Tag: Shenfu Dong

(left column) Seasonality of precipitation and 850mb winds for the monsoon regions as measured by the difference between June-July-August-September (JJAS) minus annual mean precipitation and winds. (middle-column) shows the composite difference of JJAS precipitation (shaded) and 850mb wind for each monsoon region with respect to weak minus strong SAMHT at lead-time 20 years after the anomalous SAMHT. Blue stipples indicates regions where precipitation differences are significant at 95% confidence level based on a non-parametric Kolmogorov-Smirnov test. (right-column) Lag-lead Spearman ranked correlation between SAMHT and NH monsoon index. The blue dashed lines depict the 95% significance level based on a non-parametric Kendall-t test. Negative lag indicates periods when SAMHT leads the NH monsoon index. Periods with significant correlation between the SAMHT and monsoon are shaded blue. Image Credit: NOAA AOML.

Meridional heat transport in the South Atlantic reveals links with global monsoons

A recent paper published in the Journal of Climate led by PHOD researchers Hosmay Lopez, Shenfu Dong, Sang-Ki Lee, and Gustavo Goni provides a physical mechanism on how low frequency variability of the South Atlantic Meridional Heat Transport (SAMHT) associated with the Atlantic Meridional Overturning Circulation ( AMOC) may influence decadal variability of atmospheric circulation and monsoons. This is the first attempt to link the South Atlantic Overturning Circulation variability to weather and climate.

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Interannual variations of the MOC (black) and contributions from the geostrophic (red) and Ekman (green) components at 20°S, 25°S, 30°S, and 34.5°S, respectively. The gray shading denotes the range where anomalies are not significantly different from zero. The red and green shading denote the periods of the dominance of the geostrophic and Ekman components on the MOC, respectively. Image Credit: NOAA.

Dominance of the Geostrophic and Ekman Transports on the MOC in the South Atlantic

The Meridional Overturning Circulation (MOC) plays a critical role in global and regional heat and freshwater budgets. Recent studies have suggested the possibility of a southern origin of the anomalous MOC and meridional heat transport (MHT) in the Atlantic, through changes in the transport of warm/salty waters from the Indian Ocean into the South Atlantic basin. This possibility clearly manifests the importance of understanding the South Atlantic MOC (SAMOC). Observations in the South Atlantic have been historically sparse both in space and time compared to the North Atlantic. To enhance our understanding of the MOC and MHT variability in the South Atlantic, a new methodology is recently published to estimate the MOC/MHT by combining sea surface height measurements from satellite altimetry and in situ measurements (Dong et al., 2015).

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Partial CLIVAR sections demonstrating the spread of NADW eastward into the central South Atlantic Ocean. Vertical axis is depth in meters. Panel a) partial A16S section of CFC-11 from 15 ° -30 ° S along about 25 ° W from 1000-5000 m. The lowest contour for CFC-11 concentration is 0.01 pmol kg -1 . Color scale given on the right; two neutral density surfaces are 27.9 and 28.1 kg m -3 are identified as white lines . Panel b) Same as panel a) except for oxygen concentration in m mol kg -1 . Panel c) Same as panel a) except for salinity. Panel d) partial A13 section of CFC-11 from 15 ° -30 ° S along about 5 ° -10 ° E from 1000-5000 m. The lowest contour for CFC-11 concentration is 0.005 pmol/kg. Panel e) full cross-basin section of CFC-11 along A10, 30 ° S from 1000-5000 m. Note for a, d, and e that 0.3 pmol kg -1 is the highest concentration CFC-11 contoured and higher concentrations will appear in the same shades as for 0.3 pmol kg -1.

The fate of the Deep Western Boundary Current in the South Atlantic

The pathways of recently ventilated North Atlantic Deep Water (NADW) are part of the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). In the South Atlantic these pathways have been the subject of discussion for years, mostly due to the lack of observations. Knowledge of the pathways of the AMOC in the South Atlantic is a first order prerequisite for understanding the fluxes of climatically important properties.

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(left) Location of the three repeat XBT transects (AX25, AX22, and IX28) in the Southern Ocean. (right) Altimetry-derived magnitude of surface geostrophic currents for November 10, 2010, showing the multi-front signature of the ACC, from which the SAF and APF are highlighted. Overlaid is the location of AX25 (gray line), and the location of these fronts for November 10, 2010 (shaded markers). Image Credit: NOAA AOML.

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

Researchers from PhOD and from the University of Cape Town used temperature data from the AX25 repeat XBT transect (from South Africa to Antarctica) in combination with other hydrographic and satellite observations to report a mechanism by which local winds alter the structure of the Antarctic Circumpolar Current flow south of Africa.

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