Air-sea Interaction Research Articles

Investigating air-sea interactions in the North Pacific on interannual timescales during boreal winter

We study the air-sea interaction in a perspective of interannual timescale with wintertime observed sea surface temperature (SST), turbulent heat flux (THF) and sea level pressure (SLP) based on lead-lag correlation and regression. Being forced by the atmosphere, the ocean also has a response to the atmosphere, especially in the subarctic frontal zone (SAFZ) and the eastern North Pacific. The air-sea interactions in two regions are quite similar but associated with different patterns of atmospheric variability when SST leading SLP for one month. Since the SST changes in the eastern North Pacific and the equatorial east-central Pacific are synchronous in the dominant mode (EOF1) of the SST anomalies, the SLP responses to SST in the eastern North Pacific and central Pacific (CP) ENSO both show Pacific-North American (PNA) teleconnection pattern. The CP ENSO dominates the tropical influence on the air-sea interaction in the eastern North Pacific during November. Specifically, the response of Aleutian low to CP El Niño events leads to heat transport from ocean to atmosphere in the eastern North Pacific. In CP La Niña years, the air-sea heat exchange is affected by the combination of Aleutian low and lower troposphere westerlies.

Assessing the influence of sea surface temperature and arctic sea ice cover on the uncertainty in the boreal winter future climate projections

We investigate the uncertainty (i.e., inter-model spread) in future projections of the boreal winter climate, based on the forced response of ten models from the CMIP5 following the RCP8.5 scenario. The uncertainty in the forced response of sea level pressure (SLP) is large in the North Pacific, the North Atlantic, and the Arctic. A major part of these uncertainties (31%) is marked by a pattern with a center in the northeastern Pacific and a dipole over the northeastern Atlantic that we label as the Pacific–Atlantic SLP uncertainty pattern (PA∆SLP). The PA∆SLP is associated with distinct global sea surface temperature (SST) and Arctic sea ice cover (SIC) perturbation patterns. To better understand the nature of the PA∆SLP, these SST and SIC perturbation patterns are prescribed in experiments with two atmospheric models (AGCMs): CAM4 and IFS. The AGCM responses suggest that the SST uncertainty contributes to the North Pacific SLP uncertainty in CMIP5 models, through tropical–midlatitude interactions and a forced Rossby wavetrain. The North Atlantic SLP uncertainty in CMIP5 models is better explained by the combined effect of SST and SIC uncertainties, partly related to a Rossby wavetrain from the Pacific and air-sea interaction over the North Atlantic. Major discrepancies between the CMIP5 and AGCM forced responses over northern high-latitudes and continental regions are indicative of uncertainties arising from the AGCMs. We analyze the possible dynamic mechanisms of these responses, and discuss the limitations of this work.

Weak Mesoscale Variability in the Optimum Interpolation Sea Surface Temperature (OISST)-AVHRR-Only Version 2 Data before 2007

Mesoscale sea surface temperature (SST) variability triggers mesoscale air–sea interactions and is linked to ocean subsurface mesoscale dynamics. The National Oceanic and Atmospheric Administration (NOAA) daily Optimum Interpolation SST (OISST) products, based on various satellite and in situ SST data, are widely utilized in the investigation of multi-scale SST variabilities and reconstruction of subsurface and deep-ocean fields. The quality of OISST datasets is subjected to temporal inhomogeneity due to alterations in the merged data. Yet, whether this issue can significantly affect mesoscale SST variability is unknown. The analysis of this study detects an abrupt enhancement of mesoscale SST variability after 2007 in the OISST-AVHRR-only version 2 and version 2.1 datasets (hereafter OI.v2-AVHRR-only and OI.v2.1-AVHRR-only). The contrast is most stark in the subtropical western boundary current (WBC) regions, where the average mesoscale SST variance during 2007–2018 is twofold larger than that during 1993–2006. Further comparisons with other satellite SST datasets (TMI, AMSR-E, and WindSAT) suggest that the OISST-AVHRR-only datasets have severely underestimated mesoscale SST variability before 2007. An evaluation of related documents of the OISST data indicates that this bias is mainly caused by the change of satellite AVHRR instrument in 2007. There are no corresponding changes detected in the associated fields, such as the number and activity of mesoscale eddies or the background SST gradient in these regions, confirming that the underestimation of mesoscale SST variability before 2007 is an artifact. Another OISST product, OI.v2-AVHRR-AMSR, shows a similar abrupt enhancement of mesoscale SST variability in June 2002, when the AMSR-E instrument was incorporated. This issue leaves potential influences on scientific research that utilize the OISST datasets. The composite SST anomalies of mesoscale eddies based on the OI.v2-AVHRR-only data are underestimated by up to 37% before 2007 in the subtropical WBC regions. The underestimation of mesoscale variability also affects the total (full-scale) SST variability, particularly in winter. Other SST data products based on the OISST datasets were also influenced; we identify suspicious changes in J-OFURO3 and CFSR datasets; the reconstructed three-dimensional ocean products using OISST data as input may also be inevitably affected. This study reminds caution in the usage of the OISST and relevant data products in the investigation of mesoscale processes.

Experimental investigation of the airflow structure above mechanically generated regular waves for both aligned and opposed wind–wave directions

A better understanding of the wind–wave direction effect on the wind field and on the momentum transfer at the air–sea interaction will aid in the development of numerical atmospheric and wave models, important for wind energy studies among other applications. So far, the effect of opposed wind–wave directions on the wind field is not fully understood, even though it is shown to be an occurring phenomenon at the air–sea interface. To investigate the effects of different wind–wave alignments on the wind field and on the momentum transfer, the Wind Gallery of the von Karman Institute (VKI) was modified in order to perform aligned and opposed wind–wave direction experiments. The measurements performed to define the wind field and momentum transfer consisted of a Particle Image Velocimetry (PIV) setup with one laser and one camera. Based on this setup, differences in horizontal and vertical wave perturbation velocity as well as in phase-averaged wave-perturbation stress or momentum were observed between aligned and opposed wind–wave directions in terms of location, structure and vertical depth. This study thus provides support that the wind field and phase-averaged wave-perturbation momentum are dependent on the wind–wave alignment, which in the future should be accounted for in numerical models.

INSAT-3D SST and its diurnal variability assessment using in-situ and MODIS observations

Sea Surface Temperature is an Essential Climate Variable used in oceanographic and meteorological application studies such as air-sea interaction, ocean mixing, boundary layer processes, and ocean state forecast, which require high temporal resolution SST. Indian geostationary satellite INSAT-3D imager has been designed to provide such information at 30 min intervals, 4 km spatial resolution along with several other parameters over the North Indian Ocean. In this study, the 30-minute interval INSAT-3D SST and its diurnal variability are validated (inter-compared) with collocated in-situ buoy measurements and compared with contemporary MODerate-resolution Imaging Spectroradiometer SST observations. A reasonable agreement exists between the INSAT-3D and in situ data with a correlation coefficient of 0.70 and Root Mean Square Error (RMSE) of 1.28 °C. This relationship is better for the daytime observations and during the pre and post-monsoon seasons, while the relationship is relatively weaker for the nighttime and monsoon seasons with remarked imprints from convective processes in the upper oceans. For the Arabian Sea and Bay of Bengal regions, the INSAT-3D SST shows better correlation and negative bias in comparison with in situ observations, while it is weaker with positive bias for the Equatorial Indian Ocean. Inter-comparison with MODIS Aqua/Terra day and night passes SST also shows a similar relationship, however with a much-improved correlation coefficient (∼0.95) and highly consistent in spatial and temporal variability with a cool bias of 0.14 °C. INSAT-3D SST shows a significantly high average diurnal temperature difference of 2.24 °C, whereas buoys show 0.54 °C. These high RMSE and diurnal variability results suggest that INSAT-3D SST retrieval can be improved to study the many oceanographic processes such as eddies, fronts, and estimation of air-sea fluxes. Therefore, this study helps to improve the retrieval algorithm of INSAT-3D SST, by developing the region-specific regression coefficients along with data merging and assimilation techniques.

An Extreme Tropical Cyclone Silence in the Western North Pacific in July 2020

ABSTRACT The western North Pacific (WNP) tropical cyclone (TC) number in July 2020 hit a record low since 1949, with no TC formation throughout July. It is shown that this record-breaking TC event can be largely attributed to the extremely strong lower-level anticyclone and subsidence over the TC-genesis-prone region (GPR), concurrent with an intensified western Pacific subtropical high (WPSH) there. This configuration is closely linked to both the air–sea interaction in the tropical Indo-western Pacific and the vigorous wave activities over the mid-high latitude Eurasia. The warming western Indian Ocean and the resultant active convection excited an anomalous lower-level anticyclone and descending atmospheric Kelvin wave over the GPR, together with a strengthened WPSH. Accompanied with this monthly background field, long-lasting Madden-Julian Oscillation was confined west of 90°E, which intensified the proposed anticyclonic vorticity and decreased the moisture condition in the South China Sea and tropical WNP. Meanwhile, two active wave fluxes prevailed across the jet streams over the mid-high latitude Eurasia with a quasi-barotropic cyclonic abnormity dominating from Northeast China to Japan. The resultant positive potential vorticity on its south excited the anomalous local ascending around 35°N, which favoured the upper-level convergence in the tropical WNP and reinforced the local subsidence through modulating the meridional circulation. This extratropical impact further aggravated the suppressed circulation condition for TC formation and increased the chance that the extreme tropical cyclone silence would happen.

Interannual Variability of the Indonesian Rainfall and Air–Sea Interaction over the Indo–Pacific Associated with Interdecadal Pacific Oscillation Phases in the Dry Season

Interannual Variability of the Indonesian Rainfall and Air–Sea Interaction over the Indo–Pacific Associated with Interdecadal Pacific Oscillation Phases in the Dry Season

Formation, Thermodynamic Structure, and Airflow of a Japan Sea Polar Airmass Convergence Zone

Abstract The Sea of Japan (SOJ) coast and adjoining orography of central Honshu, Japan, receive substantial snowfall each winter. A frequent contributor during cold-air outbreaks (CAOs) is the Japan Sea polar airmass convergence zone (JPCZ), which forms downstream of the highland areas of the Korean Peninsula (i.e., the Korean Highlands), extends southeastward to Honshu, and generates a mesoscale band of precipitation. Mesoscale polar vortices (MPVs) ranging in horizontal scale from tens (i.e., meso-β-scale cyclones) to several hundreds of kilometers (i.e., “polar lows”) are also common during CAOs and often interact with the JPCZ. Here we use satellite imagery and Weather Research and Forecasting Model simulations to examine the formation, thermodynamic structure, and airflow of a JPCZ that formed in the wake of an MPV during a CAO from 2 to 7 February 2018. The MPV and its associated warm seclusion and bent-back front developed in a locally warm, convergent, and convective environment over the SOJ near the base of the Korean Peninsula. The nascent JPCZ was structurally continuous with the bent-back front and lengthened as the MPV migrated southeastward. Trajectories illustrate how air–sea interactions and flow splitting around the Korean Highlands and channeling through low passes and valleys along the Asian coast affect the formation and thermodynamic structure of the JPCZ. Contrasts in airmass origin and thermodynamic modification over the SOJ affect the cross-JPCZ temperature gradient, which reverses in sign along the JPCZ from the Asian coast to Honshu. These results provide new insights into the thermodynamic structure of the JPCZ, which is an important contributor to hazardous weather over Japan.

Tropical cyclone-induced cold wakes in the northeast Indian Ocean

The physical processes during cyclone passage over the ocean. Air–sea interactions.

Bayesian maximum entropy interpolation of sea surface temperature data: A comparative assessment

ABSTRACT Sea surface temperature (SST) is an important oceanography attribute that has been used to study ocean climatic conditions, ocean dynamics and air-sea interactions. In the present work, the Bayesian maximum entropy (BME) method was used to interpolate a FY-3 C /VIRR satellite dataset in the region with coordinates 120°-150°W and 45°-60°S and during January 2020. A novel approach of constructing valuable soft data was developed by combining BME interpolation with highly correlated SST day and night data differences. The BME interpolation accuracy was assessed by cross-validation, and the results showed that the average RMSE (root mean squared error) was 0.700 and the average bias was 0.441°C. Furthermore, using the Argo data as a basis of comparison, the coverage and accuracy of the BME interpolation of the FY-3 C/VIRR satellite SST dataset were compared numerically with those of the Ordinary Kriging (OK) interpolation of the FY-3 C/VIRR satellite SST dataset and the Optimum Interpolation Sea Surface Temperature (OISST) of the AVHRR satellite SST dataset. It was found that BME had the best SST interpolation performance among the three methods with the lowest average bias and the largest correlation coefficient. Although OISST had a full product coverage rate overall (due to its use of more perfect treatment means), BME’s coverage rate (97.5%) considerably improved that of the FY-3 C/VIRR SST data. Also, both the BME and OK products maintained a 12hrs temporal resolution and a 0.05 decimal degrees longitude/latitude spatial resolution, which is an improvement over OISST data with a 24hrs time resolution and a 0.25 decimal degrees longitude/latitude spatial resolution. Another advantage of BME is that because of its broad theoretical support its performance in practice can be improved further as more knowledge sources become available (which can only be incorporated by BME).

Seasonal Variability of The Mixed Layer Depth in The Sulawesi Sea

The Sulawesi Sea is a semi-enclosed basin located in the Indonesian Seas and considered as the one of location in the west route of Indonesian Throughflow (ITF). There is less attention on the mixed layer depth investigation in the Sulawesi Sea. Concerning that the mixed layer plays an important role in influencing the ocean in air-sea interaction and affects biological activity, the estimation of mixed layer depth (MLD) in the Sulawesi Sea is important. Seasonal variation of the mixed layer in the Sulawesi Sea between 115°-125°E and 0°-8°N is estimated by using World Ocean Atlas 2013. Forcing elements on the mixed layer in terms of surface-forced turbulent mixing from mechanical forcing of wind stress and buoyancy forcing (from heat flux as well as freshwater flux) in the Sulawesi Sea is provided by using a reanalysis dataset. The MLD is estimated directly on grid profiles with interpolated levels based on chosen density fixed criterion of 0.03 kg.m<sup>-3</sup> and temperature criterion of 0.5°C difference from the surface. The results show that mixed layer depth in the Sulawesi Sea varies both spatially and temporally. Generally, the deepest MLD was occurred during the southwest monsoon (JJA), and the lowest MLD was occurred during the first transition (MAM) and second transition monsoon (SON). Strengthening and weakening MLD are influenced by mechanical forcing from wind stress and buoyancy flux. In the Sulawesi Sea, the mixed layer deepening coincides with the occurrence of a maximum in wind stress, and low buoyancy flux at the surface. This condition is the opposite when mixed layer shallowing occurs.

Wind-Stress Variations from Deep to Shallow Water during Hurricanes for Air-Sea-Land Interaction Applications

In September 2020 Hurricane Sally impacted two National Data Buoy Center (www.ndbc.noaa.gov) buoys near its track: 42040 in the deep water and 42012 in the shallow. Using pertinent air-sea interaction formulas from the literature, analyses of these buoy data indicate that, under fully rough airflow and wind sea conditions, U* = a Hs2 /Tp3 + b, here U* is the friction velocity, Hs is the significant wave height, and Tp is the peak wave period. It is found that a = 28 and b = 0.12 for the deep water environment, a = 30 and b = 0.26 for the shoaling wave environment, and a = 31 and b = 0.14 for the transitional water-depth environment, respectively. All units are in SI. Verifications and applications of these proposed formulas to estimate storm surge, wind speed, surface currents and seabed scours are presented. Because an extensive network for monitoring waves along the coastlines of the United States by the Coastal Data Information Program (CDIP) (CDIP About (ucsd.edu)), these proposed formulas are particularly useful. In addition, it is demonstrated that the parameter of wave steepness can be used to explain why there are large variations in the drag coefficient and wave-age formulations in the literature.

Response of a tropical cyclone to a subsurface ocean eddy and the role of boundary layer dynamics

AbstractWe analyse a tropical cyclone simulated for a realistic ocean‐eddy field using the global, nonhydrostatic, fully coupled atmosphere–ocean ICOsahedral Nonhydrostatic (ICON) model. After intensifying rapidly, the tropical cyclone decays following its interaction with a cold wake and subsequently reintensifies as it encounters a subsurface, warm‐core eddy. To understand the change in the azimuthal‐mean structure and intensity of the tropical cyclone, we invoke a conceptual framework, which recognises the importance of both boundary‐layer dynamics and air–sea interactions. Crucially, the framework recognises that the change in the mean radius of updraught at the boundary‐layer top is regulated by the expanding outer tangential wind field through boundary‐layer dynamics. The decrease in the average equivalent potential temperature of the boundary‐layer updraught during the early decay phase is related to an increase in the mean radius of the updraught rather than air–sea interactions. However, later in the decay phase, air–sea interactions contribute to the decrease, which is accompanied by a decrease in the vertical mass flux in the eyewall updraught and, ultimately, a more pronounced spin‐down of the tropical cyclone. Air–sea interactions are also important during reintensification, where the tendencies are reversed, that is, the mean radius of the boundary‐layer updraught decreases along with an increase in its average equivalent potential temperature and vertical mass flux. The importance of boundary‐layer dynamics to the change in the azimuthal‐mean structure is underscored by the ability of a steady‐state slab boundary‐layer model to predict an increasing and, to a lesser extent, decreasing radius of forced ascent for periods of decay and reintensification, respectively. Finally, our simulation highlights the importance of the ocean‐eddy field for tropical cyclone intensity forecasts, since the simulated warm‐core eddy does not display any sea‐surface temperature (SST) signal until it is encountered by the tropical cyclone.

Evaluation of the seasonality and spatial aspects of the Southern Annular Mode in CMIP6 models

AbstractThe simulation skills of the Southern Annular Mode (SAM) pattern for the CMIP6 models are evaluated in this study. Both the Atmospheric Model Intercomparison Project (AMIP) and historical experiments are utilized to understand the effect of SST forcing and air–sea interaction. The spatial correlation coefficient, standard deviation, and root‐mean‐square error (RMSE) are used as the objective metrics. The results suggest that the multimodel mean (MME) for both AMIP and historical data could capture the basic spatial patterns of the SAM for all seasons, featuring the opposite sign between the middle and high latitudes with a zonal three‐wave structure in the middle latitudes (positive phase). However, the overall skill of the AMIP simulation is higher than that of the historical simulation, which could be attributed to realistic SST forcing being applied. The simulation skill varies for different seasons: the skill for boreal spring (March–May [MAM]) is the lowest, while it is highest in boreal winter (December–February [DJF]). Further analysis shows that the simulations of the SAM asymmetric part are the key component for the simulation of the spatial pattern of the SAM, especially sensitive in MAM. The wave–current interaction simulation at mid–high latitudes is suggested to be improved for the advancement of SAM simulations in future model development.

Progress in understanding of Indian Ocean circulation, variability, air–sea exchange, and impacts on biogeochemistry

Abstract. Over the past decade, our understanding of the Indian Ocean has advanced through concerted efforts toward measuring the ocean circulation and air–sea exchanges, detecting changes in water masses, and linking physical processes to ecologically important variables. New circulation pathways and mechanisms have been discovered that control atmospheric and oceanic mean state and variability. This review brings together new understanding of the ocean–atmosphere system in the Indian Ocean since the last comprehensive review, describing the Indian Ocean circulation patterns, air–sea interactions, and climate variability. Coordinated international focus on the Indian Ocean has motivated the application of new technologies to deliver higher-resolution observations and models of Indian Ocean processes. As a result we are discovering the importance of small-scale processes in setting the large-scale gradients and circulation, interactions between physical and biogeochemical processes, interactions between boundary currents and the interior, and interactions between the surface and the deep ocean. A newly discovered regional climate mode in the southeast Indian Ocean, the Ningaloo Niño, has instigated more regional air–sea coupling and marine heatwave research in the global oceans. In the last decade, we have seen rapid warming of the Indian Ocean overlaid with extremes in the form of marine heatwaves. These events have motivated studies that have delivered new insight into the variability in ocean heat content and exchanges in the Indian Ocean and have highlighted the critical role of the Indian Ocean as a clearing house for anthropogenic heat. This synthesis paper reviews the advances in these areas in the last decade.

The Variability of Summer Atmospheric Water Cycle over the Tibetan Plateau and Its Response to the Indo-Pacific Warm Pool

The Tibetan Plateau (TP), atmosphere, and Indo-Pacific warm pool (IPWP) together constitute a regional land–atmosphere–ocean water vapor transport system. This study uses remote sensing data, reanalysis data, and observational data to explore the spatiotemporal variations of the summer atmospheric water cycle over the TP and its possible response to the air-sea interaction in the IPWP during the period 1958–2019. The results reveal that the atmospheric water cycle process over the TP presented an interannual and interdecadal strengthening trend. The climatic precipitation recycle ratio (PRR) over the TP was 18%, and the stronger the evapotranspiration, the higher the PRR. On the interdecadal scale, the change in evapotranspiration has a significant negative correlation with the Pacific Decadal Oscillation (PDO) index. The variability of the water vapor transport (WVT) over the TP was controlled by the dynamic and thermal conditions inside the plateau and the external air-sea interaction processes of the IPWP. When the summer monsoon over the TP was strong, there was an anomalous cyclonic WVT, which increased the water vapor budget (WVB) over the TP. The central and eastern tropical Pacific, the maritime continent and the western Indian Ocean together constituted the triple Sea Surface Temperature (SST) anomaly, which enhanced the convective activity over the IPWP and induced a significant easterly wind anomaly in the middle and lower troposphere, and then generated pronounced easterly WVT anomalies from the tropical Pacific to the maritime continent and the Bay of Bengal. Affected by the air-sea changes in the IPWP, the combined effects of the upstream strengthening and the downstream weakening in the water vapor transport process, directly and indirectly, increased the water vapor transport and budget of TP.

Mesoscale Eddy-Induced Ocean Dynamic and Thermodynamic Anomalies in the North Pacific

Oceanic mesoscale eddies are associated with large thermodynamic anomalies, yet so far they are most commonly studied in terms of surface temperature and in the sense of composite mean. Here we employ an objective eddy identification and tracking algorithm together with a novel matching and filling procedure to more thoroughly examine eddy-induced thermodynamic anomalies in the North Pacific, their relationship with eddy amplitude (SSH), and the percentage of variability they explain on various timescales from submonthly to interannual. The thermodynamic anomalies are investigated in terms of sea surface temperature (SST), isothermal layer depth (ITD), and upper ocean heat content (HCT). Most eddies are weak in amplitude and are associated with small thermodynamic anomalies. In the sense of composite mean, anticyclonic eddies are generally warm eddies with deeper isothermal layer and larger heat content, and the reverse is true for cyclonic eddies. A small fraction of eddies, most probably subsurface eddies, exhibits the opposite polarities. Linear relationships with eddy amplitude are found for each of the thermodynamic parameters but with different level of scatter and seasonality. HCT-amplitude relation scatters the least and has the smallest seasonal difference, ITD-amplitude relation has the largest scatter and seasonality, while SST-amplitude relation is in between. For the Kuroshio and Oyashio Extension region, the most eddy-rich region in the North Pacific, eddies are responsible for over 50% of the total SSH variability up to the intra-seasonal scale, and ITD and HCT variability up to interannual. Eddy-induced SST variability is the highest along the Oyashio Extension Front on the order of 40–60% on submonthly scales. These results highlight the role of mesoscale eddies in ocean thermodynamic variability and in air-sea interaction.

Air-sea coupling shapes North American hydroclimate response to ice sheets during the Last Glacial Maximum

The Western U.S. is vulnerable to hydrological stress, and insights from past climate periods are helpful for providing historical benchmarks for future climate projections. Myriad evidence from coupled models and paleoclimatic proxies suggests a major reorganization of west coast hydroclimate during the Last Glacial Maximum (LGM, ∼17–25 ka), such that the Southwest U.S. was wetter than modern day and the Pacific Northwest was drier. Yet the fundamental mechanisms underlying these hydroclimatic shifts remain unclear. Here, we employ a suite of targeted model simulations to probe the influence of LGM Northern Hemisphere ice sheets on west coast atmospheric dynamics. Whereas previous modeling studies have suggested that the southward shift of LGM west coast precipitation was driven only by the mechanical steering of atmospheric circulation by elevated ice sheet topography, we find this to be an artifact of earlier simulations that neglected realistic air-sea interaction. Instead, our simulations indicate that ice sheet albedo induced a pattern of North Pacific sea surface temperatures, reinforced by ocean-atmosphere feedbacks, that shifted the large-scale atmospheric circulation as well as the latitudinal distribution of west coast precipitation southward during the LGM. Crucially, we find that atmosphere-ocean feedbacks that sustained this ice sheet albedo-induced temperature pattern in the LGM could drive similar hydroclimatic changes today.

Development of early sea surface temperature biases in the tropical Indian Ocean in a coupled model

As part of seamless modeling strategy the ocean is increasingly becoming an important component of weather and climate forecasting systems across time scales. While the skill of sea surface temperatures (SSTs) at long-range forecasts is widely documented, few studies exist on the representation of evolving SST biases at shorter time-scales in a global coupled model. In this study, daily initialized runs of National Centre for Medium Range Weather Forecasting (NCMRWF) coupled model are analyzed over several seasons for up to two weeks of leadtimes. Analysis of source of SST biases at shorter time scales is carried out. It is found that the model captures well the spatial structure of the SSTs and their seasonal cycle. The SST correlations averaged over different regions are over 0.8 at day-14 leadtime, and RMSEs remain less than 0.6 °C over all regions except the south east equatorial Indian Ocean (SEEIO). SEEIO shows lowest of the correlations and highest of the biases among the regions considered. The difference in the skill of the model over different regions is found to be related to the representation of SST variability over the region. An assessment of the biases in different seasons suggests that the day-1 biases are related to the errors in the ocean state used to initialize the model. However, by day-14 increase in biases are seen over the regions where these are found to be related to the wind biases. Highly significant correlation (r=−0.39) is seen between winds and SST biases. Errors in winds also affect both the surface heat fluxes and the upper ocean mixing: wind biases are positively correlated to both latent heat fluxes and mixed layer depths (MLDs). It is found that the errors in atmospheric winds drive the biases in SSTs by influencing the upper ocean processes. An analysis of the relationship between latent heat fluxes and SSTs shows that the air-sea interactions are realistically represented in the model. This gives us confidence in attributing the SST biases to the upper ocean parameters. Contributions of different parameters to the SST biases are estimated using the heat budget of the upper ocean. It is shown that errors in LH fluxes and upper-ocean mixing explain large part of biases in SST tendencies. However, the errors in upper-ocean mixing explain much higher variance of biases in SST tendencies as compared to LH fluxes. This is found to be related to stronger dependence of SST tendencies to the MLD tendencies in the model. Identification of the sources of errors will help in our understanding of the biases at longer time scales and provides feedback for further model development.

Seasonal-To-Interannual Variability of Sea-Surface Temperatures in The Inter-Americas Seas: Pattern-Dependent Biases in The Regional Ocean Modeling System

The Inter-Americas Seas (IAS), involving the Gulf of Mexico, the Caribbean and a section of the eastern tropical Pacific Ocean bordering Central America, Colombia and Ecuador, exhibits very active ocean-land-atmosphere interactions that impact socio-economic activities within and beyond the region, and that are still not well understood or represented in state-of-the-art models. On seasonal-to-interannual timescales, the main source of variability of this geographical area is related to interactions between the Pacific and the Atlantic oceans, involving to anomalous sea-surface temperature (SST) patterns like El Niño-Southern Oscillation (ENSO), and regional features in the Caribbean linked to the bi-modal seasonality of the Caribbean Low-Level Jet. This study investigates seasonal-to-interannual IAS surface-temperature anomalies in observations, and their representation in am eddy-permitting, 1/9o-resolution simulation using the Regional Ocean Modeling System (ROMS), interannually-forced by the Climate Forecast System Reanalysis. Here, rather than analyzing model biases locally (i.e., gridbox-by-gridbox), a non-local SST pattern-based diagnostic was conducted via a principal component analysis. The approach allowed to identify magnitude, variance and spatial systematic errors in SST patterns related to the Western Hemisphere Warm Pool, ENSO, the Inter-American Seas Dipole, and several other variability modes. These biases are mainly related to errors in surface heat fluxes, misrepresentation of air-sea interactions impacting surface latent cooling in the Caribbean, and too strong sub-surface thermal stratification, mostly off the coast of Ecuador and northern Peru.