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Motoki Nagura, J. P. McCreary, and H. Annamalai

Abstract

This study investigates biases of the climatological mean state of the northern Arabian Sea (NAS) in 31 coupled ocean–atmosphere models. The focus is to understand the cause of the large biases in the depth of the 20°C isotherm that occur in many of them. Other prominent biases are the depth and temperature of Persian Gulf water (PGW) and the wintertime mixed-layer thickness (MLT) along the northern boundary.

For models that lack a Persian Gulf (group 1), is determined by the wintertime MLT bias through the formation of an Arabian Sea high-salinity water mass (ASHSW) that is too deep. For models with a Persian Gulf (group 2), if > MLT (group 2B), PGW remains mostly trapped to the western boundary and, again, directly controls . If MLT (group 2A), PGW spreads into the NAS and impacts because > 20°C; nevertheless still influences indirectly through its impact on .

The thick wintertime mixed layer is driven primarily by surface cooling during the fall. Nevertheless, variations in ΔMLT among the models are more strongly linked to biases in the density stratification (jump) across the bottom of the mixed layer than to biases. The jump is in turn determined primarily by sea surface salinity biases (ΔSSS) advected into the NAS by the West India Coastal Current, and the source of ΔSSS is the rainfall deficit associated with the models’ weak summer monsoon. Ultimately, then, ΔD20 is linked to this deficit.

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W. B. White and J. P. McCreary

Abstract

The El Niño studies carried out by Bjerknes indicate that the anomalous warming of the eastern boundary tropical ocean in 1939 and 1958 extended 1000 km into the interior ocean, corresponding with a diversion of the Humboldt Current offshore south of Peru. Furthermore, these features were associated with a large-scale weakening of the Southern Hemisphere wind systems. This suggests that major changes occurred in the wind-driven eastern boundary circulation during this time. To see if this is plausible from a theoretical viewpoint, we consider a mathematical model of large-scale ocean spin-down, induced by a reduction in the mean strength of the large-scale wind systems. This model shows that spin-down of the interior ocean is intensified along the eastern boundary with de-intensification propagating in time toward the west and extending out to 1000 km after one year. The interior portions of the ocean circulation are only weakly affected. Moreover, the spin-down is asymmetric, with greater de-intensification in the equatorial eastern boundary than in the poleward regions, making it appear as though the Humboldt Current is directed off-shore just south of Peru. This asymmetric aspect of gyre spin-down can be explained in terms of non-dispersive Rossby waves propagating energy [Cg=−β/(f 2/gH] to the west at a faster rate near the equator than near the poles.

The results of this study have direct application to El Niño. First, the general decrease in equatorward transport of the Humboldt Current off the coast of Peru allows for the anomalous increase in temperature simply by reducing the advection of cold subtropical waters. Moreover, the asymmetric intensification of eastern boundary spin-down has the effect of lowering the dynamic height of the sea surface in the eastern equatorial region. This situation is conducive for the cross-equatorial transport of warm water to the south; possible mechanisms for the anomalous warming have been proposed by Bjerknes and Wyrtki. The advection of warm interior water to the east does not seem to be allowed by the spin-down process and therefore, cannot be considered as a source of warm El Niño waters.

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H. Annamalai, S. P. Xie, J. P. McCreary, and R. Murtugudde

Abstract

Prior to the 1976–77 climate shift (1950–76), sea surface temperature (SST) anomalies in the tropical Indian Ocean consisted of a basinwide warming during boreal fall of the developing phase of most El Niños, whereas after the shift (1977–99) they had an east–west asymmetry—a consequence of El Niño being associated with the Indian Ocean Dipole/Zonal mode. In this study, the possible impact of these contrasting SST patterns on the ongoing El Niño is investigated, using atmospheric reanalysis products and solutions to both an atmospheric general circulation model (AGCM) and a simple atmospheric model (LBM), with the latter used to identify basic processes. Specifically, analyses of reanalysis products during the El Niño onset indicate that after the climate shift a low-level anticyclone over the South China Sea was shifted into the Bay of Bengal and that equatorial westerly anomalies in the Pacific Ocean were considerably stronger. The present study focuses on determining influence of Indian Ocean SST on these changes.

A suite of AGCM experiments, each consisting of a 10-member ensemble, is carried out to assess the relative importance of remote (Pacific) versus local (Indian Ocean) SST anomalies in determining precipitation anomalies over the equatorial Indian Ocean. Solutions indicate that both local and remote SST anomalies are necessary for realistic simulations, with convection in the tropical west Pacific and the subsequent development of the South China Sea anticyclone being particularly sensitive to Indian Ocean SST anomalies. Prior to the climate shift, the basinwide Indian Ocean SST anomalies generate an atmospheric Kelvin wave associated with easterly flow over the equatorial west-central Pacific, thereby weakening the westerly anomalies associated with the developing El Niño. In contrast, after the shift, the east–west contrast in Indian Ocean SST anomalies does not generate a significant Kelvin wave response, and there is little effect on the El Niño–induced westerlies. The Linear Baroclinic Model (LBM) solutions confirm the AGCM’s results.

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H. Annamalai, J. Potemra, R. Murtugudde, and J. P. McCreary

Abstract

Sea surface temperature observations in the eastern equatorial Indian Ocean (EEIO) during the period 1950–2003 indicate that Indian Ocean dipole/zonal mode (IODZM) events are strong in two decades, namely, the 1960s and 1990s. Atmospheric reanalysis products in conjunction with output from an ocean model are examined to investigate the possible reason for the occurrence of strong IODZM events in these two decades. Specifically, the hypothesis that the mean thermocline in the EEIO is raised or lowered depending on the phase of Pacific decadal variability (PDV), preconditioning the EEIO to favor stronger or weaker IODZM activity, is examined. Diagnostics reveal that the EEIO is preconditioned by the traditional PDV signal (SVD1 of SST), deepening or shoaling the thermocline off south Java through its influence on the Indonesian Throughflow (ITF; oceanic teleconnection), and by residual decadal variability in the western and central Pacific (SVD2 of SST) that changes the equatorial winds over the Indian Ocean (atmospheric teleconnection). Both effects produce a background state that is either favorable or unfavorable for the thermocline–mixed layer interactions, and hence for the excitation of strong IODZM events. Collectively, SVD1 and SVD2 are referred to as PDV here.

This hypothesis is tested with a suite of ocean model experiments. First, two runs are carried out, forced by climatological winds to which idealized easterly or westerly winds are added only over the equatorial Indian Ocean. As might be expected, in the easterly (westerly) run a shallower (deeper) thermocline is obtained over the EEIO. Then, observed winds from individual years are used to force the model. In these runs, anomalously cool SST in the EEIO develops only during decades when the thermocline is anomalously shallow, allowing entrainment of colder waters into the mixed layer.

Since 1999 the PDV phase has changed, and consistent with this hypothesis the depth of the mean thermocline in the EEIO has been increasing. As a consequence, no IODZM developed during the El Niño of 2002, and only a weak cooling event occurred during the summer of 2003. This hypothesis likely also explains why some strong IODZM events occur in the absence of ENSO forcing, provided that PDV has preconditioned the EEIO thermocline to be anomalously shallow.

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François Ascani, Eric Firing, Julian P. McCreary, Peter Brandt, and Richard J. Greatbatch

Abstract

We perform eddy-resolving and high vertical resolution numerical simulations of the circulation in an idealized equatorial Atlantic Ocean in order to explore the formation of the deep equatorial circulation (DEC) in this basin. Unlike in previous studies, the deep equatorial intraseasonal variability (DEIV) that is believed to be the source of the DEC is generated internally by instabilities of the upper-ocean currents. Two main simulations are discussed: solution 1, configured with a rectangular basin and with wind forcing that is zonally and temporally uniform, and solution 2, with realistic coastlines and an annual cycle of wind forcing varying zonally. Somewhat surprisingly, solution 1 produces the more realistic DEC; the large, vertical-scale currents [equatorial intermediate currents (EICs)] are found over a large zonal portion of the basin, and the small, vertical-scale equatorial currents [equatorial deep jets (EDJs)] form low-frequency, quasi-resonant, baroclinic equatorial basin modes with phase propagating mostly downward, consistent with observations. This study demonstrates that both types of currents arise from the rectification of DEIV, consistent with previous theories. The authors also find that the EDJs contribute to maintaining the EICs, suggesting that the nonlinear energy transfer is more complex than previously thought. In solution 2, the DEC is unrealistically weak and less spatially coherent than in the first simulation probably because of its weaker DEIV. Using intermediate solutions, this study finds that the main reason for this weaker DEIV is the use of realistic coastlines in solution 2. It remains to be determined what needs to be modified or included to obtain a realistic DEC in the more realistic configuration.

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Weiqing Han, Julian P. McCreary Jr., D. L. T. Anderson, and Arthur J. Mariano

Abstract

An hierarchy of ocean models is used to investigate the dynamics of the eastward surface jets that develop along the Indian Ocean equator during the spring and fall, the Wyrtki jets (WJs). The models vary in dynamical complexity from 2½-layer to 4½-layer systems, the latter including active thermodynamics, mixed layer physics, and salinity. To help identify processes, both linear and nonlinear solutions are obtained at each step in the hierarchy. Specific processes assessed are as follows: direct forcing by the wind, reflected Rossby waves, resonance, mixed layer shear, salinity effects, and the influence of the Maldive Islands. In addition, the sensitivity of solutions to forcing by different wind products is reported.

Consistent with previous studies, the authors find that direct forcing by the wind is the dominant forcing mechanism of the WJs, accounting for 81% of their amplitude when there is a mixed layer. Reflected Rossby waves, resonance, and mixed layer shear are all necessary to produce jets with realistic strength and structure. Completely new results are that precipitation during the summer and fall considerably strengthens the fall WJ in the eastern ocean by thinning the mixed layer, and that the Maldive Islands help both jets to attain roughly equal strengths.

In both the ship-drift data and the authors’ “best” solution (i.e., the solution to the highest model in the authors’ hierarchy), the semiannual response is more than twice as large as the annual one, even though the corresponding wind components have comparable amplitudes. Causes of this difference are as follows: the complex zonal structure of the annual wind, which limits the directly forced response at the annual frequency;resonance with the semiannual wind; and mixed layer shear flow, which interferes constructively (destructively) with the rest of the response for the semiannual (annual) component. Even in the most realistic solution, however, the annual component still weakens the fall WJ and strengthens the spring one in the central ocean, in contrast to the ship-drift data; this model/data discrepancy may result from model deficiencies, inaccurate driving winds, or from windage errors in the ship-drift data themselves.

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