Sea Surface Heat Flux Research Articles

Air–sea interaction over ocean fronts and eddies

Air–sea interaction at ocean fronts and eddies exhibits positive correlation between sea surface temperature (SST), wind speed, and heat fluxes out of the ocean, indicating that the ocean is forcing the atmosphere. This contrasts with larger scale climate modes where the negative correlations suggest that the atmosphere is driving the system. This paper examines the physical processes that lie behind the interaction of sharp SST gradients and the overlying marine atmospheric boundary layer and deeper atmosphere, using high resolution satellite data, field data and numerical models. The importance of different physical mechanisms of atmospheric response to SST gradients, such as the effect of surface stability variations on momentum transfer, pressure gradients, secondary circulations and cloud cover will be assessed. The atmospheric response is known to create small-scale wind stress curl and divergence anomalies, and a discussion of the feedback of these features onto the ocean will also be presented. These processes will be compared and contrasted for different regions such as the Equatorial Front in the Eastern Pacific, and oceanic fronts in mid-latitudes such as the Gulf Stream, Kuroshio, and Agulhas Return Current.

Estimate of Water Residence Times in Tudor Creek, Kenya Based on Sea Surface Heat Fluxes and Observations of the Horizontal Temperature Gradient During Different Seasons

Salinity and temperature measurements were carried out in Tudor Creek, Kenya from 1995-98 in order to determine water exchange between the creek and the adjacent Indian Ocean. Data from the Kenya Meteorological Department including gauging data from two small rivers were employed to estimate long-term rainfall, evaporation and river runoff. However, even though the observed salinity gradient in the creek appeared consistent with dry and rain periods, estimates of river runoffs were not good enough to calculate water exchange, based on salt conservation. Runoff in general was also too small to give reliable rating curves (correlation between rainfall and river runoff). For this reason, heat conservation was used for the calculation of water exchange. Although estimates of sea surface heat fluxes were based on coarse global climatology data with large seasonal variations in the net heat flux (from 50-150 Wm -2 ), the result was surprisingly consistent, with similar water exchange during all different seasons. Residence times for the creek waters in relation to the ocean water are between 3-5 days, 5 days for the waters inside the deep inlet. During spring tides, the exchange is about twice as large as during neap tides. Keywords : Kenya, water exchange, sea surface heat fluxes, heat budget, Kenya coastal meteorology West Indian Ocean Journal of Marine Science Vol. 5 (2) 2006: pp. 163-178

Upper ocean response of the Mesoamerican Barrier Reef System to Hurricane Mitch and coastal freshwater inputs: A study using Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) ocean color data and a nested‐grid ocean circulation model

The passage of category‐5 Hurricane Mitch through the Mesoamerican Barrier Reef System (MBRS) in October 1998 was an extreme event with the potential to create unusual patterns of reef connectivity. The impact of this hurricane on the upper ocean of the MBRS is investigated using a triply nested grid ocean circulation modeling system. The model results are validated with contemporaneous ocean color data from the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) satellite and oceanographic measurements in the MBRS. The nested grid system is forced by 6‐hourly National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) winds for the first 294 days prior to the arrival of the hurricane in the MBRS, and then by the combination of the NCEP/NCAR wind‐forcing and an idealized vortex representative of Mitch for the following 20 days. The system is also forced by the monthly mean sea surface heat and freshwater fluxes and buoyancy forcing associated with major river discharges and storm‐induced precipitation in the western Caribbean Sea. The simulated upper ocean circulation during Mitch is characterized by strong and divergent currents under the storm and intense near‐inertial currents and sea surface temperature cooling behind the storm. The nested grid system also reproduces the buoyant estuarine plumes extending from the coast off Honduras as inferred from SeaWiFS satellite data and detected in field measurements at Gladden Spit in Belize shortly after the passage of Hurricane Mitch. The present model results suggest that populations of site‐attached organisms associated with nearshore and offshore reef features that are dynamically isolated in normal conditions experienced greater potential for ecological connection under Mitch's extreme conditions.

Annual, Seasonal, and Interannual Variability of Air–Sea Heat Fluxes in the Indian Ocean

Abstract This study investigated the accuracy and physical representation of air–sea surface heat flux estimates for the Indian Ocean on annual, seasonal, and interannual time scales. Six heat flux products were analyzed, including the newly developed latent and sensible heat fluxes from the Objectively Analyzed Air–Sea Heat Fluxes (OAFlux) project and net shortwave and longwave radiation results from the International Satellite Cloud Climatology Project (ISCCP), the heat flux analysis from the Southampton Oceanography Centre (SOC), the National Centers for Environmental Prediction reanalysis 1 (NCEP1) and reanalysis-2 (NCEP2) datasets, and the European Centre for Medium-Range Weather Forecasts operational (ECMWF-OP) and 40-yr Re-Analysis (ERA-40) products. This paper presents the analysis of the six products in depicting the mean, the seasonal cycle, and the interannual variability of the net heat flux into the ocean. Two time series of in situ flux measurements, one taken from a 1-yr Arabian Sea Experiment field program and the other from a 1-month Joint Air–Sea Monsoon Interaction Experiment (JASMINE) field program in the Bay of Bengal were used to evaluate the statistical properties of the flux products over the measurement periods. The consistency between the six products on seasonal and interannual time scales was investigated using a standard deviation analysis and a physically based correlation analysis. The study has three findings. First of all, large differences exist in the mean value of the six heat flux products. Part of the differences may be attributable to the bias in the numerical weather prediction (NWP) models that underestimates the net heat flux into the Indian Ocean. Along the JASMINE ship tracks, the four NWP modeled mean fluxes all have a sign opposite to the observations, with NCEP1 being underestimated by 53 W m−2 (the least biased) and ECMWF-OP by 108 W m−2 (the most biased). At the Arabian Sea buoy site, the NWP mean fluxes also have an underestimation bias, with the smallest bias of 26 W m−2 (ERA-40) and the largest bias of 69 W m−2 (NCEP1). On the other hand, the OAFlux+ISCCP has the best comparison at both measurement sites. Second, the bias effect changes with the time scale. Despite the fact that the mean is biased significantly, there is no major bias in the seasonal cycle of all the products except for ECMWF-OP. The latter does not have a fixed mean due to the frequent updates of the model platform. Finally, among the four products (OAFlux+ISCCP, ERA-40, NCEP1, and NCEP2) that can be used for studying interannual variability, OAFlux+ISCCP and ERA-40 Qnet have good consistency as judged from both statistical and physical measures. NCEP1 shows broad agreement with the two products, with varying details. By comparison, NCEP2 is the least representative of the Qnet variabilities over the basin scale.

On the effects of parameterized Mediterranean overflow on North Atlantic ocean circulation and climate

A parameterized Mediterranean overflow, based on the marginal sea boundary condition of [Price, J., Yang, J., 1998. Marginal sea overflows for climate simulations. In: Chassignet, E.P., Verron, J., (Eds.), Ocean Modeling and Parameterization, Kluwer Academic, pp. 155–170], has been implemented in the ocean component of the Community Climate System Model to represent exchanges through the Strait of Gibraltar, associated entrainment and intrusion of overflow product water into the Atlantic. Previously, in coarse resolution model versions with a closed Strait, this physics has been either missing in uncoupled configurations or only partially and unphysically treated as a surface salt exchange when fully coupled. Parameter choices are evaluated by comparing climatologically forced solutions to observations and process model results. The two major criteria satisfied by the implementation in a fully coupled climate model and an uncoupled global ocean model are stable solutions and projection of the overflow signal across the Atlantic basin at about 1000 m depth. Both of these configurations are low resolution, and in both the transports of inflow, source and entrainment water are all within the range of observed estimates, but there is too little product water. This bias is attributed to inadequate modeling of water masses in the Mediterranean source region. Nevertheless, the properties of the product water differ little from observed estimates and both the uncoupled and coupled models develop a Mediterranean salt tongue that spreads west and south from the Strait with a signature reminiscent of the observed hydrography. The improvements relative to either blocking the Strait, or excavating a too wide channel are presented. In the coupled solution, the impact of the improved overflow physics on the global climate is minimal, with North Atlantic sea surface temperatures and heat fluxes changing generally by less than 1 °C and 15 W m −2, respectively. However, there is interesting spatial variability in the coupling strength, which ranges between ±20 W m −2 °C −1 in the coupled case.

Sensitivity of torrential rain events to the sea surface temperature based on high‐resolution numerical forecasts

Torrential rains often occur in the western Mediterranean region during the fall season when the Mediterranean Sea is still warm. The Mediterranean Sea acts in moistening and warming the low level of the atmosphere. Then, the southerly to easterly flow that prevails before and during torrential rainfall events, transports the conditionally unstable air toward the coasts where the convection can develop often triggered by a flow interaction with orography. This study examines the sensitivity to the sea surface temperature (SST) of very short range (18–24 hours) high‐resolution (2.4 km) forecasts of heavy precipitation events. Three torrential rainfall events were selected as representative of extreme rainfall events that occurred over southern France: two cases of quasi‐stationary mesoscale convective systems and one other case characterized by a slow moving frontal perturbation. For each case, a number of runs is performed with the MESO‐NH research model using several SST fields differing in their resolution or/and their average value over the Mediterranean basin. Results show that an increase (a decrease) of SST by several degrees, on average, intensifies (weakens) the convection. The convection can even be stopped with strong decrease of SST. Impacts on the stationary behavior of the systems have also been pointed out. A more detailed SST field influences the mesoscale patterns of the sea surface heat fluxes but have almost no significant effect on the convection and the low‐level jets forecast. Eventually, the SST has been allowed to evolve during the runs through the action of the air‐sea interface fluxes, resulting in local effects such as significant cooling of the SST beneath the low‐level jet but almost no impact on the very short range forecasts of heavy precipitation.

Seasonality of Decadal Sea Surface Temperature Anomalies in the Northwestern Pacific

Abstract The seasonality of the decadal sea surface temperature (SST) anomalies and the related physical processes in the northwestern Pacific were investigated using a three-dimensional bulk mixed layer model. In the Kuroshio–Oyashio Extension (KOE) region, the strongest decadal SST anomaly was observed during December–February, while that of the central North Pacific occurred during February–April. From an examination of the seasonal heat budget of the ocean mixed layer, it was revealed that the seasonal-scale enhancement of the decadal SST anomaly in the KOE region was controlled by horizontal Ekman temperature transport in early winter and by vertical entrainment in autumn. The temperature transport by the geostrophic current made only a slight contribution to the seasonal variation of the decadal SST anomaly, despite controlling the upper-ocean thermal conditions on decadal time scales through the slow Rossby wave adjustment to the wind stress curl. When averaging over the entire KOE region, the contribution from the net sea surface heat flux was also no longer significantly detected. By examining the horizontal distributions of the local thermal damping rate, however, it was concluded that the wintertime decadal SST anomaly in the eastern KOE region was rather damped by the net sea surface heat flux. It was due to the fact that the anomalous local thermal damping of the SST anomaly resulting from the vertical entrainment in autumn was considerably strong enough to suppress the anomalous local atmospheric thermal forcing that acted to enhance the decadal SST anomaly.

Tropical Atlantic sea surface temperature and heat flux simulations in a coupled GCM

[1] This paper contrasts the SST and heat flux errors in the Tropical Atlantic simulated by the CPTEC Coupled ocean-atmosphere General Circulation Model and its oceanic model forced by momentum and heat estimates. Comparisons between solar radiation estimated by satellite and measurements of PIRATA buoys have been made with the purpose of analyzing the impact of solar radiation in the simulation of SST in the tropical Atlantic. The radiative transfer model (ISCCP DX) has shown higher correlation with the buoys data than ECMWF ERA40 with larger differences over the eastern tropical Atlantic, where the numerical prediction models present difficulties in simulating the appearance of stratus clouds. The use of solar radiation based on satellite estimates and parameterized heat flux generated the best SST and surface heat fluxes. The stronger surface stresses generated by the CGCM contributed to generating an oceanic thermal structure in closer agreement with observations than the OGCM runs.

Air–Sea Interactions in the Tropical Atlantic: A View Based on Lagged Rotated Maximum Covariance Analysis

Abstract The dominant air–sea feedbacks that are at play in the tropical Atlantic are revisited, using the 1958–2002 NCEP reanalysis. To separate between different modes of variability and distinguish between cause and effect, a lagged rotated maximum covariance analysis (MCA) of monthly sea surface temperature (SST), wind, and surface heat flux anomalies is performed. The dominant mode is the ENSO-like zonal equatorial SST mode, which has its maximum amplitude in boreal summer and is a strongly coupled ocean–atmosphere mode sustained by a positive feedback between wind and SST. The turbulent heat flux feedback is negative, except west of 25°W where it is positive, but countered by a negative radiative feedback associated with the meridional displacement of the ITCZ. As the maximum covariance patterns change little between lead and lag conditions, the in-phase covariability between SST and the atmosphere can be used to infer the atmospheric response to the SST anomaly. The second climate mode involves an SST anomaly in the tropical North Atlantic, which is primarily generated by the surface heat flux and, in boreal winter, wind changes off the coast of Africa. After it has been generated, the SST anomaly is sustained in the deep Tropics by the positive wind–evaporation–SST feedback linked to the wind response to the SST. However, north of about 10°N where the SST anomaly is largest, the wind response is weak and the heat flux feedback is negative, thus damping the SST anomaly. As the in-phase maximum covariance patterns primarily reflect the atmospheric forcing of the SST, simultaneous correlations cannot be used to describe the atmospheric response to the SST anomaly, except in the deep Tropics. Using instead the maximum covariance patterns when SST leads the atmosphere reconciles the results of recent atmospheric general circulation model experiments with the observations.

Reduced Horizontal Sea Surface Temperature Gradients Under Conditions of Clear Skies and Weak Winds

Consideration of the dependence of various components of the sea-surface heat and momentum fluxes on sea surface temperature (SST) leads to an explanation for the observed reduction in the horizontal temperature gradients in the uppermost layer of the ocean (a few to 10 m in depth). Horizontal temperature gradients within the mixed layer can be masked by a near-surface layer of warm water. This camouflage of horizontal temperature gradients has importance for the remote sensing of SST used by the fishing industry, for the estimation of acoustic transmission, and for the forecasting of hurricane development, among many uses of SST data. Diurnal warming conditions in the Straits of Florida are examined by a simulation calculation and by analysis of observations obtained on moorings deployed on the south-east Florida shelf. When there is net heating (i.e., the solar input is stronger than the combined latent, sensible and longwave radiative heat losses) the originally warmer water experiences less heating than the colder water, leading to a weakening of the horizontal SST gradients as seen by surface buoys or satellites. The warmer water also experiences more mixing and therefore less increase in temperature. The strongest effect of the diurnal heating on wind stress occurs when the SST starts out cooler than the air temperature and the atmosphere is stably stratified. Diurnal warming can then rapidly increase the SST above the air temperature because of reduced wind stress and reduced upper-ocean mixing. After that the wind stress increases as convectively driven turbulence contributes to the atmospheric exchange.

Mechanisms Forcing an Antarctic Dipole in Simulated Sea Ice and Surface Ocean Conditions

Abstract The mechanisms forcing variability in Southern Ocean sea ice and sea surface temperature from 600 years of a control climate coupled model integration are discussed. As in the observations, the leading mode of simulated variability exhibits a dipole pattern with positive anomalies in the Pacific sector associated with negative anomalies in the Atlantic. It is found that in the Pacific ocean circulation changes associated with variable wind forcing modify the ocean heat flux convergence and sea ice transport, resulting in sea surface temperature and sea ice anomalies. The Pacific ice and ocean anomalies persist over a number of years due to reductions in ocean shortwave absorption reinforcing the initial anomalies. In the Atlantic sector, no single process dominates in forcing the anomalies. Instead there are contributions from changing ocean and sea ice circulation and surface heat fluxes. While the absorbed solar radiation in the Atlantic is modified by the changing surface albedo, the anomalies are much shorter-lived than in the Pacific because the ocean circulation transports them northward, removing them from ice formation regions. Sea ice and ocean anomalies associated with the El Niño–Southern Oscillation and the Southern Annular Mode both exhibit a dipole pattern and contribute to the leading mode of ice and ocean variability.

Sea Spray Impacts on Intensifying Midlatitude Cyclones

Abstract Air–sea transfer processes over the ocean strongly affect how hurricanes develop. High winds generate large amounts of sea spray, which can modify the transfer of momentum, heat, and moisture across the air–sea interface. However, the extent to which sea spray can modify extratropical or midlatitude hurricanes and intense cyclones has not been resolved. This paper reports simulations of extratropical Hurricanes Earl (1998) and Danielle (1998) and an intense winter cyclone from January 2000 using a mesoscale atmospheric model and a recent sea spray parameterization. These simulations show that sea spray can increase the sea surface heat flux, especially the latent heat flux, in a midlatitude cyclone and that sea spray’s impact on cyclone intensity depends on the storm structure and development and is strongest for cyclones with high winds.

Eastward propagating surface anomalies at ocean gyre boundaries

Inspection of animations and time‐distance (Hovmoller) plots of unfiltered data from a state‐of‐the‐art coupled climate model (HadCM3) reveals eastward propagating surface temperature‐salinity anomalies at subpolar/subtropical gyre boundaries throughout the world ocean. Maximum anomalies are ∼4°C and ∼0.3 practical salinity units. Horizontal length scales are ∼1000 km. The anomalies travel at a speed consistent with advection by mean near‐surface currents. Generation regions are located at western boundaries and possess an order of magnitude higher variability than all other regions. In some cases, surface heat fluxes are shown to cause anomaly generation; in other cases, surface current anomalies are likely to be responsible. Eastward anomaly propagation also occurs in the Estimating the Circulation and Climate of the Ocean model. The coarser resolution Fast Ocean Rapid Troposphere Experiment (FORTE) coupled model shows no propagating anomalies. Sea surface anomalies in the instrumental record are investigated, using ship‐based sea surface temperature (SST) and heat fluxes, and satellite SST and altimetry. Unlike the models, Hovmoller plots and animations of the unfiltered observational data showed no evidence of eastward propagation. By applying a decadal‐timescale band‐pass filter to the SST time series, we identify underlying propagation of basin‐scale anomalies in both HadCM3 and observations. Maximum anomalies are ∼1°C and length scales are ∼2500 km. These anomalies travel at speeds significantly slower than mean near‐surface currents. Ten anomalies are identified during the period 1870–2002, mostly in the eastern Atlantic basin (east of 45°W). HadCM3 displayed only one propagating anomaly of this type in a typical 100 years of data. We discuss possible reasons for the range of results in models and observations.

Impacts of the Indian Ocean on the Indian Summer Monsoon–ENSO Relationship

This study investigates the impacts of the Indian Ocean on the relationship between the Indian summer monsoon and the El Nino-Southern Oscillation (ENSO) through numerical simulations with a coupled atmosphere-ocean general circulation model and atmospheric general circulation model (AGCM) experiments with specified sea surface temperature (SST) and surface heat flux (SHF) forcing. Previous studies have shown that this particular coupled model captures many aspects of the observed Indian summer monsoon-ENSO relationship. However, it is found that, when the Indian Ocean is decoupled from the atmosphere, the Indian monsoon-ENSO relationship reverses. This change is linked to the relationships between surface evaporation, surface wind, and SST in the north Indian Ocean. In the coupled case, surface evaporation anomalies are positively correlated with surface wind anomalies during April-June and are of the same sign as SST anomalies during July-September. In the decoupled Indian Ocean case, surface evaporation anomalies are of the same sign as surface wind anomalies during the entire April-September period. Numerical experiments with an AGCM were performed with SST or SHF anomalies specified in the tropical Indian-Pacific Ocean, tropical Pacific Ocean only, and tropical Indian Ocean only. These experiments confirm the importance of local coupled air-sea feedback in the Indian Ocean for a proper simulation of the Indian monsoon-ENSO relationship.

Simulated Sea Surface Temperature and Heat Fluxes in Different Climates of the Baltic Sea

Simulated Sea Surface Temperature and Heat Fluxes in Different Climates of the Baltic Sea

Long wave dynamics of the interannual variability in a numerical hindcast of the equatorial Pacific Ocean circulation during the 1990s

A numerical hindcast of the Pacific Ocean circulation for the period of 1988–1998 is conducted to study the dynamics of interannual variations in the 1990s. The Poseidon quasi‐isopycnal ocean model is forced with an SSMI‐derived wind stress product and sea surface heat flux from an atmospheric mixed layer model. On the basis of a good comparison of the hindcast results with TAO mooring observations, the roles of the propagation and reflection of the equatorial Kelvin and Rossby waves in the evolution of El Niño and La Niña events in the 1990s are investigated. The linear waves are extracted from the nonlinear model in such a way as to preserve model nonlinearity so that the validity of linear theory of the wave propagation and reflection at the western and eastern boundaries can be examined. It is found that the reflection of the Kelvin waves at the eastern boundary is in very good agreement with a linear reflection at a straight meridional wall. In contrast, the reflection of the Rossby waves at the Pacific western boundary is significantly different from linear theory. Besides the validation of linear theory, the decomposition also shows marked differences from altimetry data analyses. Since the hindcast solution is dynamically consistent and close to observations, a significant part of the differences is attributed to errors associated with the incomplete (least squares fit) wave decomposition from the altimetry data. On the basis of the wave decomposition, the dynamics of the simulated El Niño and La Niña events in the 1990s are compared with three ENSO paradigms. The investigation shows that precursive Rossby waves, important in the Delayed Oscillator paradigm, played an important role in the termination of the 1991–1993 and the 1997–1998 El Niño events. Reflected Rossby waves from the Pacific eastern boundary, the primary negative feedback in the Advective‐Reflective paradigm, were present during the termination of the 1994–1995 and the 1997–1998 El Niño. The Western Pacific Oscillator is at best at work during the decay of the 1997–1998 El Niño; however, it played a minor role compared with the other paradigms. None of the three paradigms appears to describe the termination of the 1988–1989 and the 1995–1996 La Niña events and the onset of the 1997–1998 El Niño event well.

Simulation of extratropical Hurricane Gustav using a coupled atmosphere‐ocean‐sea spray model

Numerical simulations of extratropical Hurricane Gustav (2002) are performed using the MC2 (Mesoscale Compressible Community) atmospheric model, coupled to the Princeton Ocean Model (POM), and a sea spray parameterization. On one hand, the impact of coupling POM to MC2 generates sea surface temperature (SST) cooling, through entrainment mixing at the bottom of the mixed layer, with the passage of the storm. SST cooling reduces the sea surface heat fluxes compared to uncoupled MC2 simulations, which have time‐invariant SST. Reduced heat fluxes lead to reduced storm intensity. On the other hand, simulation of the heat and mass flux contributions of sea spray enhances sea surface heat fluxes and slightly increases maximum storm intensity compared to coupled MC2‐POM simulations without spray.

Simulated Sea Surface Temperature and Heat Fluxes in Different Climates of the Baltic Sea

The physical state of the Baltic Sea in possible future climates is approached by numerical model experiments with a regional coupled ocean-atmosphere model driven by different global simulations. Scenarios and recent climate simulations are compared to estimate changes. The sea surface is clearly warmer by 2.9 degrees C in the ensemble mean. The horizontal pattern of average annual mean warming can largely be explained in terms of ice-cover reduction. The transfer of heat from the atmosphere to the Baltic Sea shows a changed seasonal cycle: a reduced heat loss in fall, increased heat uptake in spring, and reduced heat uptake in summer. The interannual variability of surface temperature is generally increased. This is associated with a smoothed frequency distribution in northern basins. The overall heat budget shows increased solar radiation to the sea surface, which is balanced by changes of the other heat flux components.

Evaluation of the ERA-40 air–sea surface heat flux spin-up

Ocean models depend strongly on surface fluxes. When computed from atmospheric models, fluxes are affected by spin-up, i.e. they increase (or decrease) with forecast length. Such behavior may bias ocean models. The European Centre for Medium Range Forecasts (ECMWF) 40-year re-analysis (ERA-40) has been used to quantify short-range spin-ups of radiative and turbulent heat fluxes. Fluxes are compared as differences between two runs with the same initialization time. This method allows flux analysis over short-range forecasts as a function of distance from the initialization time. Results indicate that (i) latent heat flux spin-up increases with time but levels off after 24 h; (ii) sensible and radiative flux spin-ups remain constant after 6 h; (iii) regional spin-up of turbulent fluxes are systematic and can be larger than 30% for sensible heat but never exceeds 15% for latent heat; (iv) spin-up depends upon the season. The same analysis has been carried out with the ECMWF 15-year re-analysis (ERA-15); spin-ups in ERA-40 have been generally smaller than those in ERA-15.

Inverse estimation of sea surface heat flux over the equatorial Pacific Ocean: Seasonal cycle

An assimilation of satellite‐derived sea surface temperature (SST) observations into the Poseidon quasi‐isopycnal ocean model is conducted to study the seasonal cycle of surface thermodynamics of the equatorial Pacific Ocean and to estimate the sea surface heat flux inversely from the SST observations. A simplified approach in the use of the thermodynamic submodel that separates the momentum and the temperature fields in the variational assimilation procedure proves successful in improving the model simulation of the observed SST. From a comparison with several existing bulk estimates, it is seen that the seasonal cycle of sea surface heat flux has been captured well by the inversion except in the eastern equatorial Pacific, where the optimized surface heat flux shows an upward flux during the boreal summer/fall season when the cold tongue SST starts to decrease dramatically. This spurious upward surface heat flux is inconsistent with the bulk estimates and is determined to be due to model deficiencies in calculating vertical turbulent diffusion. The SST data assimilation is shown to have improved the model simulation of horizontal temperature advection significantly. A comparison of the inferred heat budget is made with an analysis by Wang and McPhaden [1999] from TAO mooring observations.