Intraseasonal oscillations and interannual variability of the Indian summer monsoon

Abstract

Several modeling studies show that the predictability of the seasonal mean Indian summer monsoon is limited due to a significant fraction of the interannual variability of the seasonal mean being governed by internal chaotic dynamics. What causes the internal low-frequency variations of the Indian summer monsoon? One possible candidate is the monsoon intraseasonal oscillations (ISOs). The Indian summer monsoon has vigorous intraseasonal oscillations in the form of 'active' and 'weak' (or 'break') spells of monsoon rainfall within the summer monsoon season. These 'active' and 'break' spells of the monsoon are associated with fluctuations of the tropical convergence zone. Temporally, ISOs of the Indian summer monsoon represent two preferred bands of periods: one between 10 and 20 days, and the other between 30 and 60 days. As the separation between the dominant ISO periods and the season is not large, the statistics of the ISOs could, in principle, influence the seasonal mean monsoon and its interannual variability. To the extent that the ISOs are intrinsically chaotic and unpredictable, the predictability of the Indian summer monsoon would depend on the relative contribution of the ISOs to the seasonal mean compared to the more predictable externally forced component.

Therefore, it is of great importance to establish (a) whether there exists a physical basis for monsoon ISOs to influence the seasonal mean, (b) Even if there exists a physical basis for the ISOs to influence the seasonal mean, is there empirical evidence of an association between some statistics of the ISOs and interannual variability of the Indian summer monsoon? (c) If such an association between monsoon ISOs and the seasonal mean monsoon exists, it would be desirable to make a quantitative estimate of the extent to which ISOs influence the seasonal mean and its interannual variability. The primary objectives of this study are to address these three issues using sufficiently long homogeneous daily circulation and convection data. Although spatial and temporal structures of the monsoon ISOs have been examined extensively, the relationship between ISOs and interannual variability has received little attention in the past. The existing literature on the subject is critically reviewed in Chapter 1.

Abstract

In an attempt to establish the physical basis for the ISOs to influence the seasonal mean, we first examine the similarity between the spatial structure of the ISOs and the seasonal mean. The large-scale nature of the Indian summer monsoon ISOs and the relationship between circulation and convection on this time scale are investigated using 42 years (1956-1997) of daily circulation data from NCEP/NCAR reanalysis and satellite-derived outgoing longwave radiation data for the period 1974-1997. Traditionally, 'active' and 'break' conditions or the dry and wet spells of the monsoon ISO are defined based on continental precipitation. Arguing that the dry and wet spells are part of large-scale fluctuations associated with the ISO, a circulation-based criterion is devised to define 'active' and 'break' monsoon conditions using zonal winds at 850 hPa over the Bay of Bengal. Although the ISOs vary in intensity and period, it is shown that the underlying spatial structure of a typical ISO cycle in circulation and convection is invariant over the years and is constructed using a composite technique. Typical ISOs have large-scale horizontal structure similar to the seasonal mean and intensify (weaken) the mean flow during its 'active' ('break') phase. A typical 'active' ('break') phase is also associated with enhanced (decreased) cyclonic low-level vorticity and convection, and anomalous upward (downward) motion in the northern position of the tropical convergence zone (TCZ), and decreased (increased) convection and anomalous downward (upward) motion in the southern position of the TCZ. The cycle evolves with a northward propagation of the TCZ and convection from the southern to the northern position of the TCZ. Thus, the ISOs result in spinning up (or spinning down) of the large-scale mean monsoon circulation in its extreme phases. (Chapter 2)

A physical basis for ISOs to influence the seasonal mean and its interannual variability is established when it is shown that the intraseasonal and interannual variations are governed by a common mode of spatial variability. The spatial pattern of the standard deviation of intraseasonal and interannual variability of low-level vorticity is shown to be similar. The spatial pattern of the dominant mode of ISO variability of the low-level winds is also shown to be similar to that of the interannual variability of the seasonal mean winds. The similarity between the spatial patterns of the two variabilities indicates that a higher frequency of occurrence of 'active' ('break') conditions would result in a 'stronger' ('weaker') than normal seasonal mean. This possibility is tested by calculating the probability density function (PDF) of the ISO activity in the low-level vorticity represented by the two dominant empirical orthogonal functions (EOFs). The PDF estimates for 'strong' monsoon years and 'weak' monsoon years are shown to be asymmetric in both cases. It is seen that the 'strong' ('weak') monsoon years are associated with a higher probability of occurrence of 'active' ('break') conditions. This result is further supported by calculation of the PDF of ISO activity from combined vorticity and outgoing longwave radiation. This result indicates that the frequency of the intraseasonal pattern determines the seasonal mean. As the ISOs are essentially chaotic, it raises an important question about the predictability of the Indian summer monsoon. (Chapter 3)

Having shown that the ISOs can influence the seasonal mean and its interannual variability, the next objective is to make quantitative estimates of the potential predictability of the monsoon climate. A measure of potential predictability of the monthly and seasonal means in a place could be obtained from the ratio of variances associated with the 'external' to the 'internal' components. A method of separating the 'external' component arising from contributions from slowly varying boundary forcing from the 'internal' components (e.g., intraseasonal oscillations) that determines the potential predictability of the monthly mean tropical climate is proposed. Based on 33 years of daily low-level wind observations and 24 years of satellite observations of outgoing longwave radiation, we show that the Indian monsoon climate is marginally predictable on monthly time scales, as the contribution from the boundary forcing in this region is comparable to that from the internal dynamics. It is further shown that excluding the Indian monsoon region, the predictable region is larger, and predictability is higher in the tropics during northern summer. Even though the boundary-forced variance is large during northern winter, the predictable region is smaller as the internal variance is larger and covers a larger region during northern winter due to stronger intraseasonal activity. It is also shown that most of the internal low-frequency variability in the Indian summer monsoon region arises from the ISOs. (Chapter 4)

An estimate of potential predictability for the Northern Hemisphere summer and winter seasons in the tropics has also been made using an established method of estimating 'climate noise'. Even on seasonal mean time scales, we show that the Indian monsoon climate is only marginally predictable as the contribution of the boundary forcing in this region is relatively low and that of the internal dynamics is relatively large. (Chapter 4)

While the monsoon ISOs seem to lead to a decrease in the predictability of monthly or seasonal mean monsoon climate, it is possible that the same ISOs lead to extended-range prediction of spells of synoptic activity. We recall that the seasonal mean monsoon is strengthened in one phase of the ISOs (active phase) while it is weakened in another ('break') phase of the monsoon. The main rain-bearing system during the monsoon season is the Low Pressure Systems (LPS), consisting of lows and depressions. Since the genesis of the LPS depends on the horizontal shear and low-level vorticity, it is possible that more LPS form in the active phase relative to the break phase. In other words, the large-scale circulation associated with the ISOs could modulate the frequency of genesis of LPS. We examined how the LPS are modulated by the intraseasonal oscillations. Using more than 40 years of LPS genesis statistics and daily circulation data, here we show that the dry and wet spells are the result of clustering of lows and depressions caused by modulation of the large-scale monsoon flow by the intraseasonal oscillations. The slow evolution of the ISOs may permit extended-range prediction of the ISO phases and, through them, dry and wet spells of the Indian summer monsoon (Chapter 5). Major results and outstanding issues are discussed in Chapter 6.