Fresh water, oceans and climate

Influences of Net Atmospheric Freshwater on the Pacific and Atlantic oceans in Decadal Ocean and Climate Variability

One of the most important permanent features of the Earth system are the Tropical Warm Pools (TWPs), especially the Indo-Pacific Warm Pool (IPWP) and the west Atlantic Warm Pool (WAWP).  The TWPs contain much of the warmest and freshest surface ocean water on the Earth.  Annual-average sea-surface temperatures (SSTs) in the TWP regions usually exceed 28°C (Fig. 1).

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Figure 1:  The Indo-Pacific Warm Pool as outlined by SSTs ≥ 28°C.

Since saturation vapor pressure is an exponential function of SST, there can be a dramatic increase in atmospheric convection over the TWPs when the SST exceeds a threshold, typically 28°C-28.5°C. Therefore, even small changes in the TWP SST can cause large changes in atmospheric convection locally.  Such a change can significantly modify atmospheric boundary layer and convective processes locally, and substantially impact global atmospheric heating and planetary-scale wave activity. The annual, long-term average area of the IPWP on the eastern Indian Ocean side is approximately 6 million sq. km and on the western Pacific Ocean side is approximately 12 million sq. kms..  The surface area of the IPWP undergoes pronounced (50-100%) variability at decadal and longer timescales (Fig. 2).

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Figure 2:  Changes in the Indo-Pacific Warm Pool surface area and warmest SST during 1909-2000.

In addition to the observational results described above, modeling and observational results by other researchers show that the IPWP region is also a “fresh pool” because it receives copious amounts of rainfall and that this can influence IPWP temperature by modifying vertical heat mixing processes.  These results strongly suggest that coupled atmosphere-ocean processes may be responsible for interannual-multidecadal variability of the IPWP, in which interactions between upper ocean and the atmosphere via the net atmospheric freshwater flux, defined as evaporation minus precipitation (EmP) at the surface, may be very important.  Therefore,  the response of the IPWP, and the Pacific and Atlantic oceans to EmP at interannual and longer timescales was studied with the MIT ocean general circulation model (OGCM). The OGCM was forced by observed, monthly EmP from 1988 to 2000, derived from evaporation estimates from the Goddard Satellite Surface Turbulent Fluxes and precipitation estimates from the Global Precipitation Climatology Project; and by climatological heat and momentum fluxes. Observed interannual variations of sea-surface salinity (SSS) in the west Pacific Warm Pool were simulated successfully (Fig. 3). Our simulations show (Huang and Mehta, 2004) that the magnitude of interannual anomalies of salinity and temperature reaches about 0.7 psu and 0.4°C, respectively. The typical timescale of these interannual variabilities is about 3 – 5 years. The diagnosed budgets of salinity and temperature (heat) to estimate the role of advection and vertical mixing in response to the surface EmP forcing indicate that the salinity anomaly in the IPWP is largely due to vertical mixing, especially in the surface layer. The vertical mixing of salinity, in turn, is associated with the surface EmP anomaly. In contrast, the temperature anomaly above 300 m is primarily due to changes in advection forced by the EmP, which is associated with basin-wide changes in major ocean currents. Because of the strong effect of advection on the interannual variability of temperature, the temperature anomaly in the surface layer lags the salinity anomaly about 14 – 15 months.

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Figure 3: Observed and simulated sea-surface salinity anomalies in the West Pacific Warm Pool.

Model simulations also showed (Huang and Mehta, 2005) that the spatial distribution of the average SSS changes during the 1988 – 2000 period in the Pacific and Atlantic oceans resembled that of average EmP changes, because SSS changes were primarily associated with anomalous vertical mixing forced by the anomalous EmP. The spatial distribution of average near-surface temperature anomalies, however, was different from those of average EmP and SSS anomalies.  Analyses indicated that temperature changes in the subtropical North and South Pacific resulted from anomalous heat advection which, in turn, resulted from changes in the subtropical gyre circulations caused by anomalous EmP. Temperature changes in the Atlantic, however, were largely associated with vertical mixing changes due to anomalous EmP.

To further explore the magnitude of salinity and temperature anomalies and their generation processes in response to EmP anomalies, we studied the response of the Pacific Ocean to idealized EmP anomalies in the tropics and subtropics using the MIT OGCM. Simulations showed (Huang et al., 2005) that salinity anomalies generated by the anomalous EmP were spread throughout the Pacific basin by mean flow advection. This redistribution of salinity anomalies caused adjustments of basin-scale ocean currents, which further resulted in basin-scale temperature anomalies due to changes in heat advection caused by anomalous currents.  The temperature anomalies propagated from the tropical Pacific to the subtropical North and South Pacific via equatorial divergent Ekman flows and poleward western boundary currents, and they propagated from the subtropical North and South Pacific to the western tropical Pacific via equatorward-propagating coastal Kelvin waves and to the eastern tropical Pacific via eastward-propagating equatorial Kelvin waves.

The slower response of ocean temperatures in these simulations due to changes in basin-scale heat advection suggests the possibility that ocean and, perhaps, climate variability at interannual and longer timescales can be generated by large-scale EmP forcing at seasonal and longer timescales.

Influences of Freshwater from Major Rivers on Ocean Circulation and Temperature

Many large dams on rivers have been built in the last 100 years or so to store the freshwater for societal uses and more large dams are in the offing as the world’s thirst for freshwater continues to increase.  Not much is known, however, about the possible consequences of blocking the river water flowing into the world’s oceans.  Therefore, we have started to study responses of global ocean circulation and temperature to freshwater runoff from major rivers.  In the initial simulations, the runoff from several major rivers was selectively blocked in the MIT global ocean general circulation model.  Runoff into the tropical Atlantic, the western North Pacific, and the Bay of Bengal and northern Arabian Sea were selectively blocked, using monthly river run-off data from the world’s major rivers. The blocking of river runoff first resulted in a significant (2 psu) salinity increase near the river mouths (e.g., Fig. 4 for the Amazon River blocking).

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Figure 4:  Salinity differences (psu) in the upper 50m between blocked and unblocked Amazon River experiments.

The saltier and, therefore, denser water was then transported to higher latitudes in the North Atlantic, North Pacific, and southern Indian Ocean by mean currents. The subsequent density contrasts between northern and southern hemispheric oceans resulted in changes in major ocean currents. These anomalous ocean currents lead to significant temperature changes (1-2ºC) by the resulting anomalous heat transports (e.g., Fig. 5 for the Amazon River blocking). The current and temperature anomalies created by the blocked river runoff propagated from one ocean basin to others via coastal and equatorial Kelvin waves.

This initial study (Huang and Mehta, 2010) suggests that river runoff may be playing an important role in oceanic salinity, temperature, and circulations; and that partially or fully blocking major rivers to divert freshwater for societal purposes might significantly change ocean salinity, circulations, temperature, and atmospheric climate.

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Figure 5:  Zonally-averaged temperature differences (°C) in the Atlantic, Pacific, and Indian oceans between blocked and unblocked Amazon River experiments.