Highlights

Oscillations & Dipoles

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"The goal of forecasting is not to predict the future but to tell you what you need to know to take meaningful action in the present." - Paul Saffo

Earth's ocean and atmosphere interact in countless ways. A striking example is the El Niño Southern Oscillation (ENSO). "El Niño" is widely recognized but what is the "Southern Oscillation"? It's the coupled system where neither the ocean nor the atmosphere is clearly the dominant driving force.

ENSO is just one of many such oscillations that occur naturally over different times and regions. Each varies among three phases; for example, a neutral ENSO means normal conditions, while El Niño and La Niña are warming and cooling phases, respectively.

El Niño conditions were first documented in the year 1525. The Indian Ocean Dipole (IOD), however, has only been recognized for about two decades. Better understanding the IOD's impact on weather – including the monsoon of South Asia - is crucial. This is a challenge because ocean-atmosphere oscillations and dipoles are erratic in strength, timing, and notoriously difficult to predict.

Adding a pinch of salt improves el nino models

  • How are ENSO and the IOD related? »
    • There are big debates on what causes what and how they’re interrelated, but the phase of one can impact the phase of the other.
    • Can interact destructively or constructively to impact monsoon rainfall (negative IOD + El Niño vs positive IOD + La Niña). A positive IOD and a La Niña constructively increase monsoon rainfall.
    • Depends heavily on the timing and intensity of both.
  • Does the IOD affect the annual monsoons in the Indian Ocean region? Part A »
    • IOD phase impacts many things, including rainfall distribution over India and the timing of/intensity of coastal Kelvin wave propagation in the Bay of Bengal → freshwater fluxes into the southeastern Arabian Sea → monsoon onset.
    • Also impacts localized eddying in the Bay of Bengal and associated precipitation because of coastal Kelvin waves at the eastern boundary of the Bay of Bengal and Andaman Sea radiate low speed (westward) propagating Rossby waves that excite eddying in the Bay of Bengal.
    • Impacts where more precipitation happens (positive IOD → Arabian Sea and Indian subcontinent have more rain; negative IOD → Maritime Continent has more rain).
    • 2019 had a very strong monsoon. The previous very strong event occurred 25 years earlier... in 1994.
  • Does the IOD affect the annual monsoons in the Indian Ocean region? Part B »
  • How far do impacts of the IOD reach? »
  • Are we close to being to predict the IOD accurately? Does predicting the IOD feel urgent to you? »
    • The IOD is being predicted by some institutions, of course through models, and monitored consistently by agencies, such as the Australian Bureau of Meteorology. Japan Agency for Marine-Earth Science and Technology (JAMSTEC) is the first institution to predict IOD events.
  • How might climate change affect the IOD? »
    • Increased frequency of positive IOD events are expected as greenhouse warming increases (frequency projected to increase by factor of three) [Cai et al., 2014].
    • Circulation trends in the Indian Ocean (IO) are more favorable for positive IOD development, where these trends are associated with weaker Walker circulations in the Pacific and enhanced land-ocean temperature gradients in the IO.
    • Still, it’s difficult to attribute these things to climate change [Cai et al., 2009].
    • "Mean climate warming in (boreal winter) austral spring is expected to lead to stronger easterly winds just south of the Equator, faster warming of SSTs in the western IO compared to the eastern basin, and a shoaling equatorial thermocline in the eastern IO. The mean climate conditions that result from these changes more closely resemble a positive dipole state" – [Cai et al., 2013].
    • "We find that a mean state change – with weakening of both equatorial westerly winds and eastward oceanic currents in association with a faster warming in the western IO facilitates more frequent occurrences of wind and oceanic current reversal." [Cai et al., 2014].
    • Normally the IOD is strongest during September through November. In 2019, however, it came in May, much earlier than previous years. This positive IOD was also anomalously strong. Predicting IOD is important but also difficult, just like ENSO. Fortunately, we have good models for ENSO, but not many models are just for IOD.
  • What is your vision for the future in terms of better understanding the IOD? »
    • Better satellite observations that are higher spatial and temporal resolution. In our recent research, we’ve noted the substantial contribution of high frequency synoptic oscillations (3-7-day period) in monsoon interannual variability and in determining the timing of monsoon onset in the southeastern Arabian Sea.
    • In order to better understand these processes, we need observations that have both high spatial resolution and temporal resolution and for that data to be assimilated into models.
    • Presently, I am not aware of any operational models that currently assimilate satellite salinity data and anything other than climatological river runoff. The assimilation of satellite salinity data could really help to improve our ocean model simulations. This would help not just with the prediction of 3-7-day events, but also it should help with monsoon forecasting and hurricane forecasting, as well.
  • How much has data from NASA and ESA satellites improved your research? »
    • As Argo data on temperature and salinity profiles started increasing in the Indian Ocean and combining this data with altimetric data, I have gained lot of experience in data analysis and started looking into interannual variability and presently on the sub-seasonal scales and synoptic scale variability in the ocean.
    • All these studies underline the importance of sea surface salinity in the tropical Indian Ocean, particularly the Bay of Bengal, where the lowest surface salinity waters occupy the upper water column making the Bay of Bengal a negative water balance region (i.e., precipitation and river runoff dominate the evaporation). It is a highly stratified oceanic region with varied mixed layer dynamics, barrier layers, many tropical weather disturbances of all intensities, coastal Kelvin wave propagation, and a seasonal salinity exchange between the Bay of Bengal and Arabian Sea.
    • For our studies, we have used satellite sea surface salinity from SMOS, Aquarius, and SMAP, which has given us an unprecedented observational view of the salinity exchange between the Arabian Sea and Bay of Bengal, especially when combined with geostrophic currents derived from satellite altimetry.
  • About Heather »
  • About Subra »

Featured Publications

El Niño/Southern Oscillation (ENSO) has far reaching global climatic impacts and extending useful ENSO forecasts would have great societal benefit. However, one key variable that has yet to be fully exploited within coupled forecast systems is accurate estimation of near‐surface ocean salinity. Satellite sea surface salinity (SSS), combined with temperature, help to improve estimates of ocean density changes and associated near‐surface mixing. In this study, the authors assess the impact of satellite SSS observations for improving near‐surface dynamics within ocean reanalyses and how these initializations impact dynamical ENSO forecasts using NASA's coupled forecast system.

Reference

Hackert, E., Kovach, R.M., Molod, A., Vernieres, G., Borovikov, A., Marshak, J., and Chang, Y. (2020). Read the full paper.

Intraseasonal oscillations (ISOs) in the Indian Ocean play a significant role in determining the active (wet) and break (dry) cycles of the southwest monsoon rainfall. In this study, we use satellite‐derived precipitation, sea level anomalies, sea surface salinity, sea surface temperature, and surface winds to monitor the 30‐90‐day, 10‐20‐day, and 3‐7‐day ISOs, and how they influence local dynamics.

Reference

Roman‐Stork, H., Subrahmanyam, B., and Trott, C. (2020). Read the full paper.

As a dominant source of tropical variability, the Madden‐Julian oscillation (MJO) influences the ocean in many ways. One approach to observe the atmosphere‐ocean relationship is by examining sea surface salinity (SSS) due to direct freshening by MJO precipitation. The convectively enhanced (suppressed) phase of the MJO is associated with negative (positive) SSS anomalies that propagate eastward along the equatorial Indian and Pacific oceans. In this study, primary MJO events are identified, and their SSS signatures are compared for the first time across multiple satellite salinity products from 2010 to 2017.

Reference

Shoup, C.G., Subrahmanyam, B., and Roman-Stork, H.L. (2019). Read the full paper.