Shenfu Dong

Multidecadal Covariability of North Atlantic Sea Surface Temperature, African Dust, Sahel Rainfall, and Atlantic Hurricanes

AbstractMost studies of African dust and North Atlantic climate have been limited to the short time period since the satellite era (1980 onward), precluding the examination of their relationship on longer time scales. Here a new dust dataset with the record extending back to the 1950s is used to show a multidecadal covariability of North Atlantic SST and aerosol, Sahel rainfall, and Atlantic hurricanes. When the North Atlantic Ocean was cold from the late 1960s to the early 1990s, the Sahel received less rainfall and the tropical North Atlantic experienced a high concentration of dust. The opposite was true when the North Atlantic Ocean was warm before the late 1960s and after the early 1990s. This suggests a novel mechanism for North Atlantic SST variability—a positive feedback between North Atlantic SST, African dust, and Sahel rainfall on multidecadal time scales. That is, a warm (cold) North Atlantic Ocean produces a wet (dry) condition in the Sahel and thus leads to low (high) concentration of dust i...

Temporal variability of the meridional overturning circulation at 34.5 S: Results from two pilot boundary arrays in the South Atlantic

Data from two boundary arrays deployed along 34.5°S are combined to produce the first continuous in situ time series observations of the basin-wide meridional overturning circulation (MOC) in the South Atlantic. Daily estimates of the MOC between March 2009 and December 2010 range between 3 Sv and 39 Sv (1 Sv = 106 m3 s−1) after a 10 day low-pass filter is applied. Much of the variability in this ∼20 month record occurs at periods shorter than 100 days. Approximately two-thirds of the MOC variability is due to changes in the geostrophic (baroclinic plus barotropic) volume transport, with the remainder associated with the direct wind-forced Ekman transport. When low-pass filtered to match previously published analyses in the North Atlantic, the observed temporal standard deviation at 34.5°S matches or somewhat exceeds that observed by time series observations at 16°N, 26.5°N, and 41°N. For periods shorter than 20 days the basin-wide MOC variations are most strongly influenced by Ekman flows, while at periods between 20 and 90 days the geostrophic flows tend to exert slightly more control over the total transport variability of the MOC. The geostrophic shear variations are roughly equally controlled by density variations on the western and eastern boundaries at all time scales captured in the record. The observed time-mean MOC vertical structure and temporal variability agree well with the limited independent observations available for confirmation.

Temporal variability of the South Atlantic Meridional Overturning Circulation between 20°S and 35°S

Altimetry-derived synthetic temperature and salinity profiles between 20°S and 34.5°S are used to estimate the Meridional Overturning Circulation (MOC) and meridional heat transport (MHT), which are assessed against estimates obtained from expendable bathythermograph (XBT) measurements. Consistent with studies from XBTs and Argo data, both the geostrophic and Ekman contributions to the MOC exhibit annual cycles and play an equal role in the MOC seasonal variations. The strongest variations on seasonal and interannual time scales in our study region are found at 34.5°S. The dominance of the geostrophic and Ekman components on the interannual variations in the MOC and MHT varies with time and latitude, with the geostrophic component being dominant during 1993–2006 and the Ekman component dominant between 2006 and 2011 at 34.5°S.

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.

Wind forced variability of the Antarctic Circumpolar Current south of Africa between 1993 and 2010

The variability of the Antarctic Circumpolar Current (ACC) system is largely linked to the atmospheric forcing. The objective of this work is to assess the link between local wind forcing mechanisms and the variability of the upper-ocean temperature and the dynamics of the different fronts in the ACC region south of South Africa. To accomplish this, in situ and satellite-derived observations are used between 1993 and 2010. The main finding of this work is that meridional changes in the westerlies linked with the Southern Annular Mode (SAM) drive temperature anomalies in the Ekman layer and changes in the Subantarctic Front (SAF) and Antarctic Polar Front (APF) transports through Ekman dynamics. The development of easterly anomalies between 35°S and 45°S during positive SAM is linked to reduced (increased) SAF (APF) transports and a warmer mixed layer in the ACC. The link between the changes in the wind stress and the SAF and APF transport variations occurs through the development of Ekman pumping anomalies near the frontal boundaries, driving an opposite response on the SAF and APF transports. The observed wind-driven changes in the frontal transports suggest small changes to the net ACC transport. In addition, observations indicate that the SAF and APF locations in this region are not linked to the local wind forcing, emphasizing the importance of other factors (e.g., baroclinic instabilities generated by bottom topography) to changes in the frontal location. Results obtained here highlight the importance of repeat XBT temperature sections and their combined analysis with other in situ and remote sensing observations.

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

Meridional Overturning Circulation Transport Variability at 34.5°S During 2009–2017: Baroclinic and Barotropic Flows and the Dueling Influence of the Boundaries

Six years of simultaneous moored observations near the western and eastern boundaries of the South Atlantic are combined with satellite winds to produce a daily time series of the basin-wide meridional overturning circulation (MOC) volume transport at 34.5°S. The results demonstrate that barotropic and baroclinic signals at both boundaries cause significant transport variations, and as such must be concurrently observed. The data, spanning ~20 months during 2009–2010 and ~4 years during 2013–2017, reveal a highly energetic MOC record with a temporal standard deviation of 8.3 Sv, and strong variations at time scales ranging from a few days to years (peak-to-peak range = 54.6 Sv). Seasonal transport variations are found to have both semiannual (baroclinic) and annual (Ekman and barotropic) timescales. Interannual MOC variations result from both barotropic and baroclinic changes, with density profile changes at the eastern boundary having the largest impact on the year-to-year variations. Plain Language Summary Changes in the meridional overturning circulation, characterized by north-south flows throughout the Atlantic Ocean basin and vertical exchange between the surface and the deep ocean, are related to changes in important ocean-atmosphere-climate signals like precipitation patterns, sea level, and extreme weather (e.g., drought, heat waves, and hurricane intensification). This study presents, for the first time, a multiyear daily record of the meridional overturning circulation flow based on direct measurements in the South Atlantic Ocean at 34.5°S. The roughly six years of observations presented in this study provided the ability to study seasonal and interannual changes in these important flows with continuous daily data, and they demonstrated a complexity of the ocean circulation as compared to other latitudes where this flow has been studied in the past.

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.