Sea Surface Temperature Tendency Research Articles

Anomalous Sea Surface Temperatures and Local Air–Sea Energy Exchange on Intraannual Timescales in the Northeastern Subtropical Pacific*

Abstract Here 11 years of surface data (1961–72, excluding 1963) taken at ocean weather ship N (OWS N) are analyzed. OWS N is located in the subtropical eastern Pacific Ocean (140°W, 30°N). Bulk formulas are employed to calculate each component of the surface heat flux (sensible, latent, longwave, and shortwave) from the 3-h measurements of sea surface temperature (SST), air temperature, surface humidity, wind speed, and cloudiness. Analyses are performed on fluxes averaged over daily and monthly intervals. Results indicate a large fraction of the variance in net surface energy flux is associated with anomalies in the latent heat flux; the latter are principally due to variability in the surface wind speed. Cross correlation and regression analyses of monthly anomalies of SST and SST tendency (∂SST/∂t) with the surface heat flux components indicate over 50% of the variance in SST-tendency anomalies is accounted for by local anomalies in the net surface energy flux. In the summer, the summed variance in th...

Decadal‐scale trends in mechanisms controlling meridional sea surface temperature gradients in the tropical Atlantic

The meridional gradient of sea surface temperature (SST) in the tropical Atlantic hydrostatically controls the atmospheric pressure gradient, thereby steering the latitude position of the Intertropical Convergence Zone (ITCZ), which affects rainfall in adjacent land areas. This study investigates the mechanisms controlling long‐term changes in meridional SST gradients, using surface and subsurface ship observations from 1951–1990. SST forcing from surface heat fluxes and entrainment are scaled by monthly climatic‐mean mixed‐layer depth. Observed SST tendency and contributions from various forcing mechanisms are computed for the 1951–1990 mean and trend. A substantial warming trend in the surface waters of the tropical South Atlantic is concentrated in the austral summer months of February–March, while SST in the tropical North Atlantic increased in boreal summer (August–September) and decreased in winter. As a consequence of the strengthened interhemispheric SST gradient in February–March, the position of the ITCZ was displaced southward, and rainfall in northeast Brazil increased over 1951–1990. A southward displacement of the ITCZ during boreal summer, accompanying warming between the equator and the ITCZ contributed to the decrease of rainfall over the west African Sahel. Strongest warming trends in the respective summer hemisphere imply an amplification of both seasonal cooling during winter and warming during summer in each hemisphere. During September–February a trend toward intensified northeast trade winds results in increased seasonal cooling due to latent heat transfer, whereas weakened southeast trades reduce latent heat loss, intensifying seasonal warming. For March–August a dominant mechanism forcing enhanced seasonal cooling in the South Atlantic is associated with entrainment as mixed‐layer depth increases during the transition to winter; this process acts to dilute warm anomalies developed during summer.

Variation in the relationship of the Indian summer monsoon with global factors

Utilizing data for the long period 1871–1990, variation in the relationships between Indian monsoon rainfall (IMR) and tendencies of the global factors. Southern Oscillation Index (SOI) and the sea surface temperature (SST) over eastern equatorial Pacific Ocean has been explored. The periods for which relationships exist have been identified. Tendencies from the season SON (Sept-Oct-Nov) to season DJF (Dec-Jan-Feb) and from DJF to MAM (Mar-Apr-May) before the Indian summer monsoon are indicated respectively by SOIT-2/SSTT-2 and SOIT-l/SSTT-1, current tendency from JJA (June-July-Aug) to SON, by SOIT0/SSTT0, tendencies from SON to DJF and DJF to MAM following monsoon, by SOIT1/SSTT1 and SOIT2/SSTT2 respectively. It is observed that while the relationships of IMR with SSTT-1, SSTT0 and SSTT2 exist almost throughout the whole period, that with SOIT-1 exists for 1942–1990, with SOIT0 for 1871–1921 and 1957–1990 and with SOIT2, for 1871–1921 only. The relationships that exist with SOIT-1, SOIT2, SSTT-1, SSTT2 and with SSTT0 (for period 1931–1990) are found to be very good and those that exist with SOIT0 for periods 1871–1921 and 1957–90 and for SSTT0 for the period 1871–1930 are good. It is thus seen that the relationships of SOIT-1, SOIT0 and SOIT2 with IMR do not correspond well with those of SSTT-1, SSTT0 and SSTT2 with IMR respectively, even though SOI and SST are closely related to each other for all the seasons. SOIT-1 and SSTT-1 can continue to be used as predictors for IMRDuring the whole period, IMR is found to play a passive, i.e. of being influenced or anticipated by SSTT-1 as well as an active role, i.e. of influencing or anticipating SSTT2. This implies a complex and perhaps non-linear interaction between IMR and SST tendency from DJF to MAM. Possibly, this is a part of the larger interaction between Asian monsoon rainfall and the tropical Pacific. A possible physical mechanism for the interaction is indicated.

Modeling the Low-Frequency Sea Surface Temperature Variability in the North Pacific

The question of whether the large-scale low-frequency sea surface temperature (SST) variability in the North Pacific can be interpreted as a response to large-scale wind anomalies is studied by an ocean general circulation model coupled to an advective model for the air temperature. Forced with observed monthly mean winds, the model is successful in reproducing the main space and time characteristics of the large-scale low-frequency SST variability. In winter also the simulated and observed SSTs are highly correlated. The dominant process in producing wintertime SST tendencies is the anomalous turbulent beat exchange with the atmosphere that is parameterized by the bulk aerodynamic formula and takes into account the simulated air temperature, the simulated SST, and the observed winds. The oceanic response to turbulent momentum fluxes is much smaller. The horizontal scale of the simulated air temperature is induced by advective transports with the observed winds and transferred to the ocean by anomalous turbulent latent and sensible heat fluxes. The ocean response is lagging the atmospheric forcing by about one month and persists over much longer time than the atmospheric anomalies, particularly in winter. Part of the observed low-frequency SST variance can be explained by teleconnection. A wind field that is directly related to the tropical El Niño–Southern Oscillation (ENSO) phenomenon produces SST anomalies with an ENSO-related variance of more than 50% instead of 10% to 30% as observed.