Hosmay Lopez

Decadal Modulations of Interhemispheric Global Atmospheric Circulations and Monsoons by the South Atlantic Meridional Overturning Circulation

This study presents a physical mechanism on how low-frequency variability of the South Atlantic meridional heat transport (SAMHT) may influence decadal variability of atmospheric circulation. A multicentury simulationofacoupledgeneralcirculationmodelisusedasbasisfor theanalysis.Thehighlightofthefindings herein is that multidecadal variability of SAMHT plays a key role in modulating global atmospheric circulation via its influence on interhemispheric redistributions of momentum, heat, and moisture. Weaker SAMHT at 308S produces anomalous ocean heat divergence over the South Atlantic, resulting in negative ocean heat content anomalies about 15‐20 years later. This forces a thermally direct anomalous interhemispheric Hadley circulation, transporting anomalous atmospheric heat from the Northern Hemisphere (NH) to the Southern Hemisphere (SH) and moisture from the SH to the NH, thereby modulating global monsoons. Further analysis shows that anomalous atmospheric eddies transport heat northward in both hemispheres, producing eddy heat flux convergence (divergence) in the NH (SH) around 158‐308, reinforcing the anomalous Hadley circulation. The effectofeddies onthe NH(SH) polewardof308 depicts heat flux divergence(convergence), whichmust be balanced by sinking (rising) motion, consistent with a poleward (equatorward) displacement of the jet stream. This study illustrates that decadal variations of SAMHT could modulate the strength of global monsoons with 15‐20 years of lead time, suggesting that SAMHT is a potential predictor of global monsoon variability. A similar mechanistic link exists between the North Atlantic meridional heat transport (NAMHT) at 308 Na nd global monsoons.

Remote influence of Interdecadal Pacific Oscillation on the South Atlantic meridional overturning circulation variability

This study explores potential factors that may influence decadal variability of the South Atlantic meridional overturning circulation (SAMOC) by using observational data as well as surface-forced ocean model runs and a fully coupled climate model run. Here we show that SAMOC is strongly correlated with the leading mode of sea surface height (SSH) variability in the South Atlantic Ocean, which displays a meridional dipole between north and south of 20°S. A significant portion (~45%) of the South Atlantic SSH dipole variability is remotely modulated by the Interdecadal Pacific Oscillation (IPO). Further analysis shows that anomalous tropical Pacific convection associated with the IPO forces robust stationary Rossby wave patterns, modulating the wind stress curl over the South Atlantic Ocean. A positive (negative) phase IPO increases (decreases) the westerlies over the South Atlantic, which increases (decreases) the strength of the subtropical gyre in the South Atlantic and thus the SAMOC.

On the Fragile Relationship Between El Niño and California Rainfall

The failed influence of the 2015–2016 El Niño on California rainfall has renewed interest in the relationship between El Niño and U.S. rainfall variability. Here we perform statistical data analyses and simple model experiments to show that sufficiently warm and persistent sea surface temperature anomalies (SSTAs) in the far eastern equatorial Pacific are required to excite an anomalous cyclone in the North Pacific that extends to the east across the U.S. West Coast and thus increases rainfall over California. Among the four most frequently recurring El Niño patterns considered in this study, only the persistent El Niño, which is often characterized by the warm SSTAs in the far eastern equatorial Pacific persisting throughout the winter and spring, is linked to such extratropical teleconnection patterns and significantly increased rainfall over the entire state of California. During the last 69 years, only three of the 25 El Niño events (i.e., 1957–1958, 1982–1983, and 1997–1998) are clearly identified as the persistent El Niño. In addition, the monthly rainfall variance explained by El Niño is less than half that caused by internal variability during the 25 El Niño. Therefore, the rarity of persistent El Niño events combined with the large influence of internal variability effectively explains the fragile relationship between El Niño and California rainfall.

Investigating the seasonal predictability of significant wave height in the West Pacific and Indian Oceans

This study investigates seasonal prediction skill of significant wave height (SWH) in the West Pacific and Indian Oceans. We forced the WAVEWATCH III model with 10m winds from the National Centers for Environmental Prediction Reanalysis-2 and from the Community Climate System Model version 4 North American Multi-Model Ensemble retrospective forecasts for the period of January 1979 to December 2013. Results indicate potential for predicting SWH with several months lead time during boreal summers after the warm phase of El Niño–Southern Oscillation (ENSO) measured by deterministic and probabilistic skill scores in the Northwest Pacific and Bay of Bengal. During these summers, SWH is smaller than normal due to reduced atmospheric synoptic activity associated with an anomalously anticyclone in the western Pacific, leading to larger signal-to-noise ratio in the 10m winds, hence increasing SWH prediction skill. It is shown that ENSO has a nonlinear influence on the number of extremely large SWH events, with reduced number of extreme occurrences during boreal summers after the warm phase of ENSO.

A reconstructed South Atlantic Meridional Overturning Circulation time series since 1870

This study reconstructs a century-long South Atlantic Meridional Overturning Circulation (SAMOC) index. The reconstruction is possible due to its covariability with sea surface temperature (SST). A singular value decomposition (SVD) method is applied to the correlation matrix of SST and SAMOC. The SVD is performed on the trained period (1993present) for which Expendable Bathythermographs (XBT) and satellite altimetry observations are available. The joint modes obtained are used in the reconstruction of a monthly SAMOC timeseries from 1870 to present. The reconstructed index is highly correlated to the observational-based SAMOC timeseries during the trained period and provides a long historical estimate. It is shown that the Interdecadal Pacific Oscillation (IPO) is the leading mode of SAMOC-SST covariability, explaining ~85% with the Atlantic Niño accounting for

Wind-driven ocean dynamics impact on the contrasting sea-ice trends around West Antarctica

Since late 1978, Antarctic sea-ice extent in the East Pacific has retreated persistently over the Amundsen and Bellingshausen Seas in warm seasons, but expanded over the Ross and Amundsen Seas in cold seasons, while almost opposite seasonal trends have occurred in the Atlantic over the Weddell Sea. By using a surface-forced ocean and sea-ice coupled model, we show that regional wind-driven ocean dynamics played a key role in driving these trends. In the East Pacific, the strengthening Southern Hemisphere (SH) westerlies in the region enhanced the Ekman upwelling of warm upper Circumpolar Deep Water and increased the northward Ekman transport of cold Antarctic surface water. The associated surface ocean warming south of 68°S and the cooling north of 68°S directly contributed to the retreat of sea ice in warm seasons and the expansion in cold seasons, respectively. In the Atlantic, the poleward shifting SH westerlies in the region strengthened the northern branch of the Weddell Gyre, which in turn increased the meridional thermal gradient across it as constrained by the thermal wind balance. Ocean heat budget analysis further suggests that the strengthened northern branch of the Weddell Gyre acted as a barrier against the poleward ocean heat transport, and thus produced anomalous heat divergence within the Weddell Gyre and anomalous heat convergence north of the gyre. The associated cooling within the Weddell Gyre and the warming north of the gyre contributed to the expansion of sea ice in warm seasons and the retreat in cold seasons, respectively.

Early emergence of anthropogenically forced heat waves in the western United States and Great Lakes

Climate projections for the twenty-first century suggest an increase in the occurrence of heat waves. However, the time at which externally forced signals of anthropogenic climate change (ACC) emerge against background natural variability (time of emergence (ToE)) has been challenging to quantify, which makes future heat-wave projections uncertain. Here we combine observations and model simulations under present and future forcing to assess how internal variability and ACC modulate US heat waves. We show that ACC dominates heat-wave occurrence over the western United States and Great Lakes regions, with ToE that occurred as early as the 2020s and 2030s, respectively. In contrast, internal variability governs heat waves in the northern and southern Great Plains, where ToE occurs in the 2050s and 2070s; this later ToE is believed to be a result of a projected increase in circulation variability, namely the Great Plain low-level jet. Thus, greater mitigation and adaptation efforts are needed in the Great Lakes and western United States regions.Heat waves have become increasingly frequent in the United States, but their occurrence is largely linked to natural variability. Model simulations reveal anthropogenically forced signals will first emerge in the western United States and Great Lakes regions by ~2030.