Daria J. Halkides

Quantifying the processes controlling intraseasonal mixed‐layer temperature variability in the tropical Indian Ocean

Spatial and temporal variation of processes that determine ocean mixed-layer (ML) temperature (MLT) variability on the timescale of the Madden-Julian Oscillation (MJO) in the Tropical Indian Ocean (TIO) are examined in a heat-conserving ocean state estimate for years 1993–2011. We introduce a new metric for representing spatial variability of the relative importance of processes. In general, horizontal advection is most important at the Equator. Subsurface processes and surface heat flux are more important away from the Equator, with surface heat flux being the more dominant factor. Analyses at key sites are discussed in the context of local dynamics and literature. At 0°, 80.5°E, for MLT events > 2 standard deviations, ocean dynamics account for more than two thirds of the net tendency during cooling and warming phases. Zonal advection alone accounts for ∼40% of the net tendency. Moderate events (1–2 standard deviations) show more differences between events, and some are dominated by surface heat flux. At 8°S, 67°E in the Seychelles-Chagos Thermocline Ridge (SCTR) area, surface heat flux accounts for ∼70% of the tendency during strong cooling and warming phases; subsurface processes linked to ML depth (MLD) deepening (shoaling) during cooling (warming) account for ∼30%. MLT is more sensitive to subsurface processes in the SCTR, due to the thin MLD, thin barrier layer and raised thermocline. Results for 8°S, 67°E support assertions by Vialard et al. (2008) not previously confirmed due to measurement error that prevented budget closure and the small number of events studied. The roles of MLD, barrier layer thickness, and thermocline depth on different timescales are examined.

Aquarius surface salinity and the Madden-Julian Oscillation: The role of salinity in surface layer density and potential energy

Sea surface salinity (SSS) data from the Aquarius satellite are analyzed along with auxiliary data to investigate the SSS signature of the Madden-Julian Oscillation (MJO) in the equatorial Indian and Pacific Oceans, the effect of evaporation-minus-precipitation (E-P), the implication for the role of ocean dynamics, and the SSS influence on surface density and potential energy. MJO-related SSS changes are consistent with E-P forcing in the western Indian Ocean throughout the MJO cycle and in the central Indian Ocean during the wet phase of the MJO cycle. However, SSS changes cannot be explained by E-P in the central Indian Ocean during the dry phase and in the eastern Indian and western Pacific Oceans throughout the MJO cycle, implying the importance of ocean dynamics. SSS has an overall larger contribution to MJO-related surface density and potential energy anomalies than SST. It partially offsets the SST effect in the western-to-central Indian Ocean and reinforces the SST effect in the eastern Indian and western Pacific Oceans. Ocean modeling and assimilation need to properly account for salinity effects in order to correctly represent mixed layer variability associated with the MJO. Our results also clarify some discrepancy in previous studies about the E-P effect on MJO-related SSS variations.

Influence of the Madden‐Julian oscillation on the Indian Ocean cross‐equatorial heat transport

The Indian Ocean cross-equatorial heat transport (CEHT) anomalies associated with the Madden-Julian oscillation (MJO) are analyzed using the National Centers for Environmental Prediction Climate Forecast System Reanalysis for the period 1979–2010. The magnitude of MJO-related CEHT anomalies, seasonal dependence, and interannual modulations are examined. The magnitude of composite MJO CEHT anomalies is ~30% (~15%) of the amplitude of the seasonal climatology in winter (summer). Interannual modulation on average accounts for only ~10% of the total magnitude of intraseasonal variability of a given year. MJO CEHT is largely contributed by temperature flux anomalies in the upper ~140 m, with notable compensation between two characteristic layers. The significance of MJO CEHT anomalies, the nonnegligible magnitude of residual CEHT accumulated from intraseasonal anomalies during specific years, and the vertical compensation of temperature flux anomalies that give rise to the CEHT, have implications to the potential importance of upper ocean thermodynamics in MJO evolution and regional climate.