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14B.1 SST Diurnal Variability and Its Influence on the Tropical

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14B.1 SST Diurnal Variability and Its Influence on the Tropical 14B.1 SST Diurnal Variability and Its Influence on the Tropical Atmospheric Intraseasonal Variability Jian Li ∗ Department of Atmospheric, Oceanic and Earth Sciences, George Mason University, Fairfax, Virgnia Bohua Huang Department of Atmospheric, Oceanic and Earth Sciences, George Mason University, Fairfax, Virginia Center for Ocean-Land-Atmosphere Studies, Institute of Global Environment and Society, Calverton, Maryland 1. Introduction temperature. The further deepening of this diurnal mixed layer is associated with the wind-induced inertial oceanic The diurnal cycle of sea surface temperature (SST) is currents within the layer. a fundamental phenomenon in the world oceans.Early ob- At the bottom of the heated water, its vertical gradi- servations of the diurnal variation of SST can be retrieved ent causes the Kelvin-Helmholtz instability when the bulk back to 1940s. For instance, Sverdrup et al. (1942) found Richardson number, i.e., the ratio of the stratification to that the diurnal SST amplitude is about 0:2 ∼ 0:6oC from the shear of the diurnal jet, drops below a critical value. in situ measurements of water temperature of upper 1 me- The resultant turbulence breaks up the layer interface and ter. With the development of remote sensing technique, we speed up entrainment. As the work of the shear stress is are able to measure water temperature within the oceans used to lift and agitate dense waters from below, the diur- skin layer of a few millimeters, which reports more dra- nal mixed layer deepens while its temperature cools down. matic day-night SST differences up to 2 ∼ 3oC, at certain The bulk Richardson number is restored to its neutral or conditions, reaching 5oC in extreme case. marginally stable value. The surface heating and inertial Partly due to its substantial magnitude, the diurnal currents then build up for the next stage of the mixing. SST variation can have impact on lower frequency vari- This progressive deepening of the mixed layer limits the ability of the climate system. Li et al. (2001) shows the surface warming phase to only about half of the period diurnal SST variability can affect the atmosphere over the while the net heat flux is into the ocean. The depth of the western Pacific warm pool through reducing the amplitude diurnal mixed layer is usually in the range of a few meters. of latent heat flux on intraseasonal time scale. The air-sea Current CGCMs generally do not resolve the physical interaction on diurnal time scale may also play an impor- processes described above because of the coarse vertical res- tant role in the Madden-Jullian Oscillation (MJO) and in olution and inadequate coupling frequency. As a result, the turn the El Nino and Southern Oscillation (ENSO), as well diurnal SST variability is often totally missing or underes- as the mean climate (Slingo et al. 2003; Dai and Trenberth timated in CGCM simulations. The effects of air-sea inter- 2004). action on the diurnal time scale have not been adequately The SST diurnal variation is fundamentally caused by examined. Given the current computational constraints, a the large variation in day/night solar radiation reaching more efficient way to simulate SST diurnal variability in the sea surface. Its amplitude and phase, however, de- CGCMs is to parameterize the effects of the diurnal mixed pend critically upon the surface wind condition, as well layer, based on its essential physics. In this study, we have as the oceanic stratification near the sea surface. A few implemented two physical parameterizations of the diurnal hours after sunrise, a shallow warm layer is formed at the mixed layer to a state-of-the-art CGCM. Both parameter- sea surface and quickly reaches its critical depth where the ization schemes have been proposed by previous studies absorbed solar radiation balances the surface heat loss. Af- but yet to be applied to CGCMs, We have fully tested terwards, growing solar heating tends to inhibit its further these parameterizations and conducted multi-year simula- deepening while the trapped heat increases its tions and sensitivity experiments with and without the pa- rameterized diurnal mixed layers. With the assistance of *Corresponding author address: Jian Li, Department of Atmo- parameterizations, we can simulate the SST diurnal vari- spheric, Oceanic and Earth Sciences, George Mason University, MSN ability realistically and investigate its potential influences 6A2, 4400 University Drive Fairfax, VA 22030. on the low frequency atmospheric variability, especially on E-mail: [email protected] 1 the intraseasonal variability over tropical Indo-Pacific re- tom of which the temperature is the same as the mean gion. The model and experiment design is described in temperature of the grid predicted by the ocean model. The the next section. The results are shown in Section 3. The temperature change within the sub-layer is parameterized summary and conclusion are given in Section 4. with a prescribed vertical profile. The skin temperature is predicted by the net heat flux at the sea surface and the 2. Model and Experiments turbulent heat transport within and at the bottom of the sub-layer. We have used the Climate Forecast System (CFS) de- The SG parameterization also considers an embedded veloped at the National Centers for Environmental Predic- sub-layer of warm water in the upper portion of the top tion (NCEP). CFS is a fully coupled climate system rep- grid of an ocean model. This sub-layer is transient because resenting the interaction between the Earths oceans, land it exists on when the total heat flux received by the model and atmosphere. Currently the CFS is being used for the grid is positive. The depth of the sub-layer changes dy- operational climate prediction at NCEP. The atmospheric namically to maintain the balance of the turbulent kinetic component of the CFS is a spectral model of atmospheric energy between the surface heat flux and wind-induced en- primitive equations with a triangular truncation at wave trainment at the bottom. Temperature is assumed to be number 62 (equivalent to nearly a 200km Gaussian grid) constant within the sub-layer and the weighted mean of the horizontally and 64 sigma levels from earths surface to sub-layer temperature and the temperatures of the rest of the top of the atmosphere (about 0:27hPa) vertically. Its the waters in the grid equals to the model-predicted grid oceanic component is the Modular Ocean Model version temperature. During the day-time, the predicted sub-layer 3 (MOM3) developed by the Geophysical Fluid Dynam- temperature is used as the SST. ics Laboratory(GFDL). The model domain encompasses After extensive testing, CFS simulations are also per- non-polar world oceans within 74oS and 64oN with zonal formed 20-year integrations with two diurnal mixed layer resolution as 1o, while the meridional resolution is 1/3o parameterizations respectively. The simulated SST diur- between10oS and 10oN, gradually increasing through Trop- nal variability and its influence on tropical intraseasonal ics until becoming fixed at 1o poleward of 30oS and 30oN. variability will be discussed in the following section. It has 40 vertical levels in height coordinator, with a 10-m resolution from the sea surface to 240m depth and 27 levels 3. Results in the upper 400m, the bottom depth is around 4.5km. The operational CFS was designed for daily coupling, a. SST Diurnal Variability which exchange daily averaged surface momentum, heat, The model simulated diurnal SST variability is com- and freshwater fluxes and SST between two components pared with the in situ SST observations from the TOGA once a day. Such coupling interval, however, totally sup- TAO/TRITON moorings in tropical region. To extract pressed the diurnal SST variability in the model. Our first the intermittent diurnal SST signals with variable ampli- experiment (control run, CTL hereafter) is to increase the tude from the total field, a novel data analysis method, the coupling frequency to every three hours and to exchange Ensemble Empirical Mode Decomposition (EEMD,Wu and 3-hourly averaged flux/SST between the two components. Huang 2009), has been applied. EEMD is a noise-assisted The CTL simulation is carried out for 20 years. This sim- algorithm to separate signals of different time scales adap- ulation allows the diurnal adjustment of the surface fluxes tive to local frequencies. Its efficiency to decompose multi- into the ocean. However, increasing the coupling frequency scaled time series into limited number of components has alone is inadequate to resolve the near-surface diurnal mix- been demonstrated by previous studies. ing processes. Our test also shows the diurnal SST change As an example, the TOGA TAO/TRITON SST ob- in CTL is negligible in most of the places. To improve the served in year 2009 at site 0oN 147oE is shown in Fig.1a and simulation of the diurnal SST variability, two parameter- its diurnal component extracted by EEMD in Fig.1b. At izations of the diurnal mixed layer by Zeng and Beljaars this location, a significant diurnal signal is superimposed on (2005), (ZB hereafter) and Schiller and Godfrey (2005)(SG the slower variations. The magnitude of the diurnal cycle hereaftet) have been implemented on CFS. can reach one degree, which is comparable with the am- The ZB parameterization assumes that a layer of strong plitude of seasonal variability. The strongest diurnal sig- diurnal cycle is embedded within the first vertical grid of an nal accompanies the SST warming trend while SST cooling ocean model. At the top of this ocean layer, they also con- primarily leads to the suppressed diurnal cycle. Such inter- sider a thin skin layer with depth about several millimeters, mittent feature of the amplitude of SST diurnal variability where molecular heat transport becomes dominant and the reflects the influences of local meteorological conditions, heat loss at the surface leads to a cold skin.
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