Upper-ocean Thermal Structure Research Articles

Can oceanic reanalyses be used to assess recent anthropogenic changes and low-frequency internal variability of upper ocean temperature?

A multivariate analysis of the upper ocean thermal structure is used to examine the recent long-term changes and decadal variability in the upper ocean heat content as represented by model-based ocean reanalyses and a model-independent objective analysis. The three variables used are the mean temperature above the 14°C isotherm, its depth and a fixed depth mean temperature (250 m mean temperature). The mean temperature above the 14°C isotherm is a convenient, albeit simple, way to isolate thermodynamical changes by filtering out dynamical changes related to thermocline vertical displacements. The global upper ocean observations and reanalyses exhibit very similar warming trends (0.045°C per decade) over the period 1965–2005, superimposed with marked decadal variability in the 1970s and 1980s. The spatial patterns of the regression between indices (representative of anthropogenic changes and known modes of internal decadal variability), and the three variables associated with the ocean heat content are used as fingerprint to separate out the different contributions. The choice of variables provides information about the local heat absorption, vertical distribution and horizontal redistribution of heat, this latter being suggestive of changes in ocean circulation. The discrepancy between the objective analysis and the reanalyses, as well as the spread among the different reanalyses, are used as a simple estimate of ocean state uncertainties. Two robust findings result from this analysis: (1) the signature of anthropogenic changes is qualitatively different from those of the internal decadal variability associated to the Pacific Interdecadal Oscillation and the Atlantic Meridional Oscillation, and (2) the anthropogenic changes in ocean heat content do not only consist of local heat absorption, but are likely related with changes in the ocean circulation, with a clear shallowing of the tropical thermocline in the Pacific and Indian oceans.

Satellite‐Derived Ocean Heat Content Improves Cyclone Predictions: Utilization of Satellite‐Derived Oceanic Heat Content for Cyclone Studies: Hyderabad, India, 25–26 March 2010

Recognizing the important role played by satellite‐derived sea surface height anomalies and upper ocean heat content (UOHC) in cyclone track and intensity prediction in the Atlantic and Pacific oceans, the National Remote Sensing Centre (NRSC) of the Indian Space Research Organisation (ISRO), Hyderabad, India, organized a 2‐day workshop. The goals of the workshop were to discuss recent advances and future work in the investigation of the role of oceans in cyclone track and intensity prediction, with special emphasis on northern Indian Ocean cyclones, and to strengthen the existing collaborative scientific investigations between NRSC and the U.S. National Oceanic and Atmospheric Administration (NOAA). The workshop also complemented other efforts by NOAA and the India Meteorological Department (IMD) geared toward the use of field experiments targeted to specific cyclones in the Bay of Bengal.Discussions at the workshop highlighted the fact that altimeter‐derived UOHC fields are more effective than sea surface temperatures alone in reducing error in cyclone track and intensity prediction. This is because with atmospheric conditions remaining the same, it is the upper ocean, not just the surface, that interacts with cyclones. Because the current network for sustained ocean observations was not specifically designed for the study or forecast of tropical cyclones globally or in particular in the northern Indian Ocean, products such as UOHC derived from satellites are the most appropriate alternatives for monitoring the upper ocean thermal structure.

A three-dimensional surface wave–ocean circulation coupled model and its initial testing

A theoretical framework to include the influences of nonbreaking surface waves in ocean general circulation models is established based on Reynolds stresses and fluxes terms derived from surface wave-induced fluctuation. An expression for the wave-induced viscosity and diffusivity as a function of the wave number spectrum is derived for infinite and finite water depths; this derivation allows the coupling of ocean circulation models with a wave number spectrum numerical model. In the case of monochromatic surface wave, the wave-induced viscosity and diffusivity are functions of the Stokes drift. The influence of the wave-induced mixing scheme on global ocean circulation models was tested with the Princeton Ocean Model, indicating significant improvement in upper ocean thermal structure and mixed layer depth compared with mixing obtained by the Mellor–Yamada scheme without the wave influence. For example, the model–observation correlation coefficient of the upper 100-m temperature along 35° N increases from 0.68 without wave influence to 0.93 with wave influence. The wave-induced Reynolds stress can reach up to about 5% of the wind stress in high latitudes, and drive 2–3 Sv transport in the global ocean in the form of mesoscale eddies with diameter of 500–1,000 km. The surface wave-induced mixing is more pronounced in middle and high latitudes during the summer in the Northern Hemisphere and in middle latitudes in the Southern Hemisphere.

Observed intra-seasonal to interannual variability of the upper ocean thermal structure in the southeastern Arabian Sea during 2002–2008

The southeastern Arabian Sea (SEAS), located in the Indian Ocean warm pool, is a key-region of the regional climate system. It is suspected to play an important role in the dynamics of the Asian summer monsoon system. The present study reports the salient features derived from a newly harvested observational dataset consisting of repeated fortnightly XBT transects in the SEAS over the period 2002–2008. The fortnightly resolution of such a multi-year record duration is unprecedented in this part of the world ocean and provides a unique opportunity to examine the observed variability of the near-surface thermal structure over a wide spectrum, from intra-seasonal to interannual timescales. We find that most of the variability is trapped in the thermocline, taking the form of upwelling and downwelling motions of the thermal stratification. The seasonal variations are consistent with past studies and confirm the role of the monsoonal wind forcing through linear baroclinic waves (coastally-trapped Kelvin and planetary Rossby waves). Sub-seasonal variability takes the form of anomalous events lasting a few weeks to a few months and occurs at two preferred timescales: in the 30–110 day band, within the frequency domain of the Madden–Julian oscillation and in the 120–180 day band. While this sub-seasonal variability appears fairly barotropic in the offshore region, the sign of the anomaly in the upper thermocline is opposite to that in its lower part on many occasions along the coast. Our dataset also reveals relatively large interannual temperature variations of about 1 °C from 50 to 200 m depth that reflect a considerable year-to-year variability of the magnitude of both upwelling and downwelling events. This study clearly demonstrates the necessity for sustained long-term temperature measurements in the SEAS.

Annual variation in the foraging behaviour of thick-billed murres in relation to upper-ocean thermal structure around St. George Island, Bering Sea

We studied the foraging behaviour of adult thick-billed murres Uria lomvia as they reared their chicks at St. George Island, Alaska, USA, relative to the thermal structure of the nearby ocean water column in the summers 2004, 2006, and 2007 using data recorders attached to the birds. The thermal structure of the upper ocean varied substantially among the years. Satellite-derived sea surface temperature (SST) and water temperature at depths >40 m were higher in 2004 than in 2006 and 2007 (9.1, 8.3 and 7.8°C for mean SST; 5.1, 4.4, and 2.2°C for median bottom temperature, respec- tively). We recorded a strong thermocline in 2004 and 2007, but not in 2006. Nonetheless, the thermo- cline was one of the important foraging habitats in all years. The foraging behaviour of thick-billed murres appeared to vary with annual variation in the intensity of the thermocline and water temper- ature at depth and evidently with associated vertical distribution of prey. Birds spent more time forag- ing in stratified waters and dived to around thermocline depth (and deeper in 2007) in 2004 and 2007. However, the birds used both stratified and mixed waters in 2006 and were less likely to dive to ther- mocline depth. Main prey items delivered to chicks varied among years. Sandlance (53% of observed items) and pollock (23%) predominated in 2004, compared with cephalopods (23%), flatfishes (17%) and pollock (15%) in 2006, and cephalopods (62%), pollock (16%) and sculpins (9%) in 2007.

On the Upper Ocean Thermal Structure in a Western North Pacific Ocean Model: Model Evaluation and Sensitivity Study

Seasonal variations in the upper water column of the western North Pacific are simulated with the Regional Ocean Model System (ROMS). The model is driven by surface fluxes of heat, momentum and freshwater without prescribing sea surface temperature or salinity. A series of numerical experiments are conducted to explore the sensitivity of the upper ocean thermal structure to the parameterized solar penetration scheme and two common practices to control model climate through a flux correction term and a nudging term. The absorption of solar radiation by the water column beneath the sea surface destabilizes the upper layers to cause a stronger mixing and deeper mixed layer in the warm season (from April to September). Therefore, removing solar penetration from the model results in an exceptionally stable surface layer, and tends to produce an overly shallow mixed layer in the warm season. The experiment with a prescribed net surface heat flux shows that the model is unable to maintain a heat balance in the upper water column, producing a cooling trend. Experiments with a flux correction term are able to keep the simulated sea surface temperature (SST) from a long term drift by adjusting the amount of the net surface heat flux. However, unrealistic net surface heat flux is produced in the experiment, when the model assumes no solar penetration. The implementation of a weak temperature nudging (1/50 days) toward a long term mean climatology prevents the model from simulating a cold bias during long term integration. The experiment with solar penetration and a weak nudging produces reasonable interannual variability during the period of 1995 - 2006 without flux corrections. The nudging terms steer advective heat fluxes towards the climatological mean state so as to avoid long-term drift in upper water column heat content. A detailed understanding of the function of nudging terms in controlling the hydrodynamics of the water column remains to be investigated.

Upper-Ocean Thermal Structure and the Western North Pacific Category 5 Typhoons. Part II: Dependence on Translation Speed

Abstract Using new in situ ocean subsurface observations from the Argo floats, best-track typhoon data from the U.S. Joint Typhoon Warning Center, an ocean mixed layer model, and other supporting datasets, this work systematically explores the interrelationships between translation speed, the ocean’s subsurface condition [characterized by the depth of the 26°C isotherm (D26) and upper-ocean heat content (UOHC)], a cyclone’s self-induced ocean cooling negative feedback, and air–sea enthalpy fluxes for the intensification of the western North Pacific category 5 typhoons. Based on a 10-yr analysis, it is found that for intensification to category 5, in addition to the warm sea surface temperature generally around 29°C, the required subsurface D26 and UOHC depend greatly on a cyclone’s translation speed. It is observed that even over a relatively shallow subsurface warm layer of D26 ∼ 60–70 m and UOHC ∼ 65–70 kJ cm−2, it is still possible to have a sufficient enthalpy flux to intensify the storm to category 5, provided that the storm can be fast moving (typically Uh ∼ 7–8 m s−1). On the contrary, a much deeper subsurface layer is needed for slow-moving typhoons. For example at Uh ∼ 2–3 m s−1, D26 and UOHC are typically ∼115–140 m and ∼115–125 kJ cm−2, respectively. A new concept named the affordable minimum translation speed Uh_min is proposed. This is the minimum required speed a storm needs to travel for its intensification to category 5, given the observed D26 and UOHC. Using more than 3000 Argo in situ profiles, a series of mixed layer numerical experiments are conducted to quantify the relationship between D26, UOHC, and Uh_min. Clear negative linear relationships with correlation coefficients R = −0.87 (−0.71) are obtained as Uh_min = −0.065 × D26 + 11.1, and Uh_min = −0.05 × UOHC + 9.4, respectively. These relationships can thus be used as a guide to predict the minimum speed a storm has to travel at for intensification to category 5, given the observed D26 and UOHC.

THE GODAE HIGH-RESOLUTION SEA SURFACE TEMPERATURE PILOT PROJECT

Sea surface temperature (SST) measurements are required by operational ocean and atmospheric forecasting systems to constrain modeled upper ocean circulation and thermal structure. The Global Ocean Data Assimilation Experiment (GODAE) High Resolution SST Pilot Project (GHRSST-PP) was initiated to address these needs by coordinating the provision of accurate, high-resolution, SST products for the global domain. The pilot project is now complete, but activities continue within the Group for High Resolution SST (GHRSST). The pilot project focused on harmonizing diverse satellite and in situ data streams that were indexed, processed, quality controlled, analyzed, and documented within a Regional/Global Task Sharing (R/GTS) framework implemented in an internationally distributed manner. Data with meaningful error estimates developed within GHRSST are provided by services within R/GTS. Currently, several terabytes of data are processed at international centers daily, creating more than 25 gigabytes of product. Ensemble SST analyses together with anomaly SST outputs are generated each day, providing confidence in SST analyses via diagnostic outputs. Diagnostic data sets are generated and Web interfaces are provided to monitor the quality of observation and analysis products. GHRSST research and development projects continue to tackle problems of instrument calibration, algorithm development, diurnal variability, skin temperature deviation, and validation/verification of GHRSST products. GHRSST also works closely with applications and users, providing a forum for discussion and feedback between SST users and producers on a regular basis. All data within the GHRSST R/GTS framework are freely available. This paper reviews the progress of GHRSST-PP, highlighting achievements that have been fundamental to the success of the pilot project.

Profiles of ocean surface heating (POSH): A new model of upper ocean diurnal warming

Shipboard radiometric measurements of diurnal warming at the ocean surface and profiles through the diurnal thermocline were utilized to assess the temporal and vertical variability and to develop a new physics‐based model of near‐surface warming. The measurements and modeled diurnal warming were compared, with the goal of comprehensively evaluating differences between the data and model results. On the basis of these results, the diurnal model was refined while attempting to maintain agreement with the measurements. Simplified bulk models commonly do not provide information on the vertical structure within the warm layer, but this new model predicts the vertical temperature profile within the diurnal thermocline using an empirically derived function dependent on wind speed. The vertical profile of temperature provides both a straightforward methodology for modeling differences due to diurnal warming between measurements made at different depths (e.g., in situ measurements at various depths and measurements of the surface temperatures by satellite radiometers) and information on upper ocean thermal structure. Additionally, the model estimates of diurnal warming at the ocean surface are important for air‐sea heat and gas flux calculations, blending satellite sea surface temperature fields, and air‐sea interaction studies.

Upper-Ocean Thermal Structure and the Western North Pacific Category 5 Typhoons. Part I: Ocean Features and the Category 5 Typhoons’ Intensification

Abstract Category 5 cyclones are the most intense and devastating cyclones on earth. With increasing observations of category 5 cyclones, such as Hurricane Katrina (2005), Rita (2005), Mitch (1998), and Supertyphoon Maemi (2003) found to intensify on warm ocean features (i.e., regions of positive sea surface height anomalies detected by satellite altimeters), there is great interest in investigating the role ocean features play in the intensification of category 5 cyclones. Based on 13 yr of satellite altimetry data, in situ and climatological upper-ocean thermal structure data, best-track typhoon data of the U.S. Joint Typhoon Warning Center, together with an ocean mixed layer model, 30 western North Pacific category 5 typhoons that occurred during the typhoon season from 1993 to 2005 are systematically examined in this study. Two different types of situations are found. The first type is the situation found in the western North Pacific south eddy zone (SEZ; 21°–26°N, 127°–170°E) and the Kuroshio (21°–30°N, 127°–170°E) region. In these regions, the background climatological warm layer is relatively shallow (typically the depth of the 26°C isotherm is around 60 m and the upper-ocean heat content is ∼50 kJ cm−2). Therefore passing over positive features is critical to meet the ocean’s part of necessary conditions in intensification because the features can effectively deepen the warm layer (depth of the 26°C isotherm reaching 100 m and upper-ocean heat content is ∼110 kJ cm−2) to restrain the typhoon’s self-induced ocean cooling. In the past 13 yr, 8 out of the 30 category 5 typhoons (i.e., 27%) belong to this situation. The second type is the situation found in the gyre central region (10°–21°N, 121°–170°E) where the background climatological warm layer is deep (typically the depth of the 26°C isotherm is ∼105–120 m and the upper-ocean heat content is ∼80–120 kJ cm−2). In this deep, warm background, passing over positive features is not critical since the background itself is already sufficient to restrain the self-induced cooling negative feedback during intensification.

On the relationship between satellite‐estimated bio‐optical and thermal properties in the Gulf of Mexico

Three years of Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) ocean color data were combined with three‐dimensional thermal fields generated by the U.S. Navy's Modular Ocean Data Assimilation System (MODAS) in order to examine the interdependencies between bio‐optical fields and their relationship to seasonal and mesoscale changes in upper ocean thermal structure. The combined data set suggests that the oceanic boundary layer within the Gulf of Mexico may be broadly defined by two seasonally occurring bio‐thermal periods. A winter mixing period, characterized by net heat losses to the atmosphere, deepening of the isothermal layer depth, and annual maxima of satellite‐estimated colored detrital matter (CDM) absorption coefficients and surface pigment concentration, was followed by a thermally stratified period characterized by net surface ocean heating, reduced isothermal layer depths, and annual minima in surface bio‐optical fields. Variability in the interdependencies of ocean color products was used to diagnose an attendant shift in the size‐structure of surface phytoplankton communities as well as identify CDM as the constituent responsible for the majority of blue‐light absorption in Gulf of Mexico surface waters. The mesoscale circulation, as resolved by MODAS thermal fields into cold and warm‐core eddies, appears to significantly modulate the seasonal bio‐optical cycle of CDM absorption and surface pigment concentration. An empirical model was developed to describe CDM absorption as a function of upper ocean thermal energy. The model accounted for nearly half the variance in the satellite‐estimate of this bio‐optical variable. Large mismatches between the model and satellite data implied episodes of shelf water export to the deep Gulf of Mexico.

Application of Oceanic Heat Content Estimation to Operational Forecasting of Recent Atlantic Category 5 Hurricanes

Abstract Research investigating the importance of the subsurface ocean structure on tropical cyclone intensity change has been ongoing for several decades. While the emergence of altimetry-derived sea height observations from satellites dates back to the 1980s, it was difficult and uncertain as to how to utilize these measurements in operations as a result of the limited coverage. As the in situ measurement coverage expanded, it became possible to estimate the upper oceanic heat content (OHC) over most ocean regions. Beginning in 2002, daily OHC analyses have been generated at the National Hurricane Center (NHC). These analyses are used qualitatively for the official NHC intensity forecast, and quantitatively to adjust the Statistical Hurricane Intensity Prediction Scheme (SHIPS) forecasts. The primary purpose of this paper is to describe how upper-ocean structure information was transitioned from research to operations, and how it is being used to generate NHC’s hurricane intensity forecasts. Examples of the utility of this information for recent category 5 hurricanes (Isabel, Ivan, Emily, Katrina, Rita, and Wilma from the 2003–05 hurricane seasons) are also presented. Results show that for a large sample of Atlantic storms, the OHC variations have a small but positive impact on the intensity forecasts. However, for intense storms, the effect of the OHC is much more significant, suggestive of its importance on rapid intensification. The OHC input improved the average intensity errors of the SHIPS forecasts by up to 5% for all cases from the category 5 storms, and up to 20% for individual storms, with the maximum improvement for the 72–96-h forecasts. The qualitative use of the OHC information on the NHC intensity forecasts is also described. These results show that knowledge of the upper-ocean thermal structure is fundamental to accurately forecasting intensity changes of tropical cyclones, and that this knowledge is making its way into operations. The statistical results obtained here indicate that the OHC only becomes important when it has values much larger than that required to support a tropical cyclone. This result suggests that the OHC is providing a measure of the upper ocean’s influence on the storm and improving the forecast.

Validation and Application of Altimetry-Derived Upper Ocean Thermal Structure in the Western North Pacific Ocean for Typhoon-Intensity Forecast

This paper uses more than 5000 colocated and near-coincident in-situ profiles from the National Oceanic and Atmospheric Administration/Global Temperature and Salinity Profile Program database spanning over the period from 2002 to 2005 to systematically validate the satellite-altimetry-derived upper ocean thermal structure in the western North Pacific ocean as such ocean thermal structure information is critical in typhoon-intensity change. It is found that this satellite-derived information is applicable in the central and the southwestern North Pacific (covering 122-170degE, 9-25degN) but not in the northern part (130-170degE, 25-40degN). However, since > 80% of the typhoons are found to intensify in the central and southern part, this regional dependence should not pose a serious constraint in studying typhoon intensification. Further comparison with the U.S. Naval Research Laboratory's North Pacific Ocean Nowcast/Forecast System (NPACNFS) hydrodynamic ocean model shows similar regional applicability, but NPACNFS is found to have a general underestimation in the upper ocean thermal structure and causes a large underestimation of the tropical cyclone heat potential (TCHP) by up to 60 kJ/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . After validation, the derived upper ocean thermal profiles are used to study the intensity change of supertyphoon Dianmu (2004). It is found that two upper ocean parameters, i.e., a typhoon's self-induced cooling and the during-typhoon TCHP, are the most sensitive parameters (with R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ~0.7) to the 6-h intensity change of Dianmu during the study period covering Dianmu's rapid intensification to category 5 and its subsequent decay to category 4. This paper suggests the usefulness of satellite-based upper ocean thermal information in future research and operation that is related to typhoon-intensity change in the western North Pacific

The upper ocean thermal structure and the genesis locations of tropical cyclones in the South China Sea

The relationship between the upper ocean thermal structure and the genesis locations of tropical cyclones (TCs) in the South China Sea (SCS) is investigated by using the Joint Typhoon Warning Center (JTWC) best-track archives and high resolution (1/4 degree) temperature analyses of the world’s oceans in this paper. In the monthly mean genesis positions of TCs from 1945 to 2005 in the SCS, the mean sea surface temperature (SST) was 28.8°C and the mean depth of 26°C water was 53.1 m. From the monthly distribution maps of genesis positions of TCs, SST and the depth of 26°C water in the SCS, we discovered that there existed regions with SST exceeding 26°C and 26°C water depth exceeding 50 m where no tropical cyclones formed from 1945 to 2005 in the SCS, which suggests that there were other factors unfavorable for TC formation in these regions.

Upper-ocean thermal structure and heat content off the US West Coast during the 1997–1998 El Niño event based on AXBT and satellite altimetry data

During the 1997/1998 El Niño event, extensive oceanic temperature profiles were taken off the coast of California in January and February 1998 using Airborne Expendable Bathythermographs (AXBTs). These AXBT measurements are compared with altimetry-based upper-ocean temperature estimates using TOPEX and ERS satellite altimetry data. The altimetry-based temperature estimates are well correlated with the AXBT data, in particular when combining the two satellite data sets together to form a blended altimeter temperature estimate. Both the AXBT and altimetry data show that the nearshore coastal El Niño signal differed from that further offshore. The AXBT data show that near shore, the warm anomalies extended to much greater depths and had greater amplitude. A time series of the satellite-derived layer-averaged temperatures, averaged separately over the nearshore and offshore halves of the AXBT analysis domain, also shows a larger El Niño signal in the nearshore half. The role of local atmospheric forcing of the coastal oceanic temperature anomalies is analyzed using NCEP reanalysis and coastal upwelling data sets. The forcing terms include Ekman pumping, radiation, surface heat fluxes, precipitation, and alongshore wind stresses that drive coastal upwelling (expressed as a coastal downwelling index, CDI). The temperature forcing from all of the terms except the CDI anomalies are small. The CDI anomalies can explain most of the slowly varying temperature changes that occur near the coast during a two-year period spanning the El Niño event, as well as some of the larger amplitude, rapid (monthly) warming episodes that appear to be part of the El Niño signal. Several distinct rapid warming episodes, however, are not correlated with the CDI anomalies, and therefore we conclude that the nearshore El Niño signal originates from a combination of both a remote oceanic pathway and local atmospheric forcing.

Sensitivity of tropical Atlantic climate to mixing in a coupled ocean–atmosphere model

The sensitivity of tropical Atlantic climate to upper ocean mixing is investigated using an ocean-only model and a coupled ocean–atmosphere model. The upper ocean thermal structure and associated atmospheric circulation prove to be strongly related to the strength of upper ocean mixing. Using the heat balance in the mixed layer it is shown that an excessively cold equatorial cold tongue can be attributed to entrainment flux at the base of the oceanic mixed layer, that is too large. Enhanced entrainment efficiency acts to deepen the mixed layer and causes strong reduction in the upper ocean divergence in the central equatorial Atlantic. As a result, the simulated sea surface temperature, thermocline structure, and upwelling velocities are close to the observed estimates. In the coupled model, the seasonal migration of the Intertropical Convergence Zone (ITCZ) reduces when the entrainment efficiency in the oceanic mixed layer is enhanced. The precipitation rates decrease in the equatorial region and increase along 10°N, resulting in a more realistic Atlantic Marine ITCZ. The reduced meridional surface temperature gradient in the eastern tropical Atlantic prohibits the development of convective precipitation in the southeastern part of the tropical Atlantic. Also, the simulation of tropical Atlantic variability as expressed in the meridional gradient mode and the eastern cold tongue mode improves when the entrainment efficiency is enhanced.

Simulation of tropical Pacific and Atlantic Oceans using a HYbrid Coordinate Ocean Model

The climatological annual cycle of the tropical Pacific and Atlantic Oceans is simulated using the HYbrid Coordinate Ocean Model (HYCOM) configured in a non-uniform horizontal grid spanning 30°S–65.5°N and 102°E–15°E. The model is initialized with climatological summer temperature and salinity and is forced by climatological atmospheric fields derived from the COADS and ECMWF ERA-15 reanalysis. The model is spun up for 20 years to reach a reasonable steady state in the primary region of interest from 20°S to 20°N, and year 20 is analyzed. The COADS simulation is primarily analyzed because it is slightly better in more respects than the ECMWF simulation, particularly in the representation of upper-ocean thermal structure. The model generally reproduces the seasonal variability of major circulation features in both oceans reasonably well when compared to climatologies derived from several observational datasets (surface drifters, TAO mooring array, COADS, Levitus, Pathfinder SST), and when compared to other model simulations. Model evaluation is complicated by the fact that the different climatologies, including the atmospheric reanalysis climatologies that drive the model, are averaged over different time intervals. In the tropics, the model thermocline reproduces the observed zonal slopes and meridional ridges/troughs in the thermocline. The simulated Equatorial Undercurrent compares favorably to observations, but is slightly deeper than observed. The model overestimates temperature in the Pacific warm pool regions, both west and east, by more than 1°C when compared to all observed climatologies. The model also tends to overestimate temperature in the eastern equatorial cold tongues in both the Atlantic and Pacific, with this overestimate being confined to a very small region of the far eastern Pacific during winter. This overestimate varies substantially depending on which observed climatology is used for the comparison, so model limitations are only partly responsible for the simulated–observed temperature differences in the cold tongues.

Study of the mixed layer depth variations within the north Indian Ocean using a 1‐D model

Mixed layer depth (MLD) over the north Indian Ocean (30°S to 30°N and 40°E to 110°E) is computed using the simple one‐dimensional model of Price et al. [1986] forced by satellite‐derived parameters (winds and chlorophyll). Seasonal chlorophyll observations obtained from the Coastal Zone Color Scanner allow us to examine how biology interacts with physics in the upper ocean by changing the absorption of light and thus the heating by penetrative solar radiation, an effect we refer to as biological heating. Our analysis focus mainly on two aspects: the importance of varying biology in the model simulations relative to runs with constant biology and secondly, the contribution of biology to the seasonal variability of the MLD. The model results are compared with observations from a surface mooring deployed for 1 year (October 1994 to October 1995) in the central Arabian Sea and also with available conductivity‐temperature‐depth (CTD) observations from the Arabian Sea during the period 1994–1995. The effect of biological heating on the upper ocean thermal structure in central Arabian Sea is found to be greatest in August. In other months it is either the wind, which is the controlling factor in mixed layer variations, or the density variations due to winter cooling and internal dynamics. A large number of CTD observations collected under the Joint Global Ocean Flux study and World Ocean Circulation Experiment have been used to validate model results. We find an overall improvement by approximately 2–3 m in root‐mean‐square error in MLD estimates when seasonally varying chlorophyll observations are used in the model.

Environmental Control of Tropical Cyclone Intensity

The influence of various environmental factors on tropical cyclone intensity is explored using a simple coupled ocean‐atmosphere model. It is first demonstrated that this model is capable of accurately replicating the intensity evolution of storms that move over oceans whose upper thermal structure is not far from monthly mean climatology and that are relatively unaffected by environmental wind shear. A parameterization of the effects of environmental wind shear is then developed and shown to work reasonably well in several cases for which the magnitude of the shear is relatively well known. When used for real-time forecasting guidance, the model is shown to perform better than other existing numerical models while being competitive with statistical methods. In the context of a limited number of case studies, the model is used to explore the sensitivity of storm intensity to its initialization and to a number of environmental factors, including potential intensity, storm track, wind shear, upper-ocean thermal structure, bathymetry, and land surface characteristics. All of these factors are shown to influence storm intensity, with their relative contributions varying greatly in space and time. It is argued that, in most cases, the greatest source of uncertainty in forecasts of storm intensity is uncertainty in forecast values of the environmental wind shear, the presence of which also reduces the inherent predictability of storm intensity.

Upper ocean thermohaline structure and its temporal variability in the southeast Indian Ocean

We examine the upper ocean thermohaline structure in the southeast Indian Ocean and its temporal variability based on XBT/XCTD observations from four cruises across the Southern Ocean from Fremantle, Australia to Prydz Bay, Antarctica. The transects were occupied in March 1998, November 1998, March 2000 and March 2002. Three major fronts—the subtropical front, subantarctic front and polar front—are clearly identified from our surveys and compared with earlier studies. Particularly, two polar fronts, separated by a few degrees of latitude, appear southeast the Kerguelen Plateau. The primary polar front is characterized by a strong horizontal thermal gradient extending deep in the water column, while the secondary polar front is identified by the 2°C isotherm at T min depth and has a relatively shallow frontal expression. Dynamic height across the Antarctic Circumpolar Current (ACC) was calculated for the transects in 2000 and 2002. With a negligible yearly variation in the total transport across the ACC, the higher sample density in 2002 reveals more detailed structure of the ACC: two jets associated with the SAF and primary polar front are embedded within the broad ACC. We find a strong temporal variation in the upper ocean thermal structure in the polar ocean southeast the Kerguelen Plateau. The depths of the mixed layer and T min layer increase over time. The mixed layer temperature decreases while the T min temperature increases during the same period. In addition, we see an increase from 1998 to 2002 in the length of the ice-free period prior to each XBT/XCTD sampling and surface wind forcing, calculated as the friction velocity cubed during the ice-free period. Our analysis suggests that the longer the ocean is exposed to the atmosphere and the stronger the wind stirring, the more enhanced is the turbulent mixing. This results in a greater mixed layer depth and more entrainment of colder water from the T min layer to the mixed layer. This surface forcing also enhances internal diffusive processes that mix the T min water with the warmer waters above and below the T min layer. The surface forcing is apparently dominant in determining the upper ocean thermal structure in this polar region.