Proceedings of 21th Conference on Climate Variability and Change, Phoenix, 11-15 January 2009 7A.6 Detection of Thermohaline Structure and Meridional Overturning Circulation Above and Below the Ocean Surface Peter C. Chu1), Charles Sun2), and Chenwu Fan1) 1) Naval Ocean Analysis and Prediction Laboratory Department of Oceanography Naval Postgraduate School, Monterey, California 2) NOAA/NODC, Silver Spring, Maryland Since rapid changes in MOC could have 1. Introduction implications for regional changes of climate, correlation analysis between our reanalyzed Ocean temperature, salinity, and currents datasets (3D ocean fields) and the surface available at the present time do not have atmospheric data (such as NCEP reanalyzed sufficient resolution to describe the variability in wind stress, air-ocean heat and moisture fluxes) Meridional Overturning Circulation (MOC). We will improve understanding of the physical use our unique data access, our strong experience mechanisms behind fluctuations in the in analysis of sparse and noisy ocean data, and thermohaline structure and MOC. our data system already in place at NOAA/NODC and the Naval Postgraduate 2. Atlantic MOC School (NPS), to produce and distribute three dimensional temperature, salinity, and velocity A major feature of the basin-scale circulation in fields using the Global Temperature-Salinity the North Atlantic is the existence of the robust Profile Program (GTSPP) and Argo profile and AMOC. Driven largely by the deep-water track data together with the Navy’s Master formation in the Labrador and Nordic seas, the Observational Oceanographic Data Set AMOC is the main mechanism for northward (MOODS), at spatial resolution equal and higher heat transport in the Atlantic Ocean. Cross- than the standard product (1o × 1o). The equatorial heat flux associated with the AMOC temporal and spatial resolution was improved by occurs through northward transport of warm merging with data from the Ocean Surface surface waters from the southern Atlantic Ocean Current Analyses – Real Time (OSCAR) derived and southward transport of cold deep waters by from satellite altimeter and scatterometer. Close the deep western boundary current (DWBC). partnership between NPS and NOAA/NODC Changes in the Labrador Cold water formation shows the impact of the temporally varying affect the MOC and subsequently the sea surface temperature, salinity, and velocity data on temperature (SST) in the tropics (Curry and operational monitoring of MOC, thermohaline McCartney, 1996; Broeker, 1997; Curry et al., structure, and associated rapid climatic change. 1998). More flexible and user-driven data processing and distributing system will be implemented, to During the World Ocean Circulation optimize data use by both the scientific and Experiment (WOCE) 1990-1998, nine operational communities. With the reanalyzed realizations conducted along “48oN” section three dimensional ocean fields for two decades (from the English Channel to the Grand Banks of (1990-2008), we indentified temporal and spatial Newfoundland) showed significant inter-annual variability of MOC and thermohaline structure. variability in climate-relevant key parameters such as heat and fresh water transports. With a phase lag of one to two years, the transport is almost linearly correlated to the changes of the dominant mode of low-frequent atmospheric variability in the North Atlantic, the North 1) Corresponding author address: Peter C. Atlantic Oscillation (NAO) [Bersh, (2002)]. Chu, Naval Postgraduate School, Monterey, Frankignoul et al. (2001), Taylor and Stephens CA 93943, email: [email protected] (1998), Rossby and Benway (2000), Volkov (2005) and others have documented an seems to be a reason of the spatial variability approximately two year lag between the shifts of along 48oN and a 1-2 years phase lag between the Gulf Stream core and NAO events. NAO and variability of oceanic heat and freshwater transports in mid-latitudes (Brand and Hypothetically, such linkages can be explained Carsten, 2005). by traveling of oceanic perturbations generated by the atmosphere. From the theoretical point of To understand the AMOC variability due to view, Rossby waves significantly affect the anomalies in surface wind and/or buoyancy large-scale thermohaline ocean circulation and forcing, it is necessary to use the planetary wave transport across the North Atlantic basin from dynamics with detecting Rossby waves their east to west at speeds of a few cm/s (depending vertical scales and structure from observations on latitude) (Gill, 1982; Rhines, 2004). It takes (Liu, 1999; Yang, 2000). The Rossby wave months or years to cross the North Atlantic signatures are clearly detected from satellite Basin. Several scenarios for ocean wave altimetry [for example, Chelton and Schlax teleconnections between mid-latitude and (1996); Osychny and Cornillon (2004)], SST tropical Atlantic were proposed in scientific [Cipollini et al. (1997); Hill et al (2000)] and literature (Doscher et al., 1994; Huang et al., color [Cipolloni e t al. (2001); Killworth et al. 2000; Hakkinen and Mo, 2002; Yang and Joyce, (2004)] in the North Atlantic that allows 2003 and others). However, the question of what estimating some propagating features of long scenario is of the most importance in transferring baroclinic Rossby waves but not their vertical decadal signals between the mid-latitude and structure. Tropical Atlantic is still unanswered. At present, observational support for detecting Numerous studies (mostly using results of the AMOC variability is quite poor. This is numerical modeling) demonstrate strong wave mainly due to lack of three dimensional ocean variability in the tropics and connections data (temperature, salinity, velocity) for the between this variability and large-scale entire Atlantic with sufficient temporal and atmospheric perturbations. After analyzing the spatial resolution. The lack of data causes less flow of Antarctic Intermediate Water along the capability in identifying long baroclinic waves equator in an idealized regional model of the in the Atlantic other than satellite observations tropical Atlantic Ocean, Jochum and Malanotte- (Brand and Carsten, 2005) although Rossby Rizzoli (2003) found that the flow is dominated wave signal should appear in various ocean by the Rossby wave activity related to the annual fields, such as velocity, temperature, salinity and cycle. Thierry et al. (2004) studied the deep biological substances at different depths. seasonal variability in realistic and simplified Following Killworth et al. (2004), baroclinic GCMs of the equatorial Atlantic Ocean and Rossby waves in the North Atlantic may induce found that the annual velocity fluctuations are vertical displacements for ocean surface up to 10 dominated by the lowest odd meridional mode cm, for bottom boundary of thermocline up to 50 Rossby wave packets. These Rossby waves are –100 m and for nitrocline up to 10 m. Below generated by the reflection of the directly wind the thermohaline, observed large perturbations of driven, shallow Kelvin wave packets at the temperature and salinity also may be associated eastern boundary. These facts show the with the vertical structure of baroclinic Rossby importance of the Rossby and Kelvin wave waves. Thus, it is urgent to produce a quality dynamics in the inter-annual variability of mid- three dimensional dataset for temperature, latitude Atlantic circulation and in teleconnection salinity, and velocity dataset with sufficient (linkage) between tropical and mid-latitude inter- spatial resolution (1o × 1o) and temporal annual variability. Boning and Schott (1993) resolution (1-3 months) for the entire Atlantic for found deep current fluctuations with magnitude determining the AMOC variability. of 5 cm s-1 induced by the seasonal cycle of the wind stress and consistent with the long 3. Detection of MOC below the Ocean equatorial Rossby waves. Surface Therefore, the propagation of long baroclinic Argo is an internationally coordinated, broad- Rossby waves generated in the eastern North scale global array of temperature and salinity Atlantic in conjunction with the meridional profiling floats (Fig. 1), and a major component movement of the zero line of wind stress curl 2 of the global ocean observing system. There are both temperature and salinity profiles, (c) sound- 3,000 temperature and salinity profiling floats speed profiles, and (d) surface temperature deployed worldwide. (drifting buoy). The measurements in the MOODS are, in general, irregular in time and space. Due to the shear size and constant influx of data to the Naval Oceanographic Office from various sources, quality control is very important. The primary editing procedure included removal of profiles with obviously erroneous location, profiles with large spikes (temperature higher than 35oC and lower than – 2oC), and profiles displaying features that do not match the characteristics of surrounding profiles such as profiles showing increase of temperature with depth. The MOODS contains more than 6 Fig. 1. Argo float (after the website: million profiles worldwide (Chu, 2006). http://scrippsnews.ucsd.edu/Releases/?releaseID=6 96). 4. Detection of MOC above the Ocean The Global Temperature and Salinity Profile Surface Program (GTSPP) is a cooperative international project to develop and maintain a global ocean Data collected from satellite altimeter, Temperature-Salinity resource with data that are scatterometer, and SST sensors can be used to both up-to-date and of the highest quality.