Linking the atmospheric Pacific-South American mode with oceanic variability and predictability

While Pacific climate variability is largely understood based on El Niño-Southern Oscillation (ENSO), the North Pacific focused Pacific decadal oscillation and the basin-wide interdecadal Pacific oscillation, the role of the South Pacific, including atmospheric drivers and cross-scale interactions, has received less attention. Using reanalysis data and model outputs, here we propose a paradigm for South Pacific climate variability whereby the atmospheric Pacific-South American (PSA) mode acts to excite multiscale spatiotemporal responses in the upper South Pacific Ocean. We find the second mid-troposphere PSA pattern is fundamental to stochastically generate a mid-latitude sea surface temperature quadrupole pattern that represents the optimal precursor for the predictability and evolution of both the South Pacific decadal oscillation and ENSO several seasons in advance. We find that the PSA mode is the key driver of oceanic variability in the South Pacific subtropics that generates a potentially predictable climate signal linked to the tropics.

Construction of the Mean Sea Surface Model Combined HY-2A With DTU18 MSS in the Antarctic Ocean

A new Mean Sea Surface (MSS) model called Shandong University of Science and Technology Antarctic Mean Sea Surface model (SDUST_ANT MSS) in the Antarctic Ocean is presented and validated in this paper. The SDUST_ANT MSS updates the DTU18 MSS with 6 years of Exact Repeat Mission (ERM) and Geodetic Mission (GM) data from HY-2A. Collinear adjustment was applied to all the ERM data to obtain the along-track mean sea surface height. Oceanic variability has been removed from the GM data. Crossover adjustment was applied to both the ERM and GM data. We constructed the HY-2A_MSS using HY-2A altimetry data based on optimal interpolation method. Several types of errors (such as white noise, residual effect of oceanic variability, and long wavelength bias) have been taken into account for the determination of MSS using optimal interpolation method. The SDUST_ANT MSS was constructed by mapping HY-2A_MSS onto the DTU18 MSS. The SDUST_ANT MSS was compared with DTU18 MSS and CNES_CLS15 MSS. At wavelengths below 150 km, differences between models are consistent with seafloor topographic gradient. At wavelengths above 150 km, differences are affected by the mesoscale activities and the altimetry errors in coastal areas. The errors of the three models, as indicated by their power spectral densities (PSDs), are of similar orders of magnitude. The absolute error is slightly smaller in SDUST_ANT than in CNES_CLS15 or DTU18.

Impact of millennial-scale oceanic variability on the Greenland ice-sheet evolution throughout the last glacial period

Temperature reconstructions from Greenland ice-sheet (GrIS) ice cores indicate the occurrence of more than 20 abrupt warmings during the last glacial period (LGP) known as Dansgaard-Oeschger (D-O) events. Although their ultimate cause is still debated, evidence from both proxy data and modelling studies robustly links these to reorganisations of the Atlantic Meridional Overturning Circulation (AMOC). During the LGP, the GrIS expanded as far as the continental shelf break and was thus more directly exposed to oceanic changes than in the present. Therefore oceanic temperature fluctuations on millennial timescales could have had a non-negligible impact on the GrIS. Here we assess the effect of millennial-scale oceanic variability on the GrIS evolution from the last interglacial to the present day. To do so, we use a three-dimensional hybrid ice-sheet–shelf model forced by subsurface oceanic temperature fluctuations, assumed to increase during D-O stadials and decrease during D-O interstadials. Since in our model the atmospheric forcing follows orbital variations only, the increase in total melting at millennial timescales is a direct result of an increase in basal melting. We show that the GrIS evolution during the LGP could have been strongly influenced by oceanic changes on millennial timescales, leading to oceanically induced ice-volume contributions above 1 m sea level equivalent (SLE). Also, our results suggest that the increased flux of GrIS icebergs as inferred from North Atlantic proxy records could have been triggered, or intensified, by peaks in melting at the base of the ice shelves resulting from increasing subsurface oceanic temperatures during D-O stadials. Several regions across the GrIS could thus have been responsible for ice mass discharge during D-O events, opening the possibility of a non-negligible role of the GrIS in oceanic reorganisations throughout the LGP.

Late-Holocene oceanic variability in the southern Benguela region driven by interplay of upwelling, fluvial discharge, and Agulhas leakage

The southern Benguela upwelling system near the St. Helena Bay has been proposed to be affected by various factors, while few investigations about the late-Holocene oceanic conditions has been carried out in this area. To determine the oceanic variability and its forcing mechanisms in the southern Benguela region during the late-Holocene, we examined organic-walled dinoflagellate cyst (dinocyst) records from two marine sediment cores located in the southernmost and central Benguela upwelling system. We compare our results with other proxies including alkenone-derived SSTs, grain size, and coccolithophore assemblages from the same samples. The results indicate a distinctive behavior between the southernmost Benguela system and the central Benguela area. We infer that the oceanic conditions in these two regions are primarily governed by an interplay of wind-induced upwelling, fluvial discharge, and advection of cold sub-Antarctic waters, which is consistent with the current understanding of the paleoclimate conditions in this area. However, the findings also suggest that the southernmost Benguela system also receives additional effects of warm and saline waters via the Agulhas leakage, which has a clear influence on the oceanic conditions in this area.