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The Indian summer monsoon intraseasonal oscillations (MISOs) induce pronounced intraseasonal sea surface temperature (SST) variability in the Bay of Bengal (BoB), which has important feedbacks to atmospheric convection. An ocean general circulation model (OGCM) is employed to investigate the upper-ocean processes affecting intraseasonal SST variability and its feedback to the MISO convection. In the BoB, the MISO induces intraseasonal SST variability predominantly through surface heat flux forcing with comparable contributions from shortwave radiation and turbulent heat flux, and to a much smaller extent through wind-driven ocean mixed layer entrainment. The ocean salinity stratification, represented by mixed layer depth (MLD) and barrier layer thickness (BLT), has a strong control on SST but weak impact on convection of the MISO. The MLD is critical for the amplitude of SST response to various forcing processes, while the BLT mainly affects entrainment by determining the temperature difference between the mixed layer and the water below. From May to mid-June, the shallow MLD and thin barrier layer greatly enhance intraseasonal SST anomalies, which can amplify convection fluctuations of the MISO through air-sea interaction and leads to intense but short-duration postconvection break spells. When either the MLD or the BLT is large, intraseasonal SSTs tend to be weak. Further investigation reveals that freshwater flux of the monsoon gives rise to the shallow MLD and thick barrier layer, and its overall effect on intraseasonal SSTs is a 20% enhancement. These results provide implications for improving the simulation and forecast of the MISO in climate models.

Previous studies suggest that both air-sea heat flux anomalies and heat advection caused by an anomalous Leeuwin Current play an important role in modulating the sea surface temperature (SST) variability associated with the Ningaloo Niño. However, the estimates of surface heat fluxes vary substantially with the datasets, and the uncertainties largely depend on the time scale and locations. This study investigates air-sea flux variability associated with the Ningaloo Niño using multiple datasets of surface fluxes. The climatological net surface heat flux off the west coast of Australia from six major air-sea flux products shows large uncertainties, which exceeds 80 W m-2, especially in the austral summer when the Ningaloo Niño develops. These uncertainties stem mainly from those in shortwave radiation and latent heat flux. The use of different bulk flux algorithms and uncertainties of bulk atmospheric variables (wind speed and air specific humidity) are mostly responsible for the difference in latent heat flux climatology between the datasets. The composite evolution of air-sea heat fluxes over the life cycle of Ningaloo Niño indicates that the anomalous latent heat flux is dominant for the net surface heat flux variations, and that the uncertainties in latent heat flux anomaly largely depend on the phase of the Ningaloo Niño. During the recovery period of Ningaloo Niño, large negative latent heat flux anomalies (cooling the ocean) are evident in all datasets and thus significantly contribute to the SST cooling. Because the recovery of winds occurs earlier than SST, high SST and strong winds favor large evaporative cooling during the recovery phase. In contrast, the role of latent heat flux during the developing phase is not clear, because the sign of the anomalies depends on the datasets in this period. The use of high-resolution SST data, which can adequately represent SST variations produced by the anomalous Leeuwin Current, could largely reduce the errors in latent heat flux anomalies during the onset and peak phases.

By analyzing satellite observations and conducting a series of ocean general circulation model experiments, this study examines the physical processes that determine intraseasonal variability (ISV) of the equatorial eastern Indian Ocean (EIO) upwelling for the 2001–2011 period. The ISV of EIO upwelling—as indicated by sea level, thermocline depth, and sea surface temperature (SST)—is predominantly forced by atmospheric intraseasonal oscillations (ISOs), and shows larger amplitudes during winter-spring season (November–April) when atmospheric ISOs are stronger than summer-fall (May–October). The chlorophyll (Chl-a) concentration, another indicator of upwelling, however reveals its largest intraseasonal variability during May–October, when the mean thermocline is shallow and seasonal upwelling occurs. For both winter-spring and summer-fall seasons, the ISV of EIO sea level and thermocline depth is dominated by remote forcing from the equatorial Indian Ocean wind stress, which drives Kelvin waves that propagate along the equator and subsequently along the Sumatra-Java coasts. Local wind forcing within the EIO plays a secondary role. The ISV of SST, however, is dominated by upwelling induced by remote equatorial wind only during summer-fall, with less contribution from surface heat fluxes for this season. During winter-spring, the ISV of SST results primarily from shortwave radiation and turbulent heat flux induced by wind speed associated with the ISOs, and local forcing dominates the SST variability. In this season, the mean thermocline is deep in the warm pool and thus thermocline variability decouples from the ISV of SST. Only in summer-fall when the mean thermocline is shallow, upwelling has important impact on SST.

The diurnal variability and the environmental conditions that support the moisture resurgence of MJO events observed during the Cooperative Indian Ocean Experiment on Intraseasonal Variability (CINDY)/DYNAMO campaign in October–December 2011 are investigated using in situ observations and the cloud-resolving fully air–ocean–wave Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS). Spectral density and wavelet analysis of the total precipitable water (TPW) constructed from the DYNAMO soundings and TRMM satellite precipitation reveal a deep layer of vapor resurgence during the observed Wheeler and Hendon real-time multivariate MJO index phases 5–8 (MJO suppressed phase), which include diurnal, quasi-2-, quasi-3–4-, quasi-6–8-, and quasi-16-day oscillations. A similar oscillatory pattern is found in the DYNAMO moorings sea surface temperature analysis, suggesting a tightly coupled atmosphere and ocean system during these periods. COAMPS hindcast focused on the 12–16 November 2011 event suggests that both the diurnal sea surface temperature (SST) pumping and horizontal and vertical moisture transport associated with the westward propagating mixed Rossby–Gravity (MRG) waves play an essential role in the moisture resurgence during this period. Idealized COAMPS simulations of MRG waves are used to estimate the MRG and diurnal SST contributions to the overall moisture increase. These idealized MRG sensitivity experiments showed the TPW increase varies from 9% to 13% with the largest changes occurring in the simulations that included a diurnal SST variation of 2.5°C as observed.

Intraseasonal sea surface temperature (SST) variability over the Seychelles–Chagos thermocline ridge (SCTR; 12°–4°S, 55°–85°E) 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 stress–driven 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 (70°–85°E), where the intraseasonal SSTs are strengthened by about 20% during the 2001–11 period. In the western part (55°–70°E), 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.

[1] The effects of solar radiation diurnal cycle on intraseasonal mixed layer variability in the tropical Indian Ocean during boreal wintertime Madden-Julian Oscillation (MJO) events are examined using the HYbrid Coordinate Ocean Model. Two parallel experiments, the main run and the experimental run, are performed for the period of 2005–2011 with daily atmospheric forcing except that an idealized hourly shortwave radiation diurnal cycle is included in the main run. The results show that the diurnal cycle of solar radiation generally warms the Indian Ocean sea surface temperature (SST) north of 10 � S, particularly during the calm phase of the MJO when sea surface wind is weak, mixed layer is thin, and the SST diurnal cycle amplitude (dSST) is large. The diurnal cycle enhances the MJO-forced intraseasonal SST variability by about 20% in key regions like the Seychelles-Chagos Thermocline Ridge (SCTR; 55 � –70 � E, 12 � –4 � S) and the central equatorial Indian Ocean (CEIO; 65 � –95 � E, 3 � S–3 � N) primarily through nonlinear rectification. The model also well reproduced the upper-ocean variations monitored by the CINDY/DYNAMO field campaign between September-November 2011. During this period, dSST reaches 0.7 � C in the CEIO region, and intraseasonal SST variability is significantly amplified. In the SCTR region where mean easterly winds are strong during this period, diurnal SST variation and its impact on intraseasonal ocean variability are much weaker. In both regions, the diurnal cycle also has a large impact on the upward surface turbulent heat flux QT and induces diurnal variation of QT with a peak-to-peak difference of O(10 W m � 2 ).

During the CINDY/DYNAMO field campaign (fall/winter 2011), intensive measurements of the upper ocean, including an array of several surface moorings and ship observations for the area around 75°E–80°E, Equator-10°S, were conducted. In this study, large-scale upper ocean variations surrounding the intensive array during the field campaign are described based on the analysis of satellite-derived data. Surface currents, sea surface height (SSH), sea surface salinity (SSS), surface winds and sea surface temperature (SST) during the CINDY/DYNAMO field campaign derived from satellite observations are analyzed. During the intensive observation period, three active episodes of large-scale convection associated with the Madden-Julian Oscillation (MJO) propagated eastward across the tropical Indian Ocean. Surface westerly winds near the equator were particularly strong during the events in late November and late December, exceeding 10 m/s. These westerlies generated strong eastward jets (&gt, 1 m/s) on the equator. Significant remote ocean responses to the equatorial westerlies were observed in both Northern and Southern Hemispheres in the central and eastern Indian Oceans. The anomalous SSH associated with strong eastward jets propagated eastward as an equatorial Kelvin wave and generated intense downwelling near the eastern boundary. The anomalous positive SSH then partly propagated westward around 4°S as a reflected equatorial Rossby wave, and it significantly influenced the upper ocean structure in the Seychelles-Chagos thermocline ridge about two months after the last MJO event during the field campaign. For the first time, it is demonstrated that subseasonal SSS variations in the central Indian Ocean can be monitored by Aquarius measurements based on the comparison with in situ observations at three locations. Subseasonal SSS variability in the central Indian Ocean observed by RAMA buoys is explained by large-scale water exchanges between the Arabian Sea and Bay of Bengal through the zonal current variation near the equator.

This study examines systematic biases in sea surface temperature (SST) under the stratus cloud deck in the southeast Pacific Ocean and upper-ocean processes relevant to the SST biases in 19 coupled general circulation models (CGCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The 20 years of simulations from each model are analyzed. Pronounced warm SST biases in a large portion of the southeast Pacific stratus region are found in all models. Processes that could contribute to the SST biases are examined in detail based on the computation of major terms in the upper-ocean heat budget. Negative biases in net surface heat fluxes are evident in most of the models, suggesting that the cause of the warm SST biases in models is not explained by errors in net surface heat fluxes. Biases in heat transport by Ekman currents largely contribute to the warm SST biases both near the coast and the open ocean. In the coastal area, southwestward Ekman currents and upwelling in most models are much weaker than observed owing to weaker alongshore winds, resulting in insufficient advection of cold water from the coast. In the open ocean, warm advection due to Ekman currents is overestimated in models because of the larger meridional temperature gradient, the smaller zonal temperature gradient, and overly weaker Ekman currents.

Sea-level rise is not globally uniform. A combination of observations and climate-model simulations reveals a pattern of sea-level changes in the Indian Ocean, with a decrease in the southern tropical Indian Ocean and a rise elsewhere, that can be attributed to changes in the atmospheric overturning circulation. Global sea level has risen during the past decades as a result of thermal expansion of the warming ocean and freshwater addition from melting continental ice1. However, sea-level rise is not globally uniform1,2,3,4,5. Regional sea levels can be affected by changes in atmospheric or oceanic circulation. As long-term observational records are scarce, regional changes in sea level in the Indian Ocean are poorly constrained. Yet estimates of future sea-level changes are essential for effective risk assessment2. Here we combine in situ and satellite observations of Indian Ocean sea level with climate-model simulations, to identify a distinct spatial pattern of sea-level rise since the 1960s. We find that sea level has decreased substantially in the south tropical Indian Ocean whereas it has increased elsewhere. This pattern is driven by changing surface winds associated with a combined invigoration of the Indian Ocean Hadley and Walker cells, patterns of atmospheric overturning circulation in the north–south and east–west direction, respectively, which is partly attributable to rising levels of atmospheric greenhouse gases. We conclude that—if ongoing anthropogenic warming dominates natural variability—the pattern we detected is likely to persist and to increase the environmental stress on some coasts and islands in the Indian Ocean.

The annual mean heat budget of the upper ocean beneath the stratocumulus/stratus cloud deck in the southeast Pacific is estimated using Simple Ocean Data Assimilation (SODA) and an eddy-resolving Hybrid Coordinate Ocean Model (HYCOM). Both are compared with estimates based on Woods Hole Oceanographic Institution (WHOI) Improved Meteorological (IMET) buoy observations at 20°S, 85°W. Net surface heat fluxes are positive (warming) over most of the area under the stratus cloud deck. Upper-ocean processes responsible for balancing the surface heat flux are examined by estimating each term in the heat equation. In contrast to surface heat fluxes, geostrophic transport in the upper 50 m causes net cooling in most of the stratus cloud deck region. Ekman transport provides net warming north of the IMET site and net cooling south of the IMET site. Although the eddy heat flux divergence term can be comparable to other terms at a particular location, such as the IMET mooring site, it is negligible for the entire stratus region when area averaged because it is not spatially coherent in the open ocean. Although cold-core eddies are often generated near the coast in the eddy-resolving model, they do not significantly impact the heat budget in the open ocean in the southeast Pacific.

Using sea surface height (SSH) data derived from recent satellite observations, we present the observed dispersion relationship of mixed Rossby Gravity (Yanai) waves in the Pacific Ocean. A wavenumber-frequency spectral analysis of SSH fields shows prominent spectral peaks along the dispersion curves of first and second baroclinic mode oceanic Yanai waves. Also, salient features of tropical instability waves (TIWs) with a period of ~17 days can be effectively isolated based on a cross-correlation analysis of sea surface temperature (SST) and SSH time series that are filtered in wavenumber-frequency domain. These statistical repre sentations of oceanic Yanai waves and TIWs are important for the evaluation of numerical model simulations of these waves and for improving our understanding of the physics of 17-day TIWs.

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.

Sea surface height (SSH), sea surface temperature (SST), and surface currents derived from satellite observations are analyzed to investigate signals of equatorial Kelvin and tropical instability waves (TIWs) in the Pacific Ocean. A wavenumber–frequency spectral analysis of SSH and SST anomalies was performed in order to examine their space and time variability. Significant spectral peaks along the dispersion curves of the first baroclinic mode Rossby and Kelvin waves are found in the SSH spectrum, indicating that the analysis can effectively identify the signals of equatorial waves in the upper ocean. A prominent peak in SSH fields at around 33 days and 1500 km wavelength along the Rossby wave dispersion curve is evident, and a similar peak is also found in SST fields. This upper ocean variability on these space and time scales is shown to be associated with TIWs. The spatial structure of 33-day TIWs is further examined based on an analysis of time series filtered in the frequency-wavenumber domain. The phase relationship between SSH, SST, and surface velocity associated with TIWs is described based on a cross-correlation analysis. Also, the interannual variability of TIW activity is compared with that of ENSO, showing a moderate correlation.

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.

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.

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.

Remote forcing of sea surface temperature (SST) variations in the Indian Ocean during the course of El Nino- 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. Composites of surface fluxes and SST over the Indian Ocean are formed based on El Nino and La Nina events during 1950-99. The surface flux variations in the eastern Indian Ocean in all three experiments are similar and realistic, confirming that much of the surface flux variation during ENSO is remotely forced from the Pacific. Furthermore, the SST anomalies in the eastern tropical Indian Ocean are well simulated by the coupled model, which supports the notion of an ''atmospheric bridge'' from the Pacific. During boreal summer and fall, when climatological winds are southeasterly over the eastern Indian Ocean, remotely forced anomalous easterlies act to increase the local wind speed. SST cools in response to increased evaporative cooling, which is partially offset by increased solar radiation associated with reduced rainfall. During winter, the climatological winds become northwesterly and the anomalous easterlies then act to reduce the wind speed and evaporative cooling. Together with increased solar radiation and a shoaling mixed layer, the SST warms rapidly. The model is less successful at reproducing the ENSO-induced SST anomalies in the western Indian Ocean, suggesting that dynamical ocean processes contribute to the east-west SST dipole that is often observed in boreal fall during ENSO events.

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 758 to 1758 Ea t 58S, 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.

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 ...

[1] This study examines the contribution of ocean dynamics to sea surface temperature (SST) biases in the eastern Pacific cold tongue region in fifteen coupled general circulation models (CGCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Twenty years (1980–1999) of the twentieth-century (20C3m) climate simulations from each model are analyzed. An excessive and narrow SST cold tongue that extends too far west into the western Pacific in comparison to observations is a common bias in CGCMs. This feature is found in CGCMs analyzed here and in many previous studies. The heat budget analysis indicates that errors in both net surface heat flux and total upper ocean heat advection significantly contribute to the excessive cold tongue in the equatorial Pacific. The stronger heat advection in the models is caused by overly strong horizontal heat advection associated with too strong zonal currents, and overly strong vertical heat advection due to excessive upwelling and the vertical gradient of temperature. The Bjerknes feedback in the coupled models is shown to be weaker than in observations, which may be related to the insufficient response of surface zonal winds to SST in the models and an erroneous subsurface temperature structure. A hypothesis that describes how the cold tongue bias is possibly developed in the CGCMs is provided based on the results of our analysis.

[1] El Nino Modoki is a variant of El Nino characterized by warming around the dateline flanked by anomalous cooling in the east and west. However, the opposite phase (La Nina Modoki) has received little attention because the prominent cooling of sea surface temperature (SST) during major La Nina events is observed in the central Pacific, and thus, it is difficult to define the two different types of cold events from the SST anomaly pattern. Here we demonstrate that cold events in 2000 and 2008 can be clearly distinguished from traditional La Nina events using surface currents derived from satellite observations. During 2000 and 2008, anomalous zonal currents in the equatorial Pacific demonstrate divergence, with westward currents west of the dateline and eastward currents east of it. These currents are opposite to the circulation pattern during the 2004 El Nino Modoki event. An empirical orthogonal function (EOF) analysis of surface currents for the period 1993–2009 shows a circulation anomaly pattern similar to that in 2000, 2004, and 2008 in the second EOF. The first EOF is consistent with traditional El Nino/La Nina events and does not exhibit a current reversal along the equator. Our results also indicate that strong cyclonic (anticyclonic) circulation anomalies occur in the tropical western Pacific around 5°N–15°N during the 2004 (2000 and 2008) El Nino (La Nina) Modoki events and during the strong traditional El Nino (La Nina) events of 1997 (1998). These circulation anomalies are related to an SST gradient in the western and central Pacific.

Processes that control the upper ocean thermohaline structure in the western equatorial Pacific are examined using a Lagrangian mixed layer model. The one-dimensional bulk mixed layer model of Garwood (1977) is integrated along the trajectories derived from a nonlinear 1 1/2 layer reduced gravity model forced with actual wind fields. The Global Precipitation Climatology Project (GPCP) data are used to estimate surface freshwater fluxes for the mixed layer model. The wind stress data which forced the 1 1/2 layer model are used for the mixed layer model. The model was run for the period 1987-1988. This simple model is able to simulate the isothermal layer below the mixed layer in the western Pacific warm pool and its variation. The subduction mechanism hypothesized by Lukas and Lindstrom (1991) is evident in the model results. During periods of strong South Equatorial Current, the warm and salty mixed layer waters in the central Pacific are subducted below the fresh shallow mixed layer in the western Pacific. However, this subduction mechanism is not evident when upwelling Rossby waves reach the western equatorial Pacific or when a prominent deepening of the mixed layer occurs in the western equatorial Pacific or when a prominent deepening of the mixed layer occurs in the western equatorial Pacific due to episodes of strong wind and light precipitation associated with the El Nino-Southern Oscillation. Comparison of the results between the Lagrangian mixed layer model and a locally forced Eulerian mixed layer model indicated that horizontal advection of salty waters from the central Pacific strongly affects the upper ocean salinity variation in the western Pacific, and that this advection is necessary to maintain the upper ocean thermohaline structure in this region.