Underlying Sea Surface Temperature Research Articles

Changes in Seasonal Predictability due to Global Warming

Abstract The change in predictability of monthly mean temperature in a future climate is quantified based on the Community Climate System Model, version 4. According to this model, the North Atlantic overtakes the El Niño–Southern Oscillation (ENSO) as the dominant area of seasonal predictability by 2095. This change arises partly because ENSO becomes less variable and partly because the ENSO teleconnection pattern expands into the Atlantic. Over land, the largest change in temperature predictability occurs in the tropics and is predominantly due to a decrease in ENSO variability. The southern peninsula of Africa and northeast South America are predicted to experience significant drying in a future climate, which decreases the effective heat capacity and memory, and hence increases variance independently of ENSO changes. Extratropical land areas experience enhanced precipitation in a future climate, which decreases temperature variance by the same mechanism. Finally, the model predicts that surface temperatures near the poles will become more predictable and less variable in a future climate, primarily because melting sea ice exposes the underlying sea surface temperature, which is more predictable owing to its longer time scale. Some of these results, especially the change in ENSO variance, are known to be model dependent. This paper also advances the use of information theory to quantify predictability, including 1) deriving a quantitative relation between predictability of the first and second kinds; 2) showing how differences in predictability can be decomposed in two dramatically different ways, facilitating physical interpretation; and 3) proposing a sample estimate of mutual information whose significance can be tested using standard techniques.

A response in the ENSO cycle to an extratropical forcing mechanism during the El Niño to La Niña transition

Current El Niño-Southern oscillation (ENSO) theory emphasizes that the forcing that drives the cycle mainly exists within tropical regions. However, these ideas are quite limited in explaining completely the occurrence of ENSO. Here, we examine whether extratropical forcing can affect ENSO cycle, specifically the transition from El Niño to La Niña. Although dispersion of the Okhotsk-Japan (OKJ) atmospheric wave train across the mid-latitude North Pacific during June terminates in the subtropics, the associated regime of southward surface wind anomalies could reach Eastern Equatorial Pacific (EEP). The OKJ wave train plays a substantial role in generating a similar underlying sea surface temperature (SST) wave train through a barotropic process in air–sea interactions and after September, it is negatively correlated strongly with the SST around EEP. Strong OKJ propagation in the positive (negative) phase during June is more (less) significantly associated with a subsequent La Niña (El Niño) episode that is matured after October. Negative SST anomalies at the southern end of the SST wave train with strong overlying OKJ propagation in the positive phase during June and the associated southward surface wind anomalies retained its strength by the further infusion of energy and gradual southward displacement joining the negative SST anomalies around EEP after the October when La Niña usually matured in-situ. Strong OKJ propagation in the positive phase during June tends to occur during a quick summer and fall transition period from El Niño to La Niña. This study strongly suggests that extratropical forcing plays an ignored role in affecting ENSO cycle especially in the formation of La Niña, which was not included in current ENSO theory.

Environmental influences on the intensity change of tropical cyclones in the western North Pacific

The atmospheric and oceanic conditions are examined during different stages of the lifecycle of western North Pacific tropical cyclones (TCs), with the intention to understand how the environment affects the intensity change of TCs in this area. It is found that the intensification usually occurs when the underlying sea surface temperature (SST) is higher than 26°C. TCs usually experience a rapid intensification when the SST is higher than 27.5°C while lower than 29.5°C. However, TCs decay or only maintain its intensity when the SST is lower than 26°C. The intensifying TCs usually experience a low-to-moderate vertical wind shear (2–10 m s−1). The larger the vertical wind shear, the slower the TCs strengthen. In addition, the convective available potential energy (CAPE) is much smaller in the developing stage than in the formation stage of TCs. For the rapidly intensifying TCs, the changes of SST, CAPE, and vertical wind shear are usually small, indicating that the rapid intensification of TCs occurs when the evolution of the environment is relatively slow.

Using the Sensitivity of Large-Eddy Simulations to Evaluate Atmospheric Boundary Layer Models

Abstract A simple framework to study the sensitivity of atmospheric boundary layer (ABL) models to the large-scale conditions and forcings is introduced. This framework minimizes the number of parameters necessary to describe the large-scale conditions, subsidence, and radiation. Using this framework, the sensitivity of the stationary ABL to the large-scale boundary conditions [underlying sea surface temperature (SST) and overlying humidity and temperature in the free troposphere] is investigated in large-eddy simulations (LESs). For increasing SST or decreasing free-tropospheric temperature, the LES exhibits a transition from a cloud-free, well-mixed ABL stationary state, through a cloudy, well-mixed stationary state and a stable shallow cumulus stationary state, to an unstable regime with a deepening shallow cumulus layer. For a warm SST, when increasing free-tropospheric humidity, the LES exhibits a transition from a stable shallow cumulus stationary state, through a stable cumulus-under-stratus stationary state, to an unstable regime with a deepening, cumulus-under-stratus layer. For a cool SST, when increasing the free-tropospheric humidity, the LES stationary state exhibits a transition from a cloud-free, well-mixed ABL regime, through a well-mixed cumulus-capped regime, to a stratus-capped regime with a decoupling between the subcloud and cloud layers. This dataset can be used to evaluate other ABL models. As an example, the sensitivity of a bulk model based on the mixing-line model is presented. This bulk model reproduces the LES sensitivity to SST and free-tropospheric temperature for the stable and unstable shallow cumulus regimes, but it is less successful at reproducing the LES sensitivity to free-tropospheric humidity for both shallow cumulus and well-mixed regimes.

Impact of 2007 and 2008 Arctic ice anomalies on the atmospheric circulation: Implications for long-range predictions

The impact on the atmospheric circulation of the unprecedented Arctic sea-ice anomalies during the summers 2007 and 2008 is evaluated using the atmospheric model of the ECMWF operational seasonal forecasting system. Results show that the ice anomaly had a significant impact on the atmospheric circulation over the Euro-Atlantic Sector, characterized by a high pressure over the Arctic (Greenland) and low pressure centres over Western Europe and Northwest America. The impact is similar for the two consecutive years, and it is consistent with the observed atmospheric anomalies. Results also show that the impact of the ice is strongly dependent on the underlying sea surface temperature. Results from partial coupling experiments indicate that the sea surface temperature over the Northwest Atlantic strongly affects the mean state of atmospheric circulation over the Euro-Atlantic sector (first-order impact), and conditions the response of the atmosphere to a given ice anomaly (second-order impact). The implications of these results for seasonal and long-term predictions are discussed. Copyright © 2010 Royal Meteorological Society

Atmospheric Sensitivity to SST near the Kuroshio Extension during the Extratropical Transition of Typhoon Tokage*

Abstract It is hypothesized that the tropical-to-extratropical transition of a cyclone in the western North Pacific can be sensitive to the underlying sea surface temperature (SST) distribution. This hypothesis was tested through a case study of Typhoon Tokage using a series of high-resolution simulations by the Weather Research Forecast (WRF) numerical weather prediction model. Simulations were carried out for a control SST distribution and for SST distributions with imposed warm and cold perturbations of 1.5°C maximum amplitude in the vicinity of the Kuroshio Extension. The simulations with the warm SST perturbation yielded a cyclone slightly weaker than in the control SST case about 2 days after transition. In contrast, the cold SST perturbation case yielded a cyclone with a central pressure 10 hPa lower than in the control case at the same point in the storm’s life cycle, apparently due to its more northward track and hence closer proximity to an approaching upper-level trough and perhaps in association with a stronger warm front. The effects of the regional SST on the simulated storms are manifested not just locally, but also cause substantial impacts on 500-hPa geopotential heights over much of the North Pacific basin. Retrospective analysis of meridional heat fluxes associated with these events using the NCEP–NCAR reanalysis was carried out for early fall (September–November) seasons with relatively warm and cool SST in the region of the imposed SST perturbations. Differences in the patterns of these fluxes between the warm and cool years are broadly consistent with the results from the warm versus cool SST simulations for Typhoon Tokage.

Changes in Potential Intensity of Tropical Cyclones Approaching Japan due to Anthropogenic Warming in Sea Surface and Upper-Air Temperatures

The intensities of the tropical cyclones (TCs) that approach or make landfall in Japan and the possible changes in these intensities due to global warming have been investigated on the basis of the theoretical model of Holland (1997). This theoretical model calculates the maximum potential intensity (MPI) of a TC as a lower limit of central sea level pressure for a given upper-air temperature profile and underlying sea surface temperature (SST). This study uses the climatology of the JRA-25 long-term reanalysis for the present environment, along with global warming anomalies derived from CMIP3 multi model climate experiments, which are added to the reanalysis climatology to form a warmed environment. A characteristic of the tropical atmosphere is that warming anomalies are amplified toward the upper troposphere, so that TC intensification due to a rise in SST is suppressed to some extent by greater warming aloft. This study evaluates the uncertainty ranges for the MPI changes by considering inter model differences in the warming structure.The MPI-SST relationship in the present climate is consistent with the historical minimum surface pressure in the western North Pacific when the eyewall relative humidity (RH), one of the model’s key parameters, is set at 88%. The average change in the MPI due to a 1°C rise in the SST is -6.7 hPa [-0.6 to -12.0 hPa], where a negative change indicates intensification and the estimated range in brackets reflects the uncertainty in the warming structure. The error of these MPI changes is estimated to be about 15%, assuming that the likely range of the RH parameter is 88 ± 2%. The fractional change in the surface pressure drop is 3.6%, 8.4%, and 19%, in response to a 0.5, 1.0, and 2.0C rise, respectively, in SST, which is basically comparable to existing studies based on observations and numerical simulations. While TC intensification due to global warming tends to be greater for intense TCs at low latitudes, the tendencies at relatively high latitudes near the main islands of Japan suggest that TC development occurs in a broader region over a more prolonged season in a warmed environment.

SST–Wind Interaction in Coastal Upwelling: Oceanic Simulation with Empirical Coupling

Abstract Observations, primarily from satellites, have shown a statistical relationship between the surface wind stress and underlying sea surface temperature (SST) on intermediate space and time scales, in many regions inclusive of eastern boundary upwelling current systems. In this paper, this empirical SST–wind stress relationship is utilized to provide a simple representation of mesoscale air–sea coupling for an oceanic model forced by surface winds, namely, the Regional Oceanic Modeling System (ROMS). This model formulation is applied to an idealized upwelling problem with prevailing equatorward winds to determine the coupling consequences on flow, SST, stratification, and wind evolutions. The initially uniform wind field adjusts through coupling to a cross-shore profile with weaker nearshore winds, similar to realistic ones. The modified wind stress weakens the nearshore upwelling circulation and increases SST in the coastal zone. The SST-induced wind stress curl strengthens offshore upwelling through Ekman suction. The total curl-driven upwelling exceeds the coastal upwelling. The SST-induced changes in the nearshore wind stress field also strengthen and broaden the poleward undercurrent. The coupling also shows significant impact on the developing mesoscale eddies by damaging cyclonic eddies more than anticyclonic eddies, which leads to dominance by the latter. Dynamically, this is a consequence of cyclones with stronger SST gradients that induce stronger wind perturbations in this particular upwelling problem and that are therefore generally more susceptible to disruption than anticyclones at finite Rossby number. The net effect is a weakening of eddy kinetic energy.

Characteristics and variability of synoptic features associated with cool season rainfall in southeastern Australia

AbstractCool season rainfall variability in southeastern Australia is investigated via classification and characterization of the predominant types of synoptic systems occurring in the region. These types are frontal systems, cut‐off low systems, and other systems. Rainfall in the region is dominated by cut‐off systems and these systems are the main influence on the interannual variability of rainfall. Both cut‐off systems and frontal systems display an enhancement of thermal (thickness) gradient as rainfall increases, but the mechanisms for intensification differ. Cut‐off systems intensify in the region in association with local increases in baroclinicity and the subtropical jet, whereas frontal systems tend to intensify via a confluence of subtropical and polar jets. Interannual rainfall variability is examined for groupings of years based on both clustering of continental rainfall patterns and on El Nino/Southern Oscillation (ENSO)/Indian Ocean Dipole (IOD) years. Cut‐off systems exhibit consistent enhancements of thermal gradients for groupings of years in which they produce more rainfall. For ENSO/IOD groupings, the cut‐off thermal gradients are consistent with the underlying sea surface temperature (SST) anomalies. Wet years in southeastern Australia are usually produced by cut‐off systems, but can also be produced by frontal systems. In those cases the mid‐tropospheric flow pattern is reminiscent of the negative Southern Annular Mode (SAM) pattern. The positive SAM pattern is also associated with enhanced rainfall in the southeast via local intensification of blocking and cut‐off systems. Copyright © 2008 Royal Meteorological Society

Observed impact of subtropical-midlatitude South Atlantic SST anomalies on the atmospheric circulation

[1] A maximum covariance analysis (MCA) of monthly anomaly data from the NCEP–NCAR reanalysis during 1981–2007 suggests that the atmosphere over subtropical-midlatitude South Atlantic during the austral late winter and spring is significantly correlated to the underlying sea surface temperature (SST) anomalies (SSTA) up to at least 4 months earlier. Such SST impact on the atmosphere is independent from the tropical Pacific and tropical South Atlantic influence, and is confirmed by the regression analysis based on the SSTA centers of action. The MCA pattern of SST resembles the dominant mode of SST variability and the monopole atmospheric signal is barotropic through the troposphere and hemispheric in extent. This implies predictability of the late winter and spring atmospheric circulation in the subtropical-midlatitude South Atlantic with a lead time of up to 4 months.

Interdecadal Changes in the Major Modes of Asian–Australian Monsoon Variability: Strengthening Relationship with ENSO since the Late 1970s*

Abstract The present paper develops an integral view of the year-to-year variability across the entire Asian–Australian monsoon (A–AM) system, which covers one-third of the global tropics between 40° and 160°E. Using season-reliant empirical orthogonal function (S-EOF) analysis, the authors identified two major modes of variability for the period 1956–2004. The first exhibits a prominent biennial tendency and concurs with the turnabout of El Niño–Southern Oscillation (ENSO), providing a new perspective of the seasonally evolving spatiotemporal structure for tropospheric biennial oscillation. The second mode leads ENSO by one year. The remote El Niño forcing, the monsoon–warm pool ocean interaction, and the influence of the annual cycle are three fundamental factors for understanding the behavior of the first mode. The monsoon–ocean interaction is characterized by a positive feedback between the off-equatorial convectively coupled Rossby waves and the underlying sea surface temperature (SST) “dipole” anomalies. Since the late 1970s the overall coupling between the A–AM system and ENSO has become strengthened. The relationships between ENSO and the western North Pacific, East Asian, and Indonesian monsoons have all become enhanced during ENSO’s developing, mature, and decaying phases, overriding the weakening of the Indian monsoon–ENSO anticorrelation during the developing phase. Prior to the late 1970s (1956–79), the first mode shows a strong biennial tendency, and the second mode does not lead ENSO. After 1980, the first mode shows a weakening biennial tendency, and the second mode provides a strong precursory signal for ENSO. These interdecadal changes are attributed to increased magnitude and periodicity of ENSO and the strengthened monsoon–ocean interaction.

Observational Study on Atmospheric and Oceanic Boundary-Layer Structures Accompanying the Okhotsk Anticyclone under Fog and Non-Fog Conditions

A series of GPS radiosonde and oceanic observations was conducted over the Okhotsk Sea for the first time in July 1998. Under the prevalent anticyclone over the Okhotsk Sea during the observations, distinct differences in the atmospheric and oceanic boundary-layer structure were observed between the fog and non-fog periods. During the fog period, the observed strong surface winds and upward sensible heat fluxes promoted vertical mixing in the atmosphere and ocean. The height of the well-mixed marine boundary layer varied from 200 to 1000m in the atmosphere, and 10 to 15 m in the ocean, respectively. The fog formation occurred when surface air temperature (SAT) was cooler than the underlying sea surface temperature (SST). By trajectory analyses for the observed well-mixed atmospheric boundary layer during the fog period, the boundary-layer air mass that has passed over warm (cool) SST tended to form stratus-like (stratocumulus-like) fog. In the non-fog period, by contrast, atmospheric and oceanic boundary layers were stably stratified due to weak surface winds and downward sensible heat fluxes. Regardless of the air mass trajectory, SAT is warmer than underlying SST during the no-fog period.

Negative ocean‐atmosphere feedback in the South Atlantic Convergence Zone

The temporal evolution of the coupled variability between the South Atlantic Convergence Zone (SACZ) and the underlying sea surface temperature (SST) during austral summer is investigated using monthly data from the NCEP/NCAR reanalysis. A maximum covariance analysis shows that the SACZ is intensified [weakened] by warm [cold] SST anomalies in the beginning of summer, drifting northward. This migration is accompanied by the cooling [warming] of the original oceanic anomalies. The results confirm earlier analyses using numerical models that suggest the existence of a negative feedback between the SACZ and the underlying South Atlantic SST field. A linear regression of daily anomalies of SST and omega at 500 hPa to the equations of a stochastic oscillator reveals a negative ocean–atmosphere feedback in the western South Atlantic, stronger during January and February directly underneath the oceanic band of the SACZ.

The Physical Basis for Predicting Atlantic Sector Seasonal-to-Interannual Climate Variability*

Abstract This paper reviews the observational and theoretical basis for the prediction of seasonal-to-interannual (S/I) climate variability in the Atlantic sector. The emphasis is on the large-scale picture rather than on regional details. The paper is divided into two main parts: a discussion of the predictability of the North Atlantic Oscillation (NAO)—the dominant pattern of variability in the North Atlantic—and a review of the tropical Atlantic prediction problem. The remote effects of El Niño are also mentioned as an important factor in Atlantic climate variability. Only a brief discussion is provided on the subject of South Atlantic climate predictability. Because of its chaotic dynamical nature, the NAO and its related rainfall and temperature variability, while highly significant over Europe and North America, are largely unpredictable. This also affects the predictive skill over the tropical Atlantic, because the NAO interferes with the remote influence of El Niño. That said, there appears to be an insufficiently understood, marginal signal in the NAO behavior that may be predictable and thus useful to certain end users. It is manifested in the deviation of the NAO temporal behavior from first-order autoregressive behavior. Tropical Atlantic climate variability centers on the sensitivity of the marine ITCZ to remote forcing from the equatorial Pacific and interactions with underlying sea surface temperature (SST) variability. Both mechanisms are potentially predictable—that is, given the underlying SSTs and the strength of El Niño, one could determine with a high degree of skill the anomalies in ITCZ position and intensity. However, local SSTs are easily affected by largely unpredictable North and South Atlantic phenomena, such as the NAO. In addition, the local ocean–atmosphere coupling in the Atlantic acts on relatively short time scales. Thus, in reality the level of skill indicated by forced model simulations are difficult to achieve. The use of coupled models may improve the prospects of tropical Atlantic prediction.

High-Frequency Variability in Hurricane Power Dissipation and Its Relationship to Global Temperature

The power dissipation of Atlantic tropical cyclones has risen dramatically during the last decades and the increase is correlated with an increase in the underlying sea surface temperature (SST) at low (decadal) frequencies. Because of the large positive correlation between global mean surface air temperature (GT) and Atlantic SST it has been speculated that increases in the power dissipation might, in part, be related to human activity. Here we investigate the question of the relationship between GT and hurricane power dissipation directly using statistical analysis and show that after removing the effect of SST, the correlation between GT and hurricane power dissipation is negative. This suggests that the positive influence of global temperature on Atlantic hurricanes appears to be limited to an indirect connection with tropical Atlantic SST. We also show that the relationship between hurricane power dissipation and Atlantic SST is significant at the high-frequency time scales. El Niño–Southern Oscillation (ENSO) plays an important role in statistically explaining the variations in hurricane power at these higher frequencies.

Estimation of the Surface Heat Flux Response to Sea Surface Temperature Anomalies over the Global Oceans

Abstract The surface heat flux response to underlying sea surface temperature (SST) anomalies (the surface heat flux feedback) is estimated using 42 yr (1956–97) of ship-derived monthly turbulent heat fluxes and 17 yr (1984–2000) of satellite-derived monthly radiative fluxes over the global oceans for individual seasons. Net surface heat flux feedback is generally negative (i.e., a damping of the underlying SST anomalies) over the global oceans, although there is considerable geographical and seasonal variation. Over the North Pacific Ocean, net surface heat flux feedback is dominated by the turbulent flux component, with maximum values (28 W m−2 K−1) in December–February and minimum values (5 W m−2 K−1) in May–July. These seasonal variations are due to changes in the strength of the climatological mean surface wind speed and the degree to which the near-surface air temperature and humidity adjust to the underlying SST anomalies. Similar features are observed over the extratropical North Atlantic Ocean with maximum (minimum) feedback values of approximately 33 W m−2 K−1 (9 W m−2 K−1) in December–February (June–August). Although the net surface heat flux feedback may be negative, individual components of the feedback can be positive depending on season and location. For example, over the midlatitude North Pacific Ocean during late spring to midsummer, the radiative flux feedback associated with marine boundary layer clouds and fog is positive, and results in a significant enhancement of the month-to-month persistence of SST anomalies, nearly doubling the SST anomaly decay time from 2.8 to 5.3 months in May–July. Several regions are identified with net positive heat flux feedback: the tropical western North Atlantic Ocean during boreal winter, the Namibian stratocumulus deck off West Africa during boreal fall, and the Indian Ocean during boreal summer and fall. These positive feedbacks are mainly associated with the following atmospheric responses to positive SST anomalies: 1) reduced surface wind speed (positive turbulent heat flux feedback) over the tropical western North Atlantic and Indian Oceans, 2) reduced marine boundary layer stratocumulus cloud fraction (positive shortwave radiative flux feedback) over the Namibian stratocumulus deck, and 3) enhanced atmospheric water vapor (positive longwave radiative flux feedback) in the vicinity of the tropical deep convection region over the Indian Ocean that exceeds the negative shortwave radiative flux feedback associated with enhanced cloudiness.

Northern Hemispheric Trends of Pressure Indices and Atmospheric Circulation Patterns in Observations, Reconstructions, and Coupled GCM Simulations

Abstract The decadal trend behavior of the Northern Hemisphere atmospheric circulation is investigated utilizing long-term simulations with different state-of-the-art coupled general circulation models (GCMs) for present-day climate conditions (1990), reconstructions of the past 500 yr, and observations. The multimodel simulations show that strong positive winter North Atlantic Oscillation (NAO) trends are connected with the underlying sea surface temperature (SST) and exhibit an SST tripole trend pattern and a northward shift of the storm-track tail. Strong negative winter trends of the Aleutian low are associated with SST changes in the El Niño–Southern Oscillation (ENSO) region and a westward shift of the storm track in the North Pacific. The observed simultaneous appearance of strong positive NAO and negative Aleutian low trends is very unlikely to occur by chance in the unforced simulations and reconstructions. The positive winter NAO trend of the last 50 yr is not statistically different from the level of internal atmosphere–ocean variability. The unforced simulations also show a strong link between positive SST trends in the ENSO region and negative Aleutian low trends. With much larger observed SST trends in the ENSO region, this suggests that the observed negative Aleutian low trend is possibly influenced by external forcing, for example, global warming, volcanism, and/or solar activity change.

Comparison of local precipitation–SST relationship between the observation and a reanalysis dataset

We defined a grid–scale time–phase relationship between precipitation (PR) and underlying sea surface temperature (SST) as the local PR–SST relationship, and compared the spatial distribution of the relationship in the observation with that in a reanalysis dataset. An analysis using observed pentad mean data shows precipitation lagging SST for two pentads over large areas of tropical oceans. A reanalysis dataset, on the other hand, has a shorter time–phase difference between precipitation and SST than that in the observation and does not match the spatial distribution of the observed local PR–SST relationship over the tropical Indian Ocean and the tropical western Pacific. A reanalysis dataset may not capture the observed local PR–SST relationship in terms of either time–phase difference or spatial distribution because a numerical model in a data assimilation system uses SST as a lower boundary condition as well as the inhomogeneity of observations the system used.

Coupling between Northward-Propagating, Intraseasonal Oscillations and Sea Surface Temperature in the Indian Ocean*

Using a hybrid atmosphere–ocean coupled model, it is shown that during the boreal summer northward-propagating, intraseasonal oscillations (NPISOs) are strongly coupled to the underlying sea surface temperature (SST) in the Indian Ocean sector. On the one hand, the intraseasonal atmospheric convection changes the SST through solar radiation, latent heat flux, and mixed-layer entrainment; on the other, the induced SST fluctuations feed back to affect the intraseasonal convection. The preferential northward, rather than southward, propagation of boreal summer ISOs in the Indian Ocean is partially explained by an interaction among the summer-mean climate state, the atmospheric disturbances, and the ocean surface temperature. A solution to an atmosphere-only model forced with daily SST produces much stronger NPISOs than a similar solution forced with monthly mean SST (AMIP-type run). The atmosphere-only model, however, even when it is forced by daily SST from the coupled model (with a small amount of noise in the initial and/or boundary conditions), is unable to reproduce the NPISOs in the coupled case. In the coupled system, intraseasonal SST anomalies are forced by intraseasonal atmospheric convection, and hence are in quadrature with the convection. In the stand-alone atmospheric model, however, SST acts only as a boundary forcing, and the resultant atmospheric convection has almost the same phase with the underlying SST. One consequence is that the intensity of the SST-forced intraseasonal convection in the stand-alone atmospheric model is considerably weaker than in the coupled model. Finally, solutions indicate that the northward movement of the off-equatorial convection in the northern Indian Ocean is more closely related to local intraseasonal SST anomalies than to the equatorial eastward-moving Madden–Julian oscillation: Positive (negative) SST anomalies in the northern Indian Ocean lead the active (break) phases of the intraseasonal convection by about 2 pentads (10 days). Therefore, intraseasonal SST anomalies in the northern Indian Ocean are potentially a useful index to forecast active (break) spells of the south Asian summer monsoon.

Baroclinically Induced Tropical Cyclogenesis

Abstract The authors investigate the transition of numerous subtropical cyclones into late season tropical storms and hurricanes during the 2000 and 2001 Atlantic tropical cyclone seasons. In all transitioning cases (10), the 900–200-hPa wind shear was initially near or in excess of the upper limit of vertical shear deemed suitable for tropical cyclogenesis. In many of these cases, the vertical shear decreased markedly near or prior to the time of tropical cyclone formation. In cases that did not become named tropical cyclones, either the tropospheric vertical shear remained in excess of 15–20 m s−1 or the underlying sea surface temperature (SST) dropped below about 26°C prior to or during the weakening of the shear. Cases in which the shear remained large featured multiple, short-wave upper-tropospheric troughs interacting with the developing lower-tropospheric disturbance such that classical occlusion did not occur. Through detailed analysis and simulation of the development of Hurricane Michael in 2000...