International Liege colloquium Influence of atmospheric circulation on the Namibian upwelling system and the oxygen minimum zone Nele Tim, Eduardo Zorita, Birgit Hünicke 09.05.2014 / University of Liège
Motivation - Focus of interests Long-term trends of the Namibian upwelling Influence of large-scale atmospheric pattern Climate modes: Antarctic Oscillation (AAO), Tropical Atlantic variability (ATL-3), Pacific ENSO, Atlantic Meridional Mode (AMM), St. Helena Index (HIX) Regional land-sea contrast of temperature and sea level pressure (SLP) Decadal variability of the Namibian upwelling Relationship with external forcings Analysis of Mode Waters and of the Oxygen Minimum Zone Shannon, L. V. and O`Toole, M. J., 2003 2
Genus Project Genus = Geochemistry and Ecology of the Namibian Upwelling System Project: Analyses of the relationships between climate change, biogeochemical cycles and ecosystem structure in the large marine ecosystem off the Namibian coast Subproject 1: Relationship between the large-scale, low-frequency climate forcing and local processes that drive upwelling intensity in this region Statistical analyses of the causes for the long-term variations at decadal and longer time scales of upwelling off the Namibian coast genus.zmaw.de 3
Data Dataset Description Time period Resolution Area HadlSST1 gridded observations 1870-2012 1 degr global STORM GENUS/MOM4 MPI-ESM-MR historical MPI-ESM-MR past millennium simulation of the global ocean model MPI-OM regional simulation of the ecosystem model Modular Ocean Model 4.0 1950-2010 0.1 degr global 07/1999-05/2012 cmip5 MPI 1850-2005 T63/L95 T0.4/L40 ~ 1.9 degr cmip5 MPI 850-1850 T63/L47 B1.5/L40 ~ 1.9 degr 0.1 degr 6.63N - 34S, 10W - 18E global global NCEP reanalysis data 1948-2011 2.5 degr global ERA Interim reanalysis data 1979-2010 0.7 degr global 4
Annual cycle Northern Benguela Southern Benguela separated at 28 S realistic modelled annual cycle 5
Long-term trends of Namibian upwelling Vertical velocities of STORM and MOM4 Slightly positive trends in STORM (except austral spring in Northern Benguela) Trends in upwelling in Northern Benguela significant: austral autumn (+), austral summer (+) 6
Correlations with atmospheric variables First EOF of sea surface temperature (SST) of HadlSST1, STORM, 10m- Temperatur of MOM4 Vertical velocities of MOM4 and STORM Strong anticyclone Strong southerly winds Strong southerly wind stress Air temperature contrast between land (positive) and ocean (negative) SSTs of HadlSST1 and STORM present well the connection between upwelling and atmosphere. MOM4 provide partly distinct results Northern Benguela: STORM: expected pattern MOM4: correlated with variables over northern Benguela Southern Benguela: STORM, MOM4: no clear pattern: SLP positively correlated over southern South Atlantic, no southerly winds 7
Correlations with climate indices Upwelling Index (SST) Upwelling Index (vertical velocity) ENSO has significant influence ENSO influences in austral summer in austral spring and summer and autumn The tropical Atlantic seems to Impact of the Antarctic Ocean have a stronger influence than stronger the of the tropical Atlantic the Antarctic Ocean HIX influences in austral winter and MOM4 upwelling is not spring (MOM4), in summer significantly influenced by (STORM) ENSO AMM in austral summer (STORM) 8
Land-sea contrast: SLP gradient Test Bakuns hypothesis (Bakun, 1990; Bakun et al. 2010) SLP differences between land and ocean NCEP: significant positive trend in austral autumn and winter Correlations with upwelling index of vertical velocities: Northern Benguela: all seasons Southern Benguela: in austral summer and autumn 9
Water masses Mohrholz et al. 2008 Oxygen poor water masses off the Namibian coast (SACW) contains regenerated nutrients, oxygen rich ESACW contains preformed nutrients. Remineralization of organic matter consume oxygen SACW: enters Benguela with the poleward untercurrent (PUC) from the north ESACW: enters Benguela with the Benguela current from the south 10
Water masses MOM4 SACW: 100 300 m depth (T, S) 200 300 m depth (T, S, nitrate, phosphate) SACW is in reality oxygen deficient, modelled oxygen is too high MPI-ESM cmip5 SACW: 82.5 645 m depth (T, S) 310 485 m depth (T, S, nitrate, phosphate, oxygen) ESACW: 200 500 m depth (T, S) 50 m depth (nitrate), 50 100 m depth (phosphate) ESACW is in reality oxygen rich, modelled oxygen is too low ESACW: 122.5 960 m depth (T, S) 6 27 m depth (nitrate), 17-82.5 m depth (phosphate), 1700 2785 m depth (oxygen) 11
Summary SST, vertical velocity and 13 C isotherm provide good indices for upwelling Slightly positive trends in upwelling ENSO influences upwelling significantly in austral summer Long-term evolution of the land-sea SLP gradient does not agree with Bakun hypothesis Water masses well defined after T and S, difficulties with nutrients and oxygen Spectral analysis show periods of 5 years, 2.5 years and decadal variability of 10 and 12.5 years nele.tim@hzg.de genus.zmaw.de 12
Thank you 13
Upwelling index derived from the 13 C isotherm Hagen defined an upwelling index derived from the 13 C isotherm of satellite data (AVHRR) Region between the 13 C isotherm and the coast is defined as the Intense Benguela Upwelling (IBU) SST of STORM Warm bias removed Unbiased SST of STORM agrees quite well with the IBU of AVHRR IBUs are larger in austral winter and spring than in summer and autumn 14
Variabilities spectral analysis Variabilities with periods of 2.5, 3.3, 5, 10 and 12 years vertical vel. Southern Benguela SST HadlSST1 vertical vel. Northern Benguela Correlations of wmo of MPI-ESM Past1000 and historical no correlation between wmo r1i1p1, wmo r2i1p1 and wmo r3i1p1 Internal variability dominates 15