Ocean Model Diagnosis of Interannual Coevolving SST Variability in the South Indian and South Atlantic Oceans (original) (raw)
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Influence of the Indian Ocean Dipole on Atmospheric Subseasonal Variability
Journal of Climate, 2005
The relationship between atmospheric subseasonal variability and interannual variation of SST over the tropical Indian Ocean is examined using winds and humidity from the NCEP-NCAR reanalysis, outgoing longwave radiation (OLR), and the monthly SST analysis. The primary focus is on whether and how the subseasonal variability is related to the zonal dipole structure of SST, which peaks during boreal fall. The level of subseasonal wind activity is measured by standard deviation of bandpass-filtered zonal wind fields on the 6-30-and 30-90-day time scales.
The Indian Ocean SST dipole simulated in a coupled general circulation model
Geophysical Research Letters, 2000
We are successful in simulating the recently discovered ocean-atmosphere coupled phenomenon called the Indian Ocean Dipole for the first time, using a coupled general circulation model without flux correction. During the analyzed 50 years of model integration, the anomalous climate events have appeared 8 times over the Indian Ocean (IO). They are characterized by the cooling of the sea surface temperature (SST) in the southeastern tropical IO and the warming of the SST in the western tropical IO, associated with the anomalous easterly winds along the equator. The spatial pattern of the anomalous SST shows an east-west dipole mode (DM) structure that is similar to the recent reports. The simulated DM events are independent of the El Niño simulated in the same model. The heat budget analysis shows that the tropical air-sea interaction, which is strongly influenced by ocean dynamics, is crucial in generating the model DM events.
Interannual SST Variability in the Southern Subtropical and Extra- tropical Ocean
2006
The interannual variability of the sea surface temperature (SST) in the southern oceans shows an unrecognized ubiquitous feature. In contrast to their northern counterparts, active SST fluctuations occur in the open oceans in all three major basins, which are unattached to the coastal processes. Using historical SST observations for 1950-2000, it is shown that these extra-tropical/subtropical SST anomalies have a tilted southwest-northeast dipole pattern in both the Atlantic and Indian Oceans and, to a certain extent, in the central and eastern Pacific. SST fluctuations in all basins show similar seasonal enhancement in austral summer. A long-term simulation of a coupled ocean-atmosphere general circulation model reproduces some of these major features realistically, especially in the South Atlantic. A composite analysis of the objectively selected major events in the South Atlantic from the observations and the simulation shows that the anomalous SST pattern is initiated by mid-latitude atmospheric fluctuations. Through a coupled air-sea feedback, the center of the subtropical branch of the SST anomalies can shift towards the tropics in the next season.
Impact of Atmospheric Intraseasonal Oscillations on the Indian Ocean Dipole during the 1990s*
Journal of Physical Oceanography, 2006
Effects of atmospheric intraseasonal oscillations (ISOs) on the Indian Ocean zonal dipole mode (IOZDM) are investigated by analyzing available observations and a suite of solutions to an ocean general circulation model, namely, the Hybrid Coordinate Ocean Model (HYCOM). Data and model solutions for the period 1991-2000 are analyzed, a period that includes two strong IOZDM events, during 1994 and 1997, and a weak one, in 1991. Both the data analysis and model results suggest that atmospheric ISOs play a significant role in causing irregularity of the two strong IOZDM events and the premature termination of the weak one. Of particular interest is a basinwide, wind-driven oceanic resonance with a period near 90 days, involving the propagation of equatorial Kelvin and first-meridional-mode Rossby waves across the basin. Before the onset of the strong 1997 dipole, wind variability had significant power near 90 days, and the resonance was strongly excited. Associated with the resonance was a deepened thermocline in the eastern basin during August and early September, which reduced the upwelling in the eastern antinode region of the IOZDM, thereby delaying the reversal of the equatorial zonal SST gradient-an important indicator of a strong IOZDM-by over a month. A similar deepened thermocline in the eastern basin also contributed to the premature termination of the weak 1991 dipole. During the 1994 IOZDM, the winds had little power near 90 days, and the resonant mode was not prominent. The ISOs influenced the IOZDM through both surface fluxes and thermocline variability. They enhanced warming in the western antinode region during October, the peak phase of the IOZDM, intensifying its strength. During November, strong winds significantly cooled the western and central basin through upwelling and surface fluxes, cooling SST there and contributing to the early and quick termination of the 1994 event.
Influence of the Indian Ocean dipole on the Southern Oscillation
Journal of The Meteorological Society of Japan, 2003
The influence of the Indian Ocean Dipole (IOD) on the interannual atmospheric pressure variability of the Indo-Pacific sector is investigated. Statistical correlation between the IOD index and the global sea level pressure anomalies demonstrates that loadings of opposite polarity occupy the western and the eastern parts of the Indian Ocean. The area of positive correlation coefficient in the eastern part even extends to the Australian region, and the IOD index has a peak correlation coefficient of about 0.4 with the Darwin pressure index, i.e. the western pole of the Southern Oscillation, when the former leads the latter by one month. The correlation analysis with seasonally stratified data further confirms the lead role of the IOD. The IOD-Darwin relation has undergone interdecadal changes; in the last 50 years the correlation is highest during the most recent decade of 1990-99, and weakest during 1980-89.
Remote Response of the Indian Ocean to Interannual SST Variations in the Tropical Pacific
Journal of Climate, 2004
Remote forcing of sea surface temperature (SST) variations in the Indian Ocean during the course of El Niño-Southern Oscillation (ENSO) events is investigated using NCEP reanalysis and general circulation model (GCM) experiments. Three experiments are conducted to elucidate how SST variations in the equatorial Pacific influence surface flux variations, and hence SST variations, across the Indian Ocean. A control experiment is conducted by prescribing observed SSTs globally for the period 1950-99. In the second experiment, observed SSTs are prescribed only in the tropical eastern Pacific, while climatological SSTs are used elsewhere over the global oceans. In the third experiment, observed SSTs are prescribed in the tropical eastern Pacific, while a variable-depth ocean mixed layer model is used at all other ocean grid points to predict the SST.
Interannual variability of the South Indian Ocean in observations and a coupled model
2008
The mean state, annual cycle, and interannual variability of the coupled ocean-atmosphere in the South Indian Ocean produced by a 300-year simulation of a coupled ocean-atmosphere general circulation model (CGCM) are compared with those from 51-year (1950-2000) observational datasets. The CGCM simulates realistically the mean annual cycles for both the sea surface temperature (SST) and lower atmospheric circulation, including the seasonal positions of the 10 o C and 20 o C SST isotherms, the zonal and meridional migration of the South Indian Ocean subtropical high, and the fluctuation of the southeast trade winds and mid-latitude westerly winds.
International Journal of Climatology, 2003
The relationship between sea-surface temperature (SST) inter-annual variability at the subtropical and midlatitudes of the southern Atlantic and Indian Oceans and its links with the atmospheric circulation in the Southern Hemisphere are investigated over the 1950-1999 period. Exploratory analysis using singular value decomposition and further investigations based on simple indices show that a large part of regional SST variability is common between the southwestern parts of both basins at subtropical and midlatitudes during the austral summer. Interestingly, these areas are also significantly associated with the far southwestern Pacific (Tasman Sea area). The patterns and time series of covariability between the southern Atlantic and Indian Oceans are shown to correspond to SST modes previously described in the literature as 'subtropical dipoles', independently for the Atlantic and Indian Oceans. Composite analyses show that austral summers characterized by simultaneous warm (and to a lesser extent cold) SST anomalies in the southwestern (northern) part of both southern oceans are related to atmospheric anomalies mainly involving a southward shift and a strengthening of the subtropical high-pressure systems over both basins. These anomalies are embedded in a hemispheric signal associating two cores of positive pressure anomalies within the South Pacific anticyclone. The global picture appears to have a wave number 4 spatial structure. The associated low-level wind and latent heat-flux anomalies and the lags between atmospheric variables and SST anomalies are consistent with an atmospheric forcing on the ocean. Potential links of these patterns with large-scale modes of climate variability in the Southern Hemisphere are discussed. Figure 1. Grid-point correlation between South African rainfall index (SARI) and SST (GISST 2.3b dataset; Rayner et al., 1996) in the Atlantic and Indian Oceans for the JFM season (same season as in Richard et al. (2001)) over the 1950-99 period
Journal of Physical Oceanography, 2014
Intraseasonal sea surface temperature (SST) variability over the Seychelles-Chagos thermocline ridge (SCTR; 128-48S, 558-858E) induced by boreal wintertime Madden-Julian oscillations (MJOs) is investigated with a series of OGCM experiments forced by the best available atmospheric data. The impact of the ocean interannual variation (OIV), for example, the thermocline depth changes in the SCTR, is assessed. The results show that surface shortwave radiation (SWR), wind speed-controlled turbulent heat fluxes, and wind stressdriven ocean processes are all important in causing the MJO-related intraseasonal SST variability. The effect of the OIV is significant in the eastern part of the SCTR (708-858E), where the intraseasonal SSTs are strengthened by about 20% during the 2001-11 period. In the western part (558-708E), such effect is relatively small and not significant. The relative importance of the three dominant forcing factors is adjusted by the OIV, with increased (decreased) contribution from wind stress (wind speed and SWR). The OIV also tends to intensify the year-to-year variability of the intraseasonal SST amplitude. In general, a stronger (weaker) SCTR favors larger (smaller) SST responses to the MJO forcing. Because of the nonlinearity of the upper-ocean thermal stratification, especially the mixed layer depth (MLD), the OIV imposes an asymmetric impact on the intraseasonal SSTs between the strong and weak SCTR conditions. In the eastern SCTR, both the heat flux forcing and entrainment are greatly amplified under the strong SCTR condition, but only slightly suppressed under the weak SCTR condition, leading to an overall strengthening effect by the OIV.
Indian Ocean dipole mode events in an ocean general circulation model
Deep-sea Research Part Ii-topical Studies in Oceanography, 2002
The evolution of the dipole mode (DM) events in the Indian Ocean is examined using an ocean model that is driven by the NCEP fluxes for the period 1975. The positive DM events during 1997 and negative DM events during 1996 and 1984-1985 are captured by the model and it reproduces both the surface and subsurface features associated with these events. In its positive phase, the DM is characterized by warmer than normal SST in the western Indian Ocean and cooler than normal SST in the eastern Indian Ocean. The DM events are accompanied by easterly wind anomalies along the equatorial Indian Ocean and upwelling-favorable alongshore wind anomalies along the coast of Sumatra. The Wyrtki jets are weak during positive DM events, and the thermocline is shallower than normal in the eastern Indian Ocean and deeper in the west. This anomaly pattern reverses during negative DM events.