik et al., 2014). In terms of eddy rotational speed, its maximum value can reach nearly 1 m/s, which is equivalent to the flow velocity of the intrusion current. Therefore, high EKE distributions of cyclonic eddies appear in this area. After that, the cyclonic eddy gradually moves northwest to the western bay, and a compensatory anticyclonic eddy is generated here in autumn (Cui et al., 2016). However, the strength of the anticyclonic eddy is weakened, but its rotational speed is still fast and just slightly lower than that of the cyclone; therefore, there is also a significant EKE distribution of anticyclonic eddies in eastern Sri Lanka. As far as the interior bay, the high EKE is still concentrated in the northwestern bay; that is, eddies accompanying the western boundary current EICC often carry more ocean energy than the eddies in other regions of the bay. At the same time, the mean zonal EKE shows the difference between the cyclonic and anticyclonic eddies in the bay is not significant (Fig. 3l), and the two have obvious EKE difference only in the low-latitude region outside the bay.
Statistical characteristics and thermohaline properties of mesoscale eddies in the Bay of Bengal
- Received Date: 2020-05-08
- Accepted Date: 2020-06-22
- mesoscale eddy /
- Bay of Bengal /
- thermohaline structures /
- satellite altimetry /
- Argo data
Abstract: The statistical characteristics and vertical thermohaline properties of mesoscale eddies in the Bay of Bengal are studied from the view of satellite altimetry data and Argo profiles. Eddy propagation preferences in different lifetimes, eddy evolution process, and geographical distribution of eddy kinetic properties are analyzed in this area. Eddies exist principally in the western Bay of Bengal, and most of them propagate westward. There is a clear southward (equatorward) preference for eddies with long lifetimes, especially for cyclones. Moreover, the eddies in different areas of the bay show different north-southward preferences. Evolution of eddy kinetic properties with lifetime shows that eddies have the significant three-stage feature: the growth period in the former one-fifth lifetime, the stable period in the middle two-fifth to four-fifth lifetime, and the dying period in the last one-fifth lifetime. Large-amplitude and high-intensity eddies occur only in the relatively confined regions of highly unstable currents, such as the East Indian Coastal Current and eastern Sri Lanka. Based on Argo profile data and climatology data, the eddy synthesis method was used to construct three-dimensional temperature and salt structures of eddies in this area. The mean temperature anomaly is negative/positive to the cyclonic/anticyclonic eddies in the upper 300 dbar, and below this depth, the anomaly becomes weak. The salinity structures of positive anomalies inside cyclonic eddies and negative anomalies inside anticyclonic eddies in the Bay of Bengal are not consistent with other regions. Due to the special characteristics of the water mass in the bay, especially under the control of the low-salinity Bay of Bengal Water at the surface and the Indian Equatorial Water in the deep ocean, the salinity of seawater shows a monotonic increase with depth. For regional varieties of temperature and salinity structures, as the eddies move westward, the temperature anomaly induced by the eddies increases, the effecting depth of the eddies deepens, and the salinity structures are more affected by inflows. In the north-south direction, the salinity structures of the eddies are associated with the local water masses, which comprise low-salinity water in the northern bay due to the inflow of freshwater from rivers and salty water in the southern bay due to the invasion of Arabian Sea High-Salinity Water from the north Indian Ocean.
|Citation:||Wei Cui, Chaojie Zhou, Jie Zhang, Jungang Yang. Statistical characteristics and thermohaline properties of mesoscale eddies in the Bay of Bengal[J]. Acta Oceanologica Sinica. doi: 10.1007/s13131-021-1723-4|