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Toshiaki Shinoda

Abstract

The mechanism by which the diurnal cycle of solar radiation modulates intraseasonal SST variability in the western Pacific warm pool is investigated using a one-dimensional mixed layer model. SSTs in the model experiments forced with hourly surface fluxes during the calm–sunny phase of intraseasonal oscillation are significantly warmer than those with daily mean surface fluxes. The difference in two experiments is explained by upper-ocean mixing processes during nighttime. Surface warming during daytime creates a shallow diurnal warm layer near the surface (0–3 m), which can be easily eroded by surface cooling during nighttime. Further cooling, however, requires a substantial amount of energy because deeper waters need to be entrained into the mixed layer. Since the shallow diurnal layer is not formed in the experiment with daily mean surface fluxes, the SST for the hourly forcing case is warmer most of the time due to the diurnally varying solar radiation.

Sensitivity of the intraseasonal SST variation to the penetrative component of solar radiation is examined, showing that the diurnal cycle plays an important role in the sensitivity. Solar radiation absorbed in the upper few meters significantly influences intraseasonal SST variations through changes in amplitude of diurnal SST variation.

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Toshiaki Shinoda
and
Jialin Lin

Abstract

Persistent stratus/stratocumulus cloud decks in the southeast Pacific near the coasts of Peru and northern Chile play an important role in regional and global climate variability. Interannual variability of the upper ocean under stratus cloud decks in the southeast Pacific is investigated using ocean general circulation model (OGCM) experiments. The model was first forced with daily surface fluxes based on the NCEP–NCAR reanalysis and satellite-derived surface shortwave and longwave radiation for the period of 1979–2004. Gridded surface heat flux estimates used in the model integration agree well with those based on Woods Hole Oceanographic Institution (WHOI) Improved Meteorology (IMET) buoy measurements at 20°S, 85°W. Also, the OGCM is able to reproduce well the observed interannual SST and sea surface height variations in this region. The results suggest that the interannual variation of the upper ocean north of 20°S is mostly associated with ENSO variability. Additional model experiments were conducted to examine the relative importance of ocean dynamics and surface heat fluxes in determining the interannual variation in SST. The results of these experiments indicate that upper-ocean dynamics play a dominant role in controlling the interannual variation of SST north of 20°S in the stratus cloud region. The upper-ocean heat budget analysis shows that meridional heat advection associated with ENSO events primarily controls the interannual SST variation in the stratus cloud region north of 20°S.

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Toshiaki Shinoda
and
Weiqing Han

Abstract

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.

During boreal fall (September–November), the interannual variation of 6–30-day (submonthly) near-surface zonal wind activity in the central and eastern equatorial Indian Ocean is highly correlated with the large-scale zonal SST gradient. The intensity of submonthly variability is largely reduced during positive dipole years. A significant reduction of intraseasonal (30–90-day) wind activity is also evident during large dipole events. However, the correlation with the zonal SST gradient is much weaker than that of submonthly variability.

The mechanism by which the Indian Ocean dipole influences equatorial submonthly winds is investigated based on a cross-correlation analysis of OLR and winds. During negative dipole years, submonthly convection is active in the southeast Indian Ocean where the anomalous convergence of surface moisture associated with dipole events is at its maximum. The submonthly convection in this region is often associated with a cyclonic circulation, and these disturbances propagate westward. Consequently, equatorial westerlies and northwesterly winds near the coast of Sumatra are generated. During positive dipole years, submonthly convective activity is highly reduced in the southeast Indian Ocean, and thus no equatorial westerly is generated.

Ocean response to submonthly disturbances is examined using OGCM experiments forced with winds from the NCEP–NCAR reanalysis. Results suggest that submonthly winds can generate significant upper-ocean response, including strong eastward surface currents near the equator and sea surface height anomalies along the coast of Sumatra where the large SST anomalies associated with dipole events are observed.

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Toshiaki Shinoda
and
Harry H. Hendon

Abstract

Sea surface temperature (SST) variations associated with the atmospheric intraseasonal oscillation in the tropical Indian and western Pacific Oceans, are examined using a one-dimensional mixed layer model. Surface fluxes associated with 10 well-defined intraseasonal events from the period 1986–93 are used to force the model. Surface winds from the European Centre for Medium-Range Weather Forecasts daily analyses and SST from the mixed layer model are used to compute latent and sensible heat fluxes and wind stress with the TOGA COARE bulk flux algorithm. Surface freshwater flux is estimated from the Microwave Sounding Unit precipitation data. Net shortwave radiation is estimated, via regression analysis, from outgoing longwave radiation. An idealized diurnal cycle of shortwave radiation is also imposed. The intraseasonal SST variation from the model, when forced by the surface fluxes estimated from gridded analyses, agrees well with the SST observed at a mooring during the COARE. The model was then integrated for the 10 well-defined intraseasonal events at grid points from 75° to 175°E at 5°S, which spans the warm pool of the equatorial Indian and western Pacific Oceans. The one-dimensional model is able to simulate the amplitude of the observed intraseasonal SST variation throughout this domain. Variations of shortwave radiation and latent heat flux are equally important for driving the SST variations in the western Pacific, while latent heat flux variations are less important in the Indian Ocean. The phasing of the intraseasonal variation of precipitation relative to wind stress results in little impact of the freshwater flux variation on the intraseasonally varying mixed layer. The diurnal cycle of shortwave radiation is found to significantly increase the intraseasonal amplitude of SST over that produced by daily mean insolation.

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Toshiaki Shinoda
and
Harry H. Hendon

Abstract

The upper-ocean heat budget in response to the atmospheric Madden–Julian oscillation (MJO) in the western equatorial Pacific is examined using a tropical Pacific basin general circulation model. The model is forced with surface fluxes associated with 10 well-defined MJO events from the period 1986–93. Surface fluxes were estimated from gridded operational analyses from the European Centre for Medium-Range Weather Forecasts and independent satellite data.

A 10-event composite of the model results was formed. The simulated composite SST agrees well with the observed composite from weekly SST analyses. Also, the simulated intraseasonal SST variation for the large MJO event during TOGA COARE (December 1992) agrees reasonably well with SST observed at a mooring. The strong equatorial jet associated with this MJO event is also well simulated.

The heat budget of the warm pool is calculated from the model output in order to investigate the role of three-dimensional processes in driving the intraseasonal SST variability. Although horizontal advection of heat is locally large, it is incoherent on the scale of MJO. It is confirmed that the intraseasonal SST variation in the western Pacific warm pool is primarily controlled by the surface heat flux variation and vertical processes.

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Toshiaki Shinoda
and
Harry H. Hendon

Abstract

Rectification of (Madden–Julian oscillation) MJO-induced wind speed and latent heat flux variations across the tropical Indian and western Pacific Oceans is estimated using 51 yr of NCEP–NCAR reanalysis. The rectified wind speed anomaly is calculated from the difference in wind speed based on 30- and 90-day low-pass-filtered winds. During periods when the MJO is active, the wind speed is typically enhanced by about 1 m s−1 south of the equator in the western Pacific. The largest rectified latent heat flux occurred during the large MJO event of March 1997 in the western Pacific warm pool. The magnitude of the rectification is found to depend strongly on the mean wind speed, and this affects the temporal and spatial variations of the rectification.

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Toshiaki Shinoda
,
Harry H. Hendon
, and
John Glick

Abstract

Composites of sea surface temperature (SST), surface heat, momentum, and freshwater flux anomalies associated with intraseasonal oscillations of convection are developed for the warm pool of the western Pacific and Indian Oceans during 1986–93. The composites are based on empirical orthogonal function analysis of intraseasonally filtered outgoing longwave radiation (OLR), which efficiently extracts the Madden–Julian oscillation (MJO) in convection. Surface fluxes are estimated using gridded analyses from the European Centre for Medium-Range Weather Forecasts, weekly SST, OLR, microwave sounding unit precipitation, and the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) bulk flux algorithm. At intraseasonal timescales, these surface flux estimates agree reasonably well with estimates based on mooring observations collected during TOGA COARE.

The amplitude of the composite SST variation produced by the MJO is about 0.25°C in the western Pacific, 0.35°C in the Indonesian region, and 0.15°C in the Indian Ocean. The intraseasonal anomalies of SST and net surface heat flux propagate eastward at about 4 m s−1 along with the convective anomaly. The amplitude of the net surface heat flux variation is 50–70 W m−2 in the western Pacific, with anomalous insolation and latent heat flux making similar contributions. Across the Indian Ocean, the net surface heat flux anomaly is weaker (30–40 W m−2), and anomalous insolation appears to make a greater contribution than anomalous latent heat flux. Across the entire warm pool, the net surface heat flux leads the SST variation by about one-quarter cycle, which is consistent with the notion that surface heat flux variations are driving the SST variations at these intraseasonal timescales. The intraseasonal SST variation, however, is estimated to significantly reduce the amplitude of the latent and sensible heat fluxes produced by the MJO.

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Toshiaki Shinoda
,
Harry H. Hendon
, and
John Glick

Abstract

Reliability of the surface fluxes from National Centers for Environmental Prediction (NCEP) reanalyses is assessed across the warm pool of the western Pacific and Indian Oceans. Emphasis is given to the spatial distribution and coherence of the fluxes on intraseasonal (25–100 day) periods, as intraseasonal variability predominates the subseasonal variability across the warm pool. Comparison is made with surface fluxes estimated from data collected at a mooring during the Coupled Ocean–Atmosphere Response Experiment and with independent gridded estimates based on operational wind and surface pressure analyses and satellite observations of rainfall, shortwave radiation, and outgoing longwave radiation. In general, fluxes that depend primarily on surface wind variations (e.g., stress and latent heat flux) agree more favorably than fluxes that are largely dependent on fluctuations of convection (e.g., surface shortwave radiation and freshwater or precipitation). In particular, the intraseasonal variance of shortwave radiation and precipitation in the NCEP reanalyses is about half of that estimated from in situ observations and from satellite observations. Composite surface flux variations for the Madden–Julian oscillation, which is the dominant mode of intraseasonal variability in the warm pool, are also constructed. Again, the composite variations of wind stress and latent heat flux from the NCEP reanalyses agree reasonably well, both in magnitude and phasing, with the composite fluxes from the independent gridded data. However, the composite intraseasonal shortwave radiation and precipitation from the NCEP reanalyses, while agreeing in phase, exhibit less than half the amplitude of the satellite-based estimates.

The impact of the underestimation of these surface flux variations in the NCEP reanalyses on the intraseasonal evolution of sea surface temperature (SST) in the warm pool is investigated in the context of a one-dimensional mixed layer model. When forced with the intraseasonal surface fluxes from the NCEP reanalyses, the amplitude of the intraseasonal SST variation is some 30%–40% smaller than observed or than that from forcing with the independent gridded fluxes. This reduced amplitude is primarily caused by the underestimation of the intraseasonal shortwave radiation variations in the NCEP reanalyses.

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Jia-Lin Lin
,
Taotao Qian
, and
Toshiaki Shinoda

Abstract

This study examines the stratocumulus clouds and associated cloud feedback in the southeast Pacific (SEP) simulated by eight global climate models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) and Cloud Feedback Model Intercomparison Project (CFMIP) using long-term observations of clouds, radiative fluxes, cloud radiative forcing (CRF), sea surface temperature (SST), and large-scale atmosphere environment. The results show that the state-of-the-art global climate models still have significant difficulty in simulating the SEP stratocumulus clouds and associated cloud feedback. Comparing with observations, the models tend to simulate significantly less cloud cover, higher cloud top, and a variety of unrealistic cloud albedo. The insufficient cloud cover leads to overly weak shortwave CRF and net CRF. Only two of the eight models capture the observed positive cloud feedback at subannual to decadal time scales. The cloud and radiation biases in the models are associated with 1) model biases in large-scale temperature structure including the lack of temperature inversion, insufficient lower troposphere stability (LTS), and insufficient reduction of LTS with local SST warming, and 2) improper model physics, especially insufficient increase of low cloud cover associated with larger LTS. The two models that arguably do best at simulating the stratocumulus clouds and associated cloud feedback are the only ones using cloud-top radiative cooling to drive boundary layer turbulence.

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Lei Zhang
,
Weiqing Han
,
Yuanlong Li
, and
Toshiaki Shinoda

Abstract

Generation and development mechanisms of the Ningaloo Niño are investigated using ocean and atmospheric general circulation model experiments. Consistent with previous studies, northerly wind anomalies off the West Australian coast are critical in generating warm sea surface temperature (SST) anomalies of the Ningaloo Niño, which induce SST warming through reduced turbulent heat loss toward the atmosphere (by decreasing surface wind speed), enhanced Leeuwin Current heat transport, and weakened coastal upwelling. Our results further reveal that northerly wind anomalies suppress the cold dry air transport from the Southern Ocean to the Ningaloo Niño region, which also contributes to the reduced turbulent heat loss. A positive cloud–radiation feedback is also found to play a role. Low stratiform cloud is reduced by the underlying warm SSTAs and the weakened air subsidence, which further enhances the SST warming by increasing downward solar radiation. The enhanced Indonesian Throughflow also contributes to the Ningaloo Niño, but only when La Niña co-occurs. Further analysis show that northerly wind anomalies along the West Australian coast can be generated by both remote forcing from the Pacific Ocean (i.e., La Niña) and internal processes of the Indian Ocean, such as the positive Indian Ocean dipole (IOD). Approximately 40% of the Ningaloo Niño events during 1950–2010 co-occurred with La Niña, and 30% co-occurred with positive IOD. There are also ~30% of the events independent of La Niña and positive IOD, suggesting the importance of other processes in triggering the Ningaloo Niño.

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