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The 1960s was a remarkable decade for research in tropical meteorology. Tropical climatology was already reasonably understood, but little was known of its variability or that of daily tropical weather. Regularly sampled data and access to computers to process data became more readily available. The excitement of looking at these data, which no one else had studied before, must have been something like that of polar explorers in the early part of the century who made their way to places no one had ever been before. The decade opened with descriptions of the remarkable Quasibiennial Oscillation (QBO) showing that neither the formally identified “Krakatoa Easterlies” nor the “Berson Westerlies” were steady features of the equatorial stratosphere (). By the mid-1960s a theory taylored specifically to waves in the equatorial region was published, and soon after some of them were observed. These were, arguably, the first identifications of large-scale waves in the atmosphere predicted by theory. By the end of the decade the tropical atmosphere was a topic of research given similar attention to that of the mid-latitudes.

As the word “monsoon” (derived from an Arabic word meaning seasons) indicates, the South Asian (SA) summer monsoon is part of an annually reversing wind system (Figure 2.1(b, e) (); ()). The winds at low levels during the summer monsoon season are characterized by the strongest westerlies anywhere at 850 hPa over the Arabian Sea, known as the low-level westerly jet (LLJ) (Figure 2.1e), and a large-scale cyclonic vorticity extending from the north Bay of Bengal (BoB) to western India known as the “monsoon trough” (Figure 2.1(e) ()). The easterly jet (Figure 2.1(f)) centered around 5N and the Tibetan anticyclone centered around 30N are important features of upper level winds over the monsoon region during northern summer. Millions of inhabitants of the region, however, attach much greater importance to the associated seasonal changes of rainfall. Wet summers and dry winters (Figure 2.1 (a, d)) associated with the seasonal changes of low-level winds are crucial for agricultural production and the economy of the region. The monsoon, or the seasonal changes of winds and rainfall, in the region could be interpreted as a result of northward seasonal migration of the east-west oriented precipitation belt (Tropical Convergence Zone, TCZ) from southern hemisphere in winter to northern hemisphere in summer (). The largest northward excursion of the rain belt takes place over the Indian monsoon region where it moves from a mean position of about 5S in winter (Figure 2.1 (a)) to about 20N in northern summer (Figure 2.1(d)) ().

The intraseasonal oscillation (ISO) is one of the major systems affecting the summer monsoon system in East Asia and the western North Pacific (EA/WNP). This has become known to the scientific community since the late 1970s and early 1980s. Studies (e.g., ; ; ; ) reported the prominent northward ISO propagation at both 10–20-day and 30–60-day periods in the Asian summer monsoon region. The passage of these intraseasonal fluctuations tended to be in phase with the onsets (i.e., beginning of wet phases) and breaks (i.e., beginning of dry phases) of the Indian summer monsoon. It was noted that northward movement also tended to occur simultaneously in EA/WNP (e.g., ). Other intraseasonal features in EA/WNP were also documented. For example, Murakami (1980) found 20–30-day perturbations propagating westward along 10°N-20°N and northward over the South China Sea.

Rains have strong socio-economic impact for the 850 million inhabitants of the American continents. Both continents depend on rainfall to sustain agriculture, hydroelectric power, and to maintain their waterways. Rainfall over Pan-America has large interannual variability (IAV) and intraseasonal variability (ISV). In the interannual band, El Nino Southern Oscillation (ENSO) has strong impact on total seasonal rainfall (, ) over the region, while the occurrence of extreme rainfall episodes is more likely modulated by intraseasonal oscillations (ISOs). Persistence of atmospheric patterns during episodes of strong intraseasonal events raises expectations of converting this information into predictability enhancement beyond the current limitation of about one week for weather forecasts. This would be of great value to optimize crop management, particularly in South America, where regional economies are largely based on agriculture and livestock.

Like its northern hemisphere counterparts (e.g., Asian monsoons of Chapters 2 and 3), the region of northern Australia and nearby longitudes, and the area immediately to its north (primarily within Indonesia), experience a marked seasonal cycle in winds and precipitation characteristic of a monsoon (e.g., ; ; ). At lower tropospheric levels, the mean winds shift from being easterly in austral winter, with correspondingly small rain totals, to westerly in summer, with much enhanced cumulonimbus convection and rainfall (e.g., Figure 5.1). This monsoonal character of the region has long been recognized. Indeed, for both northern Australia and Indonesia, reference to this nature dates back at least as far as the early 19th century.

There is a very wide variety of intraseasonal variability (ISV) in the oceans, due to many different processes beyond forcing by tropical intraseasonal winds and heat fluxes. The main focus of this chapter, however, is on the upper ocean response to the tropical atmospheric ISV that is discussed in the other chapters of this book and is most germane in this context. The prominent oceanic ISV signatures generated by other mechanisms (largely intrinsic to the ocean), and those found in other regions are briefly reviewed in Section 6.7.

Air—sea interaction associated with tropical intraseasonal variability (ISV) and, particularly, the Madden—Julian Oscillation (MJO) is of interest for three reasons. First, variations of the air—sea fluxes of heat and moisture may be fundamental to mechanisms of tropical ISV. For instance, air—sea interaction may promote the slow eastward propagation of the MJO and its northward propagation in the Indian summer monsoon. Besides playing a critical role for the interplay between convection and dynamics, surface fluxes of heat, moisture, and momentum drive sea surface temperature (SST) perturbations that may feedback to the surface fluxes and ultimately to the atmospheric dynamics, thus, for instance, contributing to the growth of the MJO. Second, the episodic variations of surface momentum, heat, and freshwater fluxes driven by atmospheric ISV may play a role in the maintenance and low-frequency variability of the warm pool in the tropical Indian and Pacific Oceans. For example, the MJO induces transports in the equatorial west Pacific that act in the mean to remove about the same amount of heat from the warm pool as is provided by the mean surface heat flux (). From the opposite perspective of the ocean driving the atmosphere, interannual variations of SST in the warm pool may also drive interannual variations in MJO activity, which may bear on the ability to predict seasonal variations of MJO activity. Third, the MJO forces surface currents that drive SST variations at the eastern edge of the warm pool (). Kelvin waves are also efficiently excited by the MJO (), which radiate into the eastern Pacific where they can perturb the SST (e.g., ; ; ). These intraseasonal SST variations may lead to a rectified coupled-response, which plays a role in the evolution of the El Niño Southern Oscillation (ENSO) (e.g., ; ).

While other chapters of this book describe the meteorological intraseasonal variability (ISV) phenomena in the atmosphere-ocean system, and examine the possible causes of the ISV or the dynamic interactions between the meteorological components that are involved, in this chapter we will study certain global effects that relate to the associated with the ISV, which for the most part occur in the atmosphere-ocean system. In particular, we will discuss the angular momentum variability of the atmosphere and its influences on Earth’s rotation; we also visit the associated mass-induced gravity variations. The observations of these can help improve our understanding of, and our modeling capability for, atmospheric-oceanic circulations in general, including those related to the ISV.

The Madden-Julian Oscillation (MJO) is the most pronounced signal in tropical intraseasonal (20–90 days) variability, and the El Niño Southern Oscillation (ENSO) is the most dominant interannual climate phenomenon in the tropical ocean-atmosphere system. Both MJO and ENSO involve major shifts in tropical convection, large-scale circulation, and weather patterns around the world. The hypothesis that the MJO and ENSO may be intrinsically linked was first proposed in the mid-1980s by Lau (1985a, b). Subsequently, many observational and modeling studies have appeared in the literature debating the merits of the hypothesis. Today, while the MJO-ENSO connection is still a topic of active research (), knowledge gained from better understanding of the causes and evolution of the MJO and ENSO has been incorporated into long-range weather forecasting and climate prediction schemes, resulting in improved forecasts not only in the tropics but also in many extra-tropical regions. A key factor in the prediction improvement is the recognition that the MJO and ENSO events do not act independently, but may interact with each other to provide the long-term pre-conditions, and the short-term fine tuning needed for better skill in long-range (> months) predictions. How can the MJO and ENSO — two phenomena with widely separate timescales, be physically linked and interact with each other? This critical question and related issues will be addressed in this chapter.

In the last two decades, many studies have been devoted to developing a theoretical understanding of the tropical intraseasonal oscillation (TISO) in order to improve the models and their predictions of these disturbances. Progress in modeling and predicting the ISO will only happen if the mechanisms underlying its complex interactions and fundamental dynamics are more fully understood. Significant progress in theoretical understanding has been achieved, although some aspects of the theories remain disputable and incomplete.