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We explore the response of wintertime Arctic sea ice growth to strong cyclones and to large-scale circulation patterns on the daily scale using Earth system model output in phase 5 of the Coupled Model Intercomparison Project (CMIP5). A combined metrics ranking method selects three CMIP5 models that are successful in reproducing the wintertime Arctic dipole (AD) pattern. A cyclone identification method is applied to select strong cyclones in two subregions in the North Atlantic to examine their different impacts on sea ice growth. The total change of sea ice growth rate (SGR) is split into those respectively driven by the dynamic and thermodynamic atmospheric forcing. Three models reproduce the downward longwave radiation anomalies that generally match thermodynamic SGR anomalies in response to both strong cyclones and large-scale circulation patterns. For large-scale circulation patterns, the negative AD outweighs the positive Arctic Oscillation in thermodynamically inhibiting SGR in both impact area and magnitude. Despite the disagreement on the spatial distribution, the three CMIP5 models agree on the weaker response of dynamic SGR than thermodynamic SGR. As the Arctic warms, the thinner sea ice results in more ice production and smaller spatial heterogeneity of thickness, dampening the SGR response to the dynamic forcing. The higher temperature increases the specific heat of sea ice, thus dampening the SGR response to the thermodynamic forcing. In this way, the atmospheric forcing is projected to contribute less to change daily SGR in the future climate.

Measurements of collocated fields of atmospheric forcing, surface waves, and mean and turbulent velocities associated with passage of Tropical Storm (TS) Barry over the U.S. Navy Tower R2 on the Georgia continental shelf are presented. A vertical-beam ADCP enables computation of directional surface wave spectra and hence of directional Stokes functions of depth and time, as well as mean (including tidal) and turbulent velocities throughout the water column. Full-depth turbulent velocity and backscatter structures observed during TS Barry are determined to be Langmuir supercells (LS). The LS appear in the present observations and in similar observations from a shallower site only when a surface growth rate g* exceeds a critical value, providing a means of predicting how deep an unstratified water column must be before LS will not be expected. When observed, LS structures at Tower R2 are less organized than archetypical LS structures: we suggest that this result is due primarily to smaller near-bottom growth rate in the deeper water column. Despite g* values above the critical value, and appropriate values of Langmuir and Rayleigh numbers, full-depth velocity/backscatter structures disappear completely for a time between the two wind maxima associated with the TS, as wind veers rapidly clockwise with eye passage to the west of Tower R2. From the observations, the most likely explanation for this hiatus is decreased wave breaking during the period of wind veering, reducing surface supply of “effective” vertical vorticity that dominates generation of Langmuir circulation (LC). This result has significant implications for LES modeling of LC.

Significant summer land surface warming has been observed in the middle latitudes over East Asia, especially after the mid-1990s, which has evidently affected the East Asian weather and climate. Using multisource observations and reanalysis data during 1979–2013, this study explores the possible reasons for recent land surface warming over this region by considering atmospheric forcing and regional land–atmosphere interaction related to extratropical cyclones (ECs). Results show that there is a close relationship between land surface warming and weakened ECs over East Asia. Recent land surface warming was attributed to local atmospheric forcing and feedback of land–atmosphere interaction associated with weakened ECs. The abnormal large-scale circulation associated with anomalous ECs produced evident dynamic forcing on the land surface. Weakened ECs are usually accompanied by an abnormal high pressure system and anticyclonic circulation around Lake Baikal, which benefit the intensification of anomalous southerly wind in the rear of the anomalous anticyclone, leading to positive temperature advection and temperature increase over East Asia. Meanwhile, the anomalous adiabatic warming caused by abnormal descending motion associated with the anticyclonic anomaly also contributes to local warming. The feedback of local land–atmosphere interaction plays an important role in land surface warming. Weakened ECs increase both incident solar radiation and precipitation. The increased precipitation reduces the soil moisture and in turn weakens the surface evaporation and local cooling effect, resulting in land surface warming. Our findings are helpful for better understanding the mechanisms responsible for recent summer land surface warming over East Asia as well as its climatic effects.

The impacts of diurnal atmospheric forcing on the summer salinity change in the East China Sea are investigated using the Regional Ocean Modeling System, forced by the hourly and daily reanalysis of wind and insolation. The differences between the forcing of these two frequencies reveal a dipole pattern of salinity change with a positive salinity deviation (1–2 psu) offshore of the Yangtze River estuary, and a negative deviation (from −1 to −0.5 psu) along the Jiangsu Coast. Further dye tracking experiments confirm that diurnal forcing strengthened the northwestward longshore freshwater transport (NLFT) of the Yangtze River by 5.2 × 109 m3 and reduced the mean water age of 7 days. Sensitivity experiments using different forcing combinations suggest that the diurnal wind, that is, the land–sea breeze, is the key to developing the dipole pattern of salinity change and the NLFT. Through the experiment, the land–sea breeze induced a mean clockwise circulation offshore of the Yangtze River estuary. The above changes resulted from both the nonlinearity of wind stress averaging (i.e., the square nature of wind stress) and the baroclinic adjustment related to the diurnal salinity variation, which is directly connected to the diurnal swing of the Yangtze River front. The baroclinic adjustment generated a dipole pattern of vorticity changes offshore of the Yangtze River estuary and a coherent northwestward jet current strengthening the NLFT. These processes developed the summer dipole pattern of the salinity change.

The Southern Ocean (SO) surface wind stress is a major atmospheric forcing for driving the Antarctic Circumpolar Current and the global overturning circulation. Here the effects of wind fluctuations at different time scales on SO wind stress in 18 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are investigated. It is found that including wind fluctuations, especially on time scales associated with synoptic storms, in the stress calculation strongly enhances the mean strength, modulates the seasonal cycle, and significantly amplifies the trends of SO wind stress. In 11 out of the 18 CMIP5 models, the SO wind stress has strengthened significantly over the period of 1960–2005. Among them, the strengthening trend of SO wind stress in one CMIP5 model is due to the increase in the intensity of wind fluctuations, while in all the other 10 models the strengthening trend is due to the increasing strength of the mean westerly wind. These discrepancies in SO wind stress trend in CMIP5 models may explain some of the diverging behaviors in the model-simulated SO circulation. Our results suggest that to reduce the uncertainty in SO responses to wind stress changes in the coupled models, both the mean wind and wind fluctuations need to be better simulated.

When subjecting ocean models to atmospheric forcing, the models exhibits two types of variability—a response to the external forcing (hereafter referred to as signal) and inherently generated (internal, intrinsic, unprovoked, chaotic) variations (hereafter referred to as noise). Based on an ensemble of simulations with an identical atmospherically forced oceanic model that differ only in the initial conditions at different times, the signal-to-noise ratio of the atmospherically forced oceanic model is determined. In the large scales, the variability of the model output is mainly induced by the external forcing and the proportion of the internal variability is small, so the signal-to-noise ratio is large. For smaller scales, the influence of the external forcing weakens and the influence of the internal variability strengthens, so the signal-to-noise ratio becomes less and less. Thus, the external forcing is dominant for large scales, while most of the variability is internally generated for small scales.

Understanding water level (WL) fluctuations in river deltas is of importance for managing water resources and minimizing the impacts of floods and droughts. Here, we demonstrate the competing effects of atmospheric and oceanic forcing on multi-timescale variability and changes in the Pearl River Delta (PRD) WLs in southern China, using 52 years (1961–2012) of in-situ observations at 13 hydrological stations. PRD WL presents significant seasonal to decadal variations, with large amplitudes upstream related to strong variability of southern China rainfall, and with relatively small amplitudes at the coastal stations determined by sea level (SL) fluctuations of the northern South China Sea. We find that the strengths of atmospheric and oceanic forcing in PRD are not mutually independent, leading to a distinct contrast of WL–forcing relationships at upstream and coastal stations. In the transition zone, because of counteracts of atmospheric and oceanic forcing, no robust relationships are identified between WL and either of the forcing. We further show that in the drought season of the warm ENSO and PDO epochs, the effect of atmospheric (oceanic) forcing on PRD WL is largely enhanced (weakened), due to increased southern China rainfall and negative SL anomalies. Over the observation period, WL significantly decreased at upstream stations, by up to 28–42 mm/year for flood season, contrasting with the upward trends of

Stochastic variability of internal atmospheric modes, known as teleconnection patterns, drives large-scale patterns of low-frequency SST variability in the extratropics. To investigate how the decadal component of this stochastically driven variability in the South and North Pacific affects the tropical Pacific and contributes to the observed basinwide pattern of decadal variability, a suite of climate model experiments was conducted. In these experiments, the models are forced with constant surface heat flux anomalies associated with the decadal component of the dominant atmospheric modes, particularly the Pacific–South American (PSA) and North Pacific Oscillation (NPO) patterns. Both the PSA and NPO modes induce basinwide SST anomalies in the tropical Pacific and beyond that resemble the observed interdecadal Pacific oscillation. The subtropical SST anomalies forced by the PSA and NPO modes propagate to the equatorial Pacific mainly through the wind–evaporation–SST feedback. This atmospheric bridge is stronger from the South Pacific than the North Pacific due to the northward displacement of the intertropical convergence zone and the associated northward advection of momentum anomalies. The equatorial ocean dynamics is also more strongly influenced by atmospheric circulation changes induced by the PSA mode than the NPO mode. In the PSA experiment, persistent and zonally coherent wind stress curl anomalies over the South Pacific affect the zonal mean depth of the equatorial thermocline and weaken the equatorial SST anomalies resulting from the atmospheric bridge. This oceanic adjustment serves as a delayed negative feedback and may be important for setting the time scales of tropical Pacific decadal variability.

A data-assimilating ⅓° regional dynamical ocean model is evaluated on its ability to synthesize components of the Tropical Pacific Ocean Observing System. The four-dimensional variational data assimilation (4DVAR) method adjusts initial conditions and atmospheric forcing for overlapping 4-month model runs, or hindcasts, that are then combined to give an ocean state estimate for the period 2010–13. Consistency within uncertainty with satellite SSH and Argo profiles is achieved. Comparison to independent observations from Tropical Atmosphere Ocean (TAO) moorings shows that for time scales shorter than 100 days, the state estimate improves estimates of TAO temperature relative to an optimally interpolated Argo product. The improvement is greater at time scales shorter than 20 days, although unpredicted variability in the TAO temperatures implies that TAO observations provide significant information in that band. Larger discrepancies between the state estimate and independent observations from Spray gliders deployed near the Galápagos, Palau, and Solomon Islands are attributed to insufficient model resolution to capture the dynamics in strong current regions and near coasts. The sea surface height forecast skill of the model is assessed. Model forecasts using climatological forcing and boundary conditions are more skillful than climatology out to 50 days compared to persistence, which is a more skillful forecast than climatology out to approximately 20 days. Hindcasts using reanalysis products for atmospheric forcing and open boundary conditions are more skillful than climatology for approximately 120 days or longer, with the exact time scale depending on the accuracy of the state estimate used for initializing and on the reanalysis forcing. Estimating the model representational error is a goal of these experiments.

There is a large uncertainty of snow water equivalent (SWE) in reanalyses and the Global Land Data Assimilation System (GLDAS), but the primary reason for this uncertainty remains unclear. Here several reanalysis products and GLDAS with different land models are evaluated and the primary reason for their deficiencies are identified using two high-resolution SWE datasets, including the Snow Data Assimilation System product and a new dataset for SWE and snowfall for the conterminous United States (CONUS) that is based on PRISM precipitation and temperature data and constrained with thousands of point snow observations of snowfall and snow thickness. The reanalyses and GLDAS products substantially underestimate SWE in the CONUS compared to the high-resolution SWE data. This occurs irrespective of biases in atmospheric forcing information or differences in model resolution. Furthermore, reanalysis and GLDAS products that predict more snow ablation at near-freezing temperatures have larger underestimates of SWE. Since many of the products do not assimilate information about SWE and snow thickness, this indicates a problem with the implementation of land models and pinpoints the need to improve the treatment of snow ablation in these systems, especially at near-freezing temperatures.

Two techniques for perturbing surface initial conditions in the regional ensemble system Aire Limitée Adaptation Dynamique Développement International-Limited Area Ensemble Forecasting (ALADIN-LAEF) are presented and investigated in this paper. The first technique is the noncycling surface breeding (NCSB), which combines short-range surface forecasts driven by perturbed atmospheric forcing and the breeding method for generating the perturbations on surface initial conditions. The second technique, which is currently used in the ALADIN-LAEF operational version, applies an ensemble of surface data assimilations (ESDA) in which the observations are randomly perturbed. Both techniques are evaluated over a two-month period from late spring to summer. The results show that the evaluation is more favorable to ESDA. In general, the ensemble forecasts of the observed near-surface meteorological variables (screen-level variables) of ESDA are more skillful than NCSB, in particular for 2-m temperature they are statistically more consistent and reliable. A slightly better statistical reliability for 2-m relative humidity and 10-m wind has been found as well. This could be attributed to the introduction of surface data assimilation in ESDA, which provides more accurate surface initial conditions. Moreover, the observation perturbation in ESDA helps to better estimate the initial condition uncertainties. For the forecast of precipitation and the upper-air variables in the lower troposphere, both ESDA and NCSB perform very similarly, having neutral impact.

One of the most robust remote impacts of El Niño–Southern Oscillation (ENSO) is the teleconnection to tropical North Atlantic (TNA) sea surface temperature (SST) in boreal spring. However, important questions still remain open. In particular, the timing of the ENSO–TNA relationship lacks understanding. The three previously proposed mechanisms rely on teleconnection dynamics involving a time lag of one season with respect to the ENSO mature phase in winter, but recent results have shown that the persistence of ENSO into spring is necessary for the development of the TNA SST anomalies. Likewise, the identification of the effective atmospheric forcing in the deep TNA to drive the regional air–sea interaction is also lacking. In this manuscript a new dynamical framework to understand the ENSO–TNA teleconnection is proposed, in which a continuous atmospheric forcing is present throughout the ENSO decaying phase. Observational datasets in the satellite era, which include reliable estimates over the ocean, are used to illustrate the mechanism at play. The dynamics rely on the remote Gill-type response to the ENSO zonally compensated heat source over the Amazon basin, associated with perturbations in the Walker circulation. For El Niño conditions, the anomalous diabatic heating in the tropical Pacific is compensated by anomalous diabatic cooling, in association with negative rainfall anomalies and descending motion over northern South America. A pair of anomalous cyclonic circulations is established at upper-tropospheric levels in the tropical Atlantic straddling the equator, displaying a characteristic baroclinic structure with height. In the TNA region, the mirrored anomalous anticyclonic circulation at lower-tropospheric levels weakens the northeasterly trade winds, leading to a reduction in evaporation and of the ocean mixed layer depth, hence to positive SST anomalies. Apart from the dominance of latent heat flux anomalies in the remote response, sensible heat flux and shortwave radiation anomalies also appear to contribute. The “lagged” relationship between mature ENSO in winter and peaking TNA SSTs in spring seems to be phase locked with the seasonal cycle in both the location of the mechanism’s centers of action and regional SST variance. This work has been supported by the EU/H2020-funded MSCA-IF-EF DPETNA project (GA 655339) and JG-S was partially supported by the Spanish MINECO-funded DANAE Project (CGL2015-68342-R). Technical support at BSC (Computational Earth Sciences group) is sincerely acknowledged. The authors are grateful to the anonymous reviewers for their comments, which helped to improve the scope of the manuscript.

The evolution of the Pacific subtropical cells (STC) is presented for the period 1948–2007. Using ocean models of different resolutions forced with interannually varying atmospheric forcing datasets, the mechanisms responsible for the observed STC weakening and late recovery during the period of study are analyzed. As a result of the STC weakening (strengthening), warming (cooling) trends are found in the equatorial Pacific sea surface temperatures (SSTs). Model results agree well with observed estimates of STC transport, STC convergence, and equatorial SST anomalies. It is shown that subtropical atmospheric variability is the primary driver of the STC and equatorial SST low-frequency evolution and is responsible for both the slowdown during the second half of the twentieth century and the rebound at the end of the century. Subtropically forced STC variability is identified as a major player in the generation of equatorial Pacific decadal SST anomalies, pacing tropical Pacific natural climate variability on interdecadal time scales, as observed in historical records. The natural mode of variability has implications for the evolution of equatorial SST in the coming decades under the concomitant effects of climate change.

Interannual to interdecadal precipitation P, evaporation E, freshwater budget (E − P), and air–sea net heat flux Q have been correlated with the North Atlantic Oscillation (NAO), eastern Atlantic (EA), eastern Atlantic–western Russia (EA-WR), and Mediterranean Oscillation (MO) climatic indices to explore the influence of atmospheric forcing in the Mediterranean freshwater and heat budget variability. The effect of the MO pattern has similarities with that of the NAO, but MO influence is more intense. On an annual basis, the MO index gives the highest correlation with all the variables considered, and during its negative phase, it exerts a stronger influence than the NAO and is associated with higher P and, especially, enhanced evaporative losses in the Levantine subbasin. The EA pattern does not significantly affect P in the Mediterranean, but a high correlation is found for E and Q from 1979. The EA-WR mode plays a significant role in annual net heat flux since variations in its sign have the potential to induce seesaw variations in the heat budgets of the eastern and western subbasins, as previously found by Josey et al., for wintertime.

The direct radiative effect of absorbing aerosols consists of absorption-induced atmospheric heating together with scattering- and absorption-induced surface cooling. It is thus important to understand whether some of the reported climate impacts of anthropogenic absorbing aerosols are mainly due to the coexistence of these two opposite effects and to what extent the nonlinearity raised from such coexistence would become a critical factor. To answer these questions specifically regarding the South Asia summer monsoon with focus on aerosol-induced changes in monsoon onset, a set of century-long simulations using the Community Earth System Model, version 1.0.3 (CESM 1.0.3), of NCAR with fully coupled atmosphere and ocean components was conducted. Prescribed direct heating to the atmosphere and cooling to the surface were applied in the simulations over the Indian subcontinent, either alone or combined, during the aerosol-laden months of May and June. Over many places in the Indian subcontinent, the nonlinear effect dominates in the changes of subcloud layer moist static energy, precipitation, and monsoon onset. The surface cooling effect of aerosols appears to shift anomalous precipitative cooling away from the aerosol-forcing region and hence turn the negative feedback to aerosol-induced atmospheric heating into a positive feedback on the monsoon circulation through latent heat release over the Himalayan foothills. Moisture processes form the critical chain mediating local aerosol direct effects and onset changes in the monsoon system. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) Published version

An analysis and physical interpretation of near-inertial waves (NIWs) propagating perpendicular to a steady, two-dimensional, strongly baroclinic, geostrophic current are presented. The analysis is appropriate for geostrophic currents with order-one Richardson numbers such as those associated with fronts experiencing strong, wintertime atmospheric forcing. This work highlights the underlying physics behind the properties of the NIWs using parcel arguments and the principles of conservation of density and absolute momentum. Baroclinicity introduces lateral gradients in density and vertical gradients in absolute momentum that significantly modify the dispersion and polarization relations and propagation of NIWs relative to classical internal wave theory. In particular, oscillations at the minimum frequency are not horizontal but, instead, are slanted along isopycnals. Furthermore, the polarization of the horizontal velocity is not necessarily circular at the minimum frequency and the spiraling of the wave’s velocity vector with time and depth can be in the opposite direction from that predicted by classical theory. Ray tracing and numerical solutions illustrate the trapping and amplification of NIWs in regions of strong baroclinicity where the wave frequency is lower than the effective Coriolis frequency. The largest amplification is found at slantwise critical layers that align with the tilted isopycnals of the current. Such slantwise critical layers are seen in wintertime observations of the Gulf Stream and, consistent with the theory, coincide with regions of intensified ageostrophic shear characterized by a banded structure that is spatially coherent along isopycnals.

To better understand error and spatial variability sources of soil moisture simulated with land surface models, observed and simulated values of soil moisture (using offline simulations with the Noah land surface model with four soil layers and approximately 1-km horizontal grid spacing) were compared. This comparison between observed and modeled daily values of soil moisture was performed over the Lower Mississippi Delta region during summer–fall months 2004–06. The Noah simulations covered the 2.5° × 2.5° latitude–longitude domain and were forced by the North American Land Data Assimilation System (NLDAS) atmospheric forcing fields. Hourly soil moisture measurements and other data, including local meteorological and soil physical properties data from 12 Soil Climate Analysis Network (SCAN) sites, were used. The results show that both the observed and simulated level of soil moisture depend critically on the specified–sampled soil texture. Soil types with a relatively high observed clay content (more than 50% of weight) retain more water as a result of low water diffusivity than silty–sandy soils with 20% or less clay, provided that other conditions are the same. This fact is in agreement with previous studies and implies a strong soil texture control (through related hydraulic parameters) on the accuracy of simulated soil moisture. Sensitivity tests using the Noah model were performed to assess the effect of using the hydraulic parameters related to the site-specific soil texture on soil moisture quality. Indeed, at some SCAN sites, the errors (root-mean-square difference and bias) were reduced. Simulated soil moisture showed at least a 50% reduction when the site-specific soil texture was used in Noah simulations compared to those derived from the State Soil Geographic (STATSGO) data. The most significant improvement of simulated soil moisture was observed within the top 0–10 cm layer where an original positive bias (an excessive wetness) was almost eliminated. Meanwhile, excessive dryness (negative soil moisture bias), which was dominant within the second and third model layers, was also reduced. These improvements are expected to be valid at sites/regions with low (

A regional model is used to study the radiative effect of boundary layer clouds over the southeast Pacific on large-scale atmosphere circulation during August–October 1999. With the standard settings, the model simulates reasonably well the large-scale circulation over the eastern Pacific, precipitation in the intertropical convergence zone (ITCZ) north of the equator, and marine boundary layer stratocumulus clouds to the south. In a sensitivity experiment with the radiative effect of liquid clouds south of the equator over the eastern Pacific artificially removed, boundary layer clouds south of the equator almost disappear and precipitation in the ITCZ is reduced by 15%–20%, indicating that the stratocumulus clouds over the southeast Pacific have both local and cross-equatorial effects. Examination of the differences between the control and sensitivity experiments indicates that clouds exert a net diabatic cooling in the inversion layer. In response to this cloud-induced cooling, an in situ anomalous high pressure system develops in the boundary layer and an anomalous shallow meridional circulation develops in the lower troposphere over the equatorial eastern Pacific. At the lower branch of this shallow circulation, anomalous boundary layer southerlies blow from the boundary layer high toward the northern ITCZ where the air ascends. An anomalous returning flow (northerly) just above the cloud layer closes the shallow circulation. This low-level anomalous shallow circulation enhances the subsidence over the southeast Pacific above the cloud layer, helping to maintain boundary layer clouds and temperature inversion there. Meanwhile, the strengthened cross-equatorial flow near the surface enhances moisture convergence and convection in the ITCZ north of the equator. This in turn strengthens the local, deep Hadley circulation and hence the large-scale subsidence and boundary layer clouds over the southeast Pacific. This positive feedback therefore enhances the interhemispheric climate asymmetry over the tropical eastern Pacific.

An El Nino of moderate intensity developed in the tropical Pacific in 2002/03. This event, though not as strong as the 1997/98 El Nino, had significant impacts on patterns of weather variability worldwide. The evolution of the 2002/03 El Nino is documented through comprehensive satellite and in situ observations from the El Nino-Southern Oscillation (ENSO) Observing System. These observations underscore the importance of both episodic atmospheric forcing and large-scale low-frequency ocean–atmosphere interactions in the development of the event.

Variability of the Pacific decadal oscillation (PDO), on both interannual and decadal timescales, is well modeled as the sum of direct forcing by El Niño Southern Oscillation (ENSO), the ``reemergence'' of North Pacific sea surface temperature anomalies in subsequent winters, and white noise atmospheric forcing. This simple model may be taken as a null hypothesis for the PDO, and may also be relevant for other climate integrators that have been previously related to the PDO.

A bulk-type (modified Kraus–Turner) mixed layer model that is embedded within the Naval Research Laboratory (NRL) Layered Ocean Model (NLOM) is introduced. It is an independent submodel loosely coupled to NLOM's dynamical core, requiring only near-surface currents, the temperature just below the mixed layer, and an estimate of the stable mixed layer depth. Coupling is achieved by explicitly distributing atmospheric forcing across the mixed layer (which can span multiple dynamic layers), and by making the heat flux and thermal expansion of seawater dependent upon the mixed layer model's sea surface temperature (SST). An advantage of this approach is that the relative independence of the dynamical solution from the mixed layer allows the initial state for simulations with the mixed layer to be defined from existing near-global model simulations spun up from rest without a mixed layer (requiring many hundreds of model years). The goal is to use the mixed layer model in near-global multidecadal simul...

The physical mechanisms of transient amplification of initial perturbations to the thermohaline circulation (THC), and of the optimal stochastic forcing of THC variability, are discussed using a simple meridional box model. Two distinct mechanisms of transient amplification are found. One such mechanism, with a transient amplification timescale of a couple of years, involves an interaction between the THC induced by rapidly decaying sea surface temperature anomalies and the THC induced by the slower-decaying salinity mode. The second mechanism of transient amplification involves an interaction between different slowly decaying salinity modes and has a typical growth timescale of decades. The optimal stochastic atmospheric forcing of heat and freshwater fluxes are calculated as well. It is shown that the optimal forcing induces low-frequency THC variability by exciting the salinity-dominated variability modes of the THC.

Regional climate simulations are long time integrations over an open system where the atmosphere over part of the model domain (boundary zones) is updated periodically. Model reinitialization after a long period of integration can allow several segments of a long simulation to be run in parallel and also minimize possible drift caused by accumulated model errors. However, the spinup problems introduced by each additional restart must be addressed. The necessity and feasibility of subdividing long integrations is investigated by means of a series of experiments in which the authors examine the effects of reinitialization frequency and the relative importance of surface forcing and atmospheric forcing. It is found that for integrations that continued without reinitialization, locations of specific meteorological features drifted downstream because simulated winds were too strong, implying the need for periodic reinitialization of the model. The results indicate also that when the model reinitialization interval is relatively long, simulated domain-averaged variables, including rainfall, were not very sensitive to model reinitialization since they are largely constrained by transient boundary conditions, suggesting the feasibility of dividing long regional climate simulations into a set of shorter ones that could be run in parallel.

Ocean–atmosphere interaction plays a key role in climate fluctuations on interdecadal timescales. In this study, different aspects of this interaction are investigated using an idealized ocean–atmosphere model, and a hierarchy of uncoupled and stochastic models derived from it. The atmospheric component is an eddy-resolving two-level global primitive equation model with simplified physical parameterizations. The oceanic component is a zonally averaged sector model of the thermohaline circulation. The coupled model exhibits spontaneous oscillations of the thermohaline circulation on interdecadal timescales. The interdecadal oscillation has qualitatively realistic features, such as dipolar sea surface temperature anomalies in the extratropics. Atmospheric forcing of the ocean plays a dominant role in exciting this oscillation. Although the coupled model is in itself deterministic, it is convenient to conceptualize the atmospheric forcing arising from weather excitation as having stochastic time dep...

The interaction between the annual and interannual variations is investigated by contrasting a pair of experiments with a general circulation model of the tropical Pacific Ocean. The atmospheric forcing applied to the model includes both annual and interannual components The phase of the annual forcing is shifted one-half year in the two runs, which are otherwise identical. Significant differences are found in the sea surface temperature (SST) evolution between the two runs that have the same interannual forcing function. SST anomalies tend to be phase-locked to the solar calendar and to appear in the cold season. A SST variance function in response to interannual forcings with a random phase distribution is constructed, which has an annual cycle and reaches its maximum in the cold season as is observed. It is suggested that this seasonality of an ocean origin is amplified by the interaction with the atmosphere, leading to the observed phase-locking. The phase-locking of the interannual cycle and...

Since the late 1940s, snowmelt and runoff have come increasingly early in the water year in many basins in northern and central California. This subtle trend is most pronounced in moderate-altitude basins, which are sensitive to changes in mean winter temperatures. Such basins have broad areas in which winter temperatures are near enough to freezing that small increases result initially in the formation of less snow and eventually in early snowmelt. In moderate-altitude basins of California, a declining fraction of the annual runoff has come in April–June. This decline has been compensated by increased fractions of runoff at other, mostly earlier, times in the water year. Weather stations in central California, including the central Sierra Nevada, have shown trends toward warmer winters since the 1940s. A series of regression analyses indicate that runoff timing responds equally to the observed decadal-scale trends in winter temperature and interannual temperature variations of the same magnitude...

We report an unprecedented description of the latitudinal structure of tropical Pacific Ocean sea level variability for time scales between 3 and 100 days. A plot of calculated sea level variance latitude and frequency is described and exposes a rich variety of phenomena. We find signals due to the fortnightly tide, equatorial waves, zonal current instabilities, and atmospheric forcing.

Observations of horizontal currents and temperature made along a mid-shelf isobath during the summer on the Southern California shelf are used to determine longshore coherent length scales. The observations are characterized by weak atmospheric forcing and strong density stratification of the water column. For periods longer than a day, coherent longshore length scales of currents are around 25 km, while coherent length scales of temperature were longer than the instrumental array (57.5 km). For periods ranging from a day to an hour, near-surface motions were dominated by diurnal fluctuations which were coherent over lengths of 5 km, whereas near-bottom motions, dominated by semidiurnal fluctuations, showed evidence of a signal propagating up coast at phase speeds of order 1 m s−1. The observed scales of motion in different frequency bands are used to estimate the dispersive properties of the shelf motions. The dispersion coefficient is found to vary with scale, as the current variance associated...

The Frontal Air-Sea Interaction Experiment (FASINEX) is a study of the response of the upper ocean to atmospheric forcing in the vicinity of an oceanic front in the subtropical convergence zone southwest of Bermuda, the response of the lower atmosphere in that vicinity to the oceanic front, and the associated two-way interaction between ocean and atmosphere. FASINEX is planned for the winter and spring of 1985-1986 with an intensive period in February and March 1986 in the vicinity of 27 deg N, 70 deg W, where sea-surface temperature fronts are climatologically common. Measurements will be made from buoys, ships, aircraft, and spacecraft. A previous article gave a brief history of FASINEX and presented its scientific goals. This article describes the FASINEX experimental plan.

The coherence of sea level is examined between a number of widely distributed stations chosen from those with the longest datasets, such as at San Francisco, where the data has been recorded since 1855. The sea-level signals in the eastern Pacific appear to be dominated by propagating Rossby waves, so that the variability, which has periods of 5–8 years, (e.g., between San Francisco and Honolulu), is coherent, but out of phase by several years. A surprising finding is that sea level is coherent on opposite sides of the Atlantic at periods near 6 years, but this is suspected to be the result of direct atmospheric forcing rather than of wave propagation. At the longest periods detectable—40–50 years—the sea-level signals have amplitudes of 5–15 cm and are “visually coherent" between the west coasts of the United States and Europe. The amplitude of these extremely long-period signals is the same as the apparent “rise of sea level over the the past century," although the rate of rise from these fluct...

Simulations of the oceanic mixed layer at Ocean Weather Ship Papa are used to study the sensitivity of the 30-day predictions of mixed-layer depth and temperature to the time resolution and averaging of the atmospheric forcing during spring, summer and autumn. The model simulations are sensitive to the length of the averaging window applied to the atmospheric forcing. Both the detail and trends in the mixed-layer depth and temperature deviate more from the control run when the length of the averaging window is increased. The effect of averaging the meteorological observations prior to calculating the surface fluxes is examined separately from the case in which the fluxes are calculated prior to the averaging. The cases which use forcing calculated from averages of the actual observations better simulate the detail and trend in mixed-layer depth of 30-day windows in the spring, summer and autumn than cases which use forcing based on the average of the calculated fluxes. By contrast, forcing based ...

Sea level observations made during a three-month period in late 1984 at four stations in and near Delaware's inland bays are used for the examination of tidal and subtidal variability in the bays. Five tidal Constituents are found to be active in the bays, With M2 being the most important one. The inland bays, with their narrow inlets and shallow depths, behave like low-pass filters for oceanic disturbances propagating into the interior of the bays. Overall, tidal variance decreases sharply inside the bays, with the semidiurnal tides suffering significantly higher attenuation than the diurnal tides. At subtidal frequencies, sea level fluctuations in the interior of the bays are primarily forced by coastal sea level fluctuations generated by atmospheric forcing on the adjacent shelf. Local wind forcing within the bays appears to play only a secondary role in modifying the coastally forced sea level. The active subtidal sea level fluctuations induced by the bay-shelf coupling effect experience no a...

The Ekman pumping mechanism for altering the depth of the main thermocline in response to wind stress curl is tested in the central midlatitude North Pacific. According to this mechanism, the depth of the main thermocline should decrease under cyclonic wind stress curl and increase under anticyclonic wind stress curl. For the two years 1976–78, temperature measurements from an XBT measurement program between North America and Japan have allowed the monthly thermal structure to be measured over an area 30–50°N, 130–170°W, accompanied with synoptic estimates of wind stress curl. Working with anomalous estimates that deviate from the normal seasonal cycle, the month-to-month secular change in the depth of the main thermocline during the nine months of each year from February to October is found to have responded to the anomalous wind stress curl according to what was expected from the Ekman pumping mechanism. The expected and observed secular changes in the thermocline depth for these times of the y...

The first-order wave equation model of Gill and Schumann, which describes the flow induced on the continental shelf by the longshore wind stress, evaluated at the coast, is extended in three ways. First, we relax the nondivergent approximation and hence are able to estimate the response in the Kelvin wave mode. Second, we include the full wind stress forcing, and allow for an offshore variation in the wind stress. We also include a representation of pressure forcing. Thus we are able to describe the response due to localized forcing systems. Third, we consider the coupling of the deep-ocean region to the shelf region across a continental slope region, modeled as a discontinuity in depth at the shelf break. The results are applied to the response on the shelf due to a localized forcing system, either traveling longshore parallel to the coast, or traveling offshore normal to the coast.

Non-tidal variability in the Chesapeake Bay and Potomac River, and its relation to atmospheric forcing, is examined from two-month sea level and bottom current measurements. The dominant sea level fluctuations in the Bay had a period of 20 days. and were the result of up-Bay propagation of coastal sea level fluctuations generated by the alongshore winds. Consequently, water was driven out of the Bay by the northward/up-Bay wind and driven into the Bay by the southward/down-Bay wind, through the coastal Ekman flux. There were also large sea level fluctuations at periods of 5 and 2.5 days. The 5-day fluctuations were driven by both the coastal sea level changes and the local lateral winds (Ekman effect). The 2.5-day fluctuations were seiche oscillations driven by the local longitudinal winds. In the Potomac River, the sea level fluctuations were induced non-locally by motions in the Bay; the associated volume fluxes appeared to have been confined to the upper layer. The near-bottom currents were ma...

A model testing procedure based on multivariate statistical analysis has been developed to provide an objective measure of the fit between ocean model simulations and observations, taking into account the uncertainties in the atmospheric forcing and the inaccuracies in the oceanic data. The method is applied to the seasonal variations in an ocean model of the tropical Atlantic. A wind-driven linear multimode model with a simple mixed-layer is tested against surface currents estimated from ship drift. The uncertainties in the observations, and in the model response due to random errors in the wind stress and its interannual variability create a substantial indeterminacy in the available sample, but it is shown that they do not explain the large discrepancies that are found between observed and modeled seasonal surface currents. Nor are the uncertainties in the wind stress bulk formulation sufficient to account for the model-data differences. These can then be attributed unambiguously to the oversi...

The subtidal frequency response of sea level to atmospheric forcing along the coastal region between Cape Hatteras and Charleston is investigated for a four-month period: 1 September-31 December, 1974. It is found that low-frequency sea level fluctuations are preferentially forced by wind stress components which are aligned with the local topography. Also, a localized, one-dimensional model of sea surface response to a clockwise rotating wind for the Charleston coastal regime is developed. The phase spectrum of the alongshore wind component versus sea level as predicted by the model is shown to compare favorably to that derived from actual observations at Charleston, an open ocean coastal site. The model results and observations also suggest that wind-induced fluctuations of coastal sea level are trapped within 40 km of the coast by the combined effects of friction, Coriolis force and bottom topography. The outer shelf is dominated by fluctuations which are less related to wind stress and are attenuated rapidly in the shoreward direction. A reasonable estimate of bottom frictional coefficient, r = 0.05 cm/s, is also established.

The Frontal Air-Sea Interaction Experiment (FASINEX) is a study of the response of the upper ocean to atmospheric forcing in the vicinity of an oceanic front in the subtropical convergence zone southwest of Bermuda, the response of the lower atmosphere in that vicinity to the oceanic front, and the response of the associated two-way interaction between ocean and atmosphere. FASINEX is planned for the winter and spying of 1985/86 with an intensive period in February and March 1986 in the vicinity of 27°N, 70°W, where sea-surface-temperature fronts are climatologically common. Measurements will be made from buoys, ships, aircraft, and spacecraft. This article gives a brief history of FASINEX and presents its scientific goals. An article in next month's BULLETIN will describe the experimental plan.

Long time series of meteorological data from Ocean Weather Ships P, V and N in the North Pacific Ocean are used to test the hypothesis that significant oceanic thermal response during September to December occurs in association with limited periods of strong atmospheric forcing. The 3 h forcing is expressed according to recent mixed layer theory in terms of u*3, where u* is the atmospheric friction velocity, and the upward surface heat flux. About 13% of the largest u*3 values contribute 50% of the total u*3, even though the total input is quite different at the three stations. Although the thermal forcing is less skewed, a significant fraction of this flux occurs during a relatively small fraction of the time. Synoptic time scale forcing events are defined as sustained periods of forcing exceeding the long-term mean fluxes for the corresponding period. Between 68 and 77% of the total u*3 occurred during the roughly one-third of the time associated with the synoptic forcing events defined in term...

This paper constitutes an attempt to elucidate and account for the characteristics of persistence of mouth-to-month, season-to-season, and year-to-year midtropospheric flow patterns-patterns that control much of the weather and climate of North America. The well-known signature of atmospheric forcing produced by normal seasonal factors is sharpened with new data, exposing a large residual of interannual variation to be documented and explained. In order to do this, several avenues are explored, including the preparation of statistics of pattern correlations stratified by month, season, year, and combinations thereof, as well as the stratification of data by quartiles representing high, low and intermediate persistence. Analyses of these data imply that certain flow patterns are more stable than others, and that the more stable patterns have stronger anomalies, are associated with mutually reinforcing teleconnections, and tend to be well reflected in persistent zonal wind speed profiles. Strong mo...

Subtidal sea level variations in the Chesapeake Bay were examined over a one-year period for evidence of wind-driven barotropic circulation. The major transport occurred at time scales of 3–5 days, whose magnitude was larger than the river runoff. It was driven by the east-west wind, as part of the coupled coastal ocean-estuary response. At shorter time scales, there was also large barotropic motion which, however, was driven by the local, north-south wind. The variance of barotropic fluctuation was larger by a factor of 4 in winter than in summer, due to the increased cyclone activities. The coupled coastal ocean-estuary response was also more pronounced in winter. In contrast, the summer season was dominated by local forcing at time scales of 3–7 days. The results suggest that the barotropic motion is an important component of the net circulation. The corresponding subtidal sea level change contributes significantly to the storm surge. Thus, the nature of barotropic response, particularly the c...

Along the west Coast of North America, the response of sea level to fluctuations in alongshore wind stress at large alongshore scales (> 1000 km) accounted for a substantial faction of the total sea level variance during summer 1973. Space-time contour plots of sea level and alongshore stress show that the response of sea level to poleward propagating wind stress events was generally stronger than the response to equatorward propagating events. Atmospheric forcing was most effective in two regions along the coast, with relatively strong forcing and response along northern California and Oregon, and somewhat weaker forcing and response along northern Baja California. The forced fluctuations in sea level propagated poleward away from these forcing regions, causing local sea level to be most correlated with alongshore wind stress earlier in time and at a distant equatorward 1ocation. Along the southern and central California coast, fluctuation in sea level were partly forced along northern Baja Cali...

A data assimilation technique using a one-dimensional, ocean mixed-layer model to advance the thermal structure observations to the analysis time is tested. The effects of insertions of erroneous temperature profiles in such a model are studied for winter and summer periods. Sets of 5, 15 or 30 model-generated profiles at random times during the 15 days prior to analysis time are modified by adding errors in layer depth and temperature and in the thermocline temperature gradient. Each of the modified profiles is advanced by the mixed-layer model to the initial forecast time. This produces a series of temperature profiles that are consistent with the prediction model and with the atmospheric forcing at that time. The profiles are combined by a simple level-by-level averaging. A prediction from this averaged profile, or a similar profile after screening for extreme temperatures, is compared to a prediction from the last available observation in the set. A control run of the model provides complete ...

Sea level and current data collected around Great South Bay, New York during December 1979 are examined in conjunction with atmospheric data for evidence of wind-forced, low frequency variability in the Bay and on the adjacent shelf. The subtidal sea level along the coast was found to be highly coherent from Sandy Hook to Montauk Point, with a single empirical mode accounted for more than 97% of the total variance. These coherent fluctuations were forced primarily by longshore winds (along 250–070°T) through the coastal Ekman effect. The sea level within the Bay exhibited large and spatially coherent subtidal fluctuations as a result of a strong coupling with the adjacent shelf. The characteristic volume exchange associated with this Bay-shelf coupling was an active simultaneous inflow or outflow through both ends of the Bay (with fluctuations in excess of 20 cm s−1) in response to the rise or fall of coastal sea level induced by longshore winds.

To assess the role of direct stochastic wind forcing in generating oceanic geostrophic eddies we calculate analytically the response of a simple ocean model to a realistic model wind-stress spectrum and compare the results with observations. The model is a continuously stratified, β-plane ocean of infinite horizontal extent and constant depth. All transfer and dissipation processes are parameterized by a linear scale-independent friction law (Rayleigh damping). The model predictions that are least sensitive to this parameterization, the total eddy energy and the subsurface displacement, are in good agreement with observations in mid-ocean regions far removed from strong currents. Properties that depend crucially on the parameterization of nonlinearities and topographic effects are not well reproduced. Observed coherences and seasonal modulations provide direct evidence of wind forcing at high frequencies where motions have little energy. Direct evidence at the more energetic low frequencies will ...

A numerical dynamic model based on primitive equations has been developed for Lake Ontario. Many experimental tests for parameter selections and alternative formulations of physical processes in the model were carried out. Two simulations, both repeated in July and December 1972, one under constant atmospheric forcing and the other under hourly time-dependent, spatially varying atmospheric forcing, are presented here. With constant southwesterly wind parallel to the major axis of the lake, as was the case in both July and December 1972, the steady-state lake circulation forms a typical two-gyre wind-driven pattern with currents flowing with the wind in both coastal regions and return flow in the middle of the lake. The monthly mean currents of the hourly time-dependent variable-forcing simulation of December have a pattern similar to that of the constant-forcing case, while the men currents of the July time-dependent variable-forcing simulation yield a single cyclonic circulation pattern with cur...

The trapped equatorial standing modes described theoretically by Gent (1979) are reproduced in a single vertical-mode numerical ocean model. integrations are carried out in domains whose longitudinal extents are characteristic of the widths of the Atlantic and Pacific Oceans, as well as in a narrow ocean in which the simplest possible standing mode can exist. The modes are shown to be very insensitive to small changes in basin width and to the inclusion of friction, and somewhat sensitive to the inclusion of the nonlinear terms and to rotation of the rectangular basin relative to the equator. Moreover, they arise spontaneously from simple atmospheric forcing or from random initial conditions. Typically, 20–40% of the energy input to the equatorial ocean remains trapped in a number of distinct standing modes after about nine years of integration time. The wider the ocean domain, the more energy remains trapped near the equator. These unexpected results have important implications for equatorial oc...

As part of a study of the large-scale response of coastal sea level to atmospheric forcing along the west coast of North America during June–September 1973, Halliwell and Allen calculate space- and time-lagged cross-correlation coefficients Rτζ between adjusted sea level ζ at fixed alongshore locations ζ(y0) and the alongshore component of the wind stress τ at general alongshore positions τ(y). Similarly, correlation coefficients Rζζ and Rττ involving, respectively, ζ(y0) versus ζ(y) and τ(y0) versus τ(y) are computed. The Rτζ correlations show a consistent asymmetry in time and space lag (tL, yL), with maximum values of Rτζ found for τ(y) to the south of ζ(y0) and leading in time. The Rττ correlations are typically symmetric in tL and in yL while Rζζ generally show sea level fluctuations to the south leading those to the north in time. It is shown here that the observed correlation coefficients involving ζ are consistent with those derived from solutions to the forced, first-order wave equation ...