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Zhao Jing and Lixin Wu

Abstract

Profiles of potential density obtained from CTD measurements during the Hawaii Ocean Time series (HOT) program in the vicinity of the island of Oahu, Hawaii, are used to evaluate low-frequency variability of turbulent kinetic dissipation rates based on a finescale parameterization method. A distinct seasonal cycle, as well as an increasing trend of dissipation rates, is found in the upper 300–600 m. The trend is mainly due to the much weaker diapycnal mixing in the first four years of the record, that is, 1988–92.

In the upper 300–600 m, enhanced diapycnal mixing is found under anticyclonic eddies with the mean dissipation rate about 53% larger than that under eddy-free conditions. The modulation of dissipation rates by anticyclonic eddies becomes more evident with increasing eddy strength. The role of cyclonic eddies in modulating diapycnal mixing is almost negligible compared with that of anticyclonic eddies. The mean dissipation rate under cyclonic eddies is comparable to that under eddy-free conditions with a difference of less than 10%. Seasonality of the dissipation rates is partly modulated by the seasonal variation of anticyclonic eddies.

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Zhao Jing and Ping Chang

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Dynamics of small-scale (<10 km) superinertial internal waves (SSIWs) of intense vertical motion are investigated theoretically and numerically. It is shown that near-inertial internal waves (NIWs) have a pronounced influence on modulation of SSIW strength. In convergence zones of NIWs, energy flux of SSIWs converge and energy is transferred from NIWs to SSIWs, leading to rapid growth of SSIWs. The opposite occurs when SSIWs enter divergence zones of NIWs. The underlying dynamics can be understood in terms of wave action conservation of SSIWs in the presence of background NIWs. The validity of the theoretical finding is verified using realistic high-resolution numerical simulations in the Gulf of Mexico. The results reveal significantly stronger small-scale superinertial vertical motions in convergence zones of NIWs than in divergence zones. By removing near-inertial wind forcing, model simulations with identical resolution show a substantial decrease in the small-scale superinertial vertical motions associated with the suppression of NIWs. Therefore, these numerical simulations support the theoretical finding of SSIW–NIW interaction.

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Liang Zhao and Jing-Song Wang

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This study provides evidence of the robust response of the East Asian monsoon rainband to the 11-yr solar cycle and first identify the exact time period within the summer half-year (1958–2012) with the strongest correlation between the mean latitude of the rainband (MLRB) over China and the sunspot number (SSN). This period just corresponds to the climatological-mean East Asian mei-yu season, characterized by a large-scale quasi-zonal monsoon rainband (i.e., 22 May–13 July). Both the statistically significant correlation and the temporal coincidence indicate a robust response of the mei-yu rainband to solar variability during the last five solar cycles. During the high SSN years, the mei-yu MLRB lies 1.2° farther north, and the amplitude of its interannual variations increases when compared with low SSN years. The robust response of monsoon rainband to solar forcing is related to an anomalous general atmospheric pattern with an up–down seesaw and a north–south seesaw over East Asia.

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Zhao Jing, Lixin Wu, and Xiaohui Ma

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The near-inertial wind work and near-inertial internal waves (NIWs) in the ocean have been extensively studied using ocean general circulation models (OGCMs) forced by 6-hourly winds or wind stress obtained from atmospheric reanalysis data. However, the OGCMs interpolate the reanalysis winds or wind stress linearly onto each time step, which partially filters out the wind stress variance in the near-inertial band. In this study, the influence of the linear interpolation on the near-inertial wind work and NIWs is quantified using an eddy-resolving (°) primitive equation ocean model. In addition, a new interpolation method is proposed—the sinc-function interpolation—that overcomes the shortages of the linear interpolation.

It is found that the linear interpolation of 6-hourly winds significantly underestimates the near-inertial wind work and NIWs at the midlatitudes. The underestimation of the near-inertial wind work and near-inertial kinetic energy is proportional to the loss of near-inertial wind stress variance due to the linear interpolation. This further weakens the diapycnal mixing in the ocean due to the reduced near-inertial shear variance. Compared to the linear interpolation, the sinc-function interpolation retains all the wind stress variance in the near-inertial band and yields correct magnitudes for the near-inertial wind work and NIWs at the midlatitudes.

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Zhao Jing, Lixin Wu, and Xiaohui Ma

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In this study, the energy exchange between mesoscale eddies and wind-forced near-inertial oscillations (NIOs) is theoretically analyzed using a slab mixed layer model modified by including the geostrophic flow. In the presence of strain, there is a permanent energy transfer from mesoscale eddies to NIOs forced by isotropic wind stress. The energy transfer efficiency, that is, the ratio of the energy transfer rate to the near-inertial wind work, is proportional to , where S 2 is the total strain variance, is the effective Coriolis frequency, and ζ is the relative vorticity. The theories derived from the modified slab mixed layer model are verified by the realistic numerical simulation obtained from a coupled regional climate model (CRCM) configured over the North Pacific. Pronounced energy transfer from mesoscale eddies to wind-forced NIOs is localized in the Kuroshio Extension region associated with both strong near-inertial wind work and strain variance. The energy transfer efficiency in anticyclonic eddies is about twice the value in cyclonic eddies in the Kuroshio Extension region because of the influence of ζ on f eff, which may contribute to shaping the dominance of cyclonic eddies than anticyclonic eddies in that region.

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Zhao Jing, Lixin Wu, and Xiaohui Ma
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Zhao Jing, Ping Chang, Steven F. DiMarco, and Lixin Wu

Abstract

Moored ADCP data collected in the northern Gulf of Mexico are analyzed to examine near-inertial internal waves and their contribution to subthermocline diapycnal mixing based on a finescale parameterization of deep ocean mixing. The focus of the study is on the impact of near-inertial internal waves generated by an extreme weather event—that is, Hurricane Katrina—and by month-to-month variation in weather patterns on the diapycnal mixing. The inferred subthermocline diapycnal mixing exhibits pronounced elevation in the wake of Katrina. Both the increased near-inertial (0.8–1.8f, where f is the Coriolis frequency) and superinertial (>1.8f) shear variances contribute to the elevated diapycnal mixing, but the former plays a more dominant role. The intense wind work on near-inertial motions by the hurricane is largely responsible for the energetic near-inertial shear variance. Energy transfer from near-inertial to superinertial internal waves, however, appears to play an important role in elevating the superinertial shear variance. The inferred subthermocline diapycnal mixing in the region also exhibits significant month-to-month variation with the estimated diffusivity in January 2006 about 3 times the values in November and December 2005. The subseasonal change in the diapycnal mixing mainly results from the subseasonal variation of the near-inertial wind work that causes intensification of the near-inertial shear in January 2006.

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Zhao Jing, Ping Chang, S. F. DiMarco, and Lixin Wu

Abstract

A long-term mooring array deployed in the northern Gulf of Mexico is used to analyze energy exchange between internal waves and low-frequency flows. In the subthermocline (245–450 m), there is a noticeable net energy transfer from low-frequency flows, defined as having a period longer than six inertial periods, to internal waves. The magnitude of energy transfer rate depends on the Okubo–Weiss parameter of low-frequency flows. A permanent energy exchange occurs only when the Okubo–Weiss parameter is positive. The near-inertial internal waves (NIWs) make major contribution to the energy exchange owing to their energetic wave stress and relatively stronger interaction with low-frequency flows compared to the high-frequency internal waves. There is some evidence that the permanent energy exchange between low-frequency flows and NIWs is attributed to the partial realization of the wave capture mechanism. In the periods favoring the occurrence of the wave capture mechanism, the horizontal propagation direction of NIWs becomes anisotropic and exhibits evident tendency toward that predicted from the wave capture mechanism, leading to pronounced energy transfer from low-frequency flows to NIWs.

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Zhao Jing, Lixin Wu, Dexing Wu, and Bo Qiu

Abstract

The westward South Equatorial Current (SEC) and eastward Equatorial Undercurrent (EUC) is a marginally stable current system due to the strong vertical shear. The existence of wavelike motions may locally reduce the Richardson number enough to trigger instabilities. Here, velocity measurements from the Tropical Atmosphere Ocean (TAO) array are used to examine the variability of oscillations within 0.125–12 cycles per day (cpd). It is found that the 0.125–12-cpd oscillations become more energetic in the presence of strong tropical instability waves (TIWs). The enhancement of shear variance is most pronounced around the EUC core (115 m), while prominent elevation of kinetic energy occurs around 85 m, where the EUC shear is strongest. Particularly, the energetic 0.125–12-cpd oscillations during strong TIW seasons do not cycle on a daily basis and are more evident during the southward phase of TIWs. The enhanced 0.125–12-cpd oscillations during strong TIW seasons can be ascribed neither to the changing background stratification nor to the vertical migration of EUC core at the corresponding time scales. Its strength is tightly correlated with the EUC shear and, to a lesser extent, the TIW shear. A partial correlation analysis suggests that the correlation to the TIW shear is mainly due to the association between EUC and TIW shear. The strength of the 0.125–12-cpd oscillations does not follow the variation of surface wind speed and wind curl, implying that they are not directly generated by surface wind forcing.

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Shuyun Zhao, Hua Zhang, Zhili Wang, and Xianwen Jing

Abstract

The comprehensive effects of anthropogenic aerosols (sulfate, black carbon, and organic carbon) on terrestrial aridity were simulated using an aerosol–climate coupled model system. The results showed that the increase in total anthropogenic aerosols in the atmosphere from 1850 to 2010 had caused global land annual mean precipitation to decrease by about 0.19 (0.18, 0.21) mm day−1, where the uncertainty range of the change (minimum, maximum) is given in parentheses following the mean change, and reference evapotranspiration ET0 (representing evapotranspiration ability) to decrease by about 0.33 (0.31, 0.35) mm day−1. The increase in anthropogenic aerosols in the atmosphere from 1850 to 2010 had caused land annual mean terrestrial aridity to decrease by about 3.0% (2.7%, 3.6%). The areal extent of global total arid and semiarid areas had reduced due to the increase in total anthropogenic aerosols in the atmosphere from preindustrial times. However, it was found that the increase in anthropogenic aerosols in the atmosphere had enhanced the terrestrial aridity and thus resulted in an expansion of arid and semiarid areas over East and South Asia. The projected decrease in anthropogenic aerosols in the atmosphere from 2010 to 2100 will increase global land annual mean precipitation by about 0.15 (0.13, 0.16) mm day−1 and ET0 by about 0.26 (0.25, 0.28) mm day−1, thereby producing a net increase in terrestrial aridity of about 2.8% (2.1%, 3.6%) and an expansion of global total arid and semiarid areas.

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