The impact of tropical Atlantic SST variability on the tropical atmosphere during boreal summer

The interannual variability of boreal summer sea surface temperature (SST) in the tropical Atlantic displays two dominant modes, the Atlantic zonal mode highlighting SST variations in the equatorial–southern tropical Atlantic (ESTA) region and the northern tropical Atlantic (NTA) mode focusing on SST fluctuations in the NTA region except in the Gulf of Guinea. Observational evidence indicates that both the boreal summer ESTA and NTA warming are accompanied by a pair of anomalous low-level anti-cyclones over the western tropical Pacific, and the NTA-related anti-cyclone is more obvious than the ESTA-related one. Both atmosphere-only and partially coupled experiments conducted with the Community Earth System Model Version 1.2 support the observed NTA–Pacific teleconnection. In contrast, the ESTA-induced atmospheric circulation response is negligible over the tropical Pacific in the atmosphere-only experiments, and though the response becomes stronger in the partially coupled experiments, obvious difference still exists between the simulations and observation. The ESTA-induced atmospheric circulation response is featured by an anomalous low-level cyclone over the western tropical Pacific in the partially coupled experiments, opposite to its observed counterpart. It is found that the ESTA warming coincides with significantly La Niña-like SST anomalies in the central–eastern equatorial Pacific, the influence of which on the tropical atmospheric circulation is opposite to that of the ESTA warming, and therefore contributes to difference between the ESTA-related simulations and observation. Moreover, the cold climatological mean SST in the ESTA region is unfavourable to enhancing the ESTA–Pacific teleconnection during boreal summer.

Inter-basin and Multi-time Scale Interactions in generating the 2019 Extreme Indian Ocean Dipole

An unprecedented extreme positive Indian Ocean Dipole event (pIOD) occurred in 2019, which has caused widespread disastrous impacts on countries bordering the Indian Ocean, including the East African floods and vast bushfires in Australia. Here we investigate the causes for the 2019 pIOD by analyzing multiple observational datasets and performing numerical model experiments. We find that the 2019 pIOD is triggered in May by easterly wind bursts over the tropical Indian Ocean associated with the dry phase of the boreal summer intraseasonal oscillation, and sustained by the local atmosphere-ocean interaction thereafter. During September-November, warm sea surface temperature anomalies (SSTA) in the central-western tropical Pacific further enhance the Indian Ocean’s easterly winds, bringing the pIOD to an extreme magnitude. The central-western tropical Pacific warm SSTA is strengthened by two consecutive Madden Julian Oscillation (MJO) events that originate from the tropical Indian Ocean. Our results highlight the important roles of cross-basin and cross-timescale interactions in generating extreme IOD events. The lack of accurate representation of these interactions may be the root for a short lead time in predicting this extreme pIOD with a state-of-the-art climate forecast model.

Regional difference of sea surface salinity variations in the western tropical pacific

Regional difference of sea surface salinity (SSS) variations in the western tropical Pacific is investigated with Ocean Reanalysis System 5 datasets. Three robust zonal bands of SSS variations have been identified in the northwestern tropical Pacific (NWTP), the western equatorial tropical Pacific (WEqP), and the southwestern tropical Pacific (SWTP), respectively. SSS in the WEqP and the SWTP has a strong interannual variability that is related to ENSO. In the WEqP, SSS variations are mainly controlled by anomalous freshwater flux, while in the SWTP they are governed by both freshwater forcing and oceanic processes. In the NWTP, SSS variations present a low-frequency variability that is correlated with Interdecadal Pacific Oscillation (IPO), which is mostly dominated by the freshwater flux and strongly adjusted by the ocean advection and mixed layer changes. After removing interannual signals, the SSS in all three regions are highly related to IPO, indicating that IPO has a general influence on the western tropical Pacific.

Sea Surface Salinity Change since 1950: Internal Variability Versus Anthropogenic Forcing

Using an eastern tropical Pacific pacemaker experiment called the Pacific Ocean–Global Atmosphere (POGA) run, this study investigated the internal variability in sea surface salinity (SSS) and its impacts on the assessment of long-term trends. By constraining the eastern tropical Pacific sea surface temperature variability with observations, the POGA experiment successfully simulated the observed variability of SSS. The long-term trend in POGA SSS shows a general pattern of salty regions becoming saltier (e.g., the northern Atlantic) and fresh regions becoming fresher, which agrees with previous studies. The 1950-2012 long-term trend in SSS is modulated by the internal variability associated with the Interdecadal Pacific Oscillation (IPO). Due to this variability, there are some regional discrepancies in the SSS 1950–2012 long-term change between POGA and the free-running simulation forced with historical radiative forcing, especially for the western tropical Pacific and southeastern Indian Ocean. Our analysis shows that the tropical Pacific cooling and intensified Walker Circulation caused the SSS to increase in the western tropical Pacific and decrease in the southeastern Indian Ocean during the 20-year period of 1993–2012. This decadal variability has led to large uncertainties in the estimation of radiative-forced trends on a regional scale. For the 63-year period of 1950–2012, the IPO caused an offset of ∼40% in the radiative-forced SSS trend in the western tropical Pacific and ∼170% enhancement in the trend in the southeastern Indian Ocean. Understanding and quantifying the contribution of internal variability to SSS trends help improve the skill for estimates and prediction of salinity/water cycle changes.

Turbulent Mixing Inferred from CTD Datasets in the Western Tropical Pacific Ocean

The spatial pattern of energetic aspect related to vertical mixing processes of the water masses in the western tropical Pacific Ocean is characterized in this study. Turbulent kinetic energy dissipation rates and vertical eddy diffusivities in this region are estimated from archived CTD profiles from World Ocean Database (WOD). The dissipation rates are estimated using the improved Thorpe method which considered the canonical Garret-Munk background dissipation rate and the typical lowest value dissipation rate from microstructure measurements, 10-10 m2s-3. Enhanced dissipation rates of 10-8-10-7 m2s-3 were found in the region known as an active area where two Pacific water masses from different sources intersect and strong mesoscale circulations exist while lower dissipation of less than 10-8 m2s-3 was found in the less active regions. A comparison with recent 3D hydrostatic model of M2 internal tide shows less agreement dissipation rates of the model with the observations, with the decreasing trend of discrepancy towards deeper. This suggested that topography roughness, homogenous stratifications yet lacking of background circulations set in the model were not sufficient to reproduce dissipation in the region with strong background mesoscale circulations. It was indicated that the main contributor for vertical overturning events occurred in this region is due to strong shear instabilities enhanced by background circulations. A direct method estimates using vertical microstructure profiler is suggested to validate this indirect method in the future.