skip to main content

Title: Near-Surface Salinity Reveals the Oceanic Sources of Moisture for Australian Precipitation Through Atmospheric Moisture Transport
The long-term trend of sea surface salinity (SSS) reveals an intensification of the global hydrological cycle due to human-induced climate change. This study demonstrates that SSS variability can also be used as a measure of terrestrial precipitation on inter-seasonal to inter-annual time scales, and to locate the source of moisture. Seasonal composites during El Niño Southern Oscillation/Indian Ocean Dipole (ENSO/IOD) events are used to understand the variations of moisture transport and precipitation over Australia, and their association with SSS variability. As ENSO/IOD events evolve, patterns of positive or negative SSS anomaly emerge in the Indo-Pacific warm pool region and are accompanied by atmospheric moisture transport anomalies towards Australia. During co-occurring La Niña and negative-IOD events, salty anomalies around the maritime continent (north of Australia) indicate freshwater export and are associated with a significant moisture transport that converges over Australia to create anomalous wet conditions. In contrast, during co-occurring El Niño and positive IOD events, there is the moisture transport divergence anomaly over Australia and results in anomalous dry conditions. The relationship between SSS and atmospheric moisture transport also holds for pure ENSO/IOD events but varies in magnitude and spatial pattern. The significant pattern correlation between the moisture flux divergence and SSS anomaly during the ENSO/IOD events highlights the associated ocean-atmosphere coupling. A case study of the extreme hydroclimatic events of Australia (e.g. 2010-11 Brisbane flood) demonstrates that the changes in SSS occur before the peak of ENSO/IOD events. This raises the prospect that tracking of SSS variability could aid the prediction of Australian rainfall.  more » « less
Award ID(s):
Author(s) / Creator(s):
; ; ; ;
Date Published:
Journal Name:
Journal of Climate
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. null (Ed.)
    Abstract This study uses sea surface salinity (SSS) as an additional precursor for improving the prediction of summer [December–February (DJF)] rainfall over northeastern Australia. From a singular value decomposition between SSS of prior seasons and DJF rainfall, we note that SSS of the Indo-Pacific warm pool region [SSSP (150°E–165°W and 10°S–10°N) and SSSI (50°–95°E and 10°S–10°N)] covaries with Australian rainfall, particularly in the northeast region. Composite analysis that is based on high or low SSS events in the SSSP and SSSI regions is performed to understand the physical links between the SSS and the atmospheric moisture originating from the regions of anomalously high or low, respectively, SSS and precipitation over Australia. The composites show the signature of co-occurring La Niña and negative Indian Ocean dipole with anomalously wet conditions over Australia and conversely show the signature of co-occurring El Niño and positive Indian Ocean dipole with anomalously dry conditions there. During the high SSS events of the SSSP and SSSI regions, the convergence of incoming moisture flux results in anomalously wet conditions over Australia with a positive soil moisture anomaly. Conversely, during the low SSS events of the SSSP and SSSI regions, the divergence of incoming moisture flux results in anomalously dry conditions over Australia with a negative soil moisture anomaly. We show from the random-forest regression analysis that the local soil moisture, El Niño–Southern Oscillation (ENSO), and SSSP are the most important precursors for the northeast Australian rainfall whereas for the Brisbane region ENSO, SSSP, and the Indian Ocean dipole are the most important. The prediction of Australian rainfall using random-forest regression shows an improvement by including SSS from the prior season. This evidence suggests that sustained observations of SSS can improve the monitoring of the Australian regional hydrological cycle. 
    more » « less
  2. The South Pacific convergence zone (SPCZ) exhibits well-known spatial displacements in response to anomalous sea surface temperatures (SSTs) associated with the El Niño–Southern Oscillation (ENSO). Although dynamic and thermodynamic changes during ENSO events are consistent with observed SPCZ shifts, explanations for these displacements have been largely qualitative. This study applies a theoretical framework based on generalizing arguments about the relationship between the zonal-mean intertropical convergence zone (ITCZ) and atmospheric energy transport (AET) to 2D, permitting quantification of SPCZ displacements during ENSO. Using either resolved atmospheric energy fluxes or estimates of column-integrated moist energy sources, this framework predicts well the observed SPCZ shifts during ENSO, at least when anomalous ENSO-region SSTs are relatively small. In large-amplitude ENSO events, such as the 1997/98 El Niño, the framework breaks down because of the large change in SPCZ precipitation intensity. The AET framework permits decomposition of the ENSO forcing into various components, such as column radiative heating versus surface turbulent fluxes, and local versus remote contributions. Column energy source anomalies in the equatorial central and eastern Pacific dominate the SPCZ shift. Furthermore, although the radiative flux anomaly is larger than the surface turbulent flux anomaly in the SPCZ region, the radiative flux anomaly, which can be viewed as a feedback on the ENSO forcing, accounts for slightly less than half of SPCZ precipitation anomalies during ENSO. This study also introduces an idealized analytical model used to illustrate AET anomalies during ENSO and to obtain a scaling for the SPCZ response to an anomalous equatorial energy source.

    more » « less
  3. Abstract

    Climatic controls regulate the coupled natural and human systems in coastal Tanzania, where mangrove wetlands provide a wealth of ecosystem services to coastal communities. Previous research has explained the precipitation seasonality of eastern Africa in terms of the local monsoons. This research examines a wider range of hydroclimatic variables, including water vapour flux, evapotranspiration, runoff, and ocean salinity, and the sources of low‐frequency atmosphere–ocean variability that support mangrove productivity and associated ecosystem services. Results confirm previous work suggesting that the northeast monsoon (kaskazi) largely corresponds to the “short rains” of October–December and extends through February, while the southeast monsoon (kusi) corresponds to the “long rains” of March–May and the drier June–September. The Indian Ocean Dipole (IOD) and, to a lesser extent, El Niño–Southern Oscillation (ENSO) are important modulators not only of precipitation (as has been shown previously) but also of water vapour flux, evapotranspiration, runoff, and salinity variability. Duringkaskazi, positive (negative) hydroclimatic anomalies occur during positive (negative) IOD, with a stronger IOD influence occurring during its positive phase, when seasonal anomalies of precipitation, evapotranspiration, and runoff exceed +50, 25, and 100%, and nearby salinity decreases by 0.5 practical salinity units. Duringkusi, the contrast between the positive and negative IOD modes is subtler, and the pattern is dictated more by variability in “long rains” months than in the dry months. The coincidence of the positive IOD and El Niño amplify this hydroclimatic signal. Because previous work suggests the likelihood of increased tendency for positive IOD and increased moisture variability associated with El Niño events in the future, wetter conditions may accompany thekaskazi, with less change expected during thekusi. These results advance understanding of the key environmental drivers controlling mangrove productivity and wetland spatial distribution that provide ecosystem services essential to the well‐being of the human population.

    more » « less
  4. Abstract

    The longitudinal location of precipitation anomalies over the equatorial Pacific shows a distinctive feature with the westernmost location for La Niña, the easternmost location for eastern Pacific (EP) El Niño, and somewhere between for central Pacific (CP) El Niño, even though the center of the sea surface temperature anomaly (SSTA) for La Niña is located slightly east of that of CP El Niño. The mechanisms for such a precipitation diversity were investigated through idealized model simulations and moisture and moist static energy budget analyses. It is revealed that the boundary layer convergence anomalies associated with the precipitation diversity are mainly induced by underlying SSTA through the Lindzen–Nigam mechanism, that is, their longitudinal locations are mainly controlled by the meridional and zonal distributions of the ENSO SSTA. The westward shift of the precipitation anomaly center during La Niña relative to that during CP El Niño is primarily caused by the combined effects of nonlinear zonal moist enthalpy advection anomalies and the Lindzen–Nigam mechanism mentioned above. Such a zonal diversity is further enhanced by the “convection–cloud–longwave radiation” feedback, the SST-induced latent heat flux anomalies, and the advection of mean moist enthalpy by anomalous winds. This diversity in the longitudinal location of precipitation anomalies has contributions to the diversities in the longitudinal locations of anomalous Walker circulation and western North Pacific anomalous anticyclone/cyclone among the three types of ENSO.

    more » « less
  5. Abstract The Indian and Pacific Oceans surround the Maritime Continent (MC). Major modes of sea surface temperature variability in both oceans, including the Indian Ocean Dipole (IOD) and El Niño–Southern Oscillation (ENSO), can strongly affect precipitation on the MC. The prevalence of fires in the MC is closely associated with precipitation amount and terrestrial water storage in September and October. Precipitation and terrestrial water storage, which is a measurement of hydrological drought conditions, are significantly modulated by Indian Ocean Dipole (IOD) and El Niño events. We utilize long-term datasets to study the combined effects of ENSO and the IOD on MC precipitation during the past 100 years (1900–2019) and find that the reductions in MC precipitation and terrestrial water storage are more pronounced during years when El Niño and a positive phase of the IOD (pIOD) coincided. The combined negative effects are produced mainly through an enhanced reduction of upward motion over the MC. Coincident El Niño-pIOD events have occurred more frequently after 1965. However, climate models do not project a higher occurrence of coincident El Niño-pIOD events in a severely warming condition, implying that not the global warming but the natural variability might be the leading cause of this phenomenon. 
    more » « less