3.1. Regional Comparison of 10–20-Day Signals
During the southwest monsoon, prevailing surface winds over the Indian Ocean are westerly in the north and easterly in the south, with winds largely being southwesterly over India (
Figure 1). The strength of these winds varies substantially interannually, which impacts the strength of the monsoon and the rainfall. To determine how the 10–20-day mode in surface wind varies with monsoon strength (meaning the strength for a monsoon season), a strong monsoon (1994), normal monsoon (1995), and weak monsoon (2002) were compared, with monsoon strength determined by the criteria from the Indian Institute of Tropical Meteorology (IITM) [
16], where a strong (weak) monsoon is defined as having total rainfall that is 10% above (below) the long-term mean. These years were chosen for this case study based on the IITM climatology and criteria, as 1994 was the last strong monsoon year prior to 2019, 2002 was an anomalously weak year, and 1995 was normal. Impacts of the ENSO or Indian Ocean Dipole (IOD) phase were not considered for this study but warrant future investigation given their impact on equatorial wind stress and Kelvin wave propagation, among other processes.
Given that the active (flood) and break (drought) phases of the monsoon associated with the 10–20-day mode are characterized by mesoscale cyclones over the Bay of Bengal, it would be expected that this basin would present the strongest 10–20-day signal of the three basins and that this signal would be most prominent in both strong and weak monsoon years. Comparative analysis of the unfiltered surface wind time series and the bandpass-filtered time series would seem to suggest that this is not necessarily the case (
Figure 2). Though the biweekly signal in the Bay can explain up to 26% of surface wind variability during a normal monsoon year (R
2 = 26%; amplitude ratio = 0.1435;
Figure 2B), this variability explained is low during strong (R
2 = 12%; amplitude ratio = 0.1174;
Figure 2A) and weak (R
2 = 14%; amplitude ratio = 0.1425;
Figure 2C) monsoon years and is not the largest amplitude signal in the Indian Ocean as the SIO has a similar amplitude. Previous studies of the 10–20-day mode in the Bay suggested that wind associated with the northernmost cell in the north/central Bay (10–15°N, 85–90°E) were largely responsible for the majority of the oceanic response in the region, particularly regarding upwelling processes in the central Bay, supporting the importance of 10–20-day winds and rain in this region [
7], even if it is not the highest amplitude signal. As other atmospheric processes, such as the MJO and tropical cyclones, are extremely active in the Bay and control a larger portion of the precipitation in the basin, it is also unsurprising that the relative amplitude of the signal might be comparatively weak.
While not the basin where the signal is generated, the Arabian Sea is the demise location for many of the mesoscale systems that comprise the 10–20-day mode over India. It is therefore surprising that it has the lowest variability of any region in the greater Indian Ocean, with no significant signals apparent and the amount of variance explained never exceeding 5% (
Figure 2D,F) of the variability identified over a year. This is consistent with previous studies of 10–20-day rainfall, which found that the double cell had largely dissipated by the time it reached the Arabian Sea [
7,
8]. However, the amplitude of the 10–20-day signal is quite substantial during many parts of each of the shown years.
The surface winds in the equatorial Indian Ocean are reasonably well characterized by the 10–20-day mode of variability (
Figure 2G–I). Regardless of monsoon strength, the signal remains low amplitude (±1 m/s) with a wide range of variance explained, ranging from 5% to 20%, suggesting that, for this basin, other modes of variability may be more dominant, such as the MJO or Boreal Summer Intraseasonal Oscillation (BSISO) during different times of the year and different monsoon regimes [
8,
15,
17,
18,
19,
20,
21,
22,
23,
24,
25]. This is reasonably consistent with previous studies, which found that the equatorial Indian Ocean has a low 10–20-day signal, despite being the location of much of the double-cell structure [
5], suggesting that monsoon strength in this region is dominated by other processes, such as the equatorially trapped MJO. If the amplitude ratios of the unfiltered and filtered signals are considered, however, it can be seen that they are even higher than those found in the Bay of Bengal, suggesting that, while the wind variability may not be high in the equatorial Indian Ocean, the 10–20-day mode does still play a major role. As the 10–20-day mode only explains roughly 20% of monsoon variability overall [
26], amplitude ratios in the equatorial Indian Ocean of 0.1592–0.2126 for surface wind are not unreasonable.
Though the Bay of Bengal and equatorial Indian Ocean are where the majority of precipitation associated with the 10–20-day mode of variability has been noted to take place [
2,
3,
4], it is the southern Indian Ocean and southeastern Indian Ocean that are best described by the 10–20-day mode for winds (
Figure 2J–L). The winds in the southern Indian Ocean remain energetic most of the year, though they do experience a slight increase in magnitude during monsoon season. The weak (R
2 = 25%; amplitude ratio = 0.1622;
Figure 2L) and strong (R
2 = 31%; amplitude ratio = 0.2443;
Figure 2J) monsoon years have the highest variability explained. Most notable about the southern Indian Ocean surface winds, however, is not the magnitude but the clearly defined 10–20-day mode of variability apparent in them, even in the unfiltered winds. Of the five regions analyzed, the southeastern Indian Ocean has the highest amplitude 10–20-day bandpass-filtered winds, reflective of the strength of this 10–20-day mode in this region regardless of monsoon regime (
Figure 2M–O). The 10–20-day mode is high amplitude during all 3 years analyzed but especially so during the strong monsoon season (R
2 = 45%; amplitude ratio = 0.5120;
Figure 2M). During this strong monsoon, the 10–20-day wind clearly aligns with peaks in the unfiltered winds throughout the season.
These results, while surprising, are critically important to this study and have implications in a much broader context, for modeling and operational studies. Atmospheric circulations that are paired across the equator typically share the upward segment of the vertical circulation. The pair of cells can move zonally with each other. Alternatively, if one cell is stationary, as has been said to be the case [
3,
4] for the northern cell, then variability in that cell would likely depend on the location of the southern cell, with equatorial convection enhanced when the southern cell is in the same longitudinal position as the northern cell. In
Section 3.3, we demonstrate that this second example is the case with the circulation that causes the 10–20-day mode of variability, consistent with the findings of [
3,
4,
8]. The relative dominance of this mode of variability on surface winds in the southern and southeastern Indian Ocean, especially during a strong monsoon season, suggests that any model analysis that does not properly capture or take into account the 10–20-day mode of variability over these regions could be missing up to 31% of surface wind variability in the southern Indian (45% in the southeastern Indian Ocean), as well as inaccurately forecasting the 10–20-day contribution to active and break cycles of the monsoon. Testing this hypothesis requires a weather modeling study, which is outside the scope of this observation-based analysis.
3.2. Signal Propagation in the Southern Indian Ocean
The 10–20-day mode of variability as characterized by rainfall is known to be a westward propagating system of Rossby waves generated by convection in the western Pacific and Bay of Bengal with a meridional double-cell structure that moves northwest over India (
Figure 1), with a matched pair of demise regions in the Arabian Sea and southern Indian Ocean [
3,
4]. In this section, we will explore how well surface winds adhere to this model of signal propagation.
To this end, time–longitude Hovmöller diagrams were 10–20-day bandpass filtered and further directionally filtered with a two-dimensional fast Fourier transform to isolate westward (top panels) and eastward (bottom panels) propagation. The Hovmöller diagrams in the southern Indian Ocean (50–100°E, 5–25°S) for all three monsoon years clearly show the strong zonal 10–20-day signal during all monsoon years (
Figure 3). The westward propagation west of 75°E may be due to the double-cell structure and mesocyclones characteristic of this mode as they move to the northwest over India from the Bay of Bengal, though it would not account for the eastward propagation also apparent, particularly in the western half of the ocean. During all three regimes, there is not a clearly defined monsoon season in the western propagation, although the strongest signals occur during this season for the normal and weak monsoon years, especially in the eastern propagating signals. The southeastern Indian Ocean experiences a peak in surface winds during the boreal summer months and a relative decrease in winds during boreal winter, which may be reflective of this seasonality [
27].
Given that the 10–20-day mode is a westward propagating system, it is therefore curious that much of the signal propagation west of 75°E in the Indian Ocean would appear to be eastward propagating. It is then proposed that the eastward propagating signal of the 10–20-day mode of variability is associated with reflected westward propagating MRG waves in the ocean, as suggested previously by [
8] in relation to the southern cell of the 10–20-day mode, as the magnitude and structure of the signal is not inconsistent with this type of wave. This would seem to be supported by the apparent coupling of eastward and westward propagating signals in the 10–20-day mode, as characterized by [
8], who found that patterns of sea level anomalies associated with the southern cell of the double-cell structure are eastward propagating and reflective of an MRG wave [
8,
28,
29].
Another possible explanation, which may not be mutually exclusive, follows the lifecycle of the 10–20-day mode set forth by [
5]. In their model for development, the southernmost cell of the double-cell structure originates as a convective anomaly in the western Indian Ocean [
5]. This convective anomaly then propagates east until it couples with the northern cell and moves westward. As the Arabian Sea is the demise location for many of these double-cell events, that would account for the westward propagation that often follows an eastward event. It is therefore plausible that the east/west pairings of signals shown here is a combination of multiple signals, both in the form of MRG waves and these convective anomalies moving back and forth across India.
To further assess the viability of this MRG wave theory, Hovmöller diagrams of the meridional component of the wind were created in the equatorial Indian Ocean (
Figure 4), the zonal propagation of which have been shown to be characteristic of MRG waves [
29]. The westward propagation in equatorial meridional winds has higher amplitude than the eastward propagation across all three monsoon regimes, although eastward propagation is still apparent, with high-amplitude eastward propagation most often being coupled with strong westward signals. These results strongly confirm the presence of MRG waves associated with the 10–20-day mode, but given the infrequency of strong eastward signals, it is likely that any reflection as previously mentioned is rare and another mechanism is chiefly responsible for the strong eastward propagation observed in the southern Indian Ocean (
Figure 3). Further research is necessary to identify the precise source of these eastward signals, and while the identified explanations are two possible mechanisms that may explain some of the variability, a more thorough study beyond the scope of this case study is required.
3.3. Composite Analysis
Given the results of previous sections, composites were constructed based on a time series of the southeastern Indian Ocean (80–100°E, 5–25°S), which was found to have the highest fraction of variance explained by the 10–20-day mode during the southwest monsoon season (R
2 = 45%) in the region (
Figure 2). It was therefore anticipated that the majority of activity seen in the composites would occur in the southeastern Indian Ocean. In contrast, previous studies [
2,
4,
30] suggest that most variability would occur in the Bay of Bengal region given that this is the location of the northern cell. In viewing the composite breakdown of the 10–20-day mode identified (
Section 2.2), the signal that is most apparent is in the southeastern Indian Ocean, but there are also prominent signals in the Bay of Bengal and Arabian Sea (
Figure 5). We will show that the variability in the northern cell is in phase with the location of the southern cell.
Little appears to change between Day −7 and Day −5. During these three days, there is a modest positive wind anomaly in the southern Indian Ocean centered around 60°E off the coast of Madagascar and a negative wind anomaly between the equator and 5°N. On Day −4, however, there is a substantial increase in the positive wind anomalies at 15°S between 55°E and 85°E and in the eastern equatorial Indian Ocean. On Day −3, a positive wind anomaly emerges in the Bay of Bengal, which likely accompanies the arrival of the northern cell into the Bay. Between Days −3 and Day −1, both of these wind anomalies continue to grow, becoming more clearly defined, and with the southern cell shifting eastward several degrees. At Day 0—the peak of the composite curve—the positive anomalies have their greatest values. After Day 0, the wind anomalies begin to rapidly weaken. By Day +4, the positive anomalies have lost most of their defined structure and the energy begins to disperse westward towards Madagascar around the Mascarene High for the southern cell and the Arabian Sea for the northern cell. This southern progression of phases has the overall effect of having a large positive wind anomaly at 15°S slosh back and forth across the basin, taking roughly seven days to cross each way and reaching its peak magnitude in the eastern portion of the basin. In contrast, the northern cell is largely confined to the Bay of Bengal and Arabian Sea, as seen in previous studies [
2,
3,
4]. This northern cell drives upwelling in the central Bay, leading to a salinification and net cooling of the Bay by Day +4 [
7]. However, the variability in the northern cell is linked to the position of the southern cell, with the greatest strength when the southern cell enhances equatorial convection in the northern cell.
The emergence of the signal at Day −4 is likely due to convective anomalies emerging out of the western Pacific that trigger a Rossby wave response in both hemispheres [
5,
6,
7,
8]. Given the longitudes at which it forms and then reverses direction, however, it would seem to almost perfectly follow the path of the southernmost cell of the convective double-cell structure characteristic of this mode [
3,
5,
6,
7,
8]. The southernmost cell has been shown to be triggered around 5°S by an MRG wave, as seen in the propagation of sea level anomalies in the eastern equatorial Indian Ocean [
8]. The precipitation associated with the vortex and the convective cell are not perfectly aligned, with the convection having a greater lateral distribution which stretches much deeper into the southern hemisphere [
4,
5]. This would suggest that the observed wind anomaly is reflective of the convective cell which is centered farther south than the precipitation associated with the vortex [
3,
4,
5,
8].
Though the complete convective double-cell structure itself is not as readily apparent in surface winds as it is in OLR or precipitation from the Tropical Rainfall Measurement Mission (TRMM), it is suggested by the positive wind anomalies, which follow a similar pattern to the OLR anomalies and precipitation of past studies [
4,
5,
7,
8]. The filtered surface wind anomalies are more fluid and encompass a larger area than the OLR anomalies. This is likely due to the nature of surface winds compared to precipitation or OLR, namely that OLR and precipitation have distinct edges that define them from the clearer air around them, whereas surface winds gridded on a 25 km map lack this definition. While OLR and precipitation are associated with patterns of pressure, surface winds instead follow the pressure gradient, which results in the observed differences seen between the distribution of these variables. OLR anomalies are not entirely consistent in past studies with regards to the location of the northern cell or anomalies to the direct east and west of India, but both patterns are reflected in the filtered surface winds, suggesting that different methods of composite creation may highlight local centers of convection or vorticity, both of which are important for our understanding of the dynamics associated with this mode. The discrepancies between all three studies can be largely attributed to data used and the manner in which the composites were created [
4,
5,
7,
8]. Additionally, OLR anomalies can be seen over land, whereas CCMP surface winds cannot, and thus the seemingly isolated patterns on either side of India may be more reflective of a greater anomaly encompassing most of the area, such as the northernmost cell (
Figure 1).
The increased activity in the Arabian Sea, particularly after Day +1, is likely reflective of a dynamic connection to the formation and dissipation of the southern cell and its associated energy. As winds in the Arabian Sea peak in response to the winds in the southern cell circulation decreasing, it is possible that the energy dissipated by the demise of the southern cell is transferred to the Arabian Sea, causing this increase in activity. Previous studies have shown consistent OLR and precipitation anomalies over the Arabian Sea throughout, so it is likely that the consistent winds over the Arabian Sea seen in our composites are, at least in part, a product of the weather patterns responsible for the precipitation and OLR in this region (
Figure 5) [
5].