How volcanism impact on the variability of the South American Monsoon System and the associated Atlantic Subtropical Cell

<p>Large volcanic eruptions can affect the global climate through changes in atmospheric and ocean circulation. Understanding the influence of volcanic eruptions on the hydroclimate over monsoon regions is of great scientific and social importance. The South America Monsoon System (SAMS) is the most important climatic feature of the continent. Both the Intertropical and the South Atlantic wind convergence zones (ITCZ and SACZ, respectively) are fundamental components of the SAMS. They show variations on a broad range of scales, dependent on complex multi-system interactions with the adjacent Atlantic Ocean and teleconnections. Also driven by the winds, the Atlantic Subtropical Cell (STC) is the link between the subduction zone in the subtropical gyre with the tropics. Hence, the STC influence equatorial sea surface temperature variability on interannual to decadal scales in the tropical Atlantic Ocean. In order to improve our understanding of the responses of the ocean-atmosphere system to the volcanic forcing, we aim to identify the dominant mechanisms of seasonal-to-interdecadal variability of the SAMS and the Atlantic STC after large Pinatubo-like (1991) and Tambora-like (1815) eruptions relying on the VolMIP model intercomparison project experiments.</p>

Multidecadal Ocean Temperature and Salinity Variability in the Tropical North Atlantic: Linking with the AMO, AMOC, and Subtropical Cell

The Atlantic multidecadal oscillation (AMO) is characterized by the sea surface warming (cooling) of the entire North Atlantic during its warm (cold) phase. Both observations and most of the phase 5 of the Coupled Model Intercomparison Project (CMIP5) models also show that the warm (cold) phase of the AMO is associated with a surface warming (cooling) and a subsurface cooling (warming) in the tropical North Atlantic (TNA). It is further shown that the warm phase of the AMO corresponds to a strengthening of the Atlantic meridional overturning circulation (AMOC) and a weakening of the Atlantic subtropical cell (STC), which both induce an anomalous northward current in the TNA subsurface ocean. Because the mean meridional temperature gradient of the subsurface ocean is positive because of the temperature dome around 9°N, the advection by the anomalous northward current cools the TNA subsurface ocean during the warm phase of the AMO. The opposite is true during the cold phase of the AMO. It is concluded that the anticorrelated ocean temperature variation in the TNA associated with the AMO is caused by the meridional current variation induced by variability of the AMOC and STC, but the AMOC plays a more important role than the STC. Observations do not seem to show an obvious anticorrelated salinity relation between the TNA surface and subsurface oceans, but most of CMIP5 models simulate an out-of-phase salinity variation. Similar to the temperature variation, the mechanism is the salinity advection by the meridional current variation induced by the AMOC and STC associated with the AMO.

The Mean and the Time Variability of the Shallow Meridional Overturning Circulation in the Tropical South Pacific Ocean

The meridional transport in the Pacific Ocean subtropical cell is studied for the period from 2004 to 2011 using gridded Argo temperature and salinity profiles and atmospheric reanalysis surface winds. The poleward Ekman and equatorward geostrophic branches of the subtropical cell exhibit an El Niño–Southern Oscillation signature with strong meridional transport occurring during La Niña and weak meridional transport during El Niño. At 7.5°S, mean basinwide geostrophic transport above 1000 dbar is 48.5 ± 2.5 Sv (Sv ≡ 106 m3 s−1) of which 30.3–38.4 Sv return to the subtropics in the surface Ekman layer, whereas 10.2–18.3 Sv flow northward, feeding the Indonesian Throughflow. Geostrophic transport within the subtropical cell is stronger in the ocean interior and weaker in the western boundary during La Niña, with changes in the interior dominating basinwide transport. Using atmospheric reanalyses, only half of the mean heat gain by the Pacific north of 7.5°S is compensated by oceanic heat transport out of the region. The National Oceanography Centre at Southampton air–sea flux climatology is more consistent for closing the oceanic heat budget. In summary, the use of Argo data for studying the Pacific subtropical cell provides an improved estimate of basinwide mean geostrophic transport, includes both interior and western boundary contributions, quantifies El Niño/La Niña transport variability, and illustrates how the meridional overturning cell dominates ocean heat transport at 7.5°S.

Tropical Oceanic Response to Extratropical Thermal Forcing in a Coupled Climate Model: A Comparison between the Atlantic and Pacific Oceans*

The tropical oceanic response to the extratropical thermal forcing is quantitatively estimated in a coupled climate model. This work focuses on comparison of the responses between the tropical Atlantic and Pacific. Under the same extratropical forcing, the tropical sea surface temperature responses are comparable. However, the responses in the tropical subsurface in the two oceans are distinct. The tropical subsurface response in the Atlantic can be twice of that in the Pacific. The maximum subsurface temperature change in the tropical Pacific occurs in the eastern lower thermocline, while that in the tropical Atlantic occurs in the west and well below the lower thermocline. The different responses in the tropical Atlantic and Pacific are closely related to the different changes in the meridional overturning circulations. The Pacific shallow overturning circulation, or the subtropical cell, tends to slow down (speed up) in response to the extratropical warming (cooling) forcing. The changes in the upwelling in the eastern equatorial Pacific as well as the shallow subduction from the extratropical southern Pacific along the eastern boundary are accountable for the eastern Pacific temperature change. The Atlantic overturning circulation consists of the shallow subtropical cell and the deep thermohaline circulation. A weakened thermohaline circulation will result in a strengthened northern subtropical cell, in which the change in the lower branch, or the low-latitude North Brazil Current, can cause strong response below the western tropical thermocline. Here the coastal Kelvin wave along the western boundary on the intermediate isopycnal level also plays an important role in the equatorward conveying of the climate anomalies in the mid-to-high-latitude Atlantic, particularly during the initial stage of the extratropical forcing.

Variability of Upper Pacific Ocean Overturning in a Coupled Climate Model

Variability of subtropical cell (STC) overturning in the upper Pacific Ocean is examined in a coupled climate model in light of large observed changes in STC transport. In a 1000-yr control run, modeled STC variations are smaller than observed, but correlate in a similar way with low-frequency ENSO-like variability. In model runs that include anthropogenically forced climate change, STC pycnocline transports decrease progressively under the influence of global warming, attaining reductions of 8% by 2000 and 46% by 2100. Although the former reduction is insufficient to fully account for the apparent observed decline in STC transport over recent decades, it does suggest that global warming may have contributed to the observed changes. Analysis of coupled model results shows that STC transports play a significant role in modulating tropical Pacific Ocean heat content, and that such changes are dominated by anomalous currents advecting mean temperature, rather than by advection of temperature anomalies by mean currents.