Samanta Dhrubajyoti

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  • Article
    Coupled model biases breed spurious low‐frequency variability in the tropical Pacific Ocean
    (John Wiley & Sons, 2018-10-07) Samanta, Dhrubajyoti ; Karnauskas, Kristopher B. ; Goodkin, Nathalie F. ; Coats, Sloan ; Smerdon, Jason E. ; Zhang, Lei
    Coupled general circulation model (GCM) biases in the tropical Pacific are substantial, including a westward extended cold sea surface temperature (SST) bias linked to El Niño–Southern Oscillation (ENSO). Investigation of internal climate variability at centennial timescales using multicentury control integrations of 27 GCMs suggests that a Pacific Centennial Oscillation emerges in GCMs with too strong ENSO variability in the equatorial Pacific, including westward extended SST variability. Using a stochastic model of climate variability (Hasselmann type), we diagnose such centennial SST variance in the western equatorial Pacific. The consistency of a simple stochastic model with complex GCMs suggests that a previously defined Pacific Centennial Oscillation may be driven by biases in high‐frequency ENSO forcing in the western equatorial Pacific. A cautious evaluation of long‐term trends in the tropical Pacific from GCMs is necessary because significant trends in historical and future simulations are possible consequences of biases in simulated internal variability alone.
  • Article
    Coral-based proxy calibrations constrain ENSO-driven sea surface temperature and salinity gradients in the Western Pacific Warm Pool
    (Elsevier, 2020-10-01) Mohtar, Ahmad T. ; Hughen, Konrad A. ; Goodkin, Nathalie F. ; Streanga, Iulia-Madalina ; Ramos, Riovie ; Samanta, Dhrubajyoti ; Cervino, James M. ; Switzer, Adam D.
    Constraining past variability in ocean conditions in the Western Pacific Warm Pool (WPWP) and examining how it has been influenced by the El-Niño Southern Oscillation (ENSO) is critical to predicting how these systems may change in the future. To characterize the spatiotemporal variability of the WPWP and ENSO during the past three decades, we analyzed climate proxies using coral cores sampled from Porites spp. from Kosrae Island (KOS) and Woleai Atoll (WOL) in the Federated States of Micronesia. Coral skeleton samples drilled along the major growth axis were analyzed for oxygen isotopes (δ18Oc) and trace element ratios (Sr/Ca), used to reconstruct sea surface salinity and temperature (SSS and SST). Pseudocoral δ18O time series (δ18Opseudo) were calculated from gridded instrumental observations and compared to δ18Oc, followed by fine-tuning using coral Sr/Ca and gridded SST, to produce age models for each coral. The thermal component of δ18Oc was removed using Sr/Ca for SST, to derive δ18O of seawater (δ18Osw), a proxy for SSS. The Sr/Ca, and δ18Osw records were compared to instrumental SST and SSS to test their fidelity as regional climate recorders. We found both sites display significant Sr/Ca-SST calibrations at monthly and interannual (dry season, wet season, mean annual) timescales. At each site, δ18Osw also exhibited significant calibrations to SSS across the same timescales. The difference between normalized dry season SST (Sr/Ca) anomalies from KOS and WOL generates a zonal SST gradient (KOSWOLSST), capturing the east-west WPWP migration observed during ENSO events. Similarly, the average of normalized dry season δ18Osw anomalies from both sites produces an SSS index (KOSWOLSSS) reflecting the regional hydrological changes. Both proxy indices, KOSWOLSST and KOSWOLSSS, are significantly correlated to regional ENSO indices. These calibration results highlight the potential for extending the climate record, revealing spatial hydrological gradients within the WPWP and ENSO variability back to the end of the Little Ice Age.
  • Article
    Natural and anthropogenic forcing of multi-decadal to centennial scale variability of sea surface temperature in the South China Sea
    (American Geophysical Union, 2021-09-23) Goodkin, Nathalie F. ; Samanta, Dhrubajyoti ; Bolton, Annette ; Ong, Maria Rosabelle ; Phan, Kim Hoang ; Vo, Si Tuan ; Karnauskas, Kristopher B. ; Hughen, Konrad A.
    Four hundred years of reconstructed sea surface temperatures (SSTs) from a coral located off the coast of Vietnam show significant multi-decadal to centennial-scale variability in wet and dry seasons. Wet and dry season SST co-vary significantly at multi-decadal timescales, and the Interdecadal Pacific Oscillation (IPO) explains the majority of variability in both seasons. A newly reconstructed wet season IPO index was compared to other IPO reconstructions, showing significant long-term agreement with varying amplitude of negative IPO signals based on geographic location. Dry season SST also correlates to sea level pressure anomalies and the East Asian Winter Monsoon, although with an inverse relationship from established interannual behavior, as previously seen with an ocean circulation proxy from the same coral. Centennial-scale variability in wet and dry season SST shows 300 years of near simultaneous changes, with an abrupt decoupling of the records around 1900, after which the dry season continues a long-term cooling trend while the wet season remains almost constant. Climate model simulations indicate greenhouse gases as the largest contributor to the decoupling of the wet and dry season SSTs and demonstrate increased heat advection to the western South China Sea in the wet season, potentially disrupting the covariance in seasonal SST.
  • Article
    Enhanced monsoon-driven upwelling in Southeast Asia during the Little Ice Age
    (American Geophysical Union, 2023-04-08) Chen, Mengli ; Martin, Patrick ; Ren, Haojia ; Zhang, Run ; Samanta, Dhrubajyoti ; Chen, Yi-Chi ; Hughen, Konrad A. ; Phan, Kim Hoang ; Vo, Si Tuan ; Goodkin, Nathalie F.
    Climate change impacts ocean nutrient availability and will likely alter the marine food web. While climate models predict decreased average ocean productivity, the extent of these changes, especially in the marginal seas upon which large human populations depend, is not well understood. Here, we reconstructed changes in seawater phosphate concentration and nitrate source over the past 400 years, which reveals a more than 50% decline in residence time of seawater phosphate, and 8%?48% decline in subsurface nitrogen supply following the coldest period of Little Ice Age. Our data indicates a link between surface ocean nutrient supply and the East Asian Summer Monsoon strength in an economically important marginal sea. As climate models predict that the East Asian Summer monsoon will strengthen in the future, our study implies that surface ocean primary productivity may increase in the South China Sea, contrary to the predicted decrease in global average ocean productivity.